extra_unused_lifetimes doesn't warn for lifetimes declared on impl and never used

[carols10cents]

I tried this code (simplified from the actual code):

pub struct Scalar;

impl<'a> std::ops::AddAssign<&Scalar> for &mut Scalar {
    fn add_assign(&mut self, _rhs: &Scalar) {
        unimplemented!();
    }
}

impl<'b> Scalar {
    pub fn something<'c>() -> Self {
        Self
    }
}

I expected to see this happen: I expected to get an unused lifetime warning for all the lifetimes in this example: 'a because it's not used anywhere in the AddAssign impl block (and AddAssign's definition doesn't have any lifetimes), 'b because it's not used anywhere in the impl Scalar block, and 'c because it's not used anywhere in fn something.

Instead, this happened: I only see a warning for 'c:

warning: this lifetime isn't used in the function definition
  --> src/lib.rs:10:22
   |
10 |     pub fn something<'c>() -> Self {
   |                      ^^
   |
   = note: `#[warn(clippy::extra_unused_lifetimes)]` on by default

Meta

• cargo clippy -V: On the playground: 0.0.212 (2020-12-03 5be3f9f)
• rustc -Vv: Stable on the playground: 1.48.0