1 //! Code for type-checking closure expressions.
3 use super::{check_fn, Expectation, FnCtxt, GeneratorTypes};
7 use rustc_hir::def_id::DefId;
8 use rustc_hir::lang_items::LangItem;
9 use rustc_hir_analysis::astconv::AstConv;
10 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
11 use rustc_infer::infer::LateBoundRegionConversionTime;
12 use rustc_infer::infer::{InferOk, InferResult};
13 use rustc_middle::ty::subst::InternalSubsts;
14 use rustc_middle::ty::visit::TypeVisitable;
15 use rustc_middle::ty::{self, Ty};
16 use rustc_span::source_map::Span;
17 use rustc_target::spec::abi::Abi;
18 use rustc_trait_selection::traits::error_reporting::ArgKind;
19 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
23 /// What signature do we *expect* the closure to have from context?
25 struct ExpectedSig<'tcx> {
26 /// Span that gave us this expectation, if we know that.
27 cause_span: Option<Span>,
28 sig: ty::PolyFnSig<'tcx>,
31 struct ClosureSignatures<'tcx> {
32 /// The signature users of the closure see.
33 bound_sig: ty::PolyFnSig<'tcx>,
34 /// The signature within the function body.
35 /// This mostly differs in the sense that lifetimes are now early bound and any
36 /// opaque types from the signature expectation are overriden in case there are
37 /// explicit hidden types written by the user in the closure signature.
38 liberated_sig: ty::FnSig<'tcx>,
41 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
42 #[instrument(skip(self, expr, _capture, decl, body_id), level = "debug")]
43 pub fn check_expr_closure(
46 _capture: hir::CaptureBy,
47 decl: &'tcx hir::FnDecl<'tcx>,
49 gen: Option<hir::Movability>,
50 expected: Expectation<'tcx>,
52 trace!("decl = {:#?}", decl);
53 trace!("expr = {:#?}", expr);
55 // It's always helpful for inference if we know the kind of
56 // closure sooner rather than later, so first examine the expected
57 // type, and see if can glean a closure kind from there.
58 let (expected_sig, expected_kind) = match expected.to_option(self) {
59 Some(ty) => self.deduce_expectations_from_expected_type(ty),
62 let body = self.tcx.hir().body(body_id);
63 self.check_closure(expr, expected_kind, decl, body, gen, expected_sig)
66 #[instrument(skip(self, expr, body, decl), level = "debug", ret)]
70 opt_kind: Option<ty::ClosureKind>,
71 decl: &'tcx hir::FnDecl<'tcx>,
72 body: &'tcx hir::Body<'tcx>,
73 gen: Option<hir::Movability>,
74 expected_sig: Option<ExpectedSig<'tcx>>,
76 trace!("decl = {:#?}", decl);
77 let expr_def_id = self.tcx.hir().local_def_id(expr.hir_id);
80 let ClosureSignatures { bound_sig, liberated_sig } =
81 self.sig_of_closure(expr.hir_id, expr_def_id.to_def_id(), decl, body, expected_sig);
83 debug!(?bound_sig, ?liberated_sig);
85 let return_type_pre_known = !liberated_sig.output().is_ty_infer();
87 let generator_types = check_fn(
89 self.param_env.without_const(),
95 return_type_pre_known,
99 let parent_substs = InternalSubsts::identity_for_item(
101 self.tcx.typeck_root_def_id(expr_def_id.to_def_id()),
104 let tupled_upvars_ty = self.next_ty_var(TypeVariableOrigin {
105 kind: TypeVariableOriginKind::ClosureSynthetic,
106 span: self.tcx.hir().span(expr.hir_id),
109 if let Some(GeneratorTypes { resume_ty, yield_ty, interior, movability }) = generator_types
111 let generator_substs = ty::GeneratorSubsts::new(
113 ty::GeneratorSubstsParts {
117 return_ty: liberated_sig.output(),
123 return self.tcx.mk_generator(
124 expr_def_id.to_def_id(),
125 generator_substs.substs,
130 // Tuple up the arguments and insert the resulting function type into
131 // the `closures` table.
