unused_attributes: false positive since nightly-2025-03-21 "unused attribute <cfg_attr>"

[taiki-e]

Code

I tried this code:

macro_rules! mac {
    () => {};
}

#[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
mac!();

I expected to see this happen: no warning

Instead, this happened: unused_attributes warns cfg_attr

warning: unused attribute `<cfg_attr>`
 --> src/lib.rs:5:1
  |
5 | #[cfg_attr(target_os = "none", cfg(target_has_atomic = "ptr"))]
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
note: the built-in attribute `<cfg_attr>` will be ignored, since it's applied to the macro invocation `mac`
 --> src/lib.rs:6:1
  |
6 | mac!();
  | ^^^
  = note: `#[warn(unused_attributes)]` on by default

Version it worked on

It most recently worked on: nightly-2025-03-20

Version with regression

nightly-2025-03-21

rustc --version --verbose:

rustc 1.87.0-nightly (78948ac25 2025-03-20)
binary: rustc
commit-hash: 78948ac259253ce89effca1e8bb64d16f4684aa4
commit-date: 2025-03-20
host: aarch64-apple-darwin
release: 1.87.0-nightly
LLVM version: 20.1.1

@rustbot modify labels: +regression-from-stable-to-nightly -regression-untriaged +A-attributes

[taiki-e]

Probably related to #138515 which added <cfg_attr>.

cc @petrochenkov

[petrochenkov]

I'll fix it today or tomorrow.

[petrochenkov]

Fixed in #138794.