compiler/rustc_hir_typeck/src/closure.rs

   1 //! Code for type-checking closure expressions.
   2
   3 use super::{check_fn, Expectation, FnCtxt, GeneratorTypes};
   4
   5 use hir::def::DefKind;
   6 use rustc_hir as hir;
   7 use rustc_hir::def_id::DefId;
   8 use rustc_hir::lang_items::LangItem;
   9 use rustc_hir_analysis::astconv::AstConv;
  10 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  11 use rustc_infer::infer::LateBoundRegionConversionTime;
  12 use rustc_infer::infer::{InferOk, InferResult};
  13 use rustc_middle::ty::subst::InternalSubsts;
  14 use rustc_middle::ty::visit::TypeVisitable;
  15 use rustc_middle::ty::{self, Ty};
  16 use rustc_span::source_map::Span;
  17 use rustc_target::spec::abi::Abi;
  18 use rustc_trait_selection::traits;
  19 use rustc_trait_selection::traits::error_reporting::ArgKind;
  20 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
  21 use std::cmp;
  22 use std::iter;
  23
  24 /// What signature do we *expect* the closure to have from context?
  25 #[derive(Debug)]
  26 struct ExpectedSig<'tcx> {
  27     /// Span that gave us this expectation, if we know that.
  28     cause_span: Option<Span>,
  29     sig: ty::PolyFnSig<'tcx>,
  30 }
  31
  32 struct ClosureSignatures<'tcx> {
  33     /// The signature users of the closure see.
  34     bound_sig: ty::PolyFnSig<'tcx>,
  35     /// The signature within the function body.
  36     /// This mostly differs in the sense that lifetimes are now early bound and any
  37     /// opaque types from the signature expectation are overriden in case there are
  38     /// explicit hidden types written by the user in the closure signature.
  39     liberated_sig: ty::FnSig<'tcx>,
  40 }
  41
  42 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  43     #[instrument(skip(self, expr, _capture, decl, body_id), level = "debug")]
  44     pub fn check_expr_closure(
  45         &self,
  46         expr: &hir::Expr<'_>,
  47         _capture: hir::CaptureBy,
  48         decl: &'tcx hir::FnDecl<'tcx>,
  49         body_id: hir::BodyId,
  50         gen: Option<hir::Movability>,
  51         expected: Expectation<'tcx>,
  52     ) -> Ty<'tcx> {
  53         trace!("decl = {:#?}", decl);
  54         trace!("expr = {:#?}", expr);
  55
  56         // It's always helpful for inference if we know the kind of
  57         // closure sooner rather than later, so first examine the expected
  58         // type, and see if can glean a closure kind from there.
  59         let (expected_sig, expected_kind) = match expected.to_option(self) {
  60             Some(ty) => self.deduce_expectations_from_expected_type(ty),
  61             None => (None, None),
  62         };
  63         let body = self.tcx.hir().body(body_id);
  64         self.check_closure(expr, expected_kind, decl, body, gen, expected_sig)
  65     }
  66
  67     #[instrument(skip(self, expr, body, decl), level = "debug", ret)]
  68     fn check_closure(
  69         &self,
  70         expr: &hir::Expr<'_>,
  71         opt_kind: Option<ty::ClosureKind>,
  72         decl: &'tcx hir::FnDecl<'tcx>,
  73         body: &'tcx hir::Body<'tcx>,
  74         gen: Option<hir::Movability>,
  75         expected_sig: Option<ExpectedSig<'tcx>>,
  76     ) -> Ty<'tcx> {
  77         trace!("decl = {:#?}", decl);
  78         let expr_def_id = self.tcx.hir().local_def_id(expr.hir_id);
  79         debug!(?expr_def_id);
  80
  81         let ClosureSignatures { bound_sig, liberated_sig } =
  82             self.sig_of_closure(expr.hir_id, expr_def_id.to_def_id(), decl, body, expected_sig);
  83
  84         debug!(?bound_sig, ?liberated_sig);
  85
  86         let generator_types = check_fn(
  87             self,
  88             self.param_env.without_const(),
  89             liberated_sig,
  90             decl,
  91             expr.hir_id,
  92             body,
  93             gen,
  94         )
  95         .1;
  96
  97         let parent_substs = InternalSubsts::identity_for_item(
  98             self.tcx,
  99             self.tcx.typeck_root_def_id(expr_def_id.to_def_id()),
 100         );
 101
 102         let tupled_upvars_ty = self.next_ty_var(TypeVariableOrigin {
 103             kind: TypeVariableOriginKind::ClosureSynthetic,
 104             span: self.tcx.hir().span(expr.hir_id),
