Macro-expanded macro definitions can be used even if they are unconfigured

[jseyfried]

For example, this compiles:

macro_rules! bar {
    {} => {
        #[cfg(foo)]
        macro_rules! baz { () => {} }
    }
}

bar! {}

fn main() { baz!(); }

If we replaced bar! {} with its expansion, #[cfg(foo)] macro_rules! { baz { () => {} } }, the example wouldn't compile.

[jseyfried]

I think we should fix this by removing unconfigured items after each macro expansion pass since we'll have to do this anyway to get accurate name resolution during macro expansion.

cc @nrc

[durka]

Same as #22250?

[jseyfried]

@durka yeah, closing in favor of #22250.