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 crate::middle::region;
   7 use rustc_hir as hir;
   8 use rustc_hir::def_id::DefId;
   9 use rustc_hir::lang_items::{FutureTraitLangItem, GeneratorTraitLangItem};
  10 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  11 use rustc_infer::infer::LateBoundRegionConversionTime;
  12 use rustc_infer::infer::{InferOk, InferResult};
  13 use rustc_middle::ty::fold::TypeFoldable;
  14 use rustc_middle::ty::subst::InternalSubsts;
  15 use rustc_middle::ty::{self, GenericParamDefKind, Ty};
  16 use rustc_span::source_map::Span;
  17 use rustc_target::spec::abi::Abi;
  18 use rustc_trait_selection::traits::error_reporting::ArgKind;
  19 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
  20 use rustc_trait_selection::traits::Obligation;
  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::FnSig<'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     pub fn check_expr_closure(
  39         &self,
  40         expr: &hir::Expr<'_>,
  41         _capture: hir::CaptureBy,
  42         decl: &'tcx hir::FnDecl<'tcx>,
  43         body_id: hir::BodyId,
  44         gen: Option<hir::Movability>,
  45         expected: Expectation<'tcx>,
  46     ) -> Ty<'tcx> {
  47         debug!("check_expr_closure(expr={:?},expected={:?})", expr, expected);
  48
  49         // It's always helpful for inference if we know the kind of
  50         // closure sooner rather than later, so first examine the expected
  51         // type, and see if can glean a closure kind from there.
  52         let (expected_sig, expected_kind) = match expected.to_option(self) {
  53             Some(ty) => self.deduce_expectations_from_expected_type(ty),
  54             None => (None, None),
  55         };
  56         let body = self.tcx.hir().body(body_id);
  57         self.check_closure(expr, expected_kind, decl, body, gen, expected_sig)
  58     }
  59
  60     fn check_closure(
  61         &self,
  62         expr: &hir::Expr<'_>,
  63         opt_kind: Option<ty::ClosureKind>,
  64         decl: &'tcx hir::FnDecl<'tcx>,
  65         body: &'tcx hir::Body<'tcx>,
  66         gen: Option<hir::Movability>,
  67         expected_sig: Option<ExpectedSig<'tcx>>,
  68     ) -> Ty<'tcx> {
  69         debug!("check_closure(opt_kind={:?}, expected_sig={:?})", opt_kind, expected_sig);
  70
  71         let expr_def_id = self.tcx.hir().local_def_id(expr.hir_id);
  72
  73         let ClosureSignatures { bound_sig, liberated_sig } =
  74             self.sig_of_closure(expr_def_id.to_def_id(), decl, body, expected_sig);
  75
  76         debug!("check_closure: ty_of_closure returns {:?}", liberated_sig);
  77
  78         let generator_types =
  79             check_fn(self, self.param_env, liberated_sig, decl, expr.hir_id, body, gen).1;
  80
  81         let base_substs = InternalSubsts::identity_for_item(
  82             self.tcx,
  83             self.tcx.closure_base_def_id(expr_def_id.to_def_id()),
  84         );
  85         // HACK(eddyb) this hardcodes indices into substs but it should rely on
  86         // `ClosureSubsts` and `GeneratorSubsts` providing constructors, instead.
  87         // That would also remove the need for most of the inference variables,
  88         // as they immediately unified with the actual type below, including
  89         // the `InferCtxt::closure_sig` and `ClosureSubsts::sig_ty` methods.
  90         let tupled_upvars_idx = base_substs.len() + if generator_types.is_some() { 4 } else { 2 };
  91         let substs =
  92             base_substs.extend_to(self.tcx, expr_def_id.to_def_id(), |param, _| match param.kind {
  93                 GenericParamDefKind::Lifetime => span_bug!(expr.span, "closure has lifetime param"),
  94                 GenericParamDefKind::Type { .. } => if param.index as usize == tupled_upvars_idx {
  95                     self.tcx.mk_tup(self.tcx.upvars(expr_def_id).iter().flat_map(|upvars| {
  96                         upvars.iter().map(|(&var_hir_id, _)| {
  97                             // Create type variables (for now) to represent the transformed
  98                             // types of upvars. These will be unified during the upvar
  99                             // inference phase (`upvar.rs`).
 100                             self.infcx.next_ty_var(TypeVariableOrigin {
 101                                 // FIXME(eddyb) distinguish upvar inference variables from the rest.
