compiler/rustc_typeck/src/check/closure.rs

   1 //! Code for type-checking closure expressions.
   2
   3 use super::{check_fn, Expectation, FnCtxt, GeneratorTypes};
   4
   5 use crate::astconv::AstConv;
   6 use crate::rustc_middle::ty::subst::Subst;
   7 use hir::OpaqueTyOrigin;
   8 use rustc_hir as hir;
   9 use rustc_hir::def_id::DefId;
  10 use rustc_hir::lang_items::LangItem;
  11 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  12 use rustc_infer::infer::LateBoundRegionConversionTime;
  13 use rustc_infer::infer::{InferOk, InferResult};
  14 use rustc_infer::traits::ObligationCause;
  15 use rustc_middle::ty::fold::TypeFoldable;
  16 use rustc_middle::ty::subst::InternalSubsts;
  17 use rustc_middle::ty::{self, Ty};
  18 use rustc_span::source_map::Span;
  19 use rustc_span::DUMMY_SP;
  20 use rustc_target::spec::abi::Abi;
  21 use rustc_trait_selection::traits::error_reporting::ArgKind;
  22 use rustc_trait_selection::traits::error_reporting::InferCtxtExt as _;
  23 use std::cmp;
  24 use std::iter;
  25
  26 /// What signature do we *expect* the closure to have from context?
  27 #[derive(Debug)]
  28 struct ExpectedSig<'tcx> {
  29     /// Span that gave us this expectation, if we know that.
  30     cause_span: Option<Span>,
  31     sig: ty::PolyFnSig<'tcx>,
  32 }
  33
  34 struct ClosureSignatures<'tcx> {
  35     bound_sig: ty::PolyFnSig<'tcx>,
  36     liberated_sig: ty::FnSig<'tcx>,
  37 }
  38
  39 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  40     #[instrument(skip(self, expr, _capture, decl, body_id), level = "debug")]
  41     pub fn check_expr_closure(
  42         &self,
  43         expr: &hir::Expr<'_>,
  44         _capture: hir::CaptureBy,
  45         decl: &'tcx hir::FnDecl<'tcx>,
  46         body_id: hir::BodyId,
  47         gen: Option<hir::Movability>,
  48         expected: Expectation<'tcx>,
  49     ) -> Ty<'tcx> {
  50         trace!("decl = {:#?}", decl);
  51         trace!("expr = {:#?}", expr);
  52
  53         // It's always helpful for inference if we know the kind of
  54         // closure sooner rather than later, so first examine the expected
  55         // type, and see if can glean a closure kind from there.
  56         let (expected_sig, expected_kind) = match expected.to_option(self) {
  57             Some(ty) => self.deduce_expectations_from_expected_type(ty),
  58             None => (None, None),
  59         };
  60         let body = self.tcx.hir().body(body_id);
  61         self.check_closure(expr, expected_kind, decl, body, gen, expected_sig)
  62     }
  63
  64     #[instrument(skip(self, expr, body, decl), level = "debug")]
  65     fn check_closure(
  66         &self,
  67         expr: &hir::Expr<'_>,
  68         opt_kind: Option<ty::ClosureKind>,
  69         decl: &'tcx hir::FnDecl<'tcx>,
  70         body: &'tcx hir::Body<'tcx>,
  71         gen: Option<hir::Movability>,
  72         expected_sig: Option<ExpectedSig<'tcx>>,
  73     ) -> Ty<'tcx> {
  74         trace!("decl = {:#?}", decl);
  75         let expr_def_id = self.tcx.hir().local_def_id(expr.hir_id);
  76         debug!(?expr_def_id);
  77
  78         let ClosureSignatures { bound_sig, liberated_sig } =
  79             self.sig_of_closure(expr.hir_id, expr_def_id.to_def_id(), decl, body, expected_sig);
  80
  81         debug!(?bound_sig, ?liberated_sig);
  82
  83         let return_type_pre_known = !liberated_sig.output().is_ty_infer();
  84
  85         let generator_types = check_fn(
  86             self,
  87             self.param_env.without_const(),
  88             liberated_sig,
  89             decl,
  90             expr.hir_id,
  91             body,
  92             gen,
  93             return_type_pre_known,
  94         )
  95         .1;
  96
  97         let parent_substs = InternalSubsts::identity_for_item(
  98             self.tcx,
  99             self.tcx.typeck_root_def_id(expr_def_id.to_def_id()),
 100         );
 101
 102         let tupled_upvars_ty = self.infcx.next_ty_var(TypeVariableOrigin {
 103             kind: TypeVariableOriginKind::ClosureSynthetic,
 104             span: self.tcx.hir().span(expr.hir_id),
 105         });
 106
 107         if let Some(GeneratorTypes { resume_ty, yield_ty, interior, movability }) = generator_types
 108         {
