unused_variables lint shouldn't warn for unused parameter to default method in trait

[sfackler]

trait Foo {
    fn bar(&self, a: i32) {}
}

fn main() {}

<anon>:2:19: 2:20 warning: unused variable: `a`, #[warn(unused_variables)] on by default
<anon>:2     fn bar(&self, a: i32) {}
                           ^

The arguments to default methods in traits form part of the documentation, but it is not uncommon for a default implementation of a method to be a no-op that uses none of its arguments.

[sanxiyn]

I implemented this in #26502, but @brson doesn't seem to think it is a good idea. You may want to comment.

[nikomatsakis]

After some discussion in the lang team meeting, our feeling was that we ought to not make this change: it's just a bit too special-cased and subtle, and to be consistent the rules ought to apply to other kinds of public functions as well. Instead, the recommended course is to add an #[allow] declaration to your trait or default method.

[SoniEx2]

I'd recommend let _=x; personally, it's essentially the Rust equivalent of the C (void)x;.

That way you neither use #[allow], which can cause issues, nor _x, which produces ugly docs.