compiler/rustc_hir_typeck/src/closure.rs

   1 //! Code for type-checking closure expressions.
   2
   3 use super::{check_fn, Expectation, FnCtxt, GeneratorTypes};
   4
   5 use hir::def::DefKind;
   6 use rustc_hir as hir;
   7 use rustc_hir::def_id::DefId;
   8 use rustc_hir::lang_items::LangItem;
   9 use rustc_hir_analysis::astconv::AstConv;
  10 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  11 use rustc_infer::infer::LateBoundRegionConversionTime;
  12 use rustc_infer::infer::{InferOk, InferResult};
  13 use rustc_middle::ty::subst::InternalSubsts;
  14 use rustc_middle::ty::visit::TypeVisitable;
  15 use rustc_middle::ty::{self, Ty};
  16 use rustc_span::source_map::Span;
  17 use rustc_target::spec::abi::Abi;
  18 use rustc_trait_selection::traits;
  19 use rustc_trait_selection::traits::error_reporting::ArgKind;
  20 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
  21 use std::cmp;
  22 use std::iter;
  23
  24 /// What signature do we *expect* the closure to have from context?
  25 #[derive(Debug)]
  26 struct ExpectedSig<'tcx> {
  27     /// Span that gave us this expectation, if we know that.
  28     cause_span: Option<Span>,
  29     sig: ty::PolyFnSig<'tcx>,
  30 }
  31
  32 struct ClosureSignatures<'tcx> {
  33     /// The signature users of the closure see.
  34     bound_sig: ty::PolyFnSig<'tcx>,
  35     /// The signature within the function body.
  36     /// This mostly differs in the sense that lifetimes are now early bound and any
  37     /// opaque types from the signature expectation are overriden in case there are
  38     /// explicit hidden types written by the user in the closure signature.
  39     liberated_sig: ty::FnSig<'tcx>,
  40 }
  41
  42 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  43     #[instrument(skip(self, expr, _capture, decl, body_id), level = "debug")]
  44     pub fn check_expr_closure(
  45         &self,
  46         expr: &hir::Expr<'_>,
  47         _capture: hir::CaptureBy,
  48         decl: &'tcx hir::FnDecl<'tcx>,
  49         body_id: hir::BodyId,
  50         gen: Option<hir::Movability>,
  51         expected: Expectation<'tcx>,
  52     ) -> Ty<'tcx> {
  53         trace!("decl = {:#?}", decl);
  54         trace!("expr = {:#?}", expr);
  55
  56         // It's always helpful for inference if we know the kind of
  57         // closure sooner rather than later, so first examine the expected
  58         // type, and see if can glean a closure kind from there.
  59         let (expected_sig, expected_kind) = match expected.to_option(self) {
  60             Some(ty) => self.deduce_expectations_from_expected_type(ty),
  61             None => (None, None),
  62         };
  63         let body = self.tcx.hir().body(body_id);
  64         self.check_closure(expr, expected_kind, decl, body, gen, expected_sig)
  65     }
  66
  67     #[instrument(skip(self, expr, body, decl), level = "debug", ret)]
  68     fn check_closure(
  69         &self,
  70         expr: &hir::Expr<'_>,
  71         opt_kind: Option<ty::ClosureKind>,
  72         decl: &'tcx hir::FnDecl<'tcx>,
  73         body: &'tcx hir::Body<'tcx>,
  74         gen: Option<hir::Movability>,
  75         expected_sig: Option<ExpectedSig<'tcx>>,
  76     ) -> Ty<'tcx> {
  77         trace!("decl = {:#?}", decl);
  78         let expr_def_id = self.tcx.hir().local_def_id(expr.hir_id);
  79         debug!(?expr_def_id);
  80
  81         let ClosureSignatures { bound_sig, liberated_sig } =
  82             self.sig_of_closure(expr.hir_id, expr_def_id.to_def_id(), decl, body, expected_sig);
  83
  84         debug!(?bound_sig, ?liberated_sig);
  85
  86         let return_type_pre_known = !liberated_sig.output().is_ty_infer();
  87
  88         let generator_types = check_fn(
  89             self,
  90             self.param_env.without_const(),
  91             liberated_sig,
  92             decl,
  93             expr.hir_id,
  94             body,
  95             gen,
  96             return_type_pre_known,
  97         )
  98         .1;
  99
 100         let parent_substs = InternalSubsts::identity_for_item(
 101             self.tcx,
 102             self.tcx.typeck_root_def_id(expr_def_id.to_def_id()),
 103         );
 104
