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_macros::{TypeFoldable, TypeVisitable};
  14 use rustc_middle::ty::subst::InternalSubsts;
  15 use rustc_middle::ty::visit::TypeVisitable;
  16 use rustc_middle::ty::{self, Ty};
  17 use rustc_span::source_map::Span;
  18 use rustc_target::spec::abi::Abi;
  19 use rustc_trait_selection::traits;
  20 use rustc_trait_selection::traits::error_reporting::ArgKind;
  21 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
  22 use std::cmp;
  23 use std::iter;
  24
  25 /// What signature do we *expect* the closure to have from context?
  26 #[derive(Debug, Clone, TypeFoldable, TypeVisitable)]
  27 struct ExpectedSig<'tcx> {
  28     /// Span that gave us this expectation, if we know that.
  29     cause_span: Option<Span>,
  30     sig: ty::PolyFnSig<'tcx>,
  31 }
  32
  33 struct ClosureSignatures<'tcx> {
  34     /// The signature users of the closure see.
  35     bound_sig: ty::PolyFnSig<'tcx>,
  36     /// The signature within the function body.
  37     /// This mostly differs in the sense that lifetimes are now early bound and any
  38     /// opaque types from the signature expectation are overriden in case there are
  39     /// explicit hidden types written by the user in the closure signature.
  40     liberated_sig: ty::FnSig<'tcx>,
  41 }
  42
  43 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  44     #[instrument(skip(self, expr, _capture, decl, body_id), level = "debug")]
  45     pub fn check_expr_closure(
  46         &self,
  47         expr: &hir::Expr<'_>,
  48         _capture: hir::CaptureBy,
  49         decl: &'tcx hir::FnDecl<'tcx>,
  50         body_id: hir::BodyId,
  51         gen: Option<hir::Movability>,
  52         expected: Expectation<'tcx>,
  53     ) -> Ty<'tcx> {
  54         trace!("decl = {:#?}", decl);
  55         trace!("expr = {:#?}", expr);
  56
  57         // It's always helpful for inference if we know the kind of
  58         // closure sooner rather than later, so first examine the expected
  59         // type, and see if can glean a closure kind from there.
  60         let (expected_sig, expected_kind) = match expected.to_option(self) {
  61             Some(ty) => self.deduce_expectations_from_expected_type(ty),
  62             None => (None, None),
  63         };
  64         let body = self.tcx.hir().body(body_id);
  65         self.check_closure(expr, expected_kind, decl, body, gen, expected_sig)
  66     }
  67
  68     #[instrument(skip(self, expr, body, decl), level = "debug", ret)]
  69     fn check_closure(
  70         &self,
  71         expr: &hir::Expr<'_>,
  72         opt_kind: Option<ty::ClosureKind>,
  73         decl: &'tcx hir::FnDecl<'tcx>,
  74         body: &'tcx hir::Body<'tcx>,
  75         gen: Option<hir::Movability>,
  76         expected_sig: Option<ExpectedSig<'tcx>>,
  77     ) -> Ty<'tcx> {
  78         trace!("decl = {:#?}", decl);
  79         let expr_def_id = self.tcx.hir().local_def_id(expr.hir_id);
  80         debug!(?expr_def_id);
  81
  82         let ClosureSignatures { bound_sig, liberated_sig } =
  83             self.sig_of_closure(expr.hir_id, expr_def_id.to_def_id(), decl, body, expected_sig);
  84
  85         debug!(?bound_sig, ?liberated_sig);
  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         )
  96         .1;
  97
  98         let parent_substs = InternalSubsts::identity_for_item(
  99             self.tcx,
 100             self.tcx.typeck_root_def_id(expr_def_id.to_def_id()),
 101         );
 102
 103         let tupled_upvars_ty = self.next_ty_var(TypeVariableOrigin {
 104             kind: TypeVariableOriginKind::ClosureSynthetic,
 105             span: self.tcx.hir().span(expr.hir_id),
 106         });
 107
 108         if let Some(GeneratorTypes { resume_ty, yield_ty, interior, movability }) = generator_types
 109         {
 110             let generator_substs = ty::GeneratorSubsts::new(
 111                 self.tcx,
 112                 ty::GeneratorSubstsParts {
 113                     parent_substs,
 114                     resume_ty,
 115                     yield_ty,
 116                     return_ty: liberated_sig.output(),
 117                     witness: interior,
 118                     tupled_upvars_ty,
 119                 },
 120             );
 121
 122             return self.tcx.mk_generator(
 123                 expr_def_id.to_def_id(),
 124                 generator_substs.substs,
 125                 movability,
 126             );
 127         }
 128
 129         // Tuple up the arguments and insert the resulting function type into
 130         // the `closures` table.
