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::lang_items::LangItem;
   8 use rustc_hir_analysis::astconv::AstConv;
   9 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  10 use rustc_infer::infer::LateBoundRegionConversionTime;
  11 use rustc_infer::infer::{InferOk, InferResult};
  12 use rustc_macros::{TypeFoldable, TypeVisitable};
  13 use rustc_middle::ty::subst::InternalSubsts;
  14 use rustc_middle::ty::visit::TypeVisitable;
  15 use rustc_middle::ty::{self, Ty, TypeSuperVisitable, TypeVisitor};
  16 use rustc_span::def_id::LocalDefId;
  17 use rustc_span::source_map::Span;
  18 use rustc_span::sym;
  19 use rustc_target::spec::abi::Abi;
  20 use rustc_trait_selection::traits;
  21 use rustc_trait_selection::traits::error_reporting::ArgKind;
  22 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
  23 use std::cmp;
  24 use std::iter;
  25 use std::ops::ControlFlow;
  26
  27 /// What signature do we *expect* the closure to have from context?
  28 #[derive(Debug, Clone, TypeFoldable, TypeVisitable)]
  29 struct ExpectedSig<'tcx> {
  30     /// Span that gave us this expectation, if we know that.
  31     cause_span: Option<Span>,
  32     sig: ty::PolyFnSig<'tcx>,
  33 }
  34
  35 struct ClosureSignatures<'tcx> {
  36     /// The signature users of the closure see.
  37     bound_sig: ty::PolyFnSig<'tcx>,
  38     /// The signature within the function body.
  39     /// This mostly differs in the sense that lifetimes are now early bound and any
  40     /// opaque types from the signature expectation are overridden in case there are
  41     /// explicit hidden types written by the user in the closure signature.
  42     liberated_sig: ty::FnSig<'tcx>,
  43 }
  44
  45 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  46     #[instrument(skip(self, closure), level = "debug")]
  47     pub fn check_expr_closure(
  48         &self,
  49         closure: &hir::Closure<'tcx>,
  50         expr_span: Span,
  51         expected: Expectation<'tcx>,
  52     ) -> Ty<'tcx> {
  53         trace!("decl = {:#?}", closure.fn_decl);
  54
  55         // It's always helpful for inference if we know the kind of
  56         // closure sooner rather than later, so first examine the expected
  57         // type, and see if can glean a closure kind from there.
  58         let (expected_sig, expected_kind) = match expected.to_option(self) {
  59             Some(ty) => self.deduce_closure_signature(ty),
  60             None => (None, None),
  61         };
  62         let body = self.tcx.hir().body(closure.body);
  63         self.check_closure(closure, expr_span, expected_kind, body, expected_sig)
  64     }
  65
  66     #[instrument(skip(self, closure, body), level = "debug", ret)]
  67     fn check_closure(
  68         &self,
  69         closure: &hir::Closure<'tcx>,
  70         expr_span: Span,
  71         opt_kind: Option<ty::ClosureKind>,
  72         body: &'tcx hir::Body<'tcx>,
  73         expected_sig: Option<ExpectedSig<'tcx>>,
  74     ) -> Ty<'tcx> {
  75         trace!("decl = {:#?}", closure.fn_decl);
  76         let expr_def_id = closure.def_id;
  77         debug!(?expr_def_id);
  78
  79         let ClosureSignatures { bound_sig, liberated_sig } =
  80             self.sig_of_closure(expr_def_id, closure.fn_decl, body, expected_sig);
  81
  82         debug!(?bound_sig, ?liberated_sig);
  83
  84         let mut fcx = FnCtxt::new(self, self.param_env.without_const(), closure.def_id);
  85         let generator_types = check_fn(
  86             &mut fcx,
  87             liberated_sig,
  88             closure.fn_decl,
  89             expr_def_id,
  90             body,
  91             closure.movability,
  92         );
  93
  94         let parent_substs = InternalSubsts::identity_for_item(
  95             self.tcx,
  96             self.tcx.typeck_root_def_id(expr_def_id.to_def_id()),
  97         );
  98
  99         let tupled_upvars_ty = self.next_ty_var(TypeVariableOrigin {
 100             kind: TypeVariableOriginKind::ClosureSynthetic,
 101             span: self.tcx.def_span(expr_def_id),
 102         });
 103
 104         if let Some(GeneratorTypes { resume_ty, yield_ty, interior, movability }) = generator_types
 105         {
 106             let generator_substs = ty::GeneratorSubsts::new(
 107                 self.tcx,
 108                 ty::GeneratorSubstsParts {
 109                     parent_substs,
 110                     resume_ty,
 111                     yield_ty,
 112                     return_ty: liberated_sig.output(),
 113                     witness: interior,
 114                     tupled_upvars_ty,
 115                 },
 116             );
 117
 118             return self.tcx.mk_generator(
 119                 expr_def_id.to_def_id(),
 120                 generator_substs.substs,
 121                 movability,
 122             );
 123         }
 124
 125         // Tuple up the arguments and insert the resulting function type into
 126         // the `closures` table.