 109             let generator_substs = ty::GeneratorSubsts::new(
 110                 self.tcx,
 111                 ty::GeneratorSubstsParts {
 112                     parent_substs,
 113                     resume_ty,
 114                     yield_ty,
 115                     return_ty: liberated_sig.output(),
 116                     witness: interior,
 117                     tupled_upvars_ty,
 118                 },
 119             );
 120
 121             return self.tcx.mk_generator(
 122                 expr_def_id.to_def_id(),
 123                 generator_substs.substs,
 124                 movability,
 125             );
 126         }
 127
 128         // Tuple up the arguments and insert the resulting function type into
 129         // the `closures` table.
 130         let sig = bound_sig.map_bound(|sig| {
 131             self.tcx.mk_fn_sig(
 132                 iter::once(self.tcx.intern_tup(sig.inputs())),
 133                 sig.output(),
 134                 sig.c_variadic,
 135                 sig.unsafety,
 136                 sig.abi,
 137             )
 138         });
 139
 140         debug!(?sig, ?opt_kind);
 141
 142         let closure_kind_ty = match opt_kind {
 143             Some(kind) => kind.to_ty(self.tcx),
 144
 145             // Create a type variable (for now) to represent the closure kind.
 146             // It will be unified during the upvar inference phase (`upvar.rs`)
 147             None => self.infcx.next_ty_var(TypeVariableOrigin {
 148                 // FIXME(eddyb) distinguish closure kind inference variables from the rest.
 149                 kind: TypeVariableOriginKind::ClosureSynthetic,
 150                 span: expr.span,
 151             }),
 152         };
 153
 154         let closure_substs = ty::ClosureSubsts::new(
 155             self.tcx,
 156             ty::ClosureSubstsParts {
 157                 parent_substs,
 158                 closure_kind_ty,
 159                 closure_sig_as_fn_ptr_ty: self.tcx.mk_fn_ptr(sig),
 160                 tupled_upvars_ty,
 161             },
 162         );
 163
 164         let closure_type = self.tcx.mk_closure(expr_def_id.to_def_id(), closure_substs.substs);
 165
 166         debug!(?expr.hir_id, ?closure_type);
 167
 168         closure_type
 169     }
 170
 171     /// Given the expected type, figures out what it can about this closure we
 172     /// are about to type check:
 173     #[instrument(skip(self), level = "debug")]
 174     fn deduce_expectations_from_expected_type(
 175         &self,
 176         expected_ty: Ty<'tcx>,
 177     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 178         match *expected_ty.kind() {
 179             ty::Opaque(def_id, substs) => {
 180                 let bounds = self.tcx.explicit_item_bounds(def_id);
 181                 let sig = bounds.iter().find_map(|(pred, span)| match pred.kind().skip_binder() {
 182                     ty::PredicateKind::Projection(proj_predicate) => self
 183                         .deduce_sig_from_projection(
 184                             Some(*span),
 185                             pred.kind().rebind(proj_predicate.subst(self.tcx, substs)),
 186                         ),
 187                     _ => None,
 188                 });
 189
 190                 let kind = bounds
 191                     .iter()
 192                     .filter_map(|(pred, _)| match pred.kind().skip_binder() {
 193                         ty::PredicateKind::Trait(tp) => {
 194                             self.tcx.fn_trait_kind_from_lang_item(tp.def_id())
 195                         }
 196                         _ => None,
 197                     })
 198                     .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
 199                 trace!(?sig, ?kind);
 200                 (sig, kind)
 201             }
 202             ty::Dynamic(ref object_type, ..) => {
 203                 let sig = object_type.projection_bounds().find_map(|pb| {
 204                     let pb = pb.with_self_ty(self.tcx, self.tcx.types.trait_object_dummy_self);
 205                     self.deduce_sig_from_projection(None, pb)
 206                 });
 207                 let kind = object_type
 208                     .principal_def_id()
 209                     .and_then(|did| self.tcx.fn_trait_kind_from_lang_item(did));
