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 = bounds
 180                     .transpose_iter()
 181                     .map(|e| e.map_bound(|e| *e).transpose_tuple2())
 182                     .find_map(|(pred, span)| match pred.0.kind().skip_binder() {
 183                         ty::PredicateKind::Projection(proj_predicate) => self
 184                             .deduce_sig_from_projection(
 185                                 Some(span.0),
 186                                 pred.0
 187                                     .kind()
 188                                     .rebind(pred.rebind(proj_predicate).subst(self.tcx, substs)),
 189                             ),
 190                         _ => None,
 191                     });
 192
 193                 let kind = bounds
 194                     .transpose_iter()
 195                     .map(|e| e.map_bound(|e| *e).transpose_tuple2())
 196                     .filter_map(|(pred, _)| match pred.0.kind().skip_binder() {
 197                         ty::PredicateKind::Trait(tp) => {
 198                             self.tcx.fn_trait_kind_from_lang_item(tp.def_id())
 199                         }
 200                         _ => None,
 201                     })
 202                     .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
 203                 trace!(?sig, ?kind);
 204                 (sig, kind)
 205             }
 206             ty::Dynamic(ref object_type, ..) => {
 207                 let sig = object_type.projection_bounds().find_map(|pb| {
 208                     let pb = pb.with_self_ty(self.tcx, self.tcx.types.trait_object_dummy_self);
 209                     self.deduce_sig_from_projection(None, pb)
 210                 });
 211                 let kind = object_type
 212                     .principal_def_id()
 213                     .and_then(|did| self.tcx.fn_trait_kind_from_lang_item(did));
 214                 (sig, kind)
 215             }
 216             ty::Infer(ty::TyVar(vid)) => self.deduce_expectations_from_obligations(vid),
 217             ty::FnPtr(sig) => {
 218                 let expected_sig = ExpectedSig { cause_span: None, sig };
 219                 (Some(expected_sig), Some(ty::ClosureKind::Fn))
 220             }
 221             _ => (None, None),
 222         }
 223     }
 224
 225     fn deduce_expectations_from_obligations(
 226         &self,
 227         expected_vid: ty::TyVid,
 228     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 229         let expected_sig =
 230             self.obligations_for_self_ty(expected_vid).find_map(|(_, obligation)| {
 231                 debug!(?obligation.predicate);
 232
 233                 let bound_predicate = obligation.predicate.kind();
 234                 if let ty::PredicateKind::Projection(proj_predicate) =
 235                     obligation.predicate.kind().skip_binder()
 236                 {
 237                     // Given a Projection predicate, we can potentially infer
 238                     // the complete signature.
 239                     self.deduce_sig_from_projection(
 240                         Some(obligation.cause.span),
 241                         bound_predicate.rebind(proj_predicate),
 242                     )
 243                 } else {
 244                     None
 245                 }
 246             });
 247
 248         // Even if we can't infer the full signature, we may be able to
 249         // infer the kind. This can occur when we elaborate a predicate
 250         // like `F : Fn<A>`. Note that due to subtyping we could encounter
 251         // many viable options, so pick the most restrictive.
 252         let expected_kind = self
 253             .obligations_for_self_ty(expected_vid)
 254             .filter_map(|(tr, _)| self.tcx.fn_trait_kind_from_lang_item(tr.def_id()))
 255             .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
 256
 257         (expected_sig, expected_kind)
 258     }
 259
 260     /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
 261     /// everything we need to know about a closure or generator.
 262     ///
 263     /// The `cause_span` should be the span that caused us to
 264     /// have this expected signature, or `None` if we can't readily
 265     /// know that.
 266     #[instrument(level = "debug", skip(self, cause_span), ret)]
 267     fn deduce_sig_from_projection(
 268         &self,
 269         cause_span: Option<Span>,
 270         projection: ty::PolyProjectionPredicate<'tcx>,
 271     ) -> Option<ExpectedSig<'tcx>> {
 272         let tcx = self.tcx;
 273
 274         let trait_def_id = projection.trait_def_id(tcx);
 275
 276         let is_fn = tcx.fn_trait_kind_from_lang_item(trait_def_id).is_some();
 277         let gen_trait = tcx.require_lang_item(LangItem::Generator, cause_span);
 278         let is_gen = gen_trait == trait_def_id;
 279         if !is_fn && !is_gen {
 280             debug!("not fn or generator");
 281             return None;
 282         }
 283
 284         if is_gen {
 285             // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
 286             // associated item and not yield.
