FP: mut_mut in external macro

[Y-Nak]

Lint name: mut_mut

I tried this code:
the example of async-book also triggered the lint, though this is not the original one.

use futures::{
    future::FutureExt, // for `.fuse()`
    pin_mut,
    select,
};

async fn task_one() { /* ... */
}
async fn task_two() { /* ... */
}

async fn race_tasks() {
    let t1 = task_one().fuse();
    let t2 = task_two().fuse();

    pin_mut!(t1, t2);

    select! {
        () = t1 => println!("task one completed first"),
        () = t2 => println!("task two completed first"),
    }
}

I expected to see this happen: None.
Instead, this happened:

error: generally you want to avoid `&mut &mut _` if possible
  --> src/lib.rs:18:5
   |
18 | /     select! {
19 | |         () = t1 => println!("task one completed first"),
20 | |         () = t2 => println!("task two completed first"),
21 | |     }
   | |_____^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mut_mut
   = note: this error originates in a macro (in Nightly builds, run with -Z macro-backtrace for more info)

Meta

• cargo clippy -V: clippy 0.1.52 (f5d8117 2021-03-16)
• rustc -Vv:

rustc 1.52.0-nightly (f5d8117c3 2021-03-16)
binary: rustc
commit-hash: f5d8117c338a788bd24abec733fd143dfceb25a0
commit-date: 2021-03-16
host: x86_64-unknown-linux-gnu
release: 1.52.0-nightly
LLVM version: 12.0.0

---

This would be caused by the missing of in_external_macro here.