 127         let sig = bound_sig.map_bound(|sig| {
 128             self.tcx.mk_fn_sig(
 129                 iter::once(self.tcx.intern_tup(sig.inputs())),
 130                 sig.output(),
 131                 sig.c_variadic,
 132                 sig.unsafety,
 133                 sig.abi,
 134             )
 135         });
 136
 137         debug!(?sig, ?opt_kind);
 138
 139         let closure_kind_ty = match opt_kind {
 140             Some(kind) => kind.to_ty(self.tcx),
 141
 142             // Create a type variable (for now) to represent the closure kind.
 143             // It will be unified during the upvar inference phase (`upvar.rs`)
 144             None => self.next_ty_var(TypeVariableOrigin {
 145                 // FIXME(eddyb) distinguish closure kind inference variables from the rest.
 146                 kind: TypeVariableOriginKind::ClosureSynthetic,
 147                 span: expr_span,
 148             }),
 149         };
 150
 151         let closure_substs = ty::ClosureSubsts::new(
 152             self.tcx,
 153             ty::ClosureSubstsParts {
 154                 parent_substs,
 155                 closure_kind_ty,
 156                 closure_sig_as_fn_ptr_ty: self.tcx.mk_fn_ptr(sig),
 157                 tupled_upvars_ty,
 158             },
 159         );
 160
 161         self.tcx.mk_closure(expr_def_id.to_def_id(), closure_substs.substs)
 162     }
 163
 164     /// Given the expected type, figures out what it can about this closure we
 165     /// are about to type check:
 166     #[instrument(skip(self), level = "debug")]
 167     fn deduce_closure_signature(
 168         &self,
 169         expected_ty: Ty<'tcx>,
 170     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 171         match *expected_ty.kind() {
 172             ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs, .. }) => self
 173                 .deduce_closure_signature_from_predicates(
 174                     expected_ty,
 175                     self.tcx.bound_explicit_item_bounds(def_id).subst_iter_copied(self.tcx, substs),
 176                 ),
 177             ty::Dynamic(ref object_type, ..) => {
 178                 let sig = object_type.projection_bounds().find_map(|pb| {
 179                     let pb = pb.with_self_ty(self.tcx, self.tcx.types.trait_object_dummy_self);
 180                     self.deduce_sig_from_projection(None, pb)
 181                 });
 182                 let kind = object_type
 183                     .principal_def_id()
 184                     .and_then(|did| self.tcx.fn_trait_kind_from_def_id(did));
 185                 (sig, kind)
 186             }
 187             ty::Infer(ty::TyVar(vid)) => self.deduce_closure_signature_from_predicates(
 188                 self.tcx.mk_ty_var(self.root_var(vid)),
 189                 self.obligations_for_self_ty(vid).map(|obl| (obl.predicate, obl.cause.span)),
 190             ),
 191             ty::FnPtr(sig) => {
 192                 let expected_sig = ExpectedSig { cause_span: None, sig };
 193                 (Some(expected_sig), Some(ty::ClosureKind::Fn))
 194             }
 195             _ => (None, None),
 196         }
 197     }
 198
 199     fn deduce_closure_signature_from_predicates(
 200         &self,
 201         expected_ty: Ty<'tcx>,
 202         predicates: impl DoubleEndedIterator<Item = (ty::Predicate<'tcx>, Span)>,
 203     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 204         let mut expected_sig = None;
 205         let mut expected_kind = None;
 206
 207         for obligation in traits::elaborate_predicates_with_span(
 208             self.tcx,
 209             // Reverse the obligations here, since `elaborate_*` uses a stack,
 210             // and we want to keep inference generally in the same order of
 211             // the registered obligations.