 102                                 kind: TypeVariableOriginKind::ClosureSynthetic,
 103                                 span: self.tcx.hir().span(var_hir_id),
 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(ref proj_predicate) = obligation.predicate {
 210                     // Given a Projection predicate, we can potentially infer
 211                     // the complete signature.
 212                     self.deduce_sig_from_projection(Some(obligation.cause.span), proj_predicate)
 213                 } else {
 214                     None
 215                 }
 216             });
 217
 218         // Even if we can't infer the full signature, we may be able to
 219         // infer the kind. This can occur if there is a trait-reference
 220         // like `F : Fn<A>`. Note that due to subtyping we could encounter
 221         // many viable options, so pick the most restrictive.
 222         let expected_kind = self
 223             .obligations_for_self_ty(expected_vid)
 224             .filter_map(|(tr, _)| self.tcx.fn_trait_kind_from_lang_item(tr.def_id()))
 225             .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
 226
 227         (expected_sig, expected_kind)
 228     }
 229
 230     /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
 231     /// everything we need to know about a closure or generator.
 232     ///
 233     /// The `cause_span` should be the span that caused us to
 234     /// have this expected signature, or `None` if we can't readily
 235     /// know that.
 236     fn deduce_sig_from_projection(
 237         &self,
 238         cause_span: Option<Span>,
 239         projection: &ty::PolyProjectionPredicate<'tcx>,
 240     ) -> Option<ExpectedSig<'tcx>> {
 241         let tcx = self.tcx;
 242
 243         debug!("deduce_sig_from_projection({:?})", projection);
 244
 245         let trait_ref = projection.to_poly_trait_ref(tcx);
 246
 247         let is_fn = tcx.fn_trait_kind_from_lang_item(trait_ref.def_id()).is_some();
 248         let gen_trait = tcx.require_lang_item(GeneratorTraitLangItem, cause_span);
 249         let is_gen = gen_trait == trait_ref.def_id();
 250         if !is_fn && !is_gen {
 251             debug!("deduce_sig_from_projection: not fn or generator");
 252             return None;
 253         }
 254
 255         if is_gen {
 256             // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
 257             // associated item and not yield.
 258             let return_assoc_item =
 259                 self.tcx.associated_items(gen_trait).in_definition_order().nth(1).unwrap().def_id;
 260             if return_assoc_item != projection.projection_def_id() {
 261                 debug!("deduce_sig_from_projection: not return assoc item of generator");
 262                 return None;
 263             }
 264         }
 265
 266         let input_tys = if is_fn {
 267             let arg_param_ty = trait_ref.skip_binder().substs.type_at(1);
 268             let arg_param_ty = self.resolve_vars_if_possible(&arg_param_ty);
 269             debug!("deduce_sig_from_projection: arg_param_ty={:?}", arg_param_ty);
 270
 271             match arg_param_ty.kind {
 272                 ty::Tuple(tys) => tys.into_iter().map(|k| k.expect_ty()).collect::<Vec<_>>(),
 273                 _ => return None,
 274             }
 275         } else {
 276             // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
 277             // else they must have exactly 1 argument. For now though, just give up in this case.
 278             return None;
 279         };
 280
 281         let ret_param_ty = projection.skip_binder().ty;
 282         let ret_param_ty = self.resolve_vars_if_possible(&ret_param_ty);
 283         debug!("deduce_sig_from_projection: ret_param_ty={:?}", ret_param_ty);
 284
 285         let sig = self.tcx.mk_fn_sig(
 286             input_tys.iter(),
 287             &ret_param_ty,
 288             false,
 289             hir::Unsafety::Normal,
 290             Abi::Rust,
 291         );
 292         debug!("deduce_sig_from_projection: sig={:?}", sig);
 293
 294         Some(ExpectedSig { cause_span, sig })
 295     }
 296
 297     fn sig_of_closure(
 298         &self,
 299         expr_def_id: DefId,
 300         decl: &hir::FnDecl<'_>,
 301         body: &hir::Body<'_>,
 302         expected_sig: Option<ExpectedSig<'tcx>>,
 303     ) -> ClosureSignatures<'tcx> {
 304         if let Some(e) = expected_sig {
 305             self.sig_of_closure_with_expectation(expr_def_id, decl, body, e)
 306         } else {
 307             self.sig_of_closure_no_expectation(expr_def_id, decl, body)
 308         }
 309     }
 310
 311     /// If there is no expected signature, then we will convert the
 312     /// types that the user gave into a signature.
