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Reproducer using another attribute:

$ cat src/main.rs
#![feature(marker_trait_attr)]

#[marker]
trait A {}

fn main() {}
$ cargo check
    Checking unused v0.1.0 (/../unused)
    Finished dev [unoptimized + debuginfo] target(s) in 0.43s
$ touch src/main.rs
$ cargo check
    Checking unused v0.1.0 (/../unused)
warning: unused attribute
 --> src/main.rs:3:1
  |
3 | #[marker]
  | ^^^^^^^^^
  |
  = note: `#[warn(unused_attributes)]` on by default

    Finished dev [unoptimized + debuginfo] target(s) in 0.25s

Looks like the general incompatibility of unused attributes lint
with incremental compilation is a known issue: #58633, #65023.

r? @petrochenkov

@bors r+

📌 Commit 34f8394 has been approved by petrochenkov

@bors p=1

Because it fixes a perf regression.

⌛ Testing commit 34f8394 with merge fbc46b7...

☀️ Test successful - checks-azure
Approved by: petrochenkov
Pushing fbc46b7 to master...