compiler/rustc_typeck/src/check/callee.rs

   1 use super::method::MethodCallee;
   2 use super::{Expectation, FnCtxt, TupleArgumentsFlag};
   3 use crate::type_error_struct;
   4
   5 use rustc_errors::{struct_span_err, Applicability, Diagnostic};
   6 use rustc_hir as hir;
   7 use rustc_hir::def::{Namespace, Res};
   8 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
   9 use rustc_infer::{
  10     infer,
  11     traits::{self, Obligation},
  12 };
  13 use rustc_infer::{
  14     infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind},
  15     traits::ObligationCause,
  16 };
  17 use rustc_middle::ty::adjustment::{
  18     Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability,
  19 };
  20 use rustc_middle::ty::subst::{Subst, SubstsRef};
  21 use rustc_middle::ty::{self, Ty, TyCtxt, TypeFoldable};
  22 use rustc_span::symbol::{sym, Ident};
  23 use rustc_span::Span;
  24 use rustc_target::spec::abi;
  25 use rustc_trait_selection::autoderef::Autoderef;
  26 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt;
  27 use std::iter;
  28
  29 /// Checks that it is legal to call methods of the trait corresponding
  30 /// to `trait_id` (this only cares about the trait, not the specific
  31 /// method that is called).
  32 pub fn check_legal_trait_for_method_call(
  33     tcx: TyCtxt<'_>,
  34     span: Span,
  35     receiver: Option<Span>,
  36     expr_span: Span,
  37     trait_id: DefId,
  38 ) {
  39     if tcx.lang_items().drop_trait() == Some(trait_id) {
  40         let mut err = struct_span_err!(tcx.sess, span, E0040, "explicit use of destructor method");
  41         err.span_label(span, "explicit destructor calls not allowed");
  42
  43         let (sp, suggestion) = receiver
  44             .and_then(|s| tcx.sess.source_map().span_to_snippet(s).ok())
  45             .filter(|snippet| !snippet.is_empty())
  46             .map(|snippet| (expr_span, format!("drop({snippet})")))
  47             .unwrap_or_else(|| (span, "drop".to_string()));
  48
  49         err.span_suggestion(
  50             sp,
  51             "consider using `drop` function",
  52             suggestion,
  53             Applicability::MaybeIncorrect,
  54         );
  55
  56         err.emit();
  57     }
  58 }
  59
  60 enum CallStep<'tcx> {
  61     Builtin(Ty<'tcx>),
  62     DeferredClosure(DefId, ty::FnSig<'tcx>),
  63     /// E.g., enum variant constructors.
  64     Overloaded(MethodCallee<'tcx>),
  65 }
  66
  67 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  68     pub fn check_call(
  69         &self,
  70         call_expr: &'tcx hir::Expr<'tcx>,
  71         callee_expr: &'tcx hir::Expr<'tcx>,
  72         arg_exprs: &'tcx [hir::Expr<'tcx>],
  73         expected: Expectation<'tcx>,
  74     ) -> Ty<'tcx> {
  75         let original_callee_ty = match &callee_expr.kind {
  76             hir::ExprKind::Path(hir::QPath::Resolved(..) | hir::QPath::TypeRelative(..)) => self
  77                 .check_expr_with_expectation_and_args(
  78                     callee_expr,
  79                     Expectation::NoExpectation,
  80                     arg_exprs,
  81                 ),
  82             _ => self.check_expr(callee_expr),
  83         };
  84
  85         let expr_ty = self.structurally_resolved_type(call_expr.span, original_callee_ty);
  86
  87         let mut autoderef = self.autoderef(callee_expr.span, expr_ty);
  88         let mut result = None;
  89         while result.is_none() && autoderef.next().is_some() {
  90             result = self.try_overloaded_call_step(call_expr, callee_expr, arg_exprs, &autoderef);
  91         }
  92         self.register_predicates(autoderef.into_obligations());
  93
  94         let output = match result {
  95             None => {
  96                 // this will report an error since original_callee_ty is not a fn
  97                 self.confirm_builtin_call(
  98                     call_expr,
  99                     callee_expr,
