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     bound_sig: ty::PolyFnSig<'tcx>,
  34     liberated_sig: ty::FnSig<'tcx>,
  35 }
  36
  37 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  38     #[instrument(skip(self, expr, _capture, decl, body_id), level = "debug")]
  39     pub fn check_expr_closure(
  40         &self,
  41         expr: &hir::Expr<'_>,
  42         _capture: hir::CaptureBy,
  43         decl: &'tcx hir::FnDecl<'tcx>,
  44         body_id: hir::BodyId,
  45         gen: Option<hir::Movability>,
  46         expected: Expectation<'tcx>,
  47     ) -> Ty<'tcx> {
  48         trace!("decl = {:#?}", decl);
  49         trace!("expr = {:#?}", expr);
  50
  51         // It's always helpful for inference if we know the kind of
  52         // closure sooner rather than later, so first examine the expected
  53         // type, and see if can glean a closure kind from there.
  54         let (expected_sig, expected_kind) = match expected.to_option(self) {
  55             Some(ty) => self.deduce_expectations_from_expected_type(ty),
  56             None => (None, None),
  57         };
  58         let body = self.tcx.hir().body(body_id);
  59         self.check_closure(expr, expected_kind, decl, body, gen, expected_sig)
  60     }
  61
  62     #[instrument(skip(self, expr, body, decl), level = "debug", ret)]
  63     fn check_closure(
  64         &self,
  65         expr: &hir::Expr<'_>,
  66         opt_kind: Option<ty::ClosureKind>,
  67         decl: &'tcx hir::FnDecl<'tcx>,
  68         body: &'tcx hir::Body<'tcx>,
  69         gen: Option<hir::Movability>,
  70         expected_sig: Option<ExpectedSig<'tcx>>,
  71     ) -> Ty<'tcx> {
  72         trace!("decl = {:#?}", decl);
  73         let expr_def_id = self.tcx.hir().local_def_id(expr.hir_id);
  74         debug!(?expr_def_id);
  75
  76         let ClosureSignatures { bound_sig, liberated_sig } =
  77             self.sig_of_closure(expr.hir_id, expr_def_id.to_def_id(), decl, body, expected_sig);
  78
  79         debug!(?bound_sig, ?liberated_sig);
  80
  81         let return_type_pre_known = !liberated_sig.output().is_ty_infer();
  82
  83         let generator_types = check_fn(
  84             self,
  85             self.param_env.without_const(),
  86             liberated_sig,
  87             decl,
  88             expr.hir_id,
  89             body,
  90             gen,
  91             return_type_pre_known,
  92         )
  93         .1;
  94
  95         let parent_substs = InternalSubsts::identity_for_item(
  96             self.tcx,
  97             self.tcx.typeck_root_def_id(expr_def_id.to_def_id()),
  98         );
  99
 100         let tupled_upvars_ty = self.next_ty_var(TypeVariableOrigin {
 101             kind: TypeVariableOriginKind::ClosureSynthetic,
 102             span: self.tcx.hir().span(expr.hir_id),
 103         });
 104
 105         if let Some(GeneratorTypes { resume_ty, yield_ty, interior, movability }) = generator_types
 106         {
 107             let generator_substs = ty::GeneratorSubsts::new(
 108                 self.tcx,
 109                 ty::GeneratorSubstsParts {
 110                     parent_substs,
 111                     resume_ty,
 112                     yield_ty,
 113                     return_ty: liberated_sig.output(),
 114                     witness: interior,
 115                     tupled_upvars_ty,
 116                 },
 117             );
 118
 119             return self.tcx.mk_generator(
 120                 expr_def_id.to_def_id(),
 121                 generator_substs.substs,
 122                 movability,
 123             );
 124         }
 125
 126         // Tuple up the arguments and insert the resulting function type into
 127         // the `closures` table.
 128         let sig = bound_sig.map_bound(|sig| {
 129             self.tcx.mk_fn_sig(
 130                 iter::once(self.tcx.intern_tup(sig.inputs())),
 131                 sig.output(),
 132                 sig.c_variadic,
 133                 sig.unsafety,
 134                 sig.abi,
 135             )
 136         });
 137
 138         debug!(?sig, ?opt_kind);
 139
 140         let closure_kind_ty = match opt_kind {
 141             Some(kind) => kind.to_ty(self.tcx),
 142
 143             // Create a type variable (for now) to represent the closure kind.
