compiler/rustc_hir_typeck/src/callee.rs

   1 use super::method::probe::{IsSuggestion, Mode, ProbeScope};
   2 use super::method::MethodCallee;
   3 use super::{Expectation, FnCtxt, TupleArgumentsFlag};
   4
   5 use crate::type_error_struct;
   6 use rustc_ast::util::parser::PREC_POSTFIX;
   7 use rustc_errors::{struct_span_err, Applicability, Diagnostic, StashKey};
   8 use rustc_hir as hir;
   9 use rustc_hir::def::{self, CtorKind, Namespace, Res};
  10 use rustc_hir::def_id::DefId;
  11 use rustc_infer::{
  12     infer,
  13     traits::{self, Obligation},
  14 };
  15 use rustc_infer::{
  16     infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind},
  17     traits::ObligationCause,
  18 };
  19 use rustc_middle::ty::adjustment::{
  20     Adjust, Adjustment, AllowTwoPhase, AutoBorrow, AutoBorrowMutability,
  21 };
  22 use rustc_middle::ty::SubstsRef;
  23 use rustc_middle::ty::{self, Ty, TyCtxt, TypeVisitable};
  24 use rustc_span::def_id::LocalDefId;
  25 use rustc_span::symbol::{sym, Ident};
  26 use rustc_span::Span;
  27 use rustc_target::spec::abi;
  28 use rustc_trait_selection::autoderef::Autoderef;
  29 use rustc_trait_selection::infer::InferCtxtExt as _;
  30 use rustc_trait_selection::traits::error_reporting::DefIdOrName;
  31 use rustc_trait_selection::traits::query::evaluate_obligation::InferCtxtExt as _;
  32
  33 use std::{iter, slice};
  34
  35 /// Checks that it is legal to call methods of the trait corresponding
  36 /// to `trait_id` (this only cares about the trait, not the specific
  37 /// method that is called).
  38 pub fn check_legal_trait_for_method_call(
  39     tcx: TyCtxt<'_>,
  40     span: Span,
  41     receiver: Option<Span>,
  42     expr_span: Span,
  43     trait_id: DefId,
  44 ) {
  45     if tcx.lang_items().drop_trait() == Some(trait_id) {
  46         let mut err = struct_span_err!(tcx.sess, span, E0040, "explicit use of destructor method");
  47         err.span_label(span, "explicit destructor calls not allowed");
  48
  49         let (sp, suggestion) = receiver
  50             .and_then(|s| tcx.sess.source_map().span_to_snippet(s).ok())
  51             .filter(|snippet| !snippet.is_empty())
  52             .map(|snippet| (expr_span, format!("drop({snippet})")))
  53             .unwrap_or_else(|| (span, "drop".to_string()));
  54
  55         err.span_suggestion(
  56             sp,
  57             "consider using `drop` function",
  58             suggestion,
  59             Applicability::MaybeIncorrect,
  60         );
  61
  62         err.emit();
  63     }
  64 }
  65
  66 #[derive(Debug)]
  67 enum CallStep<'tcx> {
  68     Builtin(Ty<'tcx>),
  69     DeferredClosure(LocalDefId, ty::FnSig<'tcx>),
  70     /// E.g., enum variant constructors.
