src/librustc_typeck/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 rustc_hir as hir;
   7 use rustc_hir::def_id::DefId;
   8 use rustc_hir::lang_items::{FutureTraitLangItem, GeneratorTraitLangItem};
   9 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  10 use rustc_infer::infer::LateBoundRegionConversionTime;
  11 use rustc_infer::infer::{InferOk, InferResult};
  12 use rustc_middle::ty::fold::TypeFoldable;
  13 use rustc_middle::ty::subst::InternalSubsts;
  14 use rustc_middle::ty::{self, GenericParamDefKind, Ty};
  15 use rustc_span::source_map::Span;
  16 use rustc_target::spec::abi::Abi;
  17 use rustc_trait_selection::traits::error_reporting::ArgKind;
  18 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
  19 use std::cmp;
  20 use std::iter;
  21
  22 /// What signature do we *expect* the closure to have from context?
  23 #[derive(Debug)]
  24 struct ExpectedSig<'tcx> {
  25     /// Span that gave us this expectation, if we know that.
  26     cause_span: Option<Span>,
  27     sig: ty::FnSig<'tcx>,
  28 }
  29
  30 struct ClosureSignatures<'tcx> {
  31     bound_sig: ty::PolyFnSig<'tcx>,
  32     liberated_sig: ty::FnSig<'tcx>,
  33 }
  34
  35 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  36     pub fn check_expr_closure(
  37         &self,
  38         expr: &hir::Expr<'_>,
  39         _capture: hir::CaptureBy,
  40         decl: &'tcx hir::FnDecl<'tcx>,
  41         body_id: hir::BodyId,
  42         gen: Option<hir::Movability>,
  43         expected: Expectation<'tcx>,
  44     ) -> Ty<'tcx> {
  45         debug!("check_expr_closure(expr={:?},expected={:?})", expr, expected);
  46
  47         // It's always helpful for inference if we know the kind of
  48         // closure sooner rather than later, so first examine the expected
  49         // type, and see if can glean a closure kind from there.
  50         let (expected_sig, expected_kind) = match expected.to_option(self) {
  51             Some(ty) => self.deduce_expectations_from_expected_type(ty),
  52             None => (None, None),
  53         };
  54         let body = self.tcx.hir().body(body_id);
  55         self.check_closure(expr, expected_kind, decl, body, gen, expected_sig)
  56     }
  57
  58     fn check_closure(
  59         &self,
  60         expr: &hir::Expr<'_>,
  61         opt_kind: Option<ty::ClosureKind>,
  62         decl: &'tcx hir::FnDecl<'tcx>,
  63         body: &'tcx hir::Body<'tcx>,
  64         gen: Option<hir::Movability>,
  65         expected_sig: Option<ExpectedSig<'tcx>>,
  66     ) -> Ty<'tcx> {
  67         debug!("check_closure(opt_kind={:?}, expected_sig={:?})", opt_kind, expected_sig);
  68
  69         let expr_def_id = self.tcx.hir().local_def_id(expr.hir_id);
  70
  71         let ClosureSignatures { bound_sig, liberated_sig } =
  72             self.sig_of_closure(expr_def_id.to_def_id(), decl, body, expected_sig);
  73
  74         debug!("check_closure: ty_of_closure returns {:?}", liberated_sig);
  75
  76         let generator_types =
  77             check_fn(self, self.param_env, liberated_sig, decl, expr.hir_id, body, gen).1;
  78
  79         let base_substs = InternalSubsts::identity_for_item(
  80             self.tcx,
  81             self.tcx.closure_base_def_id(expr_def_id.to_def_id()),
  82         );
  83         // HACK(eddyb) this hardcodes indices into substs but it should rely on
  84         // `ClosureSubsts` and `GeneratorSubsts` providing constructors, instead.
  85         // That would also remove the need for most of the inference variables,
  86         // as they immediately unified with the actual type below, including
  87         // the `InferCtxt::closure_sig` and `ClosureSubsts::sig_ty` methods.
