Implement RFC 3631: add rustdoc doc_cfg features

[GuillaumeGomez]

Implementation of rust-lang/rfcs#3631.

First commit comes from #138755.

What remains to be done:

• Handle doc(auto_cfg(show/hide)) "collisions"
• Add more tests

r? ghost

[petrochenkov]

What remains to be done:

I'd also want to block the stabilization on landing #138844 to avoid a stable rustdoc feature relying on externally observable hacks in rustc.
The crater run in #138844 returned mostly clean, so I expect it to land soon.

[GuillaumeGomez]

Noted! And that will be a nice improvement, thanks!

Just one thing left for the cfg expansion missing: #[cfg_attr(blabla, derive(Debug))]. In this case, the cfg is not kept in the generated derive items. It's been in my TODO list for a long time now. ^^'

[petrochenkov]

Just one thing left for the cfg expansion missing: #[cfg_attr(blabla, derive(Debug))]

Do you mean like in #138515? :)

[GuillaumeGomez]

You're my hero! Gonna need to handle this new attribute then. :)

[bors]

☔ The latest upstream changes (presumably #138923) made this pull request unmergeable. Please resolve the merge conflicts.

[rust-log-analyzer]

The job mingw-check failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

   |
23 | mod propagate_doc_cfg;
   | ^^^^^^^^^^^^^^^^^^^^^^
   |
   = help: to create the module `propagate_doc_cfg`, create file "src/librustdoc/passes/propagate_doc_cfg.rs" or "src/librustdoc/passes/propagate_doc_cfg/mod.rs"
   = note: if there is a `mod propagate_doc_cfg` elsewhere in the crate already, import it with `use crate::...` instead

For more information about this error, try `rustc --explain E0583`.
[RUSTC-TIMING] rustdoc test:false 3.575
error: could not compile `rustdoc` (lib) due to 1 previous error
Build completed unsuccessfully in 0:02:33