 287             let return_assoc_item = self.tcx.associated_item_def_ids(gen_trait)[1];
 288             if return_assoc_item != projection.projection_def_id() {
 289                 debug!("not return assoc item of generator");
 290                 return None;
 291             }
 292         }
 293
 294         let input_tys = if is_fn {
 295             let arg_param_ty = projection.skip_binder().projection_ty.substs.type_at(1);
 296             let arg_param_ty = self.resolve_vars_if_possible(arg_param_ty);
 297             debug!(?arg_param_ty);
 298
 299             match arg_param_ty.kind() {
 300                 &ty::Tuple(tys) => tys,
 301                 _ => return None,
 302             }
 303         } else {
 304             // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
 305             // else they must have exactly 1 argument. For now though, just give up in this case.
 306             return None;
 307         };
 308
 309         // Since this is a return parameter type it is safe to unwrap.
 310         let ret_param_ty = projection.skip_binder().term.ty().unwrap();
 311         let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
 312         debug!(?ret_param_ty);
 313
 314         let sig = projection.rebind(self.tcx.mk_fn_sig(
 315             input_tys.iter(),
 316             ret_param_ty,
 317             false,
 318             hir::Unsafety::Normal,
 319             Abi::Rust,
 320         ));
 321
 322         Some(ExpectedSig { cause_span, sig })
 323     }
 324
 325     fn sig_of_closure(
 326         &self,
 327         hir_id: hir::HirId,
 328         expr_def_id: DefId,
 329         decl: &hir::FnDecl<'_>,
 330         body: &hir::Body<'_>,
 331         expected_sig: Option<ExpectedSig<'tcx>>,
 332     ) -> ClosureSignatures<'tcx> {
 333         if let Some(e) = expected_sig {
 334             self.sig_of_closure_with_expectation(hir_id, expr_def_id, decl, body, e)
 335         } else {
 336             self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body)
 337         }
 338     }
 339
 340     /// If there is no expected signature, then we will convert the
 341     /// types that the user gave into a signature.
 342     #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
 343     fn sig_of_closure_no_expectation(
 344         &self,
 345         hir_id: hir::HirId,
 346         expr_def_id: DefId,
 347         decl: &hir::FnDecl<'_>,
 348         body: &hir::Body<'_>,
 349     ) -> ClosureSignatures<'tcx> {
 350         let bound_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 351
 352         self.closure_sigs(expr_def_id, body, bound_sig)
 353     }
 354
 355     /// Invoked to compute the signature of a closure expression. This
 356     /// combines any user-provided type annotations (e.g., `|x: u32|
 357     /// -> u32 { .. }`) with the expected signature.
 358     ///
 359     /// The approach is as follows:
 360     ///
 361     /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
 362     /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
 363     ///   - If we have no expectation `E`, then the signature of the closure is `S`.
 364     ///   - Otherwise, the signature of the closure is E. Moreover:
 365     ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
 366     ///     - Instantiate all the late-bound regions bound in the closure within `S`
 367     ///       with fresh (existential) variables, yielding `S'`
 368     ///     - Require that `E' = S'`
 369     ///       - We could use some kind of subtyping relationship here,
 370     ///         I imagine, but equality is easier and works fine for
 371     ///         our purposes.
 372     ///
 373     /// The key intuition here is that the user's types must be valid
 374     /// from "the inside" of the closure, but the expectation
 375     /// ultimately drives the overall signature.
 376     ///
 377     /// # Examples
 378     ///
 379     /// ```ignore (illustrative)
 380     /// fn with_closure<F>(_: F)
 381     ///   where F: Fn(&u32) -> &u32 { .. }
 382     ///
 383     /// with_closure(|x: &u32| { ... })
 384     /// ```
 385     ///
 386     /// Here:
 387     /// - E would be `fn(&u32) -> &u32`.
