src/librustc_typeck/check/closure.rs

   1 //! Code for type-checking closure expressions.
   2
   3 use super::{check_fn, Expectation, FnCtxt, GeneratorTypes};
   4
   5 use crate::astconv::AstConv;
   6 use crate::middle::{lang_items, region};
   7 use rustc::hir::def_id::DefId;
   8 use rustc::infer::{InferOk, InferResult};
   9 use rustc::infer::LateBoundRegionConversionTime;
  10 use rustc::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  11 use rustc::traits::Obligation;
  12 use rustc::traits::error_reporting::ArgKind;
  13 use rustc::ty::{self, Ty, GenericParamDefKind};
  14 use rustc::ty::fold::TypeFoldable;
  15 use rustc::ty::subst::InternalSubsts;
  16 use std::cmp;
  17 use std::iter;
  18 use rustc_target::spec::abi::Abi;
  19 use syntax::source_map::Span;
  20 use rustc::hir;
  21
  22 /// What signature do we *expect* the closure to have from context?
  23 #[derive(Debug)]
  24 struct ExpectedSig<'tcx> {
  25     /// Span that gave us this expectation, if we know that.
  26     cause_span: Option<Span>,
  27     sig: ty::FnSig<'tcx>,
  28 }
  29
  30 struct ClosureSignatures<'tcx> {
  31     bound_sig: ty::PolyFnSig<'tcx>,
  32     liberated_sig: ty::FnSig<'tcx>,
  33 }
  34
  35 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  36     pub fn check_expr_closure(
  37         &self,
  38         expr: &hir::Expr,
  39         _capture: hir::CaptureClause,
  40         decl: &'tcx hir::FnDecl,
  41         body_id: hir::BodyId,
  42         gen: Option<hir::GeneratorMovability>,
  43         expected: Expectation<'tcx>,
  44     ) -> Ty<'tcx> {
  45         debug!(
  46             "check_expr_closure(expr={:?},expected={:?})",
  47             expr, expected
  48         );
  49
  50         // It's always helpful for inference if we know the kind of
  51         // closure sooner rather than later, so first examine the expected
  52         // type, and see if can glean a closure kind from there.
  53         let (expected_sig, expected_kind) = match expected.to_option(self) {
  54             Some(ty) => self.deduce_expectations_from_expected_type(ty),
  55             None => (None, None),
  56         };
  57         let body = self.tcx.hir().body(body_id);
  58         self.check_closure(expr, expected_kind, decl, body, gen, expected_sig)
  59     }
  60
  61     fn check_closure(
  62         &self,
  63         expr: &hir::Expr,
  64         opt_kind: Option<ty::ClosureKind>,
  65         decl: &'tcx hir::FnDecl,
  66         body: &'tcx hir::Body,
  67         gen: Option<hir::GeneratorMovability>,
  68         expected_sig: Option<ExpectedSig<'tcx>>,
  69     ) -> Ty<'tcx> {
  70         debug!(
  71             "check_closure(opt_kind={:?}, expected_sig={:?})",
  72             opt_kind, expected_sig
  73         );
  74
  75         let expr_def_id = self.tcx.hir().local_def_id(expr.hir_id);
  76
  77         let ClosureSignatures {
  78             bound_sig,
  79             liberated_sig,
  80         } = self.sig_of_closure(expr_def_id, decl, body, expected_sig);
  81
  82         debug!("check_closure: ty_of_closure returns {:?}", liberated_sig);
  83
  84         let generator_types = check_fn(
  85             self,
  86             self.param_env,
  87             liberated_sig,
  88             decl,
  89             expr.hir_id,
  90             body,
  91             gen,
  92         ).1;
  93
  94         // Create type variables (for now) to represent the transformed
  95         // types of upvars. These will be unified during the upvar
  96         // inference phase (`upvar.rs`).