 100                     original_callee_ty,
 101                     arg_exprs,
 102                     expected,
 103                 )
 104             }
 105
 106             Some(CallStep::Builtin(callee_ty)) => {
 107                 self.confirm_builtin_call(call_expr, callee_expr, callee_ty, arg_exprs, expected)
 108             }
 109
 110             Some(CallStep::DeferredClosure(def_id, fn_sig)) => {
 111                 self.confirm_deferred_closure_call(call_expr, arg_exprs, expected, def_id, fn_sig)
 112             }
 113
 114             Some(CallStep::Overloaded(method_callee)) => {
 115                 self.confirm_overloaded_call(call_expr, arg_exprs, expected, method_callee)
 116             }
 117         };
 118
 119         // we must check that return type of called functions is WF:
 120         self.register_wf_obligation(output.into(), call_expr.span, traits::MiscObligation);
 121
 122         output
 123     }
 124
 125     fn try_overloaded_call_step(
 126         &self,
 127         call_expr: &'tcx hir::Expr<'tcx>,
 128         callee_expr: &'tcx hir::Expr<'tcx>,
 129         arg_exprs: &'tcx [hir::Expr<'tcx>],
 130         autoderef: &Autoderef<'a, 'tcx>,
 131     ) -> Option<CallStep<'tcx>> {
 132         let adjusted_ty =
 133             self.structurally_resolved_type(autoderef.span(), autoderef.final_ty(false));
 134         debug!(
 135             "try_overloaded_call_step(call_expr={:?}, adjusted_ty={:?})",
 136             call_expr, adjusted_ty
 137         );
 138
 139         // If the callee is a bare function or a closure, then we're all set.
 140         match *adjusted_ty.kind() {
 141             ty::FnDef(..) | ty::FnPtr(_) => {
 142                 let adjustments = self.adjust_steps(autoderef);
 143                 self.apply_adjustments(callee_expr, adjustments);
 144                 return Some(CallStep::Builtin(adjusted_ty));
 145             }
 146
 147             ty::Closure(def_id, substs) => {
 148                 assert_eq!(def_id.krate, LOCAL_CRATE);
 149
 150                 // Check whether this is a call to a closure where we
 151                 // haven't yet decided on whether the closure is fn vs
 152                 // fnmut vs fnonce. If so, we have to defer further processing.
 153                 if self.closure_kind(substs).is_none() {
 154                     let closure_sig = substs.as_closure().sig();
 155                     let closure_sig = self
 156                         .replace_bound_vars_with_fresh_vars(
 157                             call_expr.span,
 158                             infer::FnCall,
 159                             closure_sig,
 160                         )
 161                         .0;
 162                     let adjustments = self.adjust_steps(autoderef);
 163                     self.record_deferred_call_resolution(
 164                         def_id,
 165                         DeferredCallResolution {
 166                             call_expr,
 167                             callee_expr,
 168                             adjusted_ty,
 169                             adjustments,
 170                             fn_sig: closure_sig,
 171                             closure_substs: substs,
 172                         },
 173                     );
 174                     return Some(CallStep::DeferredClosure(def_id, closure_sig));
 175                 }
 176             }
 177
 178             // Hack: we know that there are traits implementing Fn for &F
 179             // where F:Fn and so forth. In the particular case of types
 180             // like `x: &mut FnMut()`, if there is a call `x()`, we would
 181             // normally translate to `FnMut::call_mut(&mut x, ())`, but
 182             // that winds up requiring `mut x: &mut FnMut()`. A little
 183             // over the top. The simplest fix by far is to just ignore
 184             // this case and deref again, so we wind up with
 185             // `FnMut::call_mut(&mut *x, ())`.
