Do not treat lifetimes from parent items as influencing child items

[oli-obk]

struct A;
impl Bar<'static> for A {
    const STATIC: &str = "";
    //            ^ no future incompat warning
}

has no future incompat warning, because there is no ambiguity. But

struct C;
impl Bar<'_> for C {
//       ^^ this lifeimte
    const STATIC: &'static str = {
        struct B;
        impl Bar<'static> for B {
            const STATIC: &str = "";
            // causes     ^ to emit a future incompat warning
        }
        ""
    };
}

had one before this PR, because the impl for B (which is just a copy of A) thought it was influenced by a lifetime on the impl for C.

I double checked all other lifetime_ribs iterations and all of them do check for Item boundaries. This feels very fragile tho, and I think we should do not even be able to see ribs from parent items, but that's a different refactoring that I'd rather not do at the same time as a bugfix. EDIT: ah nevermind, this is needed for improving diagnostics like "use of undeclared lifetime" being "can't use generic parameters from outer item" instead.

r? @compiler-errors

[compiler-errors]

@bors r+ rollup

[bors]

📌 Commit dabee5d has been approved by compiler-errors

It is now in the queue for this repository.