  97         let base_substs =
  98             InternalSubsts::identity_for_item(self.tcx, self.tcx.closure_base_def_id(expr_def_id));
  99         let substs = base_substs.extend_to(self.tcx,expr_def_id, |param, _| {
 100             match param.kind {
 101                 GenericParamDefKind::Lifetime => {
 102                     span_bug!(expr.span, "closure has lifetime param")
 103                 }
 104                 GenericParamDefKind::Type { .. } => {
 105                     self.infcx.next_ty_var(TypeVariableOrigin {
 106                         kind: TypeVariableOriginKind::ClosureSynthetic,
 107                         span: expr.span,
 108                     }).into()
 109                 }
 110                 GenericParamDefKind::Const => {
 111                     span_bug!(expr.span, "closure has const param")
 112                 }
 113             }
 114         });
 115         if let Some(GeneratorTypes { yield_ty, interior, movability }) = generator_types {
 116             let generator_substs = substs.as_generator();
 117             self.demand_eqtype(
 118                 expr.span,
 119                 yield_ty,
 120                 generator_substs.yield_ty(expr_def_id, self.tcx),
 121             );
 122             self.demand_eqtype(
 123                 expr.span,
 124                 liberated_sig.output(),
 125                 generator_substs.return_ty(expr_def_id, self.tcx),
 126             );
 127             self.demand_eqtype(
 128                 expr.span,
 129                 interior,
 130                 generator_substs.witness(expr_def_id, self.tcx),
 131             );
 132             return self.tcx.mk_generator(expr_def_id, substs, movability);
 133         }
 134
 135         let closure_type = self.tcx.mk_closure(expr_def_id, substs);
 136
 137         debug!(
 138             "check_closure: expr.hir_id={:?} closure_type={:?}",
 139             expr.hir_id, closure_type
 140         );
 141
 142         // Tuple up the arguments and insert the resulting function type into
 143         // the `closures` table.
 144         let sig = bound_sig.map_bound(|sig| {
 145             self.tcx.mk_fn_sig(
 146                 iter::once(self.tcx.intern_tup(sig.inputs())),
 147                 sig.output(),
 148                 sig.c_variadic,
 149                 sig.unsafety,
 150                 sig.abi,
 151             )
 152         });
 153
 154         debug!(
 155             "check_closure: expr_def_id={:?}, sig={:?}, opt_kind={:?}",
 156             expr_def_id, sig, opt_kind
 157         );
 158
 159         let sig_fn_ptr_ty = self.tcx.mk_fn_ptr(sig);
 160         self.demand_eqtype(
 161             expr.span,
 162             sig_fn_ptr_ty,
 163             substs.as_closure().sig_ty(expr_def_id, self.tcx),
 164         );
 165
 166         if let Some(kind) = opt_kind {
 167             self.demand_eqtype(
 168                 expr.span,
 169                 kind.to_ty(self.tcx),
 170                 substs.as_closure().kind_ty(expr_def_id, self.tcx),
 171             );
 172         }
 173
 174         closure_type
 175     }
 176
 177     /// Given the expected type, figures out what it can about this closure we
 178     /// are about to type check:
 179     fn deduce_expectations_from_expected_type(
 180         &self,
 181         expected_ty: Ty<'tcx>,
 182     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 183         debug!(
 184             "deduce_expectations_from_expected_type(expected_ty={:?})",
 185             expected_ty
 186         );
 187
 188         match expected_ty.kind {
 189             ty::Dynamic(ref object_type, ..) => {
 190                 let sig = object_type
 191                     .projection_bounds()
 192                     .filter_map(|pb| {
 193                         let pb = pb.with_self_ty(self.tcx, self.tcx.types.err);
 194                         self.deduce_sig_from_projection(None, &pb)
 195                     })
 196                     .next();
 197                 let kind = object_type.principal_def_id().and_then(|did| {
 198                     self.tcx.lang_items().fn_trait_kind(did)
 199                 });
 200                 (sig, kind)
 201             }
 202             ty::Infer(ty::TyVar(vid)) => self.deduce_expectations_from_obligations(vid),
 203             ty::FnPtr(sig) => {
 204                 let expected_sig = ExpectedSig {
 205                     cause_span: None,
 206                     sig: sig.skip_binder().clone(),
 207                 };
 208                 (Some(expected_sig), Some(ty::ClosureKind::Fn))
 209             }
 210             _ => (None, None),
 211         }
 212     }
 213
 214     fn deduce_expectations_from_obligations(
 215         &self,
 216         expected_vid: ty::TyVid,
 217     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 218         let expected_sig = self.obligations_for_self_ty(expected_vid)
 219             .find_map(|(_, obligation)| {
 220                 debug!(
 221                     "deduce_expectations_from_obligations: obligation.predicate={:?}",
 222                     obligation.predicate
 223                 );
 224
 225                 if let ty::Predicate::Projection(ref proj_predicate) = obligation.predicate {
 226                     // Given a Projection predicate, we can potentially infer
 227                     // the complete signature.
