compiler/rustc_hir_typeck/src/closure.rs

   1 //! Code for type-checking closure expressions.
   2
   3 use super::{check_fn, Expectation, FnCtxt, GeneratorTypes};
   4
   5 use hir::def::DefKind;
   6 use rustc_hir as hir;
   7 use rustc_hir::def_id::LocalDefId;
   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_macros::{TypeFoldable, TypeVisitable};
  14 use rustc_middle::ty::subst::InternalSubsts;
  15 use rustc_middle::ty::visit::TypeVisitable;
  16 use rustc_middle::ty::{self, Ty};
  17 use rustc_span::source_map::Span;
  18 use rustc_target::spec::abi::Abi;
  19 use rustc_trait_selection::traits;
  20 use rustc_trait_selection::traits::error_reporting::ArgKind;
  21 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
  22 use std::cmp;
  23 use std::iter;
  24
  25 /// What signature do we *expect* the closure to have from context?
  26 #[derive(Debug, Clone, TypeFoldable, TypeVisitable)]
  27 struct ExpectedSig<'tcx> {
  28     /// Span that gave us this expectation, if we know that.
  29     cause_span: Option<Span>,
  30     sig: ty::PolyFnSig<'tcx>,
  31 }
  32
  33 struct ClosureSignatures<'tcx> {
  34     /// The signature users of the closure see.
  35     bound_sig: ty::PolyFnSig<'tcx>,
  36     /// The signature within the function body.
  37     /// This mostly differs in the sense that lifetimes are now early bound and any
  38     /// opaque types from the signature expectation are overridden in case there are
  39     /// explicit hidden types written by the user in the closure signature.
  40     liberated_sig: ty::FnSig<'tcx>,
  41 }
  42
  43 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  44     #[instrument(skip(self, closure), level = "debug")]
  45     pub fn check_expr_closure(
  46         &self,
  47         closure: &hir::Closure<'tcx>,
  48         expr_span: Span,
  49         expected: Expectation<'tcx>,
  50     ) -> Ty<'tcx> {
  51         trace!("decl = {:#?}", closure.fn_decl);
  52
  53         // It's always helpful for inference if we know the kind of
  54         // closure sooner rather than later, so first examine the expected
  55         // type, and see if can glean a closure kind from there.
  56         let (expected_sig, expected_kind) = match expected.to_option(self) {
  57             Some(ty) => self.deduce_expectations_from_expected_type(ty),
  58             None => (None, None),
  59         };
  60         let body = self.tcx.hir().body(closure.body);
  61         self.check_closure(closure, expr_span, expected_kind, body, expected_sig)
  62     }
  63
  64     #[instrument(skip(self, closure, body), level = "debug", ret)]
  65     fn check_closure(
  66         &self,
  67         closure: &hir::Closure<'tcx>,
  68         expr_span: Span,
  69         opt_kind: Option<ty::ClosureKind>,
  70         body: &'tcx hir::Body<'tcx>,
  71         expected_sig: Option<ExpectedSig<'tcx>>,
  72     ) -> Ty<'tcx> {
  73         trace!("decl = {:#?}", closure.fn_decl);
  74         let expr_def_id = closure.def_id;
  75         debug!(?expr_def_id);
  76
  77         let ClosureSignatures { bound_sig, liberated_sig } =
  78             self.sig_of_closure(expr_def_id, closure.fn_decl, body, expected_sig);
  79
  80         debug!(?bound_sig, ?liberated_sig);
  81
  82         let mut fcx = FnCtxt::new(self, self.param_env.without_const(), body.value.hir_id);
  83         let generator_types = check_fn(
  84             &mut fcx,
  85             liberated_sig,
  86             closure.fn_decl,
  87             expr_def_id,
  88             body,
  89             closure.movability,
  90         );
  91
  92         let parent_substs = InternalSubsts::identity_for_item(
  93             self.tcx,
  94             self.tcx.typeck_root_def_id(expr_def_id.to_def_id()),
  95         );
  96
  97         let tupled_upvars_ty = self.next_ty_var(TypeVariableOrigin {
  98             kind: TypeVariableOriginKind::ClosureSynthetic,
  99             span: self.tcx.def_span(expr_def_id),
 100         });
 101
 102         if let Some(GeneratorTypes { resume_ty, yield_ty, interior, movability }) = generator_types
 103         {
 104             let generator_substs = ty::GeneratorSubsts::new(
 105                 self.tcx,
 106                 ty::GeneratorSubstsParts {
 107                     parent_substs,
 108                     resume_ty,
 109                     yield_ty,
 110                     return_ty: liberated_sig.output(),
 111                     witness: interior,
 112                     tupled_upvars_ty,
 113                 },
 114             );
 115
 116             return self.tcx.mk_generator(
 117                 expr_def_id.to_def_id(),
 118                 generator_substs.substs,
 119                 movability,
 120             );
 121         }
 122
 123         // Tuple up the arguments and insert the resulting function type into
 124         // the `closures` table.
