Auto merge of #94472 - JmPotato:use_maybeuninit_for_vecdeque, r=m-ou-se

Use MaybeUninit in VecDeque to remove the undefined behavior of slice

Signed-off-by: JmPotato <ghzpotato@gmail.com>

Ref https://github.com/rust-lang/rust/issues/74189. Adjust the code to follow the [doc.rust-lang.org/reference/behavior-considered-undefined.html](https://doc.rust-lang.org/reference/behavior-considered-undefined.html).

* Change the return type of `buffer_as_slice` from `&[T]` to `&[MaybeUninit<T>]`.
* Add some corresponding safety comments.

Benchmark results:

master 8d6f527530f4ba974d922269267fe89050188789

```rust
test collections::vec_deque::tests::bench_pop_back_100       ... bench:          47 ns/iter (+/- 1)
test collections::vec_deque::tests::bench_pop_front_100      ... bench:          50 ns/iter (+/- 4)
test collections::vec_deque::tests::bench_push_back_100      ... bench:          69 ns/iter (+/- 10)
test collections::vec_deque::tests::bench_push_front_100     ... bench:          72 ns/iter (+/- 6)
test collections::vec_deque::tests::bench_retain_half_10000  ... bench:     145,891 ns/iter (+/- 7,975)
test collections::vec_deque::tests::bench_retain_odd_10000   ... bench:     141,647 ns/iter (+/- 3,711)
test collections::vec_deque::tests::bench_retain_whole_10000 ... bench:     120,132 ns/iter (+/- 4,078)
```

This PR

```rust
test collections::vec_deque::tests::bench_pop_back_100       ... bench:          48 ns/iter (+/- 2)
test collections::vec_deque::tests::bench_pop_front_100      ... bench:          51 ns/iter (+/- 3)
test collections::vec_deque::tests::bench_push_back_100      ... bench:          73 ns/iter (+/- 2)
test collections::vec_deque::tests::bench_push_front_100     ... bench:          73 ns/iter (+/- 2)
test collections::vec_deque::tests::bench_retain_half_10000  ... bench:     131,796 ns/iter (+/- 5,440)
test collections::vec_deque::tests::bench_retain_odd_10000   ... bench:     137,563 ns/iter (+/- 3,349)
test collections::vec_deque::tests::bench_retain_whole_10000 ... bench:     128,815 ns/iter (+/- 3,289)
```

2 files changed, 95 insertions, 30 deletions

diff --git a/library/alloc/src/collections/vec_deque/iter.rs b/library/alloc/src/collections/vec_deque/iter.rs
index edadd666edc..e8290809276 100644
--- a/library/alloc/src/collections/vec_deque/iter.rs
+++ b/library/alloc/src/collections/vec_deque/iter.rs
@@ -1,5 +1,6 @@
 use core::fmt;
 use core::iter::{FusedIterator, TrustedLen, TrustedRandomAccess, TrustedRandomAccessNoCoerce};
+use core::mem::MaybeUninit;
 use core::ops::Try;
 
 use super::{count, wrap_index, RingSlices};
@@ -12,7 +13,7 @@ use super::{count, wrap_index, RingSlices};
 /// [`iter`]: super::VecDeque::iter
 #[stable(feature = "rust1", since = "1.0.0")]
 pub struct Iter<'a, T: 'a> {
-    pub(crate) ring: &'a [T],
+    pub(crate) ring: &'a [MaybeUninit<T>],
     pub(crate) tail: usize,
     pub(crate) head: usize,
 }
@@ -21,7 +22,15 @@ pub struct Iter<'a, T: 'a> {
 impl<T: fmt::Debug> fmt::Debug for Iter<'_, T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
-        f.debug_tuple("Iter").field(&front).field(&back).finish()
+        // Safety:
+        // - `self.head` and `self.tail` in a ring buffer are always valid indices.
+        // - `RingSlices::ring_slices` guarantees that the slices split according to `self.head` and `self.tail` are initialized.
+        unsafe {
+            f.debug_tuple("Iter")
+                .field(&MaybeUninit::slice_assume_init_ref(front))
+                .field(&MaybeUninit::slice_assume_init_ref(back))
+                .finish()
+        }
     }
 }
 