132 let sig = bound_sig.map_bound(|sig| {
134 iter::once(self.tcx.intern_tup(sig.inputs())),
142 debug!(?sig, ?opt_kind);
144 let closure_kind_ty = match opt_kind {
145 Some(kind) => kind.to_ty(self.tcx),
147 // Create a type variable (for now) to represent the closure kind.
148 // It will be unified during the upvar inference phase (`upvar.rs`)
149 None => self.next_ty_var(TypeVariableOrigin {
150 // FIXME(eddyb) distinguish closure kind inference variables from the rest.
151 kind: TypeVariableOriginKind::ClosureSynthetic,
156 let closure_substs = ty::ClosureSubsts::new(
158 ty::ClosureSubstsParts {
161 closure_sig_as_fn_ptr_ty: self.tcx.mk_fn_ptr(sig),
166 self.tcx.mk_closure(expr_def_id.to_def_id(), closure_substs.substs)
169 /// Given the expected type, figures out what it can about this closure we
170 /// are about to type check:
171 #[instrument(skip(self), level = "debug")]
172 fn deduce_expectations_from_expected_type(
174 expected_ty: Ty<'tcx>,
175 ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
176 match *expected_ty.kind() {
177 ty::Opaque(def_id, substs) => {
178 let bounds = self.tcx.bound_explicit_item_bounds(def_id);
181 .map(|e| e.map_bound(|e| *e).transpose_tuple2())
182 .find_map(|(pred, span)| match pred.0.kind().skip_binder() {
183 ty::PredicateKind::Projection(proj_predicate) => self
184 .deduce_sig_from_projection(
188 .rebind(pred.rebind(proj_predicate).subst(self.tcx, substs)),
195 .map(|e| e.map_bound(|e| *e).transpose_tuple2())
196 .filter_map(|(pred, _)| match pred.0.kind().skip_binder() {
197 ty::PredicateKind::Trait(tp) => {
198 self.tcx.fn_trait_kind_from_lang_item(tp.def_id())
202 .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
206 ty::Dynamic(ref object_type, ..) => {
207 let sig = object_type.projection_bounds().find_map(|pb| {
208 let pb = pb.with_self_ty(self.tcx, self.tcx.types.trait_object_dummy_self);
209 self.deduce_sig_from_projection(None, pb)
211 let kind = object_type
213 .and_then(|did| self.tcx.fn_trait_kind_from_lang_item(did));
216 ty::Infer(ty::TyVar(vid)) => self.deduce_expectations_from_obligations(vid),
218 let expected_sig = ExpectedSig { cause_span: None, sig };
219 (Some(expected_sig), Some(ty::ClosureKind::Fn))
225 fn deduce_expectations_from_obligations(
227 expected_vid: ty::TyVid,
228 ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
230 self.obligations_for_self_ty(expected_vid).find_map(|(_, obligation)| {
231 debug!(?obligation.predicate);
233 let bound_predicate = obligation.predicate.kind();
234 if let ty::PredicateKind::Projection(proj_predicate) =
235 obligation.predicate.kind().skip_binder()
237 // Given a Projection predicate, we can potentially infer
238 // the complete signature.
239 self.deduce_sig_from_projection(
240 Some(obligation.cause.span),
241 bound_predicate.rebind(proj_predicate),
248 // Even if we can't infer the full signature, we may be able to
249 // infer the kind. This can occur when we elaborate a predicate
250 // like `F : Fn<A>`. Note that due to subtyping we could encounter
251 // many viable options, so pick the most restrictive.
252 let expected_kind = self
253 .obligations_for_self_ty(expected_vid)
254 .filter_map(|(tr, _)| self.tcx.fn_trait_kind_from_lang_item(tr.def_id()))
255 .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
257 (expected_sig, expected_kind)
260 /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
261 /// everything we need to know about a closure or generator.