 144             // It will be unified during the upvar inference phase (`upvar.rs`)
 145             None => self.next_ty_var(TypeVariableOrigin {
 146                 // FIXME(eddyb) distinguish closure kind inference variables from the rest.
 147                 kind: TypeVariableOriginKind::ClosureSynthetic,
 148                 span: expr.span,
 149             }),
 150         };
 151
 152         let closure_substs = ty::ClosureSubsts::new(
 153             self.tcx,
 154             ty::ClosureSubstsParts {
 155                 parent_substs,
 156                 closure_kind_ty,
 157                 closure_sig_as_fn_ptr_ty: self.tcx.mk_fn_ptr(sig),
 158                 tupled_upvars_ty,
 159             },
 160         );
 161
 162         self.tcx.mk_closure(expr_def_id.to_def_id(), closure_substs.substs)
 163     }
 164
 165     /// Given the expected type, figures out what it can about this closure we
 166     /// are about to type check:
 167     #[instrument(skip(self), level = "debug")]
 168     fn deduce_expectations_from_expected_type(
 169         &self,
 170         expected_ty: Ty<'tcx>,
 171     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 172         match *expected_ty.kind() {
 173             ty::Opaque(def_id, substs) => {
 174                 let bounds = self.tcx.bound_explicit_item_bounds(def_id);
 175                 let sig = bounds
 176                     .transpose_iter()
 177                     .map(|e| e.map_bound(|e| *e).transpose_tuple2())
 178                     .find_map(|(pred, span)| match pred.0.kind().skip_binder() {
 179                         ty::PredicateKind::Projection(proj_predicate) => self
 180                             .deduce_sig_from_projection(
 181                                 Some(span.0),
 182                                 pred.0
 183                                     .kind()
 184                                     .rebind(pred.rebind(proj_predicate).subst(self.tcx, substs)),
 185                             ),
 186                         _ => None,
 187                     });
 188
 189                 let kind = bounds
 190                     .transpose_iter()
 191                     .map(|e| e.map_bound(|e| *e).transpose_tuple2())
 192                     .filter_map(|(pred, _)| match pred.0.kind().skip_binder() {
 193                         ty::PredicateKind::Trait(tp) => {
 194                             self.tcx.fn_trait_kind_from_lang_item(tp.def_id())
 195                         }
 196                         _ => None,
 197                     })
 198                     .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
 199                 trace!(?sig, ?kind);
 200                 (sig, kind)
 201             }
 202             ty::Dynamic(ref object_type, ..) => {
 203                 let sig = object_type.projection_bounds().find_map(|pb| {
 204                     let pb = pb.with_self_ty(self.tcx, self.tcx.types.trait_object_dummy_self);
 205                     self.deduce_sig_from_projection(None, pb)
 206                 });
 207                 let kind = object_type
 208                     .principal_def_id()
 209                     .and_then(|did| self.tcx.fn_trait_kind_from_lang_item(did));
 210                 (sig, kind)
 211             }
 212             ty::Infer(ty::TyVar(vid)) => self.deduce_expectations_from_obligations(vid),
 213             ty::FnPtr(sig) => {
 214                 let expected_sig = ExpectedSig { cause_span: None, sig };
 215                 (Some(expected_sig), Some(ty::ClosureKind::Fn))
 216             }
 217             _ => (None, None),
 218         }
 219     }
 220
 221     fn deduce_expectations_from_obligations(
 222         &self,
 223         expected_vid: ty::TyVid,
 224     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 225         let expected_sig =
 226             self.obligations_for_self_ty(expected_vid).find_map(|(_, obligation)| {
 227                 debug!(?obligation.predicate);
 228
 229                 let bound_predicate = obligation.predicate.kind();
 230                 if let ty::PredicateKind::Projection(proj_predicate) =
 231                     obligation.predicate.kind().skip_binder()
 232                 {
 233                     // Given a Projection predicate, we can potentially infer
 234                     // the complete signature.
 235                     self.deduce_sig_from_projection(
 236                         Some(obligation.cause.span),
 237                         bound_predicate.rebind(proj_predicate),
 238                     )
 239                 } else {
 240                     None
 241                 }
 242             });
 243
 244         // Even if we can't infer the full signature, we may be able to
 245         // infer the kind. This can occur when we elaborate a predicate
 246         // like `F : Fn<A>`. Note that due to subtyping we could encounter
 247         // many viable options, so pick the most restrictive.
