compiler/rustc_typeck/src/check/fn_ctxt/checks.rs

   1 use crate::astconv::AstConv;
   2 use crate::check::coercion::CoerceMany;
   3 use crate::check::fn_ctxt::arg_matrix::{
   4     ArgMatrix, Compatibility, Error, ExpectedIdx, ProvidedIdx,
   5 };
   6 use crate::check::gather_locals::Declaration;
   7 use crate::check::intrinsicck::InlineAsmCtxt;
   8 use crate::check::method::MethodCallee;
   9 use crate::check::Expectation::*;
  10 use crate::check::TupleArgumentsFlag::*;
  11 use crate::check::{
  12     potentially_plural_count, struct_span_err, BreakableCtxt, Diverges, Expectation, FnCtxt,
  13     LocalTy, Needs, TupleArgumentsFlag,
  14 };
  15 use crate::structured_errors::StructuredDiagnostic;
  16
  17 use rustc_ast as ast;
  18 use rustc_errors::{Applicability, Diagnostic, DiagnosticId, MultiSpan};
  19 use rustc_hir as hir;
  20 use rustc_hir::def::{CtorOf, DefKind, Res};
  21 use rustc_hir::def_id::DefId;
  22 use rustc_hir::{ExprKind, Node, QPath};
  23 use rustc_index::vec::IndexVec;
  24 use rustc_infer::infer::error_reporting::{FailureCode, ObligationCauseExt};
  25 use rustc_infer::infer::type_variable::{TypeVariableOrigin, TypeVariableOriginKind};
  26 use rustc_infer::infer::InferOk;
  27 use rustc_infer::infer::TypeTrace;
  28 use rustc_middle::ty::adjustment::AllowTwoPhase;
  29 use rustc_middle::ty::visit::TypeVisitable;
  30 use rustc_middle::ty::{self, DefIdTree, IsSuggestable, Ty};
  31 use rustc_session::Session;
  32 use rustc_span::symbol::Ident;
  33 use rustc_span::{self, Span};
  34 use rustc_trait_selection::traits::{self, ObligationCauseCode, SelectionContext};
  35
  36 use std::iter;
  37 use std::slice;
  38
  39 impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
  40     pub(in super::super) fn check_casts(&self) {
  41         let mut deferred_cast_checks = self.deferred_cast_checks.borrow_mut();
  42         debug!("FnCtxt::check_casts: {} deferred checks", deferred_cast_checks.len());
  43         for cast in deferred_cast_checks.drain(..) {
  44             cast.check(self);
  45         }
  46     }
  47
  48     pub(in super::super) fn check_transmutes(&self) {
  49         let mut deferred_transmute_checks = self.deferred_transmute_checks.borrow_mut();
  50         debug!("FnCtxt::check_transmutes: {} deferred checks", deferred_transmute_checks.len());
  51         for (from, to, span) in deferred_transmute_checks.drain(..) {
  52             self.check_transmute(span, from, to);
  53         }
  54     }
  55
  56     pub(in super::super) fn check_asms(&self) {
  57         let mut deferred_asm_checks = self.deferred_asm_checks.borrow_mut();
  58         debug!("FnCtxt::check_asm: {} deferred checks", deferred_asm_checks.len());
  59         for (asm, hir_id) in deferred_asm_checks.drain(..) {
  60             let enclosing_id = self.tcx.hir().enclosing_body_owner(hir_id);
  61             InlineAsmCtxt::new_in_fn(self).check_asm(asm, enclosing_id);
  62         }
  63     }
  64
  65     pub(in super::super) fn check_method_argument_types(
  66         &self,
  67         sp: Span,
  68         expr: &'tcx hir::Expr<'tcx>,
  69         method: Result<MethodCallee<'tcx>, ()>,
  70         args_no_rcvr: &'tcx [hir::Expr<'tcx>],
  71         tuple_arguments: TupleArgumentsFlag,
  72         expected: Expectation<'tcx>,
  73     ) -> Ty<'tcx> {
  74         let has_error = match method {
  75             Ok(method) => method.substs.references_error() || method.sig.references_error(),
  76             Err(_) => true,
  77         };
  78         if has_error {
  79             let err_inputs = self.err_args(args_no_rcvr.len());
  80
  81             let err_inputs = match tuple_arguments {
  82                 DontTupleArguments => err_inputs,
  83                 TupleArguments => vec![self.tcx.intern_tup(&err_inputs)],
  84             };
  85
  86             self.check_argument_types(
  87                 sp,
  88                 expr,
  89                 &err_inputs,
  90                 None,
  91                 args_no_rcvr,
  92                 false,
  93                 tuple_arguments,
  94                 method.ok().map(|method| method.def_id),
  95             );
  96             return self.tcx.ty_error();
  97         }
  98
  99         let method = method.unwrap();
 100         // HACK(eddyb) ignore self in the definition (see above).
 101         let expected_input_tys = self.expected_inputs_for_expected_output(
 102             sp,
 103             expected,
 104             method.sig.output(),
 105             &method.sig.inputs()[1..],
 106         );
 107         self.check_argument_types(
 108             sp,
 109             expr,
 110             &method.sig.inputs()[1..],
 111             expected_input_tys,
 112             args_no_rcvr,
 113             method.sig.c_variadic,
 114             tuple_arguments,
 115             Some(method.def_id),
 116         );
 117         method.sig.output()
 118     }
 119
 120     /// Generic function that factors out common logic from function calls,
 121     /// method calls and overloaded operators.
 122     pub(in super::super) fn check_argument_types(
 123         &self,
 124         // Span enclosing the call site
 125         call_span: Span,
 126         // Expression of the call site
 127         call_expr: &'tcx hir::Expr<'tcx>,
 128         // Types (as defined in the *signature* of the target function)
 129         formal_input_tys: &[Ty<'tcx>],
 130         // More specific expected types, after unifying with caller output types
 131         expected_input_tys: Option<Vec<Ty<'tcx>>>,
 132         // The expressions for each provided argument
 133         provided_args: &'tcx [hir::Expr<'tcx>],
 134         // Whether the function is variadic, for example when imported from C
 135         c_variadic: bool,
 136         // Whether the arguments have been bundled in a tuple (ex: closures)
 137         tuple_arguments: TupleArgumentsFlag,
 138         // The DefId for the function being called, for better error messages
 139         fn_def_id: Option<DefId>,
 140     ) {
 141         let tcx = self.tcx;
 142
 143         // Conceptually, we've got some number of expected inputs, and some number of provided aguments
 144         // and we can form a grid of whether each argument could satisfy a given input:
 145         //      in1 | in2 | in3 | ...
 146         // arg1  ?  |     |     |
 147         // arg2     |  ?  |     |
 148         // arg3     |     |  ?  |
 149         // ...
 150         // Initially, we just check the diagonal, because in the case of correct code
 151         // these are the only checks that matter
 152         // However, in the unhappy path, we'll fill in this whole grid to attempt to provide
 153         // better error messages about invalid method calls.
 154
 155         // All the input types from the fn signature must outlive the call
 156         // so as to validate implied bounds.
 157         for (&fn_input_ty, arg_expr) in iter::zip(formal_input_tys, provided_args) {
 158             self.register_wf_obligation(fn_input_ty.into(), arg_expr.span, traits::MiscObligation);
 159         }
 160
 161         let mut err_code = "E0061";
 162
 163         // If the arguments should be wrapped in a tuple (ex: closures), unwrap them here
 164         let (formal_input_tys, expected_input_tys) = if tuple_arguments == TupleArguments {
 165             let tuple_type = self.structurally_resolved_type(call_span, formal_input_tys[0]);
 166             match tuple_type.kind() {
 167                 // We expected a tuple and got a tuple
 168                 ty::Tuple(arg_types) => {
 169                     // Argument length differs
 170                     if arg_types.len() != provided_args.len() {
 171                         err_code = "E0057";
 172                     }
 173                     let expected_input_tys = match expected_input_tys {
 174                         Some(expected_input_tys) => match expected_input_tys.get(0) {
 175                             Some(ty) => match ty.kind() {
 176                                 ty::Tuple(tys) => Some(tys.iter().collect()),
 177                                 _ => None,
 178                             },
 179                             None => None,
 180                         },
 181                         None => None,
 182                     };
 183                     (arg_types.iter().collect(), expected_input_tys)
 184                 }
 185                 _ => {
 186                     // Otherwise, there's a mismatch, so clear out what we're expecting, and set
 187                     // our input types to err_args so we don't blow up the error messages
 188                     struct_span_err!(
 189                         tcx.sess,
 190                         call_span,
 191                         E0059,
 192                         "cannot use call notation; the first type parameter \
 193                          for the function trait is neither a tuple nor unit"
 194                     )
 195                     .emit();
 196                     (self.err_args(provided_args.len()), None)
 197                 }
 198             }
 199         } else {
 200             (formal_input_tys.to_vec(), expected_input_tys)
 201         };
 202
 203         // If there are no external expectations at the call site, just use the types from the function defn
 204         let expected_input_tys = if let Some(expected_input_tys) = expected_input_tys {
 205             assert_eq!(expected_input_tys.len(), formal_input_tys.len());
 206             expected_input_tys
 207         } else {
 208             formal_input_tys.clone()
 209         };
 210
 211         let minimum_input_count = expected_input_tys.len();
 212         let provided_arg_count = provided_args.len();
 213
 214         // We introduce a helper function to demand that a given argument satisfy a given input
 215         // This is more complicated than just checking type equality, as arguments could be coerced
 216         // This version writes those types back so further type checking uses the narrowed types
 217         let demand_compatible = |idx| {
 218             let formal_input_ty: Ty<'tcx> = formal_input_tys[idx];
 219             let expected_input_ty: Ty<'tcx> = expected_input_tys[idx];
 220             let provided_arg = &provided_args[idx];
 221
 222             debug!("checking argument {}: {:?} = {:?}", idx, provided_arg, formal_input_ty);
 223
 224             // We're on the happy path here, so we'll do a more involved check and write back types
 225             // To check compatibility, we'll do 3 things:
 226             // 1. Unify the provided argument with the expected type
 227             let expectation = Expectation::rvalue_hint(self, expected_input_ty);
 228
 229             let checked_ty = self.check_expr_with_expectation(provided_arg, expectation);
 230
 231             // 2. Coerce to the most detailed type that could be coerced
 232             //    to, which is `expected_ty` if `rvalue_hint` returns an
 233             //    `ExpectHasType(expected_ty)`, or the `formal_ty` otherwise.