 313     fn sig_of_closure_no_expectation(
 314         &self,
 315         expr_def_id: DefId,
 316         decl: &hir::FnDecl<'_>,
 317         body: &hir::Body<'_>,
 318     ) -> ClosureSignatures<'tcx> {
 319         debug!("sig_of_closure_no_expectation()");
 320
 321         let bound_sig = self.supplied_sig_of_closure(expr_def_id, decl, body);
 322
 323         self.closure_sigs(expr_def_id, body, bound_sig)
 324     }
 325
 326     /// Invoked to compute the signature of a closure expression. This
 327     /// combines any user-provided type annotations (e.g., `|x: u32|
 328     /// -> u32 { .. }`) with the expected signature.
 329     ///
 330     /// The approach is as follows:
 331     ///
 332     /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
 333     /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
 334     ///   - If we have no expectation `E`, then the signature of the closure is `S`.
 335     ///   - Otherwise, the signature of the closure is E. Moreover:
 336     ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
 337     ///     - Instantiate all the late-bound regions bound in the closure within `S`
 338     ///       with fresh (existential) variables, yielding `S'`
 339     ///     - Require that `E' = S'`
 340     ///       - We could use some kind of subtyping relationship here,
 341     ///         I imagine, but equality is easier and works fine for
 342     ///         our purposes.
 343     ///
 344     /// The key intuition here is that the user's types must be valid
 345     /// from "the inside" of the closure, but the expectation
 346     /// ultimately drives the overall signature.
 347     ///
 348     /// # Examples
 349     ///
 350     /// ```
 351     /// fn with_closure<F>(_: F)
 352     ///   where F: Fn(&u32) -> &u32 { .. }
 353     ///
 354     /// with_closure(|x: &u32| { ... })
 355     /// ```
 356     ///
 357     /// Here:
 358     /// - E would be `fn(&u32) -> &u32`.
 359     /// - S would be `fn(&u32) ->
 360     /// - E' is `&'!0 u32 -> &'!0 u32`
 361     /// - S' is `&'?0 u32 -> ?T`
 362     ///
 363     /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
 364     ///
 365     /// # Arguments
 366     ///
 367     /// - `expr_def_id`: the `DefId` of the closure expression
 368     /// - `decl`: the HIR declaration of the closure
 369     /// - `body`: the body of the closure
 370     /// - `expected_sig`: the expected signature (if any). Note that
 371     ///   this is missing a binder: that is, there may be late-bound
 372     ///   regions with depth 1, which are bound then by the closure.
 373     fn sig_of_closure_with_expectation(
 374         &self,
 375         expr_def_id: DefId,
 376         decl: &hir::FnDecl<'_>,
 377         body: &hir::Body<'_>,
 378         expected_sig: ExpectedSig<'tcx>,
 379     ) -> ClosureSignatures<'tcx> {
 380         debug!("sig_of_closure_with_expectation(expected_sig={:?})", expected_sig);
 381
 382         // Watch out for some surprises and just ignore the
 383         // expectation if things don't see to match up with what we
 384         // expect.
 385         if expected_sig.sig.c_variadic != decl.c_variadic {
 386             return self.sig_of_closure_no_expectation(expr_def_id, decl, body);
 387         } else if expected_sig.sig.inputs_and_output.len() != decl.inputs.len() + 1 {
 388             return self.sig_of_closure_with_mismatched_number_of_arguments(
 389                 expr_def_id,
 390                 decl,
 391                 body,
 392                 expected_sig,
 393             );
 394         }
 395
 396         // Create a `PolyFnSig`. Note the oddity that late bound
 397         // regions appearing free in `expected_sig` are now bound up
 398         // in this binder we are creating.
 399         assert!(!expected_sig.sig.has_vars_bound_above(ty::INNERMOST));
 400         let bound_sig = ty::Binder::bind(self.tcx.mk_fn_sig(
 401             expected_sig.sig.inputs().iter().cloned(),
 402             expected_sig.sig.output(),
 403             decl.c_variadic,
 404             hir::Unsafety::Normal,
 405             Abi::RustCall,
 406         ));
 407
 408         // `deduce_expectations_from_expected_type` introduces
 409         // late-bound lifetimes defined elsewhere, which we now
 410         // anonymize away, so as not to confuse the user.
