compiler/rustc_typeck/src/check/closure.rs

   1 //! Code for type-checking closure expressions.
   2
   3 use super::{check_fn, Expectation, FnCtxt, GeneratorTypes};
   4
   5 use crate::astconv::AstConv;
   6 use rustc_hir as hir;
   7 use rustc_hir::def_id::DefId;
   8 use rustc_hir::lang_items::LangItem;
   9 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  10 use rustc_infer::infer::LateBoundRegionConversionTime;
  11 use rustc_infer::infer::{InferOk, InferResult};
  12 use rustc_middle::ty::fold::TypeFoldable;
  13 use rustc_middle::ty::subst::InternalSubsts;
  14 use rustc_middle::ty::{self, Ty};
  15 use rustc_span::source_map::Span;
  16 use rustc_target::spec::abi::Abi;
  17 use rustc_trait_selection::traits::error_reporting::ArgKind;
  18 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
  19 use std::cmp;
  20 use std::iter;
  21
  22 /// What signature do we *expect* the closure to have from context?
  23 #[derive(Debug)]
  24 struct ExpectedSig<'tcx> {
  25     /// Span that gave us this expectation, if we know that.
  26     cause_span: Option<Span>,
  27     sig: ty::PolyFnSig<'tcx>,
  28 }
  29
  30 struct ClosureSignatures<'tcx> {
  31     bound_sig: ty::PolyFnSig<'tcx>,
  32     liberated_sig: ty::FnSig<'tcx>,
  33 }
  34
  35 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  36     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.hir_id, expr_def_id.to_def_id(), decl, body, expected_sig);
  73
  74         debug!("check_closure: ty_of_closure returns {:?}", liberated_sig);
  75
  76         let return_type_pre_known = !liberated_sig.output().is_ty_infer();
  77
  78         let generator_types = check_fn(
  79             self,
  80             self.param_env,
  81             liberated_sig,
  82             decl,
  83             expr.hir_id,
  84             body,
  85             gen,
  86             return_type_pre_known,
  87         )
  88         .1;
  89
  90         let parent_substs = InternalSubsts::identity_for_item(
  91             self.tcx,
  92             self.tcx.closure_base_def_id(expr_def_id.to_def_id()),
  93         );
  94
  95         let tupled_upvars_ty = self.infcx.next_ty_var(TypeVariableOrigin {
  96             kind: TypeVariableOriginKind::ClosureSynthetic,
  97             span: self.tcx.hir().span(expr.hir_id),
  98         });
  99
 100         if let Some(GeneratorTypes { resume_ty, yield_ty, interior, movability }) = generator_types
 101         {
 102             let generator_substs = ty::GeneratorSubsts::new(
 103                 self.tcx,
 104                 ty::GeneratorSubstsParts {
 105                     parent_substs,
 106                     resume_ty,
 107                     yield_ty,
 108                     return_ty: liberated_sig.output(),
 109                     witness: interior,
 110                     tupled_upvars_ty,
 111                 },
 112             );
 113
 114             return self.tcx.mk_generator(
 115                 expr_def_id.to_def_id(),
 116                 generator_substs.substs,
 117                 movability,
 118             );
 119         }
 120
 121         // Tuple up the arguments and insert the resulting function type into
 122         // the `closures` table.
 123         let sig = bound_sig.map_bound(|sig| {
 124             self.tcx.mk_fn_sig(
 125                 iter::once(self.tcx.intern_tup(sig.inputs())),
 126                 sig.output(),
 127                 sig.c_variadic,
 128                 sig.unsafety,
 129                 sig.abi,
 130             )
 131         });
 132
 133         debug!(
 134             "check_closure: expr_def_id={:?}, sig={:?}, opt_kind={:?}",
 135             expr_def_id, sig, opt_kind
 136         );
 137
 138         let closure_kind_ty = match opt_kind {
 139             Some(kind) => kind.to_ty(self.tcx),
 140
 141             // Create a type variable (for now) to represent the closure kind.