@@ -44,7 +53,10 @@ impl<'a, T> Iterator for Iter<'a, T> {
         }
         let tail = self.tail;
         self.tail = wrap_index(self.tail.wrapping_add(1), self.ring.len());
-        unsafe { Some(self.ring.get_unchecked(tail)) }
+        // Safety:
+        // - `self.tail` in a ring buffer is always a valid index.
+        // - `self.head` and `self.tail` equality is checked above.
+        unsafe { Some(self.ring.get_unchecked(tail).assume_init_ref()) }
     }
 
     #[inline]
@@ -58,8 +70,13 @@ impl<'a, T> Iterator for Iter<'a, T> {
         F: FnMut(Acc, Self::Item) -> Acc,
     {
         let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
-        accum = front.iter().fold(accum, &mut f);
-        back.iter().fold(accum, &mut f)
+        // Safety:
+        // - `self.head` and `self.tail` in a ring buffer are always valid indices.
+        // - `RingSlices::ring_slices` guarantees that the slices split according to `self.head` and `self.tail` are initialized.
+        unsafe {
+            accum = MaybeUninit::slice_assume_init_ref(front).iter().fold(accum, &mut f);
+            MaybeUninit::slice_assume_init_ref(back).iter().fold(accum, &mut f)
+        }
     }
 
     fn try_fold<B, F, R>(&mut self, init: B, mut f: F) -> R
@@ -70,17 +87,19 @@ impl<'a, T> Iterator for Iter<'a, T> {
     {
         let (mut iter, final_res);
         if self.tail <= self.head {
-            // single slice self.ring[self.tail..self.head]
-            iter = self.ring[self.tail..self.head].iter();
+            // Safety: single slice self.ring[self.tail..self.head] is initialized.
+            iter = unsafe { MaybeUninit::slice_assume_init_ref(&self.ring[self.tail..self.head]) }
+                .iter();
             final_res = iter.try_fold(init, &mut f);
         } else {
-            // two slices: self.ring[self.tail..], self.ring[..self.head]
+            // Safety: two slices: self.ring[self.tail..], self.ring[..self.head] both are initialized.
             let (front, back) = self.ring.split_at(self.tail);
-            let mut back_iter = back.iter();
+
+            let mut back_iter = unsafe { MaybeUninit::slice_assume_init_ref(back).iter() };
             let res = back_iter.try_fold(init, &mut f);
             let len = self.ring.len();
             self.tail = (self.ring.len() - back_iter.len()) & (len - 1);
-            iter = front[..self.head].iter();
+            iter = unsafe { MaybeUninit::slice_assume_init_ref(&front[..self.head]).iter() };
             final_res = iter.try_fold(res?, &mut f);
         }
         self.tail = self.head - iter.len();
@@ -109,7 +128,7 @@ impl<'a, T> Iterator for Iter<'a, T> {
         // that is in bounds.
         unsafe {
             let idx = wrap_index(self.tail.wrapping_add(idx), self.ring.len());
-            self.ring.get_unchecked(idx)
+            self.ring.get_unchecked(idx).assume_init_ref()
         }
     }
 }
@@ -122,7 +141,10 @@ impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
             return None;
         }
         self.head = wrap_index(self.head.wrapping_sub(1), self.ring.len());
-        unsafe { Some(self.ring.get_unchecked(self.head)) }
+        // Safety:
+        // - `self.head` in a ring buffer is always a valid index.
+        // - `self.head` and `self.tail` equality is checked above.
+        unsafe { Some(self.ring.get_unchecked(self.head).assume_init_ref()) }
     }
 
