Show crate features in rustdoc

[Milo123459]

Show crate features generated in rustdoc.

[epage]

There are a couple of ways features can be exposed.

One is in-work, #43781 which shows the needed features on the API items. See argfile::parse_response as an example.

Not all features are exposed on API items (and it takes extra work to make sure the ones that do do show up in rustdoc) and sometimes its helpful to get a summary. This is where the third-party document-features approach comes in. It parses comments in the manifest in a somewhat similar way to comments in source. For an example, check out kstring's manifest and docs.

For this latter case, I imagine there will be work on deciding how the feature list should be exposed via rustdoc because I doubt we would want to do it the same way as document-features (example). Maybe treat features like API items?

We'd also need to hash out the comment conventions. Having practice with third-party crates like document-features helps here.

[epage]

From Zulip

@_jsha|315412 said:

This was a big part of the discussion at https://rust-lang.zulipchat.com/#narrow/stream/266220-rustdoc/topic/display.20minimum.20rust.20version.3F and #96028.

The gist of that discussion was: showing MSRV conceptually fits within rustdoc, but not on any current rustdoc page. There are some other things that conceptually fit within rustdoc, like features and dependencies (which are currently shown on docs.rs, but are not part of rustdoc itself). It would probably be a good idea to have rustdoc generate a page with features, dependencies, and MSRV (and maybe some other things). Ideally that should make it possible to eventually remove the page on docs.rs with that information, since that page would become redundant.

[epage]

A related issue is rust-lang/cargo#4956 which is about adding descriptions to features for crates.io