 248         let expected_kind = self
 249             .obligations_for_self_ty(expected_vid)
 250             .filter_map(|(tr, _)| self.tcx.fn_trait_kind_from_lang_item(tr.def_id()))
 251             .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
 252
 253         (expected_sig, expected_kind)
 254     }
 255
 256     /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
 257     /// everything we need to know about a closure or generator.
 258     ///
 259     /// The `cause_span` should be the span that caused us to
 260     /// have this expected signature, or `None` if we can't readily
 261     /// know that.
 262     #[instrument(level = "debug", skip(self, cause_span), ret)]
 263     fn deduce_sig_from_projection(
 264         &self,
 265         cause_span: Option<Span>,
 266         projection: ty::PolyProjectionPredicate<'tcx>,
 267     ) -> Option<ExpectedSig<'tcx>> {
 268         let tcx = self.tcx;
 269
 270         let trait_def_id = projection.trait_def_id(tcx);
 271
 272         let is_fn = tcx.fn_trait_kind_from_lang_item(trait_def_id).is_some();
 273         let gen_trait = tcx.require_lang_item(LangItem::Generator, cause_span);
 274         let is_gen = gen_trait == trait_def_id;
 275         if !is_fn && !is_gen {
 276             debug!("not fn or generator");
 277             return None;
 278         }
 279
 280         if is_gen {
 281             // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
 282             // associated item and not yield.
 283             let return_assoc_item = self.tcx.associated_item_def_ids(gen_trait)[1];
 284             if return_assoc_item != projection.projection_def_id() {
 285                 debug!("not return assoc item of generator");
 286                 return None;
 287             }
 288         }
 289
 290         let input_tys = if is_fn {
 291             let arg_param_ty = projection.skip_binder().projection_ty.substs.type_at(1);
 292             let arg_param_ty = self.resolve_vars_if_possible(arg_param_ty);
 293             debug!(?arg_param_ty);
 294
 295             match arg_param_ty.kind() {
 296                 &ty::Tuple(tys) => tys,
 297                 _ => return None,
 298             }
 299         } else {
 300             // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
 301             // else they must have exactly 1 argument. For now though, just give up in this case.
 302             return None;
 303         };
 304
 305         // Since this is a return parameter type it is safe to unwrap.
 306         let ret_param_ty = projection.skip_binder().term.ty().unwrap();
 307         let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
 308         debug!(?ret_param_ty);
 309
 310         let sig = projection.rebind(self.tcx.mk_fn_sig(
 311             input_tys.iter(),
 312             ret_param_ty,
 313             false,
 314             hir::Unsafety::Normal,
 315             Abi::Rust,
 316         ));
 317
 318         Some(ExpectedSig { cause_span, sig })
 319     }
 320
 321     fn sig_of_closure(
 322         &self,
 323         hir_id: hir::HirId,
 324         expr_def_id: DefId,
 325         decl: &hir::FnDecl<'_>,
 326         body: &hir::Body<'_>,
 327         expected_sig: Option<ExpectedSig<'tcx>>,
 328     ) -> ClosureSignatures<'tcx> {
 329         if let Some(e) = expected_sig {
 330             self.sig_of_closure_with_expectation(hir_id, expr_def_id, decl, body, e)
 331         } else {
 332             self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body)
 333         }
 334     }
 335
 336     /// If there is no expected signature, then we will convert the
 337     /// types that the user gave into a signature.
 338     #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
 339     fn sig_of_closure_no_expectation(
 340         &self,
 341         hir_id: hir::HirId,
 342         expr_def_id: DefId,
 343         decl: &hir::FnDecl<'_>,
 344         body: &hir::Body<'_>,
 345     ) -> ClosureSignatures<'tcx> {
 346         let bound_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 347
 348         self.closure_sigs(expr_def_id, body, bound_sig)
 349     }
 350
 351     /// Invoked to compute the signature of a closure expression. This
 352     /// combines any user-provided type annotations (e.g., `|x: u32|
 353     /// -> u32 { .. }`) with the expected signature.
 354     ///
 355     /// The approach is as follows:
 356     ///
 357     /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
 358     /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
 359     ///   - If we have no expectation `E`, then the signature of the closure is `S`.