 388     /// - S would be `fn(&u32) ->
 389     /// - E' is `&'!0 u32 -> &'!0 u32`
 390     /// - S' is `&'?0 u32 -> ?T`
 391     ///
 392     /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
 393     ///
 394     /// # Arguments
 395     ///
 396     /// - `expr_def_id`: the `DefId` of the closure expression
 397     /// - `decl`: the HIR declaration of the closure
 398     /// - `body`: the body of the closure
 399     /// - `expected_sig`: the expected signature (if any). Note that
 400     ///   this is missing a binder: that is, there may be late-bound
 401     ///   regions with depth 1, which are bound then by the closure.
 402     #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
 403     fn sig_of_closure_with_expectation(
 404         &self,
 405         hir_id: hir::HirId,
 406         expr_def_id: DefId,
 407         decl: &hir::FnDecl<'_>,
 408         body: &hir::Body<'_>,
 409         expected_sig: ExpectedSig<'tcx>,
 410     ) -> ClosureSignatures<'tcx> {
 411         // Watch out for some surprises and just ignore the
 412         // expectation if things don't see to match up with what we
 413         // expect.
 414         if expected_sig.sig.c_variadic() != decl.c_variadic {
 415             return self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body);
 416         } else if expected_sig.sig.skip_binder().inputs_and_output.len() != decl.inputs.len() + 1 {
 417             return self.sig_of_closure_with_mismatched_number_of_arguments(
 418                 expr_def_id,
 419                 decl,
 420                 body,
 421                 expected_sig,
 422             );
 423         }
 424
 425         // Create a `PolyFnSig`. Note the oddity that late bound
 426         // regions appearing free in `expected_sig` are now bound up
 427         // in this binder we are creating.
 428         assert!(!expected_sig.sig.skip_binder().has_vars_bound_above(ty::INNERMOST));
 429         let bound_sig = expected_sig.sig.map_bound(|sig| {
 430             self.tcx.mk_fn_sig(
 431                 sig.inputs().iter().cloned(),
 432                 sig.output(),
 433                 sig.c_variadic,
 434                 hir::Unsafety::Normal,
 435                 Abi::RustCall,
 436             )
 437         });
 438
 439         // `deduce_expectations_from_expected_type` introduces
 440         // late-bound lifetimes defined elsewhere, which we now
 441         // anonymize away, so as not to confuse the user.
 442         let bound_sig = self.tcx.anonymize_late_bound_regions(bound_sig);
 443
 444         let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
 445
 446         // Up till this point, we have ignored the annotations that the user
 447         // gave. This function will check that they unify successfully.
 448         // Along the way, it also writes out entries for types that the user
 449         // wrote into our typeck results, which are then later used by the privacy
 450         // check.
 451         match self.merge_supplied_sig_with_expectation(
 452             hir_id,
 453             expr_def_id,
 454             decl,
 455             body,
 456             closure_sigs,
 457         ) {
 458             Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
 459             Err(_) => self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body),
 460         }
 461     }
 462
 463     fn sig_of_closure_with_mismatched_number_of_arguments(
 464         &self,
 465         expr_def_id: DefId,
 466         decl: &hir::FnDecl<'_>,
 467         body: &hir::Body<'_>,
 468         expected_sig: ExpectedSig<'tcx>,
 469     ) -> ClosureSignatures<'tcx> {
 470         let hir = self.tcx.hir();
 471         let expr_map_node = hir.get_if_local(expr_def_id).unwrap();
 472         let expected_args: Vec<_> = expected_sig
 473             .sig
 474             .skip_binder()
 475             .inputs()
 476             .iter()
 477             .map(|ty| ArgKind::from_expected_ty(*ty, None))
 478             .collect();
 479         let (closure_span, found_args) = match self.get_fn_like_arguments(expr_map_node) {
 480             Some((sp, args)) => (Some(sp), args),
 481             None => (None, Vec::new()),
 482         };
 483         let expected_span =
 484             expected_sig.cause_span.unwrap_or_else(|| hir.span_if_local(expr_def_id).unwrap());
 485         self.report_arg_count_mismatch(
 486             expected_span,
 487             closure_span,
 488             expected_args,
 489             found_args,
 490             true,
 491         )
 492         .emit();
 493
 494         let error_sig = self.error_sig_of_closure(decl);
 495
 496         self.closure_sigs(expr_def_id, body, error_sig)
 497     }
 498
 499     /// Enforce the user's types against the expectation. See
 500     /// `sig_of_closure_with_expectation` for details on the overall
 501     /// strategy.