  71     Overloaded(MethodCallee<'tcx>),
  72 }
  73
  74 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  75     pub fn check_call(
  76         &self,
  77         call_expr: &'tcx hir::Expr<'tcx>,
  78         callee_expr: &'tcx hir::Expr<'tcx>,
  79         arg_exprs: &'tcx [hir::Expr<'tcx>],
  80         expected: Expectation<'tcx>,
  81     ) -> Ty<'tcx> {
  82         let original_callee_ty = match &callee_expr.kind {
  83             hir::ExprKind::Path(hir::QPath::Resolved(..) | hir::QPath::TypeRelative(..)) => self
  84                 .check_expr_with_expectation_and_args(
  85                     callee_expr,
  86                     Expectation::NoExpectation,
  87                     arg_exprs,
  88                 ),
  89             _ => self.check_expr(callee_expr),
  90         };
  91
  92         let expr_ty = self.structurally_resolved_type(call_expr.span, original_callee_ty);
  93
  94         let mut autoderef = self.autoderef(callee_expr.span, expr_ty);
  95         let mut result = None;
  96         while result.is_none() && autoderef.next().is_some() {
  97             result = self.try_overloaded_call_step(call_expr, callee_expr, arg_exprs, &autoderef);
  98         }
  99         self.register_predicates(autoderef.into_obligations());
 100
 101         let output = match result {
 102             None => {
 103                 // this will report an error since original_callee_ty is not a fn
 104                 self.confirm_builtin_call(
 105                     call_expr,
 106                     callee_expr,
 107                     original_callee_ty,
 108                     arg_exprs,
 109                     expected,
 110                 )
 111             }
 112
 113             Some(CallStep::Builtin(callee_ty)) => {
 114                 self.confirm_builtin_call(call_expr, callee_expr, callee_ty, arg_exprs, expected)
 115             }
 116
 117             Some(CallStep::DeferredClosure(def_id, fn_sig)) => {
 118                 self.confirm_deferred_closure_call(call_expr, arg_exprs, expected, def_id, fn_sig)
 119             }
 120
 121             Some(CallStep::Overloaded(method_callee)) => {
 122                 self.confirm_overloaded_call(call_expr, arg_exprs, expected, method_callee)
 123             }
 124         };
 125
 126         // we must check that return type of called functions is WF:
 127         self.register_wf_obligation(output.into(), call_expr.span, traits::WellFormed(None));
 128
 129         output
 130     }
 131
 132     #[instrument(level = "debug", skip(self, call_expr, callee_expr, arg_exprs, autoderef), ret)]
 133     fn try_overloaded_call_step(
 134         &self,
 135         call_expr: &'tcx hir::Expr<'tcx>,
 136         callee_expr: &'tcx hir::Expr<'tcx>,
 137         arg_exprs: &'tcx [hir::Expr<'tcx>],
 138         autoderef: &Autoderef<'a, 'tcx>,
 139     ) -> Option<CallStep<'tcx>> {
 140         let adjusted_ty =
 141             self.structurally_resolved_type(autoderef.span(), autoderef.final_ty(false));
 142
 143         // If the callee is a bare function or a closure, then we're all set.
 144         match *adjusted_ty.kind() {
 145             ty::FnDef(..) | ty::FnPtr(_) => {
 146                 let adjustments = self.adjust_steps(autoderef);
 147                 self.apply_adjustments(callee_expr, adjustments);
 148                 return Some(CallStep::Builtin(adjusted_ty));
 149             }
 150
 151             ty::Closure(def_id, substs) => {
 152                 let def_id = def_id.expect_local();
 153
 154                 // Check whether this is a call to a closure where we
 155                 // haven't yet decided on whether the closure is fn vs
 156                 // fnmut vs fnonce. If so, we have to defer further processing.
 157                 if self.closure_kind(substs).is_none() {
 158                     let closure_sig = substs.as_closure().sig();
 159                     let closure_sig = self.replace_bound_vars_with_fresh_vars(
 160                         call_expr.span,
 161                         infer::FnCall,
 162                         closure_sig,
 163                     );
 164                     let adjustments = self.adjust_steps(autoderef);
 165                     self.record_deferred_call_resolution(
 166                         def_id,
 167                         DeferredCallResolution {
 168                             call_expr,
 169                             callee_expr,
 170                             adjusted_ty,
 171                             adjustments,
 172                             fn_sig: closure_sig,
 173                             closure_substs: substs,
 174                         },
 175                     );
 176                     return Some(CallStep::DeferredClosure(def_id, closure_sig));
 177                 }
 178             }
 179
 180             // Hack: we know that there are traits implementing Fn for &F
 181             // where F:Fn and so forth. In the particular case of types
 182             // like `f: &mut FnMut()`, if there is a call `f()`, we would
 183             // normally translate to `FnMut::call_mut(&mut f, ())`, but
 184             // that winds up potentially requiring the user to mark their
 185             // variable as `mut` which feels unnecessary and unexpected.
 186             //
 187             //     fn foo(f: &mut impl FnMut()) { f() }
 188             //            ^ without this hack `f` would have to be declared as mutable
 189             //
 190             // The simplest fix by far is to just ignore this case and deref again,
 191             // so we wind up with `FnMut::call_mut(&mut *f, ())`.
 192             ty::Ref(..) if autoderef.step_count() == 0 => {
 193                 return None;
 194             }
 195
 196             ty::Error(_) => {
 197                 return None;
 198             }
 199
 200             _ => {}
 201         }
 202
 203         // Now, we look for the implementation of a Fn trait on the object's type.
 204         // We first do it with the explicit instruction to look for an impl of
 205         // `Fn<Tuple>`, with the tuple `Tuple` having an arity corresponding
 206         // to the number of call parameters.
