Deprecated attribute on trait impls is useless

[Havvy]

I'd expect the following code to warn, but it doesn't:

struct Struct;

trait Trait {
  fn f(&self);
}

#[deprecated(since="0.0.0" note="example deprecation")]
impl Struct for Trait {
  fn f(&self) {}
}

fn main() {
  Struct.f();
}

Note: It is an error to put the deprecated attribute on the fn f in the impl itself.

[Havvy]

This should be A-Trait, not A-Impl-Trait.

[kennytm]

Duplicateof #39935.