diff --git a/src/etc/gdb_rust_pretty_printing.py b/src/etc/gdb_rust_pretty_printing.py
index 0914c22eb13f0..cae64ef6665bb 100755
--- a/src/etc/gdb_rust_pretty_printing.py
+++ b/src/etc/gdb_rust_pretty_printing.py
@@ -370,12 +370,17 @@ def to_string(self):
                 ("(len: %i)" % self.__val.get_wrapped_value()['map']['length']))
 
     def children(self):
-        root = self.__val.get_wrapped_value()['map']['root']
-        node_ptr = root['node']
-        i = 0
-        for child in children_of_node(node_ptr, root['height'], False):
-            yield (str(i), child)
-            i = i + 1
+        prev_idx = None
+        innermap = GdbValue(self.__val.get_wrapped_value()['map'])
+        if innermap.get_wrapped_value()['length'] > 0:
+            root = GdbValue(innermap.get_wrapped_value()['root'])
+            type_name = str(root.type.ty.name).replace('core::option::Option<', '')[:-1]
+            root = root.get_wrapped_value().cast(gdb.lookup_type(type_name))
+            node_ptr = root['node']
+            i = 0
+            for child in children_of_node(node_ptr, root['height'], False):
+                yield (str(i), child)
+                i = i + 1
 
 
 class RustStdBTreeMapPrinter(object):
@@ -391,13 +396,16 @@ def to_string(self):
                 ("(len: %i)" % self.__val.get_wrapped_value()['length']))
 
     def children(self):
-        root = self.__val.get_wrapped_value()['root']
-        node_ptr = root['node']
-        i = 0
-        for child in children_of_node(node_ptr, root['height'], True):
-            yield (str(i), child[0])
-            yield (str(i), child[1])
-            i = i + 1
+        if self.__val.get_wrapped_value()['length'] > 0:
+            root = GdbValue(self.__val.get_wrapped_value()['root'])
+            type_name = str(root.type.ty.name).replace('core::option::Option<', '')[:-1]
+            root = root.get_wrapped_value().cast(gdb.lookup_type(type_name))
+            node_ptr = root['node']
+            i = 0
+            for child in children_of_node(node_ptr, root['height'], True):
+                yield (str(i), child[0])
+                yield (str(i), child[1])
+                i = i + 1
 
 
 class RustStdStringPrinter(object):
diff --git a/src/liballoc/collections/btree/map.rs b/src/liballoc/collections/btree/map.rs
index 9da324ba2d4f1..3ba7befc04609 100644
--- a/src/liballoc/collections/btree/map.rs
+++ b/src/liballoc/collections/btree/map.rs
@@ -122,7 +122,7 @@ use UnderflowResult::*;
 /// ```
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct BTreeMap<K, V> {
-    root: node::Root<K, V>,
+    root: Option<node::Root<K, V>>,
     length: usize,
 }
 
@@ -147,10 +147,11 @@ impl<K: Clone, V: Clone> Clone for BTreeMap<K, V> {
         {
             match node.force() {
                 Leaf(leaf) => {
-                    let mut out_tree = BTreeMap { root: node::Root::new_leaf(), length: 0 };
+                    let mut out_tree = BTreeMap { root: Some(node::Root::new_leaf()), length: 0 };
 
                     {
-                        let mut out_node = match out_tree.root.as_mut().force() {
+                        let root = out_tree.root.as_mut().unwrap();
+                        let mut out_node = match root.as_mut().force() {
                             Leaf(leaf) => leaf,
                             Internal(_) => unreachable!(),
                         };
@@ -169,9 +170,14 @@ impl<K: Clone, V: Clone> Clone for BTreeMap<K, V> {
                 }
                 Internal(internal) => {
                     let mut out_tree = clone_subtree(internal.first_edge().descend());
+                    out_tree.ensure_root_is_owned();
 
                     {
-                        let mut out_node = out_tree.root.push_level();
+                        // Ideally we'd use the return of ensure_root_is_owned
+                        // instead of re-unwrapping here but unfortunately that
+                        // borrows all of out_tree and we need access to the
+                        // length below.
+                        let mut out_node = out_tree.root.as_mut().unwrap().push_level();
                         let mut in_edge = internal.first_edge();
                         while let Ok(kv) = in_edge.right_kv() {
                             let (k, v) = kv.into_kv();
@@ -190,7 +196,7 @@ impl<K: Clone, V: Clone> Clone for BTreeMap<K, V> {
                                 (root, length)
                             };
 
-                            out_node.push(k, v, subroot);
+                            out_node.push(k, v, subroot.unwrap_or_else(|| node::Root::new_leaf()));
                             out_tree.length += 1 + sublength;
                         }
                     }
@@ -203,9 +209,9 @@ impl<K: Clone, V: Clone> Clone for BTreeMap<K, V> {
         if self.is_empty() {
             // Ideally we'd call `BTreeMap::new` here, but that has the `K:
             // Ord` constraint, which this method lacks.
-            BTreeMap { root: node::Root::shared_empty_root(), length: 0 }
+            BTreeMap { root: None, length: 0 }
         } else {
-            clone_subtree(self.root.as_ref())
+            clone_subtree(self.root.as_ref().unwrap().as_ref())
         }
     }
 
@@ -271,14 +277,14 @@ where
     type Key = K;
 
     fn get(&self, key: &Q) -> Option<&K> {
-        match search::search_tree(self.root.as_ref(), key) {
+        match search::search_tree(self.root.as_ref()?.as_ref(), key) {
             Found(handle) => Some(handle.into_kv().0),
             GoDown(_) => None,
         }
     }
 
     fn take(&mut self, key: &Q) -> Option<K> {
-        match search::search_tree(self.root.as_mut(), key) {
+        match search::search_tree(self.root.as_mut()?.as_mut(), key) {
             Found(handle) => Some(
                 OccupiedEntry { handle, length: &mut self.length, _marker: PhantomData }
                     .remove_kv()
@@ -290,7 +296,7 @@ where
 
     fn replace(&mut self, key: K) -> Option<K> {
         self.ensure_root_is_owned();
-        match search::search_tree::<marker::Mut<'_>, K, (), K>(self.root.as_mut(), &key) {
+        match search::search_tree::<marker::Mut<'_>, K, (), K>(self.root.as_mut()?.as_mut(), &key) {
             Found(handle) => Some(mem::replace(handle.into_kv_mut().0, key)),
             GoDown(handle) => {
                 VacantEntry { key, handle, length: &mut self.length, _marker: PhantomData }
@@ -344,15 +350,18 @@ pub struct IterMut<'a, K: 'a, V: 'a> {
 /// [`BTreeMap`]: struct.BTreeMap.html
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct IntoIter<K, V> {
-    front: Handle<NodeRef<marker::Owned, K, V, marker::Leaf>, marker::Edge>,
-    back: Handle<NodeRef<marker::Owned, K, V, marker::Leaf>, marker::Edge>,
+    front: Option<Handle<NodeRef<marker::Owned, K, V, marker::Leaf>, marker::Edge>>,
+    back: Option<Handle<NodeRef<marker::Owned, K, V, marker::Leaf>, marker::Edge>>,
     length: usize,
 }
 
 #[stable(feature = "collection_debug", since = "1.17.0")]
 impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for IntoIter<K, V> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let range = Range { front: self.front.reborrow(), back: self.back.reborrow() };
+        let range = Range {
+            front: self.front.as_ref().map(|f| f.reborrow()),
+            back: self.back.as_ref().map(|b| b.reborrow()),
+        };
         f.debug_list().entries(range).finish()
     }
 }
@@ -417,8 +426,8 @@ pub struct ValuesMut<'a, K: 'a, V: 'a> {
 /// [`BTreeMap`]: struct.BTreeMap.html
 #[stable(feature = "btree_range", since = "1.17.0")]
 pub struct Range<'a, K: 'a, V: 'a> {
-    front: Handle<NodeRef<marker::Immut<'a>, K, V, marker::Leaf>, marker::Edge>,
-    back: Handle<NodeRef<marker::Immut<'a>, K, V, marker::Leaf>, marker::Edge>,
+    front: Option<Handle<NodeRef<marker::Immut<'a>, K, V, marker::Leaf>, marker::Edge>>,
+    back: Option<Handle<NodeRef<marker::Immut<'a>, K, V, marker::Leaf>, marker::Edge>>,
 }
 
 #[stable(feature = "collection_debug", since = "1.17.0")]
@@ -437,8 +446,8 @@ impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for Range<'_, K, V> {
 /// [`BTreeMap`]: struct.BTreeMap.html
 #[stable(feature = "btree_range", since = "1.17.0")]
 pub struct RangeMut<'a, K: 'a, V: 'a> {
-    front: Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>,
-    back: Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>,
+    front: Option<Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>>,
+    back: Option<Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>>,
 
     // Be invariant in `K` and `V`
     _marker: PhantomData<&'a mut (K, V)>,
@@ -447,7 +456,10 @@ pub struct RangeMut<'a, K: 'a, V: 'a> {
 #[stable(feature = "collection_debug", since = "1.17.0")]
 impl<K: fmt::Debug, V: fmt::Debug> fmt::Debug for RangeMut<'_, K, V> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        let range = Range { front: self.front.reborrow(), back: self.back.reborrow() };
+        let range = Range {
+            front: self.front.as_ref().map(|f| f.reborrow()),
+            back: self.back.as_ref().map(|b| b.reborrow()),
+        };
         f.debug_list().entries(range).finish()
     }
 }
@@ -544,7 +556,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn new() -> BTreeMap<K, V> {
-        BTreeMap { root: node::Root::shared_empty_root(), length: 0 }
+        BTreeMap { root: None, length: 0 }
     }
 
     /// Clears the map, removing all elements.
@@ -589,7 +601,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         K: Borrow<Q>,
         Q: Ord,
     {
-        match search::search_tree(self.root.as_ref(), key) {
+        match search::search_tree(self.root.as_ref()?.as_ref(), key) {
             Found(handle) => Some(handle.into_kv().1),
             GoDown(_) => None,
         }
@@ -616,7 +628,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         K: Borrow<Q>,
         Q: Ord,
     {
-        match search::search_tree(self.root.as_ref(), k) {
+        match search::search_tree(self.root.as_ref()?.as_ref(), k) {
             Found(handle) => Some(handle.into_kv()),
             GoDown(_) => None,
         }
@@ -645,7 +657,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         T: Ord,
         K: Borrow<T>,
     {
-        let front = self.root.as_ref().first_leaf_edge();
+        let front = self.root.as_ref()?.as_ref().first_leaf_edge();
         front.right_kv().ok().map(Handle::into_kv)
     }
 
@@ -674,7 +686,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         T: Ord,
         K: Borrow<T>,
     {
-        let front = self.root.as_mut().first_leaf_edge();
+        let front = self.root.as_mut()?.as_mut().first_leaf_edge();
         if let Ok(kv) = front.right_kv() {
             Some(OccupiedEntry {
                 handle: kv.forget_node_type(),
@@ -708,7 +720,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         T: Ord,
         K: Borrow<T>,
     {
-        let back = self.root.as_ref().last_leaf_edge();
+        let back = self.root.as_ref()?.as_ref().last_leaf_edge();
         back.left_kv().ok().map(Handle::into_kv)
     }
 
@@ -737,7 +749,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         T: Ord,
         K: Borrow<T>,
     {
-        let back = self.root.as_mut().last_leaf_edge();
+        let back = self.root.as_mut()?.as_mut().last_leaf_edge();
         if let Ok(kv) = back.left_kv() {
             Some(OccupiedEntry {
                 handle: kv.forget_node_type(),
@@ -801,7 +813,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         K: Borrow<Q>,
         Q: Ord,
     {
-        match search::search_tree(self.root.as_mut(), key) {
+        match search::search_tree(self.root.as_mut()?.as_mut(), key) {
             Found(handle) => Some(handle.into_kv_mut().1),
             GoDown(_) => None,
         }
@@ -896,7 +908,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         K: Borrow<Q>,
         Q: Ord,
     {
-        match search::search_tree(self.root.as_mut(), key) {
+        match search::search_tree(self.root.as_mut()?.as_mut(), key) {
             Found(handle) => Some(
                 OccupiedEntry { handle, length: &mut self.length, _marker: PhantomData }
                     .remove_entry(),
@@ -992,11 +1004,15 @@ impl<K: Ord, V> BTreeMap<K, V> {
         K: Borrow<T>,
         R: RangeBounds<T>,
     {
-        let root1 = self.root.as_ref();
-        let root2 = self.root.as_ref();
-        let (f, b) = range_search(root1, root2, range);
+        if let Some(root) = &self.root {
+            let root1 = root.as_ref();
+            let root2 = root.as_ref();
+            let (f, b) = range_search(root1, root2, range);
 
-        Range { front: f, back: b }
+            Range { front: Some(f), back: Some(b) }
+        } else {
+            Range { front: None, back: None }
+        }
     }
 
     /// Constructs a mutable double-ended iterator over a sub-range of elements in the map.
@@ -1036,11 +1052,15 @@ impl<K: Ord, V> BTreeMap<K, V> {
         K: Borrow<T>,
         R: RangeBounds<T>,
     {
-        let root1 = self.root.as_mut();
-        let root2 = unsafe { ptr::read(&root1) };
-        let (f, b) = range_search(root1, root2, range);
+        if let Some(root) = &mut self.root {
+            let root1 = root.as_mut();
+            let root2 = unsafe { ptr::read(&root1) };
+            let (f, b) = range_search(root1, root2, range);
 
-        RangeMut { front: f, back: b, _marker: PhantomData }
+            RangeMut { front: Some(f), back: Some(b), _marker: PhantomData }
+        } else {
+            RangeMut { front: None, back: None, _marker: PhantomData }
+        }
     }
 
     /// Gets the given key's corresponding entry in the map for in-place manipulation.
@@ -1065,7 +1085,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
     pub fn entry(&mut self, key: K) -> Entry<'_, K, V> {
         // FIXME(@porglezomp) Avoid allocating if we don't insert
         self.ensure_root_is_owned();
-        match search::search_tree(self.root.as_mut(), &key) {
+        match search::search_tree(self.root.as_mut().unwrap().as_mut(), &key) {
             Found(handle) => {
                 Occupied(OccupiedEntry { handle, length: &mut self.length, _marker: PhantomData })
             }
@@ -1077,7 +1097,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
 
     fn from_sorted_iter<I: Iterator<Item = (K, V)>>(&mut self, iter: I) {
         self.ensure_root_is_owned();
-        let mut cur_node = self.root.as_mut().last_leaf_edge().into_node();
+        let mut cur_node = self.root.as_mut().unwrap().as_mut().last_leaf_edge().into_node();
         // Iterate through all key-value pairs, pushing them into nodes at the right level.
         for (key, value) in iter {
             // Try to push key-value pair into the current leaf node.
@@ -1126,7 +1146,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
 
     fn fix_right_edge(&mut self) {
         // Handle underfull nodes, start from the top.
-        let mut cur_node = self.root.as_mut();
+        let mut cur_node = self.root.as_mut().unwrap().as_mut();
         while let Internal(internal) = cur_node.force() {
             // Check if right-most child is underfull.
             let mut last_edge = internal.last_edge();
@@ -1187,14 +1207,14 @@ impl<K: Ord, V> BTreeMap<K, V> {
         let total_num = self.len();
 
         let mut right = Self::new();
-        right.root = node::Root::new_leaf();
-        for _ in 0..(self.root.as_ref().height()) {
-            right.root.push_level();
+        let right_root = right.ensure_root_is_owned();
+        for _ in 0..(self.root.as_ref().unwrap().as_ref().height()) {
+            right_root.push_level();
         }
 
         {
-            let mut left_node = self.root.as_mut();
-            let mut right_node = right.root.as_mut();
+            let mut left_node = self.root.as_mut().unwrap().as_mut();
+            let mut right_node = right.root.as_mut().unwrap().as_mut();
 
             loop {
                 let mut split_edge = match search::search_node(left_node, key) {
@@ -1223,7 +1243,9 @@ impl<K: Ord, V> BTreeMap<K, V> {
         self.fix_right_border();
         right.fix_left_border();
 
-        if self.root.as_ref().height() < right.root.as_ref().height() {
+        if self.root.as_ref().unwrap().as_ref().height()
+            < right.root.as_ref().unwrap().as_ref().height()
+        {
             self.recalc_length();
             right.length = total_num - self.len();
         } else {
@@ -1261,19 +1283,19 @@ impl<K: Ord, V> BTreeMap<K, V> {
             res
         }
 
-        self.length = dfs(self.root.as_ref());
+        self.length = dfs(self.root.as_ref().unwrap().as_ref());
     }
 
     /// Removes empty levels on the top.
     fn fix_top(&mut self) {
         loop {
             {
-                let node = self.root.as_ref();
+                let node = self.root.as_ref().unwrap().as_ref();
                 if node.height() == 0 || node.len() > 0 {
                     break;
                 }
             }
-            self.root.pop_level();
+            self.root.as_mut().unwrap().pop_level();
         }
     }
 
@@ -1281,7 +1303,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         self.fix_top();
 
         {
-            let mut cur_node = self.root.as_mut();
+            let mut cur_node = self.root.as_mut().unwrap().as_mut();
 
             while let Internal(node) = cur_node.force() {
                 let mut last_kv = node.last_kv();
@@ -1307,7 +1329,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
         self.fix_top();
 
         {
-            let mut cur_node = self.root.as_mut();
+            let mut cur_node = self.root.as_mut().unwrap().as_mut();
 
             while let Internal(node) = cur_node.force() {
                 let mut first_kv = node.first_kv();
@@ -1326,13 +1348,6 @@ impl<K: Ord, V> BTreeMap<K, V> {
 
         self.fix_top();
     }
-
-    /// If the root node is the shared root node, allocate our own node.
-    fn ensure_root_is_owned(&mut self) {
-        if self.root.is_shared_root() {
-            self.root = node::Root::new_leaf();
-        }
-    }
 }
 
 #[stable(feature = "rust1", since = "1.0.0")]
@@ -1458,12 +1473,21 @@ impl<K, V> IntoIterator for BTreeMap<K, V> {
     type IntoIter = IntoIter<K, V>;
 
     fn into_iter(self) -> IntoIter<K, V> {
-        let root1 = unsafe { ptr::read(&self.root).into_ref() };
-        let root2 = unsafe { ptr::read(&self.root).into_ref() };
+        if self.root.is_none() {
+            mem::forget(self);
+            return IntoIter { front: None, back: None, length: 0 };
+        }
+
+        let root1 = unsafe { unwrap_unchecked(ptr::read(&self.root)).into_ref() };
+        let root2 = unsafe { unwrap_unchecked(ptr::read(&self.root)).into_ref() };
         let len = self.length;
         mem::forget(self);
 
-        IntoIter { front: root1.first_leaf_edge(), back: root2.last_leaf_edge(), length: len }
+        IntoIter {
+            front: Some(root1.first_leaf_edge()),
+            back: Some(root2.last_leaf_edge()),
+            length: len,
+        }
     }
 }
 
@@ -1478,9 +1502,9 @@ impl<K, V> Drop for IntoIter<K, V> {
                 // don't have to care about panics this time (they'll abort).
                 while let Some(_) = self.0.next() {}
 
-                // No need to avoid the shared root, because the tree was definitely not empty.
                 unsafe {
-                    let mut node = ptr::read(&self.0.front).into_node().forget_type();
+                    let mut node =
+                        unwrap_unchecked(ptr::read(&self.0.front)).into_node().forget_type();
                     while let Some(parent) = node.deallocate_and_ascend() {
                         node = parent.into_node().forget_type();
                     }
@@ -1495,14 +1519,13 @@ impl<K, V> Drop for IntoIter<K, V> {
         }
 
         unsafe {
-            let mut node = ptr::read(&self.front).into_node().forget_type();
-            if node.is_shared_root() {
-                return;
-            }
-            // Most of the nodes have been deallocated while traversing
-            // but one pile from a leaf up to the root is left standing.
-            while let Some(parent) = node.deallocate_and_ascend() {
-                node = parent.into_node().forget_type();
+            if let Some(front) = ptr::read(&self.front) {
+                let mut node = front.into_node().forget_type();
+                // Most of the nodes have been deallocated while traversing
+                // but one pile from a leaf up to the root is left standing.
+                while let Some(parent) = node.deallocate_and_ascend() {
+                    node = parent.into_node().forget_type();
+                }
             }
         }
     }
@@ -1517,7 +1540,7 @@ impl<K, V> Iterator for IntoIter<K, V> {
             None
         } else {
             self.length -= 1;
-            Some(unsafe { self.front.next_unchecked() })
+            Some(unsafe { self.front.as_mut().unwrap().next_unchecked() })
         }
     }
 
@@ -1533,7 +1556,7 @@ impl<K, V> DoubleEndedIterator for IntoIter<K, V> {
             None
         } else {
             self.length -= 1;
-            Some(unsafe { self.back.next_back_unchecked() })
+            Some(unsafe { self.back.as_mut().unwrap().next_back_unchecked() })
         }
     }
 }
@@ -1683,7 +1706,7 @@ impl<'a, K, V> Range<'a, K, V> {
     }
 
     unsafe fn next_unchecked(&mut self) -> (&'a K, &'a V) {
-        self.front.next_unchecked()
+        unwrap_unchecked(self.front.as_mut()).next_unchecked()
     }
 }
 
@@ -1696,7 +1719,7 @@ impl<'a, K, V> DoubleEndedIterator for Range<'a, K, V> {
 
 impl<'a, K, V> Range<'a, K, V> {
     unsafe fn next_back_unchecked(&mut self) -> (&'a K, &'a V) {
-        self.back.next_back_unchecked()
+        unwrap_unchecked(self.back.as_mut()).next_back_unchecked()
     }
 }
 
@@ -1734,7 +1757,7 @@ impl<'a, K, V> RangeMut<'a, K, V> {
     }
 
     unsafe fn next_unchecked(&mut self) -> (&'a mut K, &'a mut V) {
-        self.front.next_unchecked()
+        unwrap_unchecked(self.front.as_mut()).next_unchecked()
     }
 }
 
@@ -1755,7 +1778,7 @@ impl<K, V> FusedIterator for RangeMut<'_, K, V> {}
 
 impl<'a, K, V> RangeMut<'a, K, V> {
     unsafe fn next_back_unchecked(&mut self) -> (&'a mut K, &'a mut V) {
-        self.back.next_back_unchecked()
+        unwrap_unchecked(self.back.as_mut()).next_back_unchecked()
     }
 }
 
