coherence now uses fulfillment

[lcnr]

rust-lang/rust#121193

trait Mirror {
    type Assoc;
}
impl<T> Mirror for T {
    type Assoc = T;
}

trait Foo {}
trait Bar {}

impl<T> Foo for T where (): Mirror<Assoc = T> {}
impl<T> Foo for T where T: Bar {}

fn main() {}

now passes with the new solver as constraints from one where-bound can be used to guide another one

[lcnr]

now stabilized (ish)