Function rustc_hir_analysis::impl_wf_check::min_specialization::trait_predicates_eq

fn trait_predicates_eq<'tcx>(
    tcx: TyCtxt<'tcx>,
    predicate1: Predicate<'tcx>,
    predicate2: Predicate<'tcx>,
    span: Span
) -> bool

Checks if some predicate on the specializing impl (predicate1) is the same as some predicate on the base impl (predicate2).

This basically just checks syntactic equivalence, but is a little more forgiving since we want to equate T: Tr with T: ~const Tr so this can work:

#[rustc_specialization_trait]
trait Specialize { }

impl<T: Bound> Tr for T { }
impl<T: ~const Bound + Specialize> const Tr for T { }

However, we don’t want to allow the reverse, i.e., when the bound on the specializing impl is not as const as the bound on the base impl:

impl<T: ~const Bound> const Tr for T { }
impl<T: Bound + Specialize> const Tr for T { } // should be T: ~const Bound

So we make that check in this function and try to raise a helpful error message.