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 rustc_hir as hir;
   8 use rustc_hir::def_id::DefId;
   9 use rustc_hir::lang_items::LangItem;
  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::subst::InternalSubsts;
  14 use rustc_middle::ty::visit::TypeVisitable;
  15 use rustc_middle::ty::{self, 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 std::cmp;
  21 use std::iter;
  22
  23 /// What signature do we *expect* the closure to have from context?
  24 #[derive(Debug)]
  25 struct ExpectedSig<'tcx> {
  26     /// Span that gave us this expectation, if we know that.
  27     cause_span: Option<Span>,
  28     sig: ty::PolyFnSig<'tcx>,
  29 }
  30
  31 struct ClosureSignatures<'tcx> {
  32     bound_sig: ty::PolyFnSig<'tcx>,
  33     liberated_sig: ty::FnSig<'tcx>,
  34 }
  35
  36 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  37     #[instrument(skip(self, expr, _capture, decl, body_id), level = "debug")]
  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         trace!("decl = {:#?}", decl);
  48         trace!("expr = {:#?}", expr);
  49
  50         // It's always helpful for inference if we know the kind of
  51         // closure sooner rather than later, so first examine the expected
  52         // type, and see if can glean a closure kind from there.
  53         let (expected_sig, expected_kind) = match expected.to_option(self) {
  54             Some(ty) => self.deduce_expectations_from_expected_type(ty),
  55             None => (None, None),
  56         };
  57         let body = self.tcx.hir().body(body_id);
  58         self.check_closure(expr, expected_kind, decl, body, gen, expected_sig)
  59     }
  60
  61     #[instrument(skip(self, expr, body, decl), level = "debug", ret)]
  62     fn check_closure(
  63         &self,
  64         expr: &hir::Expr<'_>,
  65         opt_kind: Option<ty::ClosureKind>,
  66         decl: &'tcx hir::FnDecl<'tcx>,
  67         body: &'tcx hir::Body<'tcx>,
  68         gen: Option<hir::Movability>,
  69         expected_sig: Option<ExpectedSig<'tcx>>,
  70     ) -> Ty<'tcx> {
  71         trace!("decl = {:#?}", decl);
  72         let expr_def_id = self.tcx.hir().local_def_id(expr.hir_id);
  73         debug!(?expr_def_id);
  74
  75         let ClosureSignatures { bound_sig, liberated_sig } =
  76             self.sig_of_closure(expr.hir_id, expr_def_id.to_def_id(), decl, body, expected_sig);
  77
  78         debug!(?bound_sig, ?liberated_sig);
  79
  80         let return_type_pre_known = !liberated_sig.output().is_ty_infer();
  81
  82         let generator_types = check_fn(
  83             self,
  84             self.param_env.without_const(),
  85             liberated_sig,
  86             decl,
  87             expr.hir_id,
  88             body,
  89             gen,
  90             return_type_pre_known,
  91         )
  92         .1;
  93
  94         let parent_substs = InternalSubsts::identity_for_item(
  95             self.tcx,
  96             self.tcx.typeck_root_def_id(expr_def_id.to_def_id()),
  97         );
  98
  99         let tupled_upvars_ty = self.next_ty_var(TypeVariableOrigin {
 100             kind: TypeVariableOriginKind::ClosureSynthetic,
 101             span: self.tcx.hir().span(expr.hir_id),
 102         });
 103
 104         if let Some(GeneratorTypes { resume_ty, yield_ty, interior, movability }) = generator_types
 105         {
 106             let generator_substs = ty::GeneratorSubsts::new(
 107                 self.tcx,
 108                 ty::GeneratorSubstsParts {
 109                     parent_substs,
 110                     resume_ty,
 111                     yield_ty,
 112                     return_ty: liberated_sig.output(),
 113                     witness: interior,
 114                     tupled_upvars_ty,
 115                 },
 116             );
 117
 118             return self.tcx.mk_generator(
 119                 expr_def_id.to_def_id(),
 120                 generator_substs.substs,
 121                 movability,
 122             );
 123         }
 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!(?sig, ?opt_kind);
 138
 139         let closure_kind_ty = match opt_kind {
 140             Some(kind) => kind.to_ty(self.tcx),
 141
 142             // Create a type variable (for now) to represent the closure kind.