 360     ///   - Otherwise, the signature of the closure is E. Moreover:
 361     ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
 362     ///     - Instantiate all the late-bound regions bound in the closure within `S`
 363     ///       with fresh (existential) variables, yielding `S'`
 364     ///     - Require that `E' = S'`
 365     ///       - We could use some kind of subtyping relationship here,
 366     ///         I imagine, but equality is easier and works fine for
 367     ///         our purposes.
 368     ///
 369     /// The key intuition here is that the user's types must be valid
 370     /// from "the inside" of the closure, but the expectation
 371     /// ultimately drives the overall signature.
 372     ///
 373     /// # Examples
 374     ///
 375     /// ```ignore (illustrative)
 376     /// fn with_closure<F>(_: F)
 377     ///   where F: Fn(&u32) -> &u32 { .. }
 378     ///
 379     /// with_closure(|x: &u32| { ... })
 380     /// ```
 381     ///
 382     /// Here:
 383     /// - E would be `fn(&u32) -> &u32`.
 384     /// - S would be `fn(&u32) ->
 385     /// - E' is `&'!0 u32 -> &'!0 u32`
 386     /// - S' is `&'?0 u32 -> ?T`
 387     ///
 388     /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
 389     ///
 390     /// # Arguments
 391     ///
 392     /// - `expr_def_id`: the `DefId` of the closure expression
 393     /// - `decl`: the HIR declaration of the closure
 394     /// - `body`: the body of the closure
 395     /// - `expected_sig`: the expected signature (if any). Note that
 396     ///   this is missing a binder: that is, there may be late-bound
 397     ///   regions with depth 1, which are bound then by the closure.
 398     #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
 399     fn sig_of_closure_with_expectation(
 400         &self,
 401         hir_id: hir::HirId,
 402         expr_def_id: DefId,
 403         decl: &hir::FnDecl<'_>,
 404         body: &hir::Body<'_>,
 405         expected_sig: ExpectedSig<'tcx>,
 406     ) -> ClosureSignatures<'tcx> {
 407         // Watch out for some surprises and just ignore the
 408         // expectation if things don't see to match up with what we
 409         // expect.
 410         if expected_sig.sig.c_variadic() != decl.c_variadic {
 411             return self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body);
 412         } else if expected_sig.sig.skip_binder().inputs_and_output.len() != decl.inputs.len() + 1 {
 413             return self.sig_of_closure_with_mismatched_number_of_arguments(
 414                 expr_def_id,
 415                 decl,
 416                 body,
 417                 expected_sig,
 418             );
 419         }
 420
 421         // Create a `PolyFnSig`. Note the oddity that late bound
 422         // regions appearing free in `expected_sig` are now bound up
 423         // in this binder we are creating.
 424         assert!(!expected_sig.sig.skip_binder().has_vars_bound_above(ty::INNERMOST));
 425         let bound_sig = expected_sig.sig.map_bound(|sig| {
 426             self.tcx.mk_fn_sig(
 427                 sig.inputs().iter().cloned(),
 428                 sig.output(),
 429                 sig.c_variadic,
 430                 hir::Unsafety::Normal,
 431                 Abi::RustCall,
 432             )
 433         });
 434
 435         // `deduce_expectations_from_expected_type` introduces
 436         // late-bound lifetimes defined elsewhere, which we now
 437         // anonymize away, so as not to confuse the user.
 438         let bound_sig = self.tcx.anonymize_late_bound_regions(bound_sig);
 439
 440         let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
 441
 442         // Up till this point, we have ignored the annotations that the user
 443         // gave. This function will check that they unify successfully.
 444         // Along the way, it also writes out entries for types that the user
 445         // wrote into our typeck results, which are then later used by the privacy
 446         // check.