 234             let coerced_ty = expectation.only_has_type(self).unwrap_or(formal_input_ty);
 235
 236             // Cause selection errors caused by resolving a single argument to point at the
 237             // argument and not the call. This lets us customize the span pointed to in the
 238             // fulfillment error to be more accurate.
 239             let coerced_ty =
 240                 self.resolve_vars_with_obligations_and_mutate_fulfillment(coerced_ty, |errors| {
 241                     self.point_at_type_arg_instead_of_call_if_possible(errors, call_expr);
 242                     self.point_at_arg_instead_of_call_if_possible(
 243                         errors,
 244                         call_expr,
 245                         call_span,
 246                         provided_args,
 247                         &expected_input_tys,
 248                     );
 249                 });
 250
 251             let coerce_error = self
 252                 .try_coerce(provided_arg, checked_ty, coerced_ty, AllowTwoPhase::Yes, None)
 253                 .err();
 254
 255             if coerce_error.is_some() {
 256                 return Compatibility::Incompatible(coerce_error);
 257             }
 258
 259             // 3. Check if the formal type is a supertype of the checked one
 260             //    and register any such obligations for future type checks
 261             let supertype_error = self
 262                 .at(&self.misc(provided_arg.span), self.param_env)
 263                 .sup(formal_input_ty, coerced_ty);
 264             let subtyping_error = match supertype_error {
 265                 Ok(InferOk { obligations, value: () }) => {
 266                     self.register_predicates(obligations);
 267                     None
 268                 }
 269                 Err(err) => Some(err),
 270             };
 271
 272             // If neither check failed, the types are compatible
 273             match subtyping_error {
 274                 None => Compatibility::Compatible,
 275                 Some(_) => Compatibility::Incompatible(subtyping_error),
 276             }
 277         };
 278
 279         // To start, we only care "along the diagonal", where we expect every
 280         // provided arg to be in the right spot
 281         let mut compatibility_diagonal =
 282             vec![Compatibility::Incompatible(None); provided_args.len()];
 283
 284         // Keep track of whether we *could possibly* be satisfied, i.e. whether we're on the happy path
 285         // if the wrong number of arguments were supplied, we CAN'T be satisfied,
 286         // and if we're c_variadic, the supplied arguments must be >= the minimum count from the function
 287         // otherwise, they need to be identical, because rust doesn't currently support variadic functions
 288         let mut call_appears_satisfied = if c_variadic {
 289             provided_arg_count >= minimum_input_count
 290         } else {
 291             provided_arg_count == minimum_input_count
 292         };
 293
 294         // Check the arguments.
 295         // We do this in a pretty awful way: first we type-check any arguments
 296         // that are not closures, then we type-check the closures. This is so
 297         // that we have more information about the types of arguments when we
 298         // type-check the functions. This isn't really the right way to do this.
 299         for check_closures in [false, true] {
 300             // More awful hacks: before we check argument types, try to do
 301             // an "opportunistic" trait resolution of any trait bounds on
 302             // the call. This helps coercions.
 303             if check_closures {
 304                 self.select_obligations_where_possible(false, |errors| {
 305                     self.point_at_type_arg_instead_of_call_if_possible(errors, call_expr);
 306                     self.point_at_arg_instead_of_call_if_possible(
 307                         errors,
 308                         call_expr,
 309                         call_span,
 310                         &provided_args,
 311                         &expected_input_tys,
 312                     );
 313                 })
 314             }
 315
 316             // Check each argument, to satisfy the input it was provided for
 317             // Visually, we're traveling down the diagonal of the compatibility matrix
 318             for (idx, arg) in provided_args.iter().enumerate() {
 319                 // Warn only for the first loop (the "no closures" one).
 320                 // Closure arguments themselves can't be diverging, but
 321                 // a previous argument can, e.g., `foo(panic!(), || {})`.
 322                 if !check_closures {
 323                     self.warn_if_unreachable(arg.hir_id, arg.span, "expression");
 324                 }
 325
 326                 // For C-variadic functions, we don't have a declared type for all of
 327                 // the arguments hence we only do our usual type checking with
 328                 // the arguments who's types we do know. However, we *can* check
 329                 // for unreachable expressions (see above).
 330                 // FIXME: unreachable warning current isn't emitted
 331                 if idx >= minimum_input_count {
 332                     continue;
 333                 }
 334
 335                 let is_closure = matches!(arg.kind, ExprKind::Closure { .. });
 336                 if is_closure != check_closures {
 337                     continue;
 338                 }
 339
 340                 let compatible = demand_compatible(idx);
 341                 let is_compatible = matches!(compatible, Compatibility::Compatible);
 342                 compatibility_diagonal[idx] = compatible;
 343
 344                 if !is_compatible {
 345                     call_appears_satisfied = false;
 346                 }
 347             }
 348         }
 349
 350         if c_variadic && provided_arg_count < minimum_input_count {
 351             err_code = "E0060";
 352         }
 353
 354         for arg in provided_args.iter().skip(minimum_input_count) {
 355             // Make sure we've checked this expr at least once.
 356             let arg_ty = self.check_expr(&arg);
 357
 358             // If the function is c-style variadic, we skipped a bunch of arguments
 359             // so we need to check those, and write out the types
 360             // Ideally this would be folded into the above, for uniform style
 361             // but c-variadic is already a corner case
 362             if c_variadic {
 363                 fn variadic_error<'tcx>(
 364                     sess: &'tcx Session,
 365                     span: Span,
 366                     ty: Ty<'tcx>,
 367                     cast_ty: &str,
 368                 ) {
 369                     use crate::structured_errors::MissingCastForVariadicArg;
 370
 371                     MissingCastForVariadicArg { sess, span, ty, cast_ty }.diagnostic().emit();
 372                 }
 373
 374                 // There are a few types which get autopromoted when passed via varargs
 375                 // in C but we just error out instead and require explicit casts.
 376                 let arg_ty = self.structurally_resolved_type(arg.span, arg_ty);
 377                 match arg_ty.kind() {
 378                     ty::Float(ty::FloatTy::F32) => {
 379                         variadic_error(tcx.sess, arg.span, arg_ty, "c_double");
 380                     }
 381                     ty::Int(ty::IntTy::I8 | ty::IntTy::I16) | ty::Bool => {
 382                         variadic_error(tcx.sess, arg.span, arg_ty, "c_int");
 383                     }
 384                     ty::Uint(ty::UintTy::U8 | ty::UintTy::U16) => {
 385                         variadic_error(tcx.sess, arg.span, arg_ty, "c_uint");
 386                     }
 387                     ty::FnDef(..) => {
 388                         let ptr_ty = self.tcx.mk_fn_ptr(arg_ty.fn_sig(self.tcx));
 389                         let ptr_ty = self.resolve_vars_if_possible(ptr_ty);
 390                         variadic_error(tcx.sess, arg.span, arg_ty, &ptr_ty.to_string());
 391                     }
 392                     _ => {}
 393                 }
 394             }
 395         }
 396
 397         if !call_appears_satisfied {
 398             let compatibility_diagonal = IndexVec::from_raw(compatibility_diagonal);
 399             let provided_args = IndexVec::from_iter(provided_args.iter().take(if c_variadic {
 400                 minimum_input_count
 401             } else {
 402                 provided_arg_count
 403             }));
 404             debug_assert_eq!(
 405                 formal_input_tys.len(),
 406                 expected_input_tys.len(),
 407                 "expected formal_input_tys to be the same size as expected_input_tys"
 408             );
 409             let formal_and_expected_inputs = IndexVec::from_iter(
 410                 formal_input_tys
 411                     .iter()
 412                     .copied()
 413                     .zip(expected_input_tys.iter().copied())
 414                     .map(|vars| self.resolve_vars_if_possible(vars)),
 415             );
 416
 417             self.report_arg_errors(
 418                 compatibility_diagonal,
 419                 formal_and_expected_inputs,
 420                 provided_args,
 421                 c_variadic,
 422                 err_code,
 423                 fn_def_id,
 424                 call_span,
 425                 call_expr,
 426             );
 427         }
 428     }
 429
 430     fn report_arg_errors(
 431         &self,
 432         compatibility_diagonal: IndexVec<ProvidedIdx, Compatibility<'tcx>>,
 433         formal_and_expected_inputs: IndexVec<ExpectedIdx, (Ty<'tcx>, Ty<'tcx>)>,
 434         provided_args: IndexVec<ProvidedIdx, &'tcx hir::Expr<'tcx>>,
 435         c_variadic: bool,
 436         err_code: &str,
 437         fn_def_id: Option<DefId>,
 438         call_span: Span,
 439         call_expr: &hir::Expr<'tcx>,
 440     ) {
 441         // Next, let's construct the error
 442         let (error_span, full_call_span, ctor_of) = match &call_expr.kind {
 443             hir::ExprKind::Call(
 444                 hir::Expr { hir_id, span, kind: hir::ExprKind::Path(qpath), .. },
 445                 _,
 446             ) => {
 447                 if let Res::Def(DefKind::Ctor(of, _), _) =
 448                     self.typeck_results.borrow().qpath_res(qpath, *hir_id)
 449                 {
 450                     (call_span, *span, Some(of))
 451                 } else {
 452                     (call_span, *span, None)
 453                 }
 454             }
 455             hir::ExprKind::Call(hir::Expr { span, .. }, _) => (call_span, *span, None),
 456             hir::ExprKind::MethodCall(path_segment, _, span) => {
 457                 let ident_span = path_segment.ident.span;
 458                 let ident_span = if let Some(args) = path_segment.args {
 459                     ident_span.with_hi(args.span_ext.hi())
 460                 } else {
 461                     ident_span
 462                 };
 463                 (
 464                     *span, ident_span, None, // methods are never ctors
 465                 )
 466             }
 467             k => span_bug!(call_span, "checking argument types on a non-call: `{:?}`", k),
 468         };
 469         let args_span = error_span.trim_start(full_call_span).unwrap_or(error_span);
 470         let call_name = match ctor_of {
 471             Some(CtorOf::Struct) => "struct",
 472             Some(CtorOf::Variant) => "enum variant",
 473             None => "function",
 474         };
 475
 476         // Don't print if it has error types or is just plain `_`
 477         fn has_error_or_infer<'tcx>(tys: impl IntoIterator<Item = Ty<'tcx>>) -> bool {
 478             tys.into_iter().any(|ty| ty.references_error() || ty.is_ty_var())
 479         }
 480
 481         self.set_tainted_by_errors();
 482         let tcx = self.tcx;
 483
 484         // Precompute the provided types and spans, since that's all we typically need for below
 485         let provided_arg_tys: IndexVec<ProvidedIdx, (Ty<'tcx>, Span)> = provided_args
 486             .iter()
 487             .map(|expr| {
 488                 let ty = self
 489                     .typeck_results
 490                     .borrow()
 491                     .expr_ty_adjusted_opt(*expr)
 492                     .unwrap_or_else(|| tcx.ty_error());
 493                 (self.resolve_vars_if_possible(ty), expr.span)
 494             })
 495             .collect();
 496         let callee_expr = match &call_expr.peel_blocks().kind {
 497             hir::ExprKind::Call(callee, _) => Some(*callee),
 498             hir::ExprKind::MethodCall(_, callee, _) => {
 499                 if let Some((DefKind::AssocFn, def_id)) =
 500                     self.typeck_results.borrow().type_dependent_def(call_expr.hir_id)
 501                     && let Some(assoc) = tcx.opt_associated_item(def_id)
 502                     && assoc.fn_has_self_parameter
 503                 {
 504                     Some(&callee[0])
 505                 } else {
 506                     None
 507                 }
 508             }
 509             _ => None,
 510         };
 511         let callee_ty = callee_expr
 512             .and_then(|callee_expr| self.typeck_results.borrow().expr_ty_adjusted_opt(callee_expr));
 513
 514         // A "softer" version of the `demand_compatible`, which checks types without persisting them,
 515         // and treats error types differently
 516         // This will allow us to "probe" for other argument orders that would likely have been correct
 517         let check_compatible = |provided_idx: ProvidedIdx, expected_idx: ExpectedIdx| {
 518             if provided_idx.as_usize() == expected_idx.as_usize() {
 519                 return compatibility_diagonal[provided_idx].clone();
 520             }
 521
 522             let (formal_input_ty, expected_input_ty) = formal_and_expected_inputs[expected_idx];
 523             // If either is an error type, we defy the usual convention and consider them to *not* be
 524             // coercible. This prevents our error message heuristic from trying to pass errors into
 525             // every argument.
