Function rustc_hir_analysis::collect::item_bounds::opaque_type_bounds

fn opaque_type_bounds<'tcx>(
    tcx: TyCtxt<'tcx>,
    opaque_def_id: LocalDefId,
    ast_bounds: &'tcx [GenericBound<'tcx>],
    item_ty: Ty<'tcx>,
    span: Span
) -> &'tcx [(Clause<'tcx>, Span)]

Opaque types don’t inherit bounds from their parent: for return position impl trait it isn’t possible to write a suitable predicate on the containing function and for type-alias impl trait we don’t have a backwards compatibility issue.