 143             // It will be unified during the upvar inference phase (`upvar.rs`)
 144             None => self.next_ty_var(TypeVariableOrigin {
 145                 // FIXME(eddyb) distinguish closure kind inference variables from the rest.
 146                 kind: TypeVariableOriginKind::ClosureSynthetic,
 147                 span: expr.span,
 148             }),
 149         };
 150
 151         let closure_substs = ty::ClosureSubsts::new(
 152             self.tcx,
 153             ty::ClosureSubstsParts {
 154                 parent_substs,
 155                 closure_kind_ty,
 156                 closure_sig_as_fn_ptr_ty: self.tcx.mk_fn_ptr(sig),
 157                 tupled_upvars_ty,
 158             },
 159         );
 160
 161         self.tcx.mk_closure(expr_def_id.to_def_id(), closure_substs.substs)
 162     }
 163
 164     /// Given the expected type, figures out what it can about this closure we
 165     /// are about to type check:
 166     #[instrument(skip(self), level = "debug")]
 167     fn deduce_expectations_from_expected_type(
 168         &self,
 169         expected_ty: Ty<'tcx>,
 170     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 171         match *expected_ty.kind() {
 172             ty::Opaque(def_id, substs) => {
 173                 let bounds = self.tcx.bound_explicit_item_bounds(def_id);
 174                 let sig = bounds
 175                     .transpose_iter()
 176                     .map(|e| e.map_bound(|e| *e).transpose_tuple2())
 177                     .find_map(|(pred, span)| match pred.0.kind().skip_binder() {
 178                         ty::PredicateKind::Projection(proj_predicate) => self
 179                             .deduce_sig_from_projection(
 180                                 Some(span.0),
 181                                 pred.0
 182                                     .kind()
 183                                     .rebind(pred.rebind(proj_predicate).subst(self.tcx, substs)),
 184                             ),
 185                         _ => None,
 186                     });
 187
 188                 let kind = bounds
 189                     .transpose_iter()
 190                     .map(|e| e.map_bound(|e| *e).transpose_tuple2())
 191                     .filter_map(|(pred, _)| match pred.0.kind().skip_binder() {
 192                         ty::PredicateKind::Trait(tp) => {
 193                             self.tcx.fn_trait_kind_from_lang_item(tp.def_id())
 194                         }
 195                         _ => None,
 196                     })
 197                     .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
 198                 trace!(?sig, ?kind);
 199                 (sig, kind)
 200             }
 201             ty::Dynamic(ref object_type, ..) => {
 202                 let sig = object_type.projection_bounds().find_map(|pb| {
 203                     let pb = pb.with_self_ty(self.tcx, self.tcx.types.trait_object_dummy_self);
 204                     self.deduce_sig_from_projection(None, pb)
 205                 });
 206                 let kind = object_type
 207                     .principal_def_id()
 208                     .and_then(|did| self.tcx.fn_trait_kind_from_lang_item(did));
 209                 (sig, kind)
 210             }
 211             ty::Infer(ty::TyVar(vid)) => self.deduce_expectations_from_obligations(vid),
 212             ty::FnPtr(sig) => {
 213                 let expected_sig = ExpectedSig { cause_span: None, sig };
 214                 (Some(expected_sig), Some(ty::ClosureKind::Fn))
 215             }
 216             _ => (None, None),
 217         }
 218     }
 219
 220     fn deduce_expectations_from_obligations(
 221         &self,
 222         expected_vid: ty::TyVid,
 223     ) -> (Option<ExpectedSig<'tcx>>, Option<ty::ClosureKind>) {
 224         let expected_sig =
 225             self.obligations_for_self_ty(expected_vid).find_map(|(_, obligation)| {
 226                 debug!(?obligation.predicate);
 227
 228                 let bound_predicate = obligation.predicate.kind();
 229                 if let ty::PredicateKind::Projection(proj_predicate) =
 230                     obligation.predicate.kind().skip_binder()
 231                 {
 232                     // Given a Projection predicate, we can potentially infer
 233                     // the complete signature.