 142             // It will be unified during the upvar inference phase (`upvar.rs`)
 143             None => self.infcx.next_ty_var(TypeVariableOrigin {
 144                 // FIXME(eddyb) distinguish closure kind inference variables from the rest.
 145                 kind: TypeVariableOriginKind::ClosureSynthetic,
 146                 span: expr.span,
 147             }),
 148         };
 149
 150         let closure_substs = ty::ClosureSubsts::new(
 151             self.tcx,
 152             ty::ClosureSubstsParts {
 153                 parent_substs,
 154                 closure_kind_ty,
 155                 closure_sig_as_fn_ptr_ty: self.tcx.mk_fn_ptr(sig),
 156                 tupled_upvars_ty,
 157             },
 158         );
 159
 160         let closure_type = self.tcx.mk_closure(expr_def_id.to_def_id(), closure_substs.substs);
 161
 162         debug!("check_closure: expr.hir_id={:?} closure_type={:?}", expr.hir_id, closure_type);
 163
 164         closure_type
 165     }
 166
 167     /// Given the expected type, figures out what it can about this closure we
 168     /// are about to type check:
 169     fn deduce_expectations_from_expected_type(
 170         &self,
 171         expected_ty: Ty<'tcx>,
 172     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 173         debug!("deduce_expectations_from_expected_type(expected_ty={:?})", expected_ty);
 174
 175         match *expected_ty.kind() {
 176             ty::Dynamic(ref object_type, ..) => {
 177                 let sig = object_type.projection_bounds().find_map(|pb| {
 178                     let pb = pb.with_self_ty(self.tcx, self.tcx.types.trait_object_dummy_self);
 179                     self.deduce_sig_from_projection(None, pb)
 180                 });
 181                 let kind = object_type
 182                     .principal_def_id()
 183                     .and_then(|did| self.tcx.fn_trait_kind_from_lang_item(did));
 184                 (sig, kind)
 185             }
 186             ty::Infer(ty::TyVar(vid)) => self.deduce_expectations_from_obligations(vid),
 187             ty::FnPtr(sig) => {
 188                 let expected_sig = ExpectedSig { cause_span: None, sig };
 189                 (Some(expected_sig), Some(ty::ClosureKind::Fn))
 190             }
 191             _ => (None, None),
 192         }
 193     }
 194
 195     fn deduce_expectations_from_obligations(
 196         &self,
 197         expected_vid: ty::TyVid,
 198     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 199         let expected_sig =
 200             self.obligations_for_self_ty(expected_vid).find_map(|(_, obligation)| {
 201                 debug!(
 202                     "deduce_expectations_from_obligations: obligation.predicate={:?}",
 203                     obligation.predicate
 204                 );
 205
 206                 let bound_predicate = obligation.predicate.kind();
 207                 if let ty::PredicateKind::Projection(proj_predicate) =
 208                     obligation.predicate.kind().skip_binder()
 209                 {
 210                     // Given a Projection predicate, we can potentially infer
 211                     // the complete signature.
 212                     self.deduce_sig_from_projection(
 213                         Some(obligation.cause.span),
 214                         bound_predicate.rebind(proj_predicate),
 215                     )
 216                 } else {
 217                     None
 218                 }
 219             });
 220
 221         // Even if we can't infer the full signature, we may be able to
 222         // infer the kind. This can occur when we elaborate a predicate
 223         // like `F : Fn<A>`. Note that due to subtyping we could encounter
 224         // many viable options, so pick the most restrictive.
 225         let expected_kind = self
 226             .obligations_for_self_ty(expected_vid)
 227             .filter_map(|(tr, _)| self.tcx.fn_trait_kind_from_lang_item(tr.def_id()))
 228             .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
 229
 230         (expected_sig, expected_kind)
 231     }
 232
 233     /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
 234     /// everything we need to know about a closure or generator.
 235     ///
 236     /// The `cause_span` should be the span that caused us to
 237     /// have this expected signature, or `None` if we can't readily
 238     /// know that.