 526             if (formal_input_ty, expected_input_ty).references_error() {
 527                 return Compatibility::Incompatible(None);
 528             }
 529
 530             let (arg_ty, arg_span) = provided_arg_tys[provided_idx];
 531
 532             let expectation = Expectation::rvalue_hint(self, expected_input_ty);
 533             let coerced_ty = expectation.only_has_type(self).unwrap_or(formal_input_ty);
 534             let can_coerce = self.can_coerce(arg_ty, coerced_ty);
 535             if !can_coerce {
 536                 return Compatibility::Incompatible(None);
 537             }
 538
 539             // Using probe here, since we don't want this subtyping to affect inference.
 540             let subtyping_error = self.probe(|_| {
 541                 self.at(&self.misc(arg_span), self.param_env).sup(formal_input_ty, coerced_ty).err()
 542             });
 543
 544             // Same as above: if either the coerce type or the checked type is an error type,
 545             // consider them *not* compatible.
 546             let references_error = (coerced_ty, arg_ty).references_error();
 547             match (references_error, subtyping_error) {
 548                 (false, None) => Compatibility::Compatible,
 549                 (_, subtyping_error) => Compatibility::Incompatible(subtyping_error),
 550             }
 551         };
 552
 553         // The algorithm here is inspired by levenshtein distance and longest common subsequence.
 554         // We'll try to detect 4 different types of mistakes:
 555         // - An extra parameter has been provided that doesn't satisfy *any* of the other inputs
 556         // - An input is missing, which isn't satisfied by *any* of the other arguments
 557         // - Some number of arguments have been provided in the wrong order
 558         // - A type is straight up invalid
 559
 560         // First, let's find the errors
 561         let (mut errors, matched_inputs) =
 562             ArgMatrix::new(provided_args.len(), formal_and_expected_inputs.len(), check_compatible)
 563                 .find_errors();
 564
 565         // First, check if we just need to wrap some arguments in a tuple.
 566         if let Some((mismatch_idx, terr)) =
 567             compatibility_diagonal.iter().enumerate().find_map(|(i, c)| {
 568                 if let Compatibility::Incompatible(Some(terr)) = c { Some((i, terr)) } else { None }
 569             })
 570         {
 571             // Is the first bad expected argument a tuple?
 572             // Do we have as many extra provided arguments as the tuple's length?
 573             // If so, we might have just forgotten to wrap some args in a tuple.
 574             if let Some(ty::Tuple(tys)) =
 575                 formal_and_expected_inputs.get(mismatch_idx.into()).map(|tys| tys.1.kind())
 576                 // If the tuple is unit, we're not actually wrapping any arguments.
 577                 && !tys.is_empty()
 578                 && provided_arg_tys.len() == formal_and_expected_inputs.len() - 1 + tys.len()
 579             {
 580                 // Wrap up the N provided arguments starting at this position in a tuple.
 581                 let provided_as_tuple = tcx.mk_tup(
 582                     provided_arg_tys.iter().map(|(ty, _)| *ty).skip(mismatch_idx).take(tys.len()),
 583                 );
 584
 585                 let mut satisfied = true;
 586                 // Check if the newly wrapped tuple + rest of the arguments are compatible.
 587                 for ((_, expected_ty), provided_ty) in std::iter::zip(
 588                     formal_and_expected_inputs.iter().skip(mismatch_idx),
 589                     [provided_as_tuple].into_iter().chain(
 590                         provided_arg_tys.iter().map(|(ty, _)| *ty).skip(mismatch_idx + tys.len()),
 591                     ),
 592                 ) {
 593                     if !self.can_coerce(provided_ty, *expected_ty) {
 594                         satisfied = false;
 595                         break;
 596                     }
 597                 }
 598
 599                 // If they're compatible, suggest wrapping in an arg, and we're done!
 600                 // Take some care with spans, so we don't suggest wrapping a macro's
 601                 // innards in parenthesis, for example.
 602                 if satisfied
 603                     && let Some(lo) =
 604                         provided_args[mismatch_idx.into()].span.find_ancestor_inside(error_span)
 605                     && let Some(hi) = provided_args[(mismatch_idx + tys.len() - 1).into()]
 606                         .span
 607                         .find_ancestor_inside(error_span)
 608                 {
 609                     let mut err;
 610                     if tys.len() == 1 {
 611                         // A tuple wrap suggestion actually occurs within,
 612                         // so don't do anything special here.
 613                         err = self.report_and_explain_type_error(
 614                             TypeTrace::types(
 615                                 &self.misc(lo),
 616                                 true,
 617                                 formal_and_expected_inputs[mismatch_idx.into()].1,
 618                                 provided_arg_tys[mismatch_idx.into()].0,
 619                             ),
 620                             terr,
 621                         );
 622                         err.span_label(
 623                             full_call_span,
 624                             format!("arguments to this {} are incorrect", call_name),
 625                         );
 626                     } else {
 627                         err = tcx.sess.struct_span_err_with_code(
 628                             full_call_span,
 629                             &format!(
 630                                 "this {} takes {}{} but {} {} supplied",
 631                                 call_name,
 632                                 if c_variadic { "at least " } else { "" },
 633                                 potentially_plural_count(
 634                                     formal_and_expected_inputs.len(),
 635                                     "argument"
 636                                 ),
 637                                 potentially_plural_count(provided_args.len(), "argument"),
 638                                 if provided_args.len() == 1 { "was" } else { "were" }
 639                             ),
 640                             DiagnosticId::Error(err_code.to_owned()),
 641                         );
 642                         err.multipart_suggestion_verbose(
 643                             "wrap these arguments in parentheses to construct a tuple",
 644                             vec![
 645                                 (lo.shrink_to_lo(), "(".to_string()),
 646                                 (hi.shrink_to_hi(), ")".to_string()),
 647                             ],
 648                             Applicability::MachineApplicable,
 649                         );
 650                     };
 651                     self.label_fn_like(&mut err, fn_def_id, callee_ty);
 652                     err.emit();
 653                     return;
 654                 }
 655             }
 656         }
 657
 658         // Okay, so here's where it gets complicated in regards to what errors
 659         // we emit and how.
 660         // There are 3 different "types" of errors we might encounter.
 661         //   1) Missing/extra/swapped arguments
 662         //   2) Valid but incorrect arguments
 663         //   3) Invalid arguments
 664         //      - Currently I think this only comes up with `CyclicTy`
 665         //
 666         // We first need to go through, remove those from (3) and emit those
 667         // as their own error, particularly since they're error code and
 668         // message is special. From what I can tell, we *must* emit these
 669         // here (vs somewhere prior to this function) since the arguments
 670         // become invalid *because* of how they get used in the function.
 671         // It is what it is.
 672
 673         if errors.is_empty() {
 674             if cfg!(debug_assertions) {
 675                 span_bug!(error_span, "expected errors from argument matrix");
 676             } else {
 677                 tcx.sess
 678                     .struct_span_err(
 679                         error_span,
 680                         "argument type mismatch was detected, \
 681                         but rustc had trouble determining where",
 682                     )
 683                     .note(
 684                         "we would appreciate a bug report: \
 685                         https://github.com/rust-lang/rust/issues/new",
 686                     )
 687                     .emit();
 688             }
 689             return;
 690         }
 691
 692         errors.drain_filter(|error| {
 693                 let Error::Invalid(provided_idx, expected_idx, Compatibility::Incompatible(error)) = error else { return false };
 694                 let (provided_ty, provided_span) = provided_arg_tys[*provided_idx];
 695                 let (expected_ty, _) = formal_and_expected_inputs[*expected_idx];
 696                 let cause = &self.misc(provided_span);
 697                 let trace = TypeTrace::types(cause, true, expected_ty, provided_ty);
 698                 if let Some(e) = error {
 699                     if !matches!(trace.cause.as_failure_code(e), FailureCode::Error0308(_)) {
 700                         self.report_and_explain_type_error(trace, e).emit();
 701                         return true;
 702                     }
 703                 }
 704                 false
 705             });
 706
 707         // We're done if we found errors, but we already emitted them.
