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::ty::fold::TypeFoldable;
   8 use rustc::ty::subst::InternalSubsts;
   9 use rustc::ty::{self, GenericParamDefKind, Ty};
  10 use rustc_hir as hir;
  11 use rustc_hir::def_id::DefId;
  12 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  13 use rustc_infer::infer::LateBoundRegionConversionTime;
  14 use rustc_infer::infer::{InferOk, InferResult};
  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 rustc_trait_selection::traits::Obligation;
  20 use std::cmp;
  21 use std::iter;
  22
  23 /// What signature do we *expect* the closure to have from context?
  24 #[derive(Debug)]
  25 struct ExpectedSig<'tcx> {
  26     /// Span that gave us this expectation, if we know that.
  27     cause_span: Option<Span>,
  28     sig: ty::FnSig<'tcx>,
  29 }
  30
  31 struct ClosureSignatures<'tcx> {
  32     bound_sig: ty::PolyFnSig<'tcx>,
  33     liberated_sig: ty::FnSig<'tcx>,
  34 }
  35
  36 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  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         debug!("check_expr_closure(expr={:?},expected={:?})", expr, expected);
  47
  48         // It's always helpful for inference if we know the kind of
  49         // closure sooner rather than later, so first examine the expected
  50         // type, and see if can glean a closure kind from there.
  51         let (expected_sig, expected_kind) = match expected.to_option(self) {
  52             Some(ty) => self.deduce_expectations_from_expected_type(ty),
  53             None => (None, None),
  54         };
  55         let body = self.tcx.hir().body(body_id);
  56         self.check_closure(expr, expected_kind, decl, body, gen, expected_sig)
  57     }
  58
  59     fn check_closure(
  60         &self,
  61         expr: &hir::Expr<'_>,
  62         opt_kind: Option<ty::ClosureKind>,
  63         decl: &'tcx hir::FnDecl<'tcx>,
  64         body: &'tcx hir::Body<'tcx>,
  65         gen: Option<hir::Movability>,
  66         expected_sig: Option<ExpectedSig<'tcx>>,
  67     ) -> Ty<'tcx> {
  68         debug!("check_closure(opt_kind={:?}, expected_sig={:?})", opt_kind, expected_sig);
  69
  70         let expr_def_id = self.tcx.hir().local_def_id(expr.hir_id);
  71
  72         let ClosureSignatures { bound_sig, liberated_sig } =
  73             self.sig_of_closure(expr_def_id, decl, body, expected_sig);
  74
  75         debug!("check_closure: ty_of_closure returns {:?}", liberated_sig);
  76
  77         let generator_types =
  78             check_fn(self, self.param_env, liberated_sig, decl, expr.hir_id, body, gen).1;
  79
  80         // Create type variables (for now) to represent the transformed
  81         // types of upvars. These will be unified during the upvar
  82         // inference phase (`upvar.rs`).
  83         let base_substs =
  84             InternalSubsts::identity_for_item(self.tcx, self.tcx.closure_base_def_id(expr_def_id));
  85         let substs = base_substs.extend_to(self.tcx, expr_def_id, |param, _| match param.kind {
  86             GenericParamDefKind::Lifetime => span_bug!(expr.span, "closure has lifetime param"),
  87             GenericParamDefKind::Type { .. } => self
  88                 .infcx
  89                 .next_ty_var(TypeVariableOrigin {
  90                     kind: TypeVariableOriginKind::ClosureSynthetic,
  91                     span: expr.span,
  92                 })
  93                 .into(),
  94             GenericParamDefKind::Const => span_bug!(expr.span, "closure has const param"),
  95         });
  96         if let Some(GeneratorTypes { resume_ty, yield_ty, interior, movability }) = generator_types
  97         {
  98             let generator_substs = substs.as_generator();
  99             self.demand_eqtype(
 100                 expr.span,
 101                 resume_ty,
 102                 generator_substs.resume_ty(expr_def_id, self.tcx),
 103             );
 104             self.demand_eqtype(
 105                 expr.span,
 106                 yield_ty,
 107                 generator_substs.yield_ty(expr_def_id, self.tcx),
 108             );
 109             self.demand_eqtype(
 110                 expr.span,
 111                 liberated_sig.output(),
 112                 generator_substs.return_ty(expr_def_id, self.tcx),
 113             );
 114             self.demand_eqtype(
 115                 expr.span,
 116                 interior,
 117                 generator_substs.witness(expr_def_id, self.tcx),
 118             );
 119             return self.tcx.mk_generator(expr_def_id, substs, movability);
 120         }
 121
 122         let closure_type = self.tcx.mk_closure(expr_def_id, substs);
 123
 124         debug!("check_closure: expr.hir_id={:?} closure_type={:?}", expr.hir_id, closure_type);
 125
 126         // Tuple up the arguments and insert the resulting function type into
 127         // the `closures` table.