@@ -1969,8 +1992,8 @@ impl<K, V> BTreeMap<K, V> {
     pub fn iter(&self) -> Iter<'_, K, V> {
         Iter {
             range: Range {
-                front: self.root.as_ref().first_leaf_edge(),
-                back: self.root.as_ref().last_leaf_edge(),
+                front: self.root.as_ref().map(|r| r.as_ref().first_leaf_edge()),
+                back: self.root.as_ref().map(|r| r.as_ref().last_leaf_edge()),
             },
             length: self.length,
         }
@@ -1999,13 +2022,17 @@ impl<K, V> BTreeMap<K, V> {
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn iter_mut(&mut self) -> IterMut<'_, K, V> {
-        let root1 = self.root.as_mut();
-        let root2 = unsafe { ptr::read(&root1) };
         IterMut {
-            range: RangeMut {
-                front: root1.first_leaf_edge(),
-                back: root2.last_leaf_edge(),
-                _marker: PhantomData,
+            range: if let Some(root) = &mut self.root {
+                let root1 = root.as_mut();
+                let root2 = unsafe { ptr::read(&root1) };
+                RangeMut {
+                    front: Some(root1.first_leaf_edge()),
+                    back: Some(root2.last_leaf_edge()),
+                    _marker: PhantomData,
+                }
+            } else {
+                RangeMut { front: None, back: None, _marker: PhantomData }
             },
             length: self.length,
         }
@@ -2116,6 +2143,12 @@ impl<K, V> BTreeMap<K, V> {
     pub fn is_empty(&self) -> bool {
         self.len() == 0
     }
+
+    /// If the root node is the empty (non-allocated) root node, allocate our
+    /// own node.
+    fn ensure_root_is_owned(&mut self) -> &mut node::Root<K, V> {
+        self.root.get_or_insert_with(|| node::Root::new_leaf())
+    }
 }
 
 impl<'a, K: Ord, V> Entry<'a, K, V> {
diff --git a/src/liballoc/collections/btree/node.rs b/src/liballoc/collections/btree/node.rs
index 1132ffdaf8005..6ebb98c42cd4f 100644
--- a/src/liballoc/collections/btree/node.rs
+++ b/src/liballoc/collections/btree/node.rs
@@ -44,34 +44,7 @@ const B: usize = 6;
 pub const MIN_LEN: usize = B - 1;
 pub const CAPACITY: usize = 2 * B - 1;
 
-/// The underlying representation of leaf nodes. Note that it is often unsafe to actually store
-/// these, since only the first `len` keys and values are assumed to be initialized. As such,
-/// these should always be put behind pointers, and specifically behind `BoxedNode` in the owned
-/// case.
-///
-/// We have a separate type for the header and rely on it matching the prefix of `LeafNode`, in
-/// order to statically allocate a single dummy node to avoid allocations. This struct is
-/// `repr(C)` to prevent them from being reordered. `LeafNode` does not just contain a
-/// `NodeHeader` because we do not want unnecessary padding between `len` and the keys.
-/// Crucially, `NodeHeader` can be safely transmuted to different K and V. (This is exploited
-/// by `as_header`.)
-#[repr(C)]
-struct NodeHeader<K, V> {
-    /// We use `*const` as opposed to `*mut` so as to be covariant in `K` and `V`.
-    /// This either points to an actual node or is null.
-    parent: *const InternalNode<K, V>,
-
-    /// This node's index into the parent node's `edges` array.
-    /// `*node.parent.edges[node.parent_idx]` should be the same thing as `node`.
-    /// This is only guaranteed to be initialized when `parent` is non-null.
-    parent_idx: MaybeUninit<u16>,
-
-    /// The number of keys and values this node stores.
-    ///
-    /// This next to `parent_idx` to encourage the compiler to join `len` and
-    /// `parent_idx` into the same 32-bit word, reducing space overhead.
-    len: u16,
-}
+/// The underlying representation of leaf nodes.
 #[repr(C)]
 struct LeafNode<K, V> {
     /// We use `*const` as opposed to `*mut` so as to be covariant in `K` and `V`.
@@ -111,21 +84,6 @@ impl<K, V> LeafNode<K, V> {
     }
 }
 
-impl<K, V> NodeHeader<K, V> {
-    fn is_shared_root(&self) -> bool {
-        ptr::eq(self, &EMPTY_ROOT_NODE as *const _ as *const _)
-    }
-}
-
-// We need to implement Sync here in order to make a static instance.
-unsafe impl Sync for NodeHeader<(), ()> {}
-
-// An empty node used as a placeholder for the root node, to avoid allocations.
-// We use just a header in order to save space, since no operation on an empty tree will
-// ever take a pointer past the first key.
-static EMPTY_ROOT_NODE: NodeHeader<(), ()> =
-    NodeHeader { parent: ptr::null(), parent_idx: MaybeUninit::uninit(), len: 0 };
-
 /// The underlying representation of internal nodes. As with `LeafNode`s, these should be hidden
 /// behind `BoxedNode`s to prevent dropping uninitialized keys and values. Any pointer to an
 /// `InternalNode` can be directly casted to a pointer to the underlying `LeafNode` portion of the
@@ -154,12 +112,9 @@ impl<K, V> InternalNode<K, V> {
 }
 
 /// A managed, non-null pointer to a node. This is either an owned pointer to
-/// `LeafNode<K, V>`, an owned pointer to `InternalNode<K, V>`, or a (not owned)
-/// pointer to `NodeHeader<(), ()` (more specifically, the pointer to EMPTY_ROOT_NODE).
-/// All of these types have a `NodeHeader<K, V>` prefix, meaning that they have at
-/// least the same size as `NodeHeader<K, V>` and store the same kinds of data at the same
-/// offsets; and they have a pointer alignment at least as large as `NodeHeader<K, V>`'s.
-/// However, `BoxedNode` contains no information as to which of the three types
+/// `LeafNode<K, V>` or an owned pointer to `InternalNode<K, V>`.
+///
+/// However, `BoxedNode` contains no information as to which of the two types
 /// of nodes it actually contains, and, partially due to this lack of information,
 /// has no destructor.
 struct BoxedNode<K, V> {
@@ -184,8 +139,9 @@ impl<K, V> BoxedNode<K, V> {
     }
 }
 
-/// Either an owned tree or a shared, empty tree.  Note that this does not have a destructor,
-/// and must be cleaned up manually if it is an owned tree.
+/// An owned tree.
+///
+/// Note that this does not have a destructor, and must be cleaned up manually.
 pub struct Root<K, V> {
     node: BoxedNode<K, V>,
     /// The number of levels below the root node.
@@ -196,20 +152,6 @@ unsafe impl<K: Sync, V: Sync> Sync for Root<K, V> {}
 unsafe impl<K: Send, V: Send> Send for Root<K, V> {}
 
 impl<K, V> Root<K, V> {
-    /// Whether the instance of `Root` wraps a shared, empty root node. If not,
-    /// the entire tree is uniquely owned by the owner of the `Root` instance.
-    pub fn is_shared_root(&self) -> bool {
-        self.as_ref().is_shared_root()
-    }
-
-    /// Returns a shared tree, wrapping a shared root node that is eternally empty.
-    pub fn shared_empty_root() -> Self {
-        Root {
-            node: unsafe { BoxedNode::from_ptr(NonNull::from(&EMPTY_ROOT_NODE).cast()) },
-            height: 0,
-        }
-    }
-
     /// Returns a new owned tree, with its own root node that is initially empty.
     pub fn new_leaf() -> Self {
         Root { node: BoxedNode::from_leaf(Box::new(unsafe { LeafNode::new() })), height: 0 }
@@ -245,7 +187,6 @@ impl<K, V> Root<K, V> {
     /// Adds a new internal node with a single edge, pointing to the previous root, and make that
     /// new node the root. This increases the height by 1 and is the opposite of `pop_level`.
     pub fn push_level(&mut self) -> NodeRef<marker::Mut<'_>, K, V, marker::Internal> {
-        debug_assert!(!self.is_shared_root());
         let mut new_node = Box::new(unsafe { InternalNode::new() });
         new_node.edges[0].write(unsafe { BoxedNode::from_ptr(self.node.as_ptr()) });
 
@@ -308,11 +249,6 @@ impl<K, V> Root<K, V> {
 ///   `Leaf`, the `NodeRef` points to a leaf node, when this is `Internal` the
 ///   `NodeRef` points to an internal node, and when this is `LeafOrInternal` the
 ///   `NodeRef` could be pointing to either type of node.
-///   Note that in case of a leaf node, this might still be the shared root!
-///   Only turn this into a `LeafNode` reference if you know it is not the shared root!
-///   Shared references must be dereferenceable *for the entire size of their pointee*,
-///   so '&LeafNode` or `&InternalNode` pointing to the shared root is undefined behavior.
-///   Turning this into a `NodeHeader` reference is always safe.
 pub struct NodeRef<BorrowType, K, V, Type> {
     /// The number of levels below the node.
     height: usize,
@@ -354,7 +290,7 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
     /// Note that, despite being safe, calling this function can have the side effect
     /// of invalidating mutable references that unsafe code has created.
     pub fn len(&self) -> usize {
-        self.as_header().len as usize
+        self.as_leaf().len as usize
     }
 
     /// Returns the height of this node in the whole tree. Zero height denotes the
@@ -374,35 +310,24 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
         NodeRef { height: self.height, node: self.node, root: self.root, _marker: PhantomData }
     }
 
-    /// Exposes the leaf "portion" of any leaf or internal node that is not the shared root.
+    /// Exposes the leaf "portion" of any leaf or internal node.
     /// If the node is a leaf, this function simply opens up its data.
     /// If the node is an internal node, so not a leaf, it does have all the data a leaf has
     /// (header, keys and values), and this function exposes that.
-    /// Unsafe because the node must not be the shared root. For more information,
-    /// see the `NodeRef` comments.
-    unsafe fn as_leaf(&self) -> &LeafNode<K, V> {
-        debug_assert!(!self.is_shared_root());
-        self.node.as_ref()
-    }
-
-    fn as_header(&self) -> &NodeHeader<K, V> {
-        unsafe { &*(self.node.as_ptr() as *const NodeHeader<K, V>) }
-    }
-
-    /// Returns whether the node is the shared, empty root.
-    pub fn is_shared_root(&self) -> bool {
-        self.as_header().is_shared_root()
+    fn as_leaf(&self) -> &LeafNode<K, V> {
+        // The node must be valid for at least the LeafNode portion.
+        // This is not a reference in the NodeRef type because we don't know if
+        // it should be unique or shared.
+        unsafe { self.node.as_ref() }
     }
 
     /// Borrows a view into the keys stored in the node.
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    pub unsafe fn keys(&self) -> &[K] {
+    pub fn keys(&self) -> &[K] {
         self.reborrow().into_key_slice()
     }
 
     /// Borrows a view into the values stored in the node.
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn vals(&self) -> &[V] {
+    fn vals(&self) -> &[V] {
         self.reborrow().into_val_slice()
     }
 
@@ -416,7 +341,7 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
     pub fn ascend(
         self,
     ) -> Result<Handle<NodeRef<BorrowType, K, V, marker::Internal>, marker::Edge>, Self> {
-        let parent_as_leaf = self.as_header().parent as *const LeafNode<K, V>;
+        let parent_as_leaf = self.as_leaf().parent as *const LeafNode<K, V>;
         if let Some(non_zero) = NonNull::new(parent_as_leaf as *mut _) {
             Ok(Handle {
                 node: NodeRef {
@@ -425,7 +350,7 @@ impl<BorrowType, K, V, Type> NodeRef<BorrowType, K, V, Type> {
                     root: self.root,
                     _marker: PhantomData,
                 },
-                idx: unsafe { usize::from(*self.as_header().parent_idx.as_ptr()) },
+                idx: unsafe { usize::from(*self.as_leaf().parent_idx.as_ptr()) },
                 _marker: PhantomData,
             })
         } else {
@@ -464,7 +389,6 @@ impl<K, V> NodeRef<marker::Owned, K, V, marker::LeafOrInternal> {
     pub unsafe fn deallocate_and_ascend(
         self,
     ) -> Option<Handle<NodeRef<marker::Owned, K, V, marker::Internal>, marker::Edge>> {
-        assert!(!self.is_shared_root());
         let height = self.height;
         let node = self.node;
         let ret = self.ascend().ok();
@@ -507,41 +431,37 @@ impl<'a, K, V, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
     /// (header, keys and values), and this function exposes that.
     ///
     /// Returns a raw ptr to avoid asserting exclusive access to the entire node.
-    /// This also implies you can invoke this member on the shared root, but the resulting pointer
-    /// might not be properly aligned and definitely would not allow accessing keys and values.
     fn as_leaf_mut(&mut self) -> *mut LeafNode<K, V> {
         self.node.as_ptr()
     }
 
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn keys_mut(&mut self) -> &mut [K] {
-        self.reborrow_mut().into_key_slice_mut()
+    fn keys_mut(&mut self) -> &mut [K] {
+        // SAFETY: the caller will not be able to call further methods on self
+        // until the key slice reference is dropped, as we have unique access
+        // for the lifetime of the borrow.
+        unsafe { self.reborrow_mut().into_key_slice_mut() }
     }
 
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn vals_mut(&mut self) -> &mut [V] {
-        self.reborrow_mut().into_val_slice_mut()
+    fn vals_mut(&mut self) -> &mut [V] {
+        // SAFETY: the caller will not be able to call further methods on self
+        // until the value slice reference is dropped, as we have unique access
+        // for the lifetime of the borrow.
+        unsafe { self.reborrow_mut().into_val_slice_mut() }
     }
 }
 
 impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Immut<'a>, K, V, Type> {
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn into_key_slice(self) -> &'a [K] {
-        debug_assert!(!self.is_shared_root());
-        // We cannot be the shared root, so `as_leaf` is okay.
-        slice::from_raw_parts(MaybeUninit::first_ptr(&self.as_leaf().keys), self.len())
+    fn into_key_slice(self) -> &'a [K] {
+        unsafe { slice::from_raw_parts(MaybeUninit::first_ptr(&self.as_leaf().keys), self.len()) }
     }
 
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn into_val_slice(self) -> &'a [V] {
-        debug_assert!(!self.is_shared_root());
-        // We cannot be the shared root, so `as_leaf` is okay.
-        slice::from_raw_parts(MaybeUninit::first_ptr(&self.as_leaf().vals), self.len())
+    fn into_val_slice(self) -> &'a [V] {
+        unsafe { slice::from_raw_parts(MaybeUninit::first_ptr(&self.as_leaf().vals), self.len()) }
     }
 
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn into_slices(self) -> (&'a [K], &'a [V]) {
-        let k = ptr::read(&self);
+    fn into_slices(self) -> (&'a [K], &'a [V]) {
+        // SAFETY: equivalent to reborrow() except not requiring Type: 'a
+        let k = unsafe { ptr::read(&self) };
         (k.into_key_slice(), self.into_val_slice())
     }
 }
@@ -553,28 +473,27 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
         unsafe { &mut *(self.root as *mut Root<K, V>) }
     }
 
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn into_key_slice_mut(mut self) -> &'a mut [K] {
-        debug_assert!(!self.is_shared_root());
-        // We cannot be the shared root, so `as_leaf_mut` is okay.
-        slice::from_raw_parts_mut(
-            MaybeUninit::first_ptr_mut(&mut (*self.as_leaf_mut()).keys),
-            self.len(),
-        )
+    fn into_key_slice_mut(mut self) -> &'a mut [K] {
+        // SAFETY: The keys of a node must always be initialized up to length.
+        unsafe {
+            slice::from_raw_parts_mut(
+                MaybeUninit::first_ptr_mut(&mut (*self.as_leaf_mut()).keys),
+                self.len(),
+            )
+        }
     }
 
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn into_val_slice_mut(mut self) -> &'a mut [V] {
-        debug_assert!(!self.is_shared_root());
-        slice::from_raw_parts_mut(
-            MaybeUninit::first_ptr_mut(&mut (*self.as_leaf_mut()).vals),
-            self.len(),
-        )
+    fn into_val_slice_mut(mut self) -> &'a mut [V] {
+        // SAFETY: The values of a node must always be initialized up to length.
+        unsafe {
+            slice::from_raw_parts_mut(
+                MaybeUninit::first_ptr_mut(&mut (*self.as_leaf_mut()).vals),
+                self.len(),
+            )
+        }
     }
 
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn into_slices_mut(mut self) -> (&'a mut [K], &'a mut [V]) {
-        debug_assert!(!self.is_shared_root());
+    fn into_slices_mut(mut self) -> (&'a mut [K], &'a mut [V]) {
         // We cannot use the getters here, because calling the second one
         // invalidates the reference returned by the first.
         // More precisely, it is the call to `len` that is the culprit,
@@ -582,8 +501,13 @@ impl<'a, K: 'a, V: 'a, Type> NodeRef<marker::Mut<'a>, K, V, Type> {
         // overlap with the keys (and even the values, for ZST keys).
         let len = self.len();
         let leaf = self.as_leaf_mut();
-        let keys = slice::from_raw_parts_mut(MaybeUninit::first_ptr_mut(&mut (*leaf).keys), len);
-        let vals = slice::from_raw_parts_mut(MaybeUninit::first_ptr_mut(&mut (*leaf).vals), len);
+        // SAFETY: The keys and values of a node must always be initialized up to length.
+        let keys = unsafe {
+            slice::from_raw_parts_mut(MaybeUninit::first_ptr_mut(&mut (*leaf).keys), len)
+        };
+        let vals = unsafe {
+            slice::from_raw_parts_mut(MaybeUninit::first_ptr_mut(&mut (*leaf).vals), len)
+        };
         (keys, vals)
     }
 }
@@ -592,7 +516,6 @@ impl<'a, K, V> NodeRef<marker::Mut<'a>, K, V, marker::Leaf> {
     /// Adds a key/value pair the end of the node.
     pub fn push(&mut self, key: K, val: V) {
         assert!(self.len() < CAPACITY);
-        debug_assert!(!self.is_shared_root());
 
         let idx = self.len();
 
@@ -607,7 +530,6 @@ impl<'a, K, V> NodeRef<marker::Mut<'a>, K, V, marker::Leaf> {
     /// Adds a key/value pair to the beginning of the node.
     pub fn push_front(&mut self, key: K, val: V) {
         assert!(self.len() < CAPACITY);
-        debug_assert!(!self.is_shared_root());
 
         unsafe {
             slice_insert(self.keys_mut(), 0, key);
@@ -624,7 +546,6 @@ impl<'a, K, V> NodeRef<marker::Mut<'a>, K, V, marker::Internal> {
     pub fn push(&mut self, key: K, val: V, edge: Root<K, V>) {
         assert!(edge.height == self.height - 1);
         assert!(self.len() < CAPACITY);
-        debug_assert!(!self.is_shared_root());
 
         let idx = self.len();
 
@@ -658,7 +579,6 @@ impl<'a, K, V> NodeRef<marker::Mut<'a>, K, V, marker::Internal> {
     pub fn push_front(&mut self, key: K, val: V, edge: Root<K, V>) {
         assert!(edge.height == self.height - 1);
         assert!(self.len() < CAPACITY);
-        debug_assert!(!self.is_shared_root());
 
         unsafe {
             slice_insert(self.keys_mut(), 0, key);
@@ -744,8 +664,7 @@ impl<'a, K, V> NodeRef<marker::Mut<'a>, K, V, marker::LeafOrInternal> {
         }
     }
 
-    /// Unsafe because the caller must ensure that the node is not the shared root.
-    unsafe fn into_kv_pointers_mut(mut self) -> (*mut K, *mut V) {
+    fn into_kv_pointers_mut(mut self) -> (*mut K, *mut V) {
         (self.keys_mut().as_mut_ptr(), self.vals_mut().as_mut_ptr())
     }
 }
@@ -904,7 +823,6 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge
     fn insert_fit(&mut self, key: K, val: V) -> *mut V {
         // Necessary for correctness, but in a private module
         debug_assert!(self.node.len() < CAPACITY);
-        debug_assert!(!self.node.is_shared_root());
 
         unsafe {
             slice_insert(self.node.keys_mut(), self.idx, key);
@@ -1081,7 +999,6 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::KV>
     /// - All the key/value pairs to the right of this handle are put into a newly
     ///   allocated node.
     pub fn split(mut self) -> (NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, K, V, Root<K, V>) {
-        assert!(!self.node.is_shared_root());
         unsafe {
             let mut new_node = Box::new(LeafNode::new());
 
@@ -1113,7 +1030,6 @@ impl<'a, K, V> Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::KV>
     pub fn remove(
         mut self,
     ) -> (Handle<NodeRef<marker::Mut<'a>, K, V, marker::Leaf>, marker::Edge>, K, V) {
-        assert!(!self.node.is_shared_root());
         unsafe {
             let k = slice_remove(self.node.keys_mut(), self.idx);
             let v = slice_remove(self.node.vals_mut(), self.idx);
diff --git a/src/liballoc/collections/btree/search.rs b/src/liballoc/collections/btree/search.rs
index 2ba5cebbdee74..4e80f7f21ebff 100644
--- a/src/liballoc/collections/btree/search.rs
+++ b/src/liballoc/collections/btree/search.rs
@@ -67,19 +67,16 @@ where
     Q: Ord,
     K: Borrow<Q>,
 {
-    // This function is defined over all borrow types (immutable, mutable, owned),
-    // and may be called on the shared root in each case.
+    // This function is defined over all borrow types (immutable, mutable, owned).
     // Using `keys()` is fine here even if BorrowType is mutable, as all we return
     // is an index -- not a reference.
     let len = node.len();
-    if len > 0 {
-        let keys = unsafe { node.keys() }; // safe because a non-empty node cannot be the shared root
-        for (i, k) in keys.iter().enumerate() {
-            match key.cmp(k.borrow()) {
-                Ordering::Greater => {}
-                Ordering::Equal => return (i, true),
-                Ordering::Less => return (i, false),
-            }
+    let keys = node.keys();
+    for (i, k) in keys.iter().enumerate() {
+        match key.cmp(k.borrow()) {
+            Ordering::Greater => {}
+            Ordering::Equal => return (i, true),
+            Ordering::Less => return (i, false),
         }
     }
     (len, false)
diff --git a/src/liballoc/lib.rs b/src/liballoc/lib.rs
index ffa4176cc7969..d877ac6ac5c79 100644
--- a/src/liballoc/lib.rs
+++ b/src/liballoc/lib.rs
@@ -80,6 +80,7 @@
 #![feature(box_into_raw_non_null)]
 #![feature(box_patterns)]
 #![feature(box_syntax)]
+#![feature(cfg_sanitize)]
 #![feature(cfg_target_has_atomic)]
 #![feature(coerce_unsized)]
 #![feature(const_generic_impls_guard)]
diff --git a/src/liballoc/sync.rs b/src/liballoc/sync.rs
index 4a0cf2984edd9..d9b54fb0b177a 100644
--- a/src/liballoc/sync.rs
+++ b/src/liballoc/sync.rs
@@ -40,6 +40,23 @@ mod tests;
 /// necessarily) at _exactly_ `MAX_REFCOUNT + 1` references.
 const MAX_REFCOUNT: usize = (isize::MAX) as usize;
 
