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