 128         let sig = bound_sig.map_bound(|sig| {
 129             self.tcx.mk_fn_sig(
 130                 iter::once(self.tcx.intern_tup(sig.inputs())),
 131                 sig.output(),
 132                 sig.c_variadic,
 133                 sig.unsafety,
 134                 sig.abi,
 135             )
 136         });
 137
 138         debug!(
 139             "check_closure: expr_def_id={:?}, sig={:?}, opt_kind={:?}",
 140             expr_def_id, sig, opt_kind
 141         );
 142
 143         let sig_fn_ptr_ty = self.tcx.mk_fn_ptr(sig);
 144         self.demand_eqtype(
 145             expr.span,
 146             sig_fn_ptr_ty,
 147             substs.as_closure().sig_ty(expr_def_id, self.tcx),
 148         );
 149
 150         if let Some(kind) = opt_kind {
 151             self.demand_eqtype(
 152                 expr.span,
 153                 kind.to_ty(self.tcx),
 154                 substs.as_closure().kind_ty(expr_def_id, self.tcx),
 155             );
 156         }
 157
 158         closure_type
 159     }
 160
 161     /// Given the expected type, figures out what it can about this closure we
 162     /// are about to type check:
 163     fn deduce_expectations_from_expected_type(
 164         &self,
 165         expected_ty: Ty<'tcx>,
 166     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 167         debug!("deduce_expectations_from_expected_type(expected_ty={:?})", expected_ty);
 168
 169         match expected_ty.kind {
 170             ty::Dynamic(ref object_type, ..) => {
 171                 let sig = object_type
 172                     .projection_bounds()
 173                     .filter_map(|pb| {
 174                         let pb = pb.with_self_ty(self.tcx, self.tcx.types.err);
 175                         self.deduce_sig_from_projection(None, &pb)
 176                     })
 177                     .next();
 178                 let kind = object_type
 179                     .principal_def_id()
 180                     .and_then(|did| self.tcx.fn_trait_kind_from_lang_item(did));
 181                 (sig, kind)
 182             }
 183             ty::Infer(ty::TyVar(vid)) => self.deduce_expectations_from_obligations(vid),
 184             ty::FnPtr(sig) => {
 185                 let expected_sig = ExpectedSig { cause_span: None, sig: *sig.skip_binder() };
 186                 (Some(expected_sig), Some(ty::ClosureKind::Fn))
 187             }
 188             _ => (None, None),
 189         }
 190     }
 191
 192     fn deduce_expectations_from_obligations(
 193         &self,
 194         expected_vid: ty::TyVid,
 195     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 196         let expected_sig =
 197             self.obligations_for_self_ty(expected_vid).find_map(|(_, obligation)| {
 198                 debug!(
 199                     "deduce_expectations_from_obligations: obligation.predicate={:?}",
 200                     obligation.predicate
 201                 );
 202
 203                 if let ty::Predicate::Projection(ref proj_predicate) = obligation.predicate {
 204                     // Given a Projection predicate, we can potentially infer
 205                     // the complete signature.
 206                     self.deduce_sig_from_projection(Some(obligation.cause.span), proj_predicate)
 207                 } else {
 208                     None
 209                 }
 210             });
 211
 212         // Even if we can't infer the full signature, we may be able to
 213         // infer the kind. This can occur if there is a trait-reference
 214         // like `F : Fn<A>`. Note that due to subtyping we could encounter
 215         // many viable options, so pick the most restrictive.