 207         // If that fails (or_else branch), we try again without specifying the
 208         // shape of the tuple (hence the None). This allows to detect an Fn trait
 209         // is implemented, and use this information for diagnostic.
 210         self.try_overloaded_call_traits(call_expr, adjusted_ty, Some(arg_exprs))
 211             .or_else(|| self.try_overloaded_call_traits(call_expr, adjusted_ty, None))
 212             .map(|(autoref, method)| {
 213                 let mut adjustments = self.adjust_steps(autoderef);
 214                 adjustments.extend(autoref);
 215                 self.apply_adjustments(callee_expr, adjustments);
 216                 CallStep::Overloaded(method)
 217             })
 218     }
 219
 220     fn try_overloaded_call_traits(
 221         &self,
 222         call_expr: &hir::Expr<'_>,
 223         adjusted_ty: Ty<'tcx>,
 224         opt_arg_exprs: Option<&'tcx [hir::Expr<'tcx>]>,
 225     ) -> Option<(Option<Adjustment<'tcx>>, MethodCallee<'tcx>)> {
 226         // Try the options that are least restrictive on the caller first.
 227         for (opt_trait_def_id, method_name, borrow) in [
 228             (self.tcx.lang_items().fn_trait(), Ident::with_dummy_span(sym::call), true),
 229             (self.tcx.lang_items().fn_mut_trait(), Ident::with_dummy_span(sym::call_mut), true),
 230             (self.tcx.lang_items().fn_once_trait(), Ident::with_dummy_span(sym::call_once), false),
 231         ] {
 232             let Some(trait_def_id) = opt_trait_def_id else { continue };
 233
 234             let opt_input_type = opt_arg_exprs.map(|arg_exprs| {
 235                 self.tcx.mk_tup(arg_exprs.iter().map(|e| {
 236                     self.next_ty_var(TypeVariableOrigin {
 237                         kind: TypeVariableOriginKind::TypeInference,
 238                         span: e.span,
 239                     })
 240                 }))
 241             });
 242
 243             if let Some(ok) = self.lookup_method_in_trait(
 244                 call_expr.span,
 245                 method_name,
 246                 trait_def_id,
 247                 adjusted_ty,
 248                 opt_input_type.as_ref().map(slice::from_ref),
 249             ) {
 250                 let method = self.register_infer_ok_obligations(ok);
 251                 let mut autoref = None;
 252                 if borrow {
 253                     // Check for &self vs &mut self in the method signature. Since this is either
 254                     // the Fn or FnMut trait, it should be one of those.
 255                     let ty::Ref(region, _, mutbl) = method.sig.inputs()[0].kind() else {
 256                         // The `fn`/`fn_mut` lang item is ill-formed, which should have
 257                         // caused an error elsewhere.
 258                         self.tcx
 259                             .sess
 260                             .delay_span_bug(call_expr.span, "input to call/call_mut is not a ref?");
 261                         return None;
 262                     };
 263
 264                     let mutbl = match mutbl {
 265                         hir::Mutability::Not => AutoBorrowMutability::Not,
 266                         hir::Mutability::Mut => AutoBorrowMutability::Mut {
 267                             // For initial two-phase borrow
 268                             // deployment, conservatively omit
 269                             // overloaded function call ops.
 270                             allow_two_phase_borrow: AllowTwoPhase::No,
 271                         },
 272                     };
 273                     autoref = Some(Adjustment {
 274                         kind: Adjust::Borrow(AutoBorrow::Ref(*region, mutbl)),
 275                         target: method.sig.inputs()[0],
 276                     });
 277                 }
 278                 return Some((autoref, method));
 279             }
 280         }
 281
 282         None
 283     }
 284
 285     /// Give appropriate suggestion when encountering `||{/* not callable */}()`, where the
 286     /// likely intention is to call the closure, suggest `(||{})()`. (#55851)
 287     fn identify_bad_closure_def_and_call(
 288         &self,
 289         err: &mut Diagnostic,
 290         hir_id: hir::HirId,
 291         callee_node: &hir::ExprKind<'_>,
 292         callee_span: Span,
 293     ) {
 294         let hir = self.tcx.hir();
 295         let parent_hir_id = hir.get_parent_node(hir_id);
 296         let parent_node = hir.get(parent_hir_id);
 297         if let (
 298             hir::Node::Expr(hir::Expr {
 299                 kind: hir::ExprKind::Closure(&hir::Closure { fn_decl_span, body, .. }),
 300                 ..