 708         if errors.is_empty() {
 709             return;
 710         }
 711
 712         // Okay, now that we've emitted the special errors separately, we
 713         // are only left missing/extra/swapped and mismatched arguments, both
 714         // can be collated pretty easily if needed.
 715
 716         // Next special case: if there is only one "Incompatible" error, just emit that
 717         if let [
 718             Error::Invalid(provided_idx, expected_idx, Compatibility::Incompatible(Some(err))),
 719         ] = &errors[..]
 720         {
 721             let (formal_ty, expected_ty) = formal_and_expected_inputs[*expected_idx];
 722             let (provided_ty, provided_arg_span) = provided_arg_tys[*provided_idx];
 723             let cause = &self.misc(provided_arg_span);
 724             let trace = TypeTrace::types(cause, true, expected_ty, provided_ty);
 725             let mut err = self.report_and_explain_type_error(trace, err);
 726             self.emit_coerce_suggestions(
 727                 &mut err,
 728                 &provided_args[*provided_idx],
 729                 provided_ty,
 730                 Expectation::rvalue_hint(self, expected_ty)
 731                     .only_has_type(self)
 732                     .unwrap_or(formal_ty),
 733                 None,
 734                 None,
 735             );
 736             err.span_label(
 737                 full_call_span,
 738                 format!("arguments to this {} are incorrect", call_name),
 739             );
 740             // Call out where the function is defined
 741             self.label_fn_like(&mut err, fn_def_id, callee_ty);
 742             err.emit();
 743             return;
 744         }
 745
 746         let mut err = if formal_and_expected_inputs.len() == provided_args.len() {
 747             struct_span_err!(
 748                 tcx.sess,
 749                 full_call_span,
 750                 E0308,
 751                 "arguments to this {} are incorrect",
 752                 call_name,
 753             )
 754         } else {
 755             tcx.sess.struct_span_err_with_code(
 756                 full_call_span,
 757                 &format!(
 758                     "this {} takes {}{} but {} {} supplied",
 759                     call_name,
 760                     if c_variadic { "at least " } else { "" },
 761                     potentially_plural_count(formal_and_expected_inputs.len(), "argument"),
 762                     potentially_plural_count(provided_args.len(), "argument"),
 763                     if provided_args.len() == 1 { "was" } else { "were" }
 764                 ),
 765                 DiagnosticId::Error(err_code.to_owned()),
 766             )
 767         };
 768
 769         // As we encounter issues, keep track of what we want to provide for the suggestion
 770         let mut labels = vec![];
 771         // If there is a single error, we give a specific suggestion; otherwise, we change to
 772         // "did you mean" with the suggested function call
 773         enum SuggestionText {
 774             None,
 775             Provide(bool),
 776             Remove(bool),
 777             Swap,
 778             Reorder,
 779             DidYouMean,
 780         }
 781         let mut suggestion_text = SuggestionText::None;
 782
 783         let mut errors = errors.into_iter().peekable();
 784         while let Some(error) = errors.next() {
 785             match error {
 786                 Error::Invalid(provided_idx, expected_idx, compatibility) => {
 787                     let (formal_ty, expected_ty) = formal_and_expected_inputs[expected_idx];
 788                     let (provided_ty, provided_span) = provided_arg_tys[provided_idx];
 789                     if let Compatibility::Incompatible(error) = &compatibility {
 790                         let cause = &self.misc(provided_span);
 791                         let trace = TypeTrace::types(cause, true, expected_ty, provided_ty);
 792                         if let Some(e) = error {
 793                             self.note_type_err(
 794                                 &mut err,
 795                                 &trace.cause,
 796                                 None,
 797                                 Some(trace.values),
 798                                 e,
 799                                 false,
 800                                 true,
 801                             );
 802                         }
 803                     }
 804
 805                     self.emit_coerce_suggestions(
 806                         &mut err,
 807                         &provided_args[provided_idx],
 808                         provided_ty,
 809                         Expectation::rvalue_hint(self, expected_ty)
 810                             .only_has_type(self)
 811                             .unwrap_or(formal_ty),
 812                         None,
 813                         None,
 814                     );
 815                 }
 816                 Error::Extra(arg_idx) => {
 817                     let (provided_ty, provided_span) = provided_arg_tys[arg_idx];
 818                     let provided_ty_name = if !has_error_or_infer([provided_ty]) {
 819                         // FIXME: not suggestable, use something else
 820                         format!(" of type `{}`", provided_ty)
 821                     } else {
 822                         "".to_string()
 823                     };
 824                     labels
 825                         .push((provided_span, format!("argument{} unexpected", provided_ty_name)));
 826                     suggestion_text = match suggestion_text {
 827                         SuggestionText::None => SuggestionText::Remove(false),
 828                         SuggestionText::Remove(_) => SuggestionText::Remove(true),
 829                         _ => SuggestionText::DidYouMean,
 830                     };
 831                 }
 832                 Error::Missing(expected_idx) => {
 833                     // If there are multiple missing arguments adjacent to each other,
 834                     // then we can provide a single error.
 835
 836                     let mut missing_idxs = vec![expected_idx];
 837                     while let Some(e) = errors.next_if(|e| {
 838                         matches!(e, Error::Missing(next_expected_idx)
 839                             if *next_expected_idx == *missing_idxs.last().unwrap() + 1)
 840                     }) {
 841                         match e {
 842                             Error::Missing(expected_idx) => missing_idxs.push(expected_idx),
 843                             _ => unreachable!(),
 844                         }
 845                     }
 846
 847                     // NOTE: Because we might be re-arranging arguments, might have extra
 848                     // arguments, etc. it's hard to *really* know where we should provide
 849                     // this error label, so as a heuristic, we point to the provided arg, or
 850                     // to the call if the missing inputs pass the provided args.
 851                     match &missing_idxs[..] {
 852                         &[expected_idx] => {
 853                             let (_, input_ty) = formal_and_expected_inputs[expected_idx];
 854                             let span = if let Some((_, arg_span)) =
 855                                 provided_arg_tys.get(expected_idx.to_provided_idx())
 856                             {
 857                                 *arg_span
 858                             } else {
 859                                 args_span
 860                             };
 861                             let rendered = if !has_error_or_infer([input_ty]) {
 862                                 format!(" of type `{}`", input_ty)
 863                             } else {
 864                                 "".to_string()
 865                             };
 866                             labels.push((span, format!("an argument{} is missing", rendered)));
 867                             suggestion_text = match suggestion_text {
 868                                 SuggestionText::None => SuggestionText::Provide(false),
 869                                 SuggestionText::Provide(_) => SuggestionText::Provide(true),
 870                                 _ => SuggestionText::DidYouMean,
 871                             };
 872                         }
 873                         &[first_idx, second_idx] => {
 874                             let (_, first_expected_ty) = formal_and_expected_inputs[first_idx];
 875                             let (_, second_expected_ty) = formal_and_expected_inputs[second_idx];
 876                             let span = if let (Some((_, first_span)), Some((_, second_span))) = (
 877                                 provided_arg_tys.get(first_idx.to_provided_idx()),
 878                                 provided_arg_tys.get(second_idx.to_provided_idx()),
 879                             ) {
 880                                 first_span.to(*second_span)
 881                             } else {
 882                                 args_span
 883                             };
 884                             let rendered =
 885                                 if !has_error_or_infer([first_expected_ty, second_expected_ty]) {
 886                                     format!(
 887                                         " of type `{}` and `{}`",
 888                                         first_expected_ty, second_expected_ty
 889                                     )
 890                                 } else {
 891                                     "".to_string()
 892                                 };
 893                             labels.push((span, format!("two arguments{} are missing", rendered)));
 894                             suggestion_text = match suggestion_text {
 895                                 SuggestionText::None | SuggestionText::Provide(_) => {
 896                                     SuggestionText::Provide(true)
 897                                 }
 898                                 _ => SuggestionText::DidYouMean,
 899                             };
 900                         }
 901                         &[first_idx, second_idx, third_idx] => {
 902                             let (_, first_expected_ty) = formal_and_expected_inputs[first_idx];
 903                             let (_, second_expected_ty) = formal_and_expected_inputs[second_idx];
 904                             let (_, third_expected_ty) = formal_and_expected_inputs[third_idx];
 905                             let span = if let (Some((_, first_span)), Some((_, third_span))) = (
 906                                 provided_arg_tys.get(first_idx.to_provided_idx()),
 907                                 provided_arg_tys.get(third_idx.to_provided_idx()),
 908                             ) {
 909                                 first_span.to(*third_span)
 910                             } else {
 911                                 args_span
 912                             };
 913                             let rendered = if !has_error_or_infer([
 914                                 first_expected_ty,
 915                                 second_expected_ty,
 916                                 third_expected_ty,
 917                             ]) {
 918                                 format!(
 919                                     " of type `{}`, `{}`, and `{}`",
 920                                     first_expected_ty, second_expected_ty, third_expected_ty
 921                                 )
 922                             } else {
 923                                 "".to_string()
 924                             };
 925                             labels.push((span, format!("three arguments{} are missing", rendered)));
 926                             suggestion_text = match suggestion_text {
 927                                 SuggestionText::None | SuggestionText::Provide(_) => {
 928                                     SuggestionText::Provide(true)
 929                                 }
 930                                 _ => SuggestionText::DidYouMean,
 931                             };
 932                         }
 933                         missing_idxs => {
 934                             let first_idx = *missing_idxs.first().unwrap();
 935                             let last_idx = *missing_idxs.last().unwrap();
 936                             // NOTE: Because we might be re-arranging arguments, might have extra arguments, etc.
