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 generator_types = check_fn(
  83             self,
  84             self.param_env.without_const(),
  85             liberated_sig,
  86             closure.fn_decl,
  87             expr_def_id,
  88             body,
  89             closure.movability,
  90         )
  91         .1;
  92
  93         let parent_substs = InternalSubsts::identity_for_item(
  94             self.tcx,
  95             self.tcx.typeck_root_def_id(expr_def_id.to_def_id()),
  96         );
  97
  98         let tupled_upvars_ty = self.next_ty_var(TypeVariableOrigin {
  99             kind: TypeVariableOriginKind::ClosureSynthetic,
 100             span: self.tcx.def_span(expr_def_id),
 101         });
 102
 103         if let Some(GeneratorTypes { resume_ty, yield_ty, interior, movability }) = generator_types
 104         {
 105             let generator_substs = ty::GeneratorSubsts::new(
 106                 self.tcx,
 107                 ty::GeneratorSubstsParts {
 108                     parent_substs,
 109                     resume_ty,
 110                     yield_ty,
 111                     return_ty: liberated_sig.output(),
 112                     witness: interior,
 113                     tupled_upvars_ty,
 114                 },
 115             );
 116
 117             return self.tcx.mk_generator(
 118                 expr_def_id.to_def_id(),
 119                 generator_substs.substs,
 120                 movability,
 121             );
 122         }
 123
 124         // Tuple up the arguments and insert the resulting function type into
 125         // the `closures` table.
 126         let sig = bound_sig.map_bound(|sig| {
 127             self.tcx.mk_fn_sig(
 128                 iter::once(self.tcx.intern_tup(sig.inputs())),
 129                 sig.output(),
 130                 sig.c_variadic,
 131                 sig.unsafety,
 132                 sig.abi,
 133             )
 134         });
 135
 136         debug!(?sig, ?opt_kind);
 137
 138         let closure_kind_ty = match opt_kind {
 139             Some(kind) => kind.to_ty(self.tcx),
 140
 141             // Create a type variable (for now) to represent the closure kind.
 142             // It will be unified during the upvar inference phase (`upvar.rs`)
 143             None => self.next_ty_var(TypeVariableOrigin {
 144                 // FIXME(eddyb) distinguish closure kind inference variables from the rest.
 145                 kind: TypeVariableOriginKind::ClosureSynthetic,
 146                 span: expr_span,
 147             }),
 148         };
 149
 150         let closure_substs = ty::ClosureSubsts::new(
 151             self.tcx,
 152             ty::ClosureSubstsParts {
 153                 parent_substs,
 154                 closure_kind_ty,
 155                 closure_sig_as_fn_ptr_ty: self.tcx.mk_fn_ptr(sig),
 156                 tupled_upvars_ty,
 157             },
 158         );
 159
 160         self.tcx.mk_closure(expr_def_id.to_def_id(), closure_substs.substs)
 161     }
 162
 163     /// Given the expected type, figures out what it can about this closure we
 164     /// are about to type check:
 165     #[instrument(skip(self), level = "debug")]
 166     fn deduce_expectations_from_expected_type(
 167         &self,
 168         expected_ty: Ty<'tcx>,
 169     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 170         match *expected_ty.kind() {
 171             ty::Opaque(def_id, substs) => self.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_associated_types_in(
 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, found_args) = match self.get_fn_like_arguments(expr_map_node) {
 461             Some((sp, args)) => (Some(sp), args),
 462             None => (None, Vec::new()),
 463         };
 464         let expected_span =
 465             expected_sig.cause_span.unwrap_or_else(|| self.tcx.def_span(expr_def_id));
 466         self.report_arg_count_mismatch(
 467             expected_span,
 468             closure_span,
 469             expected_args,
 470             found_args,
 471             true,
 472         )
 473         .emit();
 474
 475         let error_sig = self.error_sig_of_closure(decl);
 476
 477         self.closure_sigs(expr_def_id, body, error_sig)
 478     }
 479
 480     /// Enforce the user's types against the expectation. See
 481     /// `sig_of_closure_with_expectation` for details on the overall
 482     /// strategy.