+#[cfg(not(sanitize = "thread"))]
+macro_rules! acquire {
+    ($x:expr) => {
+        atomic::fence(Acquire)
+    };
+}
+
+// ThreadSanitizer does not support memory fences. To avoid false positive
+// reports in Arc / Weak implementation use atomic loads for synchronization
+// instead.
+#[cfg(sanitize = "thread")]
+macro_rules! acquire {
+    ($x:expr) => {
+        $x.load(Acquire)
+    };
+}
+
 /// A thread-safe reference-counting pointer. 'Arc' stands for 'Atomically
 /// Reference Counted'.
 ///
@@ -402,7 +419,7 @@ impl<T> Arc<T> {
             return Err(this);
         }
 
-        atomic::fence(Acquire);
+        acquire!(this.inner().strong);
 
         unsafe {
             let elem = ptr::read(&this.ptr.as_ref().data);
@@ -739,7 +756,7 @@ impl<T: ?Sized> Arc<T> {
         ptr::drop_in_place(&mut self.ptr.as_mut().data);
 
         if self.inner().weak.fetch_sub(1, Release) == 1 {
-            atomic::fence(Acquire);
+            acquire!(self.inner().weak);
             Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref()))
         }
     }
@@ -1243,7 +1260,7 @@ unsafe impl<#[may_dangle] T: ?Sized> Drop for Arc<T> {
         //
         // [1]: (www.boost.org/doc/libs/1_55_0/doc/html/atomic/usage_examples.html)
         // [2]: (https://github.com/rust-lang/rust/pull/41714)
-        atomic::fence(Acquire);
+        acquire!(self.inner().strong);
 
         unsafe {
             self.drop_slow();
@@ -1701,7 +1718,7 @@ impl<T: ?Sized> Drop for Weak<T> {
         let inner = if let Some(inner) = self.inner() { inner } else { return };
 
         if inner.weak.fetch_sub(1, Release) == 1 {
-            atomic::fence(Acquire);
+            acquire!(inner.weak);
             unsafe { Global.dealloc(self.ptr.cast(), Layout::for_value(self.ptr.as_ref())) }
         }
     }
diff --git a/src/liballoc/tests/btree/map.rs b/src/liballoc/tests/btree/map.rs
index d05eec19346de..3a3462d546f7a 100644
--- a/src/liballoc/tests/btree/map.rs
+++ b/src/liballoc/tests/btree/map.rs
@@ -67,7 +67,7 @@ fn test_basic_large() {
 #[test]
 fn test_basic_small() {
     let mut map = BTreeMap::new();
-    // Empty, shared root:
+    // Empty, root is absent (None):
     assert_eq!(map.remove(&1), None);
     assert_eq!(map.len(), 0);
     assert_eq!(map.get(&1), None);
@@ -123,7 +123,7 @@ fn test_basic_small() {
     assert_eq!(map.values().collect::<Vec<_>>(), vec![&4]);
     assert_eq!(map.remove(&2), Some(4));
 
-    // Empty but private root:
+    // Empty but root is owned (Some(...)):
     assert_eq!(map.len(), 0);
     assert_eq!(map.get(&1), None);
     assert_eq!(map.get_mut(&1), None);
@@ -263,13 +263,6 @@ fn test_iter_mut_mutation() {
     do_test_iter_mut_mutation::<Align32>(144);
 }
 
-#[test]
-fn test_into_key_slice_with_shared_root_past_bounds() {
-    let mut map: BTreeMap<Align32, ()> = BTreeMap::new();
-    assert_eq!(map.get(&Align32(1)), None);
-    assert_eq!(map.get_mut(&Align32(1)), None);
-}
-
 #[test]
 fn test_values_mut() {
     let mut a = BTreeMap::new();
diff --git a/src/liballoc/vec.rs b/src/liballoc/vec.rs
index d1956270f135f..4769091183a37 100644
--- a/src/liballoc/vec.rs
+++ b/src/liballoc/vec.rs
@@ -1377,6 +1377,7 @@ impl<T> Vec<T> {
     /// assert_eq!(vec2, [2, 3]);
     /// ```
     #[inline]
+    #[must_use = "use `.truncate()` if you don't need the other half"]
     #[stable(feature = "split_off", since = "1.4.0")]
     pub fn split_off(&mut self, at: usize) -> Self {
         assert!(at <= self.len(), "`at` out of bounds");
diff --git a/src/libcore/future/mod.rs b/src/libcore/future/mod.rs
index 89ea4713cfdaa..8dfda7a4a3236 100644
--- a/src/libcore/future/mod.rs
+++ b/src/libcore/future/mod.rs
@@ -2,6 +2,84 @@
 
 //! Asynchronous values.
 
+#[cfg(not(bootstrap))]
+use crate::{
+    ops::{Generator, GeneratorState},
+    pin::Pin,
+    ptr::NonNull,
+    task::{Context, Poll},
+};
+
 mod future;
 #[stable(feature = "futures_api", since = "1.36.0")]
 pub use self::future::Future;
+
+/// This type is needed because:
+///
+/// a) Generators cannot implement `for<'a, 'b> Generator<&'a mut Context<'b>>`, so we need to pass
+///    a raw pointer (see https://github.com/rust-lang/rust/issues/68923).
+/// b) Raw pointers and `NonNull` aren't `Send` or `Sync`, so that would make every single future
+///    non-Send/Sync as well, and we don't want that.
+///
+/// It also simplifies the HIR lowering of `.await`.
+#[doc(hidden)]
+#[unstable(feature = "gen_future", issue = "50547")]
+#[cfg(not(bootstrap))]
+#[derive(Debug, Copy, Clone)]
+pub struct ResumeTy(NonNull<Context<'static>>);
+
+#[unstable(feature = "gen_future", issue = "50547")]
+#[cfg(not(bootstrap))]
+unsafe impl Send for ResumeTy {}
+
+#[unstable(feature = "gen_future", issue = "50547")]
+#[cfg(not(bootstrap))]
+unsafe impl Sync for ResumeTy {}
+
+/// Wrap a generator in a future.
+///
+/// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give
+/// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`).
+// This is `const` to avoid extra errors after we recover from `const async fn`
+#[doc(hidden)]
+#[unstable(feature = "gen_future", issue = "50547")]
+#[cfg(not(bootstrap))]
+#[inline]
+pub const fn from_generator<T>(gen: T) -> impl Future<Output = T::Return>
+where
+    T: Generator<ResumeTy, Yield = ()>,
+{
+    struct GenFuture<T: Generator<ResumeTy, Yield = ()>>(T);
+
+    // We rely on the fact that async/await futures are immovable in order to create
+    // self-referential borrows in the underlying generator.
+    impl<T: Generator<ResumeTy, Yield = ()>> !Unpin for GenFuture<T> {}
+
+    impl<T: Generator<ResumeTy, Yield = ()>> Future for GenFuture<T> {
+        type Output = T::Return;
+        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
+            // Safety: Safe because we're !Unpin + !Drop, and this is just a field projection.
+            let gen = unsafe { Pin::map_unchecked_mut(self, |s| &mut s.0) };
+
+            // Resume the generator, turning the `&mut Context` into a `NonNull` raw pointer. The
+            // `.await` lowering will safely cast that back to a `&mut Context`.
+            match gen.resume(ResumeTy(NonNull::from(cx).cast::<Context<'static>>())) {
+                GeneratorState::Yielded(()) => Poll::Pending,
+                GeneratorState::Complete(x) => Poll::Ready(x),
+            }
+        }
+    }
+
+    GenFuture(gen)
+}
+
+#[doc(hidden)]
+#[unstable(feature = "gen_future", issue = "50547")]
+#[cfg(not(bootstrap))]
+#[inline]
+pub unsafe fn poll_with_context<F>(f: Pin<&mut F>, mut cx: ResumeTy) -> Poll<F::Output>
+where
+    F: Future,
+{
+    F::poll(f, cx.0.as_mut())
+}
diff --git a/src/libcore/hint.rs b/src/libcore/hint.rs
index f4fb9ab1757cd..6cbd26a78de72 100644
--- a/src/libcore/hint.rs
+++ b/src/libcore/hint.rs
@@ -114,6 +114,6 @@ pub fn black_box<T>(dummy: T) -> T {
     // more than we want, but it's so far good enough.
     unsafe {
         asm!("" : : "r"(&dummy));
-        return dummy;
+        dummy
     }
 }
diff --git a/src/libcore/lib.rs b/src/libcore/lib.rs
index 5a731766054bd..94fc2fd357a06 100644
--- a/src/libcore/lib.rs
+++ b/src/libcore/lib.rs
@@ -140,6 +140,7 @@
 #![feature(associated_type_bounds)]
 #![feature(const_type_id)]
 #![feature(const_caller_location)]
+#![feature(option_zip)]
 #![feature(no_niche)] // rust-lang/rust#68303
 
 #[prelude_import]
diff --git a/src/libcore/ops/range.rs b/src/libcore/ops/range.rs
index 8ffad82b69d7c..adee8cea442b4 100644
--- a/src/libcore/ops/range.rs
+++ b/src/libcore/ops/range.rs
@@ -1,5 +1,5 @@
 use crate::fmt;
-use crate::hash::{Hash, Hasher};
+use crate::hash::Hash;
 
 /// An unbounded range (`..`).
 ///
@@ -330,7 +330,7 @@ impl<Idx: PartialOrd<Idx>> RangeTo<Idx> {
 /// assert_eq!(arr[1..=3], [  1,2,3  ]);  // RangeInclusive
 /// ```
 #[doc(alias = "..=")]
-#[derive(Clone)] // not Copy -- see #27186
+#[derive(Clone, PartialEq, Eq, Hash)] // not Copy -- see #27186
 #[stable(feature = "inclusive_range", since = "1.26.0")]
 pub struct RangeInclusive<Idx> {
     // Note that the fields here are not public to allow changing the
@@ -350,26 +350,6 @@ pub struct RangeInclusive<Idx> {
     pub(crate) exhausted: bool,
 }
 
-#[stable(feature = "inclusive_range", since = "1.26.0")]
-impl<Idx: PartialEq> PartialEq for RangeInclusive<Idx> {
-    #[inline]
-    fn eq(&self, other: &Self) -> bool {
-        self.start == other.start && self.end == other.end && self.exhausted == other.exhausted
-    }
-}
-
-#[stable(feature = "inclusive_range", since = "1.26.0")]
-impl<Idx: Eq> Eq for RangeInclusive<Idx> {}
-
-#[stable(feature = "inclusive_range", since = "1.26.0")]
-impl<Idx: Hash> Hash for RangeInclusive<Idx> {
-    fn hash<H: Hasher>(&self, state: &mut H) {
-        self.start.hash(state);
-        self.end.hash(state);
-        self.exhausted.hash(state);
-    }
-}
-
 impl<Idx> RangeInclusive<Idx> {
     /// Creates a new inclusive range. Equivalent to writing `start..=end`.
     ///
diff --git a/src/libcore/option.rs b/src/libcore/option.rs
index 9b32442371c37..3aab8b1b3337c 100644
--- a/src/libcore/option.rs
+++ b/src/libcore/option.rs
@@ -913,6 +913,63 @@ impl<T> Option<T> {
     pub fn replace(&mut self, value: T) -> Option<T> {
         mem::replace(self, Some(value))
     }
+
+    /// Zips `self` with another `Option`.
+    ///
+    /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some((s, o))`.
+    /// Otherwise, `None` is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(option_zip)]
+    /// let x = Some(1);
+    /// let y = Some("hi");
+    /// let z = None::<u8>;
+    ///
+    /// assert_eq!(x.zip(y), Some((1, "hi")));
+    /// assert_eq!(x.zip(z), None);
+    /// ```
+    #[unstable(feature = "option_zip", issue = "70086")]
+    pub fn zip<U>(self, other: Option<U>) -> Option<(T, U)> {
+        self.zip_with(other, |a, b| (a, b))
+    }
+
+    /// Zips `self` and another `Option` with function `f`.
+    ///
+    /// If `self` is `Some(s)` and `other` is `Some(o)`, this method returns `Some(f(s, o))`.
+    /// Otherwise, `None` is returned.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(option_zip)]
+    ///
+    /// #[derive(Debug, PartialEq)]
+    /// struct Point {
+    ///     x: f64,
+    ///     y: f64,
+    /// }
+    ///
+    /// impl Point {
+    ///     fn new(x: f64, y: f64) -> Self {
+    ///         Self { x, y }
+    ///     }
+    /// }
+    ///
+    /// let x = Some(17.5);
+    /// let y = Some(42.7);
+    ///
+    /// assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 }));
+    /// assert_eq!(x.zip_with(None, Point::new), None);
+    /// ```
+    #[unstable(feature = "option_zip", issue = "70086")]
+    pub fn zip_with<U, F, R>(self, other: Option<U>, f: F) -> Option<R>
+    where
+        F: FnOnce(T, U) -> R,
+    {
+        Some(f(self?, other?))
+    }
 }
 
 impl<T: Copy> Option<&T> {
diff --git a/src/libcore/tests/mem.rs b/src/libcore/tests/mem.rs
index 8337ab103419f..59588d97787b7 100644
--- a/src/libcore/tests/mem.rs
+++ b/src/libcore/tests/mem.rs
@@ -129,21 +129,3 @@ fn test_discriminant_send_sync() {
     is_send_sync::<Discriminant<Regular>>();
     is_send_sync::<Discriminant<NotSendSync>>();
 }
-
-#[test]
-fn test_const_forget() {
-    const _: () = forget(0i32);
-    const _: () = forget(Vec::<Vec<Box<i32>>>::new());
-
-    // Writing this function signature without const-forget
-    // triggers compiler errors:
-    // 1) That we use a non-const fn inside a const fn
-    // 2) without the forget, it complains about the destructor of Box
-    const fn const_forget_box<T>(x: Box<T>) {
-        forget(x);
-    }
-
-    // Call the forget_box at runtime,
-    // as we can't const-construct a box yet.
-    const_forget_box(Box::new(0i32));
-}
diff --git a/src/librustc/lint.rs b/src/librustc/lint.rs
index d4d01a716db97..4dd276d2e032c 100644
--- a/src/librustc/lint.rs
+++ b/src/librustc/lint.rs
@@ -85,7 +85,7 @@ impl LintLevelSets {
             level = cmp::min(*driver_level, level);
         }
 
-        return (level, src);
+        (level, src)
     }
 
     pub fn get_lint_id_level(
diff --git a/src/librustc/middle/region.rs b/src/librustc/middle/region.rs
index 2735c4afca2c8..1a63dc9dcf977 100644
--- a/src/librustc/middle/region.rs
+++ b/src/librustc/middle/region.rs
@@ -467,7 +467,7 @@ impl<'tcx> ScopeTree {
         }
 
         debug!("temporary_scope({:?}) = None", expr_id);
-        return None;
+        None
     }
 
     /// Returns the lifetime of the variable `id`.
@@ -498,7 +498,7 @@ impl<'tcx> ScopeTree {
 
         debug!("is_subscope_of({:?}, {:?})=true", subscope, superscope);
 
-        return true;
+        true
     }
 
     /// Returns the ID of the innermost containing body.
diff --git a/src/librustc/ty/context.rs b/src/librustc/ty/context.rs
index 742d57fb58a51..0e3776f32e0e8 100644
--- a/src/librustc/ty/context.rs
+++ b/src/librustc/ty/context.rs
@@ -1447,11 +1447,11 @@ impl<'tcx> TyCtxt<'tcx> {
             _ => return None,
         };
 
-        return Some(FreeRegionInfo {
+        Some(FreeRegionInfo {
             def_id: suitable_region_binding_scope,
             boundregion: bound_region,
             is_impl_item,
-        });
+        })
     }
 
     pub fn return_type_impl_trait(&self, scope_def_id: DefId) -> Option<(Ty<'tcx>, Span)> {
diff --git a/src/librustc/ty/relate.rs b/src/librustc/ty/relate.rs
index fb4184a9fb347..872e06e1176dc 100644
--- a/src/librustc/ty/relate.rs
+++ b/src/librustc/ty/relate.rs
@@ -440,7 +440,7 @@ pub fn super_relate_tys<R: TypeRelation<'tcx>>(
                         (Some(sz_a_val), Some(sz_b_val)) => Err(TypeError::FixedArraySize(
                             expected_found(relation, &sz_a_val, &sz_b_val),
                         )),
-                        _ => return Err(err),
+                        _ => Err(err),
                     }
                 }
             }
diff --git a/src/librustc/ty/sty.rs b/src/librustc/ty/sty.rs
index e265a2f8257fb..42cd2f52cb3ad 100644
--- a/src/librustc/ty/sty.rs
+++ b/src/librustc/ty/sty.rs
@@ -1612,7 +1612,7 @@ impl<'tcx> PolyExistentialProjection<'tcx> {
     }
 
     pub fn item_def_id(&self) -> DefId {
-        return self.skip_binder().item_def_id;
+        self.skip_binder().item_def_id
     }
 }
 
@@ -2000,8 +2000,8 @@ impl<'tcx> TyS<'tcx> {
     #[inline]
     pub fn is_unsafe_ptr(&self) -> bool {
         match self.kind {
-            RawPtr(_) => return true,
-            _ => return false,
+            RawPtr(_) => true,
+            _ => false,
         }
     }
 
diff --git a/src/librustc/ty/subst.rs b/src/librustc/ty/subst.rs
index a005581283550..a3acc14856e1f 100644
--- a/src/librustc/ty/subst.rs
+++ b/src/librustc/ty/subst.rs
@@ -524,7 +524,7 @@ impl<'a, 'tcx> TypeFolder<'tcx> for SubstFolder<'a, 'tcx> {
             self.root_ty = None;
         }
 
-        return t1;
+        t1
     }
 
     fn fold_const(&mut self, c: &'tcx ty::Const<'tcx>) -> &'tcx ty::Const<'tcx> {
diff --git a/src/librustc_ast/token.rs b/src/librustc_ast/token.rs
index 3fc6444168e24..be5d322ba1677 100644
--- a/src/librustc_ast/token.rs
+++ b/src/librustc_ast/token.rs
@@ -424,7 +424,7 @@ impl Token {
                 NtExpr(..) | NtBlock(..) | NtLiteral(..) => true,
                 _ => false,
             },
-            _ => self.can_begin_literal_or_bool(),
+            _ => self.can_begin_literal_maybe_minus(),
         }
     }
 
@@ -448,13 +448,22 @@ impl Token {
     /// Returns `true` if the token is any literal, a minus (which can prefix a literal,
     /// for example a '-42', or one of the boolean idents).
     ///
-    /// Keep this in sync with `Lit::from_token`.
-    pub fn can_begin_literal_or_bool(&self) -> bool {
+    /// In other words, would this token be a valid start of `parse_literal_maybe_minus`?
+    ///
+    /// Keep this in sync with and `Lit::from_token`, excluding unary negation.
+    pub fn can_begin_literal_maybe_minus(&self) -> bool {
         match self.uninterpolate().kind {
             Literal(..) | BinOp(Minus) => true,
             Ident(name, false) if name.is_bool_lit() => true,
             Interpolated(ref nt) => match &**nt {
-                NtExpr(e) | NtLiteral(e) => matches!(e.kind, ast::ExprKind::Lit(_)),
+                NtLiteral(_) => true,
+                NtExpr(e) => match &e.kind {
+                    ast::ExprKind::Lit(_) => true,
+                    ast::ExprKind::Unary(ast::UnOp::Neg, e) => {
+                        matches!(&e.kind, ast::ExprKind::Lit(_))
+                    }
+                    _ => false,
+                },
                 _ => false,
             },
             _ => false,
diff --git a/src/librustc_ast/util/literal.rs b/src/librustc_ast/util/literal.rs
index d1757394f3a1d..1b17f343a6d67 100644
--- a/src/librustc_ast/util/literal.rs
+++ b/src/librustc_ast/util/literal.rs
@@ -189,7 +189,7 @@ impl Lit {
 
     /// Converts arbitrary token into an AST literal.
     ///
-    /// Keep this in sync with `Token::can_begin_literal_or_bool`.
+    /// Keep this in sync with `Token::can_begin_literal_or_bool` excluding unary negation.
     pub fn from_token(token: &Token) -> Result<Lit, LitError> {
         let lit = match token.uninterpolate().kind {
             token::Ident(name, false) if name.is_bool_lit() => {
diff --git a/src/librustc_ast_lowering/expr.rs b/src/librustc_ast_lowering/expr.rs
index a4cbae5196635..a212f0a71077b 100644
--- a/src/librustc_ast_lowering/expr.rs
+++ b/src/librustc_ast_lowering/expr.rs
@@ -470,6 +470,15 @@ impl<'hir> LoweringContext<'_, 'hir> {
         }
     }
 
+    /// Lower an `async` construct to a generator that is then wrapped so it implements `Future`.
+    ///
+    /// This results in:
+    ///
+    /// ```text
+    /// std::future::from_generator(static move? |_task_context| -> <ret_ty> {
+    ///     <body>
+    /// })
+    /// ```
     pub(super) fn make_async_expr(
         &mut self,
         capture_clause: CaptureBy,
@@ -480,17 +489,42 @@ impl<'hir> LoweringContext<'_, 'hir> {
         body: impl FnOnce(&mut Self) -> hir::Expr<'hir>,
     ) -> hir::ExprKind<'hir> {
         let output = match ret_ty {
-            Some(ty) => FnRetTy::Ty(ty),
-            None => FnRetTy::Default(span),
+            Some(ty) => hir::FnRetTy::Return(self.lower_ty(&ty, ImplTraitContext::disallowed())),
+            None => hir::FnRetTy::DefaultReturn(span),
         };
-        let ast_decl = FnDecl { inputs: vec![], output };
-        let decl = self.lower_fn_decl(&ast_decl, None, /* impl trait allowed */ false, None);
-        let body_id = self.lower_fn_body(&ast_decl, |this| {
+
+        // Resume argument type. We let the compiler infer this to simplify the lowering. It is
+        // fully constrained by `future::from_generator`.
+        let input_ty = hir::Ty { hir_id: self.next_id(), kind: hir::TyKind::Infer, span };
+
+        // The closure/generator `FnDecl` takes a single (resume) argument of type `input_ty`.
+        let decl = self.arena.alloc(hir::FnDecl {
+            inputs: arena_vec![self; input_ty],
+            output,
+            c_variadic: false,
+            implicit_self: hir::ImplicitSelfKind::None,
+        });
+
+        // Lower the argument pattern/ident. The ident is used again in the `.await` lowering.
+        let (pat, task_context_hid) = self.pat_ident_binding_mode(
+            span,
+            Ident::with_dummy_span(sym::_task_context),
+            hir::BindingAnnotation::Mutable,
+        );
+        let param = hir::Param { attrs: &[], hir_id: self.next_id(), pat, span };
+        let params = arena_vec![self; param];
+
+        let body_id = self.lower_body(move |this| {
             this.generator_kind = Some(hir::GeneratorKind::Async(async_gen_kind));
-            body(this)
+
+            let old_ctx = this.task_context;
+            this.task_context = Some(task_context_hid);
+            let res = body(this);
+            this.task_context = old_ctx;
+            (params, res)
         });
 
