pub(super) fn build_custom_mir<'tcx>( tcx: TyCtxt<'tcx>, did: DefId, hir_id: HirId, thir: &Thir<'tcx>, expr: ExprId, params: &IndexVec<ParamId, Param<'tcx>>, return_ty: Ty<'tcx>, return_ty_span: Span, span: Span, attr: &Attribute) -> Body<'tcx>