Function rustc_trait_selection::traits::project::assemble_candidate_for_impl_trait_in_trait

fn assemble_candidate_for_impl_trait_in_trait<'cx, 'tcx>(
    selcx: &mut SelectionContext<'cx, 'tcx>,
    obligation: &Obligation<'tcx, AliasTy<'tcx>>,
    candidate_set: &mut ProjectionCandidateSet<'tcx>
)

If the predicate’s item is an ImplTraitPlaceholder, we do a select on the corresponding trait ref. If this yields an impl, then we’re able to project to a concrete type, since we have an impl’s method to provide the RPITIT.