 131         let sig = bound_sig.map_bound(|sig| {
 132             self.tcx.mk_fn_sig(
 133                 iter::once(self.tcx.intern_tup(sig.inputs())),
 134                 sig.output(),
 135                 sig.c_variadic,
 136                 sig.unsafety,
 137                 sig.abi,
 138             )
 139         });
 140
 141         debug!(?sig, ?opt_kind);
 142
 143         let closure_kind_ty = match opt_kind {
 144             Some(kind) => kind.to_ty(self.tcx),
 145
 146             // Create a type variable (for now) to represent the closure kind.
 147             // It will be unified during the upvar inference phase (`upvar.rs`)
 148             None => self.next_ty_var(TypeVariableOrigin {
 149                 // FIXME(eddyb) distinguish closure kind inference variables from the rest.
 150                 kind: TypeVariableOriginKind::ClosureSynthetic,
 151                 span: expr.span,
 152             }),
 153         };
 154
 155         let closure_substs = ty::ClosureSubsts::new(
 156             self.tcx,
 157             ty::ClosureSubstsParts {
 158                 parent_substs,
 159                 closure_kind_ty,
 160                 closure_sig_as_fn_ptr_ty: self.tcx.mk_fn_ptr(sig),
 161                 tupled_upvars_ty,
 162             },
 163         );
 164
 165         self.tcx.mk_closure(expr_def_id.to_def_id(), closure_substs.substs)
 166     }
 167
 168     /// Given the expected type, figures out what it can about this closure we
 169     /// are about to type check:
 170     #[instrument(skip(self), level = "debug")]
 171     fn deduce_expectations_from_expected_type(
 172         &self,
 173         expected_ty: Ty<'tcx>,
 174     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 175         match *expected_ty.kind() {
 176             ty::Opaque(def_id, substs) => {
 177                 let bounds = self.tcx.bound_explicit_item_bounds(def_id);
 178                 let sig =
 179                     bounds.subst_iter_copied(self.tcx, substs).find_map(|(pred, span)| match pred
 180                         .kind()
 181                         .skip_binder()
 182                     {
 183                         ty::PredicateKind::Projection(proj_predicate) => self
 184                             .deduce_sig_from_projection(
 185                                 Some(span),
 186                                 pred.kind().rebind(proj_predicate),
 187                             ),
 188                         _ => None,
 189                     });
 190
 191                 let kind = bounds
 192                     .0
 193                     .iter()
 194                     .filter_map(|(pred, _)| match pred.kind().skip_binder() {
 195                         ty::PredicateKind::Trait(tp) => {
 196                             self.tcx.fn_trait_kind_from_lang_item(tp.def_id())
 197                         }
 198                         _ => None,
 199                     })
 200                     .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
 201                 trace!(?sig, ?kind);
 202                 (sig, kind)
 203             }
 204             ty::Dynamic(ref object_type, ..) => {
 205                 let sig = object_type.projection_bounds().find_map(|pb| {
 206                     let pb = pb.with_self_ty(self.tcx, self.tcx.types.trait_object_dummy_self);
 207                     self.deduce_sig_from_projection(None, pb)
 208                 });
 209                 let kind = object_type
 210                     .principal_def_id()
 211                     .and_then(|did| self.tcx.fn_trait_kind_from_lang_item(did));
 212                 (sig, kind)
 213             }
 214             ty::Infer(ty::TyVar(vid)) => self.deduce_expectations_from_obligations(vid),
 215             ty::FnPtr(sig) => {
 216                 let expected_sig = ExpectedSig { cause_span: None, sig };
 217                 (Some(expected_sig), Some(ty::ClosureKind::Fn))
 218             }
 219             _ => (None, None),
 220         }
 221     }
 222
 223     fn deduce_expectations_from_obligations(
 224         &self,
 225         expected_vid: ty::TyVid,
 226     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 227         let mut expected_sig = None;
 228         let mut expected_kind = None;
 229
 230         for obligation in traits::elaborate_obligations(
 231             self.tcx,
 232             // Reverse the obligations here, since `elaborate_*` uses a stack,
 233             // and we want to keep inference generally in the same order of
 234             // the registered obligations.