 186             ty::Ref(..) if autoderef.step_count() == 0 => {
 187                 return None;
 188             }
 189
 190             _ => {}
 191         }
 192
 193         // Now, we look for the implementation of a Fn trait on the object's type.
 194         // We first do it with the explicit instruction to look for an impl of
 195         // `Fn<Tuple>`, with the tuple `Tuple` having an arity corresponding
 196         // to the number of call parameters.
 197         // If that fails (or_else branch), we try again without specifying the
 198         // shape of the tuple (hence the None). This allows to detect an Fn trait
 199         // is implemented, and use this information for diagnostic.
 200         self.try_overloaded_call_traits(call_expr, adjusted_ty, Some(arg_exprs))
 201             .or_else(|| self.try_overloaded_call_traits(call_expr, adjusted_ty, None))
 202             .map(|(autoref, method)| {
 203                 let mut adjustments = self.adjust_steps(autoderef);
 204                 adjustments.extend(autoref);
 205                 self.apply_adjustments(callee_expr, adjustments);
 206                 CallStep::Overloaded(method)
 207             })
 208     }
 209
 210     fn try_overloaded_call_traits(
 211         &self,
 212         call_expr: &hir::Expr<'_>,
 213         adjusted_ty: Ty<'tcx>,
 214         opt_arg_exprs: Option<&'tcx [hir::Expr<'tcx>]>,
 215     ) -> Option<(Option<Adjustment<'tcx>>, MethodCallee<'tcx>)> {
 216         // Try the options that are least restrictive on the caller first.
 217         for (opt_trait_def_id, method_name, borrow) in [
 218             (self.tcx.lang_items().fn_trait(), Ident::with_dummy_span(sym::call), true),
 219             (self.tcx.lang_items().fn_mut_trait(), Ident::with_dummy_span(sym::call_mut), true),
 220             (self.tcx.lang_items().fn_once_trait(), Ident::with_dummy_span(sym::call_once), false),
 221         ] {
 222             let Some(trait_def_id) = opt_trait_def_id else { continue };
 223
 224             let opt_input_types = opt_arg_exprs.map(|arg_exprs| {
 225                 [self.tcx.mk_tup(arg_exprs.iter().map(|e| {
 226                     self.next_ty_var(TypeVariableOrigin {
 227                         kind: TypeVariableOriginKind::TypeInference,
 228                         span: e.span,
 229                     })
 230                 }))]
 231             });
 232             let opt_input_types = opt_input_types.as_ref().map(AsRef::as_ref);
 233
 234             if let Some(ok) = self.lookup_method_in_trait(
 235                 call_expr.span,
 236                 method_name,
 237                 trait_def_id,
 238                 adjusted_ty,
 239                 opt_input_types,
 240             ) {
 241                 let method = self.register_infer_ok_obligations(ok);
 242                 let mut autoref = None;
 243                 if borrow {
 244                     // Check for &self vs &mut self in the method signature. Since this is either
 245                     // the Fn or FnMut trait, it should be one of those.
 246                     let ty::Ref(region, _, mutbl) = method.sig.inputs()[0].kind() else {
 247                         // The `fn`/`fn_mut` lang item is ill-formed, which should have
 248                         // caused an error elsewhere.
 249                         self.tcx
 250                             .sess
 251                             .delay_span_bug(call_expr.span, "input to call/call_mut is not a ref?");
 252                         return None;
 253                     };
 254
 255                     let mutbl = match mutbl {
 256                         hir::Mutability::Not => AutoBorrowMutability::Not,
 257                         hir::Mutability::Mut => AutoBorrowMutability::Mut {
 258                             // For initial two-phase borrow
 259                             // deployment, conservatively omit
 260                             // overloaded function call ops.
