`swap_with_temporary` mis-detects `MutexGuard`s as temporaries

[TheBlueMatt]

Summary

swap_with_temporary fires when you swap (the deref'd object behind) a MutexGuard with a stack variable as it incorrectly thinks the MutexGuard is a temporary.

Lint Name

swap_with_temporary

Reproducer

We have a bunch of cases like the following which now trigger the lint:

struct A {
   thing: Mutex<Vec<u8>>,
}
impl A {
   fn a(&self) {
      let new_vec = vec![42];
      std::mem::swap(&mut self.thing.lock().unwrap(), new_vec);
      for v in new_vec {
        // Do something with v
      }
   }

Version

Additional Labels

No response

[samueltardieu]

Compiling reproducer:

struct A {
   thing: Mutex<Vec<u8>>,
}
impl A {
   fn a(&self) {
      let mut new_vec = vec![42];
      std::mem::swap(&mut new_vec, &mut self.thing.lock().unwrap());
      for v in new_vec {
        // Do something with v
      }
   }
}

@rustbot claim
@rustbot label +I-suggestion-causes-bug