 502     #[instrument(level = "debug", skip(self, hir_id, expr_def_id, decl, body, expected_sigs))]
 503     fn merge_supplied_sig_with_expectation(
 504         &self,
 505         hir_id: hir::HirId,
 506         expr_def_id: DefId,
 507         decl: &hir::FnDecl<'_>,
 508         body: &hir::Body<'_>,
 509         mut expected_sigs: ClosureSignatures<'tcx>,
 510     ) -> InferResult<'tcx, ClosureSignatures<'tcx>> {
 511         // Get the signature S that the user gave.
 512         //
 513         // (See comment on `sig_of_closure_with_expectation` for the
 514         // meaning of these letters.)
 515         let supplied_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 516
 517         debug!(?supplied_sig);
 518
 519         // FIXME(#45727): As discussed in [this comment][c1], naively
 520         // forcing equality here actually results in suboptimal error
 521         // messages in some cases.  For now, if there would have been
 522         // an obvious error, we fallback to declaring the type of the
 523         // closure to be the one the user gave, which allows other
 524         // error message code to trigger.
 525         //
 526         // However, I think [there is potential to do even better
 527         // here][c2], since in *this* code we have the precise span of
 528         // the type parameter in question in hand when we report the
 529         // error.
 530         //
 531         // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
 532         // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
 533         self.commit_if_ok(|_| {
 534             let mut all_obligations = vec![];
 535             let inputs: Vec<_> = iter::zip(
 536                 decl.inputs,
 537                 supplied_sig.inputs().skip_binder(), // binder moved to (*) below
 538             )
 539             .map(|(hir_ty, &supplied_ty)| {
 540                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 541                 self.replace_bound_vars_with_fresh_vars(
 542                     hir_ty.span,
 543                     LateBoundRegionConversionTime::FnCall,
 544                     // (*) binder moved to here
 545                     supplied_sig.inputs().rebind(supplied_ty),
 546                 )
 547             })
 548             .collect();
 549
 550             // The liberated version of this signature should be a subtype
 551             // of the liberated form of the expectation.
 552             for ((hir_ty, &supplied_ty), expected_ty) in iter::zip(
 553                 iter::zip(decl.inputs, &inputs),
 554                 expected_sigs.liberated_sig.inputs(), // `liberated_sig` is E'.
 555             ) {
 556                 // Check that E' = S'.
 557                 let cause = self.misc(hir_ty.span);
 558                 let InferOk { value: (), obligations } =
 559                     self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
 560                 all_obligations.extend(obligations);
 561             }
 562
 563             let supplied_output_ty = self.replace_bound_vars_with_fresh_vars(
 564                 decl.output.span(),
 565                 LateBoundRegionConversionTime::FnCall,
 566                 supplied_sig.output(),
 567             );
 568             let cause = &self.misc(decl.output.span());
 569             let InferOk { value: (), obligations } = self
 570                 .at(cause, self.param_env)
 571                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 572             all_obligations.extend(obligations);
 573
 574             let inputs = inputs.into_iter().map(|ty| self.resolve_vars_if_possible(ty));
 575
 576             expected_sigs.liberated_sig = self.tcx.mk_fn_sig(
 577                 inputs,
 578                 supplied_output_ty,
 579                 expected_sigs.liberated_sig.c_variadic,
 580                 hir::Unsafety::Normal,
 581                 Abi::RustCall,
 582             );
 583
 584             Ok(InferOk { value: expected_sigs, obligations: all_obligations })
 585         })
 586     }
 587
 588     /// If there is no expected signature, then we will convert the
 589     /// types that the user gave into a signature.
 590     ///
 591     /// Also, record this closure signature for later.
 592     #[instrument(skip(self, decl, body), level = "debug", ret)]
 593     fn supplied_sig_of_closure(
 594         &self,
 595         hir_id: hir::HirId,
 596         expr_def_id: DefId,
 597         decl: &hir::FnDecl<'_>,
 598         body: &hir::Body<'_>,
 599     ) -> ty::PolyFnSig<'tcx> {
 600         let astconv: &dyn AstConv<'_> = self;
 601
 602         trace!("decl = {:#?}", decl);
 603         debug!(?body.generator_kind);
 604
 605         let bound_vars = self.tcx.late_bound_vars(hir_id);
 606
 607         // First, convert the types that the user supplied (if any).