 212             predicates.rev(),
 213         ) {
 214             debug!(?obligation.predicate);
 215             let bound_predicate = obligation.predicate.kind();
 216
 217             // Given a Projection predicate, we can potentially infer
 218             // the complete signature.
 219             if expected_sig.is_none()
 220                 && let ty::PredicateKind::Clause(ty::Clause::Projection(proj_predicate)) = bound_predicate.skip_binder()
 221             {
 222                 let inferred_sig = self.normalize(
 223                     obligation.cause.span,
 224                     self.deduce_sig_from_projection(
 225                     Some(obligation.cause.span),
 226                         bound_predicate.rebind(proj_predicate),
 227                     ),
 228                 );
 229                 // Make sure that we didn't infer a signature that mentions itself.
 230                 // This can happen when we elaborate certain supertrait bounds that
 231                 // mention projections containing the `Self` type. See #105401.
 232                 struct MentionsTy<'tcx> {
 233                     expected_ty: Ty<'tcx>,
 234                 }
 235                 impl<'tcx> TypeVisitor<'tcx> for MentionsTy<'tcx> {
 236                     type BreakTy = ();
 237
 238                     fn visit_ty(&mut self, t: Ty<'tcx>) -> ControlFlow<Self::BreakTy> {
 239                         if t == self.expected_ty {
 240                             ControlFlow::Break(())
 241                         } else {
 242                             t.super_visit_with(self)
 243                         }
 244                     }
 245                 }
 246                 if inferred_sig.visit_with(&mut MentionsTy { expected_ty }).is_continue() {
 247                     expected_sig = inferred_sig;
 248                 }
 249             }
 250
 251             // Even if we can't infer the full signature, we may be able to
 252             // infer the kind. This can occur when we elaborate a predicate
 253             // like `F : Fn<A>`. Note that due to subtyping we could encounter
 254             // many viable options, so pick the most restrictive.
 255             let trait_def_id = match bound_predicate.skip_binder() {
 256                 ty::PredicateKind::Clause(ty::Clause::Projection(data)) => {
 257                     Some(data.projection_ty.trait_def_id(self.tcx))
 258                 }
 259                 ty::PredicateKind::Clause(ty::Clause::Trait(data)) => Some(data.def_id()),
 260                 _ => None,
 261             };
 262             if let Some(closure_kind) =
 263                 trait_def_id.and_then(|def_id| self.tcx.fn_trait_kind_from_def_id(def_id))
 264             {
 265                 expected_kind = Some(
 266                     expected_kind
 267                         .map_or_else(|| closure_kind, |current| cmp::min(current, closure_kind)),
 268                 );
 269             }
 270         }
 271
 272         (expected_sig, expected_kind)
 273     }
 274
 275     /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
 276     /// everything we need to know about a closure or generator.
 277     ///
 278     /// The `cause_span` should be the span that caused us to
 279     /// have this expected signature, or `None` if we can't readily
 280     /// know that.
 281     #[instrument(level = "debug", skip(self, cause_span), ret)]
 282     fn deduce_sig_from_projection(
 283         &self,
 284         cause_span: Option<Span>,
 285         projection: ty::PolyProjectionPredicate<'tcx>,
 286     ) -> Option<ExpectedSig<'tcx>> {
 287         let tcx = self.tcx;
 288
 289         let trait_def_id = projection.trait_def_id(tcx);
 290
 291         let is_fn = tcx.is_fn_trait(trait_def_id);
 292
 293         let gen_trait = tcx.lang_items().gen_trait();
 294         let is_gen = gen_trait == Some(trait_def_id);
 295
 296         if !is_fn && !is_gen {
 297             debug!("not fn or generator");
 298             return None;
 299         }
 300
 301         // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
 302         // associated item and not yield.