 234                     self.deduce_sig_from_projection(
 235                         Some(obligation.cause.span),
 236                         bound_predicate.rebind(proj_predicate),
 237                     )
 238                 } else {
 239                     None
 240                 }
 241             });
 242
 243         // Even if we can't infer the full signature, we may be able to
 244         // infer the kind. This can occur when we elaborate a predicate
 245         // like `F : Fn<A>`. Note that due to subtyping we could encounter
 246         // many viable options, so pick the most restrictive.
 247         let expected_kind = self
 248             .obligations_for_self_ty(expected_vid)
 249             .filter_map(|(tr, _)| self.tcx.fn_trait_kind_from_lang_item(tr.def_id()))
 250             .fold(None, |best, cur| Some(best.map_or(cur, |best| cmp::min(best, cur))));
 251
 252         (expected_sig, expected_kind)
 253     }
 254
 255     /// Given a projection like "<F as Fn(X)>::Result == Y", we can deduce
 256     /// everything we need to know about a closure or generator.
 257     ///
 258     /// The `cause_span` should be the span that caused us to
 259     /// have this expected signature, or `None` if we can't readily
 260     /// know that.
 261     #[instrument(level = "debug", skip(self, cause_span), ret)]
 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         let trait_def_id = projection.trait_def_id(tcx);
 270
 271         let is_fn = tcx.fn_trait_kind_from_lang_item(trait_def_id).is_some();
 272         let gen_trait = tcx.require_lang_item(LangItem::Generator, cause_span);
 273         let is_gen = gen_trait == trait_def_id;
 274         if !is_fn && !is_gen {
 275             debug!("not fn or generator");
 276             return None;
 277         }
 278
 279         if is_gen {
 280             // Check that we deduce the signature from the `<_ as std::ops::Generator>::Return`
 281             // associated item and not yield.
 282             let return_assoc_item = self.tcx.associated_item_def_ids(gen_trait)[1];
 283             if return_assoc_item != projection.projection_def_id() {
 284                 debug!("not return assoc item of generator");
 285                 return None;
 286             }
 287         }
 288
 289         let input_tys = if is_fn {
 290             let arg_param_ty = projection.skip_binder().projection_ty.substs.type_at(1);
 291             let arg_param_ty = self.resolve_vars_if_possible(arg_param_ty);
 292             debug!(?arg_param_ty);
 293
 294             match arg_param_ty.kind() {
 295                 &ty::Tuple(tys) => tys,
 296                 _ => return None,
 297             }
 298         } else {
 299             // Generators with a `()` resume type may be defined with 0 or 1 explicit arguments,
 300             // else they must have exactly 1 argument. For now though, just give up in this case.
 301             return None;
 302         };
 303
 304         // Since this is a return parameter type it is safe to unwrap.
 305         let ret_param_ty = projection.skip_binder().term.ty().unwrap();
 306         let ret_param_ty = self.resolve_vars_if_possible(ret_param_ty);
 307         debug!(?ret_param_ty);
 308
 309         let sig = projection.rebind(self.tcx.mk_fn_sig(
 310             input_tys.iter(),
 311             ret_param_ty,
 312             false,
 313             hir::Unsafety::Normal,
 314             Abi::Rust,
 315         ));
 316
 317         Some(ExpectedSig { cause_span, sig })
 318     }
 319
 320     fn sig_of_closure(
 321         &self,
 322         hir_id: hir::HirId,
 323         expr_def_id: DefId,
 324         decl: &hir::FnDecl<'_>,
 325         body: &hir::Body<'_>,
 326         expected_sig: Option<ExpectedSig<'tcx>>,
 327     ) -> ClosureSignatures<'tcx> {
 328         if let Some(e) = expected_sig {
 329             self.sig_of_closure_with_expectation(hir_id, expr_def_id, decl, body, e)
 330         } else {
 331             self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body)
 332         }
 333     }
 334
 335     /// If there is no expected signature, then we will convert the
 336     /// types that the user gave into a signature.
