Rollup merge of rust-lang#78305 - ChrisDenton:const-layout, r=oli-obk

Stabilize alloc::Layout const functions

Stabilizes rust-lang#67521. In particular the following stable methods are stabilized as `const fn`:

* `size`
* `align`
* `from_size_align`

Stabilizing `size` and `align` should not be controversial as they are simple (usize and NonZeroUsize) fields and I don't think there's any reason to make them not const compatible in the future. That being true, the other methods are trivially `const`. The only other issue being returning a `Result` from a `const fn` but this has been made more usable by recent stabilizations.

library/core/src/alloc/layout.rs

@@ -50,7 +50,7 @@ impl Layout {
     ///    must not overflow (i.e., the rounded value must be less than
     ///    or equal to `usize::MAX`).
     #[stable(feature = "alloc_layout", since = "1.28.0")]
-    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
+    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
     #[inline]
     pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutError> {
         if !align.is_power_of_two() {
@@ -96,15 +96,15 @@ impl Layout {
 
     /// The minimum size in bytes for a memory block of this layout.
     #[stable(feature = "alloc_layout", since = "1.28.0")]
-    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
+    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
     #[inline]
     pub const fn size(&self) -> usize {
         self.size_
     }
 
     /// The minimum byte alignment for a memory block of this layout.
     #[stable(feature = "alloc_layout", since = "1.28.0")]
-    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
+    #[rustc_const_stable(feature = "const_alloc_layout", since = "1.50.0")]
     #[inline]
     pub const fn align(&self) -> usize {
         self.align_.get()