`mut_mutex_lock`: FP on `struct.mutex` where `struct` is behind `&`

Summary

(Extracted from https://github.com/rust-lang/rust-clippy/issues/9854#issuecomment-2624392084)

Given a struct like:

struct Wrapper<T>(Mutex<T>);

the lint fires in the following situation:

fn foo<T>(w: &Wrapper<T>) {
    let t = w.0.lock();
}

even though replacing that with get_mut() isn't actually possible, as w is behind &.

Reproducer (playground)

Lint Name

mut_mutex_lock

Reproducer

I tried this code:

use std::sync::Mutex;

struct Struct<'a> {
    ref_mut_mutex: &'a mut Mutex<i32>,
}

impl PartialEq for Struct<'_> {
    fn eq(&self, other: &Self) -> bool {
        let x = self.ref_mut_mutex.lock().unwrap();
        // The following suggested line doesn't compile:
        // let x = self.ref_mut_mutex.get_mut().unwrap();
        let y = other.ref_mut_mutex.lock().unwrap();
        *x == *y
    }
}

I saw this happen:

warning: calling `&mut Mutex::lock` unnecessarily locks an exclusive (mutable) reference
 --> src/lib.rs:9:36
  |
9 |         let x = self.ref_mut_mutex.lock().unwrap();
  |                                    ^^^^ help: change this to: `get_mut`
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mut_mutex_lock
  = note: `#[warn(clippy::mut_mutex_lock)]` on by default

warning: calling `&mut Mutex::lock` unnecessarily locks an exclusive (mutable) reference
  --> src/lib.rs:12:37
   |
12 |         let y = other.ref_mut_mutex.lock().unwrap();
   |                                     ^^^^ help: change this to: `get_mut`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mut_mutex_lock

I expected to see this happen:
no warning, as self, and therefore ref_mut_mutex, is behind an immutable reference.

Version

rustc 1.92.0 (ded5c06cf 2025-12-08)
binary: rustc
commit-hash: ded5c06cf21d2b93bffd5d884aa6e96934ee4234
commit-date: 2025-12-08
host: x86_64-unknown-linux-gnu
release: 1.92.0
LLVM version: 21.1.3

Additional Labels

@rustbot label I-suggestion-causes-error