 210                 (sig, kind)
 211             }
 212             ty::Infer(ty::TyVar(vid)) => self.deduce_expectations_from_obligations(vid),
 213             ty::FnPtr(sig) => {
 214                 let expected_sig = ExpectedSig { cause_span: None, sig };
 215                 (Some(expected_sig), Some(ty::ClosureKind::Fn))
 216             }
 217             _ => (None, None),
 218         }
 219     }
 220
 221     fn deduce_expectations_from_obligations(
 222         &self,
 223         expected_vid: ty::TyVid,
 224     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 225         let expected_sig =
 226             self.obligations_for_self_ty(expected_vid).find_map(|(_, obligation)| {
 227                 debug!(?obligation.predicate);
 228
 229                 let bound_predicate = obligation.predicate.kind();
 230                 if let ty::PredicateKind::Projection(proj_predicate) =
 231                     obligation.predicate.kind().skip_binder()
 232                 {
 233                     // Given a Projection predicate, we can potentially infer
 234                     // the complete signature.
 235                     self.deduce_sig_from_projection(
 236                         Some(obligation.cause.span),
 237                         bound_predicate.rebind(proj_predicate),
 238                     )
 239                 } else {
 240                     None
 241                 }
 242             });
 243
 244         // Even if we can't infer the full signature, we may be able to
 245         // infer the kind. This can occur when we elaborate a predicate
 246         // like `F : Fn<A>`. Note that due to subtyping we could encounter
 247         // many viable options, so pick the most restrictive.
 248         let expected_kind = self
 249             .obligations_for_self_ty(expected_vid)
 250             .filter_map(|(tr, _)| self.tcx.fn_trait_kind_from_lang_item(tr.def_id()))
 251             .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
 252
 253         (expected_sig, expected_kind)
 254     }
 255
 256     /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
 257     /// everything we need to know about a closure or generator.
 258     ///
 259     /// The `cause_span` should be the span that caused us to
 260     /// have this expected signature, or `None` if we can't readily
 261     /// know that.
 262     fn deduce_sig_from_projection(
 263         &self,
 264         cause_span: Option<Span>,
 265         projection: ty::PolyProjectionPredicate<'tcx>,
 266     ) -> Option<ExpectedSig<'tcx>> {
 267         let tcx = self.tcx;
 268
 269         debug!("deduce_sig_from_projection({:?})", projection);
 270
 271         let trait_def_id = projection.trait_def_id(tcx);
 272
 273         let is_fn = tcx.fn_trait_kind_from_lang_item(trait_def_id).is_some();
 274         let gen_trait = tcx.require_lang_item(LangItem::Generator, cause_span);
 275         let is_gen = gen_trait == trait_def_id;
 276         if !is_fn && !is_gen {
 277             debug!("deduce_sig_from_projection: not fn or generator");
 278             return None;
 279         }
 280
 281         if is_gen {
 282             // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
 283             // associated item and not yield.
 284             let return_assoc_item = self.tcx.associated_item_def_ids(gen_trait)[1];
 285             if return_assoc_item != projection.projection_def_id() {
 286                 debug!("deduce_sig_from_projection: not return assoc item of generator");
 287                 return None;
 288             }
 289         }
 290
 291         let input_tys = if is_fn {
 292             let arg_param_ty = projection.skip_binder().projection_ty.substs.type_at(1);
 293             let arg_param_ty = self.resolve_vars_if_possible(arg_param_ty);
 294             debug!("deduce_sig_from_projection: arg_param_ty={:?}", arg_param_ty);
 295
 296             match arg_param_ty.kind() {
 297                 ty::Tuple(tys) => tys.into_iter().map(|k| k.expect_ty()).collect::<Vec<_>>(),
 298                 _ => return None,
 299             }
 300         } else {
 301             // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
 302             // else they must have exactly 1 argument. For now though, just give up in this case.
