Make cfg imply doc(cfg)

[GuillaumeGomez]

This is a reopening of #79341, rebased and modified a bit (we made a lot of refactoring in rustdoc's types so they needed to be reflected in this PR as well):

• hidden_cfg is now in the Cache instead of DocContext because cfg information isn't stored anymore on clean::Attributes type but instead computed on-demand, so we need this information in later parts of rustdoc.
• I removed the bool_to_options feature (which makes the code a bit simpler to read for SingleExt trait implementation.
• I updated the version for the feature.

There is only one thing I couldn't figure out: this comment

I think I'll likely scrap the whole SingleExt extension trait as the diagnostics for 0 and >1 items should be different.

How/why should they differ?

EDIT: this part has been solved, the current code was fine, just needed a little simplification.

cc @Nemo157
r? @jyn514

Original PR description:

This is only active when the doc_cfg feature is active.

The implicit cfg can be overridden via #[doc(cfg(...))], so e.g. to hide a #[cfg] you can use something like:

#[cfg(unix)]
#[doc(cfg(all()))]
pub struct Unix;

By adding #![doc(cfg_hide(foobar))] to the crate attributes the cfg #[cfg(foobar)] (and only that exact cfg) will not be implicitly treated as a doc(cfg) to render a message in the documentation.

[jyn514]

This will be ignored, right, because you use single()? That doesn't seem right, either they should be ANDed or it should give a hard error.

[GuillaumeGomez]

This is valid and is supposed to work. I added it to ensure that rustdoc was happy with it.

[GuillaumeGomez]

Might be better to put it into a rustdoc test though. I'll do that.

[GuillaumeGomez]

We actually already have a test for this: https://github.com/rust-lang/rust/blob/master/src/test/rustdoc/doc-cfg.rs#L100

[jyn514]

@bors r+

[bors]

📌 Commit 09c7688 has been approved by jyn514