 411         let bound_sig = self.tcx.anonymize_late_bound_regions(&bound_sig);
 412
 413         let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
 414
 415         // Up till this point, we have ignored the annotations that the user
 416         // gave. This function will check that they unify successfully.
 417         // Along the way, it also writes out entries for types that the user
 418         // wrote into our tables, which are then later used by the privacy
 419         // check.
 420         match self.check_supplied_sig_against_expectation(expr_def_id, decl, body, &closure_sigs) {
 421             Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
 422             Err(_) => return self.sig_of_closure_no_expectation(expr_def_id, decl, body),
 423         }
 424
 425         closure_sigs
 426     }
 427
 428     fn sig_of_closure_with_mismatched_number_of_arguments(
 429         &self,
 430         expr_def_id: DefId,
 431         decl: &hir::FnDecl<'_>,
 432         body: &hir::Body<'_>,
 433         expected_sig: ExpectedSig<'tcx>,
 434     ) -> ClosureSignatures<'tcx> {
 435         let hir = self.tcx.hir();
 436         let expr_map_node = hir.get_if_local(expr_def_id).unwrap();
 437         let expected_args: Vec<_> = expected_sig
 438             .sig
 439             .inputs()
 440             .iter()
 441             .map(|ty| ArgKind::from_expected_ty(ty, None))
 442             .collect();
 443         let (closure_span, found_args) = match self.get_fn_like_arguments(expr_map_node) {
 444             Some((sp, args)) => (Some(sp), args),
 445             None => (None, Vec::new()),
 446         };
 447         let expected_span =
 448             expected_sig.cause_span.unwrap_or_else(|| hir.span_if_local(expr_def_id).unwrap());
 449         self.report_arg_count_mismatch(
 450             expected_span,
 451             closure_span,
 452             expected_args,
 453             found_args,
 454             true,
 455         )
 456         .emit();
 457
 458         let error_sig = self.error_sig_of_closure(decl);
 459
 460         self.closure_sigs(expr_def_id, body, error_sig)
 461     }
 462
 463     /// Enforce the user's types against the expectation. See
 464     /// `sig_of_closure_with_expectation` for details on the overall
 465     /// strategy.
 466     fn check_supplied_sig_against_expectation(
 467         &self,
 468         expr_def_id: DefId,
 469         decl: &hir::FnDecl<'_>,
 470         body: &hir::Body<'_>,
 471         expected_sigs: &ClosureSignatures<'tcx>,
 472     ) -> InferResult<'tcx, ()> {
 473         // Get the signature S that the user gave.
 474         //
 475         // (See comment on `sig_of_closure_with_expectation` for the
 476         // meaning of these letters.)
 477         let supplied_sig = self.supplied_sig_of_closure(expr_def_id, decl, body);
 478
 479         debug!("check_supplied_sig_against_expectation: supplied_sig={:?}", supplied_sig);
 480
 481         // FIXME(#45727): As discussed in [this comment][c1], naively
 482         // forcing equality here actually results in suboptimal error
 483         // messages in some cases.  For now, if there would have been
 484         // an obvious error, we fallback to declaring the type of the
 485         // closure to be the one the user gave, which allows other
 486         // error message code to trigger.
 487         //
 488         // However, I think [there is potential to do even better
 489         // here][c2], since in *this* code we have the precise span of
 490         // the type parameter in question in hand when we report the
 491         // error.
 492         //
 493         // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
 494         // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
 495         self.infcx.commit_if_ok(|_| {
 496             let mut all_obligations = vec![];
 497
 498             // The liberated version of this signature should be a subtype
 499             // of the liberated form of the expectation.
 500             for ((hir_ty, &supplied_ty), expected_ty) in decl
 501                 .inputs
 502                 .iter()
 503                 .zip(*supplied_sig.inputs().skip_binder()) // binder moved to (*) below
 504                 .zip(expected_sigs.liberated_sig.inputs())
 505             // `liberated_sig` is E'.
 506             {
 507                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 508                 let (supplied_ty, _) = self.infcx.replace_bound_vars_with_fresh_vars(
 509                     hir_ty.span,
 510                     LateBoundRegionConversionTime::FnCall,
 511                     &ty::Binder::bind(supplied_ty),
 512                 ); // recreated from (*) above
 513
 514                 // Check that E' = S'.
 515                 let cause = self.misc(hir_ty.span);
 516                 let InferOk { value: (), obligations } =
 517                     self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
 518                 all_obligations.extend(obligations);
 519
 520                 // Also, require that the supplied type must outlive
 521                 // the closure body.