 216         let expected_kind = self
 217             .obligations_for_self_ty(expected_vid)
 218             .filter_map(|(tr, _)| self.tcx.fn_trait_kind_from_lang_item(tr.def_id()))
 219             .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
 220
 221         (expected_sig, expected_kind)
 222     }
 223
 224     /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
 225     /// everything we need to know about a closure or generator.
 226     ///
 227     /// The `cause_span` should be the span that caused us to
 228     /// have this expected signature, or `None` if we can't readily
 229     /// know that.
 230     fn deduce_sig_from_projection(
 231         &self,
 232         cause_span: Option<Span>,
 233         projection: &ty::PolyProjectionPredicate<'tcx>,
 234     ) -> Option<ExpectedSig<'tcx>> {
 235         let tcx = self.tcx;
 236
 237         debug!("deduce_sig_from_projection({:?})", projection);
 238
 239         let trait_ref = projection.to_poly_trait_ref(tcx);
 240
 241         let is_fn = tcx.fn_trait_kind_from_lang_item(trait_ref.def_id()).is_some();
 242         let gen_trait = tcx.require_lang_item(lang_items::GeneratorTraitLangItem, cause_span);
 243         let is_gen = gen_trait == trait_ref.def_id();
 244         if !is_fn && !is_gen {
 245             debug!("deduce_sig_from_projection: not fn or generator");
 246             return None;
 247         }
 248
 249         if is_gen {
 250             // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
 251             // associated item and not yield.
 252             let return_assoc_item =
 253                 self.tcx.associated_items(gen_trait).in_definition_order().nth(1).unwrap().def_id;
 254             if return_assoc_item != projection.projection_def_id() {
 255                 debug!("deduce_sig_from_projection: not return assoc item of generator");
 256                 return None;
 257             }
 258         }
 259
 260         let input_tys = if is_fn {
 261             let arg_param_ty = trait_ref.skip_binder().substs.type_at(1);
 262             let arg_param_ty = self.resolve_vars_if_possible(&arg_param_ty);
 263             debug!("deduce_sig_from_projection: arg_param_ty={:?}", arg_param_ty);
 264
 265             match arg_param_ty.kind {
 266                 ty::Tuple(tys) => tys.into_iter().map(|k| k.expect_ty()).collect::<Vec<_>>(),
 267                 _ => return None,
 268             }
 269         } else {
 270             // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
 271             // else they must have exactly 1 argument. For now though, just give up in this case.
 272             return None;
 273         };
 274
 275         let ret_param_ty = projection.skip_binder().ty;
 276         let ret_param_ty = self.resolve_vars_if_possible(&ret_param_ty);
 277         debug!("deduce_sig_from_projection: ret_param_ty={:?}", ret_param_ty);
 278
 279         let sig = self.tcx.mk_fn_sig(
 280             input_tys.iter(),
 281             &ret_param_ty,
 282             false,
 283             hir::Unsafety::Normal,
 284             Abi::Rust,
 285         );
 286         debug!("deduce_sig_from_projection: sig={:?}", sig);
 287
 288         Some(ExpectedSig { cause_span, sig })
 289     }
 290
 291     fn sig_of_closure(
 292         &self,
 293         expr_def_id: DefId,
 294         decl: &hir::FnDecl<'_>,
 295         body: &hir::Body<'_>,
 296         expected_sig: Option<ExpectedSig<'tcx>>,
 297     ) -> ClosureSignatures<'tcx> {
 298         if let Some(e) = expected_sig {
 299             self.sig_of_closure_with_expectation(expr_def_id, decl, body, e)
 300         } else {
 301             self.sig_of_closure_no_expectation(expr_def_id, decl, body)
 302         }
 303     }
 304
 305     /// If there is no expected signature, then we will convert the
 306     /// types that the user gave into a signature.
 307     fn sig_of_closure_no_expectation(
 308         &self,
 309         expr_def_id: DefId,
 310         decl: &hir::FnDecl<'_>,
 311         body: &hir::Body<'_>,
 312     ) -> ClosureSignatures<'tcx> {
 313         debug!("sig_of_closure_no_expectation()");
 314
 315         let bound_sig = self.supplied_sig_of_closure(expr_def_id, decl, body);
 316
 317         self.closure_sigs(expr_def_id, body, bound_sig)
 318     }
 319
 320     /// Invoked to compute the signature of a closure expression. This
 321     /// combines any user-provided type annotations (e.g., `|x: u32|
 322     /// -> u32 { .. }`) with the expected signature.
