Rustdoc should indicate variance on lifetime/type parameters

[tomjakubowski]

This has come up a few times in IRC and I think it's a nice idea. It's already important for lifetime parameters, where variance is important today, and could be useful for type parameters as well if rust-lang/rfcs#738 is accepted.

[steveklabnik]

Triage: no changes here

[steveklabnik]

@rust-lang/rustdoc is this something we intend to pursue?

[QuietMisdreavus]

I'm not too sure how we could display this without cluttering up the display way more than we already have. Moreover, how do we collect this information? I don't have a good grasp of variance myself, much less whether we can just ask the compiler about a type/lifetime param to get an easy answer. I'm inclined to say "no", but the question of variance tends to crop up regularly enough that it may be worth leaving open for an intrepid compiler hacker.

[sunshowers]

This would be super useful to build tooling around even if only exposed in JSON output (#76578). Variance changes are one of the more subtle kinds of API breakage, and anything we can do to make them easier to understand would be great.

[jyn514]

@sunshowers I would be willing to take a PR for this (especially if it's only in the JSON) but I have no idea how to implement it myself.