 105         let tupled_upvars_ty = self.next_ty_var(TypeVariableOrigin {
 106             kind: TypeVariableOriginKind::ClosureSynthetic,
 107             span: self.tcx.hir().span(expr.hir_id),
 108         });
 109
 110         if let Some(GeneratorTypes { resume_ty, yield_ty, interior, movability }) = generator_types
 111         {
 112             let generator_substs = ty::GeneratorSubsts::new(
 113                 self.tcx,
 114                 ty::GeneratorSubstsParts {
 115                     parent_substs,
 116                     resume_ty,
 117                     yield_ty,
 118                     return_ty: liberated_sig.output(),
 119                     witness: interior,
 120                     tupled_upvars_ty,
 121                 },
 122             );
 123
 124             return self.tcx.mk_generator(
 125                 expr_def_id.to_def_id(),
 126                 generator_substs.substs,
 127                 movability,
 128             );
 129         }
 130
 131         // Tuple up the arguments and insert the resulting function type into
 132         // the `closures` table.
 133         let sig = bound_sig.map_bound(|sig| {
 134             self.tcx.mk_fn_sig(
 135                 iter::once(self.tcx.intern_tup(sig.inputs())),
 136                 sig.output(),
 137                 sig.c_variadic,
 138                 sig.unsafety,
 139                 sig.abi,
 140             )
 141         });
 142
 143         debug!(?sig, ?opt_kind);
 144
 145         let closure_kind_ty = match opt_kind {
 146             Some(kind) => kind.to_ty(self.tcx),
 147
 148             // Create a type variable (for now) to represent the closure kind.
 149             // It will be unified during the upvar inference phase (`upvar.rs`)
 150             None => self.next_ty_var(TypeVariableOrigin {
 151                 // FIXME(eddyb) distinguish closure kind inference variables from the rest.
 152                 kind: TypeVariableOriginKind::ClosureSynthetic,
 153                 span: expr.span,
 154             }),
 155         };
 156
 157         let closure_substs = ty::ClosureSubsts::new(
 158             self.tcx,
 159             ty::ClosureSubstsParts {
 160                 parent_substs,
 161                 closure_kind_ty,
 162                 closure_sig_as_fn_ptr_ty: self.tcx.mk_fn_ptr(sig),
 163                 tupled_upvars_ty,
 164             },
 165         );
 166
 167         self.tcx.mk_closure(expr_def_id.to_def_id(), closure_substs.substs)
 168     }
 169
 170     /// Given the expected type, figures out what it can about this closure we
 171     /// are about to type check:
 172     #[instrument(skip(self), level = "debug")]
 173     fn deduce_expectations_from_expected_type(
 174         &self,
 175         expected_ty: Ty<'tcx>,
 176     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 177         match *expected_ty.kind() {
 178             ty::Opaque(def_id, substs) => {
 179                 let bounds = self.tcx.bound_explicit_item_bounds(def_id);
 180                 let sig =
 181                     bounds.subst_iter_copied(self.tcx, substs).find_map(|(pred, span)| match pred
 182                         .kind()
 183                         .skip_binder()
 184                     {
 185                         ty::PredicateKind::Projection(proj_predicate) => self
 186                             .deduce_sig_from_projection(
 187                                 Some(span),
 188                                 pred.kind().rebind(proj_predicate),
 189                             ),
 190                         _ => None,
 191                     });
 192
 193                 let kind = bounds
 194                     .0
 195                     .iter()
 196                     .filter_map(|(pred, _)| match pred.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 mut expected_sig = None;
 230         let mut expected_kind = None;
 231
 232         for obligation in traits::elaborate_obligations(
 233             self.tcx,
 234             // Reverse the obligations here, since `elaborate_*` uses a stack,
 235             // and we want to keep inference generally in the same order of
 236             // the registered obligations.