  88         let tupled_upvars_idx = base_substs.len() + if generator_types.is_some() { 4 } else { 2 };
  89         let substs =
  90             base_substs.extend_to(self.tcx, expr_def_id.to_def_id(), |param, _| match param.kind {
  91                 GenericParamDefKind::Lifetime => span_bug!(expr.span, "closure has lifetime param"),
  92                 GenericParamDefKind::Type { .. } => if param.index as usize == tupled_upvars_idx {
  93                     self.tcx.mk_tup(self.tcx.upvars_mentioned(expr_def_id).iter().flat_map(
  94                         |upvars| {
  95                             upvars.iter().map(|(&var_hir_id, _)| {
  96                                 // Create type variables (for now) to represent the transformed
  97                                 // types of upvars. These will be unified during the upvar
  98                                 // inference phase (`upvar.rs`).
  99                                 self.infcx.next_ty_var(TypeVariableOrigin {
 100                                     // FIXME(eddyb) distinguish upvar inference variables from the rest.
 101                                     kind: TypeVariableOriginKind::ClosureSynthetic,
 102                                     span: self.tcx.hir().span(var_hir_id),
 103                                 })
 104                             })
 105                         },
 106                     ))
 107                 } else {
 108                     // Create type variables (for now) to represent the various
 109                     // pieces of information kept in `{Closure,Generic}Substs`.
 110                     // They will either be unified below, or later during the upvar
 111                     // inference phase (`upvar.rs`)
 112                     self.infcx.next_ty_var(TypeVariableOrigin {
 113                         kind: TypeVariableOriginKind::ClosureSynthetic,
 114                         span: expr.span,
 115                     })
 116                 }
 117                 .into(),
 118                 GenericParamDefKind::Const => span_bug!(expr.span, "closure has const param"),
 119             });
 120         if let Some(GeneratorTypes { resume_ty, yield_ty, interior, movability }) = generator_types
 121         {
 122             let generator_substs = substs.as_generator();
 123             self.demand_eqtype(expr.span, resume_ty, generator_substs.resume_ty());
 124             self.demand_eqtype(expr.span, yield_ty, generator_substs.yield_ty());
 125             self.demand_eqtype(expr.span, liberated_sig.output(), generator_substs.return_ty());
 126             self.demand_eqtype(expr.span, interior, generator_substs.witness());
 127
 128             // HACK(eddyb) this forces the types equated above into `substs` but
 129             // it should rely on `GeneratorSubsts` providing a constructor, instead.
 130             let substs = self.resolve_vars_if_possible(&substs);
 131
 132             return self.tcx.mk_generator(expr_def_id.to_def_id(), substs, movability);
 133         }
 134
 135         // Tuple up the arguments and insert the resulting function type into
 136         // the `closures` table.
 137         let sig = bound_sig.map_bound(|sig| {
 138             self.tcx.mk_fn_sig(
 139                 iter::once(self.tcx.intern_tup(sig.inputs())),
 140                 sig.output(),
 141                 sig.c_variadic,
 142                 sig.unsafety,
 143                 sig.abi,
 144             )
 145         });
 146
 147         debug!(
 148             "check_closure: expr_def_id={:?}, sig={:?}, opt_kind={:?}",
 149             expr_def_id, sig, opt_kind
 150         );
 151
 152         let sig_fn_ptr_ty = self.tcx.mk_fn_ptr(sig);
 153         self.demand_eqtype(expr.span, sig_fn_ptr_ty, substs.as_closure().sig_as_fn_ptr_ty());
 154
 155         if let Some(kind) = opt_kind {
 156             self.demand_eqtype(expr.span, kind.to_ty(self.tcx), substs.as_closure().kind_ty());
 157         }
 158
 159         // HACK(eddyb) this forces the types equated above into `substs` but
 160         // it should rely on `ClosureSubsts` providing a constructor, instead.