 125         let sig = bound_sig.map_bound(|sig| {
 126             self.tcx.mk_fn_sig(
 127                 iter::once(self.tcx.intern_tup(sig.inputs())),
 128                 sig.output(),
 129                 sig.c_variadic,
 130                 sig.unsafety,
 131                 sig.abi,
 132             )
 133         });
 134
 135         debug!(?sig, ?opt_kind);
 136
 137         let closure_kind_ty = match opt_kind {
 138             Some(kind) => kind.to_ty(self.tcx),
 139
 140             // Create a type variable (for now) to represent the closure kind.
 141             // It will be unified during the upvar inference phase (`upvar.rs`)
 142             None => self.next_ty_var(TypeVariableOrigin {
 143                 // FIXME(eddyb) distinguish closure kind inference variables from the rest.
 144                 kind: TypeVariableOriginKind::ClosureSynthetic,
 145                 span: expr_span,
 146             }),
 147         };
 148
 149         let closure_substs = ty::ClosureSubsts::new(
 150             self.tcx,
 151             ty::ClosureSubstsParts {
 152                 parent_substs,
 153                 closure_kind_ty,
 154                 closure_sig_as_fn_ptr_ty: self.tcx.mk_fn_ptr(sig),
 155                 tupled_upvars_ty,
 156             },
 157         );
 158
 159         self.tcx.mk_closure(expr_def_id.to_def_id(), closure_substs.substs)
 160     }
 161
 162     /// Given the expected type, figures out what it can about this closure we
 163     /// are about to type check:
 164     #[instrument(skip(self), level = "debug")]
 165     fn deduce_expectations_from_expected_type(
 166         &self,
 167         expected_ty: Ty<'tcx>,
 168     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 169         match *expected_ty.kind() {
 170             ty::Opaque(def_id, substs) => self.deduce_signature_from_predicates(
 171                 self.tcx.bound_explicit_item_bounds(def_id).subst_iter_copied(self.tcx, substs),
 172             ),
 173             ty::Dynamic(ref object_type, ..) => {
 174                 let sig = object_type.projection_bounds().find_map(|pb| {
 175                     let pb = pb.with_self_ty(self.tcx, self.tcx.types.trait_object_dummy_self);
 176                     self.deduce_sig_from_projection(None, pb)
 177                 });
 178                 let kind = object_type
 179                     .principal_def_id()
 180                     .and_then(|did| self.tcx.fn_trait_kind_from_def_id(did));
 181                 (sig, kind)
 182             }
 183             ty::Infer(ty::TyVar(vid)) => self.deduce_signature_from_predicates(
 184                 self.obligations_for_self_ty(vid).map(|obl| (obl.predicate, obl.cause.span)),
 185             ),
 186             ty::FnPtr(sig) => {
 187                 let expected_sig = ExpectedSig { cause_span: None, sig };
 188                 (Some(expected_sig), Some(ty::ClosureKind::Fn))
 189             }
 190             _ => (None, None),
 191         }
 192     }
 193
 194     fn deduce_signature_from_predicates(
 195         &self,
 196         predicates: impl DoubleEndedIterator<Item = (ty::Predicate<'tcx>, Span)>,
 197     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 198         let mut expected_sig = None;
 199         let mut expected_kind = None;
 200
 201         for obligation in traits::elaborate_predicates_with_span(
 202             self.tcx,
 203             // Reverse the obligations here, since `elaborate_*` uses a stack,
 204             // and we want to keep inference generally in the same order of
 205             // the registered obligations.
