`significant_drop_tightening` lint doesn't trigger when manually dropping

Summary

when manually dropping the lock the lint doesn't recognize, that it could be dropped earlier. it would be nice to lint that as well.

Lint Name

significant_drop_tightening

Reproducer

I tried this code:

use std::sync::Mutex;

static THING: Mutex<u32> = Mutex::new(0);

#[warn(clippy::significant_drop_tightening)]
fn main() {
	let mut thing = THING.lock().unwrap();
	*thing = 1;
	*thing += 1;

	println!("this is here to demonstrate the issue");
	println!("this lint does get triggered without the drop");

	drop(thing);
}

I expected to see this happen:

warning: temporary with significant `Drop` can be early dropped
  --> src/main.rs:7:10
   |
6  |   fn main() {
7  |       let mut thing = THING.lock().unwrap();
   |               ^^^^^
8  |       *thing = 1;
9  |       *thing += 1;
...   
14 |       drop(thing);
   |       ^^^^^^^^^^^
15 |   }
   |  _- temporary `thing` is currently being dropped later than necessary
   |
   = note: this might lead to unnecessary resource contention
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#significant_drop_tightening
note: the lint level is defined here
  --> src/main.rs:5:8
   |
5  | #[warn(clippy::significant_drop_tightening)]
   |        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
help: drop the temporary after the end of its last usage
   |
9  ~     *thing += 1;
10 +  drop(thing);
...
14 -     drop(thing);
   |

Instead, this happened:

nothing

More info:

if i remove the drop clippy correctly complains that it is dropped at the end of the scope and could be dropped earlier.

Version

No response

Start with the significant_drop_tightening lint and the Rust reproducer in the issue, comparing behavior with and without the explicit drop(thing). Confirm that manually dropping the mutex guard is recognized and that the diagnostic points to the guard's last use with the suggested earlier drop.