new lint: find unused method results (take '&mut self' into account)

Currently, the rustc unused value warning seems to just look for LHS value that is not referenced anywhere else, like

let x = 0; // unused !
let y = 1;
println!("{}", y);

However there is no warning if we call an object method that modifies the object inplace, but we don't use the new object state afterwards:

let mut x = vec![1, 3, 2];
x.sort(); // here "x" is "used"
// here "x" is no longer used so technically we can omit the entire "x.sort()"
// but there is no warning for this :(

Maybe there could be a more advanced unused variable lint that checks if we call an object method taking &mut self without side effects and not returning anything without the object being ever referenced afterwards?

I'd love to have some sort of warning for the vec.sort() example.

Start by reviewing how rustc's unused value warning handles the shown let example, then compare it with the vec.sort() example involving &mut self. Define the lint's intended side-effect boundary and verify that it warns only when the mutated object is not referenced afterward, while avoiding methods with observable side effects.