Function assemble_candidates_from_param_env

fn assemble_candidates_from_param_env<'cx, 'tcx>(
    selcx: &mut SelectionContext<'cx, 'tcx>,
    obligation: &ProjectionTermObligation<'tcx>,
    candidate_set: &mut ProjectionCandidateSet<'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.