macros: don't `cfg`-process items before applying attribute proc macros

[jseyfried]

cc #38356

[jseyfried]

cc @abonander @keeperofdakeys

[KalitaAlexey]

@jseyfried,
I can't understand the title.
Can you make it clear please?

[jseyfried]

Suppose we have an attribute proc macro attr:

#[proc_macro_attribute]
fn attr(input: TokenStream) -> TokenStream { ... }

and an invocation of attr:

#[attr] fn f() { #[cfg(any())] fn g() {} }

Today, we cfg-process fn f (resulting in fn f() {}) before we apply attr.
In other words, the input: TokenStream argument is fn f() {}.

Instead, we want the input to be fn f() { #[cfg(any())] fn g() {} }.

[KalitaAlexey]

I got it.
cfg-process means processing #[cfg] attributes.
Thank you.

[jdhorwitz]

Is this still available to be picked up with a Mentor?

[jseyfried]

@jdhorwitz
Yeah, feel free to ping me (jseyfried) on IRC (#rustc/#rust-internals) or ask questions here.

[shioju]

@jdhorwitz are you still working on this? If not, I'd like to take it up. Thx.

[jseyfried]

@shioju feel free to pick it up :)