 303         if is_gen && self.tcx.associated_item(projection.projection_def_id()).name != sym::Return {
 304             debug!("not `Return` assoc item of `Generator`");
 305             return None;
 306         }
 307
 308         let input_tys = if is_fn {
 309             let arg_param_ty = projection.skip_binder().projection_ty.substs.type_at(1);
 310             let arg_param_ty = self.resolve_vars_if_possible(arg_param_ty);
 311             debug!(?arg_param_ty);
 312
 313             match arg_param_ty.kind() {
 314                 &ty::Tuple(tys) => tys,
 315                 _ => return None,
 316             }
 317         } else {
 318             // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
 319             // else they must have exactly 1 argument. For now though, just give up in this case.
 320             return None;
 321         };
 322
 323         // Since this is a return parameter type it is safe to unwrap.
 324         let ret_param_ty = projection.skip_binder().term.ty().unwrap();
 325         let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
 326         debug!(?ret_param_ty);
 327
 328         let sig = projection.rebind(self.tcx.mk_fn_sig(
 329             input_tys.iter(),
 330             ret_param_ty,
 331             false,
 332             hir::Unsafety::Normal,
 333             Abi::Rust,
 334         ));
 335
 336         Some(ExpectedSig { cause_span, sig })
 337     }
 338
 339     fn sig_of_closure(
 340         &self,
 341         expr_def_id: LocalDefId,
 342         decl: &hir::FnDecl<'_>,
 343         body: &hir::Body<'_>,
 344         expected_sig: Option<ExpectedSig<'tcx>>,
 345     ) -> ClosureSignatures<'tcx> {
 346         if let Some(e) = expected_sig {
 347             self.sig_of_closure_with_expectation(expr_def_id, decl, body, e)
 348         } else {
 349             self.sig_of_closure_no_expectation(expr_def_id, decl, body)
 350         }
 351     }
 352
 353     /// If there is no expected signature, then we will convert the
 354     /// types that the user gave into a signature.
 355     #[instrument(skip(self, expr_def_id, decl, body), level = "debug")]
 356     fn sig_of_closure_no_expectation(
 357         &self,
 358         expr_def_id: LocalDefId,
 359         decl: &hir::FnDecl<'_>,
 360         body: &hir::Body<'_>,
 361     ) -> ClosureSignatures<'tcx> {
 362         let bound_sig = self.supplied_sig_of_closure(expr_def_id, decl, body);
 363
 364         self.closure_sigs(expr_def_id, body, bound_sig)
 365     }
 366
 367     /// Invoked to compute the signature of a closure expression. This
 368     /// combines any user-provided type annotations (e.g., `|x: u32|
 369     /// -> u32 { .. }`) with the expected signature.
 370     ///
 371     /// The approach is as follows:
 372     ///
 373     /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
 374     /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
 375     ///   - If we have no expectation `E`, then the signature of the closure is `S`.
 376     ///   - Otherwise, the signature of the closure is E. Moreover:
 377     ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
 378     ///     - Instantiate all the late-bound regions bound in the closure within `S`
 379     ///       with fresh (existential) variables, yielding `S'`
 380     ///     - Require that `E' = S'`
 381     ///       - We could use some kind of subtyping relationship here,
 382     ///         I imagine, but equality is easier and works fine for
 383     ///         our purposes.
 384     ///
 385     /// The key intuition here is that the user's types must be valid
 386     /// from "the inside" of the closure, but the expectation
 387     /// ultimately drives the overall signature.
 388     ///
 389     /// # Examples
 390     ///
 391     /// ```ignore (illustrative)
 392     /// fn with_closure<F>(_: F)
 393     ///   where F: Fn(&u32) -> &u32 { .. }
 394     ///
 395     /// with_closure(|x: &u32| { ... })
 396     /// ```
 397     ///
 398     /// Here:
 399     /// - E would be `fn(&u32) -> &u32`.
 400     /// - S would be `fn(&u32) ->
 401     /// - E' is `&'!0 u32 -> &'!0 u32`
 402     /// - S' is `&'?0 u32 -> ?T`
 403     ///
 404     /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
 405     ///
 406     /// # Arguments
 407     ///
 408     /// - `expr_def_id`: the `LocalDefId` of the closure expression
 409     /// - `decl`: the HIR declaration of the closure
 410     /// - `body`: the body of the closure
 411     /// - `expected_sig`: the expected signature (if any). Note that
 412     ///   this is missing a binder: that is, there may be late-bound
 413     ///   regions with depth 1, which are bound then by the closure.
