Pass Alignment for RawVecInner::new_in

Encodes the safety constraint that `Unique`'s pointer must be non-zero
into the API.

Author: jwnrt

library/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)]

library/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
@@ -409,7 +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 {
+    const fn new_in(alloc: A, align: Alignment) -> Self {
+        // SAFETY: `Alignment` is non-zero.
         let ptr = unsafe { core::mem::transmute(align) };
         // `cap: 0` means "unallocated". zero-sized types are ignored.
         Self { ptr, cap: ZERO_CAP, alloc }
@@ -465,7 +466,9 @@ 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()));
+            // SAFETY: `Layout.align` is a non-zero power of two.
+            let align = unsafe { Alignment::new_unchecked(elem_layout.align()) };
+            return Ok(Self::new_in(alloc, align));
         }
 
         if let Err(err) = alloc_guard(layout.size()) {

library/alloctests/lib.rs

@@ -28,6 +28,7 @@
 #![feature(iter_next_chunk)]
 #![feature(maybe_uninit_slice)]
 #![feature(maybe_uninit_uninit_array_transpose)]
+#![feature(ptr_alignment_type)]
 #![feature(ptr_internals)]
 #![feature(sized_type_properties)]
 #![feature(slice_iter_mut_as_mut_slice)]