 239     fn deduce_sig_from_projection(
 240         &self,
 241         cause_span: Option<Span>,
 242         projection: ty::PolyProjectionPredicate<'tcx>,
 243     ) -> Option<ExpectedSig<'tcx>> {
 244         let tcx = self.tcx;
 245
 246         debug!("deduce_sig_from_projection({:?})", projection);
 247
 248         let trait_def_id = projection.trait_def_id(tcx);
 249
 250         let is_fn = tcx.fn_trait_kind_from_lang_item(trait_def_id).is_some();
 251         let gen_trait = tcx.require_lang_item(LangItem::Generator, cause_span);
 252         let is_gen = gen_trait == trait_def_id;
 253         if !is_fn && !is_gen {
 254             debug!("deduce_sig_from_projection: not fn or generator");
 255             return None;
 256         }
 257
 258         if is_gen {
 259             // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
 260             // associated item and not yield.
 261             let return_assoc_item =
 262                 self.tcx.associated_items(gen_trait).in_definition_order().nth(1).unwrap().def_id;
 263             if return_assoc_item != projection.projection_def_id() {
 264                 debug!("deduce_sig_from_projection: not return assoc item of generator");
 265                 return None;
 266             }
 267         }
 268
 269         let input_tys = if is_fn {
 270             let arg_param_ty = projection.skip_binder().projection_ty.substs.type_at(1);
 271             let arg_param_ty = self.resolve_vars_if_possible(arg_param_ty);
 272             debug!("deduce_sig_from_projection: arg_param_ty={:?}", arg_param_ty);
 273
 274             match arg_param_ty.kind() {
 275                 ty::Tuple(tys) => tys.into_iter().map(|k| k.expect_ty()).collect::<Vec<_>>(),
 276                 _ => return None,
 277             }
 278         } else {
 279             // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
 280             // else they must have exactly 1 argument. For now though, just give up in this case.
 281             return None;
 282         };
 283
 284         let ret_param_ty = projection.skip_binder().ty;
 285         let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
 286         debug!("deduce_sig_from_projection: ret_param_ty={:?}", ret_param_ty);
 287
 288         let sig = projection.rebind(self.tcx.mk_fn_sig(
 289             input_tys.iter(),
 290             &ret_param_ty,
 291             false,
 292             hir::Unsafety::Normal,
 293             Abi::Rust,
 294         ));
 295         debug!("deduce_sig_from_projection: sig={:?}", sig);
 296
 297         Some(ExpectedSig { cause_span, sig })
 298     }
 299
 300     fn sig_of_closure(
 301         &self,
 302         hir_id: hir::HirId,
 303         expr_def_id: DefId,
 304         decl: &hir::FnDecl<'_>,
 305         body: &hir::Body<'_>,
 306         expected_sig: Option<ExpectedSig<'tcx>>,
 307     ) -> ClosureSignatures<'tcx> {
 308         if let Some(e) = expected_sig {
 309             self.sig_of_closure_with_expectation(hir_id, expr_def_id, decl, body, e)
 310         } else {
 311             self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body)
 312         }
 313     }
 314
 315     /// If there is no expected signature, then we will convert the
 316     /// types that the user gave into a signature.
 317     fn sig_of_closure_no_expectation(
 318         &self,
 319         hir_id: hir::HirId,
 320         expr_def_id: DefId,
 321         decl: &hir::FnDecl<'_>,
 322         body: &hir::Body<'_>,
 323     ) -> ClosureSignatures<'tcx> {
 324         debug!("sig_of_closure_no_expectation()");
 325
 326         let bound_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 327
 328         self.closure_sigs(expr_def_id, body, bound_sig)
 329     }
 330
 331     /// Invoked to compute the signature of a closure expression. This
 332     /// combines any user-provided type annotations (e.g., `|x: u32|
 333     /// -> u32 { .. }`) with the expected signature.
 334     ///
 335     /// The approach is as follows:
 336     ///
 337     /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
 338     /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
 339     ///   - If we have no expectation `E`, then the signature of the closure is `S`.
 340     ///   - Otherwise, the signature of the closure is E. Moreover:
 341     ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
 342     ///     - Instantiate all the late-bound regions bound in the closure within `S`
 343     ///       with fresh (existential) variables, yielding `S'`
 344     ///     - Require that `E' = S'`
 345     ///       - We could use some kind of subtyping relationship here,
 346     ///         I imagine, but equality is easier and works fine for
 347     ///         our purposes.
