rustdoc: Please use sticky headers for impl blocks

[joshtriplett]

If you have a long rustdoc page with different impl blocks for the same type, such as https://doc.rust-lang.org/1.82.0/std/primitive.slice.html , each of which has various methods, it's really easy to lose track of which impl block a method is in.

I'd love to see rustdoc use "sticky" headings for impl blocks, which stay visible while scrolling through the contents of the impl block, and scroll away when past that impl block. This should be possible with a bit of CSS (no JavaScript required).

This came up when discussing some methods we wanted to add to [MaybeUninit<T>].

[RalfJung]

Oh yes that would be amazing. Currently it can actually be quite hard to figure out what the type of self is when looking at a method in rustdoc.

Cc @rust-lang/rustdoc

[GuillaumeGomez]

It's a very good idea. Gonna add that.

[GuillaumeGomez]

Opened #133717.