Guarantee char layout

[joshlf]

No description provided.

[joshlf]

Ping 🙂

[ehuss]

Nominated to get lang's attention.

[joshtriplett]

Suggested change

	`char` is guaranteed to have the same size and alignment as `u32` on all platforms.
	`char` is guaranteed to have the same size and alignment as `u32` on all platforms.
	`Option<char>` is guaranteed to have the same size and alignment as `char` on all platforms.

(Unless we want to put this somewhere else.)

[joshtriplett]

This suggestion should probably be handled separately, not in this PR.

[joshlf]

Filed #1426 to track this.

[joshtriplett]

Consensus from lang is to merge.

std docs should also be updated.

[scottmcm]

TBH I was surprised this was to the reference directly, not to core like the other guarantees have been done. We currently have no guarantees written for specific alignment of anything, as far as I could find.

I was going to suggest something like

/// ```
///     use std::alloc::Layout;
///     assert_eq!(Layout::new::<char>(), Layout::new::<u32>());
///     assert_eq!(Layout::new::<Option<char>>(), Layout::new::<u32>());
/// ```

but then this wasn't a docs change...

[joshlf]

No opposition to this going in core as well of course, but we already have text in the Reference about the sizes of primitive types: https://doc.rust-lang.org/reference/type-layout.html#primitive-data-layout

[joshlf]

Put up a PR to add the same text to core: rust-lang/rust#116894

[traviscross]

@rustbot labels -I-lang-nominated