 301             }),
 302             hir::ExprKind::Block(..),
 303         ) = (parent_node, callee_node)
 304         {
 305             let fn_decl_span = if hir.body(body).generator_kind
 306                 == Some(hir::GeneratorKind::Async(hir::AsyncGeneratorKind::Closure))
 307             {
 308                 // Actually need to unwrap a few more layers of HIR to get to
 309                 // the _real_ closure...
 310                 let async_closure = hir.get_parent_node(hir.get_parent_node(parent_hir_id));
 311                 if let hir::Node::Expr(hir::Expr {
 312                     kind: hir::ExprKind::Closure(&hir::Closure { fn_decl_span, .. }),
 313                     ..
 314                 }) = hir.get(async_closure)
 315                 {
 316                     fn_decl_span
 317                 } else {
 318                     return;
 319                 }
 320             } else {
 321                 fn_decl_span
 322             };
 323
 324             let start = fn_decl_span.shrink_to_lo();
 325             let end = callee_span.shrink_to_hi();
 326             err.multipart_suggestion(
 327                 "if you meant to create this closure and immediately call it, surround the \
 328                 closure with parentheses",
 329                 vec![(start, "(".to_string()), (end, ")".to_string())],
 330                 Applicability::MaybeIncorrect,
 331             );
 332         }
 333     }
 334
 335     /// Give appropriate suggestion when encountering `[("a", 0) ("b", 1)]`, where the
 336     /// likely intention is to create an array containing tuples.
 337     fn maybe_suggest_bad_array_definition(
 338         &self,
 339         err: &mut Diagnostic,
 340         call_expr: &'tcx hir::Expr<'tcx>,
 341         callee_expr: &'tcx hir::Expr<'tcx>,
 342     ) -> bool {
 343         let hir_id = self.tcx.hir().get_parent_node(call_expr.hir_id);
 344         let parent_node = self.tcx.hir().get(hir_id);
 345         if let (
 346             hir::Node::Expr(hir::Expr { kind: hir::ExprKind::Array(_), .. }),
 347             hir::ExprKind::Tup(exp),
 348             hir::ExprKind::Call(_, args),
 349         ) = (parent_node, &callee_expr.kind, &call_expr.kind)
 350             && args.len() == exp.len()
 351         {
 352             let start = callee_expr.span.shrink_to_hi();
 353             err.span_suggestion(
 354                 start,
 355                 "consider separating array elements with a comma",
 356                 ",",
 357                 Applicability::MaybeIncorrect,
 358             );
 359             return true;
 360         }
 361         false
 362     }
 363
 364     fn confirm_builtin_call(
 365         &self,
 366         call_expr: &'tcx hir::Expr<'tcx>,
 367         callee_expr: &'tcx hir::Expr<'tcx>,
 368         callee_ty: Ty<'tcx>,
 369         arg_exprs: &'tcx [hir::Expr<'tcx>],
 370         expected: Expectation<'tcx>,
 371     ) -> Ty<'tcx> {
 372         let (fn_sig, def_id) = match *callee_ty.kind() {
 373             ty::FnDef(def_id, subst) => {
 374                 let fn_sig = self.tcx.bound_fn_sig(def_id).subst(self.tcx, subst);
 375
 376                 // Unit testing: function items annotated with
 377                 // `#[rustc_evaluate_where_clauses]` trigger special output
 378                 // to let us test the trait evaluation system.
 379                 if self.tcx.has_attr(def_id, sym::rustc_evaluate_where_clauses) {
 380                     let predicates = self.tcx.predicates_of(def_id);
 381                     let predicates = predicates.instantiate(self.tcx, subst);
 382                     for (predicate, predicate_span) in
 383                         predicates.predicates.iter().zip(&predicates.spans)
 384                     {
 385                         let obligation = Obligation::new(
 386                             self.tcx,
 387                             ObligationCause::dummy_with_span(callee_expr.span),
 388                             self.param_env,
 389                             *predicate,
 390                         );
 391                         let result = self.evaluate_obligation(&obligation);
 392                         self.tcx
 393                             .sess
 394                             .struct_span_err(
 395                                 callee_expr.span,
 396                                 &format!("evaluate({:?}) = {:?}", predicate, result),
 397                             )
 398                             .span_label(*predicate_span, "predicate")
 399                             .emit();
 400                     }
 401                 }
 402                 (fn_sig, Some(def_id))
 403             }
 404             ty::FnPtr(sig) => (sig, None),
 405             _ => {
 406                 if let hir::ExprKind::Path(hir::QPath::Resolved(_, path)) = &callee_expr.kind
 407                     && let [segment] = path.segments
 408                     && let Some(mut diag) = self
 409                         .tcx
 410                         .sess
 411                         .diagnostic()
 412                         .steal_diagnostic(segment.ident.span, StashKey::CallIntoMethod)
 413                 {
 414                     // Try suggesting `foo(a)` -> `a.foo()` if possible.