 161         let substs = self.resolve_vars_if_possible(&substs);
 162
 163         let closure_type = self.tcx.mk_closure(expr_def_id.to_def_id(), substs);
 164
 165         debug!("check_closure: expr.hir_id={:?} closure_type={:?}", expr.hir_id, closure_type);
 166
 167         closure_type
 168     }
 169
 170     /// Given the expected type, figures out what it can about this closure we
 171     /// are about to type check:
 172     fn deduce_expectations_from_expected_type(
 173         &self,
 174         expected_ty: Ty<'tcx>,
 175     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 176         debug!("deduce_expectations_from_expected_type(expected_ty={:?})", expected_ty);
 177
 178         match expected_ty.kind {
 179             ty::Dynamic(ref object_type, ..) => {
 180                 let sig = object_type.projection_bounds().find_map(|pb| {
 181                     let pb = pb.with_self_ty(self.tcx, self.tcx.types.trait_object_dummy_self);
 182                     self.deduce_sig_from_projection(None, pb)
 183                 });
 184                 let kind = object_type
 185                     .principal_def_id()
 186                     .and_then(|did| self.tcx.fn_trait_kind_from_lang_item(did));
 187                 (sig, kind)
 188             }
 189             ty::Infer(ty::TyVar(vid)) => self.deduce_expectations_from_obligations(vid),
 190             ty::FnPtr(sig) => {
 191                 let expected_sig = ExpectedSig { cause_span: None, sig: sig.skip_binder() };
 192                 (Some(expected_sig), Some(ty::ClosureKind::Fn))
 193             }
 194             _ => (None, None),
 195         }
 196     }
 197
 198     fn deduce_expectations_from_obligations(
 199         &self,
 200         expected_vid: ty::TyVid,
 201     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 202         let expected_sig =
 203             self.obligations_for_self_ty(expected_vid).find_map(|(_, obligation)| {
 204                 debug!(
 205                     "deduce_expectations_from_obligations: obligation.predicate={:?}",
 206                     obligation.predicate
 207                 );
 208
 209                 if let &ty::PredicateKind::Projection(proj_predicate) = obligation.predicate.kind()
 210                 {
 211                     // Given a Projection predicate, we can potentially infer
 212                     // the complete signature.
 213                     self.deduce_sig_from_projection(Some(obligation.cause.span), proj_predicate)
 214                 } else {
 215                     None
 216                 }
 217             });
 218
 219         // Even if we can't infer the full signature, we may be able to
 220         // infer the kind. This can occur if there is a trait-reference
 221         // like `F : Fn<A>`. Note that due to subtyping we could encounter
 222         // many viable options, so pick the most restrictive.
 223         let expected_kind = self
 224             .obligations_for_self_ty(expected_vid)
 225             .filter_map(|(tr, _)| self.tcx.fn_trait_kind_from_lang_item(tr.def_id()))
 226             .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
 227
 228         (expected_sig, expected_kind)
 229     }
 230
 231     /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
 232     /// everything we need to know about a closure or generator.
 233     ///
 234     /// The `cause_span` should be the span that caused us to
 235     /// have this expected signature, or `None` if we can't readily
 236     /// know that.
 237     fn deduce_sig_from_projection(
 238         &self,
 239         cause_span: Option<Span>,
 240         projection: ty::PolyProjectionPredicate<'tcx>,
 241     ) -> Option<ExpectedSig<'tcx>> {
 242         let tcx = self.tcx;
 243
 244         debug!("deduce_sig_from_projection({:?})", projection);
 245
 246         let trait_ref = projection.to_poly_trait_ref(tcx);
 247
 248         let is_fn = tcx.fn_trait_kind_from_lang_item(trait_ref.def_id()).is_some();
 249         let gen_trait = tcx.require_lang_item(GeneratorTraitLangItem, cause_span);
 250         let is_gen = gen_trait == trait_ref.def_id();
 251         if !is_fn && !is_gen {
 252             debug!("deduce_sig_from_projection: not fn or generator");
 253             return None;
 254         }
 255
 256         if is_gen {
 257             // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
 258             // associated item and not yield.
