Unused variable warning shouldn't apply to parameters of default trait implementations

[zwarich]

Currently, code like

trait Useless {
    fn foo(&self, descriptive_name: uint) { }
}

struct A;

impl Useless for A {
    fn foo(&self, descriptive_name: uint) { println!("{}", descriptive_name); }
}

fn main() {
    let a = A;
    a.foo(1);
}

will trigger an unused variable warning:

test.rs:2:19: 2:35 warning: unused variable: `descriptive_name`, #[warn(unused_variable)] on by default
test.rs:2     fn foo(&self, descriptive_name: uint) { }
                            ^~~~~~~~~~~~~~~~

It seems like using a descriptive name for a parameter should be allowed, even if the default implementation doesn't use the parameter at all.

[reem]

Visiting for triage.

Personally, I think this is a special case and shouldn't be exempted, since you can can have exceptions to lints with #[allow].

[huonw]

This is a dupe of #26487, which was closed: #26487 (comment).