-        // `static || -> <ret_ty> { body }`:
+        // `static |_task_context| -> <ret_ty> { body }`:
         let generator_kind = hir::ExprKind::Closure(
             capture_clause,
             decl,
@@ -523,13 +557,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
     /// ```rust
     /// match <expr> {
     ///     mut pinned => loop {
-    ///         match ::std::future::poll_with_tls_context(unsafe {
-    ///             <::std::pin::Pin>::new_unchecked(&mut pinned)
-    ///         }) {
+    ///         match unsafe { ::std::future::poll_with_context(
+    ///             <::std::pin::Pin>::new_unchecked(&mut pinned),
+    ///             task_context,
+    ///         ) } {
     ///             ::std::task::Poll::Ready(result) => break result,
     ///             ::std::task::Poll::Pending => {}
     ///         }
-    ///         yield ();
+    ///         task_context = yield ();
     ///     }
     /// }
     /// ```
@@ -561,12 +596,23 @@ impl<'hir> LoweringContext<'_, 'hir> {
         let (pinned_pat, pinned_pat_hid) =
             self.pat_ident_binding_mode(span, pinned_ident, hir::BindingAnnotation::Mutable);
 
-        // ::std::future::poll_with_tls_context(unsafe {
-        //     ::std::pin::Pin::new_unchecked(&mut pinned)
-        // })`
+        let task_context_ident = Ident::with_dummy_span(sym::_task_context);
+
+        // unsafe {
+        //     ::std::future::poll_with_context(
+        //         ::std::pin::Pin::new_unchecked(&mut pinned),
+        //         task_context,
+        //     )
+        // }
         let poll_expr = {
             let pinned = self.expr_ident(span, pinned_ident, pinned_pat_hid);
             let ref_mut_pinned = self.expr_mut_addr_of(span, pinned);
+            let task_context = if let Some(task_context_hid) = self.task_context {
+                self.expr_ident_mut(span, task_context_ident, task_context_hid)
+            } else {
+                // Use of `await` outside of an async context, we cannot use `task_context` here.
+                self.expr_err(span)
+            };
             let pin_ty_id = self.next_id();
             let new_unchecked_expr_kind = self.expr_call_std_assoc_fn(
                 pin_ty_id,
@@ -575,14 +621,13 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 "new_unchecked",
                 arena_vec![self; ref_mut_pinned],
             );
-            let new_unchecked =
-                self.arena.alloc(self.expr(span, new_unchecked_expr_kind, ThinVec::new()));
-            let unsafe_expr = self.expr_unsafe(new_unchecked);
-            self.expr_call_std_path(
+            let new_unchecked = self.expr(span, new_unchecked_expr_kind, ThinVec::new());
+            let call = self.expr_call_std_path(
                 gen_future_span,
-                &[sym::future, sym::poll_with_tls_context],
-                arena_vec![self; unsafe_expr],
-            )
+                &[sym::future, sym::poll_with_context],
+                arena_vec![self; new_unchecked, task_context],
+            );
+            self.arena.alloc(self.expr_unsafe(call))
         };
 
         // `::std::task::Poll::Ready(result) => break result`
@@ -622,6 +667,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
             self.stmt_expr(span, match_expr)
         };
 
+        // task_context = yield ();
         let yield_stmt = {
             let unit = self.expr_unit(span);
             let yield_expr = self.expr(
@@ -629,7 +675,18 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 hir::ExprKind::Yield(unit, hir::YieldSource::Await),
                 ThinVec::new(),
             );
-            self.stmt_expr(span, yield_expr)
+            let yield_expr = self.arena.alloc(yield_expr);
+
+            if let Some(task_context_hid) = self.task_context {
+                let lhs = self.expr_ident(span, task_context_ident, task_context_hid);
+                let assign =
+                    self.expr(span, hir::ExprKind::Assign(lhs, yield_expr, span), AttrVec::new());
+                self.stmt_expr(span, assign)
+            } else {
+                // Use of `await` outside of an async context. Return `yield_expr` so that we can
+                // proceed with type checking.
+                self.stmt(span, hir::StmtKind::Semi(yield_expr))
+            }
         };
 
         let loop_block = self.block_all(span, arena_vec![self; inner_match_stmt, yield_stmt], None);
diff --git a/src/librustc_ast_lowering/item.rs b/src/librustc_ast_lowering/item.rs
index 94ae2a0973ae9..c22b2812a9e3c 100644
--- a/src/librustc_ast_lowering/item.rs
+++ b/src/librustc_ast_lowering/item.rs
@@ -814,7 +814,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
     }
 
     /// Construct `ExprKind::Err` for the given `span`.
-    fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
+    crate fn expr_err(&mut self, span: Span) -> hir::Expr<'hir> {
         self.expr(span, hir::ExprKind::Err, AttrVec::new())
     }
 
@@ -960,7 +960,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
         id
     }
 
-    fn lower_body(
+    pub(super) fn lower_body(
         &mut self,
         f: impl FnOnce(&mut Self) -> (&'hir [hir::Param<'hir>], hir::Expr<'hir>),
     ) -> hir::BodyId {
diff --git a/src/librustc_ast_lowering/lib.rs b/src/librustc_ast_lowering/lib.rs
index dcedcd51f5063..24fe51fc4d0d1 100644
--- a/src/librustc_ast_lowering/lib.rs
+++ b/src/librustc_ast_lowering/lib.rs
@@ -116,6 +116,10 @@ struct LoweringContext<'a, 'hir: 'a> {
 
     generator_kind: Option<hir::GeneratorKind>,
 
+    /// When inside an `async` context, this is the `HirId` of the
+    /// `task_context` local bound to the resume argument of the generator.
+    task_context: Option<hir::HirId>,
+
     /// Used to get the current `fn`'s def span to point to when using `await`
     /// outside of an `async fn`.
     current_item: Option<Span>,
@@ -294,6 +298,7 @@ pub fn lower_crate<'a, 'hir>(
         item_local_id_counters: Default::default(),
         node_id_to_hir_id: IndexVec::new(),
         generator_kind: None,
+        task_context: None,
         current_item: None,
         lifetimes_to_define: Vec::new(),
         is_collecting_in_band_lifetimes: false,
diff --git a/src/librustc_builtin_macros/deriving/decodable.rs b/src/librustc_builtin_macros/deriving/decodable.rs
index ac5d08ba62d0d..64a810bdcf687 100644
--- a/src/librustc_builtin_macros/deriving/decodable.rs
+++ b/src/librustc_builtin_macros/deriving/decodable.rs
@@ -87,7 +87,7 @@ fn decodable_substructure(
     let blkarg = cx.ident_of("_d", trait_span);
     let blkdecoder = cx.expr_ident(trait_span, blkarg);
 
-    return match *substr.fields {
+    match *substr.fields {
         StaticStruct(_, ref summary) => {
             let nfields = match *summary {
                 Unnamed(ref fields, _) => fields.len(),
@@ -178,7 +178,7 @@ fn decodable_substructure(
             )
         }
         _ => cx.bug("expected StaticEnum or StaticStruct in derive(Decodable)"),
-    };
+    }
 }
 
 /// Creates a decoder for a single enum variant/struct:
diff --git a/src/librustc_builtin_macros/deriving/default.rs b/src/librustc_builtin_macros/deriving/default.rs
index cb85a0b1a10cc..27d5263320041 100644
--- a/src/librustc_builtin_macros/deriving/default.rs
+++ b/src/librustc_builtin_macros/deriving/default.rs
@@ -53,7 +53,7 @@ fn default_substructure(
     let default_ident = cx.std_path(&[kw::Default, sym::Default, kw::Default]);
     let default_call = |span| cx.expr_call_global(span, default_ident.clone(), Vec::new());
 
-    return match *substr.fields {
+    match *substr.fields {
         StaticStruct(_, ref summary) => match *summary {
             Unnamed(ref fields, is_tuple) => {
                 if !is_tuple {
@@ -83,5 +83,5 @@ fn default_substructure(
             DummyResult::raw_expr(trait_span, true)
         }
         _ => cx.span_bug(trait_span, "method in `derive(Default)`"),
-    };
+    }
 }
diff --git a/src/librustc_builtin_macros/deriving/encodable.rs b/src/librustc_builtin_macros/deriving/encodable.rs
index 9073085381ac1..54926ec3fd502 100644
--- a/src/librustc_builtin_macros/deriving/encodable.rs
+++ b/src/librustc_builtin_macros/deriving/encodable.rs
@@ -173,7 +173,7 @@ fn encodable_substructure(
         ],
     ));
 
-    return match *substr.fields {
+    match *substr.fields {
         Struct(_, ref fields) => {
             let emit_struct_field = cx.ident_of("emit_struct_field", trait_span);
             let mut stmts = Vec::new();
@@ -283,5 +283,5 @@ fn encodable_substructure(
         }
 
         _ => cx.bug("expected Struct or EnumMatching in derive(Encodable)"),
-    };
+    }
 }
diff --git a/src/librustc_builtin_macros/deriving/generic/mod.rs b/src/librustc_builtin_macros/deriving/generic/mod.rs
index 84ed6e96aafc8..ee32e914acba4 100644
--- a/src/librustc_builtin_macros/deriving/generic/mod.rs
+++ b/src/librustc_builtin_macros/deriving/generic/mod.rs
@@ -489,7 +489,6 @@ impl<'a> TraitDef<'a> {
                 // set earlier; see
                 // librustc_expand/expand.rs:MacroExpander::fully_expand_fragment()
                 // librustc_expand/base.rs:Annotatable::derive_allowed()
-                return;
             }
         }
     }
diff --git a/src/librustc_builtin_macros/format_foreign.rs b/src/librustc_builtin_macros/format_foreign.rs
index cc3c403450e04..e6a87e4d82586 100644
--- a/src/librustc_builtin_macros/format_foreign.rs
+++ b/src/librustc_builtin_macros/format_foreign.rs
@@ -359,7 +359,7 @@ pub mod printf {
         //
         // Note: `move` used to capture copies of the cursors as they are *now*.
         let fallback = move || {
-            return Some((
+            Some((
                 Substitution::Format(Format {
                     span: start.slice_between(next).unwrap(),
                     parameter: None,
@@ -371,7 +371,7 @@ pub mod printf {
                     position: InnerSpan::new(start.at, next.at),
                 }),
                 next.slice_after(),
-            ));
+            ))
         };
 
         // Next parsing state.
diff --git a/src/librustc_builtin_macros/source_util.rs b/src/librustc_builtin_macros/source_util.rs
index 662bbe6a287a3..718498f04b94e 100644
--- a/src/librustc_builtin_macros/source_util.rs
+++ b/src/librustc_builtin_macros/source_util.rs
@@ -4,6 +4,7 @@ use rustc_ast::token;
 use rustc_ast::tokenstream::TokenStream;
 use rustc_ast_pretty::pprust;
 use rustc_expand::base::{self, *};
+use rustc_expand::module::DirectoryOwnership;
 use rustc_expand::panictry;
 use rustc_parse::{self, new_sub_parser_from_file, parser::Parser};
 use rustc_session::lint::builtin::INCOMPLETE_INCLUDE;
@@ -11,6 +12,7 @@ use rustc_span::symbol::Symbol;
 use rustc_span::{self, Pos, Span};
 
 use smallvec::SmallVec;
+use std::rc::Rc;
 
 use rustc_data_structures::sync::Lrc;
 
@@ -101,7 +103,7 @@ pub fn expand_include<'cx>(
         None => return DummyResult::any(sp),
     };
     // The file will be added to the code map by the parser
-    let file = match cx.resolve_path(file, sp) {
+    let mut file = match cx.resolve_path(file, sp) {
         Ok(f) => f,
         Err(mut err) => {
             err.emit();
@@ -110,6 +112,15 @@ pub fn expand_include<'cx>(
     };
     let p = new_sub_parser_from_file(cx.parse_sess(), &file, None, sp);
 
+    // If in the included file we have e.g., `mod bar;`,
+    // then the path of `bar.rs` should be relative to the directory of `file`.
+    // See https://github.com/rust-lang/rust/pull/69838/files#r395217057 for a discussion.
+    // `MacroExpander::fully_expand_fragment` later restores, so "stack discipline" is maintained.
+    file.pop();
+    cx.current_expansion.directory_ownership = DirectoryOwnership::Owned { relative: None };
+    let mod_path = cx.current_expansion.module.mod_path.clone();
+    cx.current_expansion.module = Rc::new(ModuleData { mod_path, directory: file });
+
     struct ExpandResult<'a> {
         p: Parser<'a>,
     }
diff --git a/src/librustc_codegen_llvm/back/archive.rs b/src/librustc_codegen_llvm/back/archive.rs
index 239ca57ba4143..f1fe40d919eeb 100644
--- a/src/librustc_codegen_llvm/back/archive.rs
+++ b/src/librustc_codegen_llvm/back/archive.rs
@@ -146,7 +146,7 @@ impl<'a> ArchiveBuilder<'a> for LlvmArchiveBuilder<'a> {
             }
 
             // ok, don't skip this
-            return false;
+            false
         })
     }
 
diff --git a/src/librustc_codegen_llvm/back/bytecode.rs b/src/librustc_codegen_llvm/back/bytecode.rs
index db29556e70ccc..0c8ce39132abb 100644
--- a/src/librustc_codegen_llvm/back/bytecode.rs
+++ b/src/librustc_codegen_llvm/back/bytecode.rs
@@ -83,7 +83,7 @@ pub fn encode(identifier: &str, bytecode: &[u8]) -> Vec<u8> {
         encoded.push(0);
     }
 
-    return encoded;
+    encoded
 }
 
 pub struct DecodedBytecode<'a> {
@@ -132,7 +132,7 @@ impl<'a> DecodedBytecode<'a> {
     pub fn bytecode(&self) -> Vec<u8> {
         let mut data = Vec::new();
         DeflateDecoder::new(self.encoded_bytecode).read_to_end(&mut data).unwrap();
-        return data;
+        data
     }
 
     pub fn identifier(&self) -> &'a str {
diff --git a/src/librustc_codegen_llvm/common.rs b/src/librustc_codegen_llvm/common.rs
index 609ddfc1d3a80..f72060868128c 100644
--- a/src/librustc_codegen_llvm/common.rs
+++ b/src/librustc_codegen_llvm/common.rs
@@ -96,15 +96,11 @@ impl BackendTypes for CodegenCx<'ll, 'tcx> {
 
 impl CodegenCx<'ll, 'tcx> {
     pub fn const_array(&self, ty: &'ll Type, elts: &[&'ll Value]) -> &'ll Value {
-        unsafe {
-            return llvm::LLVMConstArray(ty, elts.as_ptr(), elts.len() as c_uint);
-        }
+        unsafe { llvm::LLVMConstArray(ty, elts.as_ptr(), elts.len() as c_uint) }
     }
 
     pub fn const_vector(&self, elts: &[&'ll Value]) -> &'ll Value {
-        unsafe {
-            return llvm::LLVMConstVector(elts.as_ptr(), elts.len() as c_uint);
-        }
+        unsafe { llvm::LLVMConstVector(elts.as_ptr(), elts.len() as c_uint) }
     }
 
     pub fn const_bytes(&self, bytes: &[u8]) -> &'ll Value {
@@ -330,7 +326,7 @@ pub fn val_ty(v: &Value) -> &Type {
 pub fn bytes_in_context(llcx: &'ll llvm::Context, bytes: &[u8]) -> &'ll Value {
     unsafe {
         let ptr = bytes.as_ptr() as *const c_char;
-        return llvm::LLVMConstStringInContext(llcx, ptr, bytes.len() as c_uint, True);
+        llvm::LLVMConstStringInContext(llcx, ptr, bytes.len() as c_uint, True)
     }
 }
 
diff --git a/src/librustc_codegen_llvm/context.rs b/src/librustc_codegen_llvm/context.rs
index 7b1526e9da154..4427997c2732d 100644
--- a/src/librustc_codegen_llvm/context.rs
+++ b/src/librustc_codegen_llvm/context.rs
@@ -800,7 +800,7 @@ impl CodegenCx<'b, 'tcx> {
             ifn!("llvm.dbg.declare", fn(self.type_metadata(), self.type_metadata()) -> void);
             ifn!("llvm.dbg.value", fn(self.type_metadata(), t_i64, self.type_metadata()) -> void);
         }
-        return None;
+        None
     }
 }
 
diff --git a/src/librustc_codegen_llvm/debuginfo/metadata.rs b/src/librustc_codegen_llvm/debuginfo/metadata.rs
index 6a7ed4e1dc384..f35220cc6666a 100644
--- a/src/librustc_codegen_llvm/debuginfo/metadata.rs
+++ b/src/librustc_codegen_llvm/debuginfo/metadata.rs
@@ -203,7 +203,7 @@ impl TypeMap<'ll, 'tcx> {
         let key = self.unique_id_interner.intern(&unique_type_id);
         self.type_to_unique_id.insert(type_, UniqueTypeId(key));
 
-        return UniqueTypeId(key);
+        UniqueTypeId(key)
     }
 
     /// Gets the `UniqueTypeId` for an enum variant. Enum variants are not really
@@ -314,7 +314,7 @@ impl RecursiveTypeDescription<'ll, 'tcx> {
                     member_holding_stub,
                     member_descriptions,
                 );
-                return MetadataCreationResult::new(metadata_stub, true);
+                MetadataCreationResult::new(metadata_stub, true)
             }
         }
     }
@@ -364,7 +364,7 @@ fn fixed_vec_metadata(
         )
     };
 
-    return MetadataCreationResult::new(metadata, false);
+    MetadataCreationResult::new(metadata, false)
 }
 
 fn vec_slice_metadata(
@@ -445,7 +445,7 @@ fn subroutine_type_metadata(
 
     return_if_metadata_created_in_meantime!(cx, unique_type_id);
 
-    return MetadataCreationResult::new(
+    MetadataCreationResult::new(
         unsafe {
             llvm::LLVMRustDIBuilderCreateSubroutineType(
                 DIB(cx),
@@ -454,7 +454,7 @@ fn subroutine_type_metadata(
             )
         },
         false,
-    );
+    )
 }
 
 // FIXME(1563): This is all a bit of a hack because 'trait pointer' is an ill-
@@ -781,7 +781,7 @@ fn file_metadata_raw(
     let key = (file_name, directory);
 
     match debug_context(cx).created_files.borrow_mut().entry(key) {
-        Entry::Occupied(o) => return o.get(),
+        Entry::Occupied(o) => o.get(),
         Entry::Vacant(v) => {
             let (file_name, directory) = v.key();
             debug!("file_metadata: file_name: {:?}, directory: {:?}", file_name, directory);
@@ -831,7 +831,7 @@ fn basic_type_metadata(cx: &CodegenCx<'ll, 'tcx>, t: Ty<'tcx>) -> &'ll DIType {
         )
     };
 
-    return ty_metadata;
+    ty_metadata
 }
 
 fn foreign_type_metadata(
@@ -1273,11 +1273,11 @@ fn prepare_union_metadata(
 fn use_enum_fallback(cx: &CodegenCx<'_, '_>) -> bool {
     // On MSVC we have to use the fallback mode, because LLVM doesn't
     // lower variant parts to PDB.
-    return cx.sess().target.target.options.is_like_msvc
+    cx.sess().target.target.options.is_like_msvc
         // LLVM version 7 did not release with an important bug fix;
         // but the required patch is in the LLVM 8.  Rust LLVM reports
         // 8 as well.
-        || llvm_util::get_major_version() < 8;
+        || llvm_util::get_major_version() < 8
 }
 
 // FIXME(eddyb) maybe precompute this? Right now it's computed once
@@ -2075,7 +2075,7 @@ fn prepare_enum_metadata(
         }
     };
 
-    return create_and_register_recursive_type_forward_declaration(
+    create_and_register_recursive_type_forward_declaration(
         cx,
         enum_type,
         unique_type_id,
@@ -2088,7 +2088,7 @@ fn prepare_enum_metadata(
             containing_scope,
             span,
         }),
-    );
+    )
 }
 
 /// Creates debug information for a composite type, that is, anything that
diff --git a/src/librustc_codegen_llvm/debuginfo/mod.rs b/src/librustc_codegen_llvm/debuginfo/mod.rs
index 85decff35b9e0..41829d4ee4256 100644
--- a/src/librustc_codegen_llvm/debuginfo/mod.rs
+++ b/src/librustc_codegen_llvm/debuginfo/mod.rs
@@ -444,7 +444,7 @@ impl DebugInfoMethods<'tcx> for CodegenCx<'ll, 'tcx> {
                 vec![]
             };
 
-            return create_DIArray(DIB(cx), &template_params[..]);
+            create_DIArray(DIB(cx), &template_params[..])
         }
 
         fn get_parameter_names(cx: &CodegenCx<'_, '_>, generics: &ty::Generics) -> Vec<Symbol> {
diff --git a/src/librustc_codegen_llvm/debuginfo/utils.rs b/src/librustc_codegen_llvm/debuginfo/utils.rs
index bef40decdf3ab..b42d760a77345 100644
--- a/src/librustc_codegen_llvm/debuginfo/utils.rs
+++ b/src/librustc_codegen_llvm/debuginfo/utils.rs
@@ -24,9 +24,7 @@ pub fn is_node_local_to_unit(cx: &CodegenCx<'_, '_>, def_id: DefId) -> bool {
 
 #[allow(non_snake_case)]
 pub fn create_DIArray(builder: &DIBuilder<'ll>, arr: &[Option<&'ll DIDescriptor>]) -> &'ll DIArray {
-    return unsafe {
-        llvm::LLVMRustDIBuilderGetOrCreateArray(builder, arr.as_ptr(), arr.len() as u32)
-    };
+    unsafe { llvm::LLVMRustDIBuilderGetOrCreateArray(builder, arr.as_ptr(), arr.len() as u32) }
 }
 
 #[inline]
diff --git a/src/librustc_codegen_llvm/llvm/archive_ro.rs b/src/librustc_codegen_llvm/llvm/archive_ro.rs
index ab9df4162472c..64db4f7462df8 100644
--- a/src/librustc_codegen_llvm/llvm/archive_ro.rs
+++ b/src/librustc_codegen_llvm/llvm/archive_ro.rs
@@ -27,13 +27,13 @@ impl ArchiveRO {
     /// If this archive is used with a mutable method, then an error will be
     /// raised.
     pub fn open(dst: &Path) -> Result<ArchiveRO, String> {
-        return unsafe {
+        unsafe {
             let s = path_to_c_string(dst);
             let ar = super::LLVMRustOpenArchive(s.as_ptr()).ok_or_else(|| {
                 super::last_error().unwrap_or_else(|| "failed to open archive".to_owned())
             })?;
             Ok(ArchiveRO { raw: ar })
-        };
+        }
     }
 
     pub fn iter(&self) -> Iter<'_> {
diff --git a/src/librustc_codegen_ssa/back/command.rs b/src/librustc_codegen_ssa/back/command.rs
index 30b055b313149..0208bb73abdbe 100644
--- a/src/librustc_codegen_ssa/back/command.rs
+++ b/src/librustc_codegen_ssa/back/command.rs
@@ -119,7 +119,7 @@ impl Command {
         for k in &self.env_remove {
             ret.env_remove(k);
         }
-        return ret;
+        ret
     }
 
