Constify alloc::Layout

Tracking issue rust-lang#67521, Layout::new in rust-lang#66254

Author: lukaslueg

src/libcore/alloc.rs

@@ -64,8 +64,9 @@ impl Layout {
     ///    must not overflow (i.e., the rounded value must be less than
     ///    `usize::MAX`).
     #[stable(feature = "alloc_layout", since = "1.28.0")]
+    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
     #[inline]
-    pub fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutErr> {
+    pub const fn from_size_align(size: usize, align: usize) -> Result<Self, LayoutErr> {
         if !align.is_power_of_two() {
             return Err(LayoutErr { private: () });
         }
@@ -106,15 +107,17 @@ 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")]
     #[inline]
-    pub fn size(&self) -> usize {
+    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")]
     #[inline]
-    pub fn align(&self) -> usize {
+    pub const fn align(&self) -> usize {
         self.align_.get()
     }
 
@@ -181,8 +184,9 @@ impl Layout {
     /// address for the whole allocated block of memory. One way to
     /// satisfy this constraint is to ensure `align <= self.align()`.
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
+    #[rustc_const_unstable(feature = "const_alloc_layout", issue = "67521")]
     #[inline]
-    pub fn padding_needed_for(&self, align: usize) -> usize {
+    pub const fn padding_needed_for(&self, align: usize) -> usize {
         let len = self.size();
 
         // Rounded up value is:

src/libcore/lib.rs

@@ -70,6 +70,7 @@
 #![feature(bound_cloned)]
 #![feature(cfg_target_has_atomic)]
 #![feature(concat_idents)]
+#![feature(const_alloc_layout)]
 #![feature(const_if_match)]
 #![feature(const_panic)]
 #![feature(const_fn_union)]