 483     #[instrument(level = "debug", skip(self, expr_def_id, decl, body, expected_sigs))]
 484     fn merge_supplied_sig_with_expectation(
 485         &self,
 486         expr_def_id: LocalDefId,
 487         decl: &hir::FnDecl<'_>,
 488         body: &hir::Body<'_>,
 489         mut expected_sigs: ClosureSignatures<'tcx>,
 490     ) -> InferResult<'tcx, ClosureSignatures<'tcx>> {
 491         // Get the signature S that the user gave.
 492         //
 493         // (See comment on `sig_of_closure_with_expectation` for the
 494         // meaning of these letters.)
 495         let supplied_sig = self.supplied_sig_of_closure(expr_def_id, decl, body);
 496
 497         debug!(?supplied_sig);
 498
 499         // FIXME(#45727): As discussed in [this comment][c1], naively
 500         // forcing equality here actually results in suboptimal error
 501         // messages in some cases.  For now, if there would have been
 502         // an obvious error, we fallback to declaring the type of the
 503         // closure to be the one the user gave, which allows other
 504         // error message code to trigger.
 505         //
 506         // However, I think [there is potential to do even better
 507         // here][c2], since in *this* code we have the precise span of
 508         // the type parameter in question in hand when we report the
 509         // error.
 510         //
 511         // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
 512         // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
 513         self.commit_if_ok(|_| {
 514             let mut all_obligations = vec![];
 515             let inputs: Vec<_> = iter::zip(
 516                 decl.inputs,
 517                 supplied_sig.inputs().skip_binder(), // binder moved to (*) below
 518             )
 519             .map(|(hir_ty, &supplied_ty)| {
 520                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 521                 self.replace_bound_vars_with_fresh_vars(
 522                     hir_ty.span,
 523                     LateBoundRegionConversionTime::FnCall,
 524                     // (*) binder moved to here
 525                     supplied_sig.inputs().rebind(supplied_ty),
 526                 )
 527             })
 528             .collect();
 529
 530             // The liberated version of this signature should be a subtype
 531             // of the liberated form of the expectation.
 532             for ((hir_ty, &supplied_ty), expected_ty) in iter::zip(
 533                 iter::zip(decl.inputs, &inputs),
 534                 expected_sigs.liberated_sig.inputs(), // `liberated_sig` is E'.
 535             ) {
 536                 // Check that E' = S'.
 537                 let cause = self.misc(hir_ty.span);
 538                 let InferOk { value: (), obligations } =
 539                     self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
 540                 all_obligations.extend(obligations);
 541             }
 542
 543             let supplied_output_ty = self.replace_bound_vars_with_fresh_vars(
 544                 decl.output.span(),
 545                 LateBoundRegionConversionTime::FnCall,
 546                 supplied_sig.output(),
 547             );
 548             let cause = &self.misc(decl.output.span());
 549             let InferOk { value: (), obligations } = self
 550                 .at(cause, self.param_env)
 551                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 552             all_obligations.extend(obligations);
 553
 554             let inputs = inputs.into_iter().map(|ty| self.resolve_vars_if_possible(ty));
 555
 556             expected_sigs.liberated_sig = self.tcx.mk_fn_sig(
 557                 inputs,
 558                 supplied_output_ty,
 559                 expected_sigs.liberated_sig.c_variadic,
 560                 hir::Unsafety::Normal,
 561                 Abi::RustCall,
 562             );
 563
 564             Ok(InferOk { value: expected_sigs, obligations: all_obligations })
 565         })
 566     }
 567
 568     /// If there is no expected signature, then we will convert the
 569     /// types that the user gave into a signature.
 570     ///
 571     /// Also, record this closure signature for later.
 572     #[instrument(skip(self, decl, body), level = "debug", ret)]
 573     fn supplied_sig_of_closure(
 574         &self,
 575         expr_def_id: LocalDefId,
 576         decl: &hir::FnDecl<'_>,
 577         body: &hir::Body<'_>,
 578     ) -> ty::PolyFnSig<'tcx> {
 579         let astconv: &dyn AstConv<'_> = self;
 580
 581         trace!("decl = {:#?}", decl);
 582         debug!(?body.generator_kind);
 583
 584         let hir_id = self.tcx.hir().local_def_id_to_hir_id(expr_def_id);
 585         let bound_vars = self.tcx.late_bound_vars(hir_id);
 586
 587         // First, convert the types that the user supplied (if any).