 228                     self.deduce_sig_from_projection(
 229                         Some(obligation.cause.span),
 230                         proj_predicate
 231                     )
 232                 } else {
 233                     None
 234                 }
 235             });
 236
 237         // Even if we can't infer the full signature, we may be able to
 238         // infer the kind. This can occur if there is a trait-reference
 239         // like `F : Fn<A>`. Note that due to subtyping we could encounter
 240         // many viable options, so pick the most restrictive.
 241         let expected_kind = self.obligations_for_self_ty(expected_vid)
 242             .filter_map(|(tr, _)| self.tcx.lang_items().fn_trait_kind(tr.def_id()))
 243             .fold(None, |best, cur| {
 244                 Some(best.map_or(cur, |best| cmp::min(best, cur)))
 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     fn deduce_sig_from_projection(
 257         &self,
 258         cause_span: Option<Span>,
 259         projection: &ty::PolyProjectionPredicate<'tcx>,
 260     ) -> Option<ExpectedSig<'tcx>> {
 261         let tcx = self.tcx;
 262
 263         debug!("deduce_sig_from_projection({:?})", projection);
 264
 265         let trait_ref = projection.to_poly_trait_ref(tcx);
 266
 267         let is_fn = tcx.lang_items().fn_trait_kind(trait_ref.def_id()).is_some();
 268         let gen_trait = tcx.require_lang_item(lang_items::GeneratorTraitLangItem, cause_span);
 269         let is_gen = gen_trait == trait_ref.def_id();
 270         if !is_fn && !is_gen {
 271             debug!("deduce_sig_from_projection: not fn or generator");
 272             return None;
 273         }
 274
 275         if is_gen {
 276             // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
 277             // associated item and not yield.
 278             let return_assoc_item = self.tcx.associated_items(gen_trait).nth(1).unwrap().def_id;
 279             if return_assoc_item != projection.projection_def_id() {
 280                 debug!("deduce_sig_from_projection: not return assoc item of generator");
 281                 return None;
 282             }
 283         }
 284
 285         let input_tys = if is_fn {
 286             let arg_param_ty = trait_ref.skip_binder().substs.type_at(1);
 287             let arg_param_ty = self.resolve_vars_if_possible(&arg_param_ty);
 288             debug!("deduce_sig_from_projection: arg_param_ty={:?}", arg_param_ty);
 289
 290             match arg_param_ty.kind {
 291                 ty::Tuple(tys) => tys.into_iter().map(|k| k.expect_ty()).collect::<Vec<_>>(),
 292                 _ => return None,
 293             }
 294         } else {
 295             // Generators cannot have explicit arguments.
 296             vec![]
 297         };
 298
 299         let ret_param_ty = projection.skip_binder().ty;
 300         let ret_param_ty = self.resolve_vars_if_possible(&ret_param_ty);
 301         debug!("deduce_sig_from_projection: ret_param_ty={:?}", ret_param_ty);
 302
 303         let sig = self.tcx.mk_fn_sig(
 304             input_tys.iter(),
 305             &ret_param_ty,
 306             false,
 307             hir::Unsafety::Normal,
 308             Abi::Rust,
 309         );
 310         debug!("deduce_sig_from_projection: sig={:?}", sig);
 311
 312         Some(ExpectedSig { cause_span, sig })
 313     }
 314
 315     fn sig_of_closure(
 316         &self,
 317         expr_def_id: DefId,
 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     fn sig_of_closure_no_expectation(
 332         &self,
 333         expr_def_id: DefId,
 334         decl: &hir::FnDecl,
 335         body: &hir::Body,
 336     ) -> ClosureSignatures<'tcx> {
 337         debug!("sig_of_closure_no_expectation()");
 338
 339         let bound_sig = self.supplied_sig_of_closure(expr_def_id, decl, body);
 340
 341         self.closure_sigs(expr_def_id, body, bound_sig)
 342     }
 343
 344     /// Invoked to compute the signature of a closure expression. This
 345     /// combines any user-provided type annotations (e.g., `|x: u32|
 346     /// -> u32 { .. }`) with the expected signature.