 237             self.obligations_for_self_ty(expected_vid).rev().collect(),
 238         ) {
 239             debug!(?obligation.predicate);
 240             let bound_predicate = obligation.predicate.kind();
 241
 242             // Given a Projection predicate, we can potentially infer
 243             // the complete signature.
 244             if expected_sig.is_none()
 245                 && let ty::PredicateKind::Projection(proj_predicate) = bound_predicate.skip_binder()
 246             {
 247                 expected_sig = self.deduce_sig_from_projection(
 248                     Some(obligation.cause.span),
 249                     bound_predicate.rebind(proj_predicate),
 250                 );
 251             }
 252
 253             // Even if we can't infer the full signature, we may be able to
 254             // infer the kind. This can occur when we elaborate a predicate
 255             // like `F : Fn<A>`. Note that due to subtyping we could encounter
 256             // many viable options, so pick the most restrictive.
 257             let trait_def_id = match bound_predicate.skip_binder() {
 258                 ty::PredicateKind::Projection(data) => {
 259                     Some(data.projection_ty.trait_def_id(self.tcx))
 260                 }
 261                 ty::PredicateKind::Trait(data) => Some(data.def_id()),
 262                 _ => None,
 263             };
 264             if let Some(closure_kind) =
 265                 trait_def_id.and_then(|def_id| self.tcx.fn_trait_kind_from_lang_item(def_id))
 266             {
 267                 expected_kind = Some(
 268                     expected_kind
 269                         .map_or_else(|| closure_kind, |current| cmp::min(current, closure_kind)),
 270                 );
 271             }
 272         }
 273
 274         (expected_sig, expected_kind)
 275     }
 276
 277     /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
 278     /// everything we need to know about a closure or generator.
 279     ///
 280     /// The `cause_span` should be the span that caused us to
 281     /// have this expected signature, or `None` if we can't readily
 282     /// know that.
 283     #[instrument(level = "debug", skip(self, cause_span), ret)]
 284     fn deduce_sig_from_projection(
 285         &self,
 286         cause_span: Option<Span>,
 287         projection: ty::PolyProjectionPredicate<'tcx>,
 288     ) -> Option<ExpectedSig<'tcx>> {
 289         let tcx = self.tcx;
 290
 291         let trait_def_id = projection.trait_def_id(tcx);
 292
 293         let is_fn = tcx.fn_trait_kind_from_lang_item(trait_def_id).is_some();
 294         let gen_trait = tcx.require_lang_item(LangItem::Generator, cause_span);
 295         let is_gen = gen_trait == trait_def_id;
 296         if !is_fn && !is_gen {
 297             debug!("not fn or generator");
 298             return None;
 299         }
 300
 301         if is_gen {
 302             // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
 303             // associated item and not yield.
 304             let return_assoc_item = self.tcx.associated_item_def_ids(gen_trait)[1];
 305             if return_assoc_item != projection.projection_def_id() {
 306                 debug!("not return assoc item of generator");
 307                 return None;
 308             }
 309         }
 310
 311         let input_tys = if is_fn {
 312             let arg_param_ty = projection.skip_binder().projection_ty.substs.type_at(1);
 313             let arg_param_ty = self.resolve_vars_if_possible(arg_param_ty);
 314             debug!(?arg_param_ty);
 315
 316             match arg_param_ty.kind() {
 317                 &ty::Tuple(tys) => tys,
 318                 _ => return None,
 319             }
 320         } else {
 321             // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
 322             // else they must have exactly 1 argument. For now though, just give up in this case.
 323             return None;
 324         };
 325
 326         // Since this is a return parameter type it is safe to unwrap.
