Conditionally derived traits using `cfg_attr` are not documented as being behind a feature flag.

[cobward]

When conditionally deriving a trait using cfg_attr, the doc_cfg feature gate is not documented. A minimal example can be found here, but is summarised below:

Example:

 #![feature(doc_auto_cfg)]
 #[cfg_attr(feature = "debug", derive(Debug))]
 pub struct Test();

Generates:

Expectation:

[GuillaumeGomez]

It's because the cfg information isn't kept. When I use macro expansion, here's what we get:

#![feature(prelude_import)]
#[prelude_import]
use std::prelude::rust_2021::*;
#[macro_use]
extern crate std;
pub struct Test();
#[automatically_derived]
impl ::core::fmt::Debug for Test {
    fn fmt(&self, f: &mut ::core::fmt::Formatter) -> ::core::fmt::Result {
        ::core::fmt::Formatter::write_str(f, "Test")
    }
}

[GuillaumeGomez]

It's much more complicated than what I originally imagined: when the function to generate the implementation is called, the provided item doesn't have the attribute around the derive at all. I'm investigating to see if there is a way to do it without breaking everything but the deeper I go, the less hope for that I have...

[mkroening]

There is no way to work around this manually with #[doc(cfg(...))] in affected code currently, is there?

Thank you so much for your work!

[GuillaumeGomez]

@petrochenkov I remember talking with you about this issue: is there any way to keep the cfg information in (derive proc-)macro expansion so we can use it in rustdoc to display cfg information?

[petrochenkov]

In token-based macros - no, token-based macros should not see cfg traces.
In AST based macros (derive is one of them) cfg traces should already be preserved.

Their presence may not be immediately obvious because AST pretty-printer does not print them (for reasons), but you can actually see them in HIR pretty printer (#[attr = CfgAttrTrace]).
One of the possible issues though is that all the useful data was recently removed from the cfg attr traces during migration to rustc_attr_parsing :D
It needs to be reinstated if rustdoc needs it.

[GuillaumeGomez]

Opened #158980.