 415                     if let Some(ty) =
 416                         self.suggest_call_as_method(
 417                             &mut diag,
 418                             segment,
 419                             arg_exprs,
 420                             call_expr,
 421                             expected
 422                         )
 423                     {
 424                         diag.emit();
 425                         return ty;
 426                     } else {
 427                         diag.emit();
 428                     }
 429                 }
 430
 431                 self.report_invalid_callee(call_expr, callee_expr, callee_ty, arg_exprs);
 432
 433                 // This is the "default" function signature, used in case of error.
 434                 // In that case, we check each argument against "error" in order to
 435                 // set up all the node type bindings.
 436                 (
 437                     ty::Binder::dummy(self.tcx.mk_fn_sig(
 438                         self.err_args(arg_exprs.len()).into_iter(),
 439                         self.tcx.ty_error(),
 440                         false,
 441                         hir::Unsafety::Normal,
 442                         abi::Abi::Rust,
 443                     )),
 444                     None,
 445                 )
 446             }
 447         };
 448
 449         // Replace any late-bound regions that appear in the function
 450         // signature with region variables. We also have to
 451         // renormalize the associated types at this point, since they
 452         // previously appeared within a `Binder<>` and hence would not
 453         // have been normalized before.
 454         let fn_sig = self.replace_bound_vars_with_fresh_vars(call_expr.span, infer::FnCall, fn_sig);
 455         let fn_sig = self.normalize_associated_types_in(call_expr.span, fn_sig);
 456
 457         // Call the generic checker.
 458         let expected_arg_tys = self.expected_inputs_for_expected_output(
 459             call_expr.span,
 460             expected,
 461             fn_sig.output(),
 462             fn_sig.inputs(),
 463         );
 464         self.check_argument_types(
 465             call_expr.span,
 466             call_expr,
 467             fn_sig.inputs(),
 468             expected_arg_tys,
 469             arg_exprs,
 470             fn_sig.c_variadic,
 471             TupleArgumentsFlag::DontTupleArguments,
 472             def_id,
 473         );
 474
 475         if fn_sig.abi == abi::Abi::RustCall {
 476             let sp = arg_exprs.last().map_or(call_expr.span, |expr| expr.span);
 477             if let Some(ty) = fn_sig.inputs().last().copied() {
 478                 self.register_bound(
 479                     ty,
 480                     self.tcx.require_lang_item(hir::LangItem::Tuple, Some(sp)),
 481                     traits::ObligationCause::new(sp, self.body_id, traits::RustCall),
 482                 );
 483             } else {
 484                 self.tcx.sess.span_err(
 485                         sp,
 486                         "functions with the \"rust-call\" ABI must take a single non-self tuple argument",
 487                     );
 488             }
 489         }
 490
 491         fn_sig.output()
 492     }
 493
 494     /// Attempts to reinterpret `method(rcvr, args...)` as `rcvr.method(args...)`
 495     /// and suggesting the fix if the method probe is successful.
 496     fn suggest_call_as_method(
 497         &self,
 498         diag: &mut Diagnostic,
 499         segment: &'tcx hir::PathSegment<'tcx>,
 500         arg_exprs: &'tcx [hir::Expr<'tcx>],
 501         call_expr: &'tcx hir::Expr<'tcx>,
 502         expected: Expectation<'tcx>,
 503     ) -> Option<Ty<'tcx>> {
 504         if let [callee_expr, rest @ ..] = arg_exprs {
 505             let callee_ty = self.check_expr(callee_expr);
 506             // First, do a probe with `IsSuggestion(true)` to avoid emitting
 507             // any strange errors. If it's successful, then we'll do a true
 508             // method lookup.