 261                             allow_two_phase_borrow: AllowTwoPhase::No,
 262                         },
 263                     };
 264                     autoref = Some(Adjustment {
 265                         kind: Adjust::Borrow(AutoBorrow::Ref(*region, mutbl)),
 266                         target: method.sig.inputs()[0],
 267                     });
 268                 }
 269                 return Some((autoref, method));
 270             }
 271         }
 272
 273         None
 274     }
 275
 276     /// Give appropriate suggestion when encountering `||{/* not callable */}()`, where the
 277     /// likely intention is to call the closure, suggest `(||{})()`. (#55851)
 278     fn identify_bad_closure_def_and_call(
 279         &self,
 280         err: &mut Diagnostic,
 281         hir_id: hir::HirId,
 282         callee_node: &hir::ExprKind<'_>,
 283         callee_span: Span,
 284     ) {
 285         let hir_id = self.tcx.hir().get_parent_node(hir_id);
 286         let parent_node = self.tcx.hir().get(hir_id);
 287         if let (
 288             hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Closure(_, _, _, sp, ..), .. }),
 289             hir::ExprKind::Block(..),
 290         ) = (parent_node, callee_node)
 291         {
 292             let start = sp.shrink_to_lo();
 293             let end = callee_span.shrink_to_hi();
 294             err.multipart_suggestion(
 295                 "if you meant to create this closure and immediately call it, surround the \
 296                 closure with parentheses",
 297                 vec![(start, "(".to_string()), (end, ")".to_string())],
 298                 Applicability::MaybeIncorrect,
 299             );
 300         }
 301     }
 302
 303     /// Give appropriate suggestion when encountering `[("a", 0) ("b", 1)]`, where the
 304     /// likely intention is to create an array containing tuples.
 305     fn maybe_suggest_bad_array_definition(
 306         &self,
 307         err: &mut Diagnostic,
 308         call_expr: &'tcx hir::Expr<'tcx>,
 309         callee_expr: &'tcx hir::Expr<'tcx>,
 310     ) -> bool {
 311         let hir_id = self.tcx.hir().get_parent_node(call_expr.hir_id);
 312         let parent_node = self.tcx.hir().get(hir_id);
 313         if let (
 314             hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Array(_), .. }),
 315             hir::ExprKind::Tup(exp),
 316             hir::ExprKind::Call(_, args),
 317         ) = (parent_node, &callee_expr.kind, &call_expr.kind)
 318             && args.len() == exp.len()
 319         {
 320             let start = callee_expr.span.shrink_to_hi();
 321             err.span_suggestion(
 322                 start,
 323                 "consider separating array elements with a comma",
 324                 ",".to_string(),
 325                 Applicability::MaybeIncorrect,
 326             );
 327             return true;
 328         }
 329         false
 330     }
 331
 332     fn confirm_builtin_call(
 333         &self,
 334         call_expr: &'tcx hir::Expr<'tcx>,
 335         callee_expr: &'tcx hir::Expr<'tcx>,
 336         callee_ty: Ty<'tcx>,
 337         arg_exprs: &'tcx [hir::Expr<'tcx>],
 338         expected: Expectation<'tcx>,
 339     ) -> Ty<'tcx> {
 340         let (fn_sig, def_id) = match *callee_ty.kind() {
 341             ty::FnDef(def_id, subst) => {
 342                 let fn_sig = self.tcx.fn_sig(def_id).subst(self.tcx, subst);
 343
 344                 // Unit testing: function items annotated with
 345                 // `#[rustc_evaluate_where_clauses]` trigger special output
 346                 // to let us test the trait evaluation system.