     // extensions
diff --git a/src/librustc_codegen_ssa/base.rs b/src/librustc_codegen_ssa/base.rs
index e57cae30b7795..5fd16cb121fda 100644
--- a/src/librustc_codegen_ssa/base.rs
+++ b/src/librustc_codegen_ssa/base.rs
@@ -852,7 +852,7 @@ impl CrateInfo {
             info.missing_lang_items.insert(cnum, missing);
         }
 
-        return info;
+        info
     }
 }
 
@@ -887,7 +887,7 @@ pub fn provide_both(providers: &mut Providers<'_>) {
                 }
             }
         }
-        return tcx.sess.opts.optimize;
+        tcx.sess.opts.optimize
     };
 
     providers.dllimport_foreign_items = |tcx, krate| {
diff --git a/src/librustc_data_structures/graph/dominators/mod.rs b/src/librustc_data_structures/graph/dominators/mod.rs
index 5283bd78a3029..a7f9340dead88 100644
--- a/src/librustc_data_structures/graph/dominators/mod.rs
+++ b/src/librustc_data_structures/graph/dominators/mod.rs
@@ -125,9 +125,9 @@ impl<'dom, Node: Idx> Iterator for Iter<'dom, Node> {
             } else {
                 self.node = Some(dom);
             }
-            return Some(node);
+            Some(node)
         } else {
-            return None;
+            None
         }
     }
 }
diff --git a/src/librustc_driver/lib.rs b/src/librustc_driver/lib.rs
index 34f0c182499db..e3e076e769f5d 100644
--- a/src/librustc_driver/lib.rs
+++ b/src/librustc_driver/lib.rs
@@ -752,7 +752,7 @@ impl RustcDefaultCalls {
                 PrintRequest::NativeStaticLibs => {}
             }
         }
-        return Compilation::Stop;
+        Compilation::Stop
     }
 }
 
