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