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::Alias(ty::Opaque, ty::AliasTy { def_id, substs }) => self
 171                 .deduce_signature_from_predicates(
 172                     self.tcx.bound_explicit_item_bounds(def_id).subst_iter_copied(self.tcx, substs),
 173                 ),
 174             ty::Dynamic(ref object_type, ..) => {
 175                 let sig = object_type.projection_bounds().find_map(|pb| {
 176                     let pb = pb.with_self_ty(self.tcx, self.tcx.types.trait_object_dummy_self);
 177                     self.deduce_sig_from_projection(None, pb)
 178                 });
 179                 let kind = object_type
 180                     .principal_def_id()
 181                     .and_then(|did| self.tcx.fn_trait_kind_from_def_id(did));
 182                 (sig, kind)
 183             }
 184             ty::Infer(ty::TyVar(vid)) => self.deduce_signature_from_predicates(
 185                 self.obligations_for_self_ty(vid).map(|obl| (obl.predicate, obl.cause.span)),
 186             ),
 187             ty::FnPtr(sig) => {
 188                 let expected_sig = ExpectedSig { cause_span: None, sig };
 189                 (Some(expected_sig), Some(ty::ClosureKind::Fn))
 190             }
 191             _ => (None, None),
 192         }
 193     }
 194
 195     fn deduce_signature_from_predicates(
 196         &self,
 197         predicates: impl DoubleEndedIterator<Item = (ty::Predicate<'tcx>, Span)>,
 198     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 199         let mut expected_sig = None;
 200         let mut expected_kind = None;
 201
 202         for obligation in traits::elaborate_predicates_with_span(
 203             self.tcx,
 204             // Reverse the obligations here, since `elaborate_*` uses a stack,
 205             // and we want to keep inference generally in the same order of
 206             // the registered obligations.
 207             predicates.rev(),
 208         ) {
 209             debug!(?obligation.predicate);
 210             let bound_predicate = obligation.predicate.kind();
 211
 212             // Given a Projection predicate, we can potentially infer
 213             // the complete signature.
 214             if expected_sig.is_none()
 215                 && let ty::PredicateKind::Clause(ty::Clause::Projection(proj_predicate)) = bound_predicate.skip_binder()
 216             {
 217                 expected_sig = self.normalize(
 218                     obligation.cause.span,
 219                     self.deduce_sig_from_projection(
 220                     Some(obligation.cause.span),
 221                         bound_predicate.rebind(proj_predicate),
 222                     ),
 223                 );
 224             }
 225
 226             // Even if we can't infer the full signature, we may be able to
 227             // infer the kind. This can occur when we elaborate a predicate
 228             // like `F : Fn<A>`. Note that due to subtyping we could encounter
 229             // many viable options, so pick the most restrictive.
 230             let trait_def_id = match bound_predicate.skip_binder() {
 231                 ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
 232                     Some(data.projection_ty.trait_def_id(self.tcx))
 233                 }
 234                 ty::PredicateKind::Clause(ty::Clause::Trait(data)) => Some(data.def_id()),
 235                 _ => None,
 236             };
 237             if let Some(closure_kind) =
 238                 trait_def_id.and_then(|def_id| self.tcx.fn_trait_kind_from_def_id(def_id))
 239             {
 240                 expected_kind = Some(
 241                     expected_kind
 242                         .map_or_else(|| closure_kind, |current| cmp::min(current, closure_kind)),
 243                 );
 244             }
 245         }
 246
 247         (expected_sig, expected_kind)
 248     }
 249
 250     /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
 251     /// everything we need to know about a closure or generator.
 252     ///
 253     /// The `cause_span` should be the span that caused us to
 254     /// have this expected signature, or `None` if we can't readily
 255     /// know that.
 256     #[instrument(level = "debug", skip(self, cause_span), ret)]
 257     fn deduce_sig_from_projection(
 258         &self,
 259         cause_span: Option<Span>,
 260         projection: ty::PolyProjectionPredicate<'tcx>,
 261     ) -> Option<ExpectedSig<'tcx>> {
 262         let tcx = self.tcx;
 263
 264         let trait_def_id = projection.trait_def_id(tcx);
 265
 266         let is_fn = tcx.is_fn_trait(trait_def_id);
 267         let gen_trait = tcx.require_lang_item(LangItem::Generator, cause_span);
 268         let is_gen = gen_trait == trait_def_id;
 269         if !is_fn && !is_gen {
 270             debug!("not fn or generator");
 271             return None;
 272         }
 273
 274         if is_gen {
 275             // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
 276             // associated item and not yield.