 509             let Ok(pick) = self
 510             .probe_for_name(
 511                 Mode::MethodCall,
 512                 segment.ident,
 513                 IsSuggestion(true),
 514                 callee_ty,
 515                 call_expr.hir_id,
 516                 // We didn't record the in scope traits during late resolution
 517                 // so we need to probe AllTraits unfortunately
 518                 ProbeScope::AllTraits,
 519             ) else {
 520                 return None;
 521             };
 522
 523             let pick = self.confirm_method(
 524                 call_expr.span,
 525                 callee_expr,
 526                 call_expr,
 527                 callee_ty,
 528                 pick,
 529                 segment,
 530             );
 531             if pick.illegal_sized_bound.is_some() {
 532                 return None;
 533             }
 534
 535             let up_to_rcvr_span = segment.ident.span.until(callee_expr.span);
 536             let rest_span = callee_expr.span.shrink_to_hi().to(call_expr.span.shrink_to_hi());
 537             let rest_snippet = if let Some(first) = rest.first() {
 538                 self.tcx
 539                     .sess
 540                     .source_map()
 541                     .span_to_snippet(first.span.to(call_expr.span.shrink_to_hi()))
 542             } else {
 543                 Ok(")".to_string())
 544             };
 545
 546             if let Ok(rest_snippet) = rest_snippet {
 547                 let sugg = if callee_expr.precedence().order() >= PREC_POSTFIX {
 548                     vec![
 549                         (up_to_rcvr_span, "".to_string()),
 550                         (rest_span, format!(".{}({rest_snippet}", segment.ident)),
 551                     ]
 552                 } else {
 553                     vec![
 554                         (up_to_rcvr_span, "(".to_string()),
 555                         (rest_span, format!(").{}({rest_snippet}", segment.ident)),
 556                     ]
 557                 };
 558                 let self_ty = self.resolve_vars_if_possible(pick.callee.sig.inputs()[0]);
 559                 diag.multipart_suggestion(
 560                     format!(
 561                         "use the `.` operator to call the method `{}{}` on `{self_ty}`",
 562                         self.tcx
 563                             .associated_item(pick.callee.def_id)
 564                             .trait_container(self.tcx)
 565                             .map_or_else(
 566                                 || String::new(),
 567                                 |trait_def_id| self.tcx.def_path_str(trait_def_id) + "::"
 568                             ),
 569                         segment.ident
 570                     ),
 571                     sugg,
 572                     Applicability::MaybeIncorrect,
 573                 );
 574
 575                 // Let's check the method fully now
 576                 let return_ty = self.check_method_argument_types(
 577                     segment.ident.span,
 578                     call_expr,
 579                     Ok(pick.callee),
 580                     rest,
 581                     TupleArgumentsFlag::DontTupleArguments,
 582                     expected,
 583                 );
 584
 585                 return Some(return_ty);
 586             }
 587         }
 588
 589         None
 590     }
 591
 592     fn report_invalid_callee(
 593         &self,
 594         call_expr: &'tcx hir::Expr<'tcx>,
 595         callee_expr: &'tcx hir::Expr<'tcx>,
 596         callee_ty: Ty<'tcx>,
 597         arg_exprs: &'tcx [hir::Expr<'tcx>],
 598     ) {
 599         let mut unit_variant = None;
 600         if let hir::ExprKind::Path(qpath) = &callee_expr.kind
 601             && let Res::Def(def::DefKind::Ctor(kind, CtorKind::Const), _)
 602                 = self.typeck_results.borrow().qpath_res(qpath, callee_expr.hir_id)
 603             // Only suggest removing parens if there are no arguments
 604             && arg_exprs.is_empty()
 605         {
 606             let descr = match kind {
 607                 def::CtorOf::Struct => "struct",
 608                 def::CtorOf::Variant => "enum variant",
 609             };
 610             let removal_span = callee_expr.span.shrink_to_hi().to(call_expr.span.shrink_to_hi());
 611             unit_variant = Some((removal_span, descr, rustc_hir_pretty::qpath_to_string(qpath)));
 612         }
 613
 614         let callee_ty = self.resolve_vars_if_possible(callee_ty);
 615         let mut err = type_error_struct!(
 616             self.tcx.sess,
 617             callee_expr.span,
 618             callee_ty,
 619             E0618,
 620             "expected function, found {}",
 621             match &unit_variant {
 622                 Some((_, kind, path)) => format!("{kind} `{path}`"),
 623                 None => format!("`{callee_ty}`"),
 624             }
 625         );
 626
 627         self.identify_bad_closure_def_and_call(
 628             &mut err,
 629             call_expr.hir_id,
 630             &callee_expr.kind,
 631             callee_expr.span,
 632         );
 633
 634         if let Some((removal_span, kind, path)) = &unit_variant {