 259             let return_assoc_item =
 260                 self.tcx.associated_items(gen_trait).in_definition_order().nth(1).unwrap().def_id;
 261             if return_assoc_item != projection.projection_def_id() {
 262                 debug!("deduce_sig_from_projection: not return assoc item of generator");
 263                 return None;
 264             }
 265         }
 266
 267         let input_tys = if is_fn {
 268             let arg_param_ty = trait_ref.skip_binder().substs.type_at(1);
 269             let arg_param_ty = self.resolve_vars_if_possible(&arg_param_ty);
 270             debug!("deduce_sig_from_projection: arg_param_ty={:?}", arg_param_ty);
 271
 272             match arg_param_ty.kind {
 273                 ty::Tuple(tys) => tys.into_iter().map(|k| k.expect_ty()).collect::<Vec<_>>(),
 274                 _ => return None,
 275             }
 276         } else {
 277             // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
 278             // else they must have exactly 1 argument. For now though, just give up in this case.
 279             return None;
 280         };
 281
 282         let ret_param_ty = projection.skip_binder().ty;
 283         let ret_param_ty = self.resolve_vars_if_possible(&ret_param_ty);
 284         debug!("deduce_sig_from_projection: ret_param_ty={:?}", ret_param_ty);
 285
 286         let sig = self.tcx.mk_fn_sig(
 287             input_tys.iter(),
 288             &ret_param_ty,
 289             false,
 290             hir::Unsafety::Normal,
 291             Abi::Rust,
 292         );
 293         debug!("deduce_sig_from_projection: sig={:?}", sig);
 294
 295         Some(ExpectedSig { cause_span, sig })
 296     }
 297
 298     fn sig_of_closure(
 299         &self,
 300         expr_def_id: DefId,
 301         decl: &hir::FnDecl<'_>,
 302         body: &hir::Body<'_>,
 303         expected_sig: Option<ExpectedSig<'tcx>>,
 304     ) -> ClosureSignatures<'tcx> {
 305         if let Some(e) = expected_sig {
 306             self.sig_of_closure_with_expectation(expr_def_id, decl, body, e)
 307         } else {
 308             self.sig_of_closure_no_expectation(expr_def_id, decl, body)
 309         }
 310     }
 311
 312     /// If there is no expected signature, then we will convert the
 313     /// types that the user gave into a signature.
 314     fn sig_of_closure_no_expectation(
 315         &self,
 316         expr_def_id: DefId,
 317         decl: &hir::FnDecl<'_>,
 318         body: &hir::Body<'_>,
 319     ) -> ClosureSignatures<'tcx> {
 320         debug!("sig_of_closure_no_expectation()");
 321
 322         let bound_sig = self.supplied_sig_of_closure(expr_def_id, decl, body);
 323
 324         self.closure_sigs(expr_def_id, body, bound_sig)
 325     }
 326
 327     /// Invoked to compute the signature of a closure expression. This
 328     /// combines any user-provided type annotations (e.g., `|x: u32|
 329     /// -> u32 { .. }`) with the expected signature.
 330     ///
 331     /// The approach is as follows:
 332     ///
 333     /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
 334     /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
 335     ///   - If we have no expectation `E`, then the signature of the closure is `S`.
 336     ///   - Otherwise, the signature of the closure is E. Moreover:
 337     ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
 338     ///     - Instantiate all the late-bound regions bound in the closure within `S`
 339     ///       with fresh (existential) variables, yielding `S'`
 340     ///     - Require that `E' = S'`
 341     ///       - We could use some kind of subtyping relationship here,
 342     ///         I imagine, but equality is easier and works fine for
 343     ///         our purposes.