     fn rfold<Acc, F>(self, mut accum: Acc, mut f: F) -> Acc
@@ -130,8 +152,13 @@ impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
         F: FnMut(Acc, Self::Item) -> Acc,
     {
         let (front, back) = RingSlices::ring_slices(self.ring, self.head, self.tail);
-        accum = back.iter().rfold(accum, &mut f);
-        front.iter().rfold(accum, &mut f)
+        // Safety:
+        // - `self.head` and `self.tail` in a ring buffer are always valid indices.
+        // - `RingSlices::ring_slices` guarantees that the slices split according to `self.head` and `self.tail` are initialized.
+        unsafe {
+            accum = MaybeUninit::slice_assume_init_ref(back).iter().rfold(accum, &mut f);
+            MaybeUninit::slice_assume_init_ref(front).iter().rfold(accum, &mut f)
+        }
     }
 
     fn try_rfold<B, F, R>(&mut self, init: B, mut f: F) -> R
@@ -142,16 +169,20 @@ impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
     {
         let (mut iter, final_res);
         if self.tail <= self.head {
-            // single slice self.ring[self.tail..self.head]
-            iter = self.ring[self.tail..self.head].iter();
+            // Safety: single slice self.ring[self.tail..self.head] is initialized.
+            iter = unsafe {
+                MaybeUninit::slice_assume_init_ref(&self.ring[self.tail..self.head]).iter()
+            };
             final_res = iter.try_rfold(init, &mut f);
         } else {
-            // two slices: self.ring[self.tail..], self.ring[..self.head]
+            // Safety: two slices: self.ring[self.tail..], self.ring[..self.head] both are initialized.
             let (front, back) = self.ring.split_at(self.tail);
-            let mut front_iter = front[..self.head].iter();
+
+            let mut front_iter =
+                unsafe { MaybeUninit::slice_assume_init_ref(&front[..self.head]).iter() };
             let res = front_iter.try_rfold(init, &mut f);
             self.head = front_iter.len();
-            iter = back.iter();
+            iter = unsafe { MaybeUninit::slice_assume_init_ref(back).iter() };
             final_res = iter.try_rfold(res?, &mut f);
         }
         self.head = self.tail + iter.len();
diff --git a/library/alloc/src/collections/vec_deque/mod.rs b/library/alloc/src/collections/vec_deque/mod.rs
index c3cabc754e6..63280e56332 100644
--- a/library/alloc/src/collections/vec_deque/mod.rs
+++ b/library/alloc/src/collections/vec_deque/mod.rs
@@ -12,7 +12,7 @@ use core::fmt;
 use core::hash::{Hash, Hasher};
 use core::iter::{repeat_with, FromIterator};
 use core::marker::PhantomData;
-use core::mem::{self, ManuallyDrop};
+use core::mem::{self, ManuallyDrop, MaybeUninit};
 use core::ops::{Index, IndexMut, Range, RangeBounds};
 use core::ptr::{self, NonNull};
 use core::slice;
@@ -181,16 +181,28 @@ impl<T, A: Allocator> VecDeque<T, A> {
         }
     }
 
-    /// Turn ptr into a slice
+    /// Turn ptr into a slice, since the elements of the backing buffer may be uninitialized,
+    /// we will return a slice of [`MaybeUninit<T>`].
+    ///
+    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
+    /// incorrect usage of this method.
+    ///
+    /// [zeroed]: mem::MaybeUninit::zeroed
     #[inline]
-    unsafe fn buffer_as_slice(&self) -> &[T] {
-        unsafe { slice::from_raw_parts(self.ptr(), self.cap()) }
+    unsafe fn buffer_as_slice(&self) -> &[MaybeUninit<T>] {
+        unsafe { slice::from_raw_parts(self.ptr() as *mut MaybeUninit<T>, self.cap()) }
     }
 