 327         let ret_param_ty = projection.skip_binder().term.ty().unwrap();
 328         let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
 329         debug!(?ret_param_ty);
 330
 331         let sig = projection.rebind(self.tcx.mk_fn_sig(
 332             input_tys.iter(),
 333             ret_param_ty,
 334             false,
 335             hir::Unsafety::Normal,
 336             Abi::Rust,
 337         ));
 338
 339         Some(ExpectedSig { cause_span, sig })
 340     }
 341
 342     fn sig_of_closure(
 343         &self,
 344         hir_id: hir::HirId,
 345         expr_def_id: DefId,
 346         decl: &hir::FnDecl<'_>,
 347         body: &hir::Body<'_>,
 348         expected_sig: Option<ExpectedSig<'tcx>>,
 349     ) -> ClosureSignatures<'tcx> {
 350         if let Some(e) = expected_sig {
 351             self.sig_of_closure_with_expectation(hir_id, expr_def_id, decl, body, e)
 352         } else {
 353             self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body)
 354         }
 355     }
 356
 357     /// If there is no expected signature, then we will convert the
 358     /// types that the user gave into a signature.
 359     #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
 360     fn sig_of_closure_no_expectation(
 361         &self,
 362         hir_id: hir::HirId,
 363         expr_def_id: DefId,
 364         decl: &hir::FnDecl<'_>,
 365         body: &hir::Body<'_>,
 366     ) -> ClosureSignatures<'tcx> {
 367         let bound_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 368
 369         self.closure_sigs(expr_def_id, body, bound_sig)
 370     }
 371
 372     /// Invoked to compute the signature of a closure expression. This
 373     /// combines any user-provided type annotations (e.g., `|x: u32|
 374     /// -> u32 { .. }`) with the expected signature.
 375     ///
 376     /// The approach is as follows:
 377     ///
 378     /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
 379     /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
 380     ///   - If we have no expectation `E`, then the signature of the closure is `S`.
 381     ///   - Otherwise, the signature of the closure is E. Moreover:
 382     ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
 383     ///     - Instantiate all the late-bound regions bound in the closure within `S`
 384     ///       with fresh (existential) variables, yielding `S'`
 385     ///     - Require that `E' = S'`
 386     ///       - We could use some kind of subtyping relationship here,
 387     ///         I imagine, but equality is easier and works fine for
 388     ///         our purposes.
 389     ///
 390     /// The key intuition here is that the user's types must be valid
 391     /// from "the inside" of the closure, but the expectation
 392     /// ultimately drives the overall signature.
 393     ///
 394     /// # Examples
 395     ///
 396     /// ```ignore (illustrative)
 397     /// fn with_closure<F>(_: F)
 398     ///   where F: Fn(&u32) -> &u32 { .. }
 399     ///
 400     /// with_closure(|x: &u32| { ... })
 401     /// ```
 402     ///
 403     /// Here:
 404     /// - E would be `fn(&u32) -> &u32`.
 405     /// - S would be `fn(&u32) ->
 406     /// - E' is `&'!0 u32 -> &'!0 u32`
 407     /// - S' is `&'?0 u32 -> ?T`
 408     ///
 409     /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
 410     ///
 411     /// # Arguments
 412     ///
 413     /// - `expr_def_id`: the `DefId` of the closure expression
 414     /// - `decl`: the HIR declaration of the closure
 415     /// - `body`: the body of the closure
 416     /// - `expected_sig`: the expected signature (if any). Note that
 417     ///   this is missing a binder: that is, there may be late-bound
 418     ///   regions with depth 1, which are bound then by the closure.
 419     #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
 420     fn sig_of_closure_with_expectation(
 421         &self,
 422         hir_id: hir::HirId,
 423         expr_def_id: DefId,
 424         decl: &hir::FnDecl<'_>,
 425         body: &hir::Body<'_>,
 426         expected_sig: ExpectedSig<'tcx>,
 427     ) -> ClosureSignatures<'tcx> {
 428         // Watch out for some surprises and just ignore the
 429         // expectation if things don't see to match up with what we
 430         // expect.