 303             return None;
 304         };
 305
 306         // Since this is a return parameter type it is safe to unwrap.
 307         let ret_param_ty = projection.skip_binder().term.ty().unwrap();
 308         let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
 309         debug!("deduce_sig_from_projection: ret_param_ty={:?}", ret_param_ty);
 310
 311         let sig = projection.rebind(self.tcx.mk_fn_sig(
 312             input_tys.iter(),
 313             &ret_param_ty,
 314             false,
 315             hir::Unsafety::Normal,
 316             Abi::Rust,
 317         ));
 318         debug!("deduce_sig_from_projection: sig={:?}", sig);
 319
 320         Some(ExpectedSig { cause_span, sig })
 321     }
 322
 323     fn sig_of_closure(
 324         &self,
 325         hir_id: hir::HirId,
 326         expr_def_id: DefId,
 327         decl: &hir::FnDecl<'_>,
 328         body: &hir::Body<'_>,
 329         expected_sig: Option<ExpectedSig<'tcx>>,
 330     ) -> ClosureSignatures<'tcx> {
 331         if let Some(e) = expected_sig {
 332             self.sig_of_closure_with_expectation(hir_id, expr_def_id, decl, body, e)
 333         } else {
 334             self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body)
 335         }
 336     }
 337
 338     /// If there is no expected signature, then we will convert the
 339     /// types that the user gave into a signature.
 340     #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
 341     fn sig_of_closure_no_expectation(
 342         &self,
 343         hir_id: hir::HirId,
 344         expr_def_id: DefId,
 345         decl: &hir::FnDecl<'_>,
 346         body: &hir::Body<'_>,
 347     ) -> ClosureSignatures<'tcx> {
 348         let bound_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 349
 350         self.closure_sigs(expr_def_id, body, bound_sig)
 351     }
 352
 353     /// Invoked to compute the signature of a closure expression. This
 354     /// combines any user-provided type annotations (e.g., `|x: u32|
 355     /// -> u32 { .. }`) with the expected signature.
 356     ///
 357     /// The approach is as follows:
 358     ///
 359     /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
 360     /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
 361     ///   - If we have no expectation `E`, then the signature of the closure is `S`.
 362     ///   - Otherwise, the signature of the closure is E. Moreover:
 363     ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
 364     ///     - Instantiate all the late-bound regions bound in the closure within `S`
 365     ///       with fresh (existential) variables, yielding `S'`
 366     ///     - Require that `E' = S'`
 367     ///       - We could use some kind of subtyping relationship here,
 368     ///         I imagine, but equality is easier and works fine for
 369     ///         our purposes.
 370     ///
 371     /// The key intuition here is that the user's types must be valid
 372     /// from "the inside" of the closure, but the expectation
 373     /// ultimately drives the overall signature.
 374     ///
 375     /// # Examples
 376     ///
 377     /// ```
 378     /// fn with_closure<F>(_: F)
 379     ///   where F: Fn(&u32) -> &u32 { .. }
 380     ///
 381     /// with_closure(|x: &u32| { ... })
 382     /// ```
 383     ///
 384     /// Here:
 385     /// - E would be `fn(&u32) -> &u32`.
 386     /// - S would be `fn(&u32) ->
 387     /// - E' is `&'!0 u32 -> &'!0 u32`
 388     /// - S' is `&'?0 u32 -> ?T`
 389     ///
 390     /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
 391     ///
 392     /// # Arguments
 393     ///
 394     /// - `expr_def_id`: the `DefId` of the closure expression
 395     /// - `decl`: the HIR declaration of the closure
 396     /// - `body`: the body of the closure
 397     /// - `expected_sig`: the expected signature (if any). Note that
 398     ///   this is missing a binder: that is, there may be late-bound
 399     ///   regions with depth 1, which are bound then by the closure.
