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