 347                 if self.tcx.has_attr(def_id, sym::rustc_evaluate_where_clauses) {
 348                     let predicates = self.tcx.predicates_of(def_id);
 349                     let predicates = predicates.instantiate(self.tcx, subst);
 350                     for (predicate, predicate_span) in
 351                         predicates.predicates.iter().zip(&predicates.spans)
 352                     {
 353                         let obligation = Obligation::new(
 354                             ObligationCause::dummy_with_span(callee_expr.span),
 355                             self.param_env,
 356                             *predicate,
 357                         );
 358                         let result = self.infcx.evaluate_obligation(&obligation);
 359                         self.tcx
 360                             .sess
 361                             .struct_span_err(
 362                                 callee_expr.span,
 363                                 &format!("evaluate({:?}) = {:?}", predicate, result),
 364                             )
 365                             .span_label(*predicate_span, "predicate")
 366                             .emit();
 367                     }
 368                 }
 369                 (fn_sig, Some(def_id))
 370             }
 371             ty::FnPtr(sig) => (sig, None),
 372             ref t => {
 373                 let mut unit_variant = None;
 374                 let mut removal_span = call_expr.span;
 375                 if let ty::Adt(adt_def, ..) = t
 376                     && adt_def.is_enum()
 377                     && let hir::ExprKind::Call(expr, _) = call_expr.kind
 378                 {
 379                     removal_span =
 380                         expr.span.shrink_to_hi().to(call_expr.span.shrink_to_hi());
 381                     unit_variant =
 382                         self.tcx.sess.source_map().span_to_snippet(expr.span).ok();
 383                 }
 384
 385                 let callee_ty = self.resolve_vars_if_possible(callee_ty);
 386                 let mut err = type_error_struct!(
 387                     self.tcx.sess,
 388                     callee_expr.span,
 389                     callee_ty,
 390                     E0618,
 391                     "expected function, found {}",
 392                     match unit_variant {
 393                         Some(ref path) => format!("enum variant `{path}`"),
 394                         None => format!("`{callee_ty}`"),
 395                     }
 396                 );
 397
 398                 self.identify_bad_closure_def_and_call(
 399                     &mut err,
 400                     call_expr.hir_id,
 401                     &callee_expr.kind,
 402                     callee_expr.span,
 403                 );
 404
 405                 if let Some(ref path) = unit_variant {
 406                     err.span_suggestion_verbose(
 407                         removal_span,
 408                         &format!(
 409                             "`{path}` is a unit variant, you need to write it without the parentheses",
 410                         ),
 411                         String::new(),
 412                         Applicability::MachineApplicable,
 413                     );
 414                 }
 415
 416                 let mut inner_callee_path = None;
 417                 let def = match callee_expr.kind {
 418                     hir::ExprKind::Path(ref qpath) => {
 419                         self.typeck_results.borrow().qpath_res(qpath, callee_expr.hir_id)
 420                     }
 421                     hir::ExprKind::Call(ref inner_callee, _) => {
 422                         // If the call spans more than one line and the callee kind is
 423                         // itself another `ExprCall`, that's a clue that we might just be
 424                         // missing a semicolon (Issue #51055)
 425                         let call_is_multiline =
 426                             self.tcx.sess.source_map().is_multiline(call_expr.span);
 427                         if call_is_multiline {
 428                             err.span_suggestion(
 429                                 callee_expr.span.shrink_to_hi(),
 430                                 "consider using a semicolon here",
 431                                 ";".to_owned(),
 432                                 Applicability::MaybeIncorrect,
 433                             );
 434                         }
 435                         if let hir::ExprKind::Path(ref inner_qpath) = inner_callee.kind {
 436                             inner_callee_path = Some(inner_qpath);
 437                             self.typeck_results.borrow().qpath_res(inner_qpath, inner_callee.hir_id)
 438                         } else {
 439                             Res::Err
 440                         }
 441                     }
 442                     _ => Res::Err,
 443                 };
 444
 445                 if !self.maybe_suggest_bad_array_definition(&mut err, call_expr, callee_expr) {
 446                     err.span_label(call_expr.span, "call expression requires function");
 447                 }
 448
 449                 if let Some(span) = self.tcx.hir().res_span(def) {
 450                     let callee_ty = callee_ty.to_string();
 451                     let label = match (unit_variant, inner_callee_path) {
 452                         (Some(path), _) => Some(format!("`{path}` defined here")),
 453                         (_, Some(hir::QPath::Resolved(_, path))) => self
 454                             .tcx
 455                             .sess
 456                             .source_map()
 457                             .span_to_snippet(path.span)
 458                             .ok()
 459                             .map(|p| format!("`{p}` defined here returns `{callee_ty}`")),
 460                         _ => {
 461                             match def {
 462                                 // Emit a different diagnostic for local variables, as they are not
 463                                 // type definitions themselves, but rather variables *of* that type.