 447         match self.check_supplied_sig_against_expectation(
 448             hir_id,
 449             expr_def_id,
 450             decl,
 451             body,
 452             &closure_sigs,
 453         ) {
 454             Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
 455             Err(_) => return self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body),
 456         }
 457
 458         closure_sigs
 459     }
 460
 461     fn sig_of_closure_with_mismatched_number_of_arguments(
 462         &self,
 463         expr_def_id: DefId,
 464         decl: &hir::FnDecl<'_>,
 465         body: &hir::Body<'_>,
 466         expected_sig: ExpectedSig<'tcx>,
 467     ) -> ClosureSignatures<'tcx> {
 468         let hir = self.tcx.hir();
 469         let expr_map_node = hir.get_if_local(expr_def_id).unwrap();
 470         let expected_args: Vec<_> = expected_sig
 471             .sig
 472             .skip_binder()
 473             .inputs()
 474             .iter()
 475             .map(|ty| ArgKind::from_expected_ty(*ty, None))
 476             .collect();
 477         let (closure_span, found_args) = match self.get_fn_like_arguments(expr_map_node) {
 478             Some((sp, args)) => (Some(sp), args),
 479             None => (None, Vec::new()),
 480         };
 481         let expected_span =
 482             expected_sig.cause_span.unwrap_or_else(|| hir.span_if_local(expr_def_id).unwrap());
 483         self.report_arg_count_mismatch(
 484             expected_span,
 485             closure_span,
 486             expected_args,
 487             found_args,
 488             true,
 489         )
 490         .emit();
 491
 492         let error_sig = self.error_sig_of_closure(decl);
 493
 494         self.closure_sigs(expr_def_id, body, error_sig)
 495     }
 496
 497     /// Enforce the user's types against the expectation. See
 498     /// `sig_of_closure_with_expectation` for details on the overall
 499     /// strategy.
 500     fn check_supplied_sig_against_expectation(
 501         &self,
 502         hir_id: hir::HirId,
 503         expr_def_id: DefId,
 504         decl: &hir::FnDecl<'_>,
 505         body: &hir::Body<'_>,
 506         expected_sigs: &ClosureSignatures<'tcx>,
 507     ) -> InferResult<'tcx, ()> {
 508         // Get the signature S that the user gave.
 509         //
 510         // (See comment on `sig_of_closure_with_expectation` for the
 511         // meaning of these letters.)
 512         let supplied_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 513
 514         debug!("check_supplied_sig_against_expectation: supplied_sig={:?}", supplied_sig);
 515
 516         // FIXME(#45727): As discussed in [this comment][c1], naively
 517         // forcing equality here actually results in suboptimal error
 518         // messages in some cases.  For now, if there would have been
 519         // an obvious error, we fallback to declaring the type of the
 520         // closure to be the one the user gave, which allows other
 521         // error message code to trigger.
 522         //
 523         // However, I think [there is potential to do even better
 524         // here][c2], since in *this* code we have the precise span of
 525         // the type parameter in question in hand when we report the
 526         // error.
 527         //
 528         // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
 529         // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
 530         self.commit_if_ok(|_| {
 531             let mut all_obligations = vec![];
 532
 533             // The liberated version of this signature should be a subtype
 534             // of the liberated form of the expectation.
 535             for ((hir_ty, &supplied_ty), expected_ty) in iter::zip(
 536                 iter::zip(
 537                     decl.inputs,
 538                     supplied_sig.inputs().skip_binder(), // binder moved to (*) below
 539                 ),
 540                 expected_sigs.liberated_sig.inputs(), // `liberated_sig` is E'.
 541             ) {
 542                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 543                 let supplied_ty = self.replace_bound_vars_with_fresh_vars(
 544                     hir_ty.span,
 545                     LateBoundRegionConversionTime::FnCall,
 546                     supplied_sig.inputs().rebind(supplied_ty),
 547                 ); // recreated from (*) above
 548
 549                 // Check that E' = S'.
 550                 let cause = self.misc(hir_ty.span);
 551                 let InferOk { value: (), obligations } =
 552                     self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
 553                 all_obligations.extend(obligations);
 554             }
 555
 556             let supplied_output_ty = self.replace_bound_vars_with_fresh_vars(
 557                 decl.output.span(),
 558                 LateBoundRegionConversionTime::FnCall,
 559                 supplied_sig.output(),
 560             );
 561             let cause = &self.misc(decl.output.span());
 562             let InferOk { value: (), obligations } = self
 563                 .at(cause, self.param_env)
 564                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 565             all_obligations.extend(obligations);
 566
 567             Ok(InferOk { value: (), obligations: all_obligations })
 568         })
 569     }
 570
 571     /// If there is no expected signature, then we will convert the
 572     /// types that the user gave into a signature.
 573     ///
 574     /// Also, record this closure signature for later.
