Function rustc_trait_selection::traits::project::assemble_candidates_from_param_env

pub(in traits::project) fn assemble_candidates_from_param_env<'cx, 'tcx>(
    selcx: &mut SelectionContext<'cx, 'tcx>, 
    obligation: &Obligation<'tcx, ProjectionTy<'tcx>>, 
    obligation_trait_ref: &TraitRef<'tcx>, 
    candidate_set: &mut ProjectionTyCandidateSet<'tcx>
)

The first thing we have to do is scan through the parameter environment to see whether there are any projection predicates there that can answer this question.