 337     #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
 338     fn sig_of_closure_no_expectation(
 339         &self,
 340         hir_id: hir::HirId,
 341         expr_def_id: DefId,
 342         decl: &hir::FnDecl<'_>,
 343         body: &hir::Body<'_>,
 344     ) -> ClosureSignatures<'tcx> {
 345         let bound_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 346
 347         self.closure_sigs(expr_def_id, body, bound_sig)
 348     }
 349
 350     /// Invoked to compute the signature of a closure expression. This
 351     /// combines any user-provided type annotations (e.g., `|x: u32|
 352     /// -> u32 { .. }`) with the expected signature.
 353     ///
 354     /// The approach is as follows:
 355     ///
 356     /// - Let `S` be the (higher-ranked) signature that we derive from the user's annotations.
 357     /// - Let `E` be the (higher-ranked) signature that we derive from the expectations, if any.
 358     ///   - If we have no expectation `E`, then the signature of the closure is `S`.
 359     ///   - Otherwise, the signature of the closure is E. Moreover:
 360     ///     - Skolemize the late-bound regions in `E`, yielding `E'`.
 361     ///     - Instantiate all the late-bound regions bound in the closure within `S`
 362     ///       with fresh (existential) variables, yielding `S'`
 363     ///     - Require that `E' = S'`
 364     ///       - We could use some kind of subtyping relationship here,
 365     ///         I imagine, but equality is easier and works fine for
 366     ///         our purposes.
 367     ///
 368     /// The key intuition here is that the user's types must be valid
 369     /// from "the inside" of the closure, but the expectation
 370     /// ultimately drives the overall signature.
 371     ///
 372     /// # Examples
 373     ///
 374     /// ```ignore (illustrative)
 375     /// fn with_closure<F>(_: F)
 376     ///   where F: Fn(&u32) -> &u32 { .. }
 377     ///
 378     /// with_closure(|x: &u32| { ... })
 379     /// ```
 380     ///
 381     /// Here:
 382     /// - E would be `fn(&u32) -> &u32`.
 383     /// - S would be `fn(&u32) ->
 384     /// - E' is `&'!0 u32 -> &'!0 u32`
 385     /// - S' is `&'?0 u32 -> ?T`
 386     ///
 387     /// S' can be unified with E' with `['?0 = '!0, ?T = &'!10 u32]`.
 388     ///
 389     /// # Arguments
 390     ///
 391     /// - `expr_def_id`: the `DefId` of the closure expression
 392     /// - `decl`: the HIR declaration of the closure
 393     /// - `body`: the body of the closure
 394     /// - `expected_sig`: the expected signature (if any). Note that
 395     ///   this is missing a binder: that is, there may be late-bound
 396     ///   regions with depth 1, which are bound then by the closure.
 397     #[instrument(skip(self, hir_id, expr_def_id, decl, body), level = "debug")]
 398     fn sig_of_closure_with_expectation(
 399         &self,
 400         hir_id: hir::HirId,
 401         expr_def_id: DefId,
 402         decl: &hir::FnDecl<'_>,
 403         body: &hir::Body<'_>,
 404         expected_sig: ExpectedSig<'tcx>,
 405     ) -> ClosureSignatures<'tcx> {
 406         // Watch out for some surprises and just ignore the
 407         // expectation if things don't see to match up with what we
 408         // expect.
 409         if expected_sig.sig.c_variadic() != decl.c_variadic {
 410             return self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body);
 411         } else if expected_sig.sig.skip_binder().inputs_and_output.len() != decl.inputs.len() + 1 {
 412             return self.sig_of_closure_with_mismatched_number_of_arguments(
 413                 expr_def_id,
 414                 decl,
 415                 body,
 416                 expected_sig,
 417             );
 418         }
 419
 420         // Create a `PolyFnSig`. Note the oddity that late bound
 421         // regions appearing free in `expected_sig` are now bound up
 422         // in this binder we are creating.
