Unnecessary `&mut` should be warned

[upsuper]

Given the following code:

fn foo(_: &mut usize) {}
let mut a = 0;
let mut b = &mut a;
foo(&mut b);

there is no warning saying that the &mut in the last line is unnecessary.

Some novice don't always have clear idea about where should have &mut so they may add lots of unnecessary ones which leads to more unnecessary mut variables.

It might be helpful if we can warn on this case so that they know this isn't necessary.

[ExpHP]

I'm pretty sure this would generate hundreds of warnings all over my codebase. I've always regarded this as a feature of deref coercions that makes refactoring functions less painful.

Linting against this feels much like linting against match ergonomics... which I acknowledge is something that some people want, but I don't think the standard compiler ought to do it.

[ivanbakel]

This behaviour also happens in the simpler case

fn foo (_: &i32) {}
let mut a = 0;
foo (&mut a);

Which looks a lot worthier of a lint.

[sanxiyn]

This is rust-lang/rust-clippy#1230. I agree with @ExpHP that this belongs in clippy and not rustc.