263 /// The `cause_span` should be the span that caused us to
264 /// have this expected signature, or `None` if we can't readily
266 #[instrument(level = "debug", skip(self, cause_span), ret)]
267 fn deduce_sig_from_projection(
269 cause_span: Option<Span>,
270 projection: ty::PolyProjectionPredicate<'tcx>,
271 ) -> Option<ExpectedSig<'tcx>> {
274 let trait_def_id = projection.trait_def_id(tcx);
276 let is_fn = tcx.fn_trait_kind_from_lang_item(trait_def_id).is_some();
277 let gen_trait = tcx.require_lang_item(LangItem::Generator, cause_span);
278 let is_gen = gen_trait == trait_def_id;
279 if !is_fn && !is_gen {
280 debug!("not fn or generator");
285 // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
286 // associated item and not yield.
287 let return_assoc_item = self.tcx.associated_item_def_ids(gen_trait)[1];
288 if return_assoc_item != projection.projection_def_id() {
289 debug!("not return assoc item of generator");
294 let input_tys = if is_fn {
295 let arg_param_ty = projection.skip_binder().projection_ty.substs.type_at(1);
296 let arg_param_ty = self.resolve_vars_if_possible(arg_param_ty);
297 debug!(?arg_param_ty);
299 match arg_param_ty.kind() {
300 &ty::Tuple(tys) => tys,
304 // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
305 // else they must have exactly 1 argument. For now though, just give up in this case.
309 // Since this is a return parameter type it is safe to unwrap.
310 let ret_param_ty = projection.skip_binder().term.ty().unwrap();
311 let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
312 debug!(?ret_param_ty);
314 let sig = projection.rebind(self.tcx.mk_fn_sig(
318 hir::Unsafety::Normal,
322 Some(ExpectedSig { cause_span, sig })
329 decl: &hir::FnDecl<'_>,
330 body: &hir::Body<'_>,
331 expected_sig: Option<ExpectedSig<'tcx>>,
332 ) -> ClosureSignatures<'tcx> {
333 if let Some(e) = expected_sig {
334 self.sig_of_closure_with_expectation(hir_id, expr_def_id, decl, body, e)
336 self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body)
340 /// If there is no expected signature, then we will convert the
341 /// types that the user gave into a signature.
342 #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
343 fn sig_of_closure_no_expectation(
347 decl: &hir::FnDecl<'_>,
348 body: &hir::Body<'_>,
349 ) -> ClosureSignatures<'tcx> {
350 let bound_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
352 self.closure_sigs(expr_def_id, body, bound_sig)
355 /// Invoked to compute the signature of a closure expression. This
356 /// combines any user-provided type annotations (e.g., `|x: u32|
357 /// -> u32 { .. }`) with the expected signature.
359 /// The approach is as follows:
361 /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
362 /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
363 /// - If we have no expectation `E`, then the signature of the closure is `S`.
364 /// - Otherwise, the signature of the closure is E. Moreover:
365 /// - Skolemize the late-bound regions in `E`, yielding `E'`.
366 /// - Instantiate all the late-bound regions bound in the closure within `S`
367 /// with fresh (existential) variables, yielding `S'`
368 /// - Require that `E' = S'`
369 /// - We could use some kind of subtyping relationship here,
370 /// I imagine, but equality is easier and works fine for
373 /// The key intuition here is that the user's types must be valid
374 /// from "the inside" of the closure, but the expectation
375 /// ultimately drives the overall signature.
379 /// ```ignore (illustrative)
380 /// fn with_closure<F>(_: F)
381 /// where F: Fn(&u32) -> &u32 { .. }
383 /// with_closure(|x: &u32| { ... })
387 /// - E would be `fn(&u32) -> &u32`.
388 /// - S would be `fn(&u32) ->
389 /// - E' is `&'!0 u32 -> &'!0 u32`
390 /// - S' is `&'?0 u32 -> ?T`
392 /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
396 /// - `expr_def_id`: the `DefId` of the closure expression
397 /// - `decl`: the HIR declaration of the closure
398 /// - `body`: the body of the closure
399 /// - `expected_sig`: the expected signature (if any). Note that
400 /// this is missing a binder: that is, there may be late-bound
401 /// regions with depth 1, which are bound then by the closure.