 206             predicates.rev(),
 207         ) {
 208             debug!(?obligation.predicate);
 209             let bound_predicate = obligation.predicate.kind();
 210
 211             // Given a Projection predicate, we can potentially infer
 212             // the complete signature.
 213             if expected_sig.is_none()
 214                 && let ty::PredicateKind::Clause(ty::Clause::Projection(proj_predicate)) = bound_predicate.skip_binder()
 215             {
 216                 expected_sig = self.normalize(
 217                     obligation.cause.span,
 218                     self.deduce_sig_from_projection(
 219                     Some(obligation.cause.span),
 220                         bound_predicate.rebind(proj_predicate),
 221                     ),
 222                 );
 223             }
 224
 225             // Even if we can't infer the full signature, we may be able to
 226             // infer the kind. This can occur when we elaborate a predicate
 227             // like `F : Fn<A>`. Note that due to subtyping we could encounter
 228             // many viable options, so pick the most restrictive.
 229             let trait_def_id = match bound_predicate.skip_binder() {
 230                 ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
 231                     Some(data.projection_ty.trait_def_id(self.tcx))
 232                 }
 233                 ty::PredicateKind::Clause(ty::Clause::Trait(data)) => Some(data.def_id()),
 234                 _ => None,
 235             };
 236             if let Some(closure_kind) =
 237                 trait_def_id.and_then(|def_id| self.tcx.fn_trait_kind_from_def_id(def_id))
 238             {
 239                 expected_kind = Some(
 240                     expected_kind
 241                         .map_or_else(|| closure_kind, |current| cmp::min(current, closure_kind)),
 242                 );
 243             }
 244         }
 245
 246         (expected_sig, expected_kind)
 247     }
 248
 249     /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
 250     /// everything we need to know about a closure or generator.
 251     ///
 252     /// The `cause_span` should be the span that caused us to
 253     /// have this expected signature, or `None` if we can't readily
 254     /// know that.
 255     #[instrument(level = "debug", skip(self, cause_span), ret)]
 256     fn deduce_sig_from_projection(
 257         &self,
 258         cause_span: Option<Span>,
 259         projection: ty::PolyProjectionPredicate<'tcx>,
 260     ) -> Option<ExpectedSig<'tcx>> {
 261         let tcx = self.tcx;
 262
 263         let trait_def_id = projection.trait_def_id(tcx);
 264
 265         let is_fn = tcx.is_fn_trait(trait_def_id);
 266         let gen_trait = tcx.require_lang_item(LangItem::Generator, cause_span);
 267         let is_gen = gen_trait == trait_def_id;
 268         if !is_fn && !is_gen {
 269             debug!("not fn or generator");
 270             return None;
 271         }
 272
 273         if is_gen {
 274             // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
 275             // associated item and not yield.
 276             let return_assoc_item = self.tcx.associated_item_def_ids(gen_trait)[1];
 277             if return_assoc_item != projection.projection_def_id() {
 278                 debug!("not return assoc item of generator");
 279                 return None;
 280             }
 281         }
 282
 283         let input_tys = if is_fn {
 284             let arg_param_ty = projection.skip_binder().projection_ty.substs.type_at(1);
 285             let arg_param_ty = self.resolve_vars_if_possible(arg_param_ty);
 286             debug!(?arg_param_ty);
 287
 288             match arg_param_ty.kind() {
 289                 &ty::Tuple(tys) => tys,
 290                 _ => return None,
 291             }
 292         } else {
 293             // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
 294             // else they must have exactly 1 argument. For now though, just give up in this case.
 295             return None;
 296         };
 297
 298         // Since this is a return parameter type it is safe to unwrap.
