Feature const_alloc_layout makes these methods on Layout const:

impl Layout {
  fn for_value<T>(t: &T) -> Layout
  fn align_to(&self, align: usize) -> Result<Layout, LayoutError>
  fn pad_to_align(&self) -> Layout
  fn extend(&self, next: Layout) -> Result<(Layout, usize), LayoutError>
  fn array<T>(n: usize) -> Result<Layout, LayoutError>
}

Making more methods of alloc::Layout const allows computing alignment/size information for arbitrary (sized) types at compile-time. While mem::size_of and mem::align_of are already const and Layout is solely based on those, there is no guarantee that a const derived from these functions will be exactly the same as what is used in a call to alloc::alloc. Constifying Layout makes this possible.

Implemented in #67494

Blocked on #46571 (needed for for_value)