 105         });
 106
 107         if let Some(GeneratorTypes { resume_ty, yield_ty, interior, movability }) = generator_types
 108         {
 109             let generator_substs = ty::GeneratorSubsts::new(
 110                 self.tcx,
 111                 ty::GeneratorSubstsParts {
 112                     parent_substs,
 113                     resume_ty,
 114                     yield_ty,
 115                     return_ty: liberated_sig.output(),
 116                     witness: interior,
 117                     tupled_upvars_ty,
 118                 },
 119             );
 120
 121             return self.tcx.mk_generator(
 122                 expr_def_id.to_def_id(),
 123                 generator_substs.substs,
 124                 movability,
 125             );
 126         }
 127
 128         // Tuple up the arguments and insert the resulting function type into
 129         // the `closures` table.
 130         let sig = bound_sig.map_bound(|sig| {
 131             self.tcx.mk_fn_sig(
 132                 iter::once(self.tcx.intern_tup(sig.inputs())),
 133                 sig.output(),
 134                 sig.c_variadic,
 135                 sig.unsafety,
 136                 sig.abi,
 137             )
 138         });
 139
 140         debug!(?sig, ?opt_kind);
 141
 142         let closure_kind_ty = match opt_kind {
 143             Some(kind) => kind.to_ty(self.tcx),
 144
 145             // Create a type variable (for now) to represent the closure kind.
 146             // It will be unified during the upvar inference phase (`upvar.rs`)
 147             None => self.next_ty_var(TypeVariableOrigin {
 148                 // FIXME(eddyb) distinguish closure kind inference variables from the rest.
 149                 kind: TypeVariableOriginKind::ClosureSynthetic,
 150                 span: expr.span,
 151             }),
 152         };
 153
 154         let closure_substs = ty::ClosureSubsts::new(
 155             self.tcx,
 156             ty::ClosureSubstsParts {
 157                 parent_substs,
 158                 closure_kind_ty,
 159                 closure_sig_as_fn_ptr_ty: self.tcx.mk_fn_ptr(sig),
 160                 tupled_upvars_ty,
 161             },
 162         );
 163
 164         self.tcx.mk_closure(expr_def_id.to_def_id(), closure_substs.substs)
 165     }
 166
 167     /// Given the expected type, figures out what it can about this closure we
 168     /// are about to type check:
 169     #[instrument(skip(self), level = "debug")]
 170     fn deduce_expectations_from_expected_type(
 171         &self,
 172         expected_ty: Ty<'tcx>,
 173     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 174         match *expected_ty.kind() {
 175             ty::Opaque(def_id, substs) => {
 176                 let bounds = self.tcx.bound_explicit_item_bounds(def_id);
 177                 let sig =
 178                     bounds.subst_iter_copied(self.tcx, substs).find_map(|(pred, span)| match pred
 179                         .kind()
 180                         .skip_binder()
 181                     {
 182                         ty::PredicateKind::Projection(proj_predicate) => self
 183                             .deduce_sig_from_projection(
 184                                 Some(span),
 185                                 pred.kind().rebind(proj_predicate),
 186                             ),
 187                         _ => None,
 188                     });
 189
 190                 let kind = bounds
 191                     .0
 192                     .iter()
 193                     .filter_map(|(pred, _)| match pred.kind().skip_binder() {
 194                         ty::PredicateKind::Trait(tp) => {
 195                             self.tcx.fn_trait_kind_from_lang_item(tp.def_id())
 196                         }
 197                         _ => None,
 198                     })
 199                     .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
 200                 trace!(?sig, ?kind);
 201                 (sig, kind)
 202             }
 203             ty::Dynamic(ref object_type, ..) => {
 204                 let sig = object_type.projection_bounds().find_map(|pb| {
 205                     let pb = pb.with_self_ty(self.tcx, self.tcx.types.trait_object_dummy_self);