diff --git a/src/librustc_error_codes/error_codes/E0439.md b/src/librustc_error_codes/error_codes/E0439.md
index e6da2117ac5f2..3e663df866caa 100644
--- a/src/librustc_error_codes/error_codes/E0439.md
+++ b/src/librustc_error_codes/error_codes/E0439.md
@@ -1,5 +1,6 @@
-The length of the platform-intrinsic function `simd_shuffle`
-wasn't specified. Erroneous code example:
+The length of the platform-intrinsic function `simd_shuffle` wasn't specified.
+
+Erroneous code example:
 
 ```compile_fail,E0439
 #![feature(platform_intrinsics)]
diff --git a/src/librustc_expand/mbe/macro_parser.rs b/src/librustc_expand/mbe/macro_parser.rs
index 3b9158f444519..0d777d88cad3a 100644
--- a/src/librustc_expand/mbe/macro_parser.rs
+++ b/src/librustc_expand/mbe/macro_parser.rs
@@ -778,7 +778,7 @@ fn may_begin_with(token: &Token, name: Name) -> bool {
         }
         sym::ty => token.can_begin_type(),
         sym::ident => get_macro_ident(token).is_some(),
-        sym::literal => token.can_begin_literal_or_bool(),
+        sym::literal => token.can_begin_literal_maybe_minus(),
         sym::vis => match token.kind {
             // The follow-set of :vis + "priv" keyword + interpolated
             token::Comma | token::Ident(..) | token::Interpolated(_) => true,
diff --git a/src/librustc_incremental/assert_module_sources.rs b/src/librustc_incremental/assert_module_sources.rs
index 54d7e0ece5031..c5446116f4c50 100644
--- a/src/librustc_incremental/assert_module_sources.rs
+++ b/src/librustc_incremental/assert_module_sources.rs
@@ -175,6 +175,6 @@ impl AssertModuleSource<'tcx> {
             return true;
         }
         debug!("check_config: no match found");
-        return false;
+        false
     }
 }
diff --git a/src/librustc_incremental/persist/save.rs b/src/librustc_incremental/persist/save.rs
index b465a11c99c06..ba586d0cfba04 100644
--- a/src/librustc_incremental/persist/save.rs
+++ b/src/librustc_incremental/persist/save.rs
@@ -132,7 +132,6 @@ where
         }
         Err(err) => {
             sess.err(&format!("failed to write dep-graph to `{}`: {}", path_buf.display(), err));
-            return;
         }
     }
 }
diff --git a/src/librustc_infer/infer/equate.rs b/src/librustc_infer/infer/equate.rs
index bb0c124a1892d..8f8fc4f137b73 100644
--- a/src/librustc_infer/infer/equate.rs
+++ b/src/librustc_infer/infer/equate.rs
@@ -136,7 +136,7 @@ impl TypeRelation<'tcx> for Equate<'combine, 'infcx, 'tcx> {
         } else {
             // Fast path for the common case.
             self.relate(a.skip_binder(), b.skip_binder())?;
-            return Ok(a.clone());
+            Ok(a.clone())
         }
     }
 }
diff --git a/src/librustc_infer/infer/error_reporting/nice_region_error/different_lifetimes.rs b/src/librustc_infer/infer/error_reporting/nice_region_error/different_lifetimes.rs
index 50b324c72278e..689323ce48346 100644
--- a/src/librustc_infer/infer/error_reporting/nice_region_error/different_lifetimes.rs
+++ b/src/librustc_infer/infer/error_reporting/nice_region_error/different_lifetimes.rs
@@ -142,6 +142,6 @@ impl<'a, 'tcx> NiceRegionError<'a, 'tcx> {
             .span_label(span_2, String::new())
             .span_label(span, span_label)
             .emit();
-        return Some(ErrorReported);
+        Some(ErrorReported)
     }
 }
diff --git a/src/librustc_infer/infer/higher_ranked/mod.rs b/src/librustc_infer/infer/higher_ranked/mod.rs
index 105b987f85e96..6a5a1c46d4caf 100644
--- a/src/librustc_infer/infer/higher_ranked/mod.rs
+++ b/src/librustc_infer/infer/higher_ranked/mod.rs
@@ -30,7 +30,7 @@ impl<'a, 'tcx> CombineFields<'a, 'tcx> {
 
         let span = self.trace.cause.span;
 
-        return self.infcx.commit_if_ok(|snapshot| {
+        self.infcx.commit_if_ok(|snapshot| {
             // First, we instantiate each bound region in the supertype with a
             // fresh placeholder region.
             let (b_prime, placeholder_map) = self.infcx.replace_bound_vars_with_placeholders(b);
@@ -53,7 +53,7 @@ impl<'a, 'tcx> CombineFields<'a, 'tcx> {
             debug!("higher_ranked_sub: OK result={:?}", result);
 
             Ok(ty::Binder::bind(result))
-        });
+        })
     }
 }
 
diff --git a/src/librustc_infer/infer/lexical_region_resolve/mod.rs b/src/librustc_infer/infer/lexical_region_resolve/mod.rs
index 3af10e850d534..821b9f72c0b20 100644
--- a/src/librustc_infer/infer/lexical_region_resolve/mod.rs
+++ b/src/librustc_infer/infer/lexical_region_resolve/mod.rs
@@ -452,12 +452,10 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
                 debug!("Expanding value of {:?} from {:?} to {:?}", b_vid, cur_region, lub);
 
                 *b_data = VarValue::Value(lub);
-                return true;
+                true
             }
 
-            VarValue::ErrorValue => {
-                return false;
-            }
+            VarValue::ErrorValue => false,
         }
     }
 
@@ -804,7 +802,7 @@ impl<'cx, 'tcx> LexicalResolver<'cx, 'tcx> {
             }
         }
 
-        return graph;
+        graph
     }
 
     fn collect_error_for_expanding_node(
diff --git a/src/librustc_infer/infer/nll_relate/mod.rs b/src/librustc_infer/infer/nll_relate/mod.rs
index 50bea300c5064..c194e968013eb 100644
--- a/src/librustc_infer/infer/nll_relate/mod.rs
+++ b/src/librustc_infer/infer/nll_relate/mod.rs
@@ -877,7 +877,7 @@ where
                     // If sub-roots are equal, then `for_vid` and
                     // `vid` are related via subtyping.
                     debug!("TypeGeneralizer::tys: occurs check failed");
-                    return Err(TypeError::Mismatch);
+                    Err(TypeError::Mismatch)
                 } else {
                     match variables.probe(vid) {
                         TypeVariableValue::Known { value: u } => {
@@ -898,7 +898,7 @@ where
 
                             let u = self.tcx().mk_ty_var(new_var_id);
                             debug!("generalize: replacing original vid={:?} with new={:?}", vid, u);
-                            return Ok(u);
+                            Ok(u)
                         }
                     }
                 }
diff --git a/src/librustc_infer/infer/region_constraints/mod.rs b/src/librustc_infer/infer/region_constraints/mod.rs
index 868b95043796b..38475b02e5db8 100644
--- a/src/librustc_infer/infer/region_constraints/mod.rs
+++ b/src/librustc_infer/infer/region_constraints/mod.rs
@@ -505,7 +505,7 @@ impl<'tcx> RegionConstraintCollector<'tcx> {
             self.undo_log.push(AddVar(vid));
         }
         debug!("created new region variable {:?} in {:?} with origin {:?}", vid, universe, origin);
-        return vid;
+        vid
     }
 
     /// Returns the universe for the given variable.
diff --git a/src/librustc_lexer/src/lib.rs b/src/librustc_lexer/src/lib.rs
index 25334461a113b..d3ac58a49c8d5 100644
--- a/src/librustc_lexer/src/lib.rs
+++ b/src/librustc_lexer/src/lib.rs
@@ -527,10 +527,10 @@ impl Cursor<'_> {
         if self.first() == '\'' {
             self.bump();
             let kind = Char { terminated: true };
-            return Literal { kind, suffix_start: self.len_consumed() };
+            Literal { kind, suffix_start: self.len_consumed() }
+        } else {
+            Lifetime { starts_with_number }
         }
-
-        return Lifetime { starts_with_number };
     }
 
     fn single_quoted_string(&mut self) -> bool {
diff --git a/src/librustc_lint/builtin.rs b/src/librustc_lint/builtin.rs
index 408031028b102..88f2284cd6154 100644
--- a/src/librustc_lint/builtin.rs
+++ b/src/librustc_lint/builtin.rs
@@ -269,7 +269,7 @@ impl EarlyLintPass for UnsafeCode {
                 })
             }
 
-            _ => return,
+            _ => {}
         }
     }
 
diff --git a/src/librustc_lint/types.rs b/src/librustc_lint/types.rs
index 4949c93d45eed..fcd50001cb3a9 100644
--- a/src/librustc_lint/types.rs
+++ b/src/librustc_lint/types.rs
@@ -273,7 +273,6 @@ fn lint_int_literal<'a, 'tcx>(
                     cx.sess()
                         .source_map()
                         .span_to_snippet(lit.span)
-                        .ok()
                         .expect("must get snippet from literal"),
                     t.name_str(),
                     min,
@@ -338,7 +337,6 @@ fn lint_uint_literal<'a, 'tcx>(
                     cx.sess()
                         .source_map()
                         .span_to_snippet(lit.span)
-                        .ok()
                         .expect("must get snippet from literal"),
                     t.name_str(),
                     min,
diff --git a/src/librustc_lint/unused.rs b/src/librustc_lint/unused.rs
index 229740615f707..b5826d6a5efa6 100644
--- a/src/librustc_lint/unused.rs
+++ b/src/librustc_lint/unused.rs
@@ -543,7 +543,7 @@ impl EarlyLintPass for UnusedParens {
             // Do not lint on `(..)` as that will result in the other arms being useless.
             Paren(_)
             // The other cases do not contain sub-patterns.
-            | Wild | Rest | Lit(..) | MacCall(..) | Range(..) | Ident(.., None) | Path(..) => return,
+            | Wild | Rest | Lit(..) | MacCall(..) | Range(..) | Ident(.., None) | Path(..) => {},
             // These are list-like patterns; parens can always be removed.
             TupleStruct(_, ps) | Tuple(ps) | Slice(ps) | Or(ps) => for p in ps {
                 self.check_unused_parens_pat(cx, p, false, false);
diff --git a/src/librustc_metadata/creader.rs b/src/librustc_metadata/creader.rs
index f20cdfcba15ca..9b6e427abc1fd 100644
--- a/src/librustc_metadata/creader.rs
+++ b/src/librustc_metadata/creader.rs
@@ -264,7 +264,7 @@ impl<'a> CrateLoader<'a> {
                 ret = Some(cnum);
             }
         });
-        return ret;
+        ret
     }
 
     fn verify_no_symbol_conflicts(&self, span: Span, root: &CrateRoot<'_>) {
diff --git a/src/librustc_metadata/foreign_modules.rs b/src/librustc_metadata/foreign_modules.rs
index fc988ec15cee9..60b8239a82155 100644
--- a/src/librustc_metadata/foreign_modules.rs
+++ b/src/librustc_metadata/foreign_modules.rs
@@ -6,7 +6,7 @@ use rustc_hir::itemlikevisit::ItemLikeVisitor;
 crate fn collect(tcx: TyCtxt<'_>) -> Vec<ForeignModule> {
     let mut collector = Collector { tcx, modules: Vec::new() };
     tcx.hir().krate().visit_all_item_likes(&mut collector);
-    return collector.modules;
+    collector.modules
 }
 
 struct Collector<'tcx> {
diff --git a/src/librustc_metadata/link_args.rs b/src/librustc_metadata/link_args.rs
index 13668b2423fdd..56b26efe5bf1e 100644
--- a/src/librustc_metadata/link_args.rs
+++ b/src/librustc_metadata/link_args.rs
@@ -16,7 +16,7 @@ crate fn collect(tcx: TyCtxt<'_>) -> Vec<String> {
         }
     }
 
-    return collector.args;
+    collector.args
 }
 
 struct Collector {
diff --git a/src/librustc_metadata/locator.rs b/src/librustc_metadata/locator.rs
index 1ede629e7ef7d..2f9be599ba94b 100644
--- a/src/librustc_metadata/locator.rs
+++ b/src/librustc_metadata/locator.rs
@@ -949,7 +949,7 @@ fn get_metadata_section(
     let start = Instant::now();
     let ret = get_metadata_section_imp(target, flavor, filename, loader);
     info!("reading {:?} => {:?}", filename.file_name().unwrap(), start.elapsed());
-    return ret;
+    ret
 }
 
 /// A trivial wrapper for `Mmap` that implements `StableDeref`.
diff --git a/src/librustc_metadata/native_libs.rs b/src/librustc_metadata/native_libs.rs
index 64bbf393ba0f1..19d2d620f58a7 100644
--- a/src/librustc_metadata/native_libs.rs
+++ b/src/librustc_metadata/native_libs.rs
@@ -15,7 +15,7 @@ crate fn collect(tcx: TyCtxt<'_>) -> Vec<NativeLibrary> {
     let mut collector = Collector { tcx, libs: Vec::new() };
     tcx.hir().krate().visit_all_item_likes(&mut collector);
     collector.process_command_line();
-    return collector.libs;
+    collector.libs
 }
 
 crate fn relevant_lib(sess: &Session, lib: &NativeLibrary) -> bool {
diff --git a/src/librustc_metadata/rmeta/decoder/cstore_impl.rs b/src/librustc_metadata/rmeta/decoder/cstore_impl.rs
index cc2bd51f92f3e..ca75daf1aa9b2 100644
--- a/src/librustc_metadata/rmeta/decoder/cstore_impl.rs
+++ b/src/librustc_metadata/rmeta/decoder/cstore_impl.rs
@@ -170,7 +170,7 @@ provide! { <'tcx> tcx, def_id, other, cdata,
             .iter()
             .filter_map(|&(exported_symbol, export_level)| {
                 if let ExportedSymbol::NonGeneric(def_id) = exported_symbol {
-                    return Some((def_id, export_level))
+                    Some((def_id, export_level))
                 } else {
                     None
                 }
diff --git a/src/librustc_mir/borrow_check/borrow_set.rs b/src/librustc_mir/borrow_check/borrow_set.rs
index 9d5cf3ec4bec0..9f4f0ce5620b5 100644
--- a/src/librustc_mir/borrow_check/borrow_set.rs
+++ b/src/librustc_mir/borrow_check/borrow_set.rs
@@ -273,7 +273,7 @@ impl<'a, 'tcx> Visitor<'tcx> for GatherBorrows<'a, 'tcx> {
             assert_eq!(borrow_data.borrowed_place, *place);
         }
 
-        return self.super_rvalue(rvalue, location);
+        self.super_rvalue(rvalue, location)
     }
 }
 
diff --git a/src/librustc_mir/borrow_check/diagnostics/region_name.rs b/src/librustc_mir/borrow_check/diagnostics/region_name.rs
index 7103fc596c922..d1d0ba215e08e 100644
--- a/src/librustc_mir/borrow_check/diagnostics/region_name.rs
+++ b/src/librustc_mir/borrow_check/diagnostics/region_name.rs
@@ -500,7 +500,7 @@ impl<'tcx> MirBorrowckCtxt<'_, 'tcx> {
             }
         }
 
-        return None;
+        None
     }
 
     /// We've found an enum/struct/union type with the substitutions
diff --git a/src/librustc_mir/borrow_check/type_check/input_output.rs b/src/librustc_mir/borrow_check/type_check/input_output.rs
index 37cf77b7095c6..f2194c77c8991 100644
--- a/src/librustc_mir/borrow_check/type_check/input_output.rs
+++ b/src/librustc_mir/borrow_check/type_check/input_output.rs
@@ -64,13 +64,16 @@ impl<'a, 'tcx> TypeChecker<'a, 'tcx> {
             }
         };
 
+        debug!(
+            "equate_inputs_and_outputs: normalized_input_tys = {:?}, local_decls = {:?}",
+            normalized_input_tys, body.local_decls
+        );
+
         // Equate expected input tys with those in the MIR.
         for (&normalized_input_ty, argument_index) in normalized_input_tys.iter().zip(0..) {
             // In MIR, argument N is stored in local N+1.
             let local = Local::new(argument_index + 1);
 
-            debug!("equate_inputs_and_outputs: normalized_input_ty = {:?}", normalized_input_ty);
-
             let mir_input_ty = body.local_decls[local].ty;
             let mir_input_span = body.local_decls[local].source_info.span;
             self.equate_normalized_input_or_output(
diff --git a/src/librustc_mir/const_eval/machine.rs b/src/librustc_mir/const_eval/machine.rs
index 28889486c383b..d81aae6523a45 100644
--- a/src/librustc_mir/const_eval/machine.rs
+++ b/src/librustc_mir/const_eval/machine.rs
@@ -56,7 +56,7 @@ impl<'mir, 'tcx> InterpCx<'mir, 'tcx, CompileTimeInterpreter<'mir, 'tcx>> {
 
         self.return_to_block(ret.map(|r| r.1))?;
         self.dump_place(*dest);
-        return Ok(true);
+        Ok(true)
     }
 
     /// "Intercept" a function call to a panic-related function
diff --git a/src/librustc_mir/dataflow/mod.rs b/src/librustc_mir/dataflow/mod.rs
index dd0f9ff75b9fe..c98a5e84729ab 100644
--- a/src/librustc_mir/dataflow/mod.rs
+++ b/src/librustc_mir/dataflow/mod.rs
@@ -122,7 +122,7 @@ pub(crate) fn has_rustc_mir_with(attrs: &[ast::Attribute], name: Symbol) -> Opti
             }
         }
     }
-    return None;
+    None
 }
 
 pub struct MoveDataParamEnv<'tcx> {
@@ -171,7 +171,7 @@ where
                     return None;
                 }
             }
-            return None;
+            None
         };
 
         let print_preflow_to = name_found(tcx.sess, attributes, sym::borrowck_graphviz_preflow);
diff --git a/src/librustc_mir/interpret/intrinsics/caller_location.rs b/src/librustc_mir/interpret/intrinsics/caller_location.rs
index 566601f0cae28..dc2b0e1b983dc 100644
--- a/src/librustc_mir/interpret/intrinsics/caller_location.rs
+++ b/src/librustc_mir/interpret/intrinsics/caller_location.rs
@@ -39,7 +39,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         let loc_ty = self
             .tcx
             .type_of(self.tcx.require_lang_item(PanicLocationLangItem, None))
-            .subst(*self.tcx, self.tcx.mk_substs([self.tcx.lifetimes.re_static.into()].iter()));
+            .subst(*self.tcx, self.tcx.mk_substs([self.tcx.lifetimes.re_erased.into()].iter()));
         let loc_layout = self.layout_of(loc_ty).unwrap();
         let location = self.allocate(loc_layout, MemoryKind::CallerLocation);
 
diff --git a/src/librustc_mir/interpret/operand.rs b/src/librustc_mir/interpret/operand.rs
index b39058405f5cb..4c82172ae4517 100644
--- a/src/librustc_mir/interpret/operand.rs
+++ b/src/librustc_mir/interpret/operand.rs
@@ -604,7 +604,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
                     .not_undef()
                     .and_then(|raw_discr| self.force_bits(raw_discr, discr_val.layout.size))
                     .map_err(|_| err_ub!(InvalidDiscriminant(raw_discr.erase_tag())))?;
-                let real_discr = if discr_val.layout.ty.is_signed() {
+                let real_discr = if discr_val.layout.abi.is_signed() {
                     // going from layout tag type to typeck discriminant type
                     // requires first sign extending with the discriminant layout
                     let sexted = sign_extend(bits_discr, discr_val.layout.size) as i128;
diff --git a/src/librustc_mir/interpret/operator.rs b/src/librustc_mir/interpret/operator.rs
index f2ee5e047a88e..76a5aecb9db62 100644
--- a/src/librustc_mir/interpret/operator.rs
+++ b/src/librustc_mir/interpret/operator.rs
@@ -64,7 +64,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             Ge => l >= r,
             _ => bug!("Invalid operation on char: {:?}", bin_op),
         };
-        return (Scalar::from_bool(res), false, self.tcx.types.bool);
+        (Scalar::from_bool(res), false, self.tcx.types.bool)
     }
 
     fn binary_bool_op(
@@ -87,7 +87,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             BitXor => l ^ r,
             _ => bug!("Invalid operation on bool: {:?}", bin_op),
         };
-        return (Scalar::from_bool(res), false, self.tcx.types.bool);
+        (Scalar::from_bool(res), false, self.tcx.types.bool)
     }
 
     fn binary_float_op<F: Float + Into<Scalar<M::PointerTag>>>(
@@ -113,7 +113,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
             Rem => ((l % r).value.into(), ty),
             _ => bug!("invalid float op: `{:?}`", bin_op),
         };
-        return (val, false, ty);
+        (val, false, ty)
     }
 
     fn binary_int_op(
diff --git a/src/librustc_mir/interpret/place.rs b/src/librustc_mir/interpret/place.rs
index 5313446c253c8..027e33abc7bb1 100644
--- a/src/librustc_mir/interpret/place.rs
+++ b/src/librustc_mir/interpret/place.rs
@@ -212,9 +212,7 @@ impl<'tcx, Tag> MPlaceTy<'tcx, Tag> {
         if self.layout.is_unsized() {
             // We need to consult `meta` metadata
             match self.layout.ty.kind {
-                ty::Slice(..) | ty::Str => {
-                    return self.mplace.meta.unwrap_meta().to_machine_usize(cx);
-                }
+                ty::Slice(..) | ty::Str => self.mplace.meta.unwrap_meta().to_machine_usize(cx),
                 _ => bug!("len not supported on unsized type {:?}", self.layout.ty),
             }
         } else {
@@ -920,6 +918,10 @@ where
             // most likey we *are* running `typeck` right now. Investigate whether we can bail out
             // on `typeck_tables().has_errors` at all const eval entry points.
             debug!("Size mismatch when transmuting!\nsrc: {:#?}\ndest: {:#?}", src, dest);
+            self.tcx.sess.delay_span_bug(
+                self.tcx.span,
+                "size-changing transmute, should have been caught by transmute checking",
+            );
             throw_inval!(TransmuteSizeDiff(src.layout.ty, dest.layout.ty));
         }
         // Unsized copies rely on interpreting `src.meta` with `dest.layout`, we want
diff --git a/src/librustc_mir/interpret/terminator.rs b/src/librustc_mir/interpret/terminator.rs
index 22a081a9c8e0b..a9e45a032a6be 100644
--- a/src/librustc_mir/interpret/terminator.rs
+++ b/src/librustc_mir/interpret/terminator.rs
@@ -240,7 +240,7 @@ impl<'mir, 'tcx, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         match instance.def {
             ty::InstanceDef::Intrinsic(..) => {
                 assert!(caller_abi == Abi::RustIntrinsic || caller_abi == Abi::PlatformIntrinsic);
-                return M::call_intrinsic(self, span, instance, args, ret, unwind);
+                M::call_intrinsic(self, span, instance, args, ret, unwind)
             }
             ty::InstanceDef::VtableShim(..)
             | ty::InstanceDef::ReifyShim(..)
diff --git a/src/librustc_mir/monomorphize/collector.rs b/src/librustc_mir/monomorphize/collector.rs
index 4dd037d93ce9b..a592e8d9c05fe 100644
--- a/src/librustc_mir/monomorphize/collector.rs
+++ b/src/librustc_mir/monomorphize/collector.rs
@@ -751,7 +751,7 @@ fn should_monomorphize_locally<'tcx>(tcx: TyCtxt<'tcx>, instance: &Instance<'tcx
         bug!("cannot create local mono-item for {:?}", def_id)
     }
 
-    return true;
+    true
 }
 
 /// For a given pair of source and target type that occur in an unsizing coercion,
diff --git a/src/librustc_mir/transform/cleanup_post_borrowck.rs b/src/librustc_mir/transform/cleanup_post_borrowck.rs
index 5288b6b370ddb..3d219ac2c01ec 100644
--- a/src/librustc_mir/transform/cleanup_post_borrowck.rs
+++ b/src/librustc_mir/transform/cleanup_post_borrowck.rs
@@ -32,6 +32,7 @@ impl<'tcx> MirPass<'tcx> for CleanupNonCodegenStatements {
     fn run_pass(&self, tcx: TyCtxt<'tcx>, _source: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
         let mut delete = DeleteNonCodegenStatements { tcx };
         delete.visit_body(body);
+        body.user_type_annotations.raw.clear();
     }
 }
 
diff --git a/src/librustc_mir/transform/const_prop.rs b/src/librustc_mir/transform/const_prop.rs
index ca23c44f64668..43876380c840e 100644
--- a/src/librustc_mir/transform/const_prop.rs
+++ b/src/librustc_mir/transform/const_prop.rs
@@ -483,7 +483,7 @@ impl<'mir, 'tcx> ConstPropagator<'mir, 'tcx> {
             err.span_label(source_info.span, format!("{:?}", panic));
             err.emit()
         });
-        return None;
+        None
     }
 
     fn check_unary_op(
diff --git a/src/librustc_mir/transform/erase_regions.rs b/src/librustc_mir/transform/erase_regions.rs
deleted file mode 100644
index 996b97c03b14e..0000000000000
--- a/src/librustc_mir/transform/erase_regions.rs
+++ /dev/null
@@ -1,63 +0,0 @@
-//! This pass erases all early-bound regions from the types occurring in the MIR.
-//! We want to do this once just before codegen, so codegen does not have to take
-//! care erasing regions all over the place.
-//! N.B., we do _not_ erase regions of statements that are relevant for
-//! "types-as-contracts"-validation, namely, `AcquireValid` and `ReleaseValid`.
-
-use crate::transform::{MirPass, MirSource};
-use rustc::mir::visit::{MutVisitor, TyContext};
-use rustc::mir::*;
-use rustc::ty::subst::SubstsRef;
-use rustc::ty::{self, Ty, TyCtxt};
-
-struct EraseRegionsVisitor<'tcx> {
-    tcx: TyCtxt<'tcx>,
-}
-
-impl EraseRegionsVisitor<'tcx> {
-    pub fn new(tcx: TyCtxt<'tcx>) -> Self {
-        EraseRegionsVisitor { tcx }
-    }
-}
-
-impl MutVisitor<'tcx> for EraseRegionsVisitor<'tcx> {
-    fn tcx(&self) -> TyCtxt<'tcx> {
-        self.tcx
-    }
-
-    fn visit_ty(&mut self, ty: &mut Ty<'tcx>, _: TyContext) {
-        *ty = self.tcx.erase_regions(ty);
-    }
-
-    fn visit_region(&mut self, region: &mut ty::Region<'tcx>, _: Location) {
-        *region = self.tcx.lifetimes.re_erased;
-    }
-
-    fn visit_const(&mut self, constant: &mut &'tcx ty::Const<'tcx>, _: Location) {
-        *constant = self.tcx.erase_regions(constant);
-    }
-
-    fn visit_substs(&mut self, substs: &mut SubstsRef<'tcx>, _: Location) {
-        *substs = self.tcx.erase_regions(substs);
-    }
-
-    fn process_projection_elem(&mut self, elem: &PlaceElem<'tcx>) -> Option<PlaceElem<'tcx>> {
-        if let PlaceElem::Field(field, ty) = elem {
-            let new_ty = self.tcx.erase_regions(ty);
-
-            if new_ty != *ty {
-                return Some(PlaceElem::Field(*field, new_ty));
-            }
-        }
-
-        None
-    }
-}
-
-pub struct EraseRegions;
-
-impl<'tcx> MirPass<'tcx> for EraseRegions {
-    fn run_pass(&self, tcx: TyCtxt<'tcx>, _: MirSource<'tcx>, body: &mut BodyAndCache<'tcx>) {
-        EraseRegionsVisitor::new(tcx).visit_body(body);
-    }
-}
diff --git a/src/librustc_mir/transform/generator.rs b/src/librustc_mir/transform/generator.rs
index b2906739ff1b1..82c5ac689b568 100644
--- a/src/librustc_mir/transform/generator.rs
+++ b/src/librustc_mir/transform/generator.rs
@@ -357,18 +357,11 @@ impl MutVisitor<'tcx> for TransformVisitor<'tcx> {
     }
 }
 
-fn make_generator_state_argument_indirect<'tcx>(
-    tcx: TyCtxt<'tcx>,
-    def_id: DefId,
-    body: &mut BodyAndCache<'tcx>,
-) {
+fn make_generator_state_argument_indirect<'tcx>(tcx: TyCtxt<'tcx>, body: &mut BodyAndCache<'tcx>) {
     let gen_ty = body.local_decls.raw[1].ty;
 
-    let region = ty::ReFree(ty::FreeRegion { scope: def_id, bound_region: ty::BoundRegion::BrEnv });
-
-    let region = tcx.mk_region(region);
-
-    let ref_gen_ty = tcx.mk_ref(region, ty::TypeAndMut { ty: gen_ty, mutbl: hir::Mutability::Mut });
+    let ref_gen_ty =
+        tcx.mk_ref(tcx.lifetimes.re_erased, ty::TypeAndMut { ty: gen_ty, mutbl: Mutability::Mut });
 
     // Replace the by value generator argument
     body.local_decls.raw[1].ty = ref_gen_ty;