 522                 let closure_body_region = self.tcx.mk_region(ty::ReScope(region::Scope {
 523                     id: body.value.hir_id.local_id,
 524                     data: region::ScopeData::Node,
 525                 }));
 526                 all_obligations.push(Obligation::new(
 527                     cause,
 528                     self.param_env,
 529                     ty::PredicateKind::TypeOutlives(ty::Binder::dummy(ty::OutlivesPredicate(
 530                         supplied_ty,
 531                         closure_body_region,
 532                     ))),
 533                 ));
 534             }
 535
 536             let (supplied_output_ty, _) = self.infcx.replace_bound_vars_with_fresh_vars(
 537                 decl.output.span(),
 538                 LateBoundRegionConversionTime::FnCall,
 539                 &supplied_sig.output(),
 540             );
 541             let cause = &self.misc(decl.output.span());
 542             let InferOk { value: (), obligations } = self
 543                 .at(cause, self.param_env)
 544                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 545             all_obligations.extend(obligations);
 546
 547             Ok(InferOk { value: (), obligations: all_obligations })
 548         })
 549     }
 550
 551     /// If there is no expected signature, then we will convert the
 552     /// types that the user gave into a signature.
 553     ///
 554     /// Also, record this closure signature for later.
 555     fn supplied_sig_of_closure(
 556         &self,
 557         expr_def_id: DefId,
 558         decl: &hir::FnDecl<'_>,
 559         body: &hir::Body<'_>,
 560     ) -> ty::PolyFnSig<'tcx> {
 561         let astconv: &dyn AstConv<'_> = self;
 562
 563         debug!(
 564             "supplied_sig_of_closure(decl={:?}, body.generator_kind={:?})",
 565             decl, body.generator_kind,
 566         );
 567
 568         // First, convert the types that the user supplied (if any).
 569         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 570         let supplied_return = match decl.output {
 571             hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
 572             hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
 573                 // In the case of the async block that we create for a function body,
 574                 // we expect the return type of the block to match that of the enclosing
 575                 // function.
 576                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 577                     debug!("supplied_sig_of_closure: closure is async fn body");
 578                     self.deduce_future_output_from_obligations(expr_def_id).unwrap_or_else(|| {
 579                         // AFAIK, deducing the future output
 580                         // always succeeds *except* in error cases
 581                         // like #65159. I'd like to return Error
 582                         // here, but I can't because I can't
 583                         // easily (and locally) prove that we
 584                         // *have* reported an
 585                         // error. --nikomatsakis
 586                         astconv.ty_infer(None, decl.output.span())
 587                     })
 588                 }
 589
 590                 _ => astconv.ty_infer(None, decl.output.span()),
 591             },
 592         };
 593
 594         let result = ty::Binder::bind(self.tcx.mk_fn_sig(
 595             supplied_arguments,
 596             supplied_return,
 597             decl.c_variadic,
 598             hir::Unsafety::Normal,
 599             Abi::RustCall,
 600         ));
 601
 602         debug!("supplied_sig_of_closure: result={:?}", result);
 603
 604         let c_result = self.inh.infcx.canonicalize_response(&result);
 605         self.tables.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);
 606
 607         result
 608     }
 609
 610     /// Invoked when we are translating the generator that results
 611     /// from desugaring an `async fn`. Returns the "sugared" return
 612     /// type of the `async fn` -- that is, the return type that the
 613     /// user specified. The "desugared" return type is a `impl
 614     /// Future<Output = T>`, so we do this by searching through the
 615     /// obligations to extract the `T`.
 616     fn deduce_future_output_from_obligations(&self, expr_def_id: DefId) -> Option<Ty<'tcx>> {
 617         debug!("deduce_future_output_from_obligations(expr_def_id={:?})", expr_def_id);
 618
 619         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
 620             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
 621         });
 622
 623         // In practice, the return type of the surrounding function is
 624         // always a (not yet resolved) inference variable, because it
 625         // is the hidden type for an `impl Trait` that we are going to
 626         // be inferring.
 627         let ret_ty = ret_coercion.borrow().expected_ty();
 628         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 629         let ret_vid = match ret_ty.kind {
 630             ty::Infer(ty::TyVar(ret_vid)) => ret_vid,
 631             _ => span_bug!(
 632                 self.tcx.def_span(expr_def_id),
 633                 "async fn generator return type not an inference variable"
 634             ),
 635         };
 636
 637         // Search for a pending obligation like
 638         //
 639         // `<R as Future>::Output = T`
 640         //
 641         // where R is the return type we are expecting. This type `T`
 642         // will be our output.