 235             self.obligations_for_self_ty(expected_vid).rev().collect(),
 236         ) {
 237             debug!(?obligation.predicate);
 238             let bound_predicate = obligation.predicate.kind();
 239
 240             // Given a Projection predicate, we can potentially infer
 241             // the complete signature.
 242             if expected_sig.is_none()
 243                 && let ty::PredicateKind::Projection(proj_predicate) = bound_predicate.skip_binder()
 244             {
 245                 expected_sig = self.normalize_associated_types_in(
 246                     obligation.cause.span,
 247                     self.deduce_sig_from_projection(
 248                     Some(obligation.cause.span),
 249                         bound_predicate.rebind(proj_predicate),
 250                     ),
 251                 );
 252             }
 253
 254             // Even if we can't infer the full signature, we may be able to
 255             // infer the kind. This can occur when we elaborate a predicate
 256             // like `F : Fn<A>`. Note that due to subtyping we could encounter
 257             // many viable options, so pick the most restrictive.
 258             let trait_def_id = match bound_predicate.skip_binder() {
 259                 ty::PredicateKind::Projection(data) => {
 260                     Some(data.projection_ty.trait_def_id(self.tcx))
 261                 }
 262                 ty::PredicateKind::Trait(data) => Some(data.def_id()),
 263                 _ => None,
 264             };
 265             if let Some(closure_kind) =
 266                 trait_def_id.and_then(|def_id| self.tcx.fn_trait_kind_from_lang_item(def_id))
 267             {
 268                 expected_kind = Some(
 269                     expected_kind
 270                         .map_or_else(|| closure_kind, |current| cmp::min(current, closure_kind)),
 271                 );
 272             }
 273         }
 274
 275         (expected_sig, expected_kind)
 276     }
 277
 278     /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
 279     /// everything we need to know about a closure or generator.
 280     ///
 281     /// The `cause_span` should be the span that caused us to
 282     /// have this expected signature, or `None` if we can't readily
 283     /// know that.
 284     #[instrument(level = "debug", skip(self, cause_span), ret)]
 285     fn deduce_sig_from_projection(
 286         &self,
 287         cause_span: Option<Span>,
 288         projection: ty::PolyProjectionPredicate<'tcx>,
 289     ) -> Option<ExpectedSig<'tcx>> {
 290         let tcx = self.tcx;
 291
 292         let trait_def_id = projection.trait_def_id(tcx);
 293
 294         let is_fn = tcx.fn_trait_kind_from_lang_item(trait_def_id).is_some();
 295         let gen_trait = tcx.require_lang_item(LangItem::Generator, cause_span);
 296         let is_gen = gen_trait == trait_def_id;
 297         if !is_fn && !is_gen {
 298             debug!("not fn or generator");
 299             return None;
 300         }
 301
 302         if is_gen {
 303             // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
 304             // associated item and not yield.
 305             let return_assoc_item = self.tcx.associated_item_def_ids(gen_trait)[1];
 306             if return_assoc_item != projection.projection_def_id() {
 307                 debug!("not return assoc item of generator");
 308                 return None;
 309             }
 310         }
 311
 312         let input_tys = if is_fn {
 313             let arg_param_ty = projection.skip_binder().projection_ty.substs.type_at(1);
 314             let arg_param_ty = self.resolve_vars_if_possible(arg_param_ty);
 315             debug!(?arg_param_ty);
 316
 317             match arg_param_ty.kind() {
 318                 &ty::Tuple(tys) => tys,
 319                 _ => return None,
 320             }
 321         } else {
 322             // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
 323             // else they must have exactly 1 argument. For now though, just give up in this case.