@@ -874,7 +867,6 @@ fn elaborate_generator_drops<'tcx>(
 fn create_generator_drop_shim<'tcx>(
     tcx: TyCtxt<'tcx>,
     transform: &TransformVisitor<'tcx>,
-    def_id: DefId,
     source: MirSource<'tcx>,
     gen_ty: Ty<'tcx>,
     body: &mut BodyAndCache<'tcx>,
@@ -912,7 +904,7 @@ fn create_generator_drop_shim<'tcx>(
         local_info: LocalInfo::Other,
     };
 
-    make_generator_state_argument_indirect(tcx, def_id, &mut body);
+    make_generator_state_argument_indirect(tcx, &mut body);
 
     // Change the generator argument from &mut to *mut
     body.local_decls[SELF_ARG] = LocalDecl {
@@ -1047,7 +1039,6 @@ fn can_unwind<'tcx>(tcx: TyCtxt<'tcx>, body: &Body<'tcx>) -> bool {
 fn create_generator_resume_function<'tcx>(
     tcx: TyCtxt<'tcx>,
     transform: TransformVisitor<'tcx>,
-    def_id: DefId,
     source: MirSource<'tcx>,
     body: &mut BodyAndCache<'tcx>,
     can_return: bool,
@@ -1112,7 +1103,7 @@ fn create_generator_resume_function<'tcx>(
 
     insert_switch(body, cases, &transform, TerminatorKind::Unreachable);
 
-    make_generator_state_argument_indirect(tcx, def_id, body);
+    make_generator_state_argument_indirect(tcx, body);
     make_generator_state_argument_pinned(tcx, body);
 
     no_landing_pads(tcx, body);
@@ -1332,11 +1323,11 @@ impl<'tcx> MirPass<'tcx> for StateTransform {
 
         // Create a copy of our MIR and use it to create the drop shim for the generator
         let drop_shim =
-            create_generator_drop_shim(tcx, &transform, def_id, source, gen_ty, body, drop_clean);
+            create_generator_drop_shim(tcx, &transform, source, gen_ty, body, drop_clean);
 
         body.generator_drop = Some(box drop_shim);
 
         // Create the Generator::resume function
-        create_generator_resume_function(tcx, transform, def_id, source, body, can_return);
+        create_generator_resume_function(tcx, transform, source, body, can_return);
     }
 }
diff --git a/src/librustc_mir/transform/mod.rs b/src/librustc_mir/transform/mod.rs
index 3eb9d23a32a25..50868434baa32 100644
--- a/src/librustc_mir/transform/mod.rs
+++ b/src/librustc_mir/transform/mod.rs
@@ -22,7 +22,6 @@ pub mod copy_prop;
 pub mod deaggregator;
 pub mod dump_mir;
 pub mod elaborate_drops;
-pub mod erase_regions;
 pub mod generator;
 pub mod inline;
 pub mod instcombine;
@@ -296,8 +295,6 @@ fn run_optimization_passes<'tcx>(
             &simplify::SimplifyCfg::new("elaborate-drops"),
             // No lifetime analysis based on borrowing can be done from here on out.
 
-            // From here on out, regions are gone.
-            &erase_regions::EraseRegions,
             // Optimizations begin.
             &unreachable_prop::UnreachablePropagation,
             &uninhabited_enum_branching::UninhabitedEnumBranching,
@@ -341,6 +338,9 @@ fn optimized_mir(tcx: TyCtxt<'_>, def_id: DefId) -> &BodyAndCache<'_> {
     let mut body = body.steal();
     run_optimization_passes(tcx, &mut body, def_id, None);
     body.ensure_predecessors();
+
+    debug_assert!(!body.has_free_regions(), "Free regions in optimized MIR");
+
     tcx.arena.alloc(body)
 }
 
@@ -358,5 +358,7 @@ fn promoted_mir(tcx: TyCtxt<'_>, def_id: DefId) -> &IndexVec<Promoted, BodyAndCa
         body.ensure_predecessors();
     }
 
+    debug_assert!(!promoted.has_free_regions(), "Free regions in promoted MIR");
+
     tcx.intern_promoted(promoted)
 }
diff --git a/src/librustc_mir/transform/promote_consts.rs b/src/librustc_mir/transform/promote_consts.rs
index 1336206e18626..a27c80d91cfd0 100644
--- a/src/librustc_mir/transform/promote_consts.rs
+++ b/src/librustc_mir/transform/promote_consts.rs
@@ -913,7 +913,13 @@ impl<'a, 'tcx> Promoter<'a, 'tcx> {
                         ty,
                         val: ty::ConstKind::Unevaluated(
                             def_id,
-                            InternalSubsts::identity_for_item(tcx, def_id),
+                            InternalSubsts::for_item(tcx, def_id, |param, _| {
+                                if let ty::GenericParamDefKind::Lifetime = param.kind {
+                                    tcx.lifetimes.re_erased.into()
+                                } else {
+                                    tcx.mk_param_from_def(param)
+                                }
+                            }),
                             Some(promoted_id),
                         ),
                     }),
diff --git a/src/librustc_mir_build/build/mod.rs b/src/librustc_mir_build/build/mod.rs
index 821c4d68c7e8a..c21834bfde84b 100644
--- a/src/librustc_mir_build/build/mod.rs
+++ b/src/librustc_mir_build/build/mod.rs
@@ -6,7 +6,7 @@ use rustc::middle::lang_items;
 use rustc::middle::region;
 use rustc::mir::*;
 use rustc::ty::subst::Subst;
-use rustc::ty::{self, Ty, TyCtxt};
+use rustc::ty::{self, Ty, TyCtxt, TypeFoldable};
 use rustc_attr::{self as attr, UnwindAttr};
 use rustc_hir as hir;
 use rustc_hir::def_id::DefId;
@@ -43,8 +43,7 @@ fn mir_build(tcx: TyCtxt<'_>, def_id: DefId) -> BodyAndCache<'_> {
             ..
         })
         | Node::TraitItem(hir::TraitItem {
-            kind:
-                hir::TraitItemKind::Fn(hir::FnSig { decl, .. }, hir::TraitFn::Provided(body_id)),
+            kind: hir::TraitItemKind::Fn(hir::FnSig { decl, .. }, hir::TraitFn::Provided(body_id)),
             ..
         }) => (*body_id, decl.output.span()),
         Node::Item(hir::Item { kind: hir::ItemKind::Static(ty, _, body_id), .. })
@@ -128,12 +127,8 @@ fn mir_build(tcx: TyCtxt<'_>, def_id: DefId) -> BodyAndCache<'_> {
                 let ty = if fn_sig.c_variadic && index == fn_sig.inputs().len() {
                     let va_list_did =
                         tcx.require_lang_item(lang_items::VaListTypeLangItem, Some(arg.span));
-                    let region = tcx.mk_region(ty::ReScope(region::Scope {
-                        id: body.value.hir_id.local_id,
-                        data: region::ScopeData::CallSite,
-                    }));
 
-                    tcx.type_of(va_list_did).subst(tcx, &[region.into()])
+                    tcx.type_of(va_list_did).subst(tcx, &[tcx.lifetimes.re_erased.into()])
                 } else {
                     fn_sig.inputs()[index]
                 };
@@ -189,6 +184,20 @@ fn mir_build(tcx: TyCtxt<'_>, def_id: DefId) -> BodyAndCache<'_> {
 
         let mut body = BodyAndCache::new(body);
         body.ensure_predecessors();
+
+        // The borrow checker will replace all the regions here with its own
+        // inference variables. There's no point having non-erased regions here.
+        // The exception is `body.user_type_annotations`, which is used unmodified
+        // by borrow checking.
+        debug_assert!(
+            !(body.local_decls.has_free_regions()
+                || body.basic_blocks().has_free_regions()
+                || body.var_debug_info.has_free_regions()
+                || body.yield_ty.has_free_regions()),
+            "Unexpected free regions in MIR: {:?}",
+            body,
+        );
+
         body
     })
 }
@@ -209,7 +218,7 @@ fn liberated_closure_env_ty(
     };
 
     let closure_env_ty = tcx.closure_env_ty(closure_def_id, closure_substs).unwrap();
-    tcx.liberate_late_bound_regions(closure_def_id, &closure_env_ty)
+    tcx.erase_late_bound_regions(&closure_env_ty)
 }
 
 #[derive(Debug, PartialEq, Eq)]
@@ -368,7 +377,7 @@ impl BlockContext {
             }
         }
 
-        return None;
+        None
     }
 
     /// Looks at the topmost frame on the BlockContext and reports
diff --git a/src/librustc_mir_build/hair/cx/block.rs b/src/librustc_mir_build/hair/cx/block.rs
index 8d7225c8c7b51..07a9d91cd746d 100644
--- a/src/librustc_mir_build/hair/cx/block.rs
+++ b/src/librustc_mir_build/hair/cx/block.rs
@@ -98,7 +98,7 @@ fn mirror_stmts<'a, 'tcx>(
             }
         }
     }
-    return result;
+    result
 }
 
 crate fn to_expr_ref<'a, 'tcx>(
diff --git a/src/librustc_parse/lib.rs b/src/librustc_parse/lib.rs
index c31cc1b4c9f00..58db7d286e7e6 100644
--- a/src/librustc_parse/lib.rs
+++ b/src/librustc_parse/lib.rs
@@ -320,7 +320,7 @@ pub fn nt_to_tokenstream(nt: &Nonterminal, sess: &ParseSess, span: Span) -> Toke
                 going with stringified version"
         );
     }
-    return tokens_for_real;
+    tokens_for_real
 }
 
 fn prepend_attrs(
diff --git a/src/librustc_parse/parser/expr.rs b/src/librustc_parse/parser/expr.rs
index c65e99842c5dd..b993857681a30 100644
--- a/src/librustc_parse/parser/expr.rs
+++ b/src/librustc_parse/parser/expr.rs
@@ -996,7 +996,7 @@ impl<'a> Parser<'a> {
                 let expr = self.mk_expr(lo.to(self.prev_token.span), ExprKind::Lit(literal), attrs);
                 self.maybe_recover_from_bad_qpath(expr, true)
             }
-            None => return Err(self.expected_expression_found()),
+            None => Err(self.expected_expression_found()),
         }
     }
 
@@ -1374,6 +1374,7 @@ impl<'a> Parser<'a> {
     }
 
     /// Matches `'-' lit | lit` (cf. `ast_validation::AstValidator::check_expr_within_pat`).
+    /// Keep this in sync with `Token::can_begin_literal_maybe_minus`.
     pub fn parse_literal_maybe_minus(&mut self) -> PResult<'a, P<Expr>> {
         maybe_whole_expr!(self);
 
@@ -1713,7 +1714,7 @@ impl<'a> Parser<'a> {
         }
         let hi = self.token.span;
         self.bump();
-        return Ok(self.mk_expr(lo.to(hi), ExprKind::Match(scrutinee, arms), attrs));
+        Ok(self.mk_expr(lo.to(hi), ExprKind::Match(scrutinee, arms), attrs))
     }
 
     pub(super) fn parse_arm(&mut self) -> PResult<'a, Arm> {
diff --git a/src/librustc_parse/parser/item.rs b/src/librustc_parse/parser/item.rs
index 9d70f606f3ef4..5f37069afbe4c 100644
--- a/src/librustc_parse/parser/item.rs
+++ b/src/librustc_parse/parser/item.rs
@@ -314,7 +314,7 @@ impl<'a> Parser<'a> {
                 " struct ".into(),
                 Applicability::MaybeIncorrect, // speculative
             );
-            return Err(err);
+            Err(err)
         } else if self.look_ahead(1, |t| *t == token::OpenDelim(token::Paren)) {
             let ident = self.parse_ident().unwrap();
             self.bump(); // `(`
@@ -362,7 +362,7 @@ impl<'a> Parser<'a> {
                     );
                 }
             }
-            return Err(err);
+            Err(err)
         } else if self.look_ahead(1, |t| *t == token::Lt) {
             let ident = self.parse_ident().unwrap();
             self.eat_to_tokens(&[&token::Gt]);
@@ -384,7 +384,7 @@ impl<'a> Parser<'a> {
                     Applicability::MachineApplicable,
                 );
             }
-            return Err(err);
+            Err(err)
         } else {
             Ok(())
         }
@@ -910,7 +910,7 @@ impl<'a> Parser<'a> {
         let span = self.sess.source_map().def_span(span);
         let msg = format!("{} is not supported in {}", kind.descr(), ctx);
         self.struct_span_err(span, &msg).emit();
-        return None;
+        None
     }
 
     fn error_on_foreign_const(&self, span: Span, ident: Ident) {
@@ -1509,7 +1509,7 @@ impl<'a> Parser<'a> {
                 })
             // `extern ABI fn`
             || self.check_keyword(kw::Extern)
-                && self.look_ahead(1, |t| t.can_begin_literal_or_bool())
+                && self.look_ahead(1, |t| t.can_begin_literal_maybe_minus())
                 && self.look_ahead(2, |t| t.is_keyword(kw::Fn))
     }
 
diff --git a/src/librustc_parse/parser/pat.rs b/src/librustc_parse/parser/pat.rs
index 4585941943b74..f7f7ac89a1864 100644
--- a/src/librustc_parse/parser/pat.rs
+++ b/src/librustc_parse/parser/pat.rs
@@ -696,7 +696,7 @@ impl<'a> Parser<'a> {
         self.look_ahead(dist, |t| {
             t.is_path_start() // e.g. `MY_CONST`;
                 || t.kind == token::Dot // e.g. `.5` for recovery;
-                || t.can_begin_literal_or_bool() // e.g. `42`.
+                || t.can_begin_literal_maybe_minus() // e.g. `42`.
                 || t.is_whole_expr()
         })
     }
@@ -918,7 +918,7 @@ impl<'a> Parser<'a> {
             }
             err.emit();
         }
-        return Ok((fields, etc));
+        Ok((fields, etc))
     }
 
     /// Recover on `...` as if it were `..` to avoid further errors.
diff --git a/src/librustc_parse/parser/stmt.rs b/src/librustc_parse/parser/stmt.rs
index d40597d8fcb0c..d43f5d67113a1 100644
--- a/src/librustc_parse/parser/stmt.rs
+++ b/src/librustc_parse/parser/stmt.rs
@@ -278,7 +278,7 @@ impl<'a> Parser<'a> {
             _ => {}
         }
         e.span_label(sp, "expected `{`");
-        return Err(e);
+        Err(e)
     }
 
     /// Parses a block. Inner attributes are allowed.
diff --git a/src/librustc_passes/liveness.rs b/src/librustc_passes/liveness.rs
index 70b106f5d2332..bf577d26b0fed 100644
--- a/src/librustc_passes/liveness.rs
+++ b/src/librustc_passes/liveness.rs
@@ -864,7 +864,7 @@ impl<'a, 'tcx> Liveness<'a, 'tcx> {
             first_merge,
             any_changed
         );
-        return any_changed;
+        any_changed
     }
 
     // Indicates that a local variable was *defined*; we know that no
diff --git a/src/librustc_passes/loops.rs b/src/librustc_passes/loops.rs
index 1daef45a1f591..fa2afae469c04 100644
--- a/src/librustc_passes/loops.rs
+++ b/src/librustc_passes/loops.rs
@@ -222,7 +222,7 @@ impl<'a, 'hir> CheckLoopVisitor<'a, 'hir> {
                 return true;
             }
         }
-        return false;
+        false
     }
     fn emit_unlabled_cf_in_while_condition(&mut self, span: Span, cf_type: &str) {
         struct_span_err!(
diff --git a/src/librustc_passes/reachable.rs b/src/librustc_passes/reachable.rs
index 835e7cfb62816..1e9c6c91d385f 100644
--- a/src/librustc_passes/reachable.rs
+++ b/src/librustc_passes/reachable.rs
@@ -30,9 +30,7 @@ fn item_might_be_inlined(tcx: TyCtxt<'tcx>, item: &hir::Item<'_>, attrs: Codegen
     }
 
     match item.kind {
-        hir::ItemKind::Fn(ref sig, ..) if sig.header.is_const() => {
-            return true;
-        }
+        hir::ItemKind::Fn(ref sig, ..) if sig.header.is_const() => true,
         hir::ItemKind::Impl { .. } | hir::ItemKind::Fn(..) => {
             let generics = tcx.generics_of(tcx.hir().local_def_id(item.hir_id));
             generics.requires_monomorphization(tcx)
diff --git a/src/librustc_passes/stability.rs b/src/librustc_passes/stability.rs
index 11311a3e8aa68..8fa5a4fbc61f4 100644
--- a/src/librustc_passes/stability.rs
+++ b/src/librustc_passes/stability.rs
@@ -465,7 +465,7 @@ fn new_index(tcx: TyCtxt<'tcx>) -> Index<'tcx> {
             |v| intravisit::walk_crate(v, krate),
         );
     }
-    return index;
+    index
 }
 
 /// Cross-references the feature names of unstable APIs with enabled
diff --git a/src/librustc_privacy/lib.rs b/src/librustc_privacy/lib.rs
index c8c8c2299305b..a3510737b7edc 100644
--- a/src/librustc_privacy/lib.rs
+++ b/src/librustc_privacy/lib.rs
@@ -1423,7 +1423,7 @@ impl<'a, 'tcx> ObsoleteVisiblePrivateTypesVisitor<'a, 'tcx> {
                 Some(_) | None => false,
             }
         } else {
-            return false;
+            false
         }
     }
 
@@ -1837,7 +1837,7 @@ impl SearchInterfaceForPrivateItemsVisitor<'tcx> {
             && self.tcx.is_private_dep(item_id.krate);
 
         log::debug!("leaks_private_dep(item_id={:?})={}", item_id, ret);
-        return ret;
+        ret
     }
 }
 
diff --git a/src/librustc_resolve/def_collector.rs b/src/librustc_resolve/def_collector.rs
index 505cd331a2509..0dee997f2ed96 100644
--- a/src/librustc_resolve/def_collector.rs
+++ b/src/librustc_resolve/def_collector.rs
@@ -200,7 +200,7 @@ impl<'a> visit::Visitor<'a> for DefCollector<'a> {
 
     fn visit_pat(&mut self, pat: &'a Pat) {
         match pat.kind {
-            PatKind::MacCall(..) => return self.visit_macro_invoc(pat.id),
+            PatKind::MacCall(..) => self.visit_macro_invoc(pat.id),
             _ => visit::walk_pat(self, pat),
         }
     }
diff --git a/src/librustc_resolve/imports.rs b/src/librustc_resolve/imports.rs
index 663e61ad2add4..95597e8ebf1e9 100644
--- a/src/librustc_resolve/imports.rs
+++ b/src/librustc_resolve/imports.rs
@@ -1108,7 +1108,7 @@ impl<'a, 'b> ImportResolver<'a, 'b> {
                                     match binding.kind {
                                         // Never suggest the name that has binding error
                                         // i.e., the name that cannot be previously resolved
-                                        NameBindingKind::Res(Res::Err, _) => return None,
+                                        NameBindingKind::Res(Res::Err, _) => None,
                                         _ => Some(&i.name),
                                     }
                                 }
diff --git a/src/librustc_resolve/late/diagnostics.rs b/src/librustc_resolve/late/diagnostics.rs
index 41380b2a4b78a..e1256551e24da 100644
--- a/src/librustc_resolve/late/diagnostics.rs
+++ b/src/librustc_resolve/late/diagnostics.rs
@@ -380,7 +380,7 @@ impl<'a> LateResolutionVisitor<'a, '_, '_> {
                 _ => (),
             }
         };
-        return has_self_arg;
+        has_self_arg
     }
 
     fn followed_by_brace(&self, span: Span) -> (bool, Option<(Span, String)>) {
@@ -430,7 +430,7 @@ impl<'a> LateResolutionVisitor<'a, '_, '_> {
                 break;
             }
         }
-        return (followed_by_brace, closing_brace);
+        (followed_by_brace, closing_brace)
     }
 
     /// Provides context-dependent help for errors reported by the `smart_resolve_path_fragment`
diff --git a/src/librustc_resolve/late/lifetimes.rs b/src/librustc_resolve/late/lifetimes.rs
index 51bf1f4843972..bc843fccc4c71 100644
--- a/src/librustc_resolve/late/lifetimes.rs
+++ b/src/librustc_resolve/late/lifetimes.rs
@@ -133,7 +133,7 @@ impl RegionExt for Region {
 /// that it corresponds to.
 ///
 /// FIXME. This struct gets converted to a `ResolveLifetimes` for
-/// actual use. It has the same data, but indexed by `DefIndex`.  This
+/// actual use. It has the same data, but indexed by `LocalDefId`.  This
 /// is silly.
 #[derive(Default)]
 struct NamedRegionMap {
diff --git a/src/librustc_resolve/macros.rs b/src/librustc_resolve/macros.rs
index 6f2e0bce3acaf..166fc48b44c4c 100644
--- a/src/librustc_resolve/macros.rs
+++ b/src/librustc_resolve/macros.rs
@@ -83,7 +83,7 @@ fn sub_namespace_match(candidate: Option<MacroKind>, requirement: Option<MacroKi
 // line-breaks and is slow.
 fn fast_print_path(path: &ast::Path) -> Symbol {
     if path.segments.len() == 1 {
-        return path.segments[0].ident.name;
+        path.segments[0].ident.name
     } else {
         let mut path_str = String::with_capacity(64);
         for (i, segment) in path.segments.iter().enumerate() {
diff --git a/src/librustc_save_analysis/lib.rs b/src/librustc_save_analysis/lib.rs
index 98d81c6252242..59084f1904545 100644
--- a/src/librustc_save_analysis/lib.rs
+++ b/src/librustc_save_analysis/lib.rs
@@ -534,7 +534,7 @@ impl<'l, 'tcx> SaveContext<'l, 'tcx> {
                         let variant = &def.non_enum_variant();
                         filter!(self.span_utils, ident.span);
                         let span = self.span_from_span(ident.span);
-                        return Some(Data::RefData(Ref {
+                        Some(Data::RefData(Ref {
                             kind: RefKind::Variable,
                             span,
                             ref_id: self
@@ -542,7 +542,7 @@ impl<'l, 'tcx> SaveContext<'l, 'tcx> {
                                 .find_field_index(ident, variant)
                                 .map(|index| id_from_def_id(variant.fields[index].did))
                                 .unwrap_or_else(|| null_id()),
-                        }));
+                        }))
                     }
                     ty::Tuple(..) => None,
                     _ => {
diff --git a/src/librustc_session/config.rs b/src/librustc_session/config.rs
index c273e7fdbf916..f16e4ca93d80a 100644
--- a/src/librustc_session/config.rs
+++ b/src/librustc_session/config.rs
@@ -1140,7 +1140,7 @@ pub fn parse_error_format(
         _ => {}
     }
 
-    return error_format;
+    error_format
 }
 
 fn parse_crate_edition(matches: &getopts::Matches) -> Edition {
diff --git a/src/librustc_span/caching_source_map_view.rs b/src/librustc_span/caching_source_map_view.rs
index d6725160a5d02..68b0bd1a57418 100644
--- a/src/librustc_span/caching_source_map_view.rs
+++ b/src/librustc_span/caching_source_map_view.rs
@@ -99,10 +99,6 @@ impl<'sm> CachingSourceMapView<'sm> {
         cache_entry.line_end = line_bounds.1;
         cache_entry.time_stamp = self.time_stamp;
 
-        return Some((
-            cache_entry.file.clone(),
-            cache_entry.line_number,
-            pos - cache_entry.line_start,
-        ));
+        Some((cache_entry.file.clone(), cache_entry.line_number, pos - cache_entry.line_start))
     }
 }
diff --git a/src/librustc_span/source_map.rs b/src/librustc_span/source_map.rs
index 7dd9e2f6316b4..39eb318a785e7 100644
--- a/src/librustc_span/source_map.rs
+++ b/src/librustc_span/source_map.rs
@@ -368,7 +368,7 @@ impl SourceMap {
 
     // If there is a doctest offset, applies it to the line.
     pub fn doctest_offset_line(&self, file: &FileName, orig: usize) -> usize {
-        return match file {
+        match file {
             FileName::DocTest(_, offset) => {
                 return if *offset >= 0 {
                     orig + *offset as usize
@@ -377,7 +377,7 @@ impl SourceMap {
                 };
             }
             _ => orig,
-        };
+        }
     }
 
     /// Looks up source information about a `BytePos`.
@@ -569,10 +569,10 @@ impl SourceMap {
         let local_end = self.lookup_byte_offset(sp.hi());
 
         if local_begin.sf.start_pos != local_end.sf.start_pos {
-            return Err(SpanSnippetError::DistinctSources(DistinctSources {
+            Err(SpanSnippetError::DistinctSources(DistinctSources {
                 begin: (local_begin.sf.name.clone(), local_begin.sf.start_pos),
                 end: (local_end.sf.name.clone(), local_end.sf.start_pos),
-            }));
+            }))
         } else {
             self.ensure_source_file_source_present(local_begin.sf.clone());
 
@@ -590,13 +590,11 @@ impl SourceMap {
             }
 
             if let Some(ref src) = local_begin.sf.src {
-                return extract_source(src, start_index, end_index);
+                extract_source(src, start_index, end_index)
             } else if let Some(src) = local_begin.sf.external_src.borrow().get_source() {
-                return extract_source(src, start_index, end_index);
+                extract_source(src, start_index, end_index)
             } else {
-                return Err(SpanSnippetError::SourceNotAvailable {
-                    filename: local_begin.sf.name.clone(),
-                });
+                Err(SpanSnippetError::SourceNotAvailable { filename: local_begin.sf.name.clone() })
             }
         }
     }
diff --git a/src/librustc_span/symbol.rs b/src/librustc_span/symbol.rs
index 5685505f6948d..35a97cfd3c3c2 100644
--- a/src/librustc_span/symbol.rs
+++ b/src/librustc_span/symbol.rs
@@ -544,7 +544,7 @@ symbols! {
         plugin_registrar,
         plugins,
         Poll,
-        poll_with_tls_context,
+        poll_with_context,
         powerpc_target_feature,
         precise_pointer_size_matching,
         pref_align_of,
@@ -720,6 +720,7 @@ symbols! {
         target_has_atomic_load_store,
         target_thread_local,
         task,
+        _task_context,
         tbm_target_feature,
         termination_trait,
         termination_trait_test,
diff --git a/src/librustc_target/abi/mod.rs b/src/librustc_target/abi/mod.rs
index afa30e7e632a7..635fb80b65980 100644
--- a/src/librustc_target/abi/mod.rs
+++ b/src/librustc_target/abi/mod.rs
@@ -751,7 +751,7 @@ impl Abi {
                 Primitive::Int(_, signed) => signed,
                 _ => false,
             },
-            _ => false,
+            _ => panic!("`is_signed` on non-scalar ABI {:?}", self),
         }
     }
 
diff --git a/src/librustc_trait_selection/traits/auto_trait.rs b/src/librustc_trait_selection/traits/auto_trait.rs
index d221d6886e9fb..bd980e6eb8b8a 100644
--- a/src/librustc_trait_selection/traits/auto_trait.rs
+++ b/src/librustc_trait_selection/traits/auto_trait.rs
@@ -113,7 +113,7 @@ impl<'tcx> AutoTraitFinder<'tcx> {
             return AutoTraitResult::ExplicitImpl;
         }
 
-        return tcx.infer_ctxt().enter(|infcx| {
+        tcx.infer_ctxt().enter(|infcx| {
             let mut fresh_preds = FxHashSet::default();
 
             // Due to the way projections are handled by SelectionContext, we need to run
@@ -219,8 +219,8 @@ impl<'tcx> AutoTraitFinder<'tcx> {
 
             let info = AutoTraitInfo { full_user_env, region_data, vid_to_region };
 
-            return AutoTraitResult::PositiveImpl(auto_trait_callback(&infcx, info));
-        });
+            AutoTraitResult::PositiveImpl(auto_trait_callback(&infcx, info))
+        })
     }
 }
 
@@ -384,7 +384,7 @@ impl AutoTraitFinder<'tcx> {
             ty, trait_did, new_env, final_user_env
         );
 
-        return Some((new_env, final_user_env));
+        Some((new_env, final_user_env))
     }
 
     /// This method is designed to work around the following issue:
@@ -492,7 +492,7 @@ impl AutoTraitFinder<'tcx> {
                 }
                 _ => {}
             }
-            return true;
+            true
         });
 
         if should_add_new {
@@ -591,15 +591,15 @@ impl AutoTraitFinder<'tcx> {
     }
 
     fn is_param_no_infer(&self, substs: SubstsRef<'_>) -> bool {
-        return self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types());
+        self.is_of_param(substs.type_at(0)) && !substs.types().any(|t| t.has_infer_types())
     }
 
     pub fn is_of_param(&self, ty: Ty<'_>) -> bool {
-        return match ty.kind {