 608         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 609         let supplied_return = match decl.output {
 610             hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
 611             hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
 612                 // In the case of the async block that we create for a function body,
 613                 // we expect the return type of the block to match that of the enclosing
 614                 // function.
 615                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 616                     debug!("closure is async fn body");
 617                     self.deduce_future_output_from_obligations(expr_def_id, body.id().hir_id)
 618                         .unwrap_or_else(|| {
 619                             // AFAIK, deducing the future output
 620                             // always succeeds *except* in error cases
 621                             // like #65159. I'd like to return Error
 622                             // here, but I can't because I can't
 623                             // easily (and locally) prove that we
 624                             // *have* reported an
 625                             // error. --nikomatsakis
 626                             astconv.ty_infer(None, decl.output.span())
 627                         })
 628                 }
 629
 630                 _ => astconv.ty_infer(None, decl.output.span()),
 631             },
 632         };
 633
 634         let result = ty::Binder::bind_with_vars(
 635             self.tcx.mk_fn_sig(
 636                 supplied_arguments,
 637                 supplied_return,
 638                 decl.c_variadic,
 639                 hir::Unsafety::Normal,
 640                 Abi::RustCall,
 641             ),
 642             bound_vars,
 643         );
 644         // Astconv can't normalize inputs or outputs with escaping bound vars,
 645         // so normalize them here, after we've wrapped them in a binder.
 646         let result = self.normalize_associated_types_in(self.tcx.hir().span(hir_id), result);
 647
 648         let c_result = self.inh.infcx.canonicalize_response(result);
 649         self.typeck_results.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);
 650
 651         result
 652     }
 653
 654     /// Invoked when we are translating the generator that results
 655     /// from desugaring an `async fn`. Returns the "sugared" return
 656     /// type of the `async fn` -- that is, the return type that the
 657     /// user specified. The "desugared" return type is an `impl
 658     /// Future<Output = T>`, so we do this by searching through the
 659     /// obligations to extract the `T`.
 660     #[instrument(skip(self), level = "debug", ret)]
 661     fn deduce_future_output_from_obligations(
 662         &self,
 663         expr_def_id: DefId,
 664         body_id: hir::HirId,
 665     ) -> Option<Ty<'tcx>> {
 666         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
 667             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
 668         });
 669
 670         let ret_ty = ret_coercion.borrow().expected_ty();
 671         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 672
 673         let get_future_output = |predicate: ty::Predicate<'tcx>, span| {
 674             // Search for a pending obligation like
 675             //
 676             // `<R as Future>::Output = T`
 677             //
 678             // where R is the return type we are expecting. This type `T`
 679             // will be our output.
 680             let bound_predicate = predicate.kind();
 681             if let ty::PredicateKind::Projection(proj_predicate) = bound_predicate.skip_binder() {
 682                 self.deduce_future_output_from_projection(
 683                     span,
 684                     bound_predicate.rebind(proj_predicate),
 685                 )
 686             } else {
 687                 None
 688             }
 689         };
 690
 691         let output_ty = match *ret_ty.kind() {
 692             ty::Infer(ty::TyVar(ret_vid)) => {
 693                 self.obligations_for_self_ty(ret_vid).find_map(|(_, obligation)| {
 694                     get_future_output(obligation.predicate, obligation.cause.span)
 695                 })?
 696             }
 697             ty::Opaque(def_id, substs) => self
 698                 .tcx
 699                 .bound_explicit_item_bounds(def_id)
 700                 .transpose_iter()
 701                 .map(|e| e.map_bound(|e| *e).transpose_tuple2())
 702                 .find_map(|(p, s)| get_future_output(p.subst(self.tcx, substs), s.0))?,
 703             ty::Error(_) => return None,
 704             ty::Projection(proj)
 705                 if self.tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder =>
 706             {
 707                 self.tcx
 708                     .bound_explicit_item_bounds(proj.item_def_id)
 709                     .transpose_iter()
 710                     .map(|e| e.map_bound(|e| *e).transpose_tuple2())
 711                     .find_map(|(p, s)| get_future_output(p.subst(self.tcx, proj.substs), s.0))?