 324             return None;
 325         };
 326
 327         // Since this is a return parameter type it is safe to unwrap.
 328         let ret_param_ty = projection.skip_binder().term.ty().unwrap();
 329         let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
 330         debug!(?ret_param_ty);
 331
 332         let sig = projection.rebind(self.tcx.mk_fn_sig(
 333             input_tys.iter(),
 334             ret_param_ty,
 335             false,
 336             hir::Unsafety::Normal,
 337             Abi::Rust,
 338         ));
 339
 340         Some(ExpectedSig { cause_span, sig })
 341     }
 342
 343     fn sig_of_closure(
 344         &self,
 345         hir_id: hir::HirId,
 346         expr_def_id: DefId,
 347         decl: &hir::FnDecl<'_>,
 348         body: &hir::Body<'_>,
 349         expected_sig: Option<ExpectedSig<'tcx>>,
 350     ) -> ClosureSignatures<'tcx> {
 351         if let Some(e) = expected_sig {
 352             self.sig_of_closure_with_expectation(hir_id, expr_def_id, decl, body, e)
 353         } else {
 354             self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body)
 355         }
 356     }
 357
 358     /// If there is no expected signature, then we will convert the
 359     /// types that the user gave into a signature.
 360     #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
 361     fn sig_of_closure_no_expectation(
 362         &self,
 363         hir_id: hir::HirId,
 364         expr_def_id: DefId,
 365         decl: &hir::FnDecl<'_>,
 366         body: &hir::Body<'_>,
 367     ) -> ClosureSignatures<'tcx> {
 368         let bound_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 369
 370         self.closure_sigs(expr_def_id, body, bound_sig)
 371     }
 372
 373     /// Invoked to compute the signature of a closure expression. This
 374     /// combines any user-provided type annotations (e.g., `|x: u32|
 375     /// -> u32 { .. }`) with the expected signature.
 376     ///
 377     /// The approach is as follows:
 378     ///
 379     /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
 380     /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
 381     ///   - If we have no expectation `E`, then the signature of the closure is `S`.
 382     ///   - Otherwise, the signature of the closure is E. Moreover:
 383     ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
 384     ///     - Instantiate all the late-bound regions bound in the closure within `S`
 385     ///       with fresh (existential) variables, yielding `S'`
 386     ///     - Require that `E' = S'`
 387     ///       - We could use some kind of subtyping relationship here,
 388     ///         I imagine, but equality is easier and works fine for
 389     ///         our purposes.
 390     ///
 391     /// The key intuition here is that the user's types must be valid
 392     /// from "the inside" of the closure, but the expectation
 393     /// ultimately drives the overall signature.
 394     ///
 395     /// # Examples
 396     ///
 397     /// ```ignore (illustrative)
 398     /// fn with_closure<F>(_: F)
 399     ///   where F: Fn(&u32) -> &u32 { .. }
 400     ///
 401     /// with_closure(|x: &u32| { ... })
 402     /// ```
 403     ///
 404     /// Here:
 405     /// - E would be `fn(&u32) -> &u32`.
 406     /// - S would be `fn(&u32) ->
 407     /// - E' is `&'!0 u32 -> &'!0 u32`
 408     /// - S' is `&'?0 u32 -> ?T`
 409     ///
 410     /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
 411     ///
 412     /// # Arguments
 413     ///
 414     /// - `expr_def_id`: the `DefId` of the closure expression
 415     /// - `decl`: the HIR declaration of the closure
 416     /// - `body`: the body of the closure
 417     /// - `expected_sig`: the expected signature (if any). Note that
 418     ///   this is missing a binder: that is, there may be late-bound
 419     ///   regions with depth 1, which are bound then by the closure.
