Macro-expanded unconfigured items are gated feature checked

[jseyfried]

Macro-expanded unconfigured items are gated feature checked, but ordinary unconfigured items are not. For example, the following should compile, ...

#![cfg_attr(attr, feature(rustc_attrs))]

macro_rules! mac {
    {} => {
        #[cfg(attr)]
        mod m {
            #[rustc_error] // ... but if not(attr), this complains about the lack of the rustc_attrs feature
            fn f() {}
        }
    }
}

mac! {}

If we manually expand mac! {}, it always compiles:

#![cfg_attr(attr, feature(rustc_attrs))]

#[cfg(attr)]
mod m {
    #[rustc_error]
    fn f() {}
}

[jseyfried]

It looks like this was intentional (introduced in #22899), but I don't think it's a good idea.
cc @nrc
cc #22250

[LeoTestard]

There are arguments on #32648 about why this is not a good idea: #[cfg] is typically used by crates that need to build against both stable and nightly and use nightly-specific features when available (the most important example that I have in mind is syntax extensions using plugins on nightly and syntex on stable). Checking for use of gated features inside this #[cfg]-guarded code will break those crates, and it's not a good thing, this use case is actually useful. This check pass should be removed.

[nrc]

cc @rust-lang/lang