 431         if expected_sig.sig.c_variadic() != decl.c_variadic {
 432             return self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body);
 433         } else if expected_sig.sig.skip_binder().inputs_and_output.len() != decl.inputs.len() + 1 {
 434             return self.sig_of_closure_with_mismatched_number_of_arguments(
 435                 expr_def_id,
 436                 decl,
 437                 body,
 438                 expected_sig,
 439             );
 440         }
 441
 442         // Create a `PolyFnSig`. Note the oddity that late bound
 443         // regions appearing free in `expected_sig` are now bound up
 444         // in this binder we are creating.
 445         assert!(!expected_sig.sig.skip_binder().has_vars_bound_above(ty::INNERMOST));
 446         let bound_sig = expected_sig.sig.map_bound(|sig| {
 447             self.tcx.mk_fn_sig(
 448                 sig.inputs().iter().cloned(),
 449                 sig.output(),
 450                 sig.c_variadic,
 451                 hir::Unsafety::Normal,
 452                 Abi::RustCall,
 453             )
 454         });
 455
 456         // `deduce_expectations_from_expected_type` introduces
 457         // late-bound lifetimes defined elsewhere, which we now
 458         // anonymize away, so as not to confuse the user.
 459         let bound_sig = self.tcx.anonymize_late_bound_regions(bound_sig);
 460
 461         let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
 462
 463         // Up till this point, we have ignored the annotations that the user
 464         // gave. This function will check that they unify successfully.
 465         // Along the way, it also writes out entries for types that the user
 466         // wrote into our typeck results, which are then later used by the privacy
 467         // check.
 468         match self.merge_supplied_sig_with_expectation(
 469             hir_id,
 470             expr_def_id,
 471             decl,
 472             body,
 473             closure_sigs,
 474         ) {
 475             Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
 476             Err(_) => self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body),
 477         }
 478     }
 479
 480     fn sig_of_closure_with_mismatched_number_of_arguments(
 481         &self,
 482         expr_def_id: DefId,
 483         decl: &hir::FnDecl<'_>,
 484         body: &hir::Body<'_>,
 485         expected_sig: ExpectedSig<'tcx>,
 486     ) -> ClosureSignatures<'tcx> {
 487         let hir = self.tcx.hir();
 488         let expr_map_node = hir.get_if_local(expr_def_id).unwrap();
 489         let expected_args: Vec<_> = expected_sig
 490             .sig
 491             .skip_binder()
 492             .inputs()
 493             .iter()
 494             .map(|ty| ArgKind::from_expected_ty(*ty, None))
 495             .collect();
 496         let (closure_span, found_args) = match self.get_fn_like_arguments(expr_map_node) {
 497             Some((sp, args)) => (Some(sp), args),
 498             None => (None, Vec::new()),
 499         };
 500         let expected_span =
 501             expected_sig.cause_span.unwrap_or_else(|| hir.span_if_local(expr_def_id).unwrap());
 502         self.report_arg_count_mismatch(
 503             expected_span,
 504             closure_span,
 505             expected_args,
 506             found_args,
 507             true,
 508         )
 509         .emit();
 510
 511         let error_sig = self.error_sig_of_closure(decl);
 512
 513         self.closure_sigs(expr_def_id, body, error_sig)
 514     }
 515
 516     /// Enforce the user's types against the expectation. See
 517     /// `sig_of_closure_with_expectation` for details on the overall
 518     /// strategy.
 519     #[instrument(level = "debug", skip(self, hir_id, expr_def_id, decl, body, expected_sigs))]
 520     fn merge_supplied_sig_with_expectation(
 521         &self,
 522         hir_id: hir::HirId,
 523         expr_def_id: DefId,
 524         decl: &hir::FnDecl<'_>,
 525         body: &hir::Body<'_>,
 526         mut expected_sigs: ClosureSignatures<'tcx>,
 527     ) -> InferResult<'tcx, ClosureSignatures<'tcx>> {
 528         // Get the signature S that the user gave.