 464                                 Res::Local(hir_id) => Some(format!(
 465                                     "`{}` has type `{}`",
 466                                     self.tcx.hir().name(hir_id),
 467                                     callee_ty
 468                                 )),
 469                                 Res::Def(kind, def_id) if kind.ns() == Some(Namespace::ValueNS) => {
 470                                     Some(format!(
 471                                         "`{}` defined here",
 472                                         self.tcx.def_path_str(def_id),
 473                                     ))
 474                                 }
 475                                 _ => Some(format!("`{callee_ty}` defined here")),
 476                             }
 477                         }
 478                     };
 479                     if let Some(label) = label {
 480                         err.span_label(span, label);
 481                     }
 482                 }
 483                 err.emit();
 484
 485                 // This is the "default" function signature, used in case of error.
 486                 // In that case, we check each argument against "error" in order to
 487                 // set up all the node type bindings.
 488                 (
 489                     ty::Binder::dummy(self.tcx.mk_fn_sig(
 490                         self.err_args(arg_exprs.len()).into_iter(),
 491                         self.tcx.ty_error(),
 492                         false,
 493                         hir::Unsafety::Normal,
 494                         abi::Abi::Rust,
 495                     )),
 496                     None,
 497                 )
 498             }
 499         };
 500
 501         // Replace any late-bound regions that appear in the function
 502         // signature with region variables. We also have to
 503         // renormalize the associated types at this point, since they
 504         // previously appeared within a `Binder<>` and hence would not
 505         // have been normalized before.
 506         let fn_sig =
 507             self.replace_bound_vars_with_fresh_vars(call_expr.span, infer::FnCall, fn_sig).0;
 508         let fn_sig = self.normalize_associated_types_in(call_expr.span, fn_sig);
 509
 510         // Call the generic checker.
 511         let expected_arg_tys = self.expected_inputs_for_expected_output(
 512             call_expr.span,
 513             expected,
 514             fn_sig.output(),
 515             fn_sig.inputs(),
 516         );
 517         self.check_argument_types(
 518             call_expr.span,
 519             call_expr,
 520             fn_sig.inputs(),
 521             expected_arg_tys,
 522             arg_exprs,
 523             fn_sig.c_variadic,
 524             TupleArgumentsFlag::DontTupleArguments,
 525             def_id,
 526         );
 527
 528         fn_sig.output()
 529     }
 530
 531     fn confirm_deferred_closure_call(
 532         &self,
 533         call_expr: &'tcx hir::Expr<'tcx>,
 534         arg_exprs: &'tcx [hir::Expr<'tcx>],
 535         expected: Expectation<'tcx>,
 536         closure_def_id: DefId,
 537         fn_sig: ty::FnSig<'tcx>,
 538     ) -> Ty<'tcx> {
 539         // `fn_sig` is the *signature* of the closure being called. We
 540         // don't know the full details yet (`Fn` vs `FnMut` etc), but we
 541         // do know the types expected for each argument and the return
 542         // type.