 299         let ret_param_ty = projection.skip_binder().term.ty().unwrap();
 300         let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
 301         debug!(?ret_param_ty);
 302
 303         let sig = projection.rebind(self.tcx.mk_fn_sig(
 304             input_tys.iter(),
 305             ret_param_ty,
 306             false,
 307             hir::Unsafety::Normal,
 308             Abi::Rust,
 309         ));
 310
 311         Some(ExpectedSig { cause_span, sig })
 312     }
 313
 314     fn sig_of_closure(
 315         &self,
 316         expr_def_id: LocalDefId,
 317         decl: &hir::FnDecl<'_>,
 318         body: &hir::Body<'_>,
 319         expected_sig: Option<ExpectedSig<'tcx>>,
 320     ) -> ClosureSignatures<'tcx> {
 321         if let Some(e) = expected_sig {
 322             self.sig_of_closure_with_expectation(expr_def_id, decl, body, e)
 323         } else {
 324             self.sig_of_closure_no_expectation(expr_def_id, decl, body)
 325         }
 326     }
 327
 328     /// If there is no expected signature, then we will convert the
 329     /// types that the user gave into a signature.
 330     #[instrument(skip(self, expr_def_id, decl, body), level = "debug")]
 331     fn sig_of_closure_no_expectation(
 332         &self,
 333         expr_def_id: LocalDefId,
 334         decl: &hir::FnDecl<'_>,
 335         body: &hir::Body<'_>,
 336     ) -> ClosureSignatures<'tcx> {
 337         let bound_sig = self.supplied_sig_of_closure(expr_def_id, decl, body);
 338
 339         self.closure_sigs(expr_def_id, body, bound_sig)
 340     }
 341
 342     /// Invoked to compute the signature of a closure expression. This
 343     /// combines any user-provided type annotations (e.g., `|x: u32|
 344     /// -> u32 { .. }`) with the expected signature.
 345     ///
 346     /// The approach is as follows:
 347     ///
 348     /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
 349     /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
 350     ///   - If we have no expectation `E`, then the signature of the closure is `S`.
 351     ///   - Otherwise, the signature of the closure is E. Moreover:
 352     ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
 353     ///     - Instantiate all the late-bound regions bound in the closure within `S`
 354     ///       with fresh (existential) variables, yielding `S'`
 355     ///     - Require that `E' = S'`
 356     ///       - We could use some kind of subtyping relationship here,
 357     ///         I imagine, but equality is easier and works fine for
 358     ///         our purposes.
 359     ///
 360     /// The key intuition here is that the user's types must be valid
 361     /// from "the inside" of the closure, but the expectation
 362     /// ultimately drives the overall signature.
 363     ///
 364     /// # Examples
 365     ///
 366     /// ```ignore (illustrative)
 367     /// fn with_closure<F>(_: F)
 368     ///   where F: Fn(&u32) -> &u32 { .. }
 369     ///
 370     /// with_closure(|x: &u32| { ... })
 371     /// ```
 372     ///
 373     /// Here:
 374     /// - E would be `fn(&u32) -> &u32`.
 375     /// - S would be `fn(&u32) ->
 376     /// - E' is `&'!0 u32 -> &'!0 u32`
 377     /// - S' is `&'?0 u32 -> ?T`
 378     ///
 379     /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
 380     ///
 381     /// # Arguments
 382     ///
 383     /// - `expr_def_id`: the `LocalDefId` of the closure expression
 384     /// - `decl`: the HIR declaration of the closure
 385     /// - `body`: the body of the closure
 386     /// - `expected_sig`: the expected signature (if any). Note that
 387     ///   this is missing a binder: that is, there may be late-bound
 388     ///   regions with depth 1, which are bound then by the closure.
 389     #[instrument(skip(self, expr_def_id, decl, body), level = "debug")]
 390     fn sig_of_closure_with_expectation(
 391         &self,
 392         expr_def_id: LocalDefId,
 393         decl: &hir::FnDecl<'_>,
 394         body: &hir::Body<'_>,
 395         expected_sig: ExpectedSig<'tcx>,
 396     ) -> ClosureSignatures<'tcx> {
 397         // Watch out for some surprises and just ignore the
 398         // expectation if things don't see to match up with what we
 399         // expect.