 323     ///
 324     /// The approach is as follows:
 325     ///
 326     /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
 327     /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
 328     ///   - If we have no expectation `E`, then the signature of the closure is `S`.
 329     ///   - Otherwise, the signature of the closure is E. Moreover:
 330     ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
 331     ///     - Instantiate all the late-bound regions bound in the closure within `S`
 332     ///       with fresh (existential) variables, yielding `S'`
 333     ///     - Require that `E' = S'`
 334     ///       - We could use some kind of subtyping relationship here,
 335     ///         I imagine, but equality is easier and works fine for
 336     ///         our purposes.
 337     ///
 338     /// The key intuition here is that the user's types must be valid
 339     /// from "the inside" of the closure, but the expectation
 340     /// ultimately drives the overall signature.
 341     ///
 342     /// # Examples
 343     ///
 344     /// ```
 345     /// fn with_closure<F>(_: F)
 346     ///   where F: Fn(&u32) -> &u32 { .. }
 347     ///
 348     /// with_closure(|x: &u32| { ... })
 349     /// ```
 350     ///
 351     /// Here:
 352     /// - E would be `fn(&u32) -> &u32`.
 353     /// - S would be `fn(&u32) ->
 354     /// - E' is `&'!0 u32 -> &'!0 u32`
 355     /// - S' is `&'?0 u32 -> ?T`
 356     ///
 357     /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
 358     ///
 359     /// # Arguments
 360     ///
 361     /// - `expr_def_id`: the `DefId` of the closure expression
 362     /// - `decl`: the HIR declaration of the closure
 363     /// - `body`: the body of the closure
 364     /// - `expected_sig`: the expected signature (if any). Note that
 365     ///   this is missing a binder: that is, there may be late-bound
 366     ///   regions with depth 1, which are bound then by the closure.
 367     fn sig_of_closure_with_expectation(
 368         &self,
 369         expr_def_id: DefId,
 370         decl: &hir::FnDecl<'_>,
 371         body: &hir::Body<'_>,
 372         expected_sig: ExpectedSig<'tcx>,
 373     ) -> ClosureSignatures<'tcx> {
 374         debug!("sig_of_closure_with_expectation(expected_sig={:?})", expected_sig);
 375
 376         // Watch out for some surprises and just ignore the
 377         // expectation if things don't see to match up with what we
 378         // expect.
 379         if expected_sig.sig.c_variadic != decl.c_variadic {
 380             return self.sig_of_closure_no_expectation(expr_def_id, decl, body);
 381         } else if expected_sig.sig.inputs_and_output.len() != decl.inputs.len() + 1 {
 382             return self.sig_of_closure_with_mismatched_number_of_arguments(
 383                 expr_def_id,
 384                 decl,
 385                 body,
 386                 expected_sig,
 387             );
 388         }
 389
 390         // Create a `PolyFnSig`. Note the oddity that late bound
 391         // regions appearing free in `expected_sig` are now bound up
 392         // in this binder we are creating.
 393         assert!(!expected_sig.sig.has_vars_bound_above(ty::INNERMOST));
 394         let bound_sig = ty::Binder::bind(self.tcx.mk_fn_sig(
 395             expected_sig.sig.inputs().iter().cloned(),
 396             expected_sig.sig.output(),
 397             decl.c_variadic,
 398             hir::Unsafety::Normal,
 399             Abi::RustCall,
 400         ));
 401
 402         // `deduce_expectations_from_expected_type` introduces
 403         // late-bound lifetimes defined elsewhere, which we now
 404         // anonymize away, so as not to confuse the user.
 405         let bound_sig = self.tcx.anonymize_late_bound_regions(&bound_sig);
 406
 407         let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
 408
 409         // Up till this point, we have ignored the annotations that the user
 410         // gave. This function will check that they unify successfully.
 411         // Along the way, it also writes out entries for types that the user
 412         // wrote into our tables, which are then later used by the privacy
 413         // check.