 420     #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
 421     fn sig_of_closure_with_expectation(
 422         &self,
 423         hir_id: hir::HirId,
 424         expr_def_id: DefId,
 425         decl: &hir::FnDecl<'_>,
 426         body: &hir::Body<'_>,
 427         expected_sig: ExpectedSig<'tcx>,
 428     ) -> ClosureSignatures<'tcx> {
 429         // Watch out for some surprises and just ignore the
 430         // expectation if things don't see to match up with what we
 431         // expect.
 432         if expected_sig.sig.c_variadic() != decl.c_variadic {
 433             return self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body);
 434         } else if expected_sig.sig.skip_binder().inputs_and_output.len() != decl.inputs.len() + 1 {
 435             return self.sig_of_closure_with_mismatched_number_of_arguments(
 436                 expr_def_id,
 437                 decl,
 438                 body,
 439                 expected_sig,
 440             );
 441         }
 442
 443         // Create a `PolyFnSig`. Note the oddity that late bound
 444         // regions appearing free in `expected_sig` are now bound up
 445         // in this binder we are creating.
 446         assert!(!expected_sig.sig.skip_binder().has_vars_bound_above(ty::INNERMOST));
 447         let bound_sig = expected_sig.sig.map_bound(|sig| {
 448             self.tcx.mk_fn_sig(
 449                 sig.inputs().iter().cloned(),
 450                 sig.output(),
 451                 sig.c_variadic,
 452                 hir::Unsafety::Normal,
 453                 Abi::RustCall,
 454             )
 455         });
 456
 457         // `deduce_expectations_from_expected_type` introduces
 458         // late-bound lifetimes defined elsewhere, which we now
 459         // anonymize away, so as not to confuse the user.
 460         let bound_sig = self.tcx.anonymize_late_bound_regions(bound_sig);
 461
 462         let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
 463
 464         // Up till this point, we have ignored the annotations that the user
 465         // gave. This function will check that they unify successfully.
 466         // Along the way, it also writes out entries for types that the user
 467         // wrote into our typeck results, which are then later used by the privacy
 468         // check.
 469         match self.merge_supplied_sig_with_expectation(
 470             hir_id,
 471             expr_def_id,
 472             decl,
 473             body,
 474             closure_sigs,
 475         ) {
 476             Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
 477             Err(_) => self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body),
 478         }
 479     }
 480
 481     fn sig_of_closure_with_mismatched_number_of_arguments(
 482         &self,
 483         expr_def_id: DefId,
 484         decl: &hir::FnDecl<'_>,
 485         body: &hir::Body<'_>,
 486         expected_sig: ExpectedSig<'tcx>,
 487     ) -> ClosureSignatures<'tcx> {
 488         let hir = self.tcx.hir();
 489         let expr_map_node = hir.get_if_local(expr_def_id).unwrap();
 490         let expected_args: Vec<_> = expected_sig
 491             .sig
 492             .skip_binder()
 493             .inputs()
 494             .iter()
 495             .map(|ty| ArgKind::from_expected_ty(*ty, None))
 496             .collect();
 497         let (closure_span, found_args) = match self.get_fn_like_arguments(expr_map_node) {
 498             Some((sp, args)) => (Some(sp), args),
 499             None => (None, Vec::new()),
 500         };
 501         let expected_span =
 502             expected_sig.cause_span.unwrap_or_else(|| hir.span_if_local(expr_def_id).unwrap());
 503         self.report_arg_count_mismatch(
 504             expected_span,
 505             closure_span,
 506             expected_args,
 507             found_args,
 508             true,
 509         )
 510         .emit();
 511
 512         let error_sig = self.error_sig_of_closure(decl);
 513
 514         self.closure_sigs(expr_def_id, body, error_sig)
 515     }
 516
 517     /// Enforce the user's types against the expectation. See
 518     /// `sig_of_closure_with_expectation` for details on the overall
 519     /// strategy.
 520     #[instrument(level = "debug", skip(self, hir_id, expr_def_id, decl, body, expected_sigs))]
 521     fn merge_supplied_sig_with_expectation(
 522         &self,
 523         hir_id: hir::HirId,
 524         expr_def_id: DefId,
 525         decl: &hir::FnDecl<'_>,
 526         body: &hir::Body<'_>,
 527         mut expected_sigs: ClosureSignatures<'tcx>,
 528     ) -> InferResult<'tcx, ClosureSignatures<'tcx>> {
 529         // Get the signature S that the user gave.