 414     #[instrument(skip(self, expr_def_id, decl, body), level = "debug")]
 415     fn sig_of_closure_with_expectation(
 416         &self,
 417         expr_def_id: LocalDefId,
 418         decl: &hir::FnDecl<'_>,
 419         body: &hir::Body<'_>,
 420         expected_sig: ExpectedSig<'tcx>,
 421     ) -> ClosureSignatures<'tcx> {
 422         // Watch out for some surprises and just ignore the
 423         // expectation if things don't see to match up with what we
 424         // expect.
 425         if expected_sig.sig.c_variadic() != decl.c_variadic {
 426             return self.sig_of_closure_no_expectation(expr_def_id, decl, body);
 427         } else if expected_sig.sig.skip_binder().inputs_and_output.len() != decl.inputs.len() + 1 {
 428             return self.sig_of_closure_with_mismatched_number_of_arguments(
 429                 expr_def_id,
 430                 decl,
 431                 body,
 432                 expected_sig,
 433             );
 434         }
 435
 436         // Create a `PolyFnSig`. Note the oddity that late bound
 437         // regions appearing free in `expected_sig` are now bound up
 438         // in this binder we are creating.
 439         assert!(!expected_sig.sig.skip_binder().has_vars_bound_above(ty::INNERMOST));
 440         let bound_sig = expected_sig.sig.map_bound(|sig| {
 441             self.tcx.mk_fn_sig(
 442                 sig.inputs().iter().cloned(),
 443                 sig.output(),
 444                 sig.c_variadic,
 445                 hir::Unsafety::Normal,
 446                 Abi::RustCall,
 447             )
 448         });
 449
 450         // `deduce_expectations_from_expected_type` introduces
 451         // late-bound lifetimes defined elsewhere, which we now
 452         // anonymize away, so as not to confuse the user.
 453         let bound_sig = self.tcx.anonymize_bound_vars(bound_sig);
 454
 455         let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
 456
 457         // Up till this point, we have ignored the annotations that the user
 458         // gave. This function will check that they unify successfully.
 459         // Along the way, it also writes out entries for types that the user
 460         // wrote into our typeck results, which are then later used by the privacy
 461         // check.
 462         match self.merge_supplied_sig_with_expectation(expr_def_id, decl, body, closure_sigs) {
 463             Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
 464             Err(_) => self.sig_of_closure_no_expectation(expr_def_id, decl, body),
 465         }
 466     }
 467
 468     fn sig_of_closure_with_mismatched_number_of_arguments(
 469         &self,
 470         expr_def_id: LocalDefId,
 471         decl: &hir::FnDecl<'_>,
 472         body: &hir::Body<'_>,
 473         expected_sig: ExpectedSig<'tcx>,
 474     ) -> ClosureSignatures<'tcx> {
 475         let hir = self.tcx.hir();
 476         let expr_map_node = hir.get_by_def_id(expr_def_id);
 477         let expected_args: Vec<_> = expected_sig
 478             .sig
 479             .skip_binder()
 480             .inputs()
 481             .iter()
 482             .map(|ty| ArgKind::from_expected_ty(*ty, None))
 483             .collect();
 484         let (closure_span, closure_arg_span, found_args) =
 485             match self.get_fn_like_arguments(expr_map_node) {
 486                 Some((sp, arg_sp, args)) => (Some(sp), arg_sp, args),
 487                 None => (None, None, Vec::new()),
 488             };
 489         let expected_span =
 490             expected_sig.cause_span.unwrap_or_else(|| self.tcx.def_span(expr_def_id));
 491         self.report_arg_count_mismatch(
 492             expected_span,
 493             closure_span,
 494             expected_args,
 495             found_args,
 496             true,
 497             closure_arg_span,
 498         )
 499         .emit();
 500
 501         let error_sig = self.error_sig_of_closure(decl);
 502
 503         self.closure_sigs(expr_def_id, body, error_sig)
 504     }
 505
 506     /// Enforce the user's types against the expectation. See
 507     /// `sig_of_closure_with_expectation` for details on the overall
 508     /// strategy.