 414         match self.check_supplied_sig_against_expectation(expr_def_id, decl, body, &closure_sigs) {
 415             Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
 416             Err(_) => return self.sig_of_closure_no_expectation(expr_def_id, decl, body),
 417         }
 418
 419         closure_sigs
 420     }
 421
 422     fn sig_of_closure_with_mismatched_number_of_arguments(
 423         &self,
 424         expr_def_id: DefId,
 425         decl: &hir::FnDecl<'_>,
 426         body: &hir::Body<'_>,
 427         expected_sig: ExpectedSig<'tcx>,
 428     ) -> ClosureSignatures<'tcx> {
 429         let expr_map_node = self.tcx.hir().get_if_local(expr_def_id).unwrap();
 430         let expected_args: Vec<_> = expected_sig
 431             .sig
 432             .inputs()
 433             .iter()
 434             .map(|ty| ArgKind::from_expected_ty(ty, None))
 435             .collect();
 436         let (closure_span, found_args) = self.get_fn_like_arguments(expr_map_node);
 437         let expected_span = expected_sig.cause_span.unwrap_or(closure_span);
 438         self.report_arg_count_mismatch(
 439             expected_span,
 440             Some(closure_span),
 441             expected_args,
 442             found_args,
 443             true,
 444         )
 445         .emit();
 446
 447         let error_sig = self.error_sig_of_closure(decl);
 448
 449         self.closure_sigs(expr_def_id, body, error_sig)
 450     }
 451
 452     /// Enforce the user's types against the expectation. See
 453     /// `sig_of_closure_with_expectation` for details on the overall
 454     /// strategy.
 455     fn check_supplied_sig_against_expectation(
 456         &self,
 457         expr_def_id: DefId,
 458         decl: &hir::FnDecl<'_>,
 459         body: &hir::Body<'_>,
 460         expected_sigs: &ClosureSignatures<'tcx>,
 461     ) -> InferResult<'tcx, ()> {
 462         // Get the signature S that the user gave.
 463         //
 464         // (See comment on `sig_of_closure_with_expectation` for the
 465         // meaning of these letters.)
 466         let supplied_sig = self.supplied_sig_of_closure(expr_def_id, decl, body);
 467
 468         debug!("check_supplied_sig_against_expectation: supplied_sig={:?}", supplied_sig);
 469
 470         // FIXME(#45727): As discussed in [this comment][c1], naively
 471         // forcing equality here actually results in suboptimal error
 472         // messages in some cases.  For now, if there would have been
 473         // an obvious error, we fallback to declaring the type of the
 474         // closure to be the one the user gave, which allows other
 475         // error message code to trigger.
 476         //
 477         // However, I think [there is potential to do even better
 478         // here][c2], since in *this* code we have the precise span of
 479         // the type parameter in question in hand when we report the
 480         // error.
 481         //
 482         // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
 483         // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
 484         self.infcx.commit_if_ok(|_| {
 485             let mut all_obligations = vec![];
 486
 487             // The liberated version of this signature should be a subtype
 488             // of the liberated form of the expectation.
 489             for ((hir_ty, &supplied_ty), expected_ty) in decl
 490                 .inputs
 491                 .iter()
 492                 .zip(*supplied_sig.inputs().skip_binder()) // binder moved to (*) below
 493                 .zip(expected_sigs.liberated_sig.inputs())
 494             // `liberated_sig` is E'.
 495             {
 496                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 497                 let (supplied_ty, _) = self.infcx.replace_bound_vars_with_fresh_vars(
 498                     hir_ty.span,
 499                     LateBoundRegionConversionTime::FnCall,
 500                     &ty::Binder::bind(supplied_ty),
 501                 ); // recreated from (*) above
 502
 503                 // Check that E' = S'.
 504                 let cause = self.misc(hir_ty.span);
 505                 let InferOk { value: (), obligations } =
 506                     self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
 507                 all_obligations.extend(obligations);
 508
 509                 // Also, require that the supplied type must outlive
 510                 // the closure body.
