Auto merge of rust-lang#138951 - jwnrt:alloc-raw-vec-strict-prov, r=Noratrieb

Replace last `usize` -> `ptr` transmute in `alloc` with strict provenance API

This replaces the `usize -> ptr` transmute in `RawVecInner::new_in` with a strict provenance API (`NonNull::without_provenance`).

The API is changed to take an `Alignment` which encodes the non-null constraint needed for `Unique` and allows us to do the construction safely.

Two internal-only APIs were added to let us avoid UB-checking in this hot code: `Layout::alignment` to get the `Alignment` type directly rather than as a `usize`, and `Unique::from_non_null` to create `Unique` in const context without a transmute.

Author: bors

Diff for: alloc/src/lib.rs

@@ -135,6 +135,7 @@
 #![feature(pattern)]
 #![feature(pin_coerce_unsized_trait)]
 #![feature(pointer_like_trait)]
+#![feature(ptr_alignment_type)]
 #![feature(ptr_internals)]
 #![feature(ptr_metadata)]
 #![feature(set_ptr_value)]

Diff for: alloc/src/raw_vec/mod.rs

@@ -6,7 +6,7 @@
 
 use core::marker::PhantomData;
 use core::mem::{ManuallyDrop, MaybeUninit, SizedTypeProperties};
-use core::ptr::{self, NonNull, Unique};
+use core::ptr::{self, Alignment, NonNull, Unique};
 use core::{cmp, hint};
 
 #[cfg(not(no_global_oom_handling))]
@@ -177,7 +177,7 @@ impl<T, A: Allocator> RawVec<T, A> {
     /// the returned `RawVec`.
     #[inline]
     pub(crate) const fn new_in(alloc: A) -> Self {
-        Self { inner: RawVecInner::new_in(alloc, align_of::<T>()), _marker: PhantomData }
+        Self { inner: RawVecInner::new_in(alloc, Alignment::of::<T>()), _marker: PhantomData }
     }
 
     /// Like `with_capacity`, but parameterized over the choice of
@@ -409,8 +409,8 @@ unsafe impl<#[may_dangle] T, A: Allocator> Drop for RawVec<T, A> {
 
 impl<A: Allocator> RawVecInner<A> {
     #[inline]
-    const fn new_in(alloc: A, align: usize) -> Self {
-        let ptr = unsafe { core::mem::transmute(align) };
+    const fn new_in(alloc: A, align: Alignment) -> Self {
+        let ptr = Unique::from_non_null(NonNull::without_provenance(align.as_nonzero()));
         // `cap: 0` means "unallocated". zero-sized types are ignored.
         Self { ptr, cap: ZERO_CAP, alloc }
     }
@@ -465,7 +465,7 @@ impl<A: Allocator> RawVecInner<A> {
 
         // Don't allocate here because `Drop` will not deallocate when `capacity` is 0.
         if layout.size() == 0 {
-            return Ok(Self::new_in(alloc, elem_layout.align()));
+            return Ok(Self::new_in(alloc, elem_layout.alignment()));
         }
 
         if let Err(err) = alloc_guard(layout.size()) {

Diff for: alloctests/lib.rs

@@ -28,6 +28,8 @@
 #![feature(iter_next_chunk)]
 #![feature(maybe_uninit_slice)]
 #![feature(maybe_uninit_uninit_array_transpose)]
+#![feature(nonnull_provenance)]
+#![feature(ptr_alignment_type)]
 #![feature(ptr_internals)]
 #![feature(sized_type_properties)]
 #![feature(slice_iter_mut_as_mut_slice)]

Diff for: core/src/alloc/layout.rs

@@ -520,6 +520,14 @@ impl Layout {
             unsafe { Ok(Layout::from_size_align_unchecked(array_size, align.as_usize())) }
         }
     }
+
+    /// Perma-unstable access to `align` as `Alignment` type.
+    #[unstable(issue = "none", feature = "std_internals")]
+    #[doc(hidden)]
+    #[inline]
+    pub const fn alignment(&self) -> Alignment {
+        self.align
+    }
 }
 
 #[stable(feature = "alloc_layout", since = "1.28.0")]

Diff for: core/src/ptr/unique.rs

@@ -100,6 +100,12 @@ impl<T: ?Sized> Unique<T> {
         }
     }
 
+    /// Create a new `Unique` from a `NonNull` in const context.
+    #[inline]
+    pub const fn from_non_null(pointer: NonNull<T>) -> Self {
+        Unique { pointer, _marker: PhantomData }
+    }
+
     /// Acquires the underlying `*mut` pointer.
     #[must_use = "`self` will be dropped if the result is not used"]
     #[inline]
@@ -202,6 +208,6 @@ impl<T: ?Sized> From<NonNull<T>> for Unique<T> {
     /// This conversion is infallible since `NonNull` cannot be null.
     #[inline]
     fn from(pointer: NonNull<T>) -> Self {
-        Unique { pointer, _marker: PhantomData }
+        Unique::from_non_null(pointer)
     }
 }