 347     ///
 348     /// The approach is as follows:
 349     ///
 350     /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
 351     /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
 352     ///   - If we have no expectation `E`, then the signature of the closure is `S`.
 353     ///   - Otherwise, the signature of the closure is E. Moreover:
 354     ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
 355     ///     - Instantiate all the late-bound regions bound in the closure within `S`
 356     ///       with fresh (existential) variables, yielding `S'`
 357     ///     - Require that `E' = S'`
 358     ///       - We could use some kind of subtyping relationship here,
 359     ///         I imagine, but equality is easier and works fine for
 360     ///         our purposes.
 361     ///
 362     /// The key intuition here is that the user's types must be valid
 363     /// from "the inside" of the closure, but the expectation
 364     /// ultimately drives the overall signature.
 365     ///
 366     /// # Examples
 367     ///
 368     /// ```
 369     /// fn with_closure<F>(_: F)
 370     ///   where F: Fn(&u32) -> &u32 { .. }
 371     ///
 372     /// with_closure(|x: &u32| { ... })
 373     /// ```
 374     ///
 375     /// Here:
 376     /// - E would be `fn(&u32) -> &u32`.
 377     /// - S would be `fn(&u32) ->
 378     /// - E' is `&'!0 u32 -> &'!0 u32`
 379     /// - S' is `&'?0 u32 -> ?T`
 380     ///
 381     /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
 382     ///
 383     /// # Arguments
 384     ///
 385     /// - `expr_def_id`: the `DefId` of the closure expression
 386     /// - `decl`: the HIR declaration of the closure
 387     /// - `body`: the body of the closure
 388     /// - `expected_sig`: the expected signature (if any). Note that
 389     ///   this is missing a binder: that is, there may be late-bound
 390     ///   regions with depth 1, which are bound then by the closure.
 391     fn sig_of_closure_with_expectation(
 392         &self,
 393         expr_def_id: DefId,
 394         decl: &hir::FnDecl,
 395         body: &hir::Body,
 396         expected_sig: ExpectedSig<'tcx>,
 397     ) -> ClosureSignatures<'tcx> {
 398         debug!(
 399             "sig_of_closure_with_expectation(expected_sig={:?})",
 400             expected_sig
 401         );
 402
 403         // Watch out for some surprises and just ignore the
 404         // expectation if things don't see to match up with what we
 405         // expect.
 406         if expected_sig.sig.c_variadic != decl.c_variadic {
 407             return self.sig_of_closure_no_expectation(expr_def_id, decl, body);
 408         } else if expected_sig.sig.inputs_and_output.len() != decl.inputs.len() + 1 {
 409             return self.sig_of_closure_with_mismatched_number_of_arguments(
 410                 expr_def_id,
 411                 decl,
 412                 body,
 413                 expected_sig,
 414             );
 415         }
 416
 417         // Create a `PolyFnSig`. Note the oddity that late bound
 418         // regions appearing free in `expected_sig` are now bound up
 419         // in this binder we are creating.
 420         assert!(!expected_sig.sig.has_vars_bound_above(ty::INNERMOST));
 421         let bound_sig = ty::Binder::bind(self.tcx.mk_fn_sig(
 422             expected_sig.sig.inputs().iter().cloned(),
 423             expected_sig.sig.output(),
 424             decl.c_variadic,
 425             hir::Unsafety::Normal,
 426             Abi::RustCall,
 427         ));
 428
 429         // `deduce_expectations_from_expected_type` introduces
 430         // late-bound lifetimes defined elsewhere, which we now
 431         // anonymize away, so as not to confuse the user.
 432         let bound_sig = self.tcx.anonymize_late_bound_regions(&bound_sig);
 433
 434         let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
 435
 436         // Up till this point, we have ignored the annotations that the user
 437         // gave. This function will check that they unify successfully.
 438         // Along the way, it also writes out entries for types that the user
 439         // wrote into our tables, which are then later used by the privacy
 440         // check.