 643         let output_ty = self.obligations_for_self_ty(ret_vid).find_map(|(_, obligation)| {
 644             if let ty::PredicateKind::Projection(ref proj_predicate) = obligation.predicate {
 645                 self.deduce_future_output_from_projection(obligation.cause.span, proj_predicate)
 646             } else {
 647                 None
 648             }
 649         });
 650
 651         debug!("deduce_future_output_from_obligations: output_ty={:?}", output_ty);
 652         output_ty
 653     }
 654
 655     /// Given a projection like
 656     ///
 657     /// `<X as Future>::Output = T`
 658     ///
 659     /// where `X` is some type that has no late-bound regions, returns
 660     /// `Some(T)`. If the projection is for some other trait, returns
 661     /// `None`.
 662     fn deduce_future_output_from_projection(
 663         &self,
 664         cause_span: Span,
 665         predicate: &ty::PolyProjectionPredicate<'tcx>,
 666     ) -> Option<Ty<'tcx>> {
 667         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 668
 669         // We do not expect any bound regions in our predicate, so
 670         // skip past the bound vars.
 671         let predicate = match predicate.no_bound_vars() {
 672             Some(p) => p,
 673             None => {
 674                 debug!("deduce_future_output_from_projection: has late-bound regions");
 675                 return None;
 676             }
 677         };
 678
 679         // Check that this is a projection from the `Future` trait.
 680         let trait_ref = predicate.projection_ty.trait_ref(self.tcx);
 681         let future_trait = self.tcx.require_lang_item(FutureTraitLangItem, Some(cause_span));
 682         if trait_ref.def_id != future_trait {
 683             debug!("deduce_future_output_from_projection: not a future");
 684             return None;
 685         }
 686
 687         // The `Future` trait has only one associted item, `Output`,
 688         // so check that this is what we see.
 689         let output_assoc_item =
 690             self.tcx.associated_items(future_trait).in_definition_order().next().unwrap().def_id;
 691         if output_assoc_item != predicate.projection_ty.item_def_id {
 692             span_bug!(
 693                 cause_span,
 694                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 695                 predicate.projection_ty.item_def_id,
 696                 output_assoc_item,
 697             );
 698         }
 699
 700         // Extract the type from the projection. Note that there can
 701         // be no bound variables in this type because the "self type"
 702         // does not have any regions in it.
 703         let output_ty = self.resolve_vars_if_possible(&predicate.ty);
 704         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 705         Some(output_ty)
 706     }
 707
 708     /// Converts the types that the user supplied, in case that doing
 709     /// so should yield an error, but returns back a signature where
 710     /// all parameters are of type `TyErr`.
 711     fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
 712         let astconv: &dyn AstConv<'_> = self;
 713
 714         let supplied_arguments = decl.inputs.iter().map(|a| {
 715             // Convert the types that the user supplied (if any), but ignore them.
 716             astconv.ast_ty_to_ty(a);
 717             self.tcx.types.err
 718         });
 719
 720         if let hir::FnRetTy::Return(ref output) = decl.output {
 721             astconv.ast_ty_to_ty(&output);
 722         }
 723
 724         let result = ty::Binder::bind(self.tcx.mk_fn_sig(
 725             supplied_arguments,
 726             self.tcx.types.err,
 727             decl.c_variadic,
 728             hir::Unsafety::Normal,
 729             Abi::RustCall,
 730         ));
 731
 732         debug!("supplied_sig_of_closure: result={:?}", result);
 733
 734         result
 735     }
 736
 737     fn closure_sigs(
 738         &self,
 739         expr_def_id: DefId,
 740         body: &hir::Body<'_>,
 741         bound_sig: ty::PolyFnSig<'tcx>,
 742     ) -> ClosureSignatures<'tcx> {
 743         let liberated_sig = self.tcx().liberate_late_bound_regions(expr_def_id, &bound_sig);
 744         let liberated_sig = self.inh.normalize_associated_types_in(
 745             body.value.span,
 746             body.value.hir_id,
 747             self.param_env,
 748             &liberated_sig,
 749         );
 750         ClosureSignatures { bound_sig, liberated_sig }
 751     }
 752 }