compiler/rustc_typeck/src/check/closure.rs

   1 //! Code for type-checking closure expressions.
   2
   3 use super::{check_fn, Expectation, FnCtxt, GeneratorTypes};
   4
   5 use crate::astconv::AstConv;
   6 use crate::rustc_middle::ty::subst::Subst;
   7 use hir::def::DefKind;
   8 use rustc_hir as hir;
   9 use rustc_hir::def_id::DefId;
  10 use rustc_hir::lang_items::LangItem;
  11 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  12 use rustc_infer::infer::LateBoundRegionConversionTime;
  13 use rustc_infer::infer::{InferOk, InferResult};
  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::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 = bounds
 181                     .transpose_iter()
 182                     .map(|e| e.map_bound(|e| *e).transpose_tuple2())
 183                     .find_map(|(pred, span)| match pred.0.kind().skip_binder() {
 184                         ty::PredicateKind::Projection(proj_predicate) => self
 185                             .deduce_sig_from_projection(
 186                                 Some(span.0),
 187                                 pred.0
 188                                     .kind()
 189                                     .rebind(pred.rebind(proj_predicate).subst(self.tcx, substs)),
 190                             ),
 191                         _ => None,
 192                     });
 193
 194                 let kind = bounds
 195                     .transpose_iter()
 196                     .map(|e| e.map_bound(|e| *e).transpose_tuple2())
 197                     .filter_map(|(pred, _)| match pred.0.kind().skip_binder() {
 198                         ty::PredicateKind::Trait(tp) => {
 199                             self.tcx.fn_trait_kind_from_lang_item(tp.def_id())
 200                         }
 201                         _ => None,
 202                     })
 203                     .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
 204                 trace!(?sig, ?kind);
 205                 (sig, kind)
 206             }
 207             ty::Dynamic(ref object_type, ..) => {
 208                 let sig = object_type.projection_bounds().find_map(|pb| {
 209                     let pb = pb.with_self_ty(self.tcx, self.tcx.types.trait_object_dummy_self);
 210                     self.deduce_sig_from_projection(None, pb)
 211                 });
 212                 let kind = object_type
 213                     .principal_def_id()
 214                     .and_then(|did| self.tcx.fn_trait_kind_from_lang_item(did));
 215                 (sig, kind)
 216             }
 217             ty::Infer(ty::TyVar(vid)) => self.deduce_expectations_from_obligations(vid),
 218             ty::FnPtr(sig) => {
 219                 let expected_sig = ExpectedSig { cause_span: None, sig };
 220                 (Some(expected_sig), Some(ty::ClosureKind::Fn))
 221             }
 222             _ => (None, None),
 223         }
 224     }
 225
 226     fn deduce_expectations_from_obligations(
 227         &self,
 228         expected_vid: ty::TyVid,
 229     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 230         let expected_sig =
 231             self.obligations_for_self_ty(expected_vid).find_map(|(_, obligation)| {
 232                 debug!(?obligation.predicate);
 233
 234                 let bound_predicate = obligation.predicate.kind();
 235                 if let ty::PredicateKind::Projection(proj_predicate) =
 236                     obligation.predicate.kind().skip_binder()
 237                 {
 238                     // Given a Projection predicate, we can potentially infer
 239                     // the complete signature.
 240                     self.deduce_sig_from_projection(
 241                         Some(obligation.cause.span),
 242                         bound_predicate.rebind(proj_predicate),
 243                     )
 244                 } else {
 245                     None
 246                 }
 247             });
 248
 249         // Even if we can't infer the full signature, we may be able to
 250         // infer the kind. This can occur when we elaborate a predicate
 251         // like `F : Fn<A>`. Note that due to subtyping we could encounter
 252         // many viable options, so pick the most restrictive.
 253         let expected_kind = self
 254             .obligations_for_self_ty(expected_vid)
 255             .filter_map(|(tr, _)| self.tcx.fn_trait_kind_from_lang_item(tr.def_id()))
 256             .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
 257
 258         (expected_sig, expected_kind)
 259     }
 260
 261     /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
 262     /// everything we need to know about a closure or generator.
 263     ///
 264     /// The `cause_span` should be the span that caused us to
 265     /// have this expected signature, or `None` if we can't readily
 266     /// know that.