 937                             // It's hard to *really* know where we should provide this error label, so this is a
 938                             // decent heuristic
 939                             let span = if let (Some((_, first_span)), Some((_, last_span))) = (
 940                                 provided_arg_tys.get(first_idx.to_provided_idx()),
 941                                 provided_arg_tys.get(last_idx.to_provided_idx()),
 942                             ) {
 943                                 first_span.to(*last_span)
 944                             } else {
 945                                 args_span
 946                             };
 947                             labels.push((span, format!("multiple arguments are missing")));
 948                             suggestion_text = match suggestion_text {
 949                                 SuggestionText::None | SuggestionText::Provide(_) => {
 950                                     SuggestionText::Provide(true)
 951                                 }
 952                                 _ => SuggestionText::DidYouMean,
 953                             };
 954                         }
 955                     }
 956                 }
 957                 Error::Swap(
 958                     first_provided_idx,
 959                     second_provided_idx,
 960                     first_expected_idx,
 961                     second_expected_idx,
 962                 ) => {
 963                     let (first_provided_ty, first_span) = provided_arg_tys[first_provided_idx];
 964                     let (_, first_expected_ty) = formal_and_expected_inputs[first_expected_idx];
 965                     let first_provided_ty_name = if !has_error_or_infer([first_provided_ty]) {
 966                         format!(", found `{}`", first_provided_ty)
 967                     } else {
 968                         String::new()
 969                     };
 970                     labels.push((
 971                         first_span,
 972                         format!("expected `{}`{}", first_expected_ty, first_provided_ty_name),
 973                     ));
 974
 975                     let (second_provided_ty, second_span) = provided_arg_tys[second_provided_idx];
 976                     let (_, second_expected_ty) = formal_and_expected_inputs[second_expected_idx];
 977                     let second_provided_ty_name = if !has_error_or_infer([second_provided_ty]) {
 978                         format!(", found `{}`", second_provided_ty)
 979                     } else {
 980                         String::new()
 981                     };
 982                     labels.push((
 983                         second_span,
 984                         format!("expected `{}`{}", second_expected_ty, second_provided_ty_name),
 985                     ));
 986
 987                     suggestion_text = match suggestion_text {
 988                         SuggestionText::None => SuggestionText::Swap,
 989                         _ => SuggestionText::DidYouMean,
 990                     };
 991                 }
 992                 Error::Permutation(args) => {
 993                     for (dst_arg, dest_input) in args {
 994                         let (_, expected_ty) = formal_and_expected_inputs[dst_arg];
 995                         let (provided_ty, provided_span) = provided_arg_tys[dest_input];
 996                         let provided_ty_name = if !has_error_or_infer([provided_ty]) {
 997                             format!(", found `{}`", provided_ty)
 998                         } else {
 999                             String::new()
1000                         };
1001                         labels.push((
1002                             provided_span,
1003                             format!("expected `{}`{}", expected_ty, provided_ty_name),
1004                         ));
1005                     }
1006
1007                     suggestion_text = match suggestion_text {
1008                         SuggestionText::None => SuggestionText::Reorder,
1009                         _ => SuggestionText::DidYouMean,
1010                     };
1011                 }
1012             }
1013         }
1014
1015         // If we have less than 5 things to say, it would be useful to call out exactly what's wrong
1016         if labels.len() <= 5 {
1017             for (span, label) in labels {
1018                 err.span_label(span, label);
1019             }
1020         }
1021
1022         // Call out where the function is defined
1023         self.label_fn_like(&mut err, fn_def_id, callee_ty);
1024
1025         // And add a suggestion block for all of the parameters
1026         let suggestion_text = match suggestion_text {
1027             SuggestionText::None => None,
1028             SuggestionText::Provide(plural) => {
1029                 Some(format!("provide the argument{}", if plural { "s" } else { "" }))
1030             }
1031             SuggestionText::Remove(plural) => {
1032                 Some(format!("remove the extra argument{}", if plural { "s" } else { "" }))
1033             }
1034             SuggestionText::Swap => Some("swap these arguments".to_string()),
1035             SuggestionText::Reorder => Some("reorder these arguments".to_string()),
1036             SuggestionText::DidYouMean => Some("did you mean".to_string()),
1037         };
1038         if let Some(suggestion_text) = suggestion_text {
1039             let source_map = self.sess().source_map();
1040             let mut suggestion = format!(
1041                 "{}(",
1042                 source_map.span_to_snippet(full_call_span).unwrap_or_else(|_| fn_def_id
1043                     .map_or("".to_string(), |fn_def_id| tcx.item_name(fn_def_id).to_string()))
1044             );
1045             let mut needs_comma = false;
1046             for (expected_idx, provided_idx) in matched_inputs.iter_enumerated() {
1047                 if needs_comma {
1048                     suggestion += ", ";
1049                 } else {
1050                     needs_comma = true;
1051                 }
1052                 let suggestion_text = if let Some(provided_idx) = provided_idx
1053                     && let (_, provided_span) = provided_arg_tys[*provided_idx]
1054                     && let Ok(arg_text) =
1055                         source_map.span_to_snippet(provided_span.source_callsite())
1056                 {
1057                     arg_text
1058                 } else {
1059                     // Propose a placeholder of the correct type
1060                     let (_, expected_ty) = formal_and_expected_inputs[expected_idx];
1061                     if expected_ty.is_unit() {
1062                         "()".to_string()
1063                     } else if expected_ty.is_suggestable(tcx) {
1064                         format!("/* {} */", expected_ty)
1065                     } else {
1066                         "/* value */".to_string()
1067                     }
1068                 };
1069                 suggestion += &suggestion_text;
1070             }
1071             suggestion += ")";
1072             err.span_suggestion_verbose(
1073                 error_span,
1074                 &suggestion_text,
1075                 suggestion,
1076                 Applicability::HasPlaceholders,
1077             );
1078         }
1079
1080         err.emit();
1081     }
1082
1083     // AST fragment checking
1084     pub(in super::super) fn check_lit(
1085         &self,
1086         lit: &hir::Lit,
1087         expected: Expectation<'tcx>,
1088     ) -> Ty<'tcx> {
1089         let tcx = self.tcx;
1090
1091         match lit.node {
1092             ast::LitKind::Str(..) => tcx.mk_static_str(),
1093             ast::LitKind::ByteStr(ref v) => {
1094                 tcx.mk_imm_ref(tcx.lifetimes.re_static, tcx.mk_array(tcx.types.u8, v.len() as u64))
1095             }
1096             ast::LitKind::Byte(_) => tcx.types.u8,
1097             ast::LitKind::Char(_) => tcx.types.char,
1098             ast::LitKind::Int(_, ast::LitIntType::Signed(t)) => tcx.mk_mach_int(ty::int_ty(t)),
1099             ast::LitKind::Int(_, ast::LitIntType::Unsigned(t)) => tcx.mk_mach_uint(ty::uint_ty(t)),
1100             ast::LitKind::Int(_, ast::LitIntType::Unsuffixed) => {
1101                 let opt_ty = expected.to_option(self).and_then(|ty| match ty.kind() {
1102                     ty::Int(_) | ty::Uint(_) => Some(ty),
1103                     ty::Char => Some(tcx.types.u8),
1104                     ty::RawPtr(..) => Some(tcx.types.usize),
1105                     ty::FnDef(..) | ty::FnPtr(_) => Some(tcx.types.usize),
1106                     _ => None,
1107                 });
1108                 opt_ty.unwrap_or_else(|| self.next_int_var())
1109             }
1110             ast::LitKind::Float(_, ast::LitFloatType::Suffixed(t)) => {
1111                 tcx.mk_mach_float(ty::float_ty(t))
1112             }
1113             ast::LitKind::Float(_, ast::LitFloatType::Unsuffixed) => {
1114                 let opt_ty = expected.to_option(self).and_then(|ty| match ty.kind() {
1115                     ty::Float(_) => Some(ty),
1116                     _ => None,
1117                 });
1118                 opt_ty.unwrap_or_else(|| self.next_float_var())
1119             }
1120             ast::LitKind::Bool(_) => tcx.types.bool,
1121             ast::LitKind::Err(_) => tcx.ty_error(),
1122         }
1123     }
1124
1125     pub fn check_struct_path(
1126         &self,
1127         qpath: &QPath<'_>,
1128         hir_id: hir::HirId,
1129     ) -> Option<(&'tcx ty::VariantDef, Ty<'tcx>)> {
1130         let path_span = qpath.span();
1131         let (def, ty) = self.finish_resolving_struct_path(qpath, path_span, hir_id);
1132         let variant = match def {
1133             Res::Err => {
1134                 self.set_tainted_by_errors();
1135                 return None;
1136             }
1137             Res::Def(DefKind::Variant, _) => match ty.kind() {
1138                 ty::Adt(adt, substs) => Some((adt.variant_of_res(def), adt.did(), substs)),
1139                 _ => bug!("unexpected type: {:?}", ty),
1140             },
1141             Res::Def(DefKind::Struct | DefKind::Union | DefKind::TyAlias | DefKind::AssocTy, _)
1142             | Res::SelfTy { .. } => match ty.kind() {
1143                 ty::Adt(adt, substs) if !adt.is_enum() => {
1144                     Some((adt.non_enum_variant(), adt.did(), substs))
1145                 }
1146                 _ => None,
1147             },
1148             _ => bug!("unexpected definition: {:?}", def),
1149         };
1150
1151         if let Some((variant, did, substs)) = variant {
1152             debug!("check_struct_path: did={:?} substs={:?}", did, substs);
1153             self.write_user_type_annotation_from_substs(hir_id, did, substs, None);
1154
1155             // Check bounds on type arguments used in the path.
1156             self.add_required_obligations(path_span, did, substs);
1157
1158             Some((variant, ty))
1159         } else {
1160             match ty.kind() {
1161                 ty::Error(_) => {
1162                     // E0071 might be caused by a spelling error, which will have
1163                     // already caused an error message and probably a suggestion
1164                     // elsewhere. Refrain from emitting more unhelpful errors here
1165                     // (issue #88844).
1166                 }
1167                 _ => {
1168                     struct_span_err!(
1169                         self.tcx.sess,
1170                         path_span,
1171                         E0071,
1172                         "expected struct, variant or union type, found {}",
1173                         ty.sort_string(self.tcx)
1174                     )
1175                     .span_label(path_span, "not a struct")
1176                     .emit();
1177                 }
1178             }
1179             None
1180         }
1181     }
1182
1183     pub fn check_decl_initializer(
1184         &self,
1185         hir_id: hir::HirId,
1186         pat: &'tcx hir::Pat<'tcx>,
1187         init: &'tcx hir::Expr<'tcx>,
1188     ) -> Ty<'tcx> {
1189         // FIXME(tschottdorf): `contains_explicit_ref_binding()` must be removed
1190         // for #42640 (default match binding modes).
1191         //
1192         // See #44848.