 588         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 589         let supplied_return = match decl.output {
 590             hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
 591             hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
 592                 // In the case of the async block that we create for a function body,
 593                 // we expect the return type of the block to match that of the enclosing
 594                 // function.
 595                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 596                     debug!("closure is async fn body");
 597                     self.deduce_future_output_from_obligations(expr_def_id, body.id().hir_id)
 598                         .unwrap_or_else(|| {
 599                             // AFAIK, deducing the future output
 600                             // always succeeds *except* in error cases
 601                             // like #65159. I'd like to return Error
 602                             // here, but I can't because I can't
 603                             // easily (and locally) prove that we
 604                             // *have* reported an
 605                             // error. --nikomatsakis
 606                             astconv.ty_infer(None, decl.output.span())
 607                         })
 608                 }
 609
 610                 _ => astconv.ty_infer(None, decl.output.span()),
 611             },
 612         };
 613
 614         let result = ty::Binder::bind_with_vars(
 615             self.tcx.mk_fn_sig(
 616                 supplied_arguments,
 617                 supplied_return,
 618                 decl.c_variadic,
 619                 hir::Unsafety::Normal,
 620                 Abi::RustCall,
 621             ),
 622             bound_vars,
 623         );
 624         // Astconv can't normalize inputs or outputs with escaping bound vars,
 625         // so normalize them here, after we've wrapped them in a binder.
 626         let result = self.normalize_associated_types_in(self.tcx.hir().span(hir_id), result);
 627
 628         let c_result = self.inh.infcx.canonicalize_response(result);
 629         self.typeck_results.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);
 630
 631         result
 632     }
 633
 634     /// Invoked when we are translating the generator that results
 635     /// from desugaring an `async fn`. Returns the "sugared" return
 636     /// type of the `async fn` -- that is, the return type that the
 637     /// user specified. The "desugared" return type is an `impl
 638     /// Future<Output = T>`, so we do this by searching through the
 639     /// obligations to extract the `T`.
 640     #[instrument(skip(self), level = "debug", ret)]
 641     fn deduce_future_output_from_obligations(
 642         &self,
 643         expr_def_id: LocalDefId,
 644         body_id: hir::HirId,
 645     ) -> Option<Ty<'tcx>> {
 646         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
 647             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
 648         });
 649
 650         let ret_ty = ret_coercion.borrow().expected_ty();
 651         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 652
 653         let get_future_output = |predicate: ty::Predicate<'tcx>, span| {
 654             // Search for a pending obligation like
 655             //
 656             // `<R as Future>::Output = T`
 657             //
 658             // where R is the return type we are expecting. This type `T`
 659             // will be our output.
 660             let bound_predicate = predicate.kind();
 661             if let ty::PredicateKind::Clause(ty::Clause::Projection(proj_predicate)) =
 662                 bound_predicate.skip_binder()
 663             {
 664                 self.deduce_future_output_from_projection(
 665                     span,
 666                     bound_predicate.rebind(proj_predicate),
 667                 )
 668             } else {
 669                 None
 670             }
 671         };
 672
 673         let output_ty = match *ret_ty.kind() {
 674             ty::Infer(ty::TyVar(ret_vid)) => {
 675                 self.obligations_for_self_ty(ret_vid).find_map(|obligation| {
 676                     get_future_output(obligation.predicate, obligation.cause.span)
 677                 })?
 678             }
 679             ty::Opaque(def_id, substs) => self
 680                 .tcx
 681                 .bound_explicit_item_bounds(def_id)
 682                 .subst_iter_copied(self.tcx, substs)
 683                 .find_map(|(p, s)| get_future_output(p, s))?,
 684             ty::Error(_) => return None,
 685             ty::Projection(proj)
 686                 if self.tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder =>
 687             {
 688                 self.tcx
 689                     .bound_explicit_item_bounds(proj.item_def_id)
 690                     .subst_iter_copied(self.tcx, proj.substs)
 691                     .find_map(|(p, s)| get_future_output(p, s))?
 692             }
 693             _ => span_bug!(
 694                 self.tcx.def_span(expr_def_id),
 695                 "async fn generator return type not an inference variable: {ret_ty}"
 696             ),
 697         };
 698
 699         // async fn that have opaque types in their return type need to redo the conversion to inference variables
 700         // as they fetch the still opaque version from the signature.