 267     #[instrument(level = "debug", skip(self, cause_span), ret)]
 268     fn deduce_sig_from_projection(
 269         &self,
 270         cause_span: Option<Span>,
 271         projection: ty::PolyProjectionPredicate<'tcx>,
 272     ) -> Option<ExpectedSig<'tcx>> {
 273         let tcx = self.tcx;
 274
 275         let trait_def_id = projection.trait_def_id(tcx);
 276
 277         let is_fn = tcx.fn_trait_kind_from_lang_item(trait_def_id).is_some();
 278         let gen_trait = tcx.require_lang_item(LangItem::Generator, cause_span);
 279         let is_gen = gen_trait == trait_def_id;
 280         if !is_fn && !is_gen {
 281             debug!("not fn or generator");
 282             return None;
 283         }
 284
 285         if is_gen {
 286             // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
 287             // associated item and not yield.
 288             let return_assoc_item = self.tcx.associated_item_def_ids(gen_trait)[1];
 289             if return_assoc_item != projection.projection_def_id() {
 290                 debug!("not return assoc item of generator");
 291                 return None;
 292             }
 293         }
 294
 295         let input_tys = if is_fn {
 296             let arg_param_ty = projection.skip_binder().projection_ty.substs.type_at(1);
 297             let arg_param_ty = self.resolve_vars_if_possible(arg_param_ty);
 298             debug!(?arg_param_ty);
 299
 300             match arg_param_ty.kind() {
 301                 &ty::Tuple(tys) => tys,
 302                 _ => return None,
 303             }
 304         } else {
 305             // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
 306             // else they must have exactly 1 argument. For now though, just give up in this case.
 307             return None;
 308         };
 309
 310         // Since this is a return parameter type it is safe to unwrap.
 311         let ret_param_ty = projection.skip_binder().term.ty().unwrap();
 312         let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
 313         debug!(?ret_param_ty);
 314
 315         let sig = projection.rebind(self.tcx.mk_fn_sig(
 316             input_tys.iter(),
 317             ret_param_ty,
 318             false,
 319             hir::Unsafety::Normal,
 320             Abi::Rust,
 321         ));
 322
 323         Some(ExpectedSig { cause_span, sig })
 324     }
 325
 326     fn sig_of_closure(
 327         &self,
 328         hir_id: hir::HirId,
 329         expr_def_id: DefId,
 330         decl: &hir::FnDecl<'_>,
 331         body: &hir::Body<'_>,
 332         expected_sig: Option<ExpectedSig<'tcx>>,
 333     ) -> ClosureSignatures<'tcx> {
 334         if let Some(e) = expected_sig {
 335             self.sig_of_closure_with_expectation(hir_id, expr_def_id, decl, body, e)
 336         } else {
 337             self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body)
 338         }
 339     }
 340
 341     /// If there is no expected signature, then we will convert the
 342     /// types that the user gave into a signature.
 343     #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
 344     fn sig_of_closure_no_expectation(
 345         &self,
 346         hir_id: hir::HirId,
 347         expr_def_id: DefId,
 348         decl: &hir::FnDecl<'_>,
 349         body: &hir::Body<'_>,
 350     ) -> ClosureSignatures<'tcx> {
 351         let bound_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 352
 353         self.closure_sigs(expr_def_id, body, bound_sig)
 354     }
 355
 356     /// Invoked to compute the signature of a closure expression. This
 357     /// combines any user-provided type annotations (e.g., `|x: u32|
 358     /// -> u32 { .. }`) with the expected signature.
 359     ///
 360     /// The approach is as follows:
 361     ///
 362     /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
 363     /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
 364     ///   - If we have no expectation `E`, then the signature of the closure is `S`.
 365     ///   - Otherwise, the signature of the closure is E. Moreover:
 366     ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
 367     ///     - Instantiate all the late-bound regions bound in the closure within `S`
 368     ///       with fresh (existential) variables, yielding `S'`
 369     ///     - Require that `E' = S'`
 370     ///       - We could use some kind of subtyping relationship here,
 371     ///         I imagine, but equality is easier and works fine for
 372     ///         our purposes.
 373     ///
 374     /// The key intuition here is that the user's types must be valid
 375     /// from "the inside" of the closure, but the expectation
 376     /// ultimately drives the overall signature.