 423         assert!(!expected_sig.sig.skip_binder().has_vars_bound_above(ty::INNERMOST));
 424         let bound_sig = expected_sig.sig.map_bound(|sig| {
 425             self.tcx.mk_fn_sig(
 426                 sig.inputs().iter().cloned(),
 427                 sig.output(),
 428                 sig.c_variadic,
 429                 hir::Unsafety::Normal,
 430                 Abi::RustCall,
 431             )
 432         });
 433
 434         // `deduce_expectations_from_expected_type` introduces
 435         // late-bound lifetimes defined elsewhere, which we now
 436         // anonymize away, so as not to confuse the user.
 437         let bound_sig = self.tcx.anonymize_late_bound_regions(bound_sig);
 438
 439         let closure_sigs = self.closure_sigs(expr_def_id, body, bound_sig);
 440
 441         // Up till this point, we have ignored the annotations that the user
 442         // gave. This function will check that they unify successfully.
 443         // Along the way, it also writes out entries for types that the user
 444         // wrote into our typeck results, which are then later used by the privacy
 445         // check.
 446         match self.check_supplied_sig_against_expectation(
 447             hir_id,
 448             expr_def_id,
 449             decl,
 450             body,
 451             &closure_sigs,
 452         ) {
 453             Ok(infer_ok) => self.register_infer_ok_obligations(infer_ok),
 454             Err(_) => return self.sig_of_closure_no_expectation(hir_id, expr_def_id, decl, body),
 455         }
 456
 457         closure_sigs
 458     }
 459
 460     fn sig_of_closure_with_mismatched_number_of_arguments(
 461         &self,
 462         expr_def_id: DefId,
 463         decl: &hir::FnDecl<'_>,
 464         body: &hir::Body<'_>,
 465         expected_sig: ExpectedSig<'tcx>,
 466     ) -> ClosureSignatures<'tcx> {
 467         let hir = self.tcx.hir();
 468         let expr_map_node = hir.get_if_local(expr_def_id).unwrap();
 469         let expected_args: Vec<_> = expected_sig
 470             .sig
 471             .skip_binder()
 472             .inputs()
 473             .iter()
 474             .map(|ty| ArgKind::from_expected_ty(*ty, None))
 475             .collect();
 476         let (closure_span, found_args) = match self.get_fn_like_arguments(expr_map_node) {
 477             Some((sp, args)) => (Some(sp), args),
 478             None => (None, Vec::new()),
 479         };
 480         let expected_span =
 481             expected_sig.cause_span.unwrap_or_else(|| hir.span_if_local(expr_def_id).unwrap());
 482         self.report_arg_count_mismatch(
 483             expected_span,
 484             closure_span,
 485             expected_args,
 486             found_args,
 487             true,
 488         )
 489         .emit();
 490
 491         let error_sig = self.error_sig_of_closure(decl);
 492
 493         self.closure_sigs(expr_def_id, body, error_sig)
 494     }
 495
 496     /// Enforce the user's types against the expectation. See
 497     /// `sig_of_closure_with_expectation` for details on the overall
 498     /// strategy.
 499     fn check_supplied_sig_against_expectation(
 500         &self,
 501         hir_id: hir::HirId,
 502         expr_def_id: DefId,
 503         decl: &hir::FnDecl<'_>,
 504         body: &hir::Body<'_>,
 505         expected_sigs: &ClosureSignatures<'tcx>,
 506     ) -> InferResult<'tcx, ()> {
 507         // Get the signature S that the user gave.
 508         //
 509         // (See comment on `sig_of_closure_with_expectation` for the
 510         // meaning of these letters.)