 511                 let closure_body_region = self.tcx.mk_region(ty::ReScope(region::Scope {
 512                     id: body.value.hir_id.local_id,
 513                     data: region::ScopeData::Node,
 514                 }));
 515                 all_obligations.push(Obligation::new(
 516                     cause,
 517                     self.param_env,
 518                     ty::Predicate::TypeOutlives(ty::Binder::dummy(ty::OutlivesPredicate(
 519                         supplied_ty,
 520                         closure_body_region,
 521                     ))),
 522                 ));
 523             }
 524
 525             let (supplied_output_ty, _) = self.infcx.replace_bound_vars_with_fresh_vars(
 526                 decl.output.span(),
 527                 LateBoundRegionConversionTime::FnCall,
 528                 &supplied_sig.output(),
 529             );
 530             let cause = &self.misc(decl.output.span());
 531             let InferOk { value: (), obligations } = self
 532                 .at(cause, self.param_env)
 533                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 534             all_obligations.extend(obligations);
 535
 536             Ok(InferOk { value: (), obligations: all_obligations })
 537         })
 538     }
 539
 540     /// If there is no expected signature, then we will convert the
 541     /// types that the user gave into a signature.
 542     ///
 543     /// Also, record this closure signature for later.
 544     fn supplied_sig_of_closure(
 545         &self,
 546         expr_def_id: DefId,
 547         decl: &hir::FnDecl<'_>,
 548         body: &hir::Body<'_>,
 549     ) -> ty::PolyFnSig<'tcx> {
 550         let astconv: &dyn AstConv<'_> = self;
 551
 552         debug!(
 553             "supplied_sig_of_closure(decl={:?}, body.generator_kind={:?})",
 554             decl, body.generator_kind,
 555         );
 556
 557         // First, convert the types that the user supplied (if any).
 558         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 559         let supplied_return = match decl.output {
 560             hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
 561             hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
 562                 // In the case of the async block that we create for a function body,
 563                 // we expect the return type of the block to match that of the enclosing
 564                 // function.
 565                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 566                     debug!("supplied_sig_of_closure: closure is async fn body");
 567                     self.deduce_future_output_from_obligations(expr_def_id).unwrap_or_else(|| {
 568                         // AFAIK, deducing the future output
 569                         // always succeeds *except* in error cases
 570                         // like #65159. I'd like to return Error
 571                         // here, but I can't because I can't
 572                         // easily (and locally) prove that we
 573                         // *have* reported an
 574                         // error. --nikomatsakis
 575                         astconv.ty_infer(None, decl.output.span())
 576                     })
 577                 }
 578
 579                 _ => astconv.ty_infer(None, decl.output.span()),
 580             },
 581         };
 582
 583         let result = ty::Binder::bind(self.tcx.mk_fn_sig(
 584             supplied_arguments,
 585             supplied_return,
 586             decl.c_variadic,
 587             hir::Unsafety::Normal,
 588             Abi::RustCall,
 589         ));
 590
 591         debug!("supplied_sig_of_closure: result={:?}", result);
 592
 593         let c_result = self.inh.infcx.canonicalize_response(&result);
 594         self.tables.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);
 595
 596         result
 597     }
 598
 599     /// Invoked when we are translating the generator that results
 600     /// from desugaring an `async fn`. Returns the "sugared" return
 601     /// type of the `async fn` -- that is, the return type that the
 602     /// user specified. The "desugared" return type is a `impl
 603     /// Future<Output = T>`, so we do this by searching through the
 604     /// obligations to extract the `T`.
 605     fn deduce_future_output_from_obligations(&self, expr_def_id: DefId) -> Option<Ty<'tcx>> {
 606         debug!("deduce_future_output_from_obligations(expr_def_id={:?})", expr_def_id);
 607
 608         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
 609             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
 610         });
 611
 612         // In practice, the return type of the surrounding function is
 613         // always a (not yet resolved) inference variable, because it
 614         // is the hidden type for an `impl Trait` that we are going to
 615         // be inferring.
 616         let ret_ty = ret_coercion.borrow().expected_ty();
 617         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 618         let ret_vid = match ret_ty.kind {
 619             ty::Infer(ty::TyVar(ret_vid)) => ret_vid,
 620             _ => span_bug!(
 621                 self.tcx.def_span(expr_def_id),
 622                 "async fn generator return type not an inference variable"
 623             ),
 624         };
 625
 626         // Search for a pending obligation like
 627         //
 628         // `<R as Future>::Output = T`
 629         //
 630         // where R is the return type we are expecting. This type `T`
 631         // will be our output.