 529         //
 530         // (See comment on `sig_of_closure_with_expectation` for the
 531         // meaning of these letters.)
 532         let supplied_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 533
 534         debug!(?supplied_sig);
 535
 536         // FIXME(#45727): As discussed in [this comment][c1], naively
 537         // forcing equality here actually results in suboptimal error
 538         // messages in some cases.  For now, if there would have been
 539         // an obvious error, we fallback to declaring the type of the
 540         // closure to be the one the user gave, which allows other
 541         // error message code to trigger.
 542         //
 543         // However, I think [there is potential to do even better
 544         // here][c2], since in *this* code we have the precise span of
 545         // the type parameter in question in hand when we report the
 546         // error.
 547         //
 548         // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
 549         // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
 550         self.commit_if_ok(|_| {
 551             let mut all_obligations = vec![];
 552             let inputs: Vec<_> = iter::zip(
 553                 decl.inputs,
 554                 supplied_sig.inputs().skip_binder(), // binder moved to (*) below
 555             )
 556             .map(|(hir_ty, &supplied_ty)| {
 557                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 558                 self.replace_bound_vars_with_fresh_vars(
 559                     hir_ty.span,
 560                     LateBoundRegionConversionTime::FnCall,
 561                     // (*) binder moved to here
 562                     supplied_sig.inputs().rebind(supplied_ty),
 563                 )
 564             })
 565             .collect();
 566
 567             // The liberated version of this signature should be a subtype
 568             // of the liberated form of the expectation.
 569             for ((hir_ty, &supplied_ty), expected_ty) in iter::zip(
 570                 iter::zip(decl.inputs, &inputs),
 571                 expected_sigs.liberated_sig.inputs(), // `liberated_sig` is E'.
 572             ) {
 573                 // Check that E' = S'.
 574                 let cause = self.misc(hir_ty.span);
 575                 let InferOk { value: (), obligations } =
 576                     self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
 577                 all_obligations.extend(obligations);
 578             }
 579
 580             let supplied_output_ty = self.replace_bound_vars_with_fresh_vars(
 581                 decl.output.span(),
 582                 LateBoundRegionConversionTime::FnCall,
 583                 supplied_sig.output(),
 584             );
 585             let cause = &self.misc(decl.output.span());
 586             let InferOk { value: (), obligations } = self
 587                 .at(cause, self.param_env)
 588                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 589             all_obligations.extend(obligations);
 590
 591             let inputs = inputs.into_iter().map(|ty| self.resolve_vars_if_possible(ty));
 592
 593             expected_sigs.liberated_sig = self.tcx.mk_fn_sig(
 594                 inputs,
 595                 supplied_output_ty,
 596                 expected_sigs.liberated_sig.c_variadic,
 597                 hir::Unsafety::Normal,
 598                 Abi::RustCall,
 599             );
 600
 601             Ok(InferOk { value: expected_sigs, obligations: all_obligations })
 602         })
 603     }
 604
 605     /// If there is no expected signature, then we will convert the
 606     /// types that the user gave into a signature.
 607     ///
 608     /// Also, record this closure signature for later.
 609     #[instrument(skip(self, decl, body), level = "debug", ret)]
 610     fn supplied_sig_of_closure(
 611         &self,
 612         hir_id: hir::HirId,
 613         expr_def_id: DefId,
 614         decl: &hir::FnDecl<'_>,
 615         body: &hir::Body<'_>,
 616     ) -> ty::PolyFnSig<'tcx> {
 617         let astconv: &dyn AstConv<'_> = self;
 618
 619         trace!("decl = {:#?}", decl);
 620         debug!(?body.generator_kind);
 621
 622         let bound_vars = self.tcx.late_bound_vars(hir_id);
 623
 624         // First, convert the types that the user supplied (if any).
 625         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 626         let supplied_return = match decl.output {
 627             hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
 628             hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
 629                 // In the case of the async block that we create for a function body,
 630                 // we expect the return type of the block to match that of the enclosing
 631                 // function.