 277             let return_assoc_item = self.tcx.associated_item_def_ids(gen_trait)[1];
 278             if return_assoc_item != projection.projection_def_id() {
 279                 debug!("not return assoc item of generator");
 280                 return None;
 281             }
 282         }
 283
 284         let input_tys = if is_fn {
 285             let arg_param_ty = projection.skip_binder().projection_ty.substs.type_at(1);
 286             let arg_param_ty = self.resolve_vars_if_possible(arg_param_ty);
 287             debug!(?arg_param_ty);
 288
 289             match arg_param_ty.kind() {
 290                 &ty::Tuple(tys) => tys,
 291                 _ => return None,
 292             }
 293         } else {
 294             // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
 295             // else they must have exactly 1 argument. For now though, just give up in this case.
 296             return None;
 297         };
 298
 299         // Since this is a return parameter type it is safe to unwrap.
 300         let ret_param_ty = projection.skip_binder().term.ty().unwrap();
 301         let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
 302         debug!(?ret_param_ty);
 303
 304         let sig = projection.rebind(self.tcx.mk_fn_sig(
 305             input_tys.iter(),
 306             ret_param_ty,
 307             false,
 308             hir::Unsafety::Normal,
 309             Abi::Rust,
 310         ));
 311
 312         Some(ExpectedSig { cause_span, sig })
 313     }
 314
 315     fn sig_of_closure(
 316         &self,
 317         expr_def_id: LocalDefId,
 318         decl: &hir::FnDecl<'_>,
 319         body: &hir::Body<'_>,
 320         expected_sig: Option<ExpectedSig<'tcx>>,
 321     ) -> ClosureSignatures<'tcx> {
 322         if let Some(e) = expected_sig {
 323             self.sig_of_closure_with_expectation(expr_def_id, decl, body, e)
 324         } else {
 325             self.sig_of_closure_no_expectation(expr_def_id, decl, body)
 326         }
 327     }
 328
 329     /// If there is no expected signature, then we will convert the
 330     /// types that the user gave into a signature.
 331     #[instrument(skip(self, expr_def_id, decl, body), level = "debug")]
 332     fn sig_of_closure_no_expectation(
 333         &self,
 334         expr_def_id: LocalDefId,
 335         decl: &hir::FnDecl<'_>,
 336         body: &hir::Body<'_>,
 337     ) -> ClosureSignatures<'tcx> {
 338         let bound_sig = self.supplied_sig_of_closure(expr_def_id, decl, body);
 339
 340         self.closure_sigs(expr_def_id, body, bound_sig)
 341     }
 342
 343     /// Invoked to compute the signature of a closure expression. This
 344     /// combines any user-provided type annotations (e.g., `|x: u32|
 345     /// -> u32 { .. }`) with the expected signature.
 346     ///
 347     /// The approach is as follows:
 348     ///
 349     /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
 350     /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
 351     ///   - If we have no expectation `E`, then the signature of the closure is `S`.
 352     ///   - Otherwise, the signature of the closure is E. Moreover:
 353     ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
 354     ///     - Instantiate all the late-bound regions bound in the closure within `S`
 355     ///       with fresh (existential) variables, yielding `S'`
 356     ///     - Require that `E' = S'`
 357     ///       - We could use some kind of subtyping relationship here,
 358     ///         I imagine, but equality is easier and works fine for
 359     ///         our purposes.
 360     ///
 361     /// The key intuition here is that the user's types must be valid
 362     /// from "the inside" of the closure, but the expectation
 363     /// ultimately drives the overall signature.
 364     ///
 365     /// # Examples
 366     ///
 367     /// ```ignore (illustrative)
 368     /// fn with_closure<F>(_: F)
 369     ///   where F: Fn(&u32) -> &u32 { .. }
 370     ///
 371     /// with_closure(|x: &u32| { ... })
 372     /// ```
 373     ///
 374     /// Here:
 375     /// - E would be `fn(&u32) -> &u32`.