 377     ///
 378     /// # Examples
 379     ///
 380     /// ```ignore (illustrative)
 381     /// fn with_closure<F>(_: F)
 382     ///   where F: Fn(&u32) -> &u32 { .. }
 383     ///
 384     /// with_closure(|x: &u32| { ... })
 385     /// ```
 386     ///
 387     /// Here:
 388     /// - E would be `fn(&u32) -> &u32`.
 389     /// - S would be `fn(&u32) ->
 390     /// - E' is `&'!0 u32 -> &'!0 u32`
 391     /// - S' is `&'?0 u32 -> ?T`
 392     ///
 393     /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
 394     ///
 395     /// # Arguments
 396     ///
 397     /// - `expr_def_id`: the `DefId` of the closure expression
 398     /// - `decl`: the HIR declaration of the closure
 399     /// - `body`: the body of the closure
 400     /// - `expected_sig`: the expected signature (if any). Note that
 401     ///   this is missing a binder: that is, there may be late-bound
 402     ///   regions with depth 1, which are bound then by the closure.
 403     #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
 404     fn sig_of_closure_with_expectation(
 405         &self,
 406         hir_id: hir::HirId,
 407         expr_def_id: DefId,
 408         decl: &hir::FnDecl<'_>,
 409         body: &hir::Body<'_>,
 410         expected_sig: ExpectedSig<'tcx>,
 411     ) -> ClosureSignatures<'tcx> {
 412         // Watch out for some surprises and just ignore the
 413         // expectation if things don't see to match up with what we
 414         // expect.
 415         if expected_sig.sig.c_variadic() != decl.c_variadic {
 416             return self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body);
 417         } else if expected_sig.sig.skip_binder().inputs_and_output.len() != decl.inputs.len() + 1 {
 418             return self.sig_of_closure_with_mismatched_number_of_arguments(
 419                 expr_def_id,
 420                 decl,
 421                 body,
 422                 expected_sig,
 423             );
 424         }
 425
 426         // Create a `PolyFnSig`. Note the oddity that late bound
 427         // regions appearing free in `expected_sig` are now bound up
 428         // in this binder we are creating.
 429         assert!(!expected_sig.sig.skip_binder().has_vars_bound_above(ty::INNERMOST));
 430         let bound_sig = expected_sig.sig.map_bound(|sig| {
 431             self.tcx.mk_fn_sig(
 432                 sig.inputs().iter().cloned(),
 433                 sig.output(),
 434                 sig.c_variadic,
 435                 hir::Unsafety::Normal,
 436                 Abi::RustCall,
 437             )
 438         });
 439
 440         // `deduce_expectations_from_expected_type` introduces
 441         // late-bound lifetimes defined elsewhere, which we now
 442         // anonymize away, so as not to confuse the user.
 443         let bound_sig = self.tcx.anonymize_late_bound_regions(bound_sig);
 444
 445         let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
 446
 447         // Up till this point, we have ignored the annotations that the user
 448         // gave. This function will check that they unify successfully.
 449         // Along the way, it also writes out entries for types that the user
 450         // wrote into our typeck results, which are then later used by the privacy
 451         // check.
 452         match self.merge_supplied_sig_with_expectation(
 453             hir_id,
 454             expr_def_id,
 455             decl,
 456             body,
 457             closure_sigs,
 458         ) {
 459             Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
 460             Err(_) => self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body),
 461         }
 462     }
 463
 464     fn sig_of_closure_with_mismatched_number_of_arguments(
 465         &self,
 466         expr_def_id: DefId,
 467         decl: &hir::FnDecl<'_>,
 468         body: &hir::Body<'_>,
 469         expected_sig: ExpectedSig<'tcx>,
 470     ) -> ClosureSignatures<'tcx> {
 471         let hir = self.tcx.hir();
 472         let expr_map_node = hir.get_if_local(expr_def_id).unwrap();
 473         let expected_args: Vec<_> = expected_sig
 474             .sig
 475             .skip_binder()
 476             .inputs()
 477             .iter()
 478             .map(|ty| ArgKind::from_expected_ty(*ty, None))
 479             .collect();
 480         let (closure_span, found_args) = match self.get_fn_like_arguments(expr_map_node) {
 481             Some((sp, args)) => (Some(sp), args),
 482             None => (None, Vec::new()),
 483         };
 484         let expected_span =
 485             expected_sig.cause_span.unwrap_or_else(|| hir.span_if_local(expr_def_id).unwrap());
 486         self.report_arg_count_mismatch(
 487             expected_span,
 488             closure_span,
 489             expected_args,
 490             found_args,
 491             true,
 492         )
 493         .emit();
 494
 495         let error_sig = self.error_sig_of_closure(decl);
 496
 497         self.closure_sigs(expr_def_id, body, error_sig)
 498     }
 499
 500     /// Enforce the user's types against the expectation. See
 501     /// `sig_of_closure_with_expectation` for details on the overall
 502     /// strategy.
