1 files changed, 36 insertions, 45 deletions

diff --git a/library/alloc/src/collections/vec_deque/spec_extend.rs b/library/alloc/src/collections/vec_deque/spec_extend.rs
index 97ff8b765..dccf40ccb 100644
--- a/library/alloc/src/collections/vec_deque/spec_extend.rs
+++ b/library/alloc/src/collections/vec_deque/spec_extend.rs
@@ -1,6 +1,6 @@
 use crate::alloc::Allocator;
 use crate::vec;
-use core::iter::{ByRefSized, TrustedLen};
+use core::iter::TrustedLen;
 use core::slice;
 
 use super::VecDeque;
@@ -17,19 +17,33 @@ where
     default fn spec_extend(&mut self, mut iter: I) {
         // This function should be the moral equivalent of:
         //
-        //      for item in iter {
-        //          self.push_back(item);
-        //      }
-        while let Some(element) = iter.next() {
-            if self.len() == self.capacity() {
-                let (lower, _) = iter.size_hint();
-                self.reserve(lower.saturating_add(1));
-            }
+        // for item in iter {
+        //     self.push_back(item);
+        // }
+
+        // May only be called if `deque.len() < deque.capacity()`
+        unsafe fn push_unchecked<T, A: Allocator>(deque: &mut VecDeque<T, A>, element: T) {
+            // SAFETY: Because of the precondition, it's guaranteed that there is space
+            // in the logical array after the last element.
+            unsafe { deque.buffer_write(deque.to_physical_idx(deque.len), element) };
+            // This can't overflow because `deque.len() < deque.capacity() <= usize::MAX`.
+            deque.len += 1;
+        }
 
-            let head = self.head;
-            self.head = self.wrap_add(self.head, 1);
-            unsafe {
-                self.buffer_write(head, element);
+        while let Some(element) = iter.next() {
+            let (lower, _) = iter.size_hint();
+            self.reserve(lower.saturating_add(1));
+
+            // SAFETY: We just reserved space for at least one element.
+            unsafe { push_unchecked(self, element) };
+
+            // Inner loop to avoid repeatedly calling `reserve`.
+            while self.len < self.capacity() {
+                let Some(element) = iter.next() else {
+                    return;
+                };
+                // SAFETY: The loop condition guarantees that `self.len() < self.capacity()`.
+                unsafe { push_unchecked(self, element) };
             }
         }
     }
@@ -39,7 +53,7 @@ impl<T, I, A: Allocator> SpecExtend<T, I> for VecDeque<T, A>
 where
     I: TrustedLen<Item = T>,
 {
-    default fn spec_extend(&mut self, mut iter: I) {
+    default fn spec_extend(&mut self, iter: I) {
         // This is the case for a TrustedLen iterator.
         let (low, high) = iter.size_hint();
         if let Some(additional) = high {
@@ -51,35 +65,12 @@ where
             );
             self.reserve(additional);
 
-            struct WrapAddOnDrop<'a, T, A: Allocator> {
-                vec_deque: &'a mut VecDeque<T, A>,
-                written: usize,
-            }
-
-            impl<'a, T, A: Allocator> Drop for WrapAddOnDrop<'a, T, A> {
-                fn drop(&mut self) {
-                    self.vec_deque.head =
-                        self.vec_deque.wrap_add(self.vec_deque.head, self.written);
-                }
-            }
-
-            let mut wrapper = WrapAddOnDrop { vec_deque: self, written: 0 };
-
-            let head_room = wrapper.vec_deque.cap() - wrapper.vec_deque.head;
-            unsafe {
-                wrapper.vec_deque.write_iter(
-                    wrapper.vec_deque.head,
-                    ByRefSized(&mut iter).take(head_room),
-                    &mut wrapper.written,
-                );
-
-                if additional > head_room {
-                    wrapper.vec_deque.write_iter(0, iter, &mut wrapper.written);
-                }
-            }
+            let written = unsafe {
+                self.write_iter_wrapping(self.to_physical_idx(self.len), iter, additional)
+            };
 
             debug_assert_eq!(
-                additional, wrapper.written,
+                additional, written,
                 "The number of items written to VecDeque doesn't match the TrustedLen size hint"
             );
         } else {
@@ -99,8 +90,8 @@ impl<T, A: Allocator> SpecExtend<T, vec::IntoIter<T>> for VecDeque<T, A> {
         self.reserve(slice.len());
 
         unsafe {
-            self.copy_slice(self.head, slice);
-            self.head = self.wrap_add(self.head, slice.len());
+            self.copy_slice(self.to_physical_idx(self.len), slice);
+            self.len += slice.len();
         }
         iterator.forget_remaining_elements();
     }
@@ -125,8 +116,8 @@ where
         self.reserve(slice.len());
 
         unsafe {
-            self.copy_slice(self.head, slice);
-            self.head = self.wrap_add(self.head, slice.len());
+            self.copy_slice(self.to_physical_idx(self.len), slice);
+            self.len += slice.len();
         }
     }
 }