Fix ICE when collapsing associated type with mismatched bound vars

[im-lunex]

the ICE caused by collapse_candidates_to_subtrait_pick because it decided whether one trait was a supertrait just basing on the def_id and ignoring the generic args.

the fix approach adds an check using anonymize_bound_vars before merging... if the check fails we return NONE to sign ambiguity and the existing ambiguity diagnostic (E0221) handles the rest...

added 2 test and 1 .stderr

tested with -> tests/ui/supertrait-shadowing/alpha-equivalent-duplicate-supertrait-binders.rs and tests/ui/supertrait-shadowing/ambiguous-duplicate-supertrait-binders.rs

Fixes #161547

[rustbot]

HIR ty lowering was modified

cc @fmease

[fmease]

This certainly isn't correct since it makes us start rejecting code like this:

//@ compile-flags: -Znext-solver
#![feature(supertrait_item_shadowing)]

trait A<'a> { type Ty; }
trait B: for<'a> A<'a> + A<'static> { type Ty; }

fn f<T: B>() {
    let _: T::Ty; // PASS -> AMBIGUITY
}

However, there should be no ambiguity under supertrait_item_shadowing as clearly the candidate <T as B>::Ty shadows both <T as A<'static>>::Ty and (pseudo) for<'a> <T as A<'a>>::Ty. It doesn't matter if the bound vars or the arguments mismatch. Identification by DefId is sufficient.

Similarly, this would make us reject code like below despite there being no ambiguity:

#![feature(supertrait_item_shadowing)]

trait A<D> { type Ty; }
trait B: A<i32> + A<()> { type Ty; }

fn f<T: B>() {
    let _: T::Ty; // PASS -> AMBIGUITY
}

View changes since the review

[fmease]

For reference, the fn analogue (which uses the collapse_candidates_to_subtrait_pick from method/probe.rs instead) continues to PASS under your PR meaning they now diverge in behavior:

//@ compile-flags: -Znext-solver
#![feature(supertrait_item_shadowing)]

trait A<'a> { fn f(); }
trait B: for<'a> A<'a> + A<'static> { fn f(); }

fn f<T: B>() {
    let _ = T::f(); // PASS
}

[fmease]

So the root cause is somewhere else.

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.