 376     /// - S would be `fn(&u32) ->
 377     /// - E' is `&'!0 u32 -> &'!0 u32`
 378     /// - S' is `&'?0 u32 -> ?T`
 379     ///
 380     /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
 381     ///
 382     /// # Arguments
 383     ///
 384     /// - `expr_def_id`: the `LocalDefId` of the closure expression
 385     /// - `decl`: the HIR declaration of the closure
 386     /// - `body`: the body of the closure
 387     /// - `expected_sig`: the expected signature (if any). Note that
 388     ///   this is missing a binder: that is, there may be late-bound
 389     ///   regions with depth 1, which are bound then by the closure.
 390     #[instrument(skip(self, expr_def_id, decl, body), level = "debug")]
 391     fn sig_of_closure_with_expectation(
 392         &self,
 393         expr_def_id: LocalDefId,
 394         decl: &hir::FnDecl<'_>,
 395         body: &hir::Body<'_>,
 396         expected_sig: ExpectedSig<'tcx>,
 397     ) -> ClosureSignatures<'tcx> {
 398         // Watch out for some surprises and just ignore the
 399         // expectation if things don't see to match up with what we
 400         // expect.
 401         if expected_sig.sig.c_variadic() != decl.c_variadic {
 402             return self.sig_of_closure_no_expectation(expr_def_id, decl, body);
 403         } else if expected_sig.sig.skip_binder().inputs_and_output.len() != decl.inputs.len() + 1 {
 404             return self.sig_of_closure_with_mismatched_number_of_arguments(
 405                 expr_def_id,
 406                 decl,
 407                 body,
 408                 expected_sig,
 409             );
 410         }
 411
 412         // Create a `PolyFnSig`. Note the oddity that late bound
 413         // regions appearing free in `expected_sig` are now bound up
 414         // in this binder we are creating.
 415         assert!(!expected_sig.sig.skip_binder().has_vars_bound_above(ty::INNERMOST));
 416         let bound_sig = expected_sig.sig.map_bound(|sig| {
 417             self.tcx.mk_fn_sig(
 418                 sig.inputs().iter().cloned(),
 419                 sig.output(),
 420                 sig.c_variadic,
 421                 hir::Unsafety::Normal,
 422                 Abi::RustCall,
 423             )
 424         });
 425
 426         // `deduce_expectations_from_expected_type` introduces
 427         // late-bound lifetimes defined elsewhere, which we now
 428         // anonymize away, so as not to confuse the user.
 429         let bound_sig = self.tcx.anonymize_late_bound_regions(bound_sig);
 430
 431         let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
 432
 433         // Up till this point, we have ignored the annotations that the user
 434         // gave. This function will check that they unify successfully.
 435         // Along the way, it also writes out entries for types that the user
 436         // wrote into our typeck results, which are then later used by the privacy
 437         // check.
 438         match self.merge_supplied_sig_with_expectation(expr_def_id, decl, body, closure_sigs) {
 439             Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
 440             Err(_) => self.sig_of_closure_no_expectation(expr_def_id, decl, body),
 441         }
 442     }
 443
 444     fn sig_of_closure_with_mismatched_number_of_arguments(
 445         &self,
 446         expr_def_id: LocalDefId,
 447         decl: &hir::FnDecl<'_>,
 448         body: &hir::Body<'_>,
 449         expected_sig: ExpectedSig<'tcx>,
 450     ) -> ClosureSignatures<'tcx> {
 451         let hir = self.tcx.hir();
 452         let expr_map_node = hir.get_by_def_id(expr_def_id);
 453         let expected_args: Vec<_> = expected_sig
 454             .sig
 455             .skip_binder()
 456             .inputs()
 457             .iter()
 458             .map(|ty| ArgKind::from_expected_ty(*ty, None))
 459             .collect();
 460         let (closure_span, closure_arg_span, found_args) =
 461             match self.get_fn_like_arguments(expr_map_node) {
 462                 Some((sp, arg_sp, args)) => (Some(sp), arg_sp, args),
 463                 None => (None, None, Vec::new()),
 464             };
 465         let expected_span =
 466             expected_sig.cause_span.unwrap_or_else(|| self.tcx.def_span(expr_def_id));
 467         self.report_arg_count_mismatch(
 468             expected_span,
 469             closure_span,
 470             expected_args,
 471             found_args,
 472             true,
 473             closure_arg_span,
 474         )
 475         .emit();
 476
 477         let error_sig = self.error_sig_of_closure(decl);
 478
 479         self.closure_sigs(expr_def_id, body, error_sig)
 480     }
 481
 482     /// Enforce the user's types against the expectation. See
 483     /// `sig_of_closure_with_expectation` for details on the overall
 484     /// strategy.