 503     #[instrument(level = "debug", skip(self, hir_id, expr_def_id, decl, body, expected_sigs))]
 504     fn merge_supplied_sig_with_expectation(
 505         &self,
 506         hir_id: hir::HirId,
 507         expr_def_id: DefId,
 508         decl: &hir::FnDecl<'_>,
 509         body: &hir::Body<'_>,
 510         mut expected_sigs: ClosureSignatures<'tcx>,
 511     ) -> InferResult<'tcx, ClosureSignatures<'tcx>> {
 512         // Get the signature S that the user gave.
 513         //
 514         // (See comment on `sig_of_closure_with_expectation` for the
 515         // meaning of these letters.)
 516         let supplied_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 517
 518         debug!(?supplied_sig);
 519
 520         // FIXME(#45727): As discussed in [this comment][c1], naively
 521         // forcing equality here actually results in suboptimal error
 522         // messages in some cases.  For now, if there would have been
 523         // an obvious error, we fallback to declaring the type of the
 524         // closure to be the one the user gave, which allows other
 525         // error message code to trigger.
 526         //
 527         // However, I think [there is potential to do even better
 528         // here][c2], since in *this* code we have the precise span of
 529         // the type parameter in question in hand when we report the
 530         // error.
 531         //
 532         // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
 533         // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
 534         self.commit_if_ok(|_| {
 535             let mut all_obligations = vec![];
 536             let inputs: Vec<_> = iter::zip(
 537                 decl.inputs,
 538                 supplied_sig.inputs().skip_binder(), // binder moved to (*) below
 539             )
 540             .map(|(hir_ty, &supplied_ty)| {
 541                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 542                 self.replace_bound_vars_with_fresh_vars(
 543                     hir_ty.span,
 544                     LateBoundRegionConversionTime::FnCall,
 545                     // (*) binder moved to here
 546                     supplied_sig.inputs().rebind(supplied_ty),
 547                 )
 548             })
 549             .collect();
 550
 551             // The liberated version of this signature should be a subtype
 552             // of the liberated form of the expectation.
 553             for ((hir_ty, &supplied_ty), expected_ty) in iter::zip(
 554                 iter::zip(decl.inputs, &inputs),
 555                 expected_sigs.liberated_sig.inputs(), // `liberated_sig` is E'.
 556             ) {
 557                 // Check that E' = S'.
 558                 let cause = self.misc(hir_ty.span);
 559                 let InferOk { value: (), obligations } =
 560                     self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
 561                 all_obligations.extend(obligations);
 562             }
 563
 564             let supplied_output_ty = self.replace_bound_vars_with_fresh_vars(
 565                 decl.output.span(),
 566                 LateBoundRegionConversionTime::FnCall,
 567                 supplied_sig.output(),
 568             );
 569             let cause = &self.misc(decl.output.span());
 570             let InferOk { value: (), obligations } = self
 571                 .at(cause, self.param_env)
 572                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 573             all_obligations.extend(obligations);
 574
 575             let inputs = inputs.into_iter().map(|ty| self.resolve_vars_if_possible(ty));
 576
 577             expected_sigs.liberated_sig = self.tcx.mk_fn_sig(
 578                 inputs,
 579                 supplied_output_ty,
 580                 expected_sigs.liberated_sig.c_variadic,
 581                 hir::Unsafety::Normal,
 582                 Abi::RustCall,
 583             );
 584
 585             Ok(InferOk { value: expected_sigs, obligations: all_obligations })
 586         })
 587     }
 588
 589     /// If there is no expected signature, then we will convert the
 590     /// types that the user gave into a signature.
 591     ///
 592     /// Also, record this closure signature for later.
