rustdoc: expose #[target_feature] attributes as doc(cfg) flags

[QuietMisdreavus]

This change exposes #[target_feature(enable = "feat")] attributes on an item as if they were also #[doc(cfg(target_feature = "feat"))] attributes. This gives them a banner on their documentation listing which feature is required to use the item. It also modifies the rendering code for doc(cfg) tags to handle target_feature tags. I made it print just the feature name on "short" printings (as in the function listing on a module page), and use "target feature feat" in the full banner on the item page itself.

This way, the function listing in std::arch shows which feature is required for each function:

[rust-highfive]

r? @GuillaumeGomez

(rust_highfive has picked a reviewer for you, use r? to override)

[QuietMisdreavus]

r? @GuillaumeGomez

cc @BurntSushi, @killercup

(btw, expect a PR to stdsimd to add some doc(cfg) so we can render as many arches as possible :3)

[QuietMisdreavus]

One matter this raises is that it uses the #[doc(cfg(...))] feature without using the attribute or feature flag. Is that... valid? >_>

[GuillaumeGomez]

I think we should precise this is a target feature. Like this, if I didn't know I'd have no idea what id'd be about.

[bors]

☔ The latest upstream changes (presumably #48811) made this pull request unmergeable. Please resolve the merge conflicts.

[QuietMisdreavus]

Fixed the merge conflict.

@GuillaumeGomez I'm not sure i understand. Are you saying we should say "target feature" all the time? My assumption for abbreviating it on the module view was that you're already looking in std::arch or stdsimd, which describe how to use target_feature. Also, none of the other pretty-printed forms of doc(cfg) (target_arch, target_os, target_env, etc) include which "category" of configuration item it is. Strictly speaking, to make it fit in I would need to pretty-print all the known feature names by substituting their "proper name" in, like how it's done with all the other options.

[GuillaumeGomez]

When looking and the page, if I didn't know what this text was for, I'd be completely lost. Any kind of indication would help users a lot.

[QuietMisdreavus]

And you can click through to see what it means. The "full version" is printed on the function's page. For configurations like target_arch or target_env the current pretty-printer doesn't even provide that luxury. To me, it feels like adding "target feature" to the short form would add an enormous amount of noise, considering that the x86_64 listing has pages and pages worth of functions that would now all say the same thing.

[GuillaumeGomez]

Thanks!

@bors: r+

[bors]

📌 Commit b3fb0d1 has been approved by GuillaumeGomez