Rustdoc show rustc_layout

[pickfire]

It would be good to be able to understand the size, abi and debug stuff related to rustc_layout. Instead of needing users to manually add #[rustc_layout(debug)] to struct and enum on nightly, why not just show it on rustdoc generated pages, this may only be limited to concrete struct and enum, otherwise it may not be as useful since currently rustc_layout is not so useful on those.

CC @rust-lang/rustdoc

[jonas-schievink]

If this ends up getting implemented, please make sure to clarify that the layout is implementation-defined and unstable unless #[repr] is used.

[pickfire]

Given that this have been there for quite some time and even rust-analyzer can show type size information on hover, should we stabilize this?