 575     #[instrument(skip(self, decl, body), level = "debug", ret)]
 576     fn supplied_sig_of_closure(
 577         &self,
 578         hir_id: hir::HirId,
 579         expr_def_id: DefId,
 580         decl: &hir::FnDecl<'_>,
 581         body: &hir::Body<'_>,
 582     ) -> ty::PolyFnSig<'tcx> {
 583         let astconv: &dyn AstConv<'_> = self;
 584
 585         trace!("decl = {:#?}", decl);
 586         debug!(?body.generator_kind);
 587
 588         let bound_vars = self.tcx.late_bound_vars(hir_id);
 589
 590         // First, convert the types that the user supplied (if any).
 591         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 592         let supplied_return = match decl.output {
 593             hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
 594             hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
 595                 // In the case of the async block that we create for a function body,
 596                 // we expect the return type of the block to match that of the enclosing
 597                 // function.
 598                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 599                     debug!("closure is async fn body");
 600                     self.deduce_future_output_from_obligations(expr_def_id, body.id().hir_id)
 601                         .unwrap_or_else(|| {
 602                             // AFAIK, deducing the future output
 603                             // always succeeds *except* in error cases
 604                             // like #65159. I'd like to return Error
 605                             // here, but I can't because I can't
 606                             // easily (and locally) prove that we
 607                             // *have* reported an
 608                             // error. --nikomatsakis
 609                             astconv.ty_infer(None, decl.output.span())
 610                         })
 611                 }
 612
 613                 _ => astconv.ty_infer(None, decl.output.span()),
 614             },
 615         };
 616
 617         let result = ty::Binder::bind_with_vars(
 618             self.tcx.mk_fn_sig(
 619                 supplied_arguments,
 620                 supplied_return,
 621                 decl.c_variadic,
 622                 hir::Unsafety::Normal,
 623                 Abi::RustCall,
 624             ),
 625             bound_vars,
 626         );
 627
 628         let c_result = self.inh.infcx.canonicalize_response(result);
 629         self.typeck_results.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);
 630
 631         result
 632     }
 633
 634     /// Invoked when we are translating the generator that results
 635     /// from desugaring an `async fn`. Returns the "sugared" return
 636     /// type of the `async fn` -- that is, the return type that the
 637     /// user specified. The "desugared" return type is an `impl
 638     /// Future<Output = T>`, so we do this by searching through the
 639     /// obligations to extract the `T`.
 640     #[instrument(skip(self), level = "debug", ret)]
 641     fn deduce_future_output_from_obligations(
 642         &self,
 643         expr_def_id: DefId,
 644         body_id: hir::HirId,
 645     ) -> Option<Ty<'tcx>> {
 646         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
 647             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
 648         });
 649
 650         let ret_ty = ret_coercion.borrow().expected_ty();
 651         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 652
 653         let get_future_output = |predicate: ty::Predicate<'tcx>, span| {
 654             // Search for a pending obligation like
 655             //
 656             // `<R as Future>::Output = T`
 657             //
 658             // where R is the return type we are expecting. This type `T`
 659             // will be our output.
 660             let bound_predicate = predicate.kind();
 661             if let ty::PredicateKind::Projection(proj_predicate) = bound_predicate.skip_binder() {
 662                 self.deduce_future_output_from_projection(
 663                     span,
 664                     bound_predicate.rebind(proj_predicate),
 665                 )
 666             } else {
 667                 None
 668             }
 669         };
 670
 671         let output_ty = match *ret_ty.kind() {
 672             ty::Infer(ty::TyVar(ret_vid)) => {
 673                 self.obligations_for_self_ty(ret_vid).find_map(|(_, obligation)| {
 674                     get_future_output(obligation.predicate, obligation.cause.span)
 675                 })?
 676             }
 677             ty::Opaque(def_id, substs) => self
 678                 .tcx
 679                 .bound_explicit_item_bounds(def_id)
 680                 .transpose_iter()
 681                 .map(|e| e.map_bound(|e| *e).transpose_tuple2())
 682                 .find_map(|(p, s)| get_future_output(p.subst(self.tcx, substs), s.0))?,
 683             ty::Error(_) => return None,
 684             ty::Projection(proj)
 685                 if self.tcx.def_kind(proj.item_def_id) == DefKind::ImplTraitPlaceholder =>
 686             {
 687                 self.tcx
 688                     .bound_explicit_item_bounds(proj.item_def_id)
 689                     .transpose_iter()
 690                     .map(|e| e.map_bound(|e| *e).transpose_tuple2())
 691                     .find_map(|(p, s)| get_future_output(p.subst(self.tcx, proj.substs), s.0))?