 206                     self.deduce_sig_from_projection(None, pb)
 207                 });
 208                 let kind = object_type
 209                     .principal_def_id()
 210                     .and_then(|did| self.tcx.fn_trait_kind_from_lang_item(did));
 211                 (sig, kind)
 212             }
 213             ty::Infer(ty::TyVar(vid)) => self.deduce_expectations_from_obligations(vid),
 214             ty::FnPtr(sig) => {
 215                 let expected_sig = ExpectedSig { cause_span: None, sig };
 216                 (Some(expected_sig), Some(ty::ClosureKind::Fn))
 217             }
 218             _ => (None, None),
 219         }
 220     }
 221
 222     fn deduce_expectations_from_obligations(
 223         &self,
 224         expected_vid: ty::TyVid,
 225     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 226         let mut expected_sig = None;
 227         let mut expected_kind = None;
 228
 229         for obligation in traits::elaborate_obligations(
 230             self.tcx,
 231             // Reverse the obligations here, since `elaborate_*` uses a stack,
 232             // and we want to keep inference generally in the same order of
 233             // the registered obligations.
 234             self.obligations_for_self_ty(expected_vid).rev().collect(),
 235         ) {
 236             debug!(?obligation.predicate);
 237             let bound_predicate = obligation.predicate.kind();
 238
 239             // Given a Projection predicate, we can potentially infer
 240             // the complete signature.
 241             if expected_sig.is_none()
 242                 && let ty::PredicateKind::Projection(proj_predicate) = bound_predicate.skip_binder()
 243             {
 244                 expected_sig = self.deduce_sig_from_projection(
 245                     Some(obligation.cause.span),
 246                     bound_predicate.rebind(proj_predicate),
 247                 );
 248             }
 249
 250             // Even if we can't infer the full signature, we may be able to
 251             // infer the kind. This can occur when we elaborate a predicate
 252             // like `F : Fn<A>`. Note that due to subtyping we could encounter
 253             // many viable options, so pick the most restrictive.
 254             let trait_def_id = match bound_predicate.skip_binder() {
 255                 ty::PredicateKind::Projection(data) => {
 256                     Some(data.projection_ty.trait_def_id(self.tcx))
 257                 }
 258                 ty::PredicateKind::Trait(data) => Some(data.def_id()),
 259                 _ => None,
 260             };
 261             if let Some(closure_kind) =
 262                 trait_def_id.and_then(|def_id| self.tcx.fn_trait_kind_from_lang_item(def_id))
 263             {
 264                 expected_kind = Some(
 265                     expected_kind
 266                         .map_or_else(|| closure_kind, |current| cmp::min(current, closure_kind)),
 267                 );
 268             }
 269         }
 270
 271         (expected_sig, expected_kind)
 272     }
 273
 274     /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
 275     /// everything we need to know about a closure or generator.
 276     ///
 277     /// The `cause_span` should be the span that caused us to
 278     /// have this expected signature, or `None` if we can't readily
 279     /// know that.
 280     #[instrument(level = "debug", skip(self, cause_span), ret)]
 281     fn deduce_sig_from_projection(
 282         &self,
 283         cause_span: Option<Span>,
 284         projection: ty::PolyProjectionPredicate<'tcx>,
 285     ) -> Option<ExpectedSig<'tcx>> {
 286         let tcx = self.tcx;
 287
 288         let trait_def_id = projection.trait_def_id(tcx);
 289
 290         let is_fn = tcx.fn_trait_kind_from_lang_item(trait_def_id).is_some();
 291         let gen_trait = tcx.require_lang_item(LangItem::Generator, cause_span);
 292         let is_gen = gen_trait == trait_def_id;
 293         if !is_fn && !is_gen {
 294             debug!("not fn or generator");
 295             return None;
 296         }
 297
 298         if is_gen {
 299             // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
 300             // associated item and not yield.