1193         let ref_bindings = pat.contains_explicit_ref_binding();
1194
1195         let local_ty = self.local_ty(init.span, hir_id).revealed_ty;
1196         if let Some(m) = ref_bindings {
1197             // Somewhat subtle: if we have a `ref` binding in the pattern,
1198             // we want to avoid introducing coercions for the RHS. This is
1199             // both because it helps preserve sanity and, in the case of
1200             // ref mut, for soundness (issue #23116). In particular, in
1201             // the latter case, we need to be clear that the type of the
1202             // referent for the reference that results is *equal to* the
1203             // type of the place it is referencing, and not some
1204             // supertype thereof.
1205             let init_ty = self.check_expr_with_needs(init, Needs::maybe_mut_place(m));
1206             self.demand_eqtype(init.span, local_ty, init_ty);
1207             init_ty
1208         } else {
1209             self.check_expr_coercable_to_type(init, local_ty, None)
1210         }
1211     }
1212
1213     pub(in super::super) fn check_decl(&self, decl: Declaration<'tcx>) {
1214         // Determine and write the type which we'll check the pattern against.
1215         let decl_ty = self.local_ty(decl.span, decl.hir_id).decl_ty;
1216         self.write_ty(decl.hir_id, decl_ty);
1217
1218         // Type check the initializer.
1219         if let Some(ref init) = decl.init {
1220             let init_ty = self.check_decl_initializer(decl.hir_id, decl.pat, &init);
1221             self.overwrite_local_ty_if_err(decl.hir_id, decl.pat, decl_ty, init_ty);
1222         }
1223
1224         // Does the expected pattern type originate from an expression and what is the span?
1225         let (origin_expr, ty_span) = match (decl.ty, decl.init) {
1226             (Some(ty), _) => (false, Some(ty.span)), // Bias towards the explicit user type.
1227             (_, Some(init)) => (true, Some(init.span)), // No explicit type; so use the scrutinee.
1228             _ => (false, None), // We have `let $pat;`, so the expected type is unconstrained.
1229         };
1230
1231         // Type check the pattern. Override if necessary to avoid knock-on errors.
1232         self.check_pat_top(&decl.pat, decl_ty, ty_span, origin_expr);
1233         let pat_ty = self.node_ty(decl.pat.hir_id);
1234         self.overwrite_local_ty_if_err(decl.hir_id, decl.pat, decl_ty, pat_ty);
1235
1236         if let Some(blk) = decl.els {
1237             let previous_diverges = self.diverges.get();
1238             let else_ty = self.check_block_with_expected(blk, NoExpectation);
1239             let cause = self.cause(blk.span, ObligationCauseCode::LetElse);
1240             if let Some(mut err) =
1241                 self.demand_eqtype_with_origin(&cause, self.tcx.types.never, else_ty)
1242             {
1243                 err.emit();
1244             }
1245             self.diverges.set(previous_diverges);
1246         }
1247     }
1248
1249     /// Type check a `let` statement.
1250     pub fn check_decl_local(&self, local: &'tcx hir::Local<'tcx>) {
1251         self.check_decl(local.into());
1252     }
1253
1254     pub fn check_stmt(&self, stmt: &'tcx hir::Stmt<'tcx>, is_last: bool) {
1255         // Don't do all the complex logic below for `DeclItem`.
1256         match stmt.kind {
1257             hir::StmtKind::Item(..) => return,
1258             hir::StmtKind::Local(..) | hir::StmtKind::Expr(..) | hir::StmtKind::Semi(..) => {}
1259         }
1260
1261         self.warn_if_unreachable(stmt.hir_id, stmt.span, "statement");
1262
1263         // Hide the outer diverging and `has_errors` flags.
1264         let old_diverges = self.diverges.replace(Diverges::Maybe);
1265         let old_has_errors = self.has_errors.replace(false);
1266
1267         match stmt.kind {
1268             hir::StmtKind::Local(l) => {
1269                 self.check_decl_local(l);
1270             }
1271             // Ignore for now.
1272             hir::StmtKind::Item(_) => {}
1273             hir::StmtKind::Expr(ref expr) => {
1274                 // Check with expected type of `()`.
1275                 self.check_expr_has_type_or_error(&expr, self.tcx.mk_unit(), |err| {
1276                     if expr.can_have_side_effects() {
1277                         self.suggest_semicolon_at_end(expr.span, err);
1278                     }
1279                 });
1280             }
1281             hir::StmtKind::Semi(ref expr) => {
1282                 // All of this is equivalent to calling `check_expr`, but it is inlined out here
1283                 // in order to capture the fact that this `match` is the last statement in its
1284                 // function. This is done for better suggestions to remove the `;`.
1285                 let expectation = match expr.kind {
1286                     hir::ExprKind::Match(..) if is_last => IsLast(stmt.span),
1287                     _ => NoExpectation,
1288                 };
1289                 self.check_expr_with_expectation(expr, expectation);
1290             }
1291         }
1292
1293         // Combine the diverging and `has_error` flags.
1294         self.diverges.set(self.diverges.get() | old_diverges);
1295         self.has_errors.set(self.has_errors.get() | old_has_errors);
1296     }
1297
1298     pub fn check_block_no_value(&self, blk: &'tcx hir::Block<'tcx>) {
1299         let unit = self.tcx.mk_unit();
1300         let ty = self.check_block_with_expected(blk, ExpectHasType(unit));
1301
1302         // if the block produces a `!` value, that can always be
1303         // (effectively) coerced to unit.
1304         if !ty.is_never() {
1305             self.demand_suptype(blk.span, unit, ty);
1306         }
1307     }
1308
1309     pub(in super::super) fn check_block_with_expected(
1310         &self,
1311         blk: &'tcx hir::Block<'tcx>,
1312         expected: Expectation<'tcx>,
1313     ) -> Ty<'tcx> {
1314         let prev = self.ps.replace(self.ps.get().recurse(blk));
1315
1316         // In some cases, blocks have just one exit, but other blocks
1317         // can be targeted by multiple breaks. This can happen both
1318         // with labeled blocks as well as when we desugar
1319         // a `try { ... }` expression.
1320         //
1321         // Example 1:
1322         //
1323         //    'a: { if true { break 'a Err(()); } Ok(()) }
1324         //
1325         // Here we would wind up with two coercions, one from
1326         // `Err(())` and the other from the tail expression
1327         // `Ok(())`. If the tail expression is omitted, that's a
1328         // "forced unit" -- unless the block diverges, in which
1329         // case we can ignore the tail expression (e.g., `'a: {
1330         // break 'a 22; }` would not force the type of the block
1331         // to be `()`).
1332         let tail_expr = blk.expr.as_ref();
1333         let coerce_to_ty = expected.coercion_target_type(self, blk.span);
1334         let coerce = if blk.targeted_by_break {
1335             CoerceMany::new(coerce_to_ty)
1336         } else {
1337             let tail_expr: &[&hir::Expr<'_>] = match tail_expr {
1338                 Some(e) => slice::from_ref(e),
1339                 None => &[],
1340             };
1341             CoerceMany::with_coercion_sites(coerce_to_ty, tail_expr)
1342         };
1343
1344         let prev_diverges = self.diverges.get();
1345         let ctxt = BreakableCtxt { coerce: Some(coerce), may_break: false };
1346
1347         let (ctxt, ()) = self.with_breakable_ctxt(blk.hir_id, ctxt, || {
1348             for (pos, s) in blk.stmts.iter().enumerate() {
1349                 self.check_stmt(s, blk.stmts.len() - 1 == pos);
1350             }
1351
1352             // check the tail expression **without** holding the
1353             // `enclosing_breakables` lock below.
1354             let tail_expr_ty = tail_expr.map(|t| self.check_expr_with_expectation(t, expected));
1355
1356             let mut enclosing_breakables = self.enclosing_breakables.borrow_mut();
1357             let ctxt = enclosing_breakables.find_breakable(blk.hir_id);
1358             let coerce = ctxt.coerce.as_mut().unwrap();
1359             if let Some(tail_expr_ty) = tail_expr_ty {
1360                 let tail_expr = tail_expr.unwrap();
1361                 let span = self.get_expr_coercion_span(tail_expr);
1362                 let cause = self.cause(span, ObligationCauseCode::BlockTailExpression(blk.hir_id));
1363                 let ty_for_diagnostic = coerce.merged_ty();
1364                 // We use coerce_inner here because we want to augment the error
1365                 // suggesting to wrap the block in square brackets if it might've
1366                 // been mistaken array syntax
1367                 coerce.coerce_inner(
1368                     self,
1369                     &cause,
1370                     Some(tail_expr),
1371                     tail_expr_ty,
1372                     Some(&mut |diag: &mut Diagnostic| {
1373                         self.suggest_block_to_brackets(diag, blk, tail_expr_ty, ty_for_diagnostic);
1374                     }),
1375                     false,
1376                 );
1377             } else {
1378                 // Subtle: if there is no explicit tail expression,
1379                 // that is typically equivalent to a tail expression
1380                 // of `()` -- except if the block diverges. In that
1381                 // case, there is no value supplied from the tail
1382                 // expression (assuming there are no other breaks,
1383                 // this implies that the type of the block will be
1384                 // `!`).
1385                 //
1386                 // #41425 -- label the implicit `()` as being the
1387                 // "found type" here, rather than the "expected type".
1388                 if !self.diverges.get().is_always() {
1389                     // #50009 -- Do not point at the entire fn block span, point at the return type
1390                     // span, as it is the cause of the requirement, and
1391                     // `consider_hint_about_removing_semicolon` will point at the last expression
1392                     // if it were a relevant part of the error. This improves usability in editors
1393                     // that highlight errors inline.
1394                     let mut sp = blk.span;
1395                     let mut fn_span = None;
1396                     if let Some((decl, ident)) = self.get_parent_fn_decl(blk.hir_id) {
1397                         let ret_sp = decl.output.span();
1398                         if let Some(block_sp) = self.parent_item_span(blk.hir_id) {
1399                             // HACK: on some cases (`ui/liveness/liveness-issue-2163.rs`) the
1400                             // output would otherwise be incorrect and even misleading. Make sure
1401                             // the span we're aiming at correspond to a `fn` body.
