uncertain: local projection cache strenghens inference

[lcnr]

https://crater-reports.s3.amazonaws.com/pr-133502-13/try%233cddd79d4be3c8f1e937d09ca618d192abc69373/reg/uncertain-0.3.1/log.txt

trait Trait {
    type Assoc;
}

struct W<T>(T);
impl<T: Trait> Trait for W<T> {
    type Assoc = T::Assoc;
}

impl Trait for () {
    type Assoc = f32;
}


fn foo<T: Trait>(arg: T, x: T::Assoc) -> T::Assoc {
    x
}

fn main() {
    let w = W(Default::default());
    foo(w, 1f32).abs();
    let _: W<()> = w;
}

we store <W<_> as Trait>::Assoc => ?unconstrained in the projection cache and reuse ?unconstrained for both the input and output. We only do so as normalizing <W<_> as Trait>::Assoc actually made some progress

[compiler-errors]

probably unsound according to lcnr (edit: lcnr) as the evaluation cache can rely on this, e.g. a candidate W<<W<_> as Trait>::Assoc>: Bound<?unconstrained> can differ from W<<W<_> as Trait>::Assoc>: Bound<?unrelated> without this getting tracked by the cache