402 #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
403 fn sig_of_closure_with_expectation(
407 decl: &hir::FnDecl<'_>,
408 body: &hir::Body<'_>,
409 expected_sig: ExpectedSig<'tcx>,
410 ) -> ClosureSignatures<'tcx> {
411 // Watch out for some surprises and just ignore the
412 // expectation if things don't see to match up with what we
414 if expected_sig.sig.c_variadic() != decl.c_variadic {
415 return self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body);
416 } else if expected_sig.sig.skip_binder().inputs_and_output.len() != decl.inputs.len() + 1 {
417 return self.sig_of_closure_with_mismatched_number_of_arguments(
425 // Create a `PolyFnSig`. Note the oddity that late bound
426 // regions appearing free in `expected_sig` are now bound up
427 // in this binder we are creating.
428 assert!(!expected_sig.sig.skip_binder().has_vars_bound_above(ty::INNERMOST));
429 let bound_sig = expected_sig.sig.map_bound(|sig| {
431 sig.inputs().iter().cloned(),
434 hir::Unsafety::Normal,
439 // `deduce_expectations_from_expected_type` introduces
440 // late-bound lifetimes defined elsewhere, which we now
441 // anonymize away, so as not to confuse the user.
442 let bound_sig = self.tcx.anonymize_late_bound_regions(bound_sig);
444 let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
446 // Up till this point, we have ignored the annotations that the user
447 // gave. This function will check that they unify successfully.
448 // Along the way, it also writes out entries for types that the user
449 // wrote into our typeck results, which are then later used by the privacy
451 match self.merge_supplied_sig_with_expectation(
458 Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
459 Err(_) => self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body),
463 fn sig_of_closure_with_mismatched_number_of_arguments(
466 decl: &hir::FnDecl<'_>,
467 body: &hir::Body<'_>,
468 expected_sig: ExpectedSig<'tcx>,
469 ) -> ClosureSignatures<'tcx> {
470 let hir = self.tcx.hir();
471 let expr_map_node = hir.get_if_local(expr_def_id).unwrap();
472 let expected_args: Vec<_> = expected_sig
477 .map(|ty| ArgKind::from_expected_ty(*ty, None))
479 let (closure_span, found_args) = match self.get_fn_like_arguments(expr_map_node) {
480 Some((sp, args)) => (Some(sp), args),
481 None => (None, Vec::new()),
484 expected_sig.cause_span.unwrap_or_else(|| hir.span_if_local(expr_def_id).unwrap());
485 self.report_arg_count_mismatch(
494 let error_sig = self.error_sig_of_closure(decl);
496 self.closure_sigs(expr_def_id, body, error_sig)
499 /// Enforce the user's types against the expectation. See
500 /// `sig_of_closure_with_expectation` for details on the overall
502 #[instrument(level = "debug", skip(self, hir_id, expr_def_id, decl, body, expected_sigs))]
503 fn merge_supplied_sig_with_expectation(
507 decl: &hir::FnDecl<'_>,
508 body: &hir::Body<'_>,
509 mut expected_sigs: ClosureSignatures<'tcx>,
510 ) -> InferResult<'tcx, ClosureSignatures<'tcx>> {
511 // Get the signature S that the user gave.
513 // (See comment on `sig_of_closure_with_expectation` for the
514 // meaning of these letters.)
515 let supplied_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
517 debug!(?supplied_sig);
519 // FIXME(#45727): As discussed in [this comment][c1], naively
520 // forcing equality here actually results in suboptimal error
521 // messages in some cases. For now, if there would have been
522 // an obvious error, we fallback to declaring the type of the
523 // closure to be the one the user gave, which allows other
524 // error message code to trigger.
526 // However, I think [there is potential to do even better
527 // here][c2], since in *this* code we have the precise span of
528 // the type parameter in question in hand when we report the
531 // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
532 // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
533 self.commit_if_ok(|_| {
534 let mut all_obligations = vec![];
535 let inputs: Vec<_> = iter::zip(
537 supplied_sig.inputs().skip_binder(), // binder moved to (*) below
539 .map(|(hir_ty, &supplied_ty)| {
540 // Instantiate (this part of..) S to S', i.e., with fresh variables.