 400     #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
 401     fn sig_of_closure_with_expectation(
 402         &self,
 403         hir_id: hir::HirId,
 404         expr_def_id: DefId,
 405         decl: &hir::FnDecl<'_>,
 406         body: &hir::Body<'_>,
 407         expected_sig: ExpectedSig<'tcx>,
 408     ) -> ClosureSignatures<'tcx> {
 409         // Watch out for some surprises and just ignore the
 410         // expectation if things don't see to match up with what we
 411         // expect.
 412         if expected_sig.sig.c_variadic() != decl.c_variadic {
 413             return self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body);
 414         } else if expected_sig.sig.skip_binder().inputs_and_output.len() != decl.inputs.len() + 1 {
 415             return self.sig_of_closure_with_mismatched_number_of_arguments(
 416                 expr_def_id,
 417                 decl,
 418                 body,
 419                 expected_sig,
 420             );
 421         }
 422
 423         // Create a `PolyFnSig`. Note the oddity that late bound
 424         // regions appearing free in `expected_sig` are now bound up
 425         // in this binder we are creating.
 426         assert!(!expected_sig.sig.skip_binder().has_vars_bound_above(ty::INNERMOST));
 427         let bound_sig = expected_sig.sig.map_bound(|sig| {
 428             let output = self.hide_parent_opaque_types(
 429                 sig.output(),
 430                 expected_sig.cause_span.unwrap_or(DUMMY_SP),
 431                 body.id().hir_id,
 432             );
 433             self.tcx.mk_fn_sig(
 434                 sig.inputs().iter().cloned(),
 435                 output,
 436                 sig.c_variadic,
 437                 hir::Unsafety::Normal,
 438                 Abi::RustCall,
 439             )
 440         });
 441
 442         // `deduce_expectations_from_expected_type` introduces
 443         // late-bound lifetimes defined elsewhere, which we now
 444         // anonymize away, so as not to confuse the user.
 445         let bound_sig = self.tcx.anonymize_late_bound_regions(bound_sig);
 446
 447         let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
 448
 449         // Up till this point, we have ignored the annotations that the user
 450         // gave. This function will check that they unify successfully.
 451         // Along the way, it also writes out entries for types that the user
 452         // wrote into our typeck results, which are then later used by the privacy
 453         // check.
 454         match self.check_supplied_sig_against_expectation(
 455             hir_id,
 456             expr_def_id,
 457             decl,
 458             body,
 459             &closure_sigs,
 460         ) {
 461             Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
 462             Err(_) => return self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body),
 463         }
 464
 465         closure_sigs
 466     }
 467
 468     fn sig_of_closure_with_mismatched_number_of_arguments(
 469         &self,
 470         expr_def_id: DefId,
 471         decl: &hir::FnDecl<'_>,
 472         body: &hir::Body<'_>,
 473         expected_sig: ExpectedSig<'tcx>,
 474     ) -> ClosureSignatures<'tcx> {
 475         let hir = self.tcx.hir();
 476         let expr_map_node = hir.get_if_local(expr_def_id).unwrap();
 477         let expected_args: Vec<_> = expected_sig
 478             .sig
 479             .skip_binder()
 480             .inputs()
 481             .iter()
 482             .map(|ty| ArgKind::from_expected_ty(ty, None))
 483             .collect();
 484         let (closure_span, found_args) = match self.get_fn_like_arguments(expr_map_node) {
 485             Some((sp, args)) => (Some(sp), args),
 486             None => (None, Vec::new()),
 487         };
 488         let expected_span =
 489             expected_sig.cause_span.unwrap_or_else(|| hir.span_if_local(expr_def_id).unwrap());
 490         self.report_arg_count_mismatch(
 491             expected_span,
 492             closure_span,
 493             expected_args,
 494             found_args,
 495             true,
 496         )
 497         .emit();
 498
 499         let error_sig = self.error_sig_of_closure(decl);
 500
 501         self.closure_sigs(expr_def_id, body, error_sig)
 502     }
 503
 504     /// Enforce the user's types against the expectation. See
 505     /// `sig_of_closure_with_expectation` for details on the overall
 506     /// strategy.