 301             let return_assoc_item = self.tcx.associated_item_def_ids(gen_trait)[1];
 302             if return_assoc_item != projection.projection_def_id() {
 303                 debug!("not return assoc item of generator");
 304                 return None;
 305             }
 306         }
 307
 308         let input_tys = if is_fn {
 309             let arg_param_ty = projection.skip_binder().projection_ty.substs.type_at(1);
 310             let arg_param_ty = self.resolve_vars_if_possible(arg_param_ty);
 311             debug!(?arg_param_ty);
 312
 313             match arg_param_ty.kind() {
 314                 &ty::Tuple(tys) => tys,
 315                 _ => return None,
 316             }
 317         } else {
 318             // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
 319             // else they must have exactly 1 argument. For now though, just give up in this case.
 320             return None;
 321         };
 322
 323         // Since this is a return parameter type it is safe to unwrap.
 324         let ret_param_ty = projection.skip_binder().term.ty().unwrap();
 325         let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
 326         debug!(?ret_param_ty);
 327
 328         let sig = projection.rebind(self.tcx.mk_fn_sig(
 329             input_tys.iter(),
 330             ret_param_ty,
 331             false,
 332             hir::Unsafety::Normal,
 333             Abi::Rust,
 334         ));
 335
 336         Some(ExpectedSig { cause_span, sig })
 337     }
 338
 339     fn sig_of_closure(
 340         &self,
 341         hir_id: hir::HirId,
 342         expr_def_id: DefId,
 343         decl: &hir::FnDecl<'_>,
 344         body: &hir::Body<'_>,
 345         expected_sig: Option<ExpectedSig<'tcx>>,
 346     ) -> ClosureSignatures<'tcx> {
 347         if let Some(e) = expected_sig {
 348             self.sig_of_closure_with_expectation(hir_id, expr_def_id, decl, body, e)
 349         } else {
 350             self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body)
 351         }
 352     }
 353
 354     /// If there is no expected signature, then we will convert the
 355     /// types that the user gave into a signature.
 356     #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
 357     fn sig_of_closure_no_expectation(
 358         &self,
 359         hir_id: hir::HirId,
 360         expr_def_id: DefId,
 361         decl: &hir::FnDecl<'_>,
 362         body: &hir::Body<'_>,
 363     ) -> ClosureSignatures<'tcx> {
 364         let bound_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 365
 366         self.closure_sigs(expr_def_id, body, bound_sig)
 367     }
 368
 369     /// Invoked to compute the signature of a closure expression. This
 370     /// combines any user-provided type annotations (e.g., `|x: u32|
 371     /// -> u32 { .. }`) with the expected signature.
 372     ///
 373     /// The approach is as follows:
 374     ///
 375     /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
 376     /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
 377     ///   - If we have no expectation `E`, then the signature of the closure is `S`.
 378     ///   - Otherwise, the signature of the closure is E. Moreover:
 379     ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
 380     ///     - Instantiate all the late-bound regions bound in the closure within `S`
 381     ///       with fresh (existential) variables, yielding `S'`
 382     ///     - Require that `E' = S'`
 383     ///       - We could use some kind of subtyping relationship here,
 384     ///         I imagine, but equality is easier and works fine for
 385     ///         our purposes.
 386     ///
 387     /// The key intuition here is that the user's types must be valid
 388     /// from "the inside" of the closure, but the expectation
 389     /// ultimately drives the overall signature.
 390     ///
 391     /// # Examples
 392     ///
 393     /// ```ignore (illustrative)
 394     /// fn with_closure<F>(_: F)
 395     ///   where F: Fn(&u32) -> &u32 { .. }
 396     ///
 397     /// with_closure(|x: &u32| { ... })
 398     /// ```
 399     ///
 400     /// Here:
 401     /// - E would be `fn(&u32) -> &u32`.
 402     /// - S would be `fn(&u32) ->
 403     /// - E' is `&'!0 u32 -> &'!0 u32`
 404     /// - S' is `&'?0 u32 -> ?T`
 405     ///
 406     /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
 407     ///
 408     /// # Arguments
 409     ///
 410     /// - `expr_def_id`: the `DefId` of the closure expression
 411     /// - `decl`: the HIR declaration of the closure
 412     /// - `body`: the body of the closure
 413     /// - `expected_sig`: the expected signature (if any). Note that
 414     ///   this is missing a binder: that is, there may be late-bound
 415     ///   regions with depth 1, which are bound then by the closure.