 511         let supplied_sig = self.supplied_sig_of_closure(hir_id, expr_def_id, decl, body);
 512
 513         debug!("check_supplied_sig_against_expectation: supplied_sig={:?}", supplied_sig);
 514
 515         // FIXME(#45727): As discussed in [this comment][c1], naively
 516         // forcing equality here actually results in suboptimal error
 517         // messages in some cases.  For now, if there would have been
 518         // an obvious error, we fallback to declaring the type of the
 519         // closure to be the one the user gave, which allows other
 520         // error message code to trigger.
 521         //
 522         // However, I think [there is potential to do even better
 523         // here][c2], since in *this* code we have the precise span of
 524         // the type parameter in question in hand when we report the
 525         // error.
 526         //
 527         // [c1]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341089706
 528         // [c2]: https://github.com/rust-lang/rust/pull/45072#issuecomment-341096796
 529         self.commit_if_ok(|_| {
 530             let mut all_obligations = vec![];
 531
 532             // The liberated version of this signature should be a subtype
 533             // of the liberated form of the expectation.
 534             for ((hir_ty, &supplied_ty), expected_ty) in iter::zip(
 535                 iter::zip(
 536                     decl.inputs,
 537                     supplied_sig.inputs().skip_binder(), // binder moved to (*) below
 538                 ),
 539                 expected_sigs.liberated_sig.inputs(), // `liberated_sig` is E'.
 540             ) {
 541                 // Instantiate (this part of..) S to S', i.e., with fresh variables.
 542                 let supplied_ty = self.replace_bound_vars_with_fresh_vars(
 543                     hir_ty.span,
 544                     LateBoundRegionConversionTime::FnCall,
 545                     supplied_sig.inputs().rebind(supplied_ty),
 546                 ); // recreated from (*) above
 547
 548                 // Check that E' = S'.
 549                 let cause = self.misc(hir_ty.span);
 550                 let InferOk { value: (), obligations } =
 551                     self.at(&cause, self.param_env).eq(*expected_ty, supplied_ty)?;
 552                 all_obligations.extend(obligations);
 553             }
 554
 555             let supplied_output_ty = self.replace_bound_vars_with_fresh_vars(
 556                 decl.output.span(),
 557                 LateBoundRegionConversionTime::FnCall,
 558                 supplied_sig.output(),
 559             );
 560             let cause = &self.misc(decl.output.span());
 561             let InferOk { value: (), obligations } = self
 562                 .at(cause, self.param_env)
 563                 .eq(expected_sigs.liberated_sig.output(), supplied_output_ty)?;
 564             all_obligations.extend(obligations);
 565
 566             Ok(InferOk { value: (), obligations: all_obligations })
 567         })
 568     }
 569
 570     /// If there is no expected signature, then we will convert the
 571     /// types that the user gave into a signature.
 572     ///
 573     /// Also, record this closure signature for later.
 574     #[instrument(skip(self, decl, body), level = "debug", ret)]
 575     fn supplied_sig_of_closure(
 576         &self,
 577         hir_id: hir::HirId,
 578         expr_def_id: DefId,
 579         decl: &hir::FnDecl<'_>,
 580         body: &hir::Body<'_>,
 581     ) -> ty::PolyFnSig<'tcx> {
 582         let astconv: &dyn AstConv<'_> = self;
 583
 584         trace!("decl = {:#?}", decl);
 585         debug!(?body.generator_kind);
 586
 587         let bound_vars = self.tcx.late_bound_vars(hir_id);
 588
 589         // First, convert the types that the user supplied (if any).
 590         let supplied_arguments = decl.inputs.iter().map(|a| astconv.ast_ty_to_ty(a));
 591         let supplied_return = match decl.output {
 592             hir::FnRetTy::Return(ref output) => astconv.ast_ty_to_ty(&output),
 593             hir::FnRetTy::DefaultReturn(_) => match body.generator_kind {
 594                 // In the case of the async block that we create for a function body,
 595                 // we expect the return type of the block to match that of the enclosing
 596                 // function.