 509     #[instrument(level = "debug", skip(self, expr_def_id, decl, body, expected_sigs))]
 510     fn merge_supplied_sig_with_expectation(
 511         &self,
 512         expr_def_id: LocalDefId,
 513         decl: &hir::FnDecl<'_>,
 514         body: &hir::Body<'_>,
 515         mut expected_sigs: ClosureSignatures<'tcx>,
 516     ) -> InferResult<'tcx, ClosureSignatures<'tcx>> {
 517         // Get the signature S that the user gave.
 518         //
 519         // (See comment on `sig_of_closure_with_expectation` for the
 520         // meaning of these letters.)
 521         let supplied_sig = self.supplied_sig_of_closure(expr_def_id, decl, body);
 522
 523         debug!(?supplied_sig);
 524
 525         // FIXME(#45727): As discussed in [this comment][c1], naively
 526         // forcing equality here actually results in suboptimal error
 527         // messages in some cases. For now, if there would have been
 528         // an obvious error, we fallback to declaring the type of the
 529         // closure to be the one the user gave, which allows other
 530         // error message code to trigger.
 531         //
 532         // However, I think [there is potential to do even better
 533         // here][c2], since in *this* code we have the precise span of
 534         // the type parameter in question in hand when we report the
 535         // error.
 536         //
 537         // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
 538         // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
 539         self.commit_if_ok(|_| {
 540             let mut all_obligations = vec![];
 541             let inputs: Vec<_> = iter::zip(
 542                 decl.inputs,
 543                 supplied_sig.inputs().skip_binder(), // binder moved to (*) below
 544             )
 545             .map(|(hir_ty, &supplied_ty)| {
 546                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 547                 self.replace_bound_vars_with_fresh_vars(
 548                     hir_ty.span,
 549                     LateBoundRegionConversionTime::FnCall,
 550                     // (*) binder moved to here
 551                     supplied_sig.inputs().rebind(supplied_ty),
 552                 )
 553             })
 554             .collect();
 555
 556             // The liberated version of this signature should be a subtype
 557             // of the liberated form of the expectation.
 558             for ((hir_ty, &supplied_ty), expected_ty) in iter::zip(
 559                 iter::zip(decl.inputs, &inputs),
 560                 expected_sigs.liberated_sig.inputs(), // `liberated_sig` is E'.
 561             ) {
 562                 // Check that E' = S'.
 563                 let cause = self.misc(hir_ty.span);
 564                 let InferOk { value: (), obligations } =
 565                     self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
 566                 all_obligations.extend(obligations);
 567             }
 568
 569             let supplied_output_ty = self.replace_bound_vars_with_fresh_vars(
 570                 decl.output.span(),
 571                 LateBoundRegionConversionTime::FnCall,
 572                 supplied_sig.output(),
 573             );
 574             let cause = &self.misc(decl.output.span());
 575             let InferOk { value: (), obligations } = self
 576                 .at(cause, self.param_env)
 577                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 578             all_obligations.extend(obligations);
 579
 580             let inputs = inputs.into_iter().map(|ty| self.resolve_vars_if_possible(ty));
 581
 582             expected_sigs.liberated_sig = self.tcx.mk_fn_sig(
 583                 inputs,
 584                 supplied_output_ty,
 585                 expected_sigs.liberated_sig.c_variadic,
 586                 hir::Unsafety::Normal,
 587                 Abi::RustCall,
 588             );
 589
 590             Ok(InferOk { value: expected_sigs, obligations: all_obligations })
 591         })
 592     }
 593
 594     /// If there is no expected signature, then we will convert the
 595     /// types that the user gave into a signature.
 596     ///
 597     /// Also, record this closure signature for later.
 598     #[instrument(skip(self, decl, body), level = "debug", ret)]
 599     fn supplied_sig_of_closure(
 600         &self,
 601         expr_def_id: LocalDefId,
 602         decl: &hir::FnDecl<'_>,
 603         body: &hir::Body<'_>,
 604     ) -> ty::PolyFnSig<'tcx> {
 605         let astconv: &dyn AstConv<'_> = self;
 606
 607         trace!("decl = {:#?}", decl);
 608         debug!(?body.generator_kind);
 609
 610         let hir_id = self.tcx.hir().local_def_id_to_hir_id(expr_def_id);
 611         let bound_vars = self.tcx.late_bound_vars(hir_id);
 612
 613         // First, convert the types that the user supplied (if any).