 416     #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
 417     fn sig_of_closure_with_expectation(
 418         &self,
 419         hir_id: hir::HirId,
 420         expr_def_id: DefId,
 421         decl: &hir::FnDecl<'_>,
 422         body: &hir::Body<'_>,
 423         expected_sig: ExpectedSig<'tcx>,
 424     ) -> ClosureSignatures<'tcx> {
 425         // Watch out for some surprises and just ignore the
 426         // expectation if things don't see to match up with what we
 427         // expect.
 428         if expected_sig.sig.c_variadic() != decl.c_variadic {
 429             return self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body);
 430         } else if expected_sig.sig.skip_binder().inputs_and_output.len() != decl.inputs.len() + 1 {
 431             return self.sig_of_closure_with_mismatched_number_of_arguments(
 432                 expr_def_id,
 433                 decl,
 434                 body,
 435                 expected_sig,
 436             );
 437         }
 438
 439         // Create a `PolyFnSig`. Note the oddity that late bound
 440         // regions appearing free in `expected_sig` are now bound up
 441         // in this binder we are creating.
 442         assert!(!expected_sig.sig.skip_binder().has_vars_bound_above(ty::INNERMOST));
 443         let bound_sig = expected_sig.sig.map_bound(|sig| {
 444             self.tcx.mk_fn_sig(
 445                 sig.inputs().iter().cloned(),
 446                 sig.output(),
 447                 sig.c_variadic,
 448                 hir::Unsafety::Normal,
 449                 Abi::RustCall,
 450             )
 451         });
 452
 453         // `deduce_expectations_from_expected_type` introduces
 454         // late-bound lifetimes defined elsewhere, which we now
 455         // anonymize away, so as not to confuse the user.
 456         let bound_sig = self.tcx.anonymize_late_bound_regions(bound_sig);
 457
 458         let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
 459
 460         // Up till this point, we have ignored the annotations that the user
 461         // gave. This function will check that they unify successfully.
 462         // Along the way, it also writes out entries for types that the user
 463         // wrote into our typeck results, which are then later used by the privacy
 464         // check.
 465         match self.merge_supplied_sig_with_expectation(
 466             hir_id,
 467             expr_def_id,
 468             decl,
 469             body,
 470             closure_sigs,
 471         ) {
 472             Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
 473             Err(_) => self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body),
 474         }
 475     }
 476
 477     fn sig_of_closure_with_mismatched_number_of_arguments(
 478         &self,
 479         expr_def_id: DefId,
 480         decl: &hir::FnDecl<'_>,
 481         body: &hir::Body<'_>,
 482         expected_sig: ExpectedSig<'tcx>,
 483     ) -> ClosureSignatures<'tcx> {
 484         let hir = self.tcx.hir();
 485         let expr_map_node = hir.get_if_local(expr_def_id).unwrap();
 486         let expected_args: Vec<_> = expected_sig
 487             .sig
 488             .skip_binder()
 489             .inputs()
 490             .iter()
 491             .map(|ty| ArgKind::from_expected_ty(*ty, None))
 492             .collect();
 493         let (closure_span, found_args) = match self.get_fn_like_arguments(expr_map_node) {
 494             Some((sp, args)) => (Some(sp), args),
 495             None => (None, Vec::new()),
 496         };
 497         let expected_span =
 498             expected_sig.cause_span.unwrap_or_else(|| hir.span_if_local(expr_def_id).unwrap());
 499         self.report_arg_count_mismatch(
 500             expected_span,
 501             closure_span,
 502             expected_args,
 503             found_args,
 504             true,
 505         )
 506         .emit();
 507
 508         let error_sig = self.error_sig_of_closure(decl);
 509
 510         self.closure_sigs(expr_def_id, body, error_sig)
 511     }
 512
 513     /// Enforce the user's types against the expectation. See
 514     /// `sig_of_closure_with_expectation` for details on the overall
 515     /// strategy.