 485     #[instrument(level = "debug", skip(self, expr_def_id, decl, body, expected_sigs))]
 486     fn merge_supplied_sig_with_expectation(
 487         &self,
 488         expr_def_id: LocalDefId,
 489         decl: &hir::FnDecl<'_>,
 490         body: &hir::Body<'_>,
 491         mut expected_sigs: ClosureSignatures<'tcx>,
 492     ) -> InferResult<'tcx, ClosureSignatures<'tcx>> {
 493         // Get the signature S that the user gave.
 494         //
 495         // (See comment on `sig_of_closure_with_expectation` for the
 496         // meaning of these letters.)
 497         let supplied_sig = self.supplied_sig_of_closure(expr_def_id, decl, body);
 498
 499         debug!(?supplied_sig);
 500
 501         // FIXME(#45727): As discussed in [this comment][c1], naively
 502         // forcing equality here actually results in suboptimal error
 503         // messages in some cases.  For now, if there would have been
 504         // an obvious error, we fallback to declaring the type of the
 505         // closure to be the one the user gave, which allows other
 506         // error message code to trigger.
 507         //
 508         // However, I think [there is potential to do even better
 509         // here][c2], since in *this* code we have the precise span of
 510         // the type parameter in question in hand when we report the
 511         // error.
 512         //
 513         // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
 514         // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
 515         self.commit_if_ok(|_| {
 516             let mut all_obligations = vec![];
 517             let inputs: Vec<_> = iter::zip(
 518                 decl.inputs,
 519                 supplied_sig.inputs().skip_binder(), // binder moved to (*) below
 520             )
 521             .map(|(hir_ty, &supplied_ty)| {
 522                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 523                 self.replace_bound_vars_with_fresh_vars(
 524                     hir_ty.span,
 525                     LateBoundRegionConversionTime::FnCall,
 526                     // (*) binder moved to here
 527                     supplied_sig.inputs().rebind(supplied_ty),
 528                 )
 529             })
 530             .collect();
 531
 532             // The liberated version of this signature should be a subtype
 533             // of the liberated form of the expectation.
 534             for ((hir_ty, &supplied_ty), expected_ty) in iter::zip(
 535                 iter::zip(decl.inputs, &inputs),
 536                 expected_sigs.liberated_sig.inputs(), // `liberated_sig` is E'.
 537             ) {
 538                 // Check that E' = S'.
 539                 let cause = self.misc(hir_ty.span);
 540                 let InferOk { value: (), obligations } =
 541                     self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
 542                 all_obligations.extend(obligations);
 543             }
 544
 545             let supplied_output_ty = self.replace_bound_vars_with_fresh_vars(
 546                 decl.output.span(),
 547                 LateBoundRegionConversionTime::FnCall,
 548                 supplied_sig.output(),
 549             );
 550             let cause = &self.misc(decl.output.span());
 551             let InferOk { value: (), obligations } = self
 552                 .at(cause, self.param_env)
 553                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 554             all_obligations.extend(obligations);
 555
 556             let inputs = inputs.into_iter().map(|ty| self.resolve_vars_if_possible(ty));
 557
 558             expected_sigs.liberated_sig = self.tcx.mk_fn_sig(
 559                 inputs,
 560                 supplied_output_ty,
 561                 expected_sigs.liberated_sig.c_variadic,
 562                 hir::Unsafety::Normal,
 563                 Abi::RustCall,
 564             );
 565
 566             Ok(InferOk { value: expected_sigs, obligations: all_obligations })
 567         })
 568     }
 569
 570     /// If there is no expected signature, then we will convert the
 571     /// types that the user gave into a signature.
 572     ///
 573     /// Also, record this closure signature for later.
 574     #[instrument(skip(self, decl, body), level = "debug", ret)]
 575     fn supplied_sig_of_closure(
 576         &self,
 577         expr_def_id: LocalDefId,
 578         decl: &hir::FnDecl<'_>,
 579         body: &hir::Body<'_>,
 580     ) -> ty::PolyFnSig<'tcx> {
 581         let astconv: &dyn AstConv<'_> = self;
 582
 583         trace!("decl = {:#?}", decl);
 584         debug!(?body.generator_kind);
 585
 586         let hir_id = self.tcx.hir().local_def_id_to_hir_id(expr_def_id);
 587         let bound_vars = self.tcx.late_bound_vars(hir_id);
 588
 589         // First, convert the types that the user supplied (if any).