 344     ///
 345     /// The key intuition here is that the user's types must be valid
 346     /// from "the inside" of the closure, but the expectation
 347     /// ultimately drives the overall signature.
 348     ///
 349     /// # Examples
 350     ///
 351     /// ```
 352     /// fn with_closure<F>(_: F)
 353     ///   where F: Fn(&u32) -> &u32 { .. }
 354     ///
 355     /// with_closure(|x: &u32| { ... })
 356     /// ```
 357     ///
 358     /// Here:
 359     /// - E would be `fn(&u32) -> &u32`.
 360     /// - S would be `fn(&u32) ->
 361     /// - E' is `&'!0 u32 -> &'!0 u32`
 362     /// - S' is `&'?0 u32 -> ?T`
 363     ///
 364     /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
 365     ///
 366     /// # Arguments
 367     ///
 368     /// - `expr_def_id`: the `DefId` of the closure expression
 369     /// - `decl`: the HIR declaration of the closure
 370     /// - `body`: the body of the closure
 371     /// - `expected_sig`: the expected signature (if any). Note that
 372     ///   this is missing a binder: that is, there may be late-bound
 373     ///   regions with depth 1, which are bound then by the closure.
 374     fn sig_of_closure_with_expectation(
 375         &self,
 376         expr_def_id: DefId,
 377         decl: &hir::FnDecl<'_>,
 378         body: &hir::Body<'_>,
 379         expected_sig: ExpectedSig<'tcx>,
 380     ) -> ClosureSignatures<'tcx> {
 381         debug!("sig_of_closure_with_expectation(expected_sig={:?})", expected_sig);
 382
 383         // Watch out for some surprises and just ignore the
 384         // expectation if things don't see to match up with what we
 385         // expect.
 386         if expected_sig.sig.c_variadic != decl.c_variadic {
 387             return self.sig_of_closure_no_expectation(expr_def_id, decl, body);
 388         } else if expected_sig.sig.inputs_and_output.len() != decl.inputs.len() + 1 {
 389             return self.sig_of_closure_with_mismatched_number_of_arguments(
 390                 expr_def_id,
 391                 decl,
 392                 body,
 393                 expected_sig,
 394             );
 395         }
 396
 397         // Create a `PolyFnSig`. Note the oddity that late bound
 398         // regions appearing free in `expected_sig` are now bound up
 399         // in this binder we are creating.
 400         assert!(!expected_sig.sig.has_vars_bound_above(ty::INNERMOST));
 401         let bound_sig = ty::Binder::bind(self.tcx.mk_fn_sig(
 402             expected_sig.sig.inputs().iter().cloned(),
 403             expected_sig.sig.output(),
 404             decl.c_variadic,
 405             hir::Unsafety::Normal,
 406             Abi::RustCall,
 407         ));
 408
 409         // `deduce_expectations_from_expected_type` introduces
 410         // late-bound lifetimes defined elsewhere, which we now
 411         // anonymize away, so as not to confuse the user.
 412         let bound_sig = self.tcx.anonymize_late_bound_regions(&bound_sig);
 413
 414         let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
 415
 416         // Up till this point, we have ignored the annotations that the user
 417         // gave. This function will check that they unify successfully.
 418         // Along the way, it also writes out entries for types that the user
 419         // wrote into our typeck results, which are then later used by the privacy
 420         // check.