541 self.replace_bound_vars_with_fresh_vars(
543 LateBoundRegionConversionTime::FnCall,
544 // (*) binder moved to here
545 supplied_sig.inputs().rebind(supplied_ty),
550 // The liberated version of this signature should be a subtype
551 // of the liberated form of the expectation.
552 for ((hir_ty, &supplied_ty), expected_ty) in iter::zip(
553 iter::zip(decl.inputs, &inputs),
554 expected_sigs.liberated_sig.inputs(), // `liberated_sig` is E'.
556 // Check that E' = S'.
557 let cause = self.misc(hir_ty.span);
558 let InferOk { value: (), obligations } =
559 self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
560 all_obligations.extend(obligations);
563 let supplied_output_ty = self.replace_bound_vars_with_fresh_vars(
565 LateBoundRegionConversionTime::FnCall,
566 supplied_sig.output(),
568 let cause = &self.misc(decl.output.span());
569 let InferOk { value: (), obligations } = self
570 .at(cause, self.param_env)
571 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
572 all_obligations.extend(obligations);
574 let inputs = inputs.into_iter().map(|ty| self.resolve_vars_if_possible(ty));
576 expected_sigs.liberated_sig = self.tcx.mk_fn_sig(
579 expected_sigs.liberated_sig.c_variadic,
580 hir::Unsafety::Normal,
584 Ok(InferOk { value: expected_sigs, obligations: all_obligations })
588 /// If there is no expected signature, then we will convert the
589 /// types that the user gave into a signature.
591 /// Also, record this closure signature for later.
592 #[instrument(skip(self, decl, body), level = "debug", ret)]
593 fn supplied_sig_of_closure(
597 decl: &hir::FnDecl<'_>,
598 body: &hir::Body<'_>,
599 ) -> ty::PolyFnSig<'tcx> {
600 let astconv: &dyn AstConv<'_> = self;
602 trace!("decl = {:#?}", decl);
603 debug!(?body.generator_kind);
605 let bound_vars = self.tcx.late_bound_vars(hir_id);
607 // First, convert the types that the user supplied (if any).
608 let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
609 let supplied_return = match decl.output {
610 hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
611 hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
612 // In the case of the async block that we create for a function body,
613 // we expect the return type of the block to match that of the enclosing
615 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
616 debug!("closure is async fn body");
617 self.deduce_future_output_from_obligations(expr_def_id, body.id().hir_id)
619 // AFAIK, deducing the future output
620 // always succeeds *except* in error cases
621 // like #65159. I'd like to return Error
622 // here, but I can't because I can't
623 // easily (and locally) prove that we
624 // *have* reported an
625 // error. --nikomatsakis
626 astconv.ty_infer(None, decl.output.span())
630 _ => astconv.ty_infer(None, decl.output.span()),
634 let result = ty::Binder::bind_with_vars(
639 hir::Unsafety::Normal,
644 // Astconv can't normalize inputs or outputs with escaping bound vars,
645 // so normalize them here, after we've wrapped them in a binder.
646 let result = self.normalize_associated_types_in(self.tcx.hir().span(hir_id), result);
648 let c_result = self.inh.infcx.canonicalize_response(result);
649 self.typeck_results.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);
654 /// Invoked when we are translating the generator that results
655 /// from desugaring an `async fn`. Returns the "sugared" return
656 /// type of the `async fn` -- that is, the return type that the
657 /// user specified. The "desugared" return type is an `impl
658 /// Future<Output = T>`, so we do this by searching through the
659 /// obligations to extract the `T`.
660 #[instrument(skip(self), level = "debug", ret)]
661 fn deduce_future_output_from_obligations(
665 ) -> Option<Ty<'tcx>> {
666 let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
667 span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
670 let ret_ty = ret_coercion.borrow().expected_ty();
671 let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
673 let get_future_output = |predicate: ty::Predicate<'tcx>, span| {
674 // Search for a pending obligation like
676 // `<R as Future>::Output = T`
678 // where R is the return type we are expecting. This type `T`
679 // will be our output.