 597                 Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Fn)) => {
 598                     debug!("closure is async fn body");
 599                     self.deduce_future_output_from_obligations(expr_def_id, body.id().hir_id)
 600                         .unwrap_or_else(|| {
 601                             // AFAIK, deducing the future output
 602                             // always succeeds *except* in error cases
 603                             // like #65159. I'd like to return Error
 604                             // here, but I can't because I can't
 605                             // easily (and locally) prove that we
 606                             // *have* reported an
 607                             // error. --nikomatsakis
 608                             astconv.ty_infer(None, decl.output.span())
 609                         })
 610                 }
 611
 612                 _ => astconv.ty_infer(None, decl.output.span()),
 613             },
 614         };
 615
 616         let result = ty::Binder::bind_with_vars(
 617             self.tcx.mk_fn_sig(
 618                 supplied_arguments,
 619                 supplied_return,
 620                 decl.c_variadic,
 621                 hir::Unsafety::Normal,
 622                 Abi::RustCall,
 623             ),
 624             bound_vars,
 625         );
 626
 627         let c_result = self.inh.infcx.canonicalize_response(result);
 628         self.typeck_results.borrow_mut().user_provided_sigs.insert(expr_def_id, c_result);
 629
 630         result
 631     }
 632
 633     /// Invoked when we are translating the generator that results
 634     /// from desugaring an `async fn`. Returns the "sugared" return
 635     /// type of the `async fn` -- that is, the return type that the
 636     /// user specified. The "desugared" return type is an `impl
 637     /// Future<Output = T>`, so we do this by searching through the
 638     /// obligations to extract the `T`.
 639     #[instrument(skip(self), level = "debug", ret)]
 640     fn deduce_future_output_from_obligations(
 641         &self,
 642         expr_def_id: DefId,
 643         body_id: hir::HirId,
 644     ) -> Option<Ty<'tcx>> {
 645         let ret_coercion = self.ret_coercion.as_ref().unwrap_or_else(|| {
 646             span_bug!(self.tcx.def_span(expr_def_id), "async fn generator outside of a fn")
 647         });
 648
 649         let ret_ty = ret_coercion.borrow().expected_ty();
 650         let ret_ty = self.inh.infcx.shallow_resolve(ret_ty);
 651
 652         let get_future_output = |predicate: ty::Predicate<'tcx>, span| {
 653             // Search for a pending obligation like
 654             //
 655             // `<R as Future>::Output = T`
 656             //
 657             // where R is the return type we are expecting. This type `T`
 658             // will be our output.
 659             let bound_predicate = predicate.kind();
 660             if let ty::PredicateKind::Projection(proj_predicate) = bound_predicate.skip_binder() {
 661                 self.deduce_future_output_from_projection(
 662                     span,
 663                     bound_predicate.rebind(proj_predicate),
 664                 )
 665             } else {
 666                 None
 667             }
 668         };
 669
 670         let output_ty = match *ret_ty.kind() {
 671             ty::Infer(ty::TyVar(ret_vid)) => {
 672                 self.obligations_for_self_ty(ret_vid).find_map(|(_, obligation)| {
 673                     get_future_output(obligation.predicate, obligation.cause.span)
 674                 })?
 675             }
 676             ty::Opaque(def_id, substs) => self
 677                 .tcx
 678                 .bound_explicit_item_bounds(def_id)
 679                 .transpose_iter()
 680                 .map(|e| e.map_bound(|e| *e).transpose_tuple2())
 681                 .find_map(|(p, s)| get_future_output(p.subst(self.tcx, substs), s.0))?,
 682             ty::Error(_) => return None,
 683             _ => span_bug!(
 684                 self.tcx.def_span(expr_def_id),
 685                 "async fn generator return type not an inference variable"
 686             ),
 687         };
 688
 689         // async fn that have opaque types in their return type need to redo the conversion to inference variables
 690         // as they fetch the still opaque version from the signature.
 691         let InferOk { value: output_ty, obligations } = self
 692             .replace_opaque_types_with_inference_vars(
 693                 output_ty,
 694                 body_id,
 695                 self.tcx.def_span(expr_def_id),
 696                 self.param_env,
 697             );
 698         self.register_predicates(obligations);
 699
 700         Some(output_ty)
 701     }
 702
 703     /// Given a projection like
 704     ///
 705     /// `<X as Future>::Output = T`
 706     ///
 707     /// where `X` is some type that has no late-bound regions, returns
 708     /// `Some(T)`. If the projection is for some other trait, returns
 709     /// `None`.