 614         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 615         let supplied_return = match decl.output {
 616             hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
 617             hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
 618                 // In the case of the async block that we create for a function body,
 619                 // we expect the return type of the block to match that of the enclosing
 620                 // function.
 621                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 622                     debug!("closure is async fn body");
 623                     let def_id = self.tcx.hir().body_owner_def_id(body.id());
 624                     self.deduce_future_output_from_obligations(expr_def_id, def_id).unwrap_or_else(
 625                         || {
 626                             // AFAIK, deducing the future output
 627                             // always succeeds *except* in error cases
 628                             // like #65159. I'd like to return Error
 629                             // here, but I can't because I can't
 630                             // easily (and locally) prove that we
 631                             // *have* reported an
 632                             // error. --nikomatsakis
 633                             astconv.ty_infer(None, decl.output.span())
 634                         },
 635                     )
 636                 }
 637
 638                 _ => astconv.ty_infer(None, decl.output.span()),
 639             },
 640         };
 641
 642         let result = ty::Binder::bind_with_vars(
 643             self.tcx.mk_fn_sig(
 644                 supplied_arguments,
 645                 supplied_return,
 646                 decl.c_variadic,
 647                 hir::Unsafety::Normal,
 648                 Abi::RustCall,
 649             ),
 650             bound_vars,
 651         );
 652
 653         let c_result = self.inh.infcx.canonicalize_response(result);
 654         self.typeck_results.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);
 655
 656         // Normalize only after registering in `user_provided_sigs`.
 657         self.normalize(self.tcx.hir().span(hir_id), result)
 658     }
 659
 660     /// Invoked when we are translating the generator that results
 661     /// from desugaring an `async fn`. Returns the "sugared" return
 662     /// type of the `async fn` -- that is, the return type that the
 663     /// user specified. The "desugared" return type is an `impl
 664     /// Future<Output = T>`, so we do this by searching through the
 665     /// obligations to extract the `T`.
 666     #[instrument(skip(self), level = "debug", ret)]
 667     fn deduce_future_output_from_obligations(
 668         &self,
 669         expr_def_id: LocalDefId,
 670         body_def_id: LocalDefId,
 671     ) -> Option<Ty<'tcx>> {
 672         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
 673             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
 674         });
 675
 676         let ret_ty = ret_coercion.borrow().expected_ty();
 677         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 678
 679         let get_future_output = |predicate: ty::Predicate<'tcx>, span| {
 680             // Search for a pending obligation like
 681             //
 682             // `<R as Future>::Output = T`
 683             //
 684             // where R is the return type we are expecting. This type `T`
 685             // will be our output.
 686             let bound_predicate = predicate.kind();
 687             if let ty::PredicateKind::Clause(ty::Clause::Projection(proj_predicate)) =
 688                 bound_predicate.skip_binder()
 689             {
 690                 self.deduce_future_output_from_projection(
 691                     span,
 692                     bound_predicate.rebind(proj_predicate),
 693                 )
 694             } else {
 695                 None
 696             }
 697         };
 698
 699         let output_ty = match *ret_ty.kind() {
 700             ty::Infer(ty::TyVar(ret_vid)) => {
 701                 self.obligations_for_self_ty(ret_vid).find_map(|obligation| {
 702                     get_future_output(obligation.predicate, obligation.cause.span)
 703                 })?
 704             }
 705             ty::Alias(ty::Opaque, ty::AliasTy { def_id, substs, .. }) => self
 706                 .tcx
 707                 .bound_explicit_item_bounds(def_id)
 708                 .subst_iter_copied(self.tcx, substs)
 709                 .find_map(|(p, s)| get_future_output(p, s))?,
 710             ty::Error(_) => return None,
 711             ty::Alias(ty::Projection, proj)
 712                 if self.tcx.def_kind(proj.def_id) == DefKind::ImplTraitPlaceholder =>
 713             {
 714                 self.tcx
 715                     .bound_explicit_item_bounds(proj.def_id)
 716                     .subst_iter_copied(self.tcx, proj.substs)
 717                     .find_map(|(p, s)| get_future_output(p, s))?