 516     #[instrument(level = "debug", skip(self, hir_id, expr_def_id, decl, body, expected_sigs))]
 517     fn merge_supplied_sig_with_expectation(
 518         &self,
 519         hir_id: hir::HirId,
 520         expr_def_id: DefId,
 521         decl: &hir::FnDecl<'_>,
 522         body: &hir::Body<'_>,
 523         mut expected_sigs: ClosureSignatures<'tcx>,
 524     ) -> InferResult<'tcx, ClosureSignatures<'tcx>> {
 525         // Get the signature S that the user gave.
 526         //
 527         // (See comment on `sig_of_closure_with_expectation` for the
 528         // meaning of these letters.)
 529         let supplied_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 530
 531         debug!(?supplied_sig);
 532
 533         // FIXME(#45727): As discussed in [this comment][c1], naively
 534         // forcing equality here actually results in suboptimal error
 535         // messages in some cases.  For now, if there would have been
 536         // an obvious error, we fallback to declaring the type of the
 537         // closure to be the one the user gave, which allows other
 538         // error message code to trigger.
 539         //
 540         // However, I think [there is potential to do even better
 541         // here][c2], since in *this* code we have the precise span of
 542         // the type parameter in question in hand when we report the
 543         // error.
 544         //
 545         // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
 546         // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
 547         self.commit_if_ok(|_| {
 548             let mut all_obligations = vec![];
 549             let inputs: Vec<_> = iter::zip(
 550                 decl.inputs,
 551                 supplied_sig.inputs().skip_binder(), // binder moved to (*) below
 552             )
 553             .map(|(hir_ty, &supplied_ty)| {
 554                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 555                 self.replace_bound_vars_with_fresh_vars(
 556                     hir_ty.span,
 557                     LateBoundRegionConversionTime::FnCall,
 558                     // (*) binder moved to here
 559                     supplied_sig.inputs().rebind(supplied_ty),
 560                 )
 561             })
 562             .collect();
 563
 564             // The liberated version of this signature should be a subtype
 565             // of the liberated form of the expectation.
 566             for ((hir_ty, &supplied_ty), expected_ty) in iter::zip(
 567                 iter::zip(decl.inputs, &inputs),
 568                 expected_sigs.liberated_sig.inputs(), // `liberated_sig` is E'.
 569             ) {
 570                 // Check that E' = S'.
 571                 let cause = self.misc(hir_ty.span);
 572                 let InferOk { value: (), obligations } =
 573                     self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
 574                 all_obligations.extend(obligations);
 575             }
 576
 577             let supplied_output_ty = self.replace_bound_vars_with_fresh_vars(
 578                 decl.output.span(),
 579                 LateBoundRegionConversionTime::FnCall,
 580                 supplied_sig.output(),
 581             );
 582             let cause = &self.misc(decl.output.span());
 583             let InferOk { value: (), obligations } = self
 584                 .at(cause, self.param_env)
 585                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 586             all_obligations.extend(obligations);
 587
 588             let inputs = inputs.into_iter().map(|ty| self.resolve_vars_if_possible(ty));
 589
 590             expected_sigs.liberated_sig = self.tcx.mk_fn_sig(
 591                 inputs,
 592                 supplied_output_ty,
 593                 expected_sigs.liberated_sig.c_variadic,
 594                 hir::Unsafety::Normal,
 595                 Abi::RustCall,
 596             );
 597
 598             Ok(InferOk { value: expected_sigs, obligations: all_obligations })
 599         })
 600     }
 601
 602     /// If there is no expected signature, then we will convert the
 603     /// types that the user gave into a signature.
 604     ///
 605     /// Also, record this closure signature for later.
 606     #[instrument(skip(self, decl, body), level = "debug", ret)]
 607     fn supplied_sig_of_closure(
 608         &self,
 609         hir_id: hir::HirId,
 610         expr_def_id: DefId,
 611         decl: &hir::FnDecl<'_>,
 612         body: &hir::Body<'_>,
 613     ) -> ty::PolyFnSig<'tcx> {
 614         let astconv: &dyn AstConv<'_> = self;
 615
 616         trace!("decl = {:#?}", decl);
 617         debug!(?body.generator_kind);
 618
 619         let bound_vars = self.tcx.late_bound_vars(hir_id);
 620
 621         // First, convert the types that the user supplied (if any).