 348     ///
 349     /// The key intuition here is that the user's types must be valid
 350     /// from "the inside" of the closure, but the expectation
 351     /// ultimately drives the overall signature.
 352     ///
 353     /// # Examples
 354     ///
 355     /// ```
 356     /// fn with_closure<F>(_: F)
 357     ///   where F: Fn(&u32) -> &u32 { .. }
 358     ///
 359     /// with_closure(|x: &u32| { ... })
 360     /// ```
 361     ///
 362     /// Here:
 363     /// - E would be `fn(&u32) -> &u32`.
 364     /// - S would be `fn(&u32) ->
 365     /// - E' is `&'!0 u32 -> &'!0 u32`
 366     /// - S' is `&'?0 u32 -> ?T`
 367     ///
 368     /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
 369     ///
 370     /// # Arguments
 371     ///
 372     /// - `expr_def_id`: the `DefId` of the closure expression
 373     /// - `decl`: the HIR declaration of the closure
 374     /// - `body`: the body of the closure
 375     /// - `expected_sig`: the expected signature (if any). Note that
 376     ///   this is missing a binder: that is, there may be late-bound
 377     ///   regions with depth 1, which are bound then by the closure.
 378     fn sig_of_closure_with_expectation(
 379         &self,
 380         hir_id: hir::HirId,
 381         expr_def_id: DefId,
 382         decl: &hir::FnDecl<'_>,
 383         body: &hir::Body<'_>,
 384         expected_sig: ExpectedSig<'tcx>,
 385     ) -> ClosureSignatures<'tcx> {
 386         debug!("sig_of_closure_with_expectation(expected_sig={:?})", expected_sig);
 387
 388         // Watch out for some surprises and just ignore the
 389         // expectation if things don't see to match up with what we
 390         // expect.
 391         if expected_sig.sig.c_variadic() != decl.c_variadic {
 392             return self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body);
 393         } else if expected_sig.sig.skip_binder().inputs_and_output.len() != decl.inputs.len() + 1 {
 394             return self.sig_of_closure_with_mismatched_number_of_arguments(
 395                 expr_def_id,
 396                 decl,
 397                 body,
 398                 expected_sig,
 399             );
 400         }
 401
 402         // Create a `PolyFnSig`. Note the oddity that late bound
 403         // regions appearing free in `expected_sig` are now bound up
 404         // in this binder we are creating.
 405         assert!(!expected_sig.sig.skip_binder().has_vars_bound_above(ty::INNERMOST));
 406         let bound_sig = expected_sig.sig.map_bound(|sig| {
 407             self.tcx.mk_fn_sig(
 408                 sig.inputs().iter().cloned(),
 409                 sig.output(),
 410                 sig.c_variadic,
 411                 hir::Unsafety::Normal,
 412                 Abi::RustCall,
 413             )
 414         });
 415
 416         // `deduce_expectations_from_expected_type` introduces
 417         // late-bound lifetimes defined elsewhere, which we now
 418         // anonymize away, so as not to confuse the user.
 419         let bound_sig = self.tcx.anonymize_late_bound_regions(bound_sig);
 420
 421         let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
 422
 423         // Up till this point, we have ignored the annotations that the user
 424         // gave. This function will check that they unify successfully.
 425         // Along the way, it also writes out entries for types that the user
 426         // wrote into our typeck results, which are then later used by the privacy
 427         // check.