 710     fn deduce_future_output_from_projection(
 711         &self,
 712         cause_span: Span,
 713         predicate: ty::PolyProjectionPredicate<'tcx>,
 714     ) -> Option<Ty<'tcx>> {
 715         debug!("deduce_future_output_from_projection(predicate={:?})", predicate);
 716
 717         // We do not expect any bound regions in our predicate, so
 718         // skip past the bound vars.
 719         let Some(predicate) = predicate.no_bound_vars() else {
 720             debug!("deduce_future_output_from_projection: has late-bound regions");
 721             return None;
 722         };
 723
 724         // Check that this is a projection from the `Future` trait.
 725         let trait_def_id = predicate.projection_ty.trait_def_id(self.tcx);
 726         let future_trait = self.tcx.require_lang_item(LangItem::Future, Some(cause_span));
 727         if trait_def_id != future_trait {
 728             debug!("deduce_future_output_from_projection: not a future");
 729             return None;
 730         }
 731
 732         // The `Future` trait has only one associated item, `Output`,
 733         // so check that this is what we see.
 734         let output_assoc_item = self.tcx.associated_item_def_ids(future_trait)[0];
 735         if output_assoc_item != predicate.projection_ty.item_def_id {
 736             span_bug!(
 737                 cause_span,
 738                 "projecting associated item `{:?}` from future, which is not Output `{:?}`",
 739                 predicate.projection_ty.item_def_id,
 740                 output_assoc_item,
 741             );
 742         }
 743
 744         // Extract the type from the projection. Note that there can
 745         // be no bound variables in this type because the "self type"
 746         // does not have any regions in it.
 747         let output_ty = self.resolve_vars_if_possible(predicate.term);
 748         debug!("deduce_future_output_from_projection: output_ty={:?}", output_ty);
 749         // This is a projection on a Fn trait so will always be a type.
 750         Some(output_ty.ty().unwrap())
 751     }
 752
 753     /// Converts the types that the user supplied, in case that doing
 754     /// so should yield an error, but returns back a signature where
 755     /// all parameters are of type `TyErr`.
 756     fn error_sig_of_closure(&self, decl: &hir::FnDecl<'_>) -> ty::PolyFnSig<'tcx> {
 757         let astconv: &dyn AstConv<'_> = self;
 758
 759         let supplied_arguments = decl.inputs.iter().map(|a| {
 760             // Convert the types that the user supplied (if any), but ignore them.
 761             astconv.ast_ty_to_ty(a);
 762             self.tcx.ty_error()
 763         });
 764
 765         if let hir::FnRetTy::Return(ref output) = decl.output {
 766             astconv.ast_ty_to_ty(&output);
 767         }
 768
 769         let result = ty::Binder::dummy(self.tcx.mk_fn_sig(
 770             supplied_arguments,
 771             self.tcx.ty_error(),
 772             decl.c_variadic,
 773             hir::Unsafety::Normal,
 774             Abi::RustCall,
 775         ));
 776
 777         debug!("supplied_sig_of_closure: result={:?}", result);
 778
 779         result
 780     }
 781
 782     fn closure_sigs(
 783         &self,
 784         expr_def_id: DefId,
 785         body: &hir::Body<'_>,
 786         bound_sig: ty::PolyFnSig<'tcx>,
 787     ) -> ClosureSignatures<'tcx> {
 788         let liberated_sig = self.tcx().liberate_late_bound_regions(expr_def_id, bound_sig);
 789         let liberated_sig = self.inh.normalize_associated_types_in(
 790             body.value.span,
 791             body.value.hir_id,
 792             self.param_env,
 793             liberated_sig,
 794         );
 795         ClosureSignatures { bound_sig, liberated_sig }
 796     }
 797 }