 530         //
 531         // (See comment on `sig_of_closure_with_expectation` for the
 532         // meaning of these letters.)
 533         let supplied_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 534
 535         debug!(?supplied_sig);
 536
 537         // FIXME(#45727): As discussed in [this comment][c1], naively
 538         // forcing equality here actually results in suboptimal error
 539         // messages in some cases.  For now, if there would have been
 540         // an obvious error, we fallback to declaring the type of the
 541         // closure to be the one the user gave, which allows other
 542         // error message code to trigger.
 543         //
 544         // However, I think [there is potential to do even better
 545         // here][c2], since in *this* code we have the precise span of
 546         // the type parameter in question in hand when we report the
 547         // error.
 548         //
 549         // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
 550         // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
 551         self.commit_if_ok(|_| {
 552             let mut all_obligations = vec![];
 553             let inputs: Vec<_> = iter::zip(
 554                 decl.inputs,
 555                 supplied_sig.inputs().skip_binder(), // binder moved to (*) below
 556             )
 557             .map(|(hir_ty, &supplied_ty)| {
 558                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 559                 self.replace_bound_vars_with_fresh_vars(
 560                     hir_ty.span,
 561                     LateBoundRegionConversionTime::FnCall,
 562                     // (*) binder moved to here
 563                     supplied_sig.inputs().rebind(supplied_ty),
 564                 )
 565             })
 566             .collect();
 567
 568             // The liberated version of this signature should be a subtype
 569             // of the liberated form of the expectation.
 570             for ((hir_ty, &supplied_ty), expected_ty) in iter::zip(
 571                 iter::zip(decl.inputs, &inputs),
 572                 expected_sigs.liberated_sig.inputs(), // `liberated_sig` is E'.
 573             ) {
 574                 // Check that E' = S'.
 575                 let cause = self.misc(hir_ty.span);
 576                 let InferOk { value: (), obligations } =
 577                     self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
 578                 all_obligations.extend(obligations);
 579             }
 580
 581             let supplied_output_ty = self.replace_bound_vars_with_fresh_vars(
 582                 decl.output.span(),
 583                 LateBoundRegionConversionTime::FnCall,
 584                 supplied_sig.output(),
 585             );
 586             let cause = &self.misc(decl.output.span());
 587             let InferOk { value: (), obligations } = self
 588                 .at(cause, self.param_env)
 589                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 590             all_obligations.extend(obligations);
 591
 592             let inputs = inputs.into_iter().map(|ty| self.resolve_vars_if_possible(ty));
 593
 594             expected_sigs.liberated_sig = self.tcx.mk_fn_sig(
 595                 inputs,
 596                 supplied_output_ty,
 597                 expected_sigs.liberated_sig.c_variadic,
 598                 hir::Unsafety::Normal,
 599                 Abi::RustCall,
 600             );
 601
 602             Ok(InferOk { value: expected_sigs, obligations: all_obligations })
 603         })
 604     }
 605
 606     /// If there is no expected signature, then we will convert the
 607     /// types that the user gave into a signature.
 608     ///
 609     /// Also, record this closure signature for later.
 610     #[instrument(skip(self, decl, body), level = "debug", ret)]
 611     fn supplied_sig_of_closure(
 612         &self,
 613         hir_id: hir::HirId,
 614         expr_def_id: DefId,
 615         decl: &hir::FnDecl<'_>,
 616         body: &hir::Body<'_>,
 617     ) -> ty::PolyFnSig<'tcx> {
 618         let astconv: &dyn AstConv<'_> = self;
 619
 620         trace!("decl = {:#?}", decl);
 621         debug!(?body.generator_kind);
 622
 623         let bound_vars = self.tcx.late_bound_vars(hir_id);
 624
 625         // First, convert the types that the user supplied (if any).
 626         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 627         let supplied_return = match decl.output {
 628             hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
 629             hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
 630                 // In the case of the async block that we create for a function body,
 631                 // we expect the return type of the block to match that of the enclosing
 632                 // function.
