Document builtin attributes (like `no_mangle`, `repr`, ...) in the standard library docs

[Veykril]

Location

https://doc.rust-lang.org/std/

Summary

The standard library docs currently document keywords on the main index https://doc.rust-lang.org/std/#keywords

I propose we should do the same for builtin language attributes (no_mangle, repr, no_std), but notably not builtin attribute macros (test, bench) as those are already described by nature of being proper macros exposed from the standard libraries.

They should be using a similar mechanism as to how keywords are documented, dummy modules with doc strings. That way tooling like IDEs can render these docs on hover of the builtin language attributes.

cc @GuillaumeGomez

[Noratrieb]

I played around with this a bit in a branch: https://github.com/Noratrieb/rust/tree/rustdoc-inert-attributes
The code would need quite some cleanup and thinking before being ready to merge I think which I don't think I'm gonna do, but in case anyone wants to pick this up and continue

[GuillaumeGomez]

I was planning to get to it at some point. The code for this will likely be very similar to doc(keyword). But it's always appreciated @Noratrieb, so thanks! :)

[GuillaumeGomez]

Ok implementing it. With the code from @Noratrieb (thanks!), hopefully shouldn't take too long.