 400         if expected_sig.sig.c_variadic() != decl.c_variadic {
 401             return self.sig_of_closure_no_expectation(expr_def_id, decl, body);
 402         } else if expected_sig.sig.skip_binder().inputs_and_output.len() != decl.inputs.len() + 1 {
 403             return self.sig_of_closure_with_mismatched_number_of_arguments(
 404                 expr_def_id,
 405                 decl,
 406                 body,
 407                 expected_sig,
 408             );
 409         }
 410
 411         // Create a `PolyFnSig`. Note the oddity that late bound
 412         // regions appearing free in `expected_sig` are now bound up
 413         // in this binder we are creating.
 414         assert!(!expected_sig.sig.skip_binder().has_vars_bound_above(ty::INNERMOST));
 415         let bound_sig = expected_sig.sig.map_bound(|sig| {
 416             self.tcx.mk_fn_sig(
 417                 sig.inputs().iter().cloned(),
 418                 sig.output(),
 419                 sig.c_variadic,
 420                 hir::Unsafety::Normal,
 421                 Abi::RustCall,
 422             )
 423         });
 424
 425         // `deduce_expectations_from_expected_type` introduces
 426         // late-bound lifetimes defined elsewhere, which we now
 427         // anonymize away, so as not to confuse the user.
 428         let bound_sig = self.tcx.anonymize_late_bound_regions(bound_sig);
 429
 430         let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
 431
 432         // Up till this point, we have ignored the annotations that the user
 433         // gave. This function will check that they unify successfully.
 434         // Along the way, it also writes out entries for types that the user
 435         // wrote into our typeck results, which are then later used by the privacy
 436         // check.
 437         match self.merge_supplied_sig_with_expectation(expr_def_id, decl, body, closure_sigs) {
 438             Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
 439             Err(_) => self.sig_of_closure_no_expectation(expr_def_id, decl, body),
 440         }
 441     }
 442
 443     fn sig_of_closure_with_mismatched_number_of_arguments(
 444         &self,
 445         expr_def_id: LocalDefId,
 446         decl: &hir::FnDecl<'_>,
 447         body: &hir::Body<'_>,
 448         expected_sig: ExpectedSig<'tcx>,
 449     ) -> ClosureSignatures<'tcx> {
 450         let hir = self.tcx.hir();
 451         let expr_map_node = hir.get_by_def_id(expr_def_id);
 452         let expected_args: Vec<_> = expected_sig
 453             .sig
 454             .skip_binder()
 455             .inputs()
 456             .iter()
 457             .map(|ty| ArgKind::from_expected_ty(*ty, None))
 458             .collect();
 459         let (closure_span, found_args) = match self.get_fn_like_arguments(expr_map_node) {
 460             Some((sp, args)) => (Some(sp), args),
 461             None => (None, Vec::new()),
 462         };
 463         let expected_span =
 464             expected_sig.cause_span.unwrap_or_else(|| self.tcx.def_span(expr_def_id));
 465         self.report_arg_count_mismatch(
 466             expected_span,
 467             closure_span,
 468             expected_args,
 469             found_args,
 470             true,
 471         )
 472         .emit();
 473
 474         let error_sig = self.error_sig_of_closure(decl);
 475
 476         self.closure_sigs(expr_def_id, body, error_sig)
 477     }
 478
 479     /// Enforce the user's types against the expectation. See
 480     /// `sig_of_closure_with_expectation` for details on the overall
 481     /// strategy.
 482     #[instrument(level = "debug", skip(self, expr_def_id, decl, body, expected_sigs))]
 483     fn merge_supplied_sig_with_expectation(
 484         &self,
 485         expr_def_id: LocalDefId,
 486         decl: &hir::FnDecl<'_>,
 487         body: &hir::Body<'_>,
 488         mut expected_sigs: ClosureSignatures<'tcx>,
 489     ) -> InferResult<'tcx, ClosureSignatures<'tcx>> {
 490         // Get the signature S that the user gave.
