Fix FP of identity_op when encountering Default::default()

[relaxcn]

Fixes: #14932

changelog: Fix false positive of [identity_op] when encountering Default::default().

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[relaxcn]

The current fix has false negatives at this code:

fn test() -> usize {
    0usize + &Default::default()
    //~^ identity_op
}

fn main() {
    let _n: usize = 0usize + &Default::default();
    //~^ identity_op
}

The lint also need to check whether the compiler can infer the type of Default::default().

I will make it ready when I finish.

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[relaxcn]

@rustbot ready

[samueltardieu]

I have the feeling that this only solves one edge case but doesn't address the real problem. For example, the following code will fail as well:

trait Def {
    fn def() -> Self;
}

impl Def for usize {
    fn def() -> Self {
        0
    }
}

_ = 0usize + &Def::def();

The problem is not really the Default::default() call, it is the fact that the associated function call returns a type which is neither given explicitly nor derived from the function arguments. Maybe checking this would solve the general case.

[relaxcn]

Thank you!

I have modified the code. I have tried to find more cases about non-assoc function, but I can't.

Now, the lint will exclude checking AssocFn which return Self and doesn't specify fully-qualified.

@rustbot ready

[relaxcn]

It's strange. I passed the cargo test localy but it still failed in github action.

Now fixed.

[samueltardieu]

That's a reasonable approach, thanks!