Tracking Issue for built-in attribute macro `#[cfg_eval]`

[petrochenkov]

Feature gate: #![feature(cfg_eval)]

This is a tracking issue for built-in attribute macro #[cfg_eval] that is used for eagerly expanding all #[cfg] and #[cfg_attr] attributes in its input ("fully configuring" the input).
The effect is identical to effect of #[derive(Foo, Bar)] which also fully configures its input before passing it to macros Foo and Bar.

Public API

#[cfg_eval]
#[my_attr] // Receives `struct S {}` as input, the field is configured away by `#[cfg_eval]`
struct S {
    #[cfg(FALSE)]
    field: u8,
}

Steps / History

• Implementation: Implement built-in attribute macro #[cfg_eval] + some refactoring #82682
• Final commenting period (FCP)
• Stabilization PR

Unresolved Questions

• None yet.

[Aaron1011]

It looks like cfg_eval is currently supported as an inner attribute:

#![feature(custom_inner_attributes)]
#![feature(cfg_eval)]
#![feature(stmt_expr_attributes)]
fn main() {
    let a = #[cfg_attr(FALSE, rustc_dummy)] {
        #![cfg_eval]
    };
}

Is this intentional?

[petrochenkov]

@Aaron1011
I didn't thought about it, cfg_eval is supposed to be a regular macro attribute, and macro attributes are supported in inner attribute positions in general.
It can be prohibited if it e.g. enables token collection optimizations.

[Aaron1011]

I had thought that allowing it as inner attribute would interfere with token collection optimizations, but it turned out to be straightforward to deal with.