 428         match self.check_supplied_sig_against_expectation(
 429             hir_id,
 430             expr_def_id,
 431             decl,
 432             body,
 433             &closure_sigs,
 434         ) {
 435             Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
 436             Err(_) => return self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body),
 437         }
 438
 439         closure_sigs
 440     }
 441
 442     fn sig_of_closure_with_mismatched_number_of_arguments(
 443         &self,
 444         expr_def_id: DefId,
 445         decl: &hir::FnDecl<'_>,
 446         body: &hir::Body<'_>,
 447         expected_sig: ExpectedSig<'tcx>,
 448     ) -> ClosureSignatures<'tcx> {
 449         let hir = self.tcx.hir();
 450         let expr_map_node = hir.get_if_local(expr_def_id).unwrap();
 451         let expected_args: Vec<_> = expected_sig
 452             .sig
 453             .skip_binder()
 454             .inputs()
 455             .iter()
 456             .map(|ty| ArgKind::from_expected_ty(ty, None))
 457             .collect();
 458         let (closure_span, found_args) = match self.get_fn_like_arguments(expr_map_node) {
 459             Some((sp, args)) => (Some(sp), args),
 460             None => (None, Vec::new()),
 461         };
 462         let expected_span =
 463             expected_sig.cause_span.unwrap_or_else(|| hir.span_if_local(expr_def_id).unwrap());
 464         self.report_arg_count_mismatch(
 465             expected_span,
 466             closure_span,
 467             expected_args,
 468             found_args,
 469             true,
 470         )
 471         .emit();
 472
 473         let error_sig = self.error_sig_of_closure(decl);
 474
 475         self.closure_sigs(expr_def_id, body, error_sig)
 476     }
 477
 478     /// Enforce the user's types against the expectation. See
 479     /// `sig_of_closure_with_expectation` for details on the overall
 480     /// strategy.
 481     fn check_supplied_sig_against_expectation(
 482         &self,
 483         hir_id: hir::HirId,
 484         expr_def_id: DefId,
 485         decl: &hir::FnDecl<'_>,
 486         body: &hir::Body<'_>,
 487         expected_sigs: &ClosureSignatures<'tcx>,
 488     ) -> InferResult<'tcx, ()> {
 489         // Get the signature S that the user gave.
 490         //
 491         // (See comment on `sig_of_closure_with_expectation` for the
 492         // meaning of these letters.)
 493         let supplied_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 494
 495         debug!("check_supplied_sig_against_expectation: supplied_sig={:?}", supplied_sig);
 496
 497         // FIXME(#45727): As discussed in [this comment][c1], naively
 498         // forcing equality here actually results in suboptimal error
 499         // messages in some cases.  For now, if there would have been
 500         // an obvious error, we fallback to declaring the type of the
 501         // closure to be the one the user gave, which allows other
 502         // error message code to trigger.
 503         //
 504         // However, I think [there is potential to do even better
 505         // here][c2], since in *this* code we have the precise span of
 506         // the type parameter in question in hand when we report the
 507         // error.
 508         //
 509         // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
 510         // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
 511         self.infcx.commit_if_ok(|_| {
 512             let mut all_obligations = vec![];
 513
 514             // The liberated version of this signature should be a subtype
 515             // of the liberated form of the expectation.
 516             for ((hir_ty, &supplied_ty), expected_ty) in iter::zip(
 517                 iter::zip(
 518                     decl.inputs,
 519                     supplied_sig.inputs().skip_binder(), // binder moved to (*) below
 520                 ),
 521                 expected_sigs.liberated_sig.inputs(), // `liberated_sig` is E'.
 522             ) {
 523                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 524                 let (supplied_ty, _) = self.infcx.replace_bound_vars_with_fresh_vars(
 525                     hir_ty.span,
 526                     LateBoundRegionConversionTime::FnCall,
 527                     supplied_sig.inputs().rebind(supplied_ty),
 528                 ); // recreated from (*) above
 529
 530                 // Check that E' = S'.
 531                 let cause = self.misc(hir_ty.span);
 532                 let InferOk { value: (), obligations } =
 533                     self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
 534                 all_obligations.extend(obligations);
 535             }
 536
 537             let (supplied_output_ty, _) = self.infcx.replace_bound_vars_with_fresh_vars(
 538                 decl.output.span(),
 539                 LateBoundRegionConversionTime::FnCall,
 540                 supplied_sig.output(),
 541             );
 542             let cause = &self.misc(decl.output.span());
 543             let InferOk { value: (), obligations } = self
 544                 .at(cause, self.param_env)
 545                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 546             all_obligations.extend(obligations);
 547
 548             Ok(InferOk { value: (), obligations: all_obligations })
 549         })
 550     }
 551
 552     /// If there is no expected signature, then we will convert the
 553     /// types that the user gave into a signature.