 543
 544         let expected_arg_tys = self.expected_inputs_for_expected_output(
 545             call_expr.span,
 546             expected,
 547             fn_sig.output(),
 548             fn_sig.inputs(),
 549         );
 550
 551         self.check_argument_types(
 552             call_expr.span,
 553             call_expr,
 554             fn_sig.inputs(),
 555             expected_arg_tys,
 556             arg_exprs,
 557             fn_sig.c_variadic,
 558             TupleArgumentsFlag::TupleArguments,
 559             Some(closure_def_id),
 560         );
 561
 562         fn_sig.output()
 563     }
 564
 565     fn confirm_overloaded_call(
 566         &self,
 567         call_expr: &'tcx hir::Expr<'tcx>,
 568         arg_exprs: &'tcx [hir::Expr<'tcx>],
 569         expected: Expectation<'tcx>,
 570         method_callee: MethodCallee<'tcx>,
 571     ) -> Ty<'tcx> {
 572         let output_type = self.check_method_argument_types(
 573             call_expr.span,
 574             call_expr,
 575             Ok(method_callee),
 576             arg_exprs,
 577             TupleArgumentsFlag::TupleArguments,
 578             expected,
 579         );
 580
 581         self.write_method_call(call_expr.hir_id, method_callee);
 582         output_type
 583     }
 584 }
 585
 586 #[derive(Debug)]
 587 pub struct DeferredCallResolution<'tcx> {
 588     call_expr: &'tcx hir::Expr<'tcx>,
 589     callee_expr: &'tcx hir::Expr<'tcx>,
 590     adjusted_ty: Ty<'tcx>,
 591     adjustments: Vec<Adjustment<'tcx>>,
 592     fn_sig: ty::FnSig<'tcx>,
 593     closure_substs: SubstsRef<'tcx>,
 594 }
 595
 596 impl<'a, 'tcx> DeferredCallResolution<'tcx> {
 597     pub fn resolve(self, fcx: &FnCtxt<'a, 'tcx>) {
 598         debug!("DeferredCallResolution::resolve() {:?}", self);
 599
 600         // we should not be invoked until the closure kind has been
 601         // determined by upvar inference
 602         assert!(fcx.closure_kind(self.closure_substs).is_some());
 603
 604         // We may now know enough to figure out fn vs fnmut etc.
 605         match fcx.try_overloaded_call_traits(self.call_expr, self.adjusted_ty, None) {
 606             Some((autoref, method_callee)) => {
 607                 // One problem is that when we get here, we are going
 608                 // to have a newly instantiated function signature
 609                 // from the call trait. This has to be reconciled with
 610                 // the older function signature we had before. In
 611                 // principle we *should* be able to fn_sigs(), but we
 612                 // can't because of the annoying need for a TypeTrace.
 613                 // (This always bites me, should find a way to
 614                 // refactor it.)
 615                 let method_sig = method_callee.sig;
 616
 617                 debug!("attempt_resolution: method_callee={:?}", method_callee);
 618
 619                 for (method_arg_ty, self_arg_ty) in
 620                     iter::zip(method_sig.inputs().iter().skip(1), self.fn_sig.inputs())
 621                 {
 622                     fcx.demand_eqtype(self.call_expr.span, *self_arg_ty, *method_arg_ty);
 623                 }
 624
 625                 fcx.demand_eqtype(self.call_expr.span, method_sig.output(), self.fn_sig.output());
 626
 627                 let mut adjustments = self.adjustments;
 628                 adjustments.extend(autoref);
 629                 fcx.apply_adjustments(self.callee_expr, adjustments);
 630
 631                 fcx.write_method_call(self.call_expr.hir_id, method_callee);
 632             }
 633             None => {
 634                 // This can happen if `#![no_core]` is used and the `fn/fn_mut/fn_once`
 635                 // lang items are not defined (issue #86238).
 636                 let mut err = fcx.inh.tcx.sess.struct_span_err(
 637                     self.call_expr.span,
 638                     "failed to find an overloaded call trait for closure call",
 639                 );
 640                 err.help(
 641                     "make sure the `fn`/`fn_mut`/`fn_once` lang items are defined \
 642                      and have associated `call`/`call_mut`/`call_once` functions",
 643                 );
 644                 err.emit();
 645             }
 646         }
 647     }
 648 }