Suggest unused lifetime parameters

[mu001999]

Code

type Foo<'a> = ();

fn foo<'a>() -> Foo<'a> { () }

fn main() {
    foo();
}

Current output

Success

Desired output

warning: lifetime parameter `'a` is never used
 --> src/main.rs:1:10
  |
1 | type Foo<'a> = ();
  |          ^^
  |
  = note: `#[warn(dead_code)]` on by default

warning: lifetime parameter `'a` is never used
 --> src/main.rs:3:8
  |
3 | fn foo<'a>() -> Foo<'a> { () }
  |        ^^

Rationale and extra context

No response

Other cases

No response

Anything else?

No response

[fmease]

This is only tangentially related but the specific code you posted will stop compiling anyway once lazy type aliases (#112792) become the default in Rust 202X:

error[E0392]: parameter `'a` is never used
 --> src/main.rs:3:10
  |
3 | type Foo<'a> = ();
  |          ^^ unused parameter
  |
  = help: consider removing `'a`, referring to it in a field, or using a marker such as `PhantomData`

[sanxiyn]

Note that Clippy already has a warning about this.

fn foo<'a>() {}

fn main() {
    foo();
}

warning: this lifetime isn't used in the function definition
 --> src/main.rs:1:8
  |
1 | fn foo<'a>() {}
  |        ^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#extra_unused_lifetimes
  = note: `#[warn(clippy::extra_unused_lifetimes)]` on by default