680 let bound_predicate = predicate.kind();
681 if let ty::PredicateKind::Projection(proj_predicate) = bound_predicate.skip_binder() {
682 self.deduce_future_output_from_projection(
684 bound_predicate.rebind(proj_predicate),
691 let output_ty = match *ret_ty.kind() {
692 ty::Infer(ty::TyVar(ret_vid)) => {
693 self.obligations_for_self_ty(ret_vid).find_map(|(_, obligation)| {
694 get_future_output(obligation.predicate, obligation.cause.span)
697 ty::Opaque(def_id, substs) => self
699 .bound_explicit_item_bounds(def_id)
701 .map(|e| e.map_bound(|e| *e).transpose_tuple2())
702 .find_map(|(p, s)| get_future_output(p.subst(self.tcx, substs), s.0))?,
703 ty::Error(_) => return None,
705 if self.tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder =>
708 .bound_explicit_item_bounds(proj.item_def_id)
710 .map(|e| e.map_bound(|e| *e).transpose_tuple2())
711 .find_map(|(p, s)| get_future_output(p.subst(self.tcx, proj.substs), s.0))?
714 self.tcx.def_span(expr_def_id),
715 "async fn generator return type not an inference variable: {ret_ty}"
719 // async fn that have opaque types in their return type need to redo the conversion to inference variables
720 // as they fetch the still opaque version from the signature.
721 let InferOk { value: output_ty, obligations } = self
722 .replace_opaque_types_with_inference_vars(
725 self.tcx.def_span(expr_def_id),
728 self.register_predicates(obligations);
733 /// Given a projection like
735 /// `<X as Future>::Output = T`
737 /// where `X` is some type that has no late-bound regions, returns
738 /// `Some(T)`. If the projection is for some other trait, returns
740 fn deduce_future_output_from_projection(
743 predicate: ty::PolyProjectionPredicate<'tcx>,
744 ) -> Option<Ty<'tcx>> {
745 debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
747 // We do not expect any bound regions in our predicate, so
748 // skip past the bound vars.
749 let Some(predicate) = predicate.no_bound_vars() else {
750 debug!("deduce_future_output_from_projection: has late-bound regions");
754 // Check that this is a projection from the `Future` trait.
755 let trait_def_id = predicate.projection_ty.trait_def_id(self.tcx);
756 let future_trait = self.tcx.require_lang_item(LangItem::Future, Some(cause_span));
757 if trait_def_id != future_trait {
758 debug!("deduce_future_output_from_projection: not a future");
762 // The `Future` trait has only one associated item, `Output`,
763 // so check that this is what we see.
764 let output_assoc_item = self.tcx.associated_item_def_ids(future_trait)[0];
765 if output_assoc_item != predicate.projection_ty.item_def_id {
768 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
769 predicate.projection_ty.item_def_id,
774 // Extract the type from the projection. Note that there can
775 // be no bound variables in this type because the "self type"
776 // does not have any regions in it.
777 let output_ty = self.resolve_vars_if_possible(predicate.term);
778 debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
779 // This is a projection on a Fn trait so will always be a type.
780 Some(output_ty.ty().unwrap())
783 /// Converts the types that the user supplied, in case that doing
784 /// so should yield an error, but returns back a signature where
785 /// all parameters are of type `TyErr`.
786 fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
787 let astconv: &dyn AstConv<'_> = self;
789 let supplied_arguments = decl.inputs.iter().map(|a| {
790 // Convert the types that the user supplied (if any), but ignore them.
791 astconv.ast_ty_to_ty(a);
795 if let hir::FnRetTy::Return(ref output) = decl.output {
796 astconv.ast_ty_to_ty(&output);
799 let result = ty::Binder::dummy(self.tcx.mk_fn_sig(
803 hir::Unsafety::Normal,
807 debug!("supplied_sig_of_closure: result={:?}", result);
815 body: &hir::Body<'_>,
816 bound_sig: ty::PolyFnSig<'tcx>,
817 ) -> ClosureSignatures<'tcx> {
818 let liberated_sig = self.tcx().liberate_late_bound_regions(expr_def_id, bound_sig);
819 let liberated_sig = self.inh.normalize_associated_types_in(
825 ClosureSignatures { bound_sig, liberated_sig }