 632                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 633                     debug!("closure is async fn body");
 634                     self.deduce_future_output_from_obligations(expr_def_id, body.id().hir_id)
 635                         .unwrap_or_else(|| {
 636                             // AFAIK, deducing the future output
 637                             // always succeeds *except* in error cases
 638                             // like #65159. I'd like to return Error
 639                             // here, but I can't because I can't
 640                             // easily (and locally) prove that we
 641                             // *have* reported an
 642                             // error. --nikomatsakis
 643                             astconv.ty_infer(None, decl.output.span())
 644                         })
 645                 }
 646
 647                 _ => astconv.ty_infer(None, decl.output.span()),
 648             },
 649         };
 650
 651         let result = ty::Binder::bind_with_vars(
 652             self.tcx.mk_fn_sig(
 653                 supplied_arguments,
 654                 supplied_return,
 655                 decl.c_variadic,
 656                 hir::Unsafety::Normal,
 657                 Abi::RustCall,
 658             ),
 659             bound_vars,
 660         );
 661         // Astconv can't normalize inputs or outputs with escaping bound vars,
 662         // so normalize them here, after we've wrapped them in a binder.
 663         let result = self.normalize_associated_types_in(self.tcx.hir().span(hir_id), result);
 664
 665         let c_result = self.inh.infcx.canonicalize_response(result);
 666         self.typeck_results.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);
 667
 668         result
 669     }
 670
 671     /// Invoked when we are translating the generator that results
 672     /// from desugaring an `async fn`. Returns the "sugared" return
 673     /// type of the `async fn` -- that is, the return type that the
 674     /// user specified. The "desugared" return type is an `impl
 675     /// Future<Output = T>`, so we do this by searching through the
 676     /// obligations to extract the `T`.
 677     #[instrument(skip(self), level = "debug", ret)]
 678     fn deduce_future_output_from_obligations(
 679         &self,
 680         expr_def_id: DefId,
 681         body_id: hir::HirId,
 682     ) -> Option<Ty<'tcx>> {
 683         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
 684             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
 685         });
 686
 687         let ret_ty = ret_coercion.borrow().expected_ty();
 688         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 689
 690         let get_future_output = |predicate: ty::Predicate<'tcx>, span| {
 691             // Search for a pending obligation like
 692             //
 693             // `<R as Future>::Output = T`
 694             //
 695             // where R is the return type we are expecting. This type `T`
 696             // will be our output.
 697             let bound_predicate = predicate.kind();
 698             if let ty::PredicateKind::Projection(proj_predicate) = bound_predicate.skip_binder() {
 699                 self.deduce_future_output_from_projection(
 700                     span,
 701                     bound_predicate.rebind(proj_predicate),
 702                 )
 703             } else {
 704                 None
 705             }
 706         };
 707
 708         let output_ty = match *ret_ty.kind() {
 709             ty::Infer(ty::TyVar(ret_vid)) => {
 710                 self.obligations_for_self_ty(ret_vid).find_map(|obligation| {
 711                     get_future_output(obligation.predicate, obligation.cause.span)
 712                 })?
 713             }
 714             ty::Opaque(def_id, substs) => self
 715                 .tcx
 716                 .bound_explicit_item_bounds(def_id)
 717                 .subst_iter_copied(self.tcx, substs)
 718                 .find_map(|(p, s)| get_future_output(p, s))?,
 719             ty::Error(_) => return None,
 720             ty::Projection(proj)
 721                 if self.tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder =>
 722             {
 723                 self.tcx
 724                     .bound_explicit_item_bounds(proj.item_def_id)
 725                     .subst_iter_copied(self.tcx, proj.substs)
 726                     .find_map(|(p, s)| get_future_output(p, s))?
 727             }
 728             _ => span_bug!(
 729                 self.tcx.def_span(expr_def_id),
 730                 "async fn generator return type not an inference variable: {ret_ty}"
 731             ),
 732         };
 733
 734         // async fn that have opaque types in their return type need to redo the conversion to inference variables
 735         // as they fetch the still opaque version from the signature.