 441         match self.check_supplied_sig_against_expectation(expr_def_id, decl, body, &closure_sigs) {
 442             Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
 443             Err(_) => return self.sig_of_closure_no_expectation(expr_def_id, decl, body),
 444         }
 445
 446         closure_sigs
 447     }
 448
 449     fn sig_of_closure_with_mismatched_number_of_arguments(
 450         &self,
 451         expr_def_id: DefId,
 452         decl: &hir::FnDecl,
 453         body: &hir::Body,
 454         expected_sig: ExpectedSig<'tcx>,
 455     ) -> ClosureSignatures<'tcx> {
 456         let expr_map_node = self.tcx.hir().get_if_local(expr_def_id).unwrap();
 457         let expected_args: Vec<_> = expected_sig
 458             .sig
 459             .inputs()
 460             .iter()
 461             .map(|ty| ArgKind::from_expected_ty(ty, None))
 462             .collect();
 463         let (closure_span, found_args) = self.get_fn_like_arguments(expr_map_node);
 464         let expected_span = expected_sig.cause_span.unwrap_or(closure_span);
 465         self.report_arg_count_mismatch(
 466             expected_span,
 467             Some(closure_span),
 468             expected_args,
 469             found_args,
 470             true,
 471         ).emit();
 472
 473         let error_sig = self.error_sig_of_closure(decl);
 474
 475         self.closure_sigs(expr_def_id, body, error_sig)
 476     }
 477
 478     /// Enforce the user's types against the expectation. See
 479     /// `sig_of_closure_with_expectation` for details on the overall
 480     /// strategy.
 481     fn check_supplied_sig_against_expectation(
 482         &self,
 483         expr_def_id: DefId,
 484         decl: &hir::FnDecl,
 485         body: &hir::Body,
 486         expected_sigs: &ClosureSignatures<'tcx>,
 487     ) -> InferResult<'tcx, ()> {
 488         // Get the signature S that the user gave.
 489         //
 490         // (See comment on `sig_of_closure_with_expectation` for the
 491         // meaning of these letters.)
 492         let supplied_sig = self.supplied_sig_of_closure(expr_def_id, decl, body);
 493
 494         debug!(
 495             "check_supplied_sig_against_expectation: supplied_sig={:?}",
 496             supplied_sig
 497         );
 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.infcx.commit_if_ok(|_| {
 514             let mut all_obligations = vec![];
 515
 516             // The liberated version of this signature should be a subtype
 517             // of the liberated form of the expectation.
 518             for ((hir_ty, &supplied_ty), expected_ty) in decl.inputs.iter()
 519                .zip(*supplied_sig.inputs().skip_binder()) // binder moved to (*) below
 520                .zip(expected_sigs.liberated_sig.inputs())
 521             // `liberated_sig` is E'.
 522             {
 523                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 524                 let (supplied_ty, _) = self.infcx.replace_bound_vars_with_fresh_vars(
 525                     hir_ty.span,
 526                     LateBoundRegionConversionTime::FnCall,
 527                     &ty::Binder::bind(supplied_ty),
 528                 ); // recreated from (*) above
 529
 530                 // Check that E' = S'.
 531                 let cause = self.misc(hir_ty.span);
 532                 let InferOk {
 533                     value: (),
 534                     obligations,
 535                 } = self.at(&cause, self.param_env)
 536                     .eq(*expected_ty, supplied_ty)?;
 537                 all_obligations.extend(obligations);
 538
 539                 // Also, require that the supplied type must outlive
 540                 // the closure body.
 541                 let closure_body_region = self.tcx.mk_region(
 542                     ty::ReScope(
 543                         region::Scope {
 544                             id: body.value.hir_id.local_id,
 545                             data: region::ScopeData::Node,
 546                         },
 547                     ),
 548                 );
 549                 all_obligations.push(
 550                     Obligation::new(
 551                         cause,
 552                         self.param_env,
 553                         ty::Predicate::TypeOutlives(
 554                             ty::Binder::dummy(
 555                                 ty::OutlivesPredicate(
 556                                     supplied_ty,
 557                                     closure_body_region,
 558                                 ),
 559                             ),
 560                         ),
 561                     ),
 562                 );
 563             }
 564
 565             let (supplied_output_ty, _) = self.infcx.replace_bound_vars_with_fresh_vars(
 566                 decl.output.span(),
 567                 LateBoundRegionConversionTime::FnCall,
 568                 &supplied_sig.output(),
 569             );
 570             let cause = &self.misc(decl.output.span());
 571             let InferOk {
 572                 value: (),
 573                 obligations,
 574             } = self.at(cause, self.param_env)
 575                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 576             all_obligations.extend(obligations);
 577
 578             Ok(InferOk {
 579                 value: (),
 580                 obligations: all_obligations,
 581             })
 582         })
 583     }
 584
 585     /// If there is no expected signature, then we will convert the
 586     /// types that the user gave into a signature.