 718             }
 719             _ => span_bug!(
 720                 self.tcx.def_span(expr_def_id),
 721                 "async fn generator return type not an inference variable: {ret_ty}"
 722             ),
 723         };
 724
 725         // async fn that have opaque types in their return type need to redo the conversion to inference variables
 726         // as they fetch the still opaque version from the signature.
 727         let InferOk { value: output_ty, obligations } = self
 728             .replace_opaque_types_with_inference_vars(
 729                 output_ty,
 730                 body_def_id,
 731                 self.tcx.def_span(expr_def_id),
 732                 self.param_env,
 733             );
 734         self.register_predicates(obligations);
 735
 736         Some(output_ty)
 737     }
 738
 739     /// Given a projection like
 740     ///
 741     /// `<X as Future>::Output = T`
 742     ///
 743     /// where `X` is some type that has no late-bound regions, returns
 744     /// `Some(T)`. If the projection is for some other trait, returns
 745     /// `None`.
 746     fn deduce_future_output_from_projection(
 747         &self,
 748         cause_span: Span,
 749         predicate: ty::PolyProjectionPredicate<'tcx>,
 750     ) -> Option<Ty<'tcx>> {
 751         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 752
 753         // We do not expect any bound regions in our predicate, so
 754         // skip past the bound vars.
 755         let Some(predicate) = predicate.no_bound_vars() else {
 756             debug!("deduce_future_output_from_projection: has late-bound regions");
 757             return None;
 758         };
 759
 760         // Check that this is a projection from the `Future` trait.
 761         let trait_def_id = predicate.projection_ty.trait_def_id(self.tcx);
 762         let future_trait = self.tcx.require_lang_item(LangItem::Future, Some(cause_span));
 763         if trait_def_id != future_trait {
 764             debug!("deduce_future_output_from_projection: not a future");
 765             return None;
 766         }
 767
 768         // The `Future` trait has only one associated item, `Output`,
 769         // so check that this is what we see.
 770         let output_assoc_item = self.tcx.associated_item_def_ids(future_trait)[0];
 771         if output_assoc_item != predicate.projection_ty.def_id {
 772             span_bug!(
 773                 cause_span,
 774                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 775                 predicate.projection_ty.def_id,
 776                 output_assoc_item,
 777             );
 778         }
 779
 780         // Extract the type from the projection. Note that there can
 781         // be no bound variables in this type because the "self type"
 782         // does not have any regions in it.
 783         let output_ty = self.resolve_vars_if_possible(predicate.term);
 784         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 785         // This is a projection on a Fn trait so will always be a type.
 786         Some(output_ty.ty().unwrap())
 787     }
 788
 789     /// Converts the types that the user supplied, in case that doing
 790     /// so should yield an error, but returns back a signature where
 791     /// all parameters are of type `TyErr`.
 792     fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
 793         let astconv: &dyn AstConv<'_> = self;
 794
 795         let supplied_arguments = decl.inputs.iter().map(|a| {
 796             // Convert the types that the user supplied (if any), but ignore them.
 797             astconv.ast_ty_to_ty(a);
 798             self.tcx.ty_error()
 799         });
 800
 801         if let hir::FnRetTy::Return(ref output) = decl.output {
 802             astconv.ast_ty_to_ty(&output);
 803         }
 804
 805         let result = ty::Binder::dummy(self.tcx.mk_fn_sig(
 806             supplied_arguments,
 807             self.tcx.ty_error(),
 808             decl.c_variadic,
 809             hir::Unsafety::Normal,
 810             Abi::RustCall,
 811         ));
 812
 813         debug!("supplied_sig_of_closure: result={:?}", result);
 814
 815         result
 816     }
 817
 818     fn closure_sigs(
 819         &self,
 820         expr_def_id: LocalDefId,
 821         body: &hir::Body<'_>,
 822         bound_sig: ty::PolyFnSig<'tcx>,
 823     ) -> ClosureSignatures<'tcx> {
 824         let liberated_sig =
 825             self.tcx().liberate_late_bound_regions(expr_def_id.to_def_id(), bound_sig);
 826         let liberated_sig = self.normalize(body.value.span, liberated_sig);
 827         ClosureSignatures { bound_sig, liberated_sig }
 828     }
 829 }