 507     fn check_supplied_sig_against_expectation(
 508         &self,
 509         hir_id: hir::HirId,
 510         expr_def_id: DefId,
 511         decl: &hir::FnDecl<'_>,
 512         body: &hir::Body<'_>,
 513         expected_sigs: &ClosureSignatures<'tcx>,
 514     ) -> InferResult<'tcx, ()> {
 515         // Get the signature S that the user gave.
 516         //
 517         // (See comment on `sig_of_closure_with_expectation` for the
 518         // meaning of these letters.)
 519         let supplied_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 520
 521         debug!("check_supplied_sig_against_expectation: supplied_sig={:?}", supplied_sig);
 522
 523         // FIXME(#45727): As discussed in [this comment][c1], naively
 524         // forcing equality here actually results in suboptimal error
 525         // messages in some cases.  For now, if there would have been
 526         // an obvious error, we fallback to declaring the type of the
 527         // closure to be the one the user gave, which allows other
 528         // error message code to trigger.
 529         //
 530         // However, I think [there is potential to do even better
 531         // here][c2], since in *this* code we have the precise span of
 532         // the type parameter in question in hand when we report the
 533         // error.
 534         //
 535         // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
 536         // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
 537         self.infcx.commit_if_ok(|_| {
 538             let mut all_obligations = vec![];
 539
 540             // The liberated version of this signature should be a subtype
 541             // of the liberated form of the expectation.
 542             for ((hir_ty, &supplied_ty), expected_ty) in iter::zip(
 543                 iter::zip(
 544                     decl.inputs,
 545                     supplied_sig.inputs().skip_binder(), // binder moved to (*) below
 546                 ),
 547                 expected_sigs.liberated_sig.inputs(), // `liberated_sig` is E'.
 548             ) {
 549                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 550                 let (supplied_ty, _) = self.infcx.replace_bound_vars_with_fresh_vars(
 551                     hir_ty.span,
 552                     LateBoundRegionConversionTime::FnCall,
 553                     supplied_sig.inputs().rebind(supplied_ty),
 554                 ); // recreated from (*) above
 555
 556                 // Check that E' = S'.
 557                 let cause = self.misc(hir_ty.span);
 558                 let InferOk { value: (), obligations } =
 559                     self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
 560                 all_obligations.extend(obligations);
 561             }
 562
 563             let (supplied_output_ty, _) = self.infcx.replace_bound_vars_with_fresh_vars(
 564                 decl.output.span(),
 565                 LateBoundRegionConversionTime::FnCall,
 566                 supplied_sig.output(),
 567             );
 568             let cause = &self.misc(decl.output.span());
 569             let InferOk { value: (), obligations } = self
 570                 .at(cause, self.param_env)
 571                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 572             all_obligations.extend(obligations);
 573
 574             Ok(InferOk { value: (), obligations: all_obligations })
 575         })
 576     }
 577
 578     /// If there is no expected signature, then we will convert the
 579     /// types that the user gave into a signature.
 580     ///
 581     /// Also, record this closure signature for later.
 582     #[instrument(skip(self, decl, body), level = "debug")]
 583     fn supplied_sig_of_closure(
 584         &self,
 585         hir_id: hir::HirId,
 586         expr_def_id: DefId,
 587         decl: &hir::FnDecl<'_>,
 588         body: &hir::Body<'_>,
 589     ) -> ty::PolyFnSig<'tcx> {
 590         let astconv: &dyn AstConv<'_> = self;
 591
 592         trace!("decl = {:#?}", decl);
 593         debug!(?body.generator_kind);
 594
 595         let bound_vars = self.tcx.late_bound_vars(hir_id);
 596
 597         // First, convert the types that the user supplied (if any).
 598         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 599         let supplied_return = match decl.output {
 600             hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
 601             hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
 602                 // In the case of the async block that we create for a function body,
 603                 // we expect the return type of the block to match that of the enclosing
 604                 // function.