 587     ///
 588     /// Also, record this closure signature for later.
 589     fn supplied_sig_of_closure(
 590         &self,
 591         expr_def_id: DefId,
 592         decl: &hir::FnDecl,
 593         body: &hir::Body,
 594     ) -> ty::PolyFnSig<'tcx> {
 595         let astconv: &dyn AstConv<'_> = self;
 596
 597         debug!(
 598             "supplied_sig_of_closure(decl={:?}, body.generator_kind={:?})",
 599             decl,
 600             body.generator_kind,
 601         );
 602
 603         // First, convert the types that the user supplied (if any).
 604         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 605         let supplied_return = match decl.output {
 606             hir::Return(ref output) => astconv.ast_ty_to_ty(&output),
 607             hir::DefaultReturn(_) => match body.generator_kind {
 608                 // In the case of the async block that we create for a function body,
 609                 // we expect the return type of the block to match that of the enclosing
 610                 // function.
 611                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 612                     debug!("supplied_sig_of_closure: closure is async fn body");
 613                     self.deduce_future_output_from_obligations(expr_def_id)
 614                         .unwrap_or_else(|| {
 615                             // AFAIK, deducing the future output
 616                             // always succeeds *except* in error cases
 617                             // like #65159. I'd like to return Error
 618                             // here, but I can't because I can't
 619                             // easily (and locally) prove that we
 620                             // *have* reported an
 621                             // error. --nikomatsakis
 622                             astconv.ty_infer(None, decl.output.span())
 623                         })
 624                 }
 625
 626                 _ => astconv.ty_infer(None, decl.output.span()),
 627             }
 628         };
 629
 630         let result = ty::Binder::bind(self.tcx.mk_fn_sig(
 631             supplied_arguments,
 632             supplied_return,
 633             decl.c_variadic,
 634             hir::Unsafety::Normal,
 635             Abi::RustCall,
 636         ));
 637
 638         debug!("supplied_sig_of_closure: result={:?}", result);
 639
 640         let c_result = self.inh.infcx.canonicalize_response(&result);
 641         self.tables.borrow_mut().user_provided_sigs.insert(
 642             expr_def_id,
 643             c_result,
 644         );
 645
 646         result
 647     }
 648
 649     /// Invoked when we are translating the generator that results
 650     /// from desugaring an `async fn`. Returns the "sugared" return
 651     /// type of the `async fn` -- that is, the return type that the
 652     /// user specified. The "desugared" return type is a `impl
 653     /// Future<Output = T>`, so we do this by searching through the
 654     /// obligations to extract the `T`.
 655     fn deduce_future_output_from_obligations(
 656         &self,
 657         expr_def_id: DefId,
 658     ) -> Option<Ty<'tcx>> {
 659         debug!("deduce_future_output_from_obligations(expr_def_id={:?})", expr_def_id);
 660
 661         let ret_coercion =
 662             self.ret_coercion
 663             .as_ref()
 664             .unwrap_or_else(|| span_bug!(
 665                 self.tcx.def_span(expr_def_id),
 666                 "async fn generator outside of a fn"
 667             ));
 668
 669         // In practice, the return type of the surrounding function is
 670         // always a (not yet resolved) inference variable, because it
 671         // is the hidden type for an `impl Trait` that we are going to
 672         // be inferring.
 673         let ret_ty = ret_coercion.borrow().expected_ty();
 674         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 675         let ret_vid = match ret_ty.kind {
 676             ty::Infer(ty::TyVar(ret_vid)) => ret_vid,
 677             _ => {
 678                 span_bug!(
 679                     self.tcx.def_span(expr_def_id),
 680                     "async fn generator return type not an inference variable"
 681                 )
 682             }
 683         };
 684
 685         // Search for a pending obligation like
 686         //
 687         // `<R as Future>::Output = T`
 688         //
 689         // where R is the return type we are expecting. This type `T`
 690         // will be our output.