+        match ty.kind {
             ty::Param(_) => true,
             ty::Projection(p) => self.is_of_param(p.self_ty()),
             _ => false,
-        };
+        }
     }
 
     fn is_self_referential_projection(&self, p: ty::PolyProjectionPredicate<'_>) -> bool {
@@ -804,7 +804,7 @@ impl AutoTraitFinder<'tcx> {
                 _ => panic!("Unexpected predicate {:?} {:?}", ty, predicate),
             };
         }
-        return true;
+        true
     }
 
     pub fn clean_pred(
diff --git a/src/librustc_trait_selection/traits/coherence.rs b/src/librustc_trait_selection/traits/coherence.rs
index 5f542e7e13be5..dc13af99fec6c 100644
--- a/src/librustc_trait_selection/traits/coherence.rs
+++ b/src/librustc_trait_selection/traits/coherence.rs
@@ -221,10 +221,10 @@ pub fn trait_ref_is_knowable<'tcx>(
     // we are an owner.
     if orphan_check_trait_ref(tcx, trait_ref, InCrate::Local).is_ok() {
         debug!("trait_ref_is_knowable: orphan check passed");
-        return None;
+        None
     } else {
         debug!("trait_ref_is_knowable: nonlocal, nonfundamental, unowned");
-        return Some(Conflict::Upstream);
+        Some(Conflict::Upstream)
     }
 }
 
diff --git a/src/librustc_trait_selection/traits/error_reporting/on_unimplemented.rs b/src/librustc_trait_selection/traits/error_reporting/on_unimplemented.rs
index 3d0dd73f03c18..18b2ca8983720 100644
--- a/src/librustc_trait_selection/traits/error_reporting/on_unimplemented.rs
+++ b/src/librustc_trait_selection/traits/error_reporting/on_unimplemented.rs
@@ -111,11 +111,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             }),
             hir::Node::Expr(hir::Expr { .. }) => {
                 let parent_hid = hir.get_parent_node(hir_id);
-                if parent_hid != hir_id {
-                    return self.describe_enclosure(parent_hid);
-                } else {
-                    None
-                }
+                if parent_hid != hir_id { self.describe_enclosure(parent_hid) } else { None }
             }
             _ => None,
         }
diff --git a/src/librustc_trait_selection/traits/error_reporting/suggestions.rs b/src/librustc_trait_selection/traits/error_reporting/suggestions.rs
index 0523a2019861c..522a8084cdc0d 100644
--- a/src/librustc_trait_selection/traits/error_reporting/suggestions.rs
+++ b/src/librustc_trait_selection/traits/error_reporting/suggestions.rs
@@ -351,7 +351,7 @@ impl<'a, 'tcx> InferCtxtExt<'tcx> for InferCtxt<'a, 'tcx> {
             // Different to previous arm because one is `&hir::Local` and the other
             // is `P<hir::Local>`.
             Some(hir::Node::Local(local)) => get_name(err, &local.pat.kind),
-            _ => return None,
+            _ => None,
         }
     }
 
diff --git a/src/librustc_trait_selection/traits/query/normalize.rs b/src/librustc_trait_selection/traits/query/normalize.rs
index adec2ddb25322..99412fafcfa8d 100644
--- a/src/librustc_trait_selection/traits/query/normalize.rs
+++ b/src/librustc_trait_selection/traits/query/normalize.rs
@@ -169,12 +169,12 @@ impl<'cx, 'tcx> TypeFolder<'tcx> for QueryNormalizer<'cx, 'tcx> {
                                 debug!("QueryNormalizer: result = {:#?}", result);
                                 debug!("QueryNormalizer: obligations = {:#?}", obligations);
                                 self.obligations.extend(obligations);
-                                return result.normalized_ty;
+                                result.normalized_ty
                             }
 
                             Err(_) => {
                                 self.error = true;
-                                return ty;
+                                ty
                             }
                         }
                     }
diff --git a/src/librustc_trait_selection/traits/wf.rs b/src/librustc_trait_selection/traits/wf.rs
index b69c5bdce2abc..5f40c1cefca45 100644
--- a/src/librustc_trait_selection/traits/wf.rs
+++ b/src/librustc_trait_selection/traits/wf.rs
@@ -595,7 +595,7 @@ impl<'a, 'tcx> WfPredicates<'a, 'tcx> {
         }
 
         // if we made it through that loop above, we made progress!
-        return true;
+        true
     }
 
     fn nominal_obligations(
diff --git a/src/librustc_traits/lowering/environment.rs b/src/librustc_traits/lowering/environment.rs
index 69d0bd0929687..ed6259d457361 100644
--- a/src/librustc_traits/lowering/environment.rs
+++ b/src/librustc_traits/lowering/environment.rs
@@ -146,7 +146,7 @@ crate fn program_clauses_for_env<'tcx>(
 
     debug!("program_clauses_for_env: closure = {:#?}", closure);
 
-    return tcx.mk_clauses(closure.into_iter());
+    tcx.mk_clauses(closure.into_iter())
 }
 
 crate fn environment(tcx: TyCtxt<'_>, def_id: DefId) -> Environment<'_> {
diff --git a/src/librustc_ty/needs_drop.rs b/src/librustc_ty/needs_drop.rs
index 0f71246c73759..3b72da23bafae 100644
--- a/src/librustc_ty/needs_drop.rs
+++ b/src/librustc_ty/needs_drop.rs
@@ -132,7 +132,7 @@ where
             }
         }
 
-        return None;
+        None
     }
 }
 
diff --git a/src/librustc_typeck/astconv.rs b/src/librustc_typeck/astconv.rs
index 9a8d161572bcf..3ee6d5df7356b 100644
--- a/src/librustc_typeck/astconv.rs
+++ b/src/librustc_typeck/astconv.rs
@@ -2128,7 +2128,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
                 return Err(ErrorReported);
             }
         }
-        return Ok(bound);
+        Ok(bound)
     }
 
     fn complain_about_assoc_type_not_found<I>(
@@ -2709,7 +2709,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             }
             Res::Err => {
                 self.set_tainted_by_errors();
-                return self.tcx().types.err;
+                self.tcx().types.err
             }
             _ => span_bug!(span, "unexpected resolution: {:?}", path.res),
         }
@@ -3047,7 +3047,7 @@ impl<'o, 'tcx> dyn AstConv<'tcx> + 'o {
             )
             .emit();
         }
-        return Some(r);
+        Some(r)
     }
 }
 
diff --git a/src/librustc_typeck/check/mod.rs b/src/librustc_typeck/check/mod.rs
index 368f64e4d41aa..d194223117195 100644
--- a/src/librustc_typeck/check/mod.rs
+++ b/src/librustc_typeck/check/mod.rs
@@ -1580,7 +1580,7 @@ fn check_union_fields(tcx: TyCtxt<'_>, span: Span, item_def_id: DefId) -> bool {
     } else {
         span_bug!(span, "unions must be ty::Adt, but got {:?}", item_type.kind);
     }
-    return true;
+    true
 }
 
 /// Checks that an opaque type does not contain cycles and does not use `Self` or `T::Foo`
@@ -2313,7 +2313,7 @@ fn check_representable(tcx: TyCtxt<'_>, sp: Span, item_def_id: DefId) -> bool {
         }
         Representability::Representable | Representability::ContainsRecursive => (),
     }
-    return true;
+    true
 }
 
 pub fn check_simd(tcx: TyCtxt<'_>, sp: Span, def_id: DefId) {
diff --git a/src/librustc_typeck/coherence/inherent_impls.rs b/src/librustc_typeck/coherence/inherent_impls.rs
index 60e5df68b5842..c6ee9ab60abf3 100644
--- a/src/librustc_typeck/coherence/inherent_impls.rs
+++ b/src/librustc_typeck/coherence/inherent_impls.rs
@@ -272,9 +272,7 @@ impl ItemLikeVisitor<'v> for InherentCollect<'tcx> {
                     item.span,
                 );
             }
-            ty::Error => {
-                return;
-            }
+            ty::Error => {}
             _ => {
                 struct_span_err!(
                     self.tcx.sess,
@@ -288,7 +286,6 @@ impl ItemLikeVisitor<'v> for InherentCollect<'tcx> {
                        to wrap it instead",
                 )
                 .emit();
-                return;
             }
         }
     }
diff --git a/src/librustc_typeck/collect/type_of.rs b/src/librustc_typeck/collect/type_of.rs
index 41c205bc11b35..44ef4ebd463d7 100644
--- a/src/librustc_typeck/collect/type_of.rs
+++ b/src/librustc_typeck/collect/type_of.rs
@@ -290,7 +290,7 @@ pub(super) fn type_of(tcx: TyCtxt<'_>, def_id: DefId) -> Ty<'_> {
                             DUMMY_SP,
                             &format!("unexpected const parent path {:?}", parent_node,),
                         );
-                        return tcx.types.err;
+                        tcx.types.err
                     }
                 }
 
diff --git a/src/librustc_typeck/impl_wf_check/min_specialization.rs b/src/librustc_typeck/impl_wf_check/min_specialization.rs
index cae8837611846..ef94500f5c444 100644
--- a/src/librustc_typeck/impl_wf_check/min_specialization.rs
+++ b/src/librustc_typeck/impl_wf_check/min_specialization.rs
@@ -270,7 +270,7 @@ fn check_static_lifetimes<'tcx>(
     span: Span,
 ) {
     if tcx.any_free_region_meets(parent_substs, |r| *r == ty::ReStatic) {
-        tcx.sess.struct_span_err(span, &format!("cannot specialize on `'static` lifetime")).emit();
+        tcx.sess.struct_span_err(span, "cannot specialize on `'static` lifetime").emit();
     }
 }
 
diff --git a/src/librustdoc/passes/calculate_doc_coverage.rs b/src/librustdoc/passes/calculate_doc_coverage.rs
index f48224512ba4f..98300385c8fb8 100644
--- a/src/librustdoc/passes/calculate_doc_coverage.rs
+++ b/src/librustdoc/passes/calculate_doc_coverage.rs
@@ -8,7 +8,6 @@ use rustc_ast::attr;
 use rustc_span::symbol::sym;
 use rustc_span::FileName;
 use serde::Serialize;
-use serde_json;
 
 use std::collections::BTreeMap;
 use std::ops;
diff --git a/src/libstd/backtrace.rs b/src/libstd/backtrace.rs
index 34317c7a2ee3d..e10d466030f0b 100644
--- a/src/libstd/backtrace.rs
+++ b/src/libstd/backtrace.rs
@@ -250,7 +250,7 @@ impl Backtrace {
             },
         };
         ENABLED.store(enabled as usize + 1, SeqCst);
-        return enabled;
+        enabled
     }
 
     /// Capture a stack backtrace of the current thread.
diff --git a/src/libstd/future.rs b/src/libstd/future.rs
index c1ca6771326cb..c0675eeba98da 100644
--- a/src/libstd/future.rs
+++ b/src/libstd/future.rs
@@ -1,12 +1,14 @@
 //! Asynchronous values.
 
-use core::cell::Cell;
-use core::marker::Unpin;
-use core::ops::{Drop, Generator, GeneratorState};
-use core::option::Option;
-use core::pin::Pin;
-use core::ptr::NonNull;
-use core::task::{Context, Poll};
+#[cfg(bootstrap)]
+use core::{
+    cell::Cell,
+    marker::Unpin,
+    ops::{Drop, Generator, GeneratorState},
+    pin::Pin,
+    ptr::NonNull,
+    task::{Context, Poll},
+};
 
 #[doc(inline)]
 #[stable(feature = "futures_api", since = "1.36.0")]
@@ -17,6 +19,7 @@ pub use core::future::*;
 /// This function returns a `GenFuture` underneath, but hides it in `impl Trait` to give
 /// better error messages (`impl Future` rather than `GenFuture<[closure.....]>`).
 // This is `const` to avoid extra errors after we recover from `const async fn`
+#[cfg(bootstrap)]
 #[doc(hidden)]
 #[unstable(feature = "gen_future", issue = "50547")]
 pub const fn from_generator<T: Generator<Yield = ()>>(x: T) -> impl Future<Output = T::Return> {
@@ -24,6 +27,7 @@ pub const fn from_generator<T: Generator<Yield = ()>>(x: T) -> impl Future<Outpu
 }
 
 /// A wrapper around generators used to implement `Future` for `async`/`await` code.
+#[cfg(bootstrap)]
 #[doc(hidden)]
 #[unstable(feature = "gen_future", issue = "50547")]
 #[derive(Copy, Clone, Debug, Eq, PartialEq, Ord, PartialOrd, Hash)]
@@ -31,8 +35,10 @@ struct GenFuture<T: Generator<Yield = ()>>(T);
 
 // We rely on the fact that async/await futures are immovable in order to create
 // self-referential borrows in the underlying generator.
+#[cfg(bootstrap)]
 impl<T: Generator<Yield = ()>> !Unpin for GenFuture<T> {}
 
+#[cfg(bootstrap)]
 #[doc(hidden)]
 #[unstable(feature = "gen_future", issue = "50547")]
 impl<T: Generator<Yield = ()>> Future for GenFuture<T> {
@@ -48,12 +54,15 @@ impl<T: Generator<Yield = ()>> Future for GenFuture<T> {
     }
 }
 
+#[cfg(bootstrap)]
 thread_local! {
     static TLS_CX: Cell<Option<NonNull<Context<'static>>>> = Cell::new(None);
 }
 
+#[cfg(bootstrap)]
 struct SetOnDrop(Option<NonNull<Context<'static>>>);
 
+#[cfg(bootstrap)]
 impl Drop for SetOnDrop {
     fn drop(&mut self) {
         TLS_CX.with(|tls_cx| {
@@ -64,6 +73,7 @@ impl Drop for SetOnDrop {
 
 // Safety: the returned guard must drop before `cx` is dropped and before
 // any previous guard is dropped.
+#[cfg(bootstrap)]
 unsafe fn set_task_context(cx: &mut Context<'_>) -> SetOnDrop {
     // transmute the context's lifetime to 'static so we can store it.
     let cx = core::mem::transmute::<&mut Context<'_>, &mut Context<'static>>(cx);
@@ -71,6 +81,7 @@ unsafe fn set_task_context(cx: &mut Context<'_>) -> SetOnDrop {
     SetOnDrop(old_cx)
 }
 
+#[cfg(bootstrap)]
 #[doc(hidden)]
 #[unstable(feature = "gen_future", issue = "50547")]
 /// Polls a future in the current thread-local task waker.
diff --git a/src/libstd/sys_common/backtrace.rs b/src/libstd/sys_common/backtrace.rs
index 2c7ba8f8ea1fd..e9b1e86d7ae49 100644
--- a/src/libstd/sys_common/backtrace.rs
+++ b/src/libstd/sys_common/backtrace.rs
@@ -28,7 +28,7 @@ pub fn lock() -> impl Drop {
 
     unsafe {
         LOCK.lock();
-        return Guard;
+        Guard
     }
 }
 
diff --git a/src/test/debuginfo/pretty-std-collections.rs b/src/test/debuginfo/pretty-std-collections.rs
index f8997fad9a53f..3d2d88a676d0d 100644
--- a/src/test/debuginfo/pretty-std-collections.rs
+++ b/src/test/debuginfo/pretty-std-collections.rs
@@ -17,35 +17,43 @@
 // gdb-command: print btree_set
 // gdb-check:$1 = BTreeSet<i32>(len: 15) = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14}
 
+// gdb-command: print empty_btree_set
+// gdb-check:$2 = BTreeSet<i32>(len: 0)
+
 // gdb-command: print btree_map
-// gdb-check:$2 = BTreeMap<i32, i32>(len: 15) = {[0] = 0, [1] = 1, [2] = 2, [3] = 3, [4] = 4, [5] = 5, [6] = 6, [7] = 7, [8] = 8, [9] = 9, [10] = 10, [11] = 11, [12] = 12, [13] = 13, [14] = 14}
+// gdb-check:$3 = BTreeMap<i32, i32>(len: 15) = {[0] = 0, [1] = 1, [2] = 2, [3] = 3, [4] = 4, [5] = 5, [6] = 6, [7] = 7, [8] = 8, [9] = 9, [10] = 10, [11] = 11, [12] = 12, [13] = 13, [14] = 14}
+
+// gdb-command: print empty_btree_map
+// gdb-check:$4 = BTreeMap<i32, u32>(len: 0)
 
 // gdb-command: print vec_deque
-// gdb-check:$3 = VecDeque<i32>(len: 3, cap: 8) = {5, 3, 7}
+// gdb-check:$5 = VecDeque<i32>(len: 3, cap: 8) = {5, 3, 7}
 
 // gdb-command: print vec_deque2
-// gdb-check:$4 = VecDeque<i32>(len: 7, cap: 8) = {2, 3, 4, 5, 6, 7, 8}
+// gdb-check:$6 = VecDeque<i32>(len: 7, cap: 8) = {2, 3, 4, 5, 6, 7, 8}
 
 #![allow(unused_variables)]
-use std::collections::BTreeSet;
 use std::collections::BTreeMap;
+use std::collections::BTreeSet;
 use std::collections::VecDeque;
 
-
 fn main() {
-
     // BTreeSet
     let mut btree_set = BTreeSet::new();
     for i in 0..15 {
         btree_set.insert(i);
     }
 
+    let mut empty_btree_set: BTreeSet<i32> = BTreeSet::new();
+
     // BTreeMap
     let mut btree_map = BTreeMap::new();
     for i in 0..15 {
         btree_map.insert(i, i);
     }
 
+    let mut empty_btree_map: BTreeMap<i32, u32> = BTreeMap::new();
+
     // VecDeque
     let mut vec_deque = VecDeque::new();
     vec_deque.push_back(5);
@@ -63,4 +71,6 @@ fn main() {
     zzz(); // #break
 }
 
-fn zzz() { () }
+fn zzz() {
+    ()
+}
diff --git a/src/test/mir-opt/array-index-is-temporary.rs b/src/test/mir-opt/array-index-is-temporary.rs
index 096f98bade25a..57f30acb3a643 100644
--- a/src/test/mir-opt/array-index-is-temporary.rs
+++ b/src/test/mir-opt/array-index-is-temporary.rs
@@ -15,7 +15,7 @@ fn main() {
 }
 
 // END RUST SOURCE
-// START rustc.main.EraseRegions.after.mir
+// START rustc.main.SimplifyCfg-elaborate-drops.after.mir
 //     bb0: {
 //         ...
 //         _4 = &mut _2;
@@ -38,4 +38,4 @@ fn main() {
 //         ...
 //         return;
 //     }
-// END rustc.main.EraseRegions.after.mir
+// END rustc.main.SimplifyCfg-elaborate-drops.after.mir
diff --git a/src/test/mir-opt/byte_slice.rs b/src/test/mir-opt/byte_slice.rs
index 7edfa3e1124db..0fb685c3c4e6d 100644
--- a/src/test/mir-opt/byte_slice.rs
+++ b/src/test/mir-opt/byte_slice.rs
@@ -6,10 +6,10 @@ fn main() {
 }
 
 // END RUST SOURCE
-// START rustc.main.EraseRegions.after.mir
+// START rustc.main.SimplifyCfg-elaborate-drops.after.mir
 // ...
 // _1 = const b"foo";
 // ...
 // _2 = [const 5u8, const 120u8];
 // ...
-// END rustc.main.EraseRegions.after.mir
+// END rustc.main.SimplifyCfg-elaborate-drops.after.mir
diff --git a/src/test/mir-opt/packed-struct-drop-aligned.rs b/src/test/mir-opt/packed-struct-drop-aligned.rs
index 113f81c441f7c..39b9006017958 100644
--- a/src/test/mir-opt/packed-struct-drop-aligned.rs
+++ b/src/test/mir-opt/packed-struct-drop-aligned.rs
@@ -15,7 +15,7 @@ impl Drop for Droppy {
 }
 
 // END RUST SOURCE
-// START rustc.main.EraseRegions.before.mir
+// START rustc.main.SimplifyCfg-elaborate-drops.after.mir
 // fn main() -> () {
 //     let mut _0: ();
 //     let mut _1: Packed;
@@ -56,4 +56,4 @@ impl Drop for Droppy {
 //         drop(_1) -> [return: bb2, unwind: bb1];
 //     }
 // }
-// END rustc.main.EraseRegions.before.mir
+// END rustc.main.SimplifyCfg-elaborate-drops.after.mir
diff --git a/src/test/mir-opt/retag.rs b/src/test/mir-opt/retag.rs
index 1c88a9e4d5a32..e917441200b8a 100644
--- a/src/test/mir-opt/retag.rs
+++ b/src/test/mir-opt/retag.rs
@@ -8,8 +8,12 @@ struct Test(i32);
 
 impl Test {
     // Make sure we run the pass on a method, not just on bare functions.
-    fn foo<'x>(&self, x: &'x mut i32) -> &'x mut i32 { x }
-    fn foo_shr<'x>(&self, x: &'x i32) -> &'x i32 { x }
+    fn foo<'x>(&self, x: &'x mut i32) -> &'x mut i32 {
+        x
+    }
+    fn foo_shr<'x>(&self, x: &'x i32) -> &'x i32 {
+        x
+    }
 }
 
 impl Drop for Test {
@@ -27,7 +31,10 @@ fn main() {
     }
 
     // Also test closures
-    let c: fn(&i32) -> &i32 = |x: &i32| -> &i32 { let _y = x; x };
+    let c: fn(&i32) -> &i32 = |x: &i32| -> &i32 {
+        let _y = x;
+        x
+    };
     let _w = c(&x);
 
     // need to call `foo_shr` or it doesn't even get generated
@@ -38,7 +45,7 @@ fn main() {
 }
 
 // END RUST SOURCE
-// START rustc.{{impl}}-foo.EraseRegions.after.mir
+// START rustc.{{impl}}-foo.SimplifyCfg-elaborate-drops.after.mir
 //     bb0: {
 //         Retag([fn entry] _1);
 //         Retag([fn entry] _2);
@@ -48,8 +55,8 @@ fn main() {
 //         ...
 //         return;
 //     }
-// END rustc.{{impl}}-foo.EraseRegions.after.mir
-// START rustc.{{impl}}-foo_shr.EraseRegions.after.mir
+// END rustc.{{impl}}-foo.SimplifyCfg-elaborate-drops.after.mir
+// START rustc.{{impl}}-foo_shr.SimplifyCfg-elaborate-drops.after.mir
 //     bb0: {
 //         Retag([fn entry] _1);
 //         Retag([fn entry] _2);
@@ -59,8 +66,8 @@ fn main() {
 //         ...
 //         return;
 //     }
-// END rustc.{{impl}}-foo_shr.EraseRegions.after.mir
-// START rustc.main.EraseRegions.after.mir
+// END rustc.{{impl}}-foo_shr.SimplifyCfg-elaborate-drops.after.mir
+// START rustc.main.SimplifyCfg-elaborate-drops.after.mir
 // fn main() -> () {
 //     ...
 //     bb0: {
@@ -96,8 +103,8 @@ fn main() {
 //
 //     ...
 // }
-// END rustc.main.EraseRegions.after.mir
-// START rustc.main-{{closure}}.EraseRegions.after.mir
+// END rustc.main.SimplifyCfg-elaborate-drops.after.mir
+// START rustc.main-{{closure}}.SimplifyCfg-elaborate-drops.after.mir
 // fn main::{{closure}}#0(_1: &[closure@main::{{closure}}#0], _2: &i32) -> &i32 {
 //     ...
 //     bb0: {
@@ -112,7 +119,7 @@ fn main() {
 //         return;
 //     }
 // }
-// END rustc.main-{{closure}}.EraseRegions.after.mir
+// END rustc.main-{{closure}}.SimplifyCfg-elaborate-drops.after.mir
 // START rustc.ptr-drop_in_place.Test.SimplifyCfg-make_shim.after.mir
 // fn  std::intrinsics::drop_in_place(_1: *mut Test) -> () {
 //     ...
diff --git a/src/test/ui/async-await/issues/issue-62009-1.stderr b/src/test/ui/async-await/issues/issue-62009-1.stderr
index cd6670923c2c6..0624c049048c7 100644
--- a/src/test/ui/async-await/issues/issue-62009-1.stderr
+++ b/src/test/ui/async-await/issues/issue-62009-1.stderr
@@ -33,10 +33,10 @@ error[E0277]: the trait bound `[closure@$DIR/issue-62009-1.rs:16:5: 16:15]: std:
 LL |     (|_| 2333).await;
    |     ^^^^^^^^^^^^^^^^ the trait `std::future::Future` is not implemented for `[closure@$DIR/issue-62009-1.rs:16:5: 16:15]`
    | 
-  ::: $SRC_DIR/libstd/future.rs:LL:COL
+  ::: $SRC_DIR/libcore/future/mod.rs:LL:COL
    |
 LL |     F: Future,
-   |        ------ required by this bound in `std::future::poll_with_tls_context`
+   |        ------ required by this bound in `std::future::poll_with_context`
 
 error: aborting due to 4 previous errors
 
diff --git a/src/test/ui/async-await/no-std.rs b/src/test/ui/async-await/no-std.rs
new file mode 100644
index 0000000000000..63e93cdff7e77
--- /dev/null
+++ b/src/test/ui/async-await/no-std.rs
@@ -0,0 +1,13 @@
+// edition:2018
+// check-pass
+
+#![no_std]
+#![crate_type = "rlib"]
+
+use core::future::Future;
+
+async fn a(f: impl Future) {
+    f.await;
+}
+
+fn main() {}
diff --git a/src/test/ui/consts/const_forget.rs b/src/test/ui/consts/const_forget.rs
new file mode 100644
index 0000000000000..2dcb72a5a09cb
--- /dev/null
+++ b/src/test/ui/consts/const_forget.rs
@@ -0,0 +1,22 @@
+// check-pass
+
+#![feature(const_forget)]
+
+use std::mem::forget;
+
+const _: () = forget(0i32);
+const _: () = forget(Vec::<Vec<Box<i32>>>::new());
+
+// Writing this function signature without const-forget
+// triggers compiler errors:
+// 1) That we use a non-const fn inside a const fn
+// 2) without the forget, it complains about the destructor of Box
+//
+// FIXME: this method cannot be called in const-eval yet, as Box isn't
+// const constructable
+#[allow(unused)]
+const fn const_forget_box<T: ?Sized>(b: Box<T>) {
+    forget(b);
+}
+
+fn main() {}
diff --git a/src/test/ui/issues/issue-58319.rs b/src/test/ui/issues/issue-58319.rs
new file mode 100644
index 0000000000000..757307d944f18
--- /dev/null
+++ b/src/test/ui/issues/issue-58319.rs
@@ -0,0 +1,621 @@
+// run-pass
+fn main() {}
+#[derive(Clone)]
+pub struct Little;
+#[derive(Clone)]
+pub struct Big(
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+    Little,
+);
diff --git a/src/test/ui/issues/issue-65131.rs b/src/test/ui/issues/issue-65131.rs
new file mode 100644
index 0000000000000..8b5345da900aa
--- /dev/null
+++ b/src/test/ui/issues/issue-65131.rs
@@ -0,0 +1,18 @@
+fn get_pair(_a: &mut u32, _b: &mut u32) {}
+
+macro_rules! x10 {
+    ($($t:tt)*) => {
+        $($t)* $($t)* $($t)* $($t)* $($t)*
+        $($t)* $($t)* $($t)* $($t)* $($t)*
+    }
+}
+
+#[allow(unused_assignments)]
+fn main() {
+    let mut x = 1;
+
+    get_pair(&mut x, &mut x);
+    //~^ ERROR: cannot borrow `x` as mutable more than once at a time
+
+    x10! { x10!{ x10!{ if x > 0 { x += 2 } else { x += 1 } } } }
+}
diff --git a/src/test/ui/issues/issue-65131.stderr b/src/test/ui/issues/issue-65131.stderr
new file mode 100644
index 0000000000000..e234e6da552a8
--- /dev/null
+++ b/src/test/ui/issues/issue-65131.stderr
@@ -0,0 +1,12 @@
+error[E0499]: cannot borrow `x` as mutable more than once at a time
+  --> $DIR/issue-65131.rs:14:22
+   |
+LL |     get_pair(&mut x, &mut x);
+   |     -------- ------  ^^^^^^ second mutable borrow occurs here
+   |     |        |
+   |     |        first mutable borrow occurs here
+   |     first borrow later used by call
+
+error: aborting due to previous error
+
+For more information about this error, try `rustc --explain E0499`.
diff --git a/src/test/ui/macros/issue-69838-dir/bar.rs b/src/test/ui/macros/issue-69838-dir/bar.rs
new file mode 100644
index 0000000000000..ec12f8c5cb442
--- /dev/null
+++ b/src/test/ui/macros/issue-69838-dir/bar.rs
@@ -0,0 +1,3 @@
+// ignore-test -- this is an auxiliary file as part of another test.
+
+pub fn i_am_in_bar() {}
diff --git a/src/test/ui/macros/issue-69838-dir/included.rs b/src/test/ui/macros/issue-69838-dir/included.rs
new file mode 100644
index 0000000000000..9900b8fd5092c
--- /dev/null
+++ b/src/test/ui/macros/issue-69838-dir/included.rs
@@ -0,0 +1,3 @@
+// ignore-test -- this is an auxiliary file as part of another test.
+
+pub mod bar;
diff --git a/src/test/ui/macros/issue-69838-mods-relative-to-included-path.rs b/src/test/ui/macros/issue-69838-mods-relative-to-included-path.rs
new file mode 100644
index 0000000000000..2a4e97f0ef5f1
--- /dev/null
+++ b/src/test/ui/macros/issue-69838-mods-relative-to-included-path.rs
@@ -0,0 +1,7 @@
+// check-pass
+
+include!("issue-69838-dir/included.rs");
+
+fn main() {
+    bar::i_am_in_bar();
+}
diff --git a/src/test/ui/parser/extern-abi-from-mac-literal-frag.rs b/src/test/ui/parser/extern-abi-from-mac-literal-frag.rs
index cb23f2c808c34..4ecb21d26ab9b 100644
--- a/src/test/ui/parser/extern-abi-from-mac-literal-frag.rs
+++ b/src/test/ui/parser/extern-abi-from-mac-literal-frag.rs
@@ -1,7 +1,7 @@
 // check-pass
 
 // In this test we check that the parser accepts an ABI string when it
-// comes from a macro `literal` fragment as opposed to a hardcoded string.
+// comes from a macro `literal` or `expr` fragment as opposed to a hardcoded string.
 
 fn main() {}
 
@@ -17,6 +17,18 @@ macro_rules! abi_from_lit_frag {
     }
 }
 
+macro_rules! abi_from_expr_frag {
+    ($abi:expr) => {
+        extern $abi {
+            fn _import();
+        }
+
+        extern $abi fn _export() {}
+
+        type _PTR = extern $abi fn();
+    };
+}
+
 mod rust {
     abi_from_lit_frag!("Rust");
 }
@@ -24,3 +36,11 @@ mod rust {
 mod c {
     abi_from_lit_frag!("C");
 }
+
+mod rust_expr {
+    abi_from_expr_frag!("Rust");
+}
+
+mod c_expr {
+    abi_from_expr_frag!("C");
+}
diff --git a/src/test/ui/parser/issue-70050-ntliteral-accepts-negated-lit.rs b/src/test/ui/parser/issue-70050-ntliteral-accepts-negated-lit.rs
new file mode 100644
index 0000000000000..aca9d9eb0a5b4
--- /dev/null
+++ b/src/test/ui/parser/issue-70050-ntliteral-accepts-negated-lit.rs
@@ -0,0 +1,16 @@
+// check-pass
+
+macro_rules! foo {
+    ($a:literal) => {
+        bar!($a)
+    };
+}
+
+macro_rules! bar {
+    ($b:literal) => {};
+}
+
+fn main() {
+    foo!(-2);
+    bar!(-2);
+}