 590         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 591         let supplied_return = match decl.output {
 592             hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
 593             hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
 594                 // In the case of the async block that we create for a function body,
 595                 // we expect the return type of the block to match that of the enclosing
 596                 // function.
 597                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 598                     debug!("closure is async fn body");
 599                     self.deduce_future_output_from_obligations(expr_def_id, body.id().hir_id)
 600                         .unwrap_or_else(|| {
 601                             // AFAIK, deducing the future output
 602                             // always succeeds *except* in error cases
 603                             // like #65159. I'd like to return Error
 604                             // here, but I can't because I can't
 605                             // easily (and locally) prove that we
 606                             // *have* reported an
 607                             // error. --nikomatsakis
 608                             astconv.ty_infer(None, decl.output.span())
 609                         })
 610                 }
 611
 612                 _ => astconv.ty_infer(None, decl.output.span()),
 613             },
 614         };
 615
 616         let result = ty::Binder::bind_with_vars(
 617             self.tcx.mk_fn_sig(
 618                 supplied_arguments,
 619                 supplied_return,
 620                 decl.c_variadic,
 621                 hir::Unsafety::Normal,
 622                 Abi::RustCall,
 623             ),
 624             bound_vars,
 625         );
 626         // Astconv can't normalize inputs or outputs with escaping bound vars,
 627         // so normalize them here, after we've wrapped them in a binder.
 628         let result = self.normalize(self.tcx.hir().span(hir_id), result);
 629
 630         let c_result = self.inh.infcx.canonicalize_response(result);
 631         self.typeck_results.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);
 632
 633         result
 634     }
 635
 636     /// Invoked when we are translating the generator that results
 637     /// from desugaring an `async fn`. Returns the "sugared" return
 638     /// type of the `async fn` -- that is, the return type that the
 639     /// user specified. The "desugared" return type is an `impl
 640     /// Future<Output = T>`, so we do this by searching through the
 641     /// obligations to extract the `T`.
 642     #[instrument(skip(self), level = "debug", ret)]
 643     fn deduce_future_output_from_obligations(
 644         &self,
 645         expr_def_id: LocalDefId,
 646         body_id: hir::HirId,
 647     ) -> Option<Ty<'tcx>> {
 648         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
 649             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
 650         });
 651
 652         let ret_ty = ret_coercion.borrow().expected_ty();
 653         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 654
 655         let get_future_output = |predicate: ty::Predicate<'tcx>, span| {
 656             // Search for a pending obligation like
 657             //
 658             // `<R as Future>::Output = T`
 659             //
 660             // where R is the return type we are expecting. This type `T`
 661             // will be our output.
 662             let bound_predicate = predicate.kind();
 663             if let ty::PredicateKind::Clause(ty::Clause::Projection(proj_predicate)) =
 664                 bound_predicate.skip_binder()
 665             {
 666                 self.deduce_future_output_from_projection(
 667                     span,
 668                     bound_predicate.rebind(proj_predicate),
 669                 )
 670             } else {
 671                 None
 672             }
 673         };
 674
 675         let output_ty = match *ret_ty.kind() {
 676             ty::Infer(ty::TyVar(ret_vid)) => {
 677                 self.obligations_for_self_ty(ret_vid).find_map(|obligation| {
 678                     get_future_output(obligation.predicate, obligation.cause.span)
 679                 })?
 680             }
 681             ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs }) => self
 682                 .tcx
 683                 .bound_explicit_item_bounds(def_id)
 684                 .subst_iter_copied(self.tcx, substs)
 685                 .find_map(|(p, s)| get_future_output(p, s))?,
 686             ty::Error(_) => return None,
 687             ty::Alias(ty::Projection, proj)
 688                 if self.tcx.def_kind(proj.def_id) == DefKind::ImplTraitPlaceholder =>
 689             {
 690                 self.tcx
 691                     .bound_explicit_item_bounds(proj.def_id)
 692                     .subst_iter_copied(self.tcx, proj.substs)
 693                     .find_map(|(p, s)| get_future_output(p, s))?