 691         let output_ty = self.obligations_for_self_ty(ret_vid)
 692             .find_map(|(_, obligation)| {
 693                 if let ty::Predicate::Projection(ref proj_predicate) = obligation.predicate {
 694                     self.deduce_future_output_from_projection(
 695                         obligation.cause.span,
 696                         proj_predicate
 697                     )
 698                 } else {
 699                     None
 700                 }
 701             });
 702
 703         debug!("deduce_future_output_from_obligations: output_ty={:?}", output_ty);
 704         output_ty
 705     }
 706
 707     /// Given a projection like
 708     ///
 709     /// `<X as Future>::Output = T`
 710     ///
 711     /// where `X` is some type that has no late-bound regions, returns
 712     /// `Some(T)`. If the projection is for some other trait, returns
 713     /// `None`.
 714     fn deduce_future_output_from_projection(
 715         &self,
 716         cause_span: Span,
 717         predicate: &ty::PolyProjectionPredicate<'tcx>,
 718     ) -> Option<Ty<'tcx>> {
 719         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 720
 721         // We do not expect any bound regions in our predicate, so
 722         // skip past the bound vars.
 723         let predicate = match predicate.no_bound_vars() {
 724             Some(p) => p,
 725             None => {
 726                 debug!("deduce_future_output_from_projection: has late-bound regions");
 727                 return None;
 728             }
 729         };
 730
 731         // Check that this is a projection from the `Future` trait.
 732         let trait_ref = predicate.projection_ty.trait_ref(self.tcx);
 733         let future_trait = self.tcx.lang_items().future_trait().unwrap();
 734         if trait_ref.def_id != future_trait {
 735             debug!("deduce_future_output_from_projection: not a future");
 736             return None;
 737         }
 738
 739         // The `Future` trait has only one associted item, `Output`,
 740         // so check that this is what we see.
 741         let output_assoc_item = self.tcx.associated_items(future_trait).nth(0).unwrap().def_id;
 742         if output_assoc_item != predicate.projection_ty.item_def_id {
 743             span_bug!(
 744                 cause_span,
 745                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 746                 predicate.projection_ty.item_def_id,
 747                 output_assoc_item,
 748             );
 749         }
 750
 751         // Extract the type from the projection. Note that there can
 752         // be no bound variables in this type because the "self type"
 753         // does not have any regions in it.
 754         let output_ty = self.resolve_vars_if_possible(&predicate.ty);
 755         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 756         Some(output_ty)
 757     }
 758
 759     /// Converts the types that the user supplied, in case that doing
 760     /// so should yield an error, but returns back a signature where
 761     /// all parameters are of type `TyErr`.
 762     fn error_sig_of_closure(&self, decl: &hir::FnDecl) -> ty::PolyFnSig<'tcx> {
 763         let astconv: &dyn AstConv<'_> = self;
 764
 765         let supplied_arguments = decl.inputs.iter().map(|a| {
 766             // Convert the types that the user supplied (if any), but ignore them.
 767             astconv.ast_ty_to_ty(a);
 768             self.tcx.types.err
 769         });
 770
 771         if let hir::Return(ref output) = decl.output {
 772             astconv.ast_ty_to_ty(&output);
 773         }
 774
 775         let result = ty::Binder::bind(self.tcx.mk_fn_sig(
 776             supplied_arguments,
 777             self.tcx.types.err,
 778             decl.c_variadic,
 779             hir::Unsafety::Normal,
 780             Abi::RustCall,
 781         ));
 782
 783         debug!("supplied_sig_of_closure: result={:?}", result);
 784
 785         result
 786     }
 787
 788     fn closure_sigs(
 789         &self,
 790         expr_def_id: DefId,
 791         body: &hir::Body,
 792         bound_sig: ty::PolyFnSig<'tcx>,
 793     ) -> ClosureSignatures<'tcx> {
 794         let liberated_sig = self.tcx()
 795             .liberate_late_bound_regions(expr_def_id, &bound_sig);
 796         let liberated_sig = self.inh.normalize_associated_types_in(
 797             body.value.span,
 798             body.value.hir_id,
 799             self.param_env,
 800             &liberated_sig,
 801         );
 802         ClosureSignatures {
 803             bound_sig,
 804             liberated_sig,
 805         }
 806     }
 807 }