 421         match self.check_supplied_sig_against_expectation(expr_def_id, decl, body, &closure_sigs) {
 422             Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
 423             Err(_) => return self.sig_of_closure_no_expectation(expr_def_id, decl, body),
 424         }
 425
 426         closure_sigs
 427     }
 428
 429     fn sig_of_closure_with_mismatched_number_of_arguments(
 430         &self,
 431         expr_def_id: DefId,
 432         decl: &hir::FnDecl<'_>,
 433         body: &hir::Body<'_>,
 434         expected_sig: ExpectedSig<'tcx>,
 435     ) -> ClosureSignatures<'tcx> {
 436         let hir = self.tcx.hir();
 437         let expr_map_node = hir.get_if_local(expr_def_id).unwrap();
 438         let expected_args: Vec<_> = expected_sig
 439             .sig
 440             .inputs()
 441             .iter()
 442             .map(|ty| ArgKind::from_expected_ty(ty, None))
 443             .collect();
 444         let (closure_span, found_args) = match self.get_fn_like_arguments(expr_map_node) {
 445             Some((sp, args)) => (Some(sp), args),
 446             None => (None, Vec::new()),
 447         };
 448         let expected_span =
 449             expected_sig.cause_span.unwrap_or_else(|| hir.span_if_local(expr_def_id).unwrap());
 450         self.report_arg_count_mismatch(
 451             expected_span,
 452             closure_span,
 453             expected_args,
 454             found_args,
 455             true,
 456         )
 457         .emit();
 458
 459         let error_sig = self.error_sig_of_closure(decl);
 460
 461         self.closure_sigs(expr_def_id, body, error_sig)
 462     }
 463
 464     /// Enforce the user's types against the expectation. See
 465     /// `sig_of_closure_with_expectation` for details on the overall
 466     /// strategy.
 467     fn check_supplied_sig_against_expectation(
 468         &self,
 469         expr_def_id: DefId,
 470         decl: &hir::FnDecl<'_>,
 471         body: &hir::Body<'_>,
 472         expected_sigs: &ClosureSignatures<'tcx>,
 473     ) -> InferResult<'tcx, ()> {
 474         // Get the signature S that the user gave.
 475         //
 476         // (See comment on `sig_of_closure_with_expectation` for the
 477         // meaning of these letters.)
 478         let supplied_sig = self.supplied_sig_of_closure(expr_def_id, decl, body);
 479
 480         debug!("check_supplied_sig_against_expectation: supplied_sig={:?}", supplied_sig);
 481
 482         // FIXME(#45727): As discussed in [this comment][c1], naively
 483         // forcing equality here actually results in suboptimal error
 484         // messages in some cases.  For now, if there would have been
 485         // an obvious error, we fallback to declaring the type of the
 486         // closure to be the one the user gave, which allows other
 487         // error message code to trigger.
 488         //
 489         // However, I think [there is potential to do even better
 490         // here][c2], since in *this* code we have the precise span of
 491         // the type parameter in question in hand when we report the
 492         // error.
 493         //
 494         // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
 495         // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
 496         self.infcx.commit_if_ok(|_| {
 497             let mut all_obligations = vec![];
 498
 499             // The liberated version of this signature should be a subtype
 500             // of the liberated form of the expectation.
 501             for ((hir_ty, &supplied_ty), expected_ty) in decl
 502                 .inputs
 503                 .iter()
 504                 .zip(supplied_sig.inputs().skip_binder()) // binder moved to (*) below
 505                 .zip(expected_sigs.liberated_sig.inputs())
 506             // `liberated_sig` is E'.
 507             {
 508                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 509                 let (supplied_ty, _) = self.infcx.replace_bound_vars_with_fresh_vars(
 510                     hir_ty.span,
 511                     LateBoundRegionConversionTime::FnCall,
 512                     &ty::Binder::bind(supplied_ty),
 513                 ); // recreated from (*) above
 514
 515                 // Check that E' = S'.
 516                 let cause = self.misc(hir_ty.span);
 517                 let InferOk { value: (), obligations } =
 518                     self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
 519                 all_obligations.extend(obligations);
 520             }
 521
 522             let (supplied_output_ty, _) = self.infcx.replace_bound_vars_with_fresh_vars(
 523                 decl.output.span(),
 524                 LateBoundRegionConversionTime::FnCall,
 525                 &supplied_sig.output(),
 526             );
 527             let cause = &self.misc(decl.output.span());
 528             let InferOk { value: (), obligations } = self
 529                 .at(cause, self.param_env)
 530                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 531             all_obligations.extend(obligations);
 532
 533             Ok(InferOk { value: (), obligations: all_obligations })
 534         })
 535     }
 536
 537     /// If there is no expected signature, then we will convert the
 538     /// types that the user gave into a signature.
