`needless_borrowed_reference` does not handle mutable references

[Jarcho]

Summary

needless_borrowed_reference currently only lints on shared references (e.g. &ref x), but mutable references can be linted in some cases.

Lint Name

needless_borrowed_reference

Reproducer

This should lint:

fn f(x: Option<&mut i32>) {
    if let Some(&mut ref mut x) = x {
        *x = 0;
    }
}

And suggest:

fn f(x: Option<&mut i32>) {
    if let Some(x) = x {
        *x = 0;
    }
}

Note that this can only be done if the reference can be moved. None of these should lint:

fn f(x: Option<&mut &mut i32>) {
    if let Some(&mut &mut ref mut x) = x {
        *x = 0;
    }
}

fn f(x: Option<&mut i32>) -> Option<&mut i32> {
    if let Some(x) = x {
        *x = 0;
    }
    x
}

struct WithDrop<'a>(Option<&'a mut i32>);
impl Drop for WithDrop<'_> { fn drop(&mut self) {} }

fn f(x: WithDrop) {
    if let Some(&mut ref mut x) = x.0 {
        *x = 0;
    }
}

[ada4a]

the codeblocks seem to be broken..

[Jarcho]

Fixed.

[profetia]

@rustbot label S-fixed