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