 539     ///
 540     /// Also, record this closure signature for later.
 541     fn supplied_sig_of_closure(
 542         &self,
 543         expr_def_id: DefId,
 544         decl: &hir::FnDecl<'_>,
 545         body: &hir::Body<'_>,
 546     ) -> ty::PolyFnSig<'tcx> {
 547         let astconv: &dyn AstConv<'_> = self;
 548
 549         debug!(
 550             "supplied_sig_of_closure(decl={:?}, body.generator_kind={:?})",
 551             decl, body.generator_kind,
 552         );
 553
 554         // First, convert the types that the user supplied (if any).
 555         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 556         let supplied_return = match decl.output {
 557             hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
 558             hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
 559                 // In the case of the async block that we create for a function body,
 560                 // we expect the return type of the block to match that of the enclosing
 561                 // function.
 562                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 563                     debug!("supplied_sig_of_closure: closure is async fn body");
 564                     self.deduce_future_output_from_obligations(expr_def_id).unwrap_or_else(|| {
 565                         // AFAIK, deducing the future output
 566                         // always succeeds *except* in error cases
 567                         // like #65159. I'd like to return Error
 568                         // here, but I can't because I can't
 569                         // easily (and locally) prove that we
 570                         // *have* reported an
 571                         // error. --nikomatsakis
 572                         astconv.ty_infer(None, decl.output.span())
 573                     })
 574                 }
 575
 576                 _ => astconv.ty_infer(None, decl.output.span()),
 577             },
 578         };
 579
 580         let result = ty::Binder::bind(self.tcx.mk_fn_sig(
 581             supplied_arguments,
 582             supplied_return,
 583             decl.c_variadic,
 584             hir::Unsafety::Normal,
 585             Abi::RustCall,
 586         ));
 587
 588         debug!("supplied_sig_of_closure: result={:?}", result);
 589
 590         let c_result = self.inh.infcx.canonicalize_response(&result);
 591         self.typeck_results.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);
 592
 593         result
 594     }
 595
 596     /// Invoked when we are translating the generator that results
 597     /// from desugaring an `async fn`. Returns the "sugared" return
 598     /// type of the `async fn` -- that is, the return type that the
 599     /// user specified. The "desugared" return type is a `impl
 600     /// Future<Output = T>`, so we do this by searching through the
 601     /// obligations to extract the `T`.
 602     fn deduce_future_output_from_obligations(&self, expr_def_id: DefId) -> Option<Ty<'tcx>> {
 603         debug!("deduce_future_output_from_obligations(expr_def_id={:?})", expr_def_id);
 604
 605         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
 606             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
 607         });
 608
 609         // In practice, the return type of the surrounding function is
 610         // always a (not yet resolved) inference variable, because it
 611         // is the hidden type for an `impl Trait` that we are going to
 612         // be inferring.
 613         let ret_ty = ret_coercion.borrow().expected_ty();
 614         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 615         let ret_vid = match ret_ty.kind {
 616             ty::Infer(ty::TyVar(ret_vid)) => ret_vid,
 617             _ => span_bug!(
 618                 self.tcx.def_span(expr_def_id),
 619                 "async fn generator return type not an inference variable"
 620             ),
 621         };
 622
 623         // Search for a pending obligation like
 624         //
 625         // `<R as Future>::Output = T`
 626         //
 627         // where R is the return type we are expecting. This type `T`
 628         // will be our output.