 635             err.span_suggestion_verbose(
 636                 *removal_span,
 637                 &format!(
 638                     "`{path}` is a unit {kind}, and does not take parentheses to be constructed",
 639                 ),
 640                 "",
 641                 Applicability::MachineApplicable,
 642             );
 643         }
 644
 645         let mut inner_callee_path = None;
 646         let def = match callee_expr.kind {
 647             hir::ExprKind::Path(ref qpath) => {
 648                 self.typeck_results.borrow().qpath_res(qpath, callee_expr.hir_id)
 649             }
 650             hir::ExprKind::Call(ref inner_callee, _) => {
 651                 // If the call spans more than one line and the callee kind is
 652                 // itself another `ExprCall`, that's a clue that we might just be
 653                 // missing a semicolon (Issue #51055)
 654                 let call_is_multiline = self.tcx.sess.source_map().is_multiline(call_expr.span);
 655                 if call_is_multiline {
 656                     err.span_suggestion(
 657                         callee_expr.span.shrink_to_hi(),
 658                         "consider using a semicolon here",
 659                         ";",
 660                         Applicability::MaybeIncorrect,
 661                     );
 662                 }
 663                 if let hir::ExprKind::Path(ref inner_qpath) = inner_callee.kind {
 664                     inner_callee_path = Some(inner_qpath);
 665                     self.typeck_results.borrow().qpath_res(inner_qpath, inner_callee.hir_id)
 666                 } else {
 667                     Res::Err
 668                 }
 669             }
 670             _ => Res::Err,
 671         };
 672
 673         if !self.maybe_suggest_bad_array_definition(&mut err, call_expr, callee_expr) {
 674             if let Some((maybe_def, output_ty, _)) =
 675                 self.extract_callable_info(callee_expr, callee_ty)
 676                 && !self.type_is_sized_modulo_regions(self.param_env, output_ty, callee_expr.span)
 677             {
 678                 let descr = match maybe_def {
 679                     DefIdOrName::DefId(def_id) => self.tcx.def_kind(def_id).descr(def_id),
 680                     DefIdOrName::Name(name) => name,
 681                 };
 682                 err.span_label(
 683                     callee_expr.span,
 684                     format!("this {descr} returns an unsized value `{output_ty}`, so it cannot be called")
 685                 );
 686                 if let DefIdOrName::DefId(def_id) = maybe_def
 687                     && let Some(def_span) = self.tcx.hir().span_if_local(def_id)
 688                 {
 689                     err.span_label(def_span, "the callable type is defined here");
 690                 }
 691             } else {
 692                 err.span_label(call_expr.span, "call expression requires function");
 693             }
 694         }
 695
 696         if let Some(span) = self.tcx.hir().res_span(def) {
 697             let callee_ty = callee_ty.to_string();
 698             let label = match (unit_variant, inner_callee_path) {
 699                 (Some((_, kind, path)), _) => Some(format!("{kind} `{path}` defined here")),
 700                 (_, Some(hir::QPath::Resolved(_, path))) => self
 701                     .tcx
 702                     .sess
 703                     .source_map()
 704                     .span_to_snippet(path.span)
 705                     .ok()
 706                     .map(|p| format!("`{p}` defined here returns `{callee_ty}`")),
 707                 _ => {
 708                     match def {
 709                         // Emit a different diagnostic for local variables, as they are not
 710                         // type definitions themselves, but rather variables *of* that type.
 711                         Res::Local(hir_id) => Some(format!(
 712                             "`{}` has type `{}`",
 713                             self.tcx.hir().name(hir_id),
 714                             callee_ty
 715                         )),
 716                         Res::Def(kind, def_id) if kind.ns() == Some(Namespace::ValueNS) => {
 717                             Some(format!("`{}` defined here", self.tcx.def_path_str(def_id),))
 718                         }
 719                         _ => Some(format!("`{callee_ty}` defined here")),
 720                     }
 721                 }
 722             };
 723             if let Some(label) = label {
 724                 err.span_label(span, label);
 725             }
 726         }
 727         err.emit();
 728     }
 729
 730     fn confirm_deferred_closure_call(
 731         &self,
 732         call_expr: &'tcx hir::Expr<'tcx>,
 733         arg_exprs: &'tcx [hir::Expr<'tcx>],
 734         expected: Expectation<'tcx>,
 735         closure_def_id: LocalDefId,
 736         fn_sig: ty::FnSig<'tcx>,
 737     ) -> Ty<'tcx> {
 738         // `fn_sig` is the *signature* of the closure being called. We
 739         // don't know the full details yet (`Fn` vs `FnMut` etc), but we
 740         // do know the types expected for each argument and the return
 741         // type.