 491         //
 492         // (See comment on `sig_of_closure_with_expectation` for the
 493         // meaning of these letters.)
 494         let supplied_sig = self.supplied_sig_of_closure(expr_def_id, decl, body);
 495
 496         debug!(?supplied_sig);
 497
 498         // FIXME(#45727): As discussed in [this comment][c1], naively
 499         // forcing equality here actually results in suboptimal error
 500         // messages in some cases.  For now, if there would have been
 501         // an obvious error, we fallback to declaring the type of the
 502         // closure to be the one the user gave, which allows other
 503         // error message code to trigger.
 504         //
 505         // However, I think [there is potential to do even better
 506         // here][c2], since in *this* code we have the precise span of
 507         // the type parameter in question in hand when we report the
 508         // error.
 509         //
 510         // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
 511         // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
 512         self.commit_if_ok(|_| {
 513             let mut all_obligations = vec![];
 514             let inputs: Vec<_> = iter::zip(
 515                 decl.inputs,
 516                 supplied_sig.inputs().skip_binder(), // binder moved to (*) below
 517             )
 518             .map(|(hir_ty, &supplied_ty)| {
 519                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 520                 self.replace_bound_vars_with_fresh_vars(
 521                     hir_ty.span,
 522                     LateBoundRegionConversionTime::FnCall,
 523                     // (*) binder moved to here
 524                     supplied_sig.inputs().rebind(supplied_ty),
 525                 )
 526             })
 527             .collect();
 528
 529             // The liberated version of this signature should be a subtype
 530             // of the liberated form of the expectation.
 531             for ((hir_ty, &supplied_ty), expected_ty) in iter::zip(
 532                 iter::zip(decl.inputs, &inputs),
 533                 expected_sigs.liberated_sig.inputs(), // `liberated_sig` is E'.
 534             ) {
 535                 // Check that E' = S'.
 536                 let cause = self.misc(hir_ty.span);
 537                 let InferOk { value: (), obligations } =
 538                     self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
 539                 all_obligations.extend(obligations);
 540             }
 541
 542             let supplied_output_ty = self.replace_bound_vars_with_fresh_vars(
 543                 decl.output.span(),
 544                 LateBoundRegionConversionTime::FnCall,
 545                 supplied_sig.output(),
 546             );
 547             let cause = &self.misc(decl.output.span());
 548             let InferOk { value: (), obligations } = self
 549                 .at(cause, self.param_env)
 550                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 551             all_obligations.extend(obligations);
 552
 553             let inputs = inputs.into_iter().map(|ty| self.resolve_vars_if_possible(ty));
 554
 555             expected_sigs.liberated_sig = self.tcx.mk_fn_sig(
 556                 inputs,
 557                 supplied_output_ty,
 558                 expected_sigs.liberated_sig.c_variadic,
 559                 hir::Unsafety::Normal,
 560                 Abi::RustCall,
 561             );
 562
 563             Ok(InferOk { value: expected_sigs, obligations: all_obligations })
 564         })
 565     }
 566
 567     /// If there is no expected signature, then we will convert the
 568     /// types that the user gave into a signature.
 569     ///
 570     /// Also, record this closure signature for later.
 571     #[instrument(skip(self, decl, body), level = "debug", ret)]
 572     fn supplied_sig_of_closure(
 573         &self,
 574         expr_def_id: LocalDefId,
 575         decl: &hir::FnDecl<'_>,
 576         body: &hir::Body<'_>,
 577     ) -> ty::PolyFnSig<'tcx> {
 578         let astconv: &dyn AstConv<'_> = self;
 579
 580         trace!("decl = {:#?}", decl);
 581         debug!(?body.generator_kind);
 582
 583         let hir_id = self.tcx.hir().local_def_id_to_hir_id(expr_def_id);
 584         let bound_vars = self.tcx.late_bound_vars(hir_id);
 585
 586         // First, convert the types that the user supplied (if any).
