optimize inplace collection of Vec

library/alloc/src/lib.rs

@@ -160,6 +160,7 @@
 #![feature(tuple_trait)]
 #![feature(unicode_internals)]
 #![feature(unsize)]
+#![feature(unwrap_infallible)]
 #![feature(vec_pop_if)]
 // tidy-alphabetical-end
 //

library/alloc/src/vec/in_place_collect.rs

@@ -259,7 +259,8 @@ where
             inner.cap,
             inner.buf.cast::<T>(),
             inner.end as *const T,
-            inner.cap * mem::size_of::<I::Src>() / mem::size_of::<T>(),
+            // SAFETY: the multiplication can not overflow, since `inner.cap * size_of::<I::SRC>()` is the size of the allocation.
+            inner.cap.unchecked_mul(mem::size_of::<I::Src>()) / mem::size_of::<T>(),
         )
     };
 
@@ -374,7 +375,7 @@ where
         // - it lets us thread the write pointer through its innards and get it back in the end
         let sink = InPlaceDrop { inner: dst_buf, dst: dst_buf };
         let sink =
-            self.try_fold::<_, _, Result<_, !>>(sink, write_in_place_with_drop(end)).unwrap();
+            self.try_fold::<_, _, Result<_, !>>(sink, write_in_place_with_drop(end)).into_ok();
         // iteration succeeded, don't drop head
         unsafe { ManuallyDrop::new(sink).dst.sub_ptr(dst_buf) }
     }

library/alloc/src/vec/into_iter.rs

@@ -289,6 +289,60 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
         };
     }
 
+    fn fold<B, F>(mut self, mut accum: B, mut f: F) -> B
+    where
+        F: FnMut(B, Self::Item) -> B,
+    {
+        if T::IS_ZST {
+            while self.ptr.as_ptr() != self.end.cast_mut() {
+                // SAFETY: we just checked that `self.ptr` is in bounds.
+                let tmp = unsafe { self.ptr.read() };
+                // See `next` for why we subtract from `end` here.
+                self.end = self.end.wrapping_byte_sub(1);
+                accum = f(accum, tmp);
+            }
+        } else {
+            // SAFETY: `self.end` can only be null if `T` is a ZST.
+            while self.ptr != non_null!(self.end, T) {
+                // SAFETY: we just checked that `self.ptr` is in bounds.
+                let tmp = unsafe { self.ptr.read() };
+                // SAFETY: the maximum this can be is `self.end`.
+                // Increment `self.ptr` first to avoid double dropping in the event of a panic.
+                self.ptr = unsafe { self.ptr.add(1) };
+                accum = f(accum, tmp);
+            }
+        }
+        accum
+    }
+
+    fn try_fold<B, F, R>(&mut self, mut accum: B, mut f: F) -> R
+    where
+        Self: Sized,
+        F: FnMut(B, Self::Item) -> R,
+        R: core::ops::Try<Output = B>,
+    {
+        if T::IS_ZST {
+            while self.ptr.as_ptr() != self.end.cast_mut() {
+                // SAFETY: we just checked that `self.ptr` is in bounds.
+                let tmp = unsafe { self.ptr.read() };
+                // See `next` for why we subtract from `end` here.
+                self.end = self.end.wrapping_byte_sub(1);
+                accum = f(accum, tmp)?;
+            }
+        } else {
+            // SAFETY: `self.end` can only be null if `T` is a ZST.
+            while self.ptr != non_null!(self.end, T) {
+                // SAFETY: we just checked that `self.ptr` is in bounds.
+                let tmp = unsafe { self.ptr.read() };
+                // SAFETY: the maximum this can be is `self.end`.
+                // Increment `self.ptr` first to avoid double dropping in the event of a panic.
+                self.ptr = unsafe { self.ptr.add(1) };
+                accum = f(accum, tmp)?;
+            }
+        }
+        R::from_output(accum)
+    }
+
     unsafe fn __iterator_get_unchecked(&mut self, i: usize) -> Self::Item
     where
         Self: TrustedRandomAccessNoCoerce,