 742
 743         let expected_arg_tys = self.expected_inputs_for_expected_output(
 744             call_expr.span,
 745             expected,
 746             fn_sig.output(),
 747             fn_sig.inputs(),
 748         );
 749
 750         self.check_argument_types(
 751             call_expr.span,
 752             call_expr,
 753             fn_sig.inputs(),
 754             expected_arg_tys,
 755             arg_exprs,
 756             fn_sig.c_variadic,
 757             TupleArgumentsFlag::TupleArguments,
 758             Some(closure_def_id.to_def_id()),
 759         );
 760
 761         fn_sig.output()
 762     }
 763
 764     fn confirm_overloaded_call(
 765         &self,
 766         call_expr: &'tcx hir::Expr<'tcx>,
 767         arg_exprs: &'tcx [hir::Expr<'tcx>],
 768         expected: Expectation<'tcx>,
 769         method_callee: MethodCallee<'tcx>,
 770     ) -> Ty<'tcx> {
 771         let output_type = self.check_method_argument_types(
 772             call_expr.span,
 773             call_expr,
 774             Ok(method_callee),
 775             arg_exprs,
 776             TupleArgumentsFlag::TupleArguments,
 777             expected,
 778         );
 779
 780         self.write_method_call(call_expr.hir_id, method_callee);
 781         output_type
 782     }
 783 }
 784
 785 #[derive(Debug)]
 786 pub struct DeferredCallResolution<'tcx> {
 787     call_expr: &'tcx hir::Expr<'tcx>,
 788     callee_expr: &'tcx hir::Expr<'tcx>,
 789     adjusted_ty: Ty<'tcx>,
 790     adjustments: Vec<Adjustment<'tcx>>,
 791     fn_sig: ty::FnSig<'tcx>,
 792     closure_substs: SubstsRef<'tcx>,
 793 }
 794
 795 impl<'a, 'tcx> DeferredCallResolution<'tcx> {
 796     pub fn resolve(self, fcx: &FnCtxt<'a, 'tcx>) {
 797         debug!("DeferredCallResolution::resolve() {:?}", self);
 798
 799         // we should not be invoked until the closure kind has been
 800         // determined by upvar inference
 801         assert!(fcx.closure_kind(self.closure_substs).is_some());
 802
 803         // We may now know enough to figure out fn vs fnmut etc.
 804         match fcx.try_overloaded_call_traits(self.call_expr, self.adjusted_ty, None) {
 805             Some((autoref, method_callee)) => {
 806                 // One problem is that when we get here, we are going
 807                 // to have a newly instantiated function signature
 808                 // from the call trait. This has to be reconciled with
 809                 // the older function signature we had before. In
 810                 // principle we *should* be able to fn_sigs(), but we
 811                 // can't because of the annoying need for a TypeTrace.
 812                 // (This always bites me, should find a way to
 813                 // refactor it.)
 814                 let method_sig = method_callee.sig;
 815
 816                 debug!("attempt_resolution: method_callee={:?}", method_callee);
 817
 818                 for (method_arg_ty, self_arg_ty) in
 819                     iter::zip(method_sig.inputs().iter().skip(1), self.fn_sig.inputs())
 820                 {
 821                     fcx.demand_eqtype(self.call_expr.span, *self_arg_ty, *method_arg_ty);
 822                 }
 823
 824                 fcx.demand_eqtype(self.call_expr.span, method_sig.output(), self.fn_sig.output());
 825
 826                 let mut adjustments = self.adjustments;
 827                 adjustments.extend(autoref);
 828                 fcx.apply_adjustments(self.callee_expr, adjustments);
 829
 830                 fcx.write_method_call(self.call_expr.hir_id, method_callee);
 831             }
 832             None => {
 833                 // This can happen if `#![no_core]` is used and the `fn/fn_mut/fn_once`
 834                 // lang items are not defined (issue #86238).
 835                 let mut err = fcx.inh.tcx.sess.struct_span_err(
 836                     self.call_expr.span,
 837                     "failed to find an overloaded call trait for closure call",
 838                 );
 839                 err.help(
 840                     "make sure the `fn`/`fn_mut`/`fn_once` lang items are defined \
 841                      and have associated `call`/`call_mut`/`call_once` functions",
 842                 );
 843                 err.emit();
 844             }
 845         }
 846     }
 847 }