wfcheck incorrectly assumes unnormalized types to be wf

[lcnr]

struct Foo<T>(T);

trait GoodBye {
    type Forget;
}
impl<T> GoodBye for T {
    type Forget = ();
}

trait NeedsWf<'a, 'b> {
    type Assoc;
}

impl<'a, 'b> NeedsWf<'a, 'b> for Foo<<&'a &'b () as GoodBye>::Forget> {
    type Assoc = &'a &'b ();
}

fn main() {}

this compiles with #100676 because we now assume unnormalized types in the impl header to be well formed when computing implied bounds. We don't check that when using the impl so we should be able to transmute lifetimes with this, though I haven't tried that yet.

Going to fix that myself once I am back home in 2 weeks. cc @rust-lang/types

[lcnr]

assigning P-critical after a very long and deep discussion in https://rust-lang.zulipchat.com/#narrow/stream/245100-t-compiler.2Fwg-prioritization.2Falerts/topic/.23100910.20wfcheck.20incorrectly.20assumes.20unnormalized.20types.20to.20b.E2.80.A6

[pnkfelix]

(FYI I also nominated PR #100676 for discussion at T-compiler meeting, essentially because of this associated issue; I hadn't seen that this was tagged as P-critical at that time. I'm leaving it nominated for now, just to ensure doubly that we discuss the matter.)

[lcnr]

this is an existing issue on stable if we prevent the projection from normalizing in the impl while normalizing it when using the impl:

struct Foo<T>(T);

trait GoodBye {
    type Forget;
}
impl<T> GoodBye for T {
    type Forget = ();
}

trait NeedsWf<'a, 'b> {
    type Assoc;
}

impl<'a, 'b> NeedsWf<'a, 'b> for Foo<<&'a &'b () as GoodBye>::Forget>
where
    &'a &'b (): GoodBye, // this line prevents us from normalizing in the param env.
{
    type Assoc = &'a &'b ();
}

fn needs_wf<'a, 'b, T: NeedsWf<'a, 'b>>() {}

fn foo<'a: 'a, 'b: 'b>(_: &'b String) {
    needs_wf::<'a, 'b, Foo<()>>();
}

fn main() {
    let x = String::from("hello");
    foo::<'static, '_>(&x);
}

that's the same issue as #98543, which has been only fixed for function signatures

edit: that's #100051