 554     ///
 555     /// Also, record this closure signature for later.
 556     fn supplied_sig_of_closure(
 557         &self,
 558         hir_id: hir::HirId,
 559         expr_def_id: DefId,
 560         decl: &hir::FnDecl<'_>,
 561         body: &hir::Body<'_>,
 562     ) -> ty::PolyFnSig<'tcx> {
 563         let astconv: &dyn AstConv<'_> = self;
 564
 565         debug!(
 566             "supplied_sig_of_closure(decl={:?}, body.generator_kind={:?})",
 567             decl, body.generator_kind,
 568         );
 569
 570         let bound_vars = self.tcx.late_bound_vars(hir_id);
 571
 572         // First, convert the types that the user supplied (if any).
 573         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 574         let supplied_return = match decl.output {
 575             hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
 576             hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
 577                 // In the case of the async block that we create for a function body,
 578                 // we expect the return type of the block to match that of the enclosing
 579                 // function.
 580                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 581                     debug!("supplied_sig_of_closure: closure is async fn body");
 582                     self.deduce_future_output_from_obligations(expr_def_id).unwrap_or_else(|| {
 583                         // AFAIK, deducing the future output
 584                         // always succeeds *except* in error cases
 585                         // like #65159. I'd like to return Error
 586                         // here, but I can't because I can't
 587                         // easily (and locally) prove that we
 588                         // *have* reported an
 589                         // error. --nikomatsakis
 590                         astconv.ty_infer(None, decl.output.span())
 591                     })
 592                 }
 593
 594                 _ => astconv.ty_infer(None, decl.output.span()),
 595             },
 596         };
 597
 598         let result = ty::Binder::bind_with_vars(
 599             self.tcx.mk_fn_sig(
 600                 supplied_arguments,
 601                 supplied_return,
 602                 decl.c_variadic,
 603                 hir::Unsafety::Normal,
 604                 Abi::RustCall,
 605             ),
 606             bound_vars,
 607         );
 608
 609         debug!("supplied_sig_of_closure: result={:?}", result);
 610
 611         let c_result = self.inh.infcx.canonicalize_response(result);
 612         self.typeck_results.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);
 613
 614         result
 615     }
 616
 617     /// Invoked when we are translating the generator that results
 618     /// from desugaring an `async fn`. Returns the "sugared" return
 619     /// type of the `async fn` -- that is, the return type that the
 620     /// user specified. The "desugared" return type is an `impl
 621     /// Future<Output = T>`, so we do this by searching through the
 622     /// obligations to extract the `T`.
 623     fn deduce_future_output_from_obligations(&self, expr_def_id: DefId) -> Option<Ty<'tcx>> {
 624         debug!("deduce_future_output_from_obligations(expr_def_id={:?})", expr_def_id);
 625
 626         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
 627             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
 628         });
 629
 630         // In practice, the return type of the surrounding function is
 631         // always a (not yet resolved) inference variable, because it
 632         // is the hidden type for an `impl Trait` that we are going to
 633         // be inferring.
 634         let ret_ty = ret_coercion.borrow().expected_ty();
 635         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 636         let ret_vid = match *ret_ty.kind() {
 637             ty::Infer(ty::TyVar(ret_vid)) => ret_vid,
 638             ty::Error(_) => return None,
 639             _ => span_bug!(
 640                 self.tcx.def_span(expr_def_id),
 641                 "async fn generator return type not an inference variable"
 642             ),
 643         };
 644
 645         // Search for a pending obligation like
 646         //
 647         // `<R as Future>::Output = T`
 648         //
 649         // where R is the return type we are expecting. This type `T`
 650         // will be our output.