 593     #[instrument(skip(self, decl, body), level = "debug", ret)]
 594     fn supplied_sig_of_closure(
 595         &self,
 596         hir_id: hir::HirId,
 597         expr_def_id: DefId,
 598         decl: &hir::FnDecl<'_>,
 599         body: &hir::Body<'_>,
 600     ) -> ty::PolyFnSig<'tcx> {
 601         let astconv: &dyn AstConv<'_> = self;
 602
 603         trace!("decl = {:#?}", decl);
 604         debug!(?body.generator_kind);
 605
 606         let bound_vars = self.tcx.late_bound_vars(hir_id);
 607
 608         // First, convert the types that the user supplied (if any).
 609         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 610         let supplied_return = match decl.output {
 611             hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
 612             hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
 613                 // In the case of the async block that we create for a function body,
 614                 // we expect the return type of the block to match that of the enclosing
 615                 // function.
 616                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 617                     debug!("closure is async fn body");
 618                     self.deduce_future_output_from_obligations(expr_def_id, body.id().hir_id)
 619                         .unwrap_or_else(|| {
 620                             // AFAIK, deducing the future output
 621                             // always succeeds *except* in error cases
 622                             // like #65159. I'd like to return Error
 623                             // here, but I can't because I can't
 624                             // easily (and locally) prove that we
 625                             // *have* reported an
 626                             // error. --nikomatsakis
 627                             astconv.ty_infer(None, decl.output.span())
 628                         })
 629                 }
 630
 631                 _ => astconv.ty_infer(None, decl.output.span()),
 632             },
 633         };
 634
 635         let result = ty::Binder::bind_with_vars(
 636             self.tcx.mk_fn_sig(
 637                 supplied_arguments,
 638                 supplied_return,
 639                 decl.c_variadic,
 640                 hir::Unsafety::Normal,
 641                 Abi::RustCall,
 642             ),
 643             bound_vars,
 644         );
 645
 646         let c_result = self.inh.infcx.canonicalize_response(result);
 647         self.typeck_results.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);
 648
 649         result
 650     }
 651
 652     /// Invoked when we are translating the generator that results
 653     /// from desugaring an `async fn`. Returns the "sugared" return
 654     /// type of the `async fn` -- that is, the return type that the
 655     /// user specified. The "desugared" return type is an `impl
 656     /// Future<Output = T>`, so we do this by searching through the
 657     /// obligations to extract the `T`.
 658     #[instrument(skip(self), level = "debug", ret)]
 659     fn deduce_future_output_from_obligations(
 660         &self,
 661         expr_def_id: DefId,
 662         body_id: hir::HirId,
 663     ) -> Option<Ty<'tcx>> {
 664         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
 665             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
 666         });
 667
 668         let ret_ty = ret_coercion.borrow().expected_ty();
 669         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 670
 671         let get_future_output = |predicate: ty::Predicate<'tcx>, span| {
 672             // Search for a pending obligation like
 673             //
 674             // `<R as Future>::Output = T`
 675             //
 676             // where R is the return type we are expecting. This type `T`
 677             // will be our output.
 678             let bound_predicate = predicate.kind();
 679             if let ty::PredicateKind::Projection(proj_predicate) = bound_predicate.skip_binder() {
 680                 self.deduce_future_output_from_projection(
 681                     span,
 682                     bound_predicate.rebind(proj_predicate),
 683                 )
 684             } else {
 685                 None
 686             }
 687         };
 688
 689         let output_ty = match *ret_ty.kind() {
 690             ty::Infer(ty::TyVar(ret_vid)) => {
 691                 self.obligations_for_self_ty(ret_vid).find_map(|(_, obligation)| {
 692                     get_future_output(obligation.predicate, obligation.cause.span)
 693                 })?
 694             }
 695             ty::Opaque(def_id, substs) => self
 696                 .tcx
 697                 .bound_explicit_item_bounds(def_id)
 698                 .transpose_iter()
 699                 .map(|e| e.map_bound(|e| *e).transpose_tuple2())
 700                 .find_map(|(p, s)| get_future_output(p.subst(self.tcx, substs), s.0))?,
 701             ty::Error(_) => return None,
 702             ty::Projection(proj)
 703                 if self.tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder =>
 704             {
 705                 self.tcx
 706                     .bound_explicit_item_bounds(proj.item_def_id)
 707                     .transpose_iter()
 708                     .map(|e| e.map_bound(|e| *e).transpose_tuple2())
 709                     .find_map(|(p, s)| get_future_output(p.subst(self.tcx, proj.substs), s.0))?
