Add lint checking for no-op uses of Option::{as_deref,as_deref_mut}

[LeSeulArtichaut]

What it does

Checks for no-op uses of Option::{as_deref,as_deref_mut}.

Categories (optional)

• Kind: complexity

What is the advantage of the recommended code over the original code

Removes useless code, removing clutter and improving readability.

Drawbacks

None.

Example

let a = Some(&1);
let b = a.as_deref(); // goes from Option<&i32> to Option<&i32>

Could be written as:

let a = Some(&1);
let b = a;

[camsteffen]

Should be easy as two expr_ty calls and check that they are the same type.

[lengyijun]

Result<T,E> as_deref to Result<&<T as Deref>::Target, &E>
Due to &E is not E, we shouldn't lint on Result.

[LeSeulArtichaut]

@lengyijun Correct, I missed that. Edited the issue

[lengyijun]

@rustbot claim