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