 701         let InferOk { value: output_ty, obligations } = self
 702             .replace_opaque_types_with_inference_vars(
 703                 output_ty,
 704                 body_id,
 705                 self.tcx.def_span(expr_def_id),
 706                 self.param_env,
 707             );
 708         self.register_predicates(obligations);
 709
 710         Some(output_ty)
 711     }
 712
 713     /// Given a projection like
 714     ///
 715     /// `<X as Future>::Output = T`
 716     ///
 717     /// where `X` is some type that has no late-bound regions, returns
 718     /// `Some(T)`. If the projection is for some other trait, returns
 719     /// `None`.
 720     fn deduce_future_output_from_projection(
 721         &self,
 722         cause_span: Span,
 723         predicate: ty::PolyProjectionPredicate<'tcx>,
 724     ) -> Option<Ty<'tcx>> {
 725         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 726
 727         // We do not expect any bound regions in our predicate, so
 728         // skip past the bound vars.
 729         let Some(predicate) = predicate.no_bound_vars() else {
 730             debug!("deduce_future_output_from_projection: has late-bound regions");
 731             return None;
 732         };
 733
 734         // Check that this is a projection from the `Future` trait.
 735         let trait_def_id = predicate.projection_ty.trait_def_id(self.tcx);
 736         let future_trait = self.tcx.require_lang_item(LangItem::Future, Some(cause_span));
 737         if trait_def_id != future_trait {
 738             debug!("deduce_future_output_from_projection: not a future");
 739             return None;
 740         }
 741
 742         // The `Future` trait has only one associated item, `Output`,
 743         // so check that this is what we see.
 744         let output_assoc_item = self.tcx.associated_item_def_ids(future_trait)[0];
 745         if output_assoc_item != predicate.projection_ty.item_def_id {
 746             span_bug!(
 747                 cause_span,
 748                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 749                 predicate.projection_ty.item_def_id,
 750                 output_assoc_item,
 751             );
 752         }
 753
 754         // Extract the type from the projection. Note that there can
 755         // be no bound variables in this type because the "self type"
 756         // does not have any regions in it.
 757         let output_ty = self.resolve_vars_if_possible(predicate.term);
 758         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 759         // This is a projection on a Fn trait so will always be a type.
 760         Some(output_ty.ty().unwrap())
 761     }
 762
 763     /// Converts the types that the user supplied, in case that doing
 764     /// so should yield an error, but returns back a signature where
 765     /// all parameters are of type `TyErr`.
 766     fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
 767         let astconv: &dyn AstConv<'_> = self;
 768
 769         let supplied_arguments = decl.inputs.iter().map(|a| {
 770             // Convert the types that the user supplied (if any), but ignore them.
 771             astconv.ast_ty_to_ty(a);
 772             self.tcx.ty_error()
 773         });
 774
 775         if let hir::FnRetTy::Return(ref output) = decl.output {
 776             astconv.ast_ty_to_ty(&output);
 777         }
 778
 779         let result = ty::Binder::dummy(self.tcx.mk_fn_sig(
 780             supplied_arguments,
 781             self.tcx.ty_error(),
 782             decl.c_variadic,
 783             hir::Unsafety::Normal,
 784             Abi::RustCall,
 785         ));
 786
 787         debug!("supplied_sig_of_closure: result={:?}", result);
 788
 789         result
 790     }
 791
 792     fn closure_sigs(
 793         &self,
 794         expr_def_id: LocalDefId,
 795         body: &hir::Body<'_>,
 796         bound_sig: ty::PolyFnSig<'tcx>,
 797     ) -> ClosureSignatures<'tcx> {
 798         let liberated_sig =
 799             self.tcx().liberate_late_bound_regions(expr_def_id.to_def_id(), bound_sig);
 800         let liberated_sig = self.inh.normalize_associated_types_in(
 801             body.value.span,
 802             self.tcx.hir().local_def_id_to_hir_id(expr_def_id),
 803             self.param_env,
 804             liberated_sig,
 805         );
 806         ClosureSignatures { bound_sig, liberated_sig }
 807     }
 808 }