 629         let output_ty = self.obligations_for_self_ty(ret_vid).find_map(|(_, obligation)| {
 630             if let &ty::PredicateKind::Projection(proj_predicate) = obligation.predicate.kind() {
 631                 self.deduce_future_output_from_projection(obligation.cause.span, proj_predicate)
 632             } else {
 633                 None
 634             }
 635         });
 636
 637         debug!("deduce_future_output_from_obligations: output_ty={:?}", output_ty);
 638         output_ty
 639     }
 640
 641     /// Given a projection like
 642     ///
 643     /// `<X as Future>::Output = T`
 644     ///
 645     /// where `X` is some type that has no late-bound regions, returns
 646     /// `Some(T)`. If the projection is for some other trait, returns
 647     /// `None`.
 648     fn deduce_future_output_from_projection(
 649         &self,
 650         cause_span: Span,
 651         predicate: ty::PolyProjectionPredicate<'tcx>,
 652     ) -> Option<Ty<'tcx>> {
 653         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 654
 655         // We do not expect any bound regions in our predicate, so
 656         // skip past the bound vars.
 657         let predicate = match predicate.no_bound_vars() {
 658             Some(p) => p,
 659             None => {
 660                 debug!("deduce_future_output_from_projection: has late-bound regions");
 661                 return None;
 662             }
 663         };
 664
 665         // Check that this is a projection from the `Future` trait.
 666         let trait_ref = predicate.projection_ty.trait_ref(self.tcx);
 667         let future_trait = self.tcx.require_lang_item(FutureTraitLangItem, Some(cause_span));
 668         if trait_ref.def_id != future_trait {
 669             debug!("deduce_future_output_from_projection: not a future");
 670             return None;
 671         }
 672
 673         // The `Future` trait has only one associted item, `Output`,
 674         // so check that this is what we see.
 675         let output_assoc_item =
 676             self.tcx.associated_items(future_trait).in_definition_order().next().unwrap().def_id;
 677         if output_assoc_item != predicate.projection_ty.item_def_id {
 678             span_bug!(
 679                 cause_span,
 680                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 681                 predicate.projection_ty.item_def_id,
 682                 output_assoc_item,
 683             );
 684         }
 685
 686         // Extract the type from the projection. Note that there can
 687         // be no bound variables in this type because the "self type"
 688         // does not have any regions in it.
 689         let output_ty = self.resolve_vars_if_possible(&predicate.ty);
 690         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 691         Some(output_ty)
 692     }
 693
 694     /// Converts the types that the user supplied, in case that doing
 695     /// so should yield an error, but returns back a signature where
 696     /// all parameters are of type `TyErr`.
 697     fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
 698         let astconv: &dyn AstConv<'_> = self;
 699
 700         let supplied_arguments = decl.inputs.iter().map(|a| {
 701             // Convert the types that the user supplied (if any), but ignore them.
 702             astconv.ast_ty_to_ty(a);
 703             self.tcx.ty_error()
 704         });
 705
 706         if let hir::FnRetTy::Return(ref output) = decl.output {
 707             astconv.ast_ty_to_ty(&output);
 708         }
 709
 710         let result = ty::Binder::bind(self.tcx.mk_fn_sig(
 711             supplied_arguments,
 712             self.tcx.ty_error(),
 713             decl.c_variadic,
 714             hir::Unsafety::Normal,
 715             Abi::RustCall,
 716         ));
 717
 718         debug!("supplied_sig_of_closure: result={:?}", result);
 719
 720         result
 721     }
 722
 723     fn closure_sigs(
 724         &self,
 725         expr_def_id: DefId,
 726         body: &hir::Body<'_>,
 727         bound_sig: ty::PolyFnSig<'tcx>,
 728     ) -> ClosureSignatures<'tcx> {
 729         let liberated_sig = self.tcx().liberate_late_bound_regions(expr_def_id, &bound_sig);
 730         let liberated_sig = self.inh.normalize_associated_types_in(
 731             body.value.span,
 732             body.value.hir_id,
 733             self.param_env,
 734             &liberated_sig,
 735         );
 736         ClosureSignatures { bound_sig, liberated_sig }
 737     }
 738 }