 694             }
 695             _ => span_bug!(
 696                 self.tcx.def_span(expr_def_id),
 697                 "async fn generator return type not an inference variable: {ret_ty}"
 698             ),
 699         };
 700
 701         // async fn that have opaque types in their return type need to redo the conversion to inference variables
 702         // as they fetch the still opaque version from the signature.
 703         let InferOk { value: output_ty, obligations } = self
 704             .replace_opaque_types_with_inference_vars(
 705                 output_ty,
 706                 body_id,
 707                 self.tcx.def_span(expr_def_id),
 708                 self.param_env,
 709             );
 710         self.register_predicates(obligations);
 711
 712         Some(output_ty)
 713     }
 714
 715     /// Given a projection like
 716     ///
 717     /// `<X as Future>::Output = T`
 718     ///
 719     /// where `X` is some type that has no late-bound regions, returns
 720     /// `Some(T)`. If the projection is for some other trait, returns
 721     /// `None`.
 722     fn deduce_future_output_from_projection(
 723         &self,
 724         cause_span: Span,
 725         predicate: ty::PolyProjectionPredicate<'tcx>,
 726     ) -> Option<Ty<'tcx>> {
 727         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 728
 729         // We do not expect any bound regions in our predicate, so
 730         // skip past the bound vars.
 731         let Some(predicate) = predicate.no_bound_vars() else {
 732             debug!("deduce_future_output_from_projection: has late-bound regions");
 733             return None;
 734         };
 735
 736         // Check that this is a projection from the `Future` trait.
 737         let trait_def_id = predicate.projection_ty.trait_def_id(self.tcx);
 738         let future_trait = self.tcx.require_lang_item(LangItem::Future, Some(cause_span));
 739         if trait_def_id != future_trait {
 740             debug!("deduce_future_output_from_projection: not a future");
 741             return None;
 742         }
 743
 744         // The `Future` trait has only one associated item, `Output`,
 745         // so check that this is what we see.
 746         let output_assoc_item = self.tcx.associated_item_def_ids(future_trait)[0];
 747         if output_assoc_item != predicate.projection_ty.def_id {
 748             span_bug!(
 749                 cause_span,
 750                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 751                 predicate.projection_ty.def_id,
 752                 output_assoc_item,
 753             );
 754         }
 755
 756         // Extract the type from the projection. Note that there can
 757         // be no bound variables in this type because the "self type"
 758         // does not have any regions in it.
 759         let output_ty = self.resolve_vars_if_possible(predicate.term);
 760         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 761         // This is a projection on a Fn trait so will always be a type.
 762         Some(output_ty.ty().unwrap())
 763     }
 764
 765     /// Converts the types that the user supplied, in case that doing
 766     /// so should yield an error, but returns back a signature where
 767     /// all parameters are of type `TyErr`.
 768     fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
 769         let astconv: &dyn AstConv<'_> = self;
 770
 771         let supplied_arguments = decl.inputs.iter().map(|a| {
 772             // Convert the types that the user supplied (if any), but ignore them.
 773             astconv.ast_ty_to_ty(a);
 774             self.tcx.ty_error()
 775         });
 776
 777         if let hir::FnRetTy::Return(ref output) = decl.output {
 778             astconv.ast_ty_to_ty(&output);
 779         }
 780
 781         let result = ty::Binder::dummy(self.tcx.mk_fn_sig(
 782             supplied_arguments,
 783             self.tcx.ty_error(),
 784             decl.c_variadic,
 785             hir::Unsafety::Normal,
 786             Abi::RustCall,
 787         ));
 788
 789         debug!("supplied_sig_of_closure: result={:?}", result);
 790
 791         result
 792     }
 793
 794     fn closure_sigs(
 795         &self,
 796         expr_def_id: LocalDefId,
 797         body: &hir::Body<'_>,
 798         bound_sig: ty::PolyFnSig<'tcx>,
 799     ) -> ClosureSignatures<'tcx> {
 800         let liberated_sig =
 801             self.tcx().liberate_late_bound_regions(expr_def_id.to_def_id(), bound_sig);
 802         let liberated_sig = self.normalize(body.value.span, liberated_sig);
 803         ClosureSignatures { bound_sig, liberated_sig }
 804     }
 805 }