Make an ensure_root_is_owned method to reduce duplication

Also remove some unnecessary debug_assert! when creating the shared
root, since the root should be stored in the rodata and thus be
impossible to accidentally modify.

Author: porglezomp

src/liballoc/btree/map.rs

@@ -246,9 +246,7 @@ impl<K, Q: ?Sized> super::Recover<Q> for BTreeMap<K, ()>
     }
 
     fn replace(&mut self, key: K) -> Option<K> {
-        if self.root.is_shared_root() {
-            self.root = node::Root::new_leaf();
-        }
+        self.ensure_root_is_owned();
         match search::search_tree::<marker::Mut, K, (), K>(self.root.as_mut(), &key) {
             Found(handle) => Some(mem::replace(handle.into_kv_mut().0, key)),
             GoDown(handle) => {
@@ -893,10 +891,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn entry(&mut self, key: K) -> Entry<K, V> {
         // FIXME(@porglezomp) Avoid allocating if we don't insert
-        if self.root.is_shared_root() {
-            self.root = node::Root::new_leaf();
-        }
-
+        self.ensure_root_is_owned();
         match search::search_tree(self.root.as_mut(), &key) {
             Found(handle) => {
                 Occupied(OccupiedEntry {
@@ -917,10 +912,7 @@ impl<K: Ord, V> BTreeMap<K, V> {
     }
 
     fn from_sorted_iter<I: Iterator<Item = (K, V)>>(&mut self, iter: I) {
-        if self.root.is_shared_root() {
-            self.root = node::Root::new_leaf();
-        }
-
+        self.ensure_root_is_owned();
         let mut cur_node = last_leaf_edge(self.root.as_mut()).into_node();
         // Iterate through all key-value pairs, pushing them into nodes at the right level.
         for (key, value) in iter {
@@ -1165,6 +1157,13 @@ 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")]

src/liballoc/btree/node.rs

@@ -195,10 +195,6 @@ impl<K, V> Root<K, V> {
     }
 
     pub fn shared_empty_root() -> Self {
-        // Ensuring that the shared node hasn't been corrupted by any mutations
-        debug_assert!(EMPTY_ROOT_NODE.parent == ptr::null());
-        debug_assert!(EMPTY_ROOT_NODE.parent_idx == 0);
-        debug_assert!(EMPTY_ROOT_NODE.len == 0);
         Root {
             node: unsafe {
                 BoxedNode::from_ptr(NonNull::new_unchecked(