1402                             if block_sp == blk.span {
1403                                 sp = ret_sp;
1404                                 fn_span = Some(ident.span);
1405                             }
1406                         }
1407                     }
1408                     coerce.coerce_forced_unit(
1409                         self,
1410                         &self.misc(sp),
1411                         &mut |err| {
1412                             if let Some(expected_ty) = expected.only_has_type(self) {
1413                                 if !self.consider_removing_semicolon(blk, expected_ty, err) {
1414                                     self.consider_returning_binding(blk, expected_ty, err);
1415                                 }
1416                                 if expected_ty == self.tcx.types.bool {
1417                                     // If this is caused by a missing `let` in a `while let`,
1418                                     // silence this redundant error, as we already emit E0070.
1419
1420                                     // Our block must be a `assign desugar local; assignment`
1421                                     if let Some(hir::Node::Block(hir::Block {
1422                                         stmts:
1423                                             [
1424                                                 hir::Stmt {
1425                                                     kind:
1426                                                         hir::StmtKind::Local(hir::Local {
1427                                                             source:
1428                                                                 hir::LocalSource::AssignDesugar(_),
1429                                                             ..
1430                                                         }),
1431                                                     ..
1432                                                 },
1433                                                 hir::Stmt {
1434                                                     kind:
1435                                                         hir::StmtKind::Expr(hir::Expr {
1436                                                             kind: hir::ExprKind::Assign(..),
1437                                                             ..
1438                                                         }),
1439                                                     ..
1440                                                 },
1441                                             ],
1442                                         ..
1443                                     })) = self.tcx.hir().find(blk.hir_id)
1444                                     {
1445                                         self.comes_from_while_condition(blk.hir_id, |_| {
1446                                             err.downgrade_to_delayed_bug();
1447                                         })
1448                                     }
1449                                 }
1450                             }
1451                             if let Some(fn_span) = fn_span {
1452                                 err.span_label(
1453                                     fn_span,
1454                                     "implicitly returns `()` as its body has no tail or `return` \
1455                                      expression",
1456                                 );
1457                             }
1458                         },
1459                         false,
1460                     );
1461                 }
1462             }
1463         });
1464
1465         if ctxt.may_break {
1466             // If we can break from the block, then the block's exit is always reachable
1467             // (... as long as the entry is reachable) - regardless of the tail of the block.
1468             self.diverges.set(prev_diverges);
1469         }
1470
1471         let mut ty = ctxt.coerce.unwrap().complete(self);
1472
1473         if self.has_errors.get() || ty.references_error() {
1474             ty = self.tcx.ty_error()
1475         }
1476
1477         self.write_ty(blk.hir_id, ty);
1478
1479         self.ps.set(prev);
1480         ty
1481     }
1482
1483     fn parent_item_span(&self, id: hir::HirId) -> Option<Span> {
1484         let node = self.tcx.hir().get_by_def_id(self.tcx.hir().get_parent_item(id));
1485         match node {
1486             Node::Item(&hir::Item { kind: hir::ItemKind::Fn(_, _, body_id), .. })
1487             | Node::ImplItem(&hir::ImplItem { kind: hir::ImplItemKind::Fn(_, body_id), .. }) => {
1488                 let body = self.tcx.hir().body(body_id);
1489                 if let ExprKind::Block(block, _) = &body.value.kind {
1490                     return Some(block.span);
1491                 }
1492             }
1493             _ => {}
1494         }
1495         None
1496     }
1497
1498     /// Given a function block's `HirId`, returns its `FnDecl` if it exists, or `None` otherwise.
1499     fn get_parent_fn_decl(&self, blk_id: hir::HirId) -> Option<(&'tcx hir::FnDecl<'tcx>, Ident)> {
1500         let parent = self.tcx.hir().get_by_def_id(self.tcx.hir().get_parent_item(blk_id));
1501         self.get_node_fn_decl(parent).map(|(fn_decl, ident, _)| (fn_decl, ident))
1502     }
1503
1504     /// If `expr` is a `match` expression that has only one non-`!` arm, use that arm's tail
1505     /// expression's `Span`, otherwise return `expr.span`. This is done to give better errors
1506     /// when given code like the following:
1507     /// ```text
1508     /// if false { return 0i32; } else { 1u32 }
1509     /// //                               ^^^^ point at this instead of the whole `if` expression
1510     /// ```
1511     fn get_expr_coercion_span(&self, expr: &hir::Expr<'_>) -> rustc_span::Span {
1512         let check_in_progress = |elem: &hir::Expr<'_>| {
1513             self.in_progress_typeck_results
1514                 .and_then(|typeck_results| typeck_results.borrow().node_type_opt(elem.hir_id))
1515                 .and_then(|ty| {
1516                     if ty.is_never() {
1517                         None
1518                     } else {
1519                         Some(match elem.kind {
1520                             // Point at the tail expression when possible.
1521                             hir::ExprKind::Block(block, _) => {
1522                                 block.expr.map_or(block.span, |e| e.span)
1523                             }
1524                             _ => elem.span,
1525                         })
1526                     }
1527                 })
1528         };
1529
1530         if let hir::ExprKind::If(_, _, Some(el)) = expr.kind {
1531             if let Some(rslt) = check_in_progress(el) {
1532                 return rslt;
1533             }
1534         }
1535
1536         if let hir::ExprKind::Match(_, arms, _) = expr.kind {
1537             let mut iter = arms.iter().filter_map(|arm| check_in_progress(arm.body));
1538             if let Some(span) = iter.next() {
1539                 if iter.next().is_none() {
1540                     return span;
1541                 }
1542             }
1543         }
1544
1545         expr.span
1546     }
1547
1548     fn overwrite_local_ty_if_err(
1549         &self,
1550         hir_id: hir::HirId,
1551         pat: &'tcx hir::Pat<'tcx>,
1552         decl_ty: Ty<'tcx>,
1553         ty: Ty<'tcx>,
1554     ) {
1555         if ty.references_error() {
1556             // Override the types everywhere with `err()` to avoid knock on errors.
1557             self.write_ty(hir_id, ty);
1558             self.write_ty(pat.hir_id, ty);
1559             let local_ty = LocalTy { decl_ty, revealed_ty: ty };
1560             self.locals.borrow_mut().insert(hir_id, local_ty);
1561             self.locals.borrow_mut().insert(pat.hir_id, local_ty);
1562         }
1563     }
1564
1565     // Finish resolving a path in a struct expression or pattern `S::A { .. }` if necessary.
1566     // The newly resolved definition is written into `type_dependent_defs`.
1567     fn finish_resolving_struct_path(
1568         &self,
1569         qpath: &QPath<'_>,
1570         path_span: Span,
1571         hir_id: hir::HirId,
1572     ) -> (Res, Ty<'tcx>) {
1573         match *qpath {
1574             QPath::Resolved(ref maybe_qself, ref path) => {
1575                 let self_ty = maybe_qself.as_ref().map(|qself| self.to_ty(qself));
1576                 let ty = <dyn AstConv<'_>>::res_to_ty(self, self_ty, path, true);
1577                 (path.res, ty)
1578             }
1579             QPath::TypeRelative(ref qself, ref segment) => {
1580                 let ty = self.to_ty(qself);
1581
1582                 let result = <dyn AstConv<'_>>::associated_path_to_ty(
1583                     self, hir_id, path_span, ty, qself, segment, true,
1584                 );
1585                 let ty = result.map(|(ty, _, _)| ty).unwrap_or_else(|_| self.tcx().ty_error());
1586                 let result = result.map(|(_, kind, def_id)| (kind, def_id));
1587
1588                 // Write back the new resolution.
1589                 self.write_resolution(hir_id, result);
1590
1591                 (result.map_or(Res::Err, |(kind, def_id)| Res::Def(kind, def_id)), ty)
1592             }
1593             QPath::LangItem(lang_item, span, id) => {
1594                 self.resolve_lang_item_path(lang_item, span, hir_id, id)
1595             }
1596         }
1597     }
1598
1599     /// Given a vec of evaluated `FulfillmentError`s and an `fn` call argument expressions, we walk
1600     /// the checked and coerced types for each argument to see if any of the `FulfillmentError`s
1601     /// reference a type argument. The reason to walk also the checked type is that the coerced type
1602     /// can be not easily comparable with predicate type (because of coercion). If the types match
1603     /// for either checked or coerced type, and there's only *one* argument that does, we point at
1604     /// the corresponding argument's expression span instead of the `fn` call path span.
1605     fn point_at_arg_instead_of_call_if_possible(
1606         &self,
1607         errors: &mut Vec<traits::FulfillmentError<'tcx>>,
1608         expr: &'tcx hir::Expr<'tcx>,
1609         call_sp: Span,
1610         args: &'tcx [hir::Expr<'tcx>],
1611         expected_tys: &[Ty<'tcx>],
1612     ) {
1613         // We *do not* do this for desugared call spans to keep good diagnostics when involving
1614         // the `?` operator.
1615         if call_sp.desugaring_kind().is_some() {
1616             return;
1617         }
1618
1619         'outer: for error in errors {
1620             // Only if the cause is somewhere inside the expression we want try to point at arg.
1621             // Otherwise, it means that the cause is somewhere else and we should not change
1622             // anything because we can break the correct span.
1623             if !call_sp.contains(error.obligation.cause.span) {
1624                 continue;
1625             }
1626
1627             // Peel derived obligation, because it's the type that originally
1628             // started this inference chain that matters, not the one we wound
1629             // up with at the end.
1630             fn unpeel_to_top<'a, 'tcx>(
1631                 mut code: &'a ObligationCauseCode<'tcx>,
1632             ) -> &'a ObligationCauseCode<'tcx> {
1633                 let mut result_code = code;
1634                 loop {
1635                     let parent = match code {
1636                         ObligationCauseCode::ImplDerivedObligation(c) => &c.derived.parent_code,
1637                         ObligationCauseCode::BuiltinDerivedObligation(c)
1638                         | ObligationCauseCode::DerivedObligation(c) => &c.parent_code,
1639                         _ => break result_code,
1640                     };
1641                     (result_code, code) = (code, parent);
1642                 }
1643             }
1644             let self_: ty::subst::GenericArg<'_> =
1645                 match unpeel_to_top(error.obligation.cause.code()) {
1646                     ObligationCauseCode::BuiltinDerivedObligation(code)
1647                     | ObligationCauseCode::DerivedObligation(code) => {
1648                         code.parent_trait_pred.self_ty().skip_binder().into()
1649                     }
1650                     ObligationCauseCode::ImplDerivedObligation(code) => {
1651                         code.derived.parent_trait_pred.self_ty().skip_binder().into()
1652                     }
1653                     _ if let ty::PredicateKind::Trait(predicate) =
1654                         error.obligation.predicate.kind().skip_binder() =>
1655                     {
1656                         predicate.self_ty().into()
1657                     }
1658                     _ => continue,
1659                 };
1660             let self_ = self.resolve_vars_if_possible(self_);
1661             let ty_matches_self = |ty: Ty<'tcx>| ty.walk().any(|arg| arg == self_);
1662
1663             let typeck_results = self.typeck_results.borrow();
1664
1665             for (idx, arg) in args.iter().enumerate() {
1666                 // Don't adjust the span if we already have a more precise span
1667                 // within one of the args.