-    /// Turn ptr into a mut slice
+    /// Turn ptr into a mut slice, since the elements of the backing buffer may be uninitialized,
+    /// we will return a slice of [`MaybeUninit<T>`].
+    ///
+    /// See [`MaybeUninit::zeroed`][zeroed] for examples of correct and
+    /// incorrect usage of this method.
+    ///
+    /// [zeroed]: mem::MaybeUninit::zeroed
     #[inline]
-    unsafe fn buffer_as_mut_slice(&mut self) -> &mut [T] {
-        unsafe { slice::from_raw_parts_mut(self.ptr(), self.cap()) }
+    unsafe fn buffer_as_mut_slice(&mut self) -> &mut [MaybeUninit<T>] {
+        unsafe { slice::from_raw_parts_mut(self.ptr() as *mut MaybeUninit<T>, self.cap()) }
     }
 
     /// Moves an element out of the buffer
@@ -1055,9 +1067,13 @@ impl<T, A: Allocator> VecDeque<T, A> {
     #[inline]
     #[stable(feature = "deque_extras_15", since = "1.5.0")]
     pub fn as_slices(&self) -> (&[T], &[T]) {
+        // Safety:
+        // - `self.head` and `self.tail` in a ring buffer are always valid indices.
+        // - `RingSlices::ring_slices` guarantees that the slices split according to `self.head` and `self.tail` are initialized.
         unsafe {
             let buf = self.buffer_as_slice();
-            RingSlices::ring_slices(buf, self.head, self.tail)
+            let (front, back) = RingSlices::ring_slices(buf, self.head, self.tail);
+            (MaybeUninit::slice_assume_init_ref(front), MaybeUninit::slice_assume_init_ref(back))
         }
     }
 
@@ -1089,11 +1105,15 @@ impl<T, A: Allocator> VecDeque<T, A> {
     #[inline]
     #[stable(feature = "deque_extras_15", since = "1.5.0")]
     pub fn as_mut_slices(&mut self) -> (&mut [T], &mut [T]) {
+        // Safety:
+        // - `self.head` and `self.tail` in a ring buffer are always valid indices.
+        // - `RingSlices::ring_slices` guarantees that the slices split according to `self.head` and `self.tail` are initialized.
         unsafe {
             let head = self.head;
             let tail = self.tail;
             let buf = self.buffer_as_mut_slice();
-            RingSlices::ring_slices(buf, head, tail)
+            let (front, back) = RingSlices::ring_slices(buf, head, tail);
+            (MaybeUninit::slice_assume_init_mut(front), MaybeUninit::slice_assume_init_mut(back))
         }
     }
 
@@ -2327,7 +2347,14 @@ impl<T, A: Allocator> VecDeque<T, A> {
         if self.is_contiguous() {
             let tail = self.tail;
             let head = self.head;
-            return unsafe { RingSlices::ring_slices(self.buffer_as_mut_slice(), head, tail).0 };
+            // Safety:
+            // - `self.head` and `self.tail` in a ring buffer are always valid indices.
+            // - `RingSlices::ring_slices` guarantees that the slices split according to `self.head` and `self.tail` are initialized.
+            return unsafe {
+                MaybeUninit::slice_assume_init_mut(
+                    RingSlices::ring_slices(self.buffer_as_mut_slice(), head, tail).0,
+                )
+            };
         }
 
         let buf = self.buf.ptr();
@@ -2413,7 +2440,14 @@ impl<T, A: Allocator> VecDeque<T, A> {
 
         let tail = self.tail;
         let head = self.head;
-        unsafe { RingSlices::ring_slices(self.buffer_as_mut_slice(), head, tail).0 }
+        // Safety:
+        // - `self.head` and `self.tail` in a ring buffer are always valid indices.
+        // - `RingSlices::ring_slices` guarantees that the slices split according to `self.head` and `self.tail` are initialized.
+        unsafe {
+            MaybeUninit::slice_assume_init_mut(
+                RingSlices::ring_slices(self.buffer_as_mut_slice(), head, tail).0,
+            )
+        }
     }
 
     /// Rotates the double-ended queue `mid` places to the left.