 632         let output_ty = self.obligations_for_self_ty(ret_vid).find_map(|(_, obligation)| {
 633             if let ty::Predicate::Projection(ref proj_predicate) = obligation.predicate {
 634                 self.deduce_future_output_from_projection(obligation.cause.span, proj_predicate)
 635             } else {
 636                 None
 637             }
 638         });
 639
 640         debug!("deduce_future_output_from_obligations: output_ty={:?}", output_ty);
 641         output_ty
 642     }
 643
 644     /// Given a projection like
 645     ///
 646     /// `<X as Future>::Output = T`
 647     ///
 648     /// where `X` is some type that has no late-bound regions, returns
 649     /// `Some(T)`. If the projection is for some other trait, returns
 650     /// `None`.
 651     fn deduce_future_output_from_projection(
 652         &self,
 653         cause_span: Span,
 654         predicate: &ty::PolyProjectionPredicate<'tcx>,
 655     ) -> Option<Ty<'tcx>> {
 656         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 657
 658         // We do not expect any bound regions in our predicate, so
 659         // skip past the bound vars.
 660         let predicate = match predicate.no_bound_vars() {
 661             Some(p) => p,
 662             None => {
 663                 debug!("deduce_future_output_from_projection: has late-bound regions");
 664                 return None;
 665             }
 666         };
 667
 668         // Check that this is a projection from the `Future` trait.
 669         let trait_ref = predicate.projection_ty.trait_ref(self.tcx);
 670         let future_trait = self.tcx.lang_items().future_trait().unwrap();
 671         if trait_ref.def_id != future_trait {
 672             debug!("deduce_future_output_from_projection: not a future");
 673             return None;
 674         }
 675
 676         // The `Future` trait has only one associted item, `Output`,
 677         // so check that this is what we see.
 678         let output_assoc_item =
 679             self.tcx.associated_items(future_trait).in_definition_order().next().unwrap().def_id;
 680         if output_assoc_item != predicate.projection_ty.item_def_id {
 681             span_bug!(
 682                 cause_span,
 683                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 684                 predicate.projection_ty.item_def_id,
 685                 output_assoc_item,
 686             );
 687         }
 688
 689         // Extract the type from the projection. Note that there can
 690         // be no bound variables in this type because the "self type"
 691         // does not have any regions in it.
 692         let output_ty = self.resolve_vars_if_possible(&predicate.ty);
 693         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 694         Some(output_ty)
 695     }
 696
 697     /// Converts the types that the user supplied, in case that doing
 698     /// so should yield an error, but returns back a signature where
 699     /// all parameters are of type `TyErr`.
 700     fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
 701         let astconv: &dyn AstConv<'_> = self;
 702
 703         let supplied_arguments = decl.inputs.iter().map(|a| {
 704             // Convert the types that the user supplied (if any), but ignore them.
 705             astconv.ast_ty_to_ty(a);
 706             self.tcx.types.err
 707         });
 708
 709         if let hir::FnRetTy::Return(ref output) = decl.output {
 710             astconv.ast_ty_to_ty(&output);
 711         }
 712
 713         let result = ty::Binder::bind(self.tcx.mk_fn_sig(
 714             supplied_arguments,
 715             self.tcx.types.err,
 716             decl.c_variadic,
 717             hir::Unsafety::Normal,
 718             Abi::RustCall,
 719         ));
 720
 721         debug!("supplied_sig_of_closure: result={:?}", result);
 722
 723         result
 724     }
 725
 726     fn closure_sigs(
 727         &self,
 728         expr_def_id: DefId,
 729         body: &hir::Body<'_>,
 730         bound_sig: ty::PolyFnSig<'tcx>,
 731     ) -> ClosureSignatures<'tcx> {
 732         let liberated_sig = self.tcx().liberate_late_bound_regions(expr_def_id, &bound_sig);
 733         let liberated_sig = self.inh.normalize_associated_types_in(
 734             body.value.span,
 735             body.value.hir_id,
 736             self.param_env,
 737             &liberated_sig,
 738         );
 739         ClosureSignatures { bound_sig, liberated_sig }
 740     }
 741 }