1668                 if arg.span.contains(error.obligation.cause.span) {
1669                     let references_arg =
1670                         typeck_results.expr_ty_opt(arg).map_or(false, &ty_matches_self)
1671                             || expected_tys.get(idx).copied().map_or(false, &ty_matches_self);
1672                     if references_arg && !arg.span.from_expansion() {
1673                         error.obligation.cause.map_code(|parent_code| {
1674                             ObligationCauseCode::FunctionArgumentObligation {
1675                                 arg_hir_id: args[idx].hir_id,
1676                                 call_hir_id: expr.hir_id,
1677                                 parent_code,
1678                             }
1679                         })
1680                     }
1681                     continue 'outer;
1682                 }
1683             }
1684
1685             // Collect the argument position for all arguments that could have caused this
1686             // `FulfillmentError`.
1687             let mut referenced_in: Vec<_> = std::iter::zip(expected_tys, args)
1688                 .enumerate()
1689                 .flat_map(|(idx, (expected_ty, arg))| {
1690                     if let Some(arg_ty) = typeck_results.expr_ty_opt(arg) {
1691                         vec![(idx, arg_ty), (idx, *expected_ty)]
1692                     } else {
1693                         vec![]
1694                     }
1695                 })
1696                 .filter_map(|(i, ty)| {
1697                     let ty = self.resolve_vars_if_possible(ty);
1698                     // We walk the argument type because the argument's type could have
1699                     // been `Option<T>`, but the `FulfillmentError` references `T`.
1700                     if ty_matches_self(ty) { Some(i) } else { None }
1701                 })
1702                 .collect();
1703
1704             // Both checked and coerced types could have matched, thus we need to remove
1705             // duplicates.
1706
1707             // We sort primitive type usize here and can use unstable sort
1708             referenced_in.sort_unstable();
1709             referenced_in.dedup();
1710
1711             if let &[idx] = &referenced_in[..] {
1712                 // Do not point at the inside of a macro.
1713                 // That would often result in poor error messages.
1714                 if args[idx].span.from_expansion() {
1715                     continue;
1716                 }
1717                 // We make sure that only *one* argument matches the obligation failure
1718                 // and we assign the obligation's span to its expression's.
1719                 error.obligation.cause.span = args[idx].span;
1720                 error.obligation.cause.map_code(|parent_code| {
1721                     ObligationCauseCode::FunctionArgumentObligation {
1722                         arg_hir_id: args[idx].hir_id,
1723                         call_hir_id: expr.hir_id,
1724                         parent_code,
1725                     }
1726                 });
1727             } else if error.obligation.cause.span == call_sp {
1728                 // Make function calls point at the callee, not the whole thing.
1729                 if let hir::ExprKind::Call(callee, _) = expr.kind {
1730                     error.obligation.cause.span = callee.span;
1731                 }
1732             }
1733         }
1734     }
1735
1736     /// Given a vec of evaluated `FulfillmentError`s and an `fn` call expression, we walk the
1737     /// `PathSegment`s and resolve their type parameters to see if any of the `FulfillmentError`s
1738     /// were caused by them. If they were, we point at the corresponding type argument's span
1739     /// instead of the `fn` call path span.
1740     fn point_at_type_arg_instead_of_call_if_possible(
1741         &self,
1742         errors: &mut Vec<traits::FulfillmentError<'tcx>>,
1743         call_expr: &'tcx hir::Expr<'tcx>,
1744     ) {
1745         if let hir::ExprKind::Call(path, _) = &call_expr.kind {
1746             if let hir::ExprKind::Path(hir::QPath::Resolved(_, path)) = &path.kind {
1747                 for error in errors {
1748                     if let ty::PredicateKind::Trait(predicate) =
1749                         error.obligation.predicate.kind().skip_binder()
1750                     {
1751                         // If any of the type arguments in this path segment caused the
1752                         // `FulfillmentError`, point at its span (#61860).
1753                         for arg in path
1754                             .segments
1755                             .iter()
1756                             .filter_map(|seg| seg.args.as_ref())
1757                             .flat_map(|a| a.args.iter())
1758                         {
1759                             if let hir::GenericArg::Type(hir_ty) = &arg {
1760                                 let ty = self.resolve_vars_if_possible(
1761                                     self.typeck_results.borrow().node_type(hir_ty.hir_id),
1762                                 );
1763                                 if ty == predicate.self_ty() {
1764                                     error.obligation.cause.span = hir_ty.span;
1765                                 }
1766                             }
1767                         }
1768                     }
1769                 }
1770             }
1771         }
1772     }
1773
1774     fn label_fn_like(
1775         &self,
1776         err: &mut rustc_errors::DiagnosticBuilder<'tcx, rustc_errors::ErrorGuaranteed>,
1777         callable_def_id: Option<DefId>,
1778         callee_ty: Option<Ty<'tcx>>,
1779     ) {
1780         let Some(mut def_id) = callable_def_id else {
1781             return;
1782         };
1783
1784         if let Some(assoc_item) = self.tcx.opt_associated_item(def_id)
1785             // Possibly points at either impl or trait item, so try to get it
1786             // to point to trait item, then get the parent.
1787             // This parent might be an impl in the case of an inherent function,
1788             // but the next check will fail.
1789             && let maybe_trait_item_def_id = assoc_item.trait_item_def_id.unwrap_or(def_id)
1790             && let maybe_trait_def_id = self.tcx.parent(maybe_trait_item_def_id)
1791             // Just an easy way to check "trait_def_id == Fn/FnMut/FnOnce"
1792             && let Some(call_kind) = ty::ClosureKind::from_def_id(self.tcx, maybe_trait_def_id)
1793             && let Some(callee_ty) = callee_ty
1794         {
1795             let callee_ty = callee_ty.peel_refs();
1796             match *callee_ty.kind() {
1797                 ty::Param(param) => {
1798                     let param =
1799                         self.tcx.generics_of(self.body_id.owner).type_param(&param, self.tcx);
1800                     if param.kind.is_synthetic() {
1801                         // if it's `impl Fn() -> ..` then just fall down to the def-id based logic
1802                         def_id = param.def_id;
1803                     } else {
1804                         // Otherwise, find the predicate that makes this generic callable,
1805                         // and point at that.
1806                         let instantiated = self
1807                             .tcx
1808                             .explicit_predicates_of(self.body_id.owner)
1809                             .instantiate_identity(self.tcx);
1810                         // FIXME(compiler-errors): This could be problematic if something has two
1811                         // fn-like predicates with different args, but callable types really never
1812                         // do that, so it's OK.
1813                         for (predicate, span) in
1814                             std::iter::zip(instantiated.predicates, instantiated.spans)
1815                         {
1816                             if let ty::PredicateKind::Trait(pred) = predicate.kind().skip_binder()
1817                                 && pred.self_ty().peel_refs() == callee_ty
1818                                 && ty::ClosureKind::from_def_id(self.tcx, pred.def_id()).is_some()
1819                             {
1820                                 err.span_note(span, "callable defined here");
1821                                 return;
1822                             }
1823                         }
1824                     }
1825                 }
1826                 ty::Opaque(new_def_id, _)
1827                 | ty::Closure(new_def_id, _)
1828                 | ty::FnDef(new_def_id, _) => {
1829                     def_id = new_def_id;
1830                 }
1831                 _ => {
1832                     // Look for a user-provided impl of a `Fn` trait, and point to it.
1833                     let new_def_id = self.probe(|_| {
1834                         let trait_ref = ty::TraitRef::new(
1835                             call_kind.to_def_id(self.tcx),
1836                             self.tcx.mk_substs([
1837                                 ty::GenericArg::from(callee_ty),
1838                                 self.next_ty_var(TypeVariableOrigin {
1839                                     kind: TypeVariableOriginKind::MiscVariable,
1840                                     span: rustc_span::DUMMY_SP,
1841                                 })
1842                                 .into(),
1843                             ].into_iter()),
1844                         );
1845                         let obligation = traits::Obligation::new(
1846                             traits::ObligationCause::dummy(),
1847                             self.param_env,
1848                             ty::Binder::dummy(ty::TraitPredicate {
1849                                 trait_ref,
1850                                 constness: ty::BoundConstness::NotConst,
1851                                 polarity: ty::ImplPolarity::Positive,
1852                             }),
1853                         );
1854                         match SelectionContext::new(&self).select(&obligation) {
1855                             Ok(Some(traits::ImplSource::UserDefined(impl_source))) => {
1856                                 Some(impl_source.impl_def_id)
1857                             }
1858                             _ => None
1859                         }
1860                     });
1861                     if let Some(new_def_id) = new_def_id {
1862                         def_id = new_def_id;
1863                     } else {
1864                         return;
1865                     }
1866                 }
1867             }
1868         }
1869
1870         if let Some(def_span) = self.tcx.def_ident_span(def_id) && !def_span.is_dummy() {
1871             let mut spans: MultiSpan = def_span.into();
1872
1873             let params = self
1874                 .tcx
1875                 .hir()
1876                 .get_if_local(def_id)
1877                 .and_then(|node| node.body_id())
1878                 .into_iter()
1879                 .flat_map(|id| self.tcx.hir().body(id).params);
1880
1881             for param in params {
1882                 spans.push_span_label(param.span, "");
1883             }
1884
1885             let def_kind = self.tcx.def_kind(def_id);
1886             err.span_note(spans, &format!("{} defined here", def_kind.descr(def_id)));
1887         } else {
1888             let def_kind = self.tcx.def_kind(def_id);
1889             err.span_note(
1890                 self.tcx.def_span(def_id),
1891                 &format!("{} defined here", def_kind.descr(def_id)),
1892             );
1893         }
1894     }
1895 }