 587         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 588         let supplied_return = match decl.output {
 589             hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
 590             hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
 591                 // In the case of the async block that we create for a function body,
 592                 // we expect the return type of the block to match that of the enclosing
 593                 // function.
 594                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 595                     debug!("closure is async fn body");
 596                     self.deduce_future_output_from_obligations(expr_def_id, body.id().hir_id)
 597                         .unwrap_or_else(|| {
 598                             // AFAIK, deducing the future output
 599                             // always succeeds *except* in error cases
 600                             // like #65159. I'd like to return Error
 601                             // here, but I can't because I can't
 602                             // easily (and locally) prove that we
 603                             // *have* reported an
 604                             // error. --nikomatsakis
 605                             astconv.ty_infer(None, decl.output.span())
 606                         })
 607                 }
 608
 609                 _ => astconv.ty_infer(None, decl.output.span()),
 610             },
 611         };
 612
 613         let result = ty::Binder::bind_with_vars(
 614             self.tcx.mk_fn_sig(
 615                 supplied_arguments,
 616                 supplied_return,
 617                 decl.c_variadic,
 618                 hir::Unsafety::Normal,
 619                 Abi::RustCall,
 620             ),
 621             bound_vars,
 622         );
 623         // Astconv can't normalize inputs or outputs with escaping bound vars,
 624         // so normalize them here, after we've wrapped them in a binder.
 625         let result = self.normalize(self.tcx.hir().span(hir_id), result);
 626
 627         let c_result = self.inh.infcx.canonicalize_response(result);
 628         self.typeck_results.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);
 629
 630         result
 631     }
 632
 633     /// Invoked when we are translating the generator that results
 634     /// from desugaring an `async fn`. Returns the "sugared" return
 635     /// type of the `async fn` -- that is, the return type that the
 636     /// user specified. The "desugared" return type is an `impl
 637     /// Future<Output = T>`, so we do this by searching through the
 638     /// obligations to extract the `T`.
 639     #[instrument(skip(self), level = "debug", ret)]
 640     fn deduce_future_output_from_obligations(
 641         &self,
 642         expr_def_id: LocalDefId,
 643         body_id: hir::HirId,
 644     ) -> Option<Ty<'tcx>> {
 645         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
 646             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
 647         });
 648
 649         let ret_ty = ret_coercion.borrow().expected_ty();
 650         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 651
 652         let get_future_output = |predicate: ty::Predicate<'tcx>, span| {
 653             // Search for a pending obligation like
 654             //
 655             // `<R as Future>::Output = T`
 656             //
 657             // where R is the return type we are expecting. This type `T`
 658             // will be our output.
 659             let bound_predicate = predicate.kind();
 660             if let ty::PredicateKind::Clause(ty::Clause::Projection(proj_predicate)) =
 661                 bound_predicate.skip_binder()
 662             {
 663                 self.deduce_future_output_from_projection(
 664                     span,
 665                     bound_predicate.rebind(proj_predicate),
 666                 )
 667             } else {
 668                 None
 669             }
 670         };
 671
 672         let output_ty = match *ret_ty.kind() {
 673             ty::Infer(ty::TyVar(ret_vid)) => {
 674                 self.obligations_for_self_ty(ret_vid).find_map(|obligation| {
 675                     get_future_output(obligation.predicate, obligation.cause.span)
 676                 })?
 677             }
 678             ty::Opaque(def_id, substs) => self
 679                 .tcx
 680                 .bound_explicit_item_bounds(def_id)
 681                 .subst_iter_copied(self.tcx, substs)
 682                 .find_map(|(p, s)| get_future_output(p, s))?,
 683             ty::Error(_) => return None,
 684             ty::Projection(proj)
 685                 if self.tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder =>
 686             {
 687                 self.tcx
 688                     .bound_explicit_item_bounds(proj.item_def_id)
 689                     .subst_iter_copied(self.tcx, proj.substs)
 690                     .find_map(|(p, s)| get_future_output(p, s))?
 691             }
 692             _ => span_bug!(
 693                 self.tcx.def_span(expr_def_id),
 694                 "async fn generator return type not an inference variable: {ret_ty}"
 695             ),
 696         };
 697
 698         // async fn that have opaque types in their return type need to redo the conversion to inference variables
 699         // as they fetch the still opaque version from the signature.