 633                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 634                     debug!("closure is async fn body");
 635                     self.deduce_future_output_from_obligations(expr_def_id, body.id().hir_id)
 636                         .unwrap_or_else(|| {
 637                             // AFAIK, deducing the future output
 638                             // always succeeds *except* in error cases
 639                             // like #65159. I'd like to return Error
 640                             // here, but I can't because I can't
 641                             // easily (and locally) prove that we
 642                             // *have* reported an
 643                             // error. --nikomatsakis
 644                             astconv.ty_infer(None, decl.output.span())
 645                         })
 646                 }
 647
 648                 _ => astconv.ty_infer(None, decl.output.span()),
 649             },
 650         };
 651
 652         let result = ty::Binder::bind_with_vars(
 653             self.tcx.mk_fn_sig(
 654                 supplied_arguments,
 655                 supplied_return,
 656                 decl.c_variadic,
 657                 hir::Unsafety::Normal,
 658                 Abi::RustCall,
 659             ),
 660             bound_vars,
 661         );
 662         // Astconv can't normalize inputs or outputs with escaping bound vars,
 663         // so normalize them here, after we've wrapped them in a binder.
 664         let result = self.normalize_associated_types_in(self.tcx.hir().span(hir_id), result);
 665
 666         let c_result = self.inh.infcx.canonicalize_response(result);
 667         self.typeck_results
 668             .borrow_mut()
 669             .user_provided_sigs
 670             .insert(expr_def_id.expect_local(), c_result);
 671
 672         result
 673     }
 674
 675     /// Invoked when we are translating the generator that results
 676     /// from desugaring an `async fn`. Returns the "sugared" return
 677     /// type of the `async fn` -- that is, the return type that the
 678     /// user specified. The "desugared" return type is an `impl
 679     /// Future<Output = T>`, so we do this by searching through the
 680     /// obligations to extract the `T`.
 681     #[instrument(skip(self), level = "debug", ret)]
 682     fn deduce_future_output_from_obligations(
 683         &self,
 684         expr_def_id: DefId,
 685         body_id: hir::HirId,
 686     ) -> Option<Ty<'tcx>> {
 687         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
 688             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
 689         });
 690
 691         let ret_ty = ret_coercion.borrow().expected_ty();
 692         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 693
 694         let get_future_output = |predicate: ty::Predicate<'tcx>, span| {
 695             // Search for a pending obligation like
 696             //
 697             // `<R as Future>::Output = T`
 698             //
 699             // where R is the return type we are expecting. This type `T`
 700             // will be our output.
 701             let bound_predicate = predicate.kind();
 702             if let ty::PredicateKind::Projection(proj_predicate) = bound_predicate.skip_binder() {
 703                 self.deduce_future_output_from_projection(
 704                     span,
 705                     bound_predicate.rebind(proj_predicate),
 706                 )
 707             } else {
 708                 None
 709             }
 710         };
 711
 712         let output_ty = match *ret_ty.kind() {
 713             ty::Infer(ty::TyVar(ret_vid)) => {
 714                 self.obligations_for_self_ty(ret_vid).find_map(|obligation| {
 715                     get_future_output(obligation.predicate, obligation.cause.span)
 716                 })?
 717             }
 718             ty::Opaque(def_id, substs) => self
 719                 .tcx
 720                 .bound_explicit_item_bounds(def_id)
 721                 .subst_iter_copied(self.tcx, substs)
 722                 .find_map(|(p, s)| get_future_output(p, s))?,
 723             ty::Error(_) => return None,
 724             ty::Projection(proj)
 725                 if self.tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder =>
 726             {
 727                 self.tcx
 728                     .bound_explicit_item_bounds(proj.item_def_id)
 729                     .subst_iter_copied(self.tcx, proj.substs)
 730                     .find_map(|(p, s)| get_future_output(p, s))?
 731             }
 732             _ => span_bug!(
 733                 self.tcx.def_span(expr_def_id),
 734                 "async fn generator return type not an inference variable: {ret_ty}"
 735             ),
 736         };
 737
 738         // async fn that have opaque types in their return type need to redo the conversion to inference variables
 739         // as they fetch the still opaque version from the signature.