 605                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 606                     debug!("closure is async fn body");
 607                     self.deduce_future_output_from_obligations(expr_def_id).unwrap_or_else(|| {
 608                         // AFAIK, deducing the future output
 609                         // always succeeds *except* in error cases
 610                         // like #65159. I'd like to return Error
 611                         // here, but I can't because I can't
 612                         // easily (and locally) prove that we
 613                         // *have* reported an
 614                         // error. --nikomatsakis
 615                         astconv.ty_infer(None, decl.output.span())
 616                     })
 617                 }
 618
 619                 _ => astconv.ty_infer(None, decl.output.span()),
 620             },
 621         };
 622         let supplied_return =
 623             self.hide_parent_opaque_types(supplied_return, decl.output.span(), body.id().hir_id);
 624
 625         let result = ty::Binder::bind_with_vars(
 626             self.tcx.mk_fn_sig(
 627                 supplied_arguments,
 628                 supplied_return,
 629                 decl.c_variadic,
 630                 hir::Unsafety::Normal,
 631                 Abi::RustCall,
 632             ),
 633             bound_vars,
 634         );
 635
 636         debug!(?result);
 637
 638         let c_result = self.inh.infcx.canonicalize_response(result);
 639         self.typeck_results.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);
 640
 641         result
 642     }
 643
 644     fn hide_parent_opaque_types(&self, ty: Ty<'tcx>, span: Span, body_id: hir::HirId) -> Ty<'tcx> {
 645         ty.fold_with(&mut ty::fold::BottomUpFolder {
 646             tcx: self.infcx.tcx,
 647             lt_op: |lt| lt,
 648             ct_op: |ct| ct,
 649             ty_op: |ty| match *ty.kind() {
 650                 // Closures can't create hidden types for opaque types of their parent, as they
 651                 // do not have all the outlives information available. Also `type_of` looks for
 652                 // hidden types in the owner (so the closure's parent), so it would not find these
 653                 // definitions.
 654                 ty::Opaque(def_id, _substs)
 655                     if matches!(
 656                         self.infcx.opaque_type_origin(def_id, DUMMY_SP),
 657                         Some(OpaqueTyOrigin::FnReturn(..))
 658                     ) =>
 659                 {
 660                     let ty_var = self.next_ty_var(TypeVariableOrigin {
 661                         kind: TypeVariableOriginKind::TypeInference,
 662                         span,
 663                     });
 664                     let cause = ObligationCause::misc(span, body_id);
 665                     self.register_predicates(vec![self.infcx.opaque_ty_obligation(
 666                         ty,
 667                         ty_var,
 668                         true,
 669                         self.param_env,
 670                         cause,
 671                     )]);
 672                     ty_var
 673                 }
 674                 _ => ty,
 675             },
 676         })
 677     }
 678
 679     /// Invoked when we are translating the generator that results
 680     /// from desugaring an `async fn`. Returns the "sugared" return
 681     /// type of the `async fn` -- that is, the return type that the
 682     /// user specified. The "desugared" return type is an `impl
 683     /// Future<Output = T>`, so we do this by searching through the
 684     /// obligations to extract the `T`.
 685     #[instrument(skip(self), level = "debug")]
 686     fn deduce_future_output_from_obligations(&self, expr_def_id: DefId) -> Option<Ty<'tcx>> {
 687         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
 688             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
 689         });
 690
 691         let ret_ty = ret_coercion.borrow().expected_ty();
 692         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 693         let (def_id, substs) = match *ret_ty.kind() {
 694             ty::Opaque(def_id, substs) => (def_id, substs),
 695             ty::Error(_) => return None,
 696             _ => span_bug!(
 697                 self.tcx.def_span(expr_def_id),
 698                 "async fn generator return type not an inference variable"
 699             ),
 700         };
 701
 702         let item_bounds = self.tcx.explicit_item_bounds(def_id);
 703
 704         // Search for a pending obligation like
 705         //
 706         // `<R as Future>::Output = T`
 707         //
 708         // where R is the return type we are expecting. This type `T`
 709         // will be our output.