 700         let InferOk { value: output_ty, obligations } = self
 701             .replace_opaque_types_with_inference_vars(
 702                 output_ty,
 703                 body_id,
 704                 self.tcx.def_span(expr_def_id),
 705                 self.param_env,
 706             );
 707         self.register_predicates(obligations);
 708
 709         Some(output_ty)
 710     }
 711
 712     /// Given a projection like
 713     ///
 714     /// `<X as Future>::Output = T`
 715     ///
 716     /// where `X` is some type that has no late-bound regions, returns
 717     /// `Some(T)`. If the projection is for some other trait, returns
 718     /// `None`.
 719     fn deduce_future_output_from_projection(
 720         &self,
 721         cause_span: Span,
 722         predicate: ty::PolyProjectionPredicate<'tcx>,
 723     ) -> Option<Ty<'tcx>> {
 724         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 725
 726         // We do not expect any bound regions in our predicate, so
 727         // skip past the bound vars.
 728         let Some(predicate) = predicate.no_bound_vars() else {
 729             debug!("deduce_future_output_from_projection: has late-bound regions");
 730             return None;
 731         };
 732
 733         // Check that this is a projection from the `Future` trait.
 734         let trait_def_id = predicate.projection_ty.trait_def_id(self.tcx);
 735         let future_trait = self.tcx.require_lang_item(LangItem::Future, Some(cause_span));
 736         if trait_def_id != future_trait {
 737             debug!("deduce_future_output_from_projection: not a future");
 738             return None;
 739         }
 740
 741         // The `Future` trait has only one associated item, `Output`,
 742         // so check that this is what we see.
 743         let output_assoc_item = self.tcx.associated_item_def_ids(future_trait)[0];
 744         if output_assoc_item != predicate.projection_ty.item_def_id {
 745             span_bug!(
 746                 cause_span,
 747                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 748                 predicate.projection_ty.item_def_id,
 749                 output_assoc_item,
 750             );
 751         }
 752
 753         // Extract the type from the projection. Note that there can
 754         // be no bound variables in this type because the "self type"
 755         // does not have any regions in it.
 756         let output_ty = self.resolve_vars_if_possible(predicate.term);
 757         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 758         // This is a projection on a Fn trait so will always be a type.
 759         Some(output_ty.ty().unwrap())
 760     }
 761
 762     /// Converts the types that the user supplied, in case that doing
 763     /// so should yield an error, but returns back a signature where
 764     /// all parameters are of type `TyErr`.
 765     fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
 766         let astconv: &dyn AstConv<'_> = self;
 767
 768         let supplied_arguments = decl.inputs.iter().map(|a| {
 769             // Convert the types that the user supplied (if any), but ignore them.
 770             astconv.ast_ty_to_ty(a);
 771             self.tcx.ty_error()
 772         });
 773
 774         if let hir::FnRetTy::Return(ref output) = decl.output {
 775             astconv.ast_ty_to_ty(&output);
 776         }
 777
 778         let result = ty::Binder::dummy(self.tcx.mk_fn_sig(
 779             supplied_arguments,
 780             self.tcx.ty_error(),
 781             decl.c_variadic,
 782             hir::Unsafety::Normal,
 783             Abi::RustCall,
 784         ));
 785
 786         debug!("supplied_sig_of_closure: result={:?}", result);
 787
 788         result
 789     }
 790
 791     fn closure_sigs(
 792         &self,
 793         expr_def_id: LocalDefId,
 794         body: &hir::Body<'_>,
 795         bound_sig: ty::PolyFnSig<'tcx>,
 796     ) -> ClosureSignatures<'tcx> {
 797         let liberated_sig =
 798             self.tcx().liberate_late_bound_regions(expr_def_id.to_def_id(), bound_sig);
 799         let liberated_sig = self.normalize(body.value.span, liberated_sig);
 800         ClosureSignatures { bound_sig, liberated_sig }
 801     }
 802 }