Lint unused assoc tys although the trait is used

[mu001999]

r? @pnkfelix

[mu001999]

  error: associated type `Bar` is never used
     --> library/core/tests/any.rs:137:14
      |
  136 |     trait Foo {
      |           --- associated type in this trait
  137 |         type Bar;
      |              ^^^
      |
      = note: `-D dead-code` implied by `-D warnings`
      = help: to override `-D warnings` add `#[allow(dead_code)]`

the code is:

#[test]
fn dyn_type_name() {
    trait Foo {
        type Bar;
    }

    assert_eq!(
        "dyn core::ops::function::Fn(i32, i32) -> i32",
        std::any::type_name::<dyn Fn(i32, i32) -> i32>()
    );
    assert_eq!(
        "dyn coretests::any::dyn_type_name::Foo<Bar = i32> \
        + core::marker::Send + core::marker::Sync",
        std::any::type_name::<dyn Foo<Bar = i32> + Send + Sync>()
    );
}

seems the assoc type is not marked live if it was used in the trait object.

[bors]

☔ The latest upstream changes (presumably #125443) made this pull request unmergeable. Please resolve the merge conflicts.

[bors]

☔ The latest upstream changes (presumably #128404) made this pull request unmergeable. Please resolve the merge conflicts.