 712             }
 713             _ => span_bug!(
 714                 self.tcx.def_span(expr_def_id),
 715                 "async fn generator return type not an inference variable: {ret_ty}"
 716             ),
 717         };
 718
 719         // async fn that have opaque types in their return type need to redo the conversion to inference variables
 720         // as they fetch the still opaque version from the signature.
 721         let InferOk { value: output_ty, obligations } = self
 722             .replace_opaque_types_with_inference_vars(
 723                 output_ty,
 724                 body_id,
 725                 self.tcx.def_span(expr_def_id),
 726                 self.param_env,
 727             );
 728         self.register_predicates(obligations);
 729
 730         Some(output_ty)
 731     }
 732
 733     /// Given a projection like
 734     ///
 735     /// `<X as Future>::Output = T`
 736     ///
 737     /// where `X` is some type that has no late-bound regions, returns
 738     /// `Some(T)`. If the projection is for some other trait, returns
 739     /// `None`.
 740     fn deduce_future_output_from_projection(
 741         &self,
 742         cause_span: Span,
 743         predicate: ty::PolyProjectionPredicate<'tcx>,
 744     ) -> Option<Ty<'tcx>> {
 745         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 746
 747         // We do not expect any bound regions in our predicate, so
 748         // skip past the bound vars.
 749         let Some(predicate) = predicate.no_bound_vars() else {
 750             debug!("deduce_future_output_from_projection: has late-bound regions");
 751             return None;
 752         };
 753
 754         // Check that this is a projection from the `Future` trait.
 755         let trait_def_id = predicate.projection_ty.trait_def_id(self.tcx);
 756         let future_trait = self.tcx.require_lang_item(LangItem::Future, Some(cause_span));
 757         if trait_def_id != future_trait {
 758             debug!("deduce_future_output_from_projection: not a future");
 759             return None;
 760         }
 761
 762         // The `Future` trait has only one associated item, `Output`,
 763         // so check that this is what we see.
 764         let output_assoc_item = self.tcx.associated_item_def_ids(future_trait)[0];
 765         if output_assoc_item != predicate.projection_ty.item_def_id {
 766             span_bug!(
 767                 cause_span,
 768                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 769                 predicate.projection_ty.item_def_id,
 770                 output_assoc_item,
 771             );
 772         }
 773
 774         // Extract the type from the projection. Note that there can
 775         // be no bound variables in this type because the "self type"
 776         // does not have any regions in it.
 777         let output_ty = self.resolve_vars_if_possible(predicate.term);
 778         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 779         // This is a projection on a Fn trait so will always be a type.
 780         Some(output_ty.ty().unwrap())
 781     }
 782
 783     /// Converts the types that the user supplied, in case that doing
 784     /// so should yield an error, but returns back a signature where
 785     /// all parameters are of type `TyErr`.
 786     fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
 787         let astconv: &dyn AstConv<'_> = self;
 788
 789         let supplied_arguments = decl.inputs.iter().map(|a| {
 790             // Convert the types that the user supplied (if any), but ignore them.
 791             astconv.ast_ty_to_ty(a);
 792             self.tcx.ty_error()
 793         });
 794
 795         if let hir::FnRetTy::Return(ref output) = decl.output {
 796             astconv.ast_ty_to_ty(&output);
 797         }
 798
 799         let result = ty::Binder::dummy(self.tcx.mk_fn_sig(
 800             supplied_arguments,
 801             self.tcx.ty_error(),
 802             decl.c_variadic,
 803             hir::Unsafety::Normal,
 804             Abi::RustCall,
 805         ));
 806
 807         debug!("supplied_sig_of_closure: result={:?}", result);
 808
 809         result
 810     }
 811
 812     fn closure_sigs(
 813         &self,
 814         expr_def_id: DefId,
 815         body: &hir::Body<'_>,
 816         bound_sig: ty::PolyFnSig<'tcx>,
 817     ) -> ClosureSignatures<'tcx> {
 818         let liberated_sig = self.tcx().liberate_late_bound_regions(expr_def_id, bound_sig);
 819         let liberated_sig = self.inh.normalize_associated_types_in(
 820             body.value.span,
 821             body.value.hir_id,
 822             self.param_env,
 823             liberated_sig,
 824         );
 825         ClosureSignatures { bound_sig, liberated_sig }
 826     }
 827 }