 710         let output_ty = item_bounds.iter().find_map(|&(predicate, span)| {
 711             let bound_predicate = predicate.subst(self.tcx, substs).kind();
 712             if let ty::PredicateKind::Projection(proj_predicate) = bound_predicate.skip_binder() {
 713                 self.deduce_future_output_from_projection(
 714                     span,
 715                     bound_predicate.rebind(proj_predicate),
 716                 )
 717             } else {
 718                 None
 719             }
 720         });
 721
 722         debug!("deduce_future_output_from_obligations: output_ty={:?}", output_ty);
 723         output_ty
 724     }
 725
 726     /// Given a projection like
 727     ///
 728     /// `<X as Future>::Output = T`
 729     ///
 730     /// where `X` is some type that has no late-bound regions, returns
 731     /// `Some(T)`. If the projection is for some other trait, returns
 732     /// `None`.
 733     fn deduce_future_output_from_projection(
 734         &self,
 735         cause_span: Span,
 736         predicate: ty::PolyProjectionPredicate<'tcx>,
 737     ) -> Option<Ty<'tcx>> {
 738         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 739
 740         // We do not expect any bound regions in our predicate, so
 741         // skip past the bound vars.
 742         let predicate = match predicate.no_bound_vars() {
 743             Some(p) => p,
 744             None => {
 745                 debug!("deduce_future_output_from_projection: has late-bound regions");
 746                 return None;
 747             }
 748         };
 749
 750         // Check that this is a projection from the `Future` trait.
 751         let trait_def_id = predicate.projection_ty.trait_def_id(self.tcx);
 752         let future_trait = self.tcx.require_lang_item(LangItem::Future, Some(cause_span));
 753         if trait_def_id != future_trait {
 754             debug!("deduce_future_output_from_projection: not a future");
 755             return None;
 756         }
 757
 758         // The `Future` trait has only one associted item, `Output`,
 759         // so check that this is what we see.
 760         let output_assoc_item = self.tcx.associated_item_def_ids(future_trait)[0];
 761         if output_assoc_item != predicate.projection_ty.item_def_id {
 762             span_bug!(
 763                 cause_span,
 764                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 765                 predicate.projection_ty.item_def_id,
 766                 output_assoc_item,
 767             );
 768         }
 769
 770         // Extract the type from the projection. Note that there can
 771         // be no bound variables in this type because the "self type"
 772         // does not have any regions in it.
 773         let output_ty = self.resolve_vars_if_possible(predicate.term);
 774         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 775         // This is a projection on a Fn trait so will always be a type.
 776         Some(output_ty.ty().unwrap())
 777     }
 778
 779     /// Converts the types that the user supplied, in case that doing
 780     /// so should yield an error, but returns back a signature where
 781     /// all parameters are of type `TyErr`.
 782     fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
 783         let astconv: &dyn AstConv<'_> = self;
 784
 785         let supplied_arguments = decl.inputs.iter().map(|a| {
 786             // Convert the types that the user supplied (if any), but ignore them.
 787             astconv.ast_ty_to_ty(a);
 788             self.tcx.ty_error()
 789         });
 790
 791         if let hir::FnRetTy::Return(ref output) = decl.output {
 792             astconv.ast_ty_to_ty(&output);
 793         }
 794
 795         let result = ty::Binder::dummy(self.tcx.mk_fn_sig(
 796             supplied_arguments,
 797             self.tcx.ty_error(),
 798             decl.c_variadic,
 799             hir::Unsafety::Normal,
 800             Abi::RustCall,
 801         ));
 802
 803         debug!("supplied_sig_of_closure: result={:?}", result);
 804
 805         result
 806     }
 807
 808     fn closure_sigs(
 809         &self,
 810         expr_def_id: DefId,
 811         body: &hir::Body<'_>,
 812         bound_sig: ty::PolyFnSig<'tcx>,
 813     ) -> ClosureSignatures<'tcx> {
 814         let liberated_sig = self.tcx().liberate_late_bound_regions(expr_def_id, bound_sig);
 815         let liberated_sig = self.inh.normalize_associated_types_in(
 816             body.value.span,
 817             body.value.hir_id,
 818             self.param_env,
 819             liberated_sig,
 820         );
 821         ClosureSignatures { bound_sig, liberated_sig }
 822     }
 823 }