 651         let output_ty = self.obligations_for_self_ty(ret_vid).find_map(|(_, obligation)| {
 652             let bound_predicate = obligation.predicate.kind();
 653             if let ty::PredicateKind::Projection(proj_predicate) = bound_predicate.skip_binder() {
 654                 self.deduce_future_output_from_projection(
 655                     obligation.cause.span,
 656                     bound_predicate.rebind(proj_predicate),
 657                 )
 658             } else {
 659                 None
 660             }
 661         });
 662
 663         debug!("deduce_future_output_from_obligations: output_ty={:?}", output_ty);
 664         output_ty
 665     }
 666
 667     /// Given a projection like
 668     ///
 669     /// `<X as Future>::Output = T`
 670     ///
 671     /// where `X` is some type that has no late-bound regions, returns
 672     /// `Some(T)`. If the projection is for some other trait, returns
 673     /// `None`.
 674     fn deduce_future_output_from_projection(
 675         &self,
 676         cause_span: Span,
 677         predicate: ty::PolyProjectionPredicate<'tcx>,
 678     ) -> Option<Ty<'tcx>> {
 679         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 680
 681         // We do not expect any bound regions in our predicate, so
 682         // skip past the bound vars.
 683         let predicate = match predicate.no_bound_vars() {
 684             Some(p) => p,
 685             None => {
 686                 debug!("deduce_future_output_from_projection: has late-bound regions");
 687                 return None;
 688             }
 689         };
 690
 691         // Check that this is a projection from the `Future` trait.
 692         let trait_def_id = predicate.projection_ty.trait_def_id(self.tcx);
 693         let future_trait = self.tcx.require_lang_item(LangItem::Future, Some(cause_span));
 694         if trait_def_id != future_trait {
 695             debug!("deduce_future_output_from_projection: not a future");
 696             return None;
 697         }
 698
 699         // The `Future` trait has only one associted item, `Output`,
 700         // so check that this is what we see.
 701         let output_assoc_item =
 702             self.tcx.associated_items(future_trait).in_definition_order().next().unwrap().def_id;
 703         if output_assoc_item != predicate.projection_ty.item_def_id {
 704             span_bug!(
 705                 cause_span,
 706                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 707                 predicate.projection_ty.item_def_id,
 708                 output_assoc_item,
 709             );
 710         }
 711
 712         // Extract the type from the projection. Note that there can
 713         // be no bound variables in this type because the "self type"
 714         // does not have any regions in it.
 715         let output_ty = self.resolve_vars_if_possible(predicate.ty);
 716         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 717         Some(output_ty)
 718     }
 719
 720     /// Converts the types that the user supplied, in case that doing
 721     /// so should yield an error, but returns back a signature where
 722     /// all parameters are of type `TyErr`.
 723     fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
 724         let astconv: &dyn AstConv<'_> = self;
 725
 726         let supplied_arguments = decl.inputs.iter().map(|a| {
 727             // Convert the types that the user supplied (if any), but ignore them.
 728             astconv.ast_ty_to_ty(a);
 729             self.tcx.ty_error()
 730         });
 731
 732         if let hir::FnRetTy::Return(ref output) = decl.output {
 733             astconv.ast_ty_to_ty(&output);
 734         }
 735
 736         let result = ty::Binder::dummy(self.tcx.mk_fn_sig(
 737             supplied_arguments,
 738             self.tcx.ty_error(),
 739             decl.c_variadic,
 740             hir::Unsafety::Normal,
 741             Abi::RustCall,
 742         ));
 743
 744         debug!("supplied_sig_of_closure: result={:?}", result);
 745
 746         result
 747     }
 748
 749     fn closure_sigs(
 750         &self,
 751         expr_def_id: DefId,
 752         body: &hir::Body<'_>,
 753         bound_sig: ty::PolyFnSig<'tcx>,
 754     ) -> ClosureSignatures<'tcx> {
 755         let liberated_sig = self.tcx().liberate_late_bound_regions(expr_def_id, bound_sig);
 756         let liberated_sig = self.inh.normalize_associated_types_in(
 757             body.value.span,
 758             body.value.hir_id,
 759             self.param_env,
 760             liberated_sig,
 761         );
 762         ClosureSignatures { bound_sig, liberated_sig }
 763     }
 764 }