 740         let InferOk { value: output_ty, obligations } = self
 741             .replace_opaque_types_with_inference_vars(
 742                 output_ty,
 743                 body_id,
 744                 self.tcx.def_span(expr_def_id),
 745                 self.param_env,
 746             );
 747         self.register_predicates(obligations);
 748
 749         Some(output_ty)
 750     }
 751
 752     /// Given a projection like
 753     ///
 754     /// `<X as Future>::Output = T`
 755     ///
 756     /// where `X` is some type that has no late-bound regions, returns
 757     /// `Some(T)`. If the projection is for some other trait, returns
 758     /// `None`.
 759     fn deduce_future_output_from_projection(
 760         &self,
 761         cause_span: Span,
 762         predicate: ty::PolyProjectionPredicate<'tcx>,
 763     ) -> Option<Ty<'tcx>> {
 764         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 765
 766         // We do not expect any bound regions in our predicate, so
 767         // skip past the bound vars.
 768         let Some(predicate) = predicate.no_bound_vars() else {
 769             debug!("deduce_future_output_from_projection: has late-bound regions");
 770             return None;
 771         };
 772
 773         // Check that this is a projection from the `Future` trait.
 774         let trait_def_id = predicate.projection_ty.trait_def_id(self.tcx);
 775         let future_trait = self.tcx.require_lang_item(LangItem::Future, Some(cause_span));
 776         if trait_def_id != future_trait {
 777             debug!("deduce_future_output_from_projection: not a future");
 778             return None;
 779         }
 780
 781         // The `Future` trait has only one associated item, `Output`,
 782         // so check that this is what we see.
 783         let output_assoc_item = self.tcx.associated_item_def_ids(future_trait)[0];
 784         if output_assoc_item != predicate.projection_ty.item_def_id {
 785             span_bug!(
 786                 cause_span,
 787                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 788                 predicate.projection_ty.item_def_id,
 789                 output_assoc_item,
 790             );
 791         }
 792
 793         // Extract the type from the projection. Note that there can
 794         // be no bound variables in this type because the "self type"
 795         // does not have any regions in it.
 796         let output_ty = self.resolve_vars_if_possible(predicate.term);
 797         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 798         // This is a projection on a Fn trait so will always be a type.
 799         Some(output_ty.ty().unwrap())
 800     }
 801
 802     /// Converts the types that the user supplied, in case that doing
 803     /// so should yield an error, but returns back a signature where
 804     /// all parameters are of type `TyErr`.
 805     fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
 806         let astconv: &dyn AstConv<'_> = self;
 807
 808         let supplied_arguments = decl.inputs.iter().map(|a| {
 809             // Convert the types that the user supplied (if any), but ignore them.
 810             astconv.ast_ty_to_ty(a);
 811             self.tcx.ty_error()
 812         });
 813
 814         if let hir::FnRetTy::Return(ref output) = decl.output {
 815             astconv.ast_ty_to_ty(&output);
 816         }
 817
 818         let result = ty::Binder::dummy(self.tcx.mk_fn_sig(
 819             supplied_arguments,
 820             self.tcx.ty_error(),
 821             decl.c_variadic,
 822             hir::Unsafety::Normal,
 823             Abi::RustCall,
 824         ));
 825
 826         debug!("supplied_sig_of_closure: result={:?}", result);
 827
 828         result
 829     }
 830
 831     fn closure_sigs(
 832         &self,
 833         expr_def_id: DefId,
 834         body: &hir::Body<'_>,
 835         bound_sig: ty::PolyFnSig<'tcx>,
 836     ) -> ClosureSignatures<'tcx> {
 837         let liberated_sig = self.tcx().liberate_late_bound_regions(expr_def_id, bound_sig);
 838         let liberated_sig = self.inh.normalize_associated_types_in(
 839             body.value.span,
 840             body.value.hir_id,
 841             self.param_env,
 842             liberated_sig,
 843         );
 844         ClosureSignatures { bound_sig, liberated_sig }
 845     }
 846 }