 692             }
 693             _ => span_bug!(
 694                 self.tcx.def_span(expr_def_id),
 695                 "async fn generator return type not an inference variable: {ret_ty}"
 696             ),
 697         };
 698
 699         // async fn that have opaque types in their return type need to redo the conversion to inference variables
 700         // as they fetch the still opaque version from the signature.
 701         let InferOk { value: output_ty, obligations } = self
 702             .replace_opaque_types_with_inference_vars(
 703                 output_ty,
 704                 body_id,
 705                 self.tcx.def_span(expr_def_id),
 706                 self.param_env,
 707             );
 708         self.register_predicates(obligations);
 709
 710         Some(output_ty)
 711     }
 712
 713     /// Given a projection like
 714     ///
 715     /// `<X as Future>::Output = T`
 716     ///
 717     /// where `X` is some type that has no late-bound regions, returns
 718     /// `Some(T)`. If the projection is for some other trait, returns
 719     /// `None`.
 720     fn deduce_future_output_from_projection(
 721         &self,
 722         cause_span: Span,
 723         predicate: ty::PolyProjectionPredicate<'tcx>,
 724     ) -> Option<Ty<'tcx>> {
 725         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 726
 727         // We do not expect any bound regions in our predicate, so
 728         // skip past the bound vars.
 729         let Some(predicate) = predicate.no_bound_vars() else {
 730             debug!("deduce_future_output_from_projection: has late-bound regions");
 731             return None;
 732         };
 733
 734         // Check that this is a projection from the `Future` trait.
 735         let trait_def_id = predicate.projection_ty.trait_def_id(self.tcx);
 736         let future_trait = self.tcx.require_lang_item(LangItem::Future, Some(cause_span));
 737         if trait_def_id != future_trait {
 738             debug!("deduce_future_output_from_projection: not a future");
 739             return None;
 740         }
 741
 742         // The `Future` trait has only one associated item, `Output`,
 743         // so check that this is what we see.
 744         let output_assoc_item = self.tcx.associated_item_def_ids(future_trait)[0];
 745         if output_assoc_item != predicate.projection_ty.item_def_id {
 746             span_bug!(
 747                 cause_span,
 748                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 749                 predicate.projection_ty.item_def_id,
 750                 output_assoc_item,
 751             );
 752         }
 753
 754         // Extract the type from the projection. Note that there can
 755         // be no bound variables in this type because the "self type"
 756         // does not have any regions in it.
 757         let output_ty = self.resolve_vars_if_possible(predicate.term);
 758         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 759         // This is a projection on a Fn trait so will always be a type.
 760         Some(output_ty.ty().unwrap())
 761     }
 762
 763     /// Converts the types that the user supplied, in case that doing
 764     /// so should yield an error, but returns back a signature where
 765     /// all parameters are of type `TyErr`.
 766     fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
 767         let astconv: &dyn AstConv<'_> = self;
 768
 769         let supplied_arguments = decl.inputs.iter().map(|a| {
 770             // Convert the types that the user supplied (if any), but ignore them.
 771             astconv.ast_ty_to_ty(a);
 772             self.tcx.ty_error()
 773         });
 774
 775         if let hir::FnRetTy::Return(ref output) = decl.output {
 776             astconv.ast_ty_to_ty(&output);
 777         }
 778
 779         let result = ty::Binder::dummy(self.tcx.mk_fn_sig(
 780             supplied_arguments,
 781             self.tcx.ty_error(),
 782             decl.c_variadic,
 783             hir::Unsafety::Normal,
 784             Abi::RustCall,
 785         ));
 786
 787         debug!("supplied_sig_of_closure: result={:?}", result);
 788
 789         result
 790     }
 791
 792     fn closure_sigs(
 793         &self,
 794         expr_def_id: DefId,
 795         body: &hir::Body<'_>,
 796         bound_sig: ty::PolyFnSig<'tcx>,
 797     ) -> ClosureSignatures<'tcx> {
 798         let liberated_sig = self.tcx().liberate_late_bound_regions(expr_def_id, bound_sig);
 799         let liberated_sig = self.inh.normalize_associated_types_in(
 800             body.value.span,
 801             body.value.hir_id,
 802             self.param_env,
 803             liberated_sig,
 804         );
 805         ClosureSignatures { bound_sig, liberated_sig }
 806     }
 807 }