 622         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 623         let supplied_return = match decl.output {
 624             hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
 625             hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
 626                 // In the case of the async block that we create for a function body,
 627                 // we expect the return type of the block to match that of the enclosing
 628                 // function.
 629                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 630                     debug!("closure is async fn body");
 631                     self.deduce_future_output_from_obligations(expr_def_id, body.id().hir_id)
 632                         .unwrap_or_else(|| {
 633                             // AFAIK, deducing the future output
 634                             // always succeeds *except* in error cases
 635                             // like #65159. I'd like to return Error
 636                             // here, but I can't because I can't
 637                             // easily (and locally) prove that we
 638                             // *have* reported an
 639                             // error. --nikomatsakis
 640                             astconv.ty_infer(None, decl.output.span())
 641                         })
 642                 }
 643
 644                 _ => astconv.ty_infer(None, decl.output.span()),
 645             },
 646         };
 647
 648         let result = ty::Binder::bind_with_vars(
 649             self.tcx.mk_fn_sig(
 650                 supplied_arguments,
 651                 supplied_return,
 652                 decl.c_variadic,
 653                 hir::Unsafety::Normal,
 654                 Abi::RustCall,
 655             ),
 656             bound_vars,
 657         );
 658         // Astconv can't normalize inputs or outputs with escaping bound vars,
 659         // so normalize them here, after we've wrapped them in a binder.
 660         let result = self.normalize_associated_types_in(self.tcx.hir().span(hir_id), result);
 661
 662         let c_result = self.inh.infcx.canonicalize_response(result);
 663         self.typeck_results.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);
 664
 665         result
 666     }
 667
 668     /// Invoked when we are translating the generator that results
 669     /// from desugaring an `async fn`. Returns the "sugared" return
 670     /// type of the `async fn` -- that is, the return type that the
 671     /// user specified. The "desugared" return type is an `impl
 672     /// Future<Output = T>`, so we do this by searching through the
 673     /// obligations to extract the `T`.
 674     #[instrument(skip(self), level = "debug", ret)]
 675     fn deduce_future_output_from_obligations(
 676         &self,
 677         expr_def_id: DefId,
 678         body_id: hir::HirId,
 679     ) -> Option<Ty<'tcx>> {
 680         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
 681             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
 682         });
 683
 684         let ret_ty = ret_coercion.borrow().expected_ty();
 685         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 686
 687         let get_future_output = |predicate: ty::Predicate<'tcx>, span| {
 688             // Search for a pending obligation like
 689             //
 690             // `<R as Future>::Output = T`
 691             //
 692             // where R is the return type we are expecting. This type `T`
 693             // will be our output.
 694             let bound_predicate = predicate.kind();
 695             if let ty::PredicateKind::Projection(proj_predicate) = bound_predicate.skip_binder() {
 696                 self.deduce_future_output_from_projection(
 697                     span,
 698                     bound_predicate.rebind(proj_predicate),
 699                 )
 700             } else {
 701                 None
 702             }
 703         };
 704
 705         let output_ty = match *ret_ty.kind() {
 706             ty::Infer(ty::TyVar(ret_vid)) => {
 707                 self.obligations_for_self_ty(ret_vid).find_map(|obligation| {
 708                     get_future_output(obligation.predicate, obligation.cause.span)
 709                 })?
 710             }
 711             ty::Opaque(def_id, substs) => self
 712                 .tcx
 713                 .bound_explicit_item_bounds(def_id)
 714                 .subst_iter_copied(self.tcx, substs)
 715                 .find_map(|(p, s)| get_future_output(p, s))?,
 716             ty::Error(_) => return None,
 717             ty::Projection(proj)
 718                 if self.tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder =>
 719             {
 720                 self.tcx
 721                     .bound_explicit_item_bounds(proj.item_def_id)
 722                     .subst_iter_copied(self.tcx, proj.substs)
 723                     .find_map(|(p, s)| get_future_output(p, s))?
 724             }
 725             _ => span_bug!(
 726                 self.tcx.def_span(expr_def_id),
 727                 "async fn generator return type not an inference variable: {ret_ty}"
 728             ),
 729         };
 730
 731         // async fn that have opaque types in their return type need to redo the conversion to inference variables
 732         // as they fetch the still opaque version from the signature.
