Types FCP v2: Supertrait item shadowing stabilization

[jackh726]

The stabilization of supertrait item shadowing is being proposed in #148605.

Since the initial FCP, item shadowing of associated types and consts has been added in #152225 (see https://github.com/rust-lang/rust/blob/main/tests/ui/supertrait-shadowing/assoc-type.rs).

We should redo FCP since we're now stabilizing more.

(I can add more info here if needed.)

[jackh726]

@rfcbot merge types

[rust-rfcbot]

@jackh726 has proposed to merge this. The next step is review by the rest of the tagged team members:

• @BoxyUwU
• @jackh726
• @lcnr
• @oli-obk
• @spastorino

No concerns currently listed.

Once a majority of reviewers approve (and at most 2 approvals are outstanding), this will enter its final comment period. If you spot a major issue that hasn't been raised at any point in this process, please speak up!

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[lcnr]

#![feature(supertrait_item_shadowing)]
trait Trait {
    const C: usize;
}

trait Sub: Trait {
    const C: usize;
}

impl Trait for () {
    const C: usize = 0;
}
impl Sub for () {
    const C: usize = 1;
}

fn foo<T: Sub>() -> usize {
    T::C
}

this compiles as expected. We do not emit the supertrait_item_shadowing_definition lint however, neither do we for

#![feature(supertrait_item_shadowing)]
trait Trait {
    fn foo() {}
}

trait Sub: Trait {
    fn foo() {}
}

impl Trait for () {}
impl Sub for () {}

fn foo<T: Sub>() {
    let _ = T::foo;
}

The FCP states "supertrait_item_shadowing_definition warns by default.", so I am confused why we don't lint here?