 710             }
 711             _ => span_bug!(
 712                 self.tcx.def_span(expr_def_id),
 713                 "async fn generator return type not an inference variable: {ret_ty}"
 714             ),
 715         };
 716
 717         // async fn that have opaque types in their return type need to redo the conversion to inference variables
 718         // as they fetch the still opaque version from the signature.
 719         let InferOk { value: output_ty, obligations } = self
 720             .replace_opaque_types_with_inference_vars(
 721                 output_ty,
 722                 body_id,
 723                 self.tcx.def_span(expr_def_id),
 724                 self.param_env,
 725             );
 726         self.register_predicates(obligations);
 727
 728         Some(output_ty)
 729     }
 730
 731     /// Given a projection like
 732     ///
 733     /// `<X as Future>::Output = T`
 734     ///
 735     /// where `X` is some type that has no late-bound regions, returns
 736     /// `Some(T)`. If the projection is for some other trait, returns
 737     /// `None`.
 738     fn deduce_future_output_from_projection(
 739         &self,
 740         cause_span: Span,
 741         predicate: ty::PolyProjectionPredicate<'tcx>,
 742     ) -> Option<Ty<'tcx>> {
 743         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 744
 745         // We do not expect any bound regions in our predicate, so
 746         // skip past the bound vars.
 747         let Some(predicate) = predicate.no_bound_vars() else {
 748             debug!("deduce_future_output_from_projection: has late-bound regions");
 749             return None;
 750         };
 751
 752         // Check that this is a projection from the `Future` trait.
 753         let trait_def_id = predicate.projection_ty.trait_def_id(self.tcx);
 754         let future_trait = self.tcx.require_lang_item(LangItem::Future, Some(cause_span));
 755         if trait_def_id != future_trait {
 756             debug!("deduce_future_output_from_projection: not a future");
 757             return None;
 758         }
 759
 760         // The `Future` trait has only one associated item, `Output`,
 761         // so check that this is what we see.
 762         let output_assoc_item = self.tcx.associated_item_def_ids(future_trait)[0];
 763         if output_assoc_item != predicate.projection_ty.item_def_id {
 764             span_bug!(
 765                 cause_span,
 766                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 767                 predicate.projection_ty.item_def_id,
 768                 output_assoc_item,
 769             );
 770         }
 771
 772         // Extract the type from the projection. Note that there can
 773         // be no bound variables in this type because the "self type"
 774         // does not have any regions in it.
 775         let output_ty = self.resolve_vars_if_possible(predicate.term);
 776         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 777         // This is a projection on a Fn trait so will always be a type.
 778         Some(output_ty.ty().unwrap())
 779     }
 780
 781     /// Converts the types that the user supplied, in case that doing
 782     /// so should yield an error, but returns back a signature where
 783     /// all parameters are of type `TyErr`.
 784     fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
 785         let astconv: &dyn AstConv<'_> = self;
 786
 787         let supplied_arguments = decl.inputs.iter().map(|a| {
 788             // Convert the types that the user supplied (if any), but ignore them.
 789             astconv.ast_ty_to_ty(a);
 790             self.tcx.ty_error()
 791         });
 792
 793         if let hir::FnRetTy::Return(ref output) = decl.output {
 794             astconv.ast_ty_to_ty(&output);
 795         }
 796
 797         let result = ty::Binder::dummy(self.tcx.mk_fn_sig(
 798             supplied_arguments,
 799             self.tcx.ty_error(),
 800             decl.c_variadic,
 801             hir::Unsafety::Normal,
 802             Abi::RustCall,
 803         ));
 804
 805         debug!("supplied_sig_of_closure: result={:?}", result);
 806
 807         result
 808     }
 809
 810     fn closure_sigs(
 811         &self,
 812         expr_def_id: DefId,
 813         body: &hir::Body<'_>,
 814         bound_sig: ty::PolyFnSig<'tcx>,
 815     ) -> ClosureSignatures<'tcx> {
 816         let liberated_sig = self.tcx().liberate_late_bound_regions(expr_def_id, bound_sig);
 817         let liberated_sig = self.inh.normalize_associated_types_in(
 818             body.value.span,
 819             body.value.hir_id,
 820             self.param_env,
 821             liberated_sig,
 822         );
 823         ClosureSignatures { bound_sig, liberated_sig }
 824     }
 825 }