 733         let InferOk { value: output_ty, obligations } = self
 734             .replace_opaque_types_with_inference_vars(
 735                 output_ty,
 736                 body_id,
 737                 self.tcx.def_span(expr_def_id),
 738                 self.param_env,
 739             );
 740         self.register_predicates(obligations);
 741
 742         Some(output_ty)
 743     }
 744
 745     /// Given a projection like
 746     ///
 747     /// `<X as Future>::Output = T`
 748     ///
 749     /// where `X` is some type that has no late-bound regions, returns
 750     /// `Some(T)`. If the projection is for some other trait, returns
 751     /// `None`.
 752     fn deduce_future_output_from_projection(
 753         &self,
 754         cause_span: Span,
 755         predicate: ty::PolyProjectionPredicate<'tcx>,
 756     ) -> Option<Ty<'tcx>> {
 757         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 758
 759         // We do not expect any bound regions in our predicate, so
 760         // skip past the bound vars.
 761         let Some(predicate) = predicate.no_bound_vars() else {
 762             debug!("deduce_future_output_from_projection: has late-bound regions");
 763             return None;
 764         };
 765
 766         // Check that this is a projection from the `Future` trait.
 767         let trait_def_id = predicate.projection_ty.trait_def_id(self.tcx);
 768         let future_trait = self.tcx.require_lang_item(LangItem::Future, Some(cause_span));
 769         if trait_def_id != future_trait {
 770             debug!("deduce_future_output_from_projection: not a future");
 771             return None;
 772         }
 773
 774         // The `Future` trait has only one associated item, `Output`,
 775         // so check that this is what we see.
 776         let output_assoc_item = self.tcx.associated_item_def_ids(future_trait)[0];
 777         if output_assoc_item != predicate.projection_ty.item_def_id {
 778             span_bug!(
 779                 cause_span,
 780                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 781                 predicate.projection_ty.item_def_id,
 782                 output_assoc_item,
 783             );
 784         }
 785
 786         // Extract the type from the projection. Note that there can
 787         // be no bound variables in this type because the "self type"
 788         // does not have any regions in it.
 789         let output_ty = self.resolve_vars_if_possible(predicate.term);
 790         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 791         // This is a projection on a Fn trait so will always be a type.
 792         Some(output_ty.ty().unwrap())
 793     }
 794
 795     /// Converts the types that the user supplied, in case that doing
 796     /// so should yield an error, but returns back a signature where
 797     /// all parameters are of type `TyErr`.
 798     fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
 799         let astconv: &dyn AstConv<'_> = self;
 800
 801         let supplied_arguments = decl.inputs.iter().map(|a| {
 802             // Convert the types that the user supplied (if any), but ignore them.
 803             astconv.ast_ty_to_ty(a);
 804             self.tcx.ty_error()
 805         });
 806
 807         if let hir::FnRetTy::Return(ref output) = decl.output {
 808             astconv.ast_ty_to_ty(&output);
 809         }
 810
 811         let result = ty::Binder::dummy(self.tcx.mk_fn_sig(
 812             supplied_arguments,
 813             self.tcx.ty_error(),
 814             decl.c_variadic,
 815             hir::Unsafety::Normal,
 816             Abi::RustCall,
 817         ));
 818
 819         debug!("supplied_sig_of_closure: result={:?}", result);
 820
 821         result
 822     }
 823
 824     fn closure_sigs(
 825         &self,
 826         expr_def_id: DefId,
 827         body: &hir::Body<'_>,
 828         bound_sig: ty::PolyFnSig<'tcx>,
 829     ) -> ClosureSignatures<'tcx> {
 830         let liberated_sig = self.tcx().liberate_late_bound_regions(expr_def_id, bound_sig);
 831         let liberated_sig = self.inh.normalize_associated_types_in(
 832             body.value.span,
 833             body.value.hir_id,
 834             self.param_env,
 835             liberated_sig,
 836         );
 837         ClosureSignatures { bound_sig, liberated_sig }
 838     }
 839 }