 736         let InferOk { value: output_ty, obligations } = self
 737             .replace_opaque_types_with_inference_vars(
 738                 output_ty,
 739                 body_id,
 740                 self.tcx.def_span(expr_def_id),
 741                 self.param_env,
 742             );
 743         self.register_predicates(obligations);
 744
 745         Some(output_ty)
 746     }
 747
 748     /// Given a projection like
 749     ///
 750     /// `<X as Future>::Output = T`
 751     ///
 752     /// where `X` is some type that has no late-bound regions, returns
 753     /// `Some(T)`. If the projection is for some other trait, returns
 754     /// `None`.
 755     fn deduce_future_output_from_projection(
 756         &self,
 757         cause_span: Span,
 758         predicate: ty::PolyProjectionPredicate<'tcx>,
 759     ) -> Option<Ty<'tcx>> {
 760         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 761
 762         // We do not expect any bound regions in our predicate, so
 763         // skip past the bound vars.
 764         let Some(predicate) = predicate.no_bound_vars() else {
 765             debug!("deduce_future_output_from_projection: has late-bound regions");
 766             return None;
 767         };
 768
 769         // Check that this is a projection from the `Future` trait.
 770         let trait_def_id = predicate.projection_ty.trait_def_id(self.tcx);
 771         let future_trait = self.tcx.require_lang_item(LangItem::Future, Some(cause_span));
 772         if trait_def_id != future_trait {
 773             debug!("deduce_future_output_from_projection: not a future");
 774             return None;
 775         }
 776
 777         // The `Future` trait has only one associated item, `Output`,
 778         // so check that this is what we see.
 779         let output_assoc_item = self.tcx.associated_item_def_ids(future_trait)[0];
 780         if output_assoc_item != predicate.projection_ty.item_def_id {
 781             span_bug!(
 782                 cause_span,
 783                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 784                 predicate.projection_ty.item_def_id,
 785                 output_assoc_item,
 786             );
 787         }
 788
 789         // Extract the type from the projection. Note that there can
 790         // be no bound variables in this type because the "self type"
 791         // does not have any regions in it.
 792         let output_ty = self.resolve_vars_if_possible(predicate.term);
 793         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 794         // This is a projection on a Fn trait so will always be a type.
 795         Some(output_ty.ty().unwrap())
 796     }
 797
 798     /// Converts the types that the user supplied, in case that doing
 799     /// so should yield an error, but returns back a signature where
 800     /// all parameters are of type `TyErr`.
 801     fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
 802         let astconv: &dyn AstConv<'_> = self;
 803
 804         let supplied_arguments = decl.inputs.iter().map(|a| {
 805             // Convert the types that the user supplied (if any), but ignore them.
 806             astconv.ast_ty_to_ty(a);
 807             self.tcx.ty_error()
 808         });
 809
 810         if let hir::FnRetTy::Return(ref output) = decl.output {
 811             astconv.ast_ty_to_ty(&output);
 812         }
 813
 814         let result = ty::Binder::dummy(self.tcx.mk_fn_sig(
 815             supplied_arguments,
 816             self.tcx.ty_error(),
 817             decl.c_variadic,
 818             hir::Unsafety::Normal,
 819             Abi::RustCall,
 820         ));
 821
 822         debug!("supplied_sig_of_closure: result={:?}", result);
 823
 824         result
 825     }
 826
 827     fn closure_sigs(
 828         &self,
 829         expr_def_id: DefId,
 830         body: &hir::Body<'_>,
 831         bound_sig: ty::PolyFnSig<'tcx>,
 832     ) -> ClosureSignatures<'tcx> {
 833         let liberated_sig = self.tcx().liberate_late_bound_regions(expr_def_id, bound_sig);
 834         let liberated_sig = self.inh.normalize_associated_types_in(
 835             body.value.span,
 836             body.value.hir_id,
 837             self.param_env,
 838             liberated_sig,
 839         );
 840         ClosureSignatures { bound_sig, liberated_sig }
 841     }
 842 }