Function confirm_impl_candidate

fn confirm_impl_candidate<'cx, 'tcx>(
    selcx: &mut SelectionContext<'cx, 'tcx>,
    obligation: &ProjectionTermObligation<'tcx>,
    impl_impl_source: ImplSourceUserDefinedData<'tcx, PredicateObligation<'tcx>>,
) -> Result<Projected<'tcx>, ProjectionError<'tcx>>