src/librustc_const_eval/check_match.rs

   1 // Copyright 2012-2016 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use _match::{MatchCheckCtxt, Matrix, expand_pattern, is_useful};
  12 use _match::Usefulness::*;
  13 use _match::WitnessPreference::*;
  14
  15 use pattern::{Pattern, PatternContext, PatternError, PatternKind};
  16
  17 use eval::report_const_eval_err;
  18
  19 use rustc::dep_graph::DepNode;
  20
  21 use rustc::middle::expr_use_visitor::{ConsumeMode, Delegate, ExprUseVisitor};
  22 use rustc::middle::expr_use_visitor::{LoanCause, MutateMode};
  23 use rustc::middle::expr_use_visitor as euv;
  24 use rustc::middle::mem_categorization::{cmt};
  25 use rustc::session::Session;
  26 use rustc::traits::Reveal;
  27 use rustc::ty::{self, Ty, TyCtxt};
  28 use rustc::lint;
  29 use rustc_errors::{Diagnostic, Level, DiagnosticBuilder};
  30
  31 use rustc::hir::def::*;
  32 use rustc::hir::intravisit::{self, Visitor, FnKind, NestedVisitorMap};
  33 use rustc::hir::{self, Pat, PatKind};
  34
  35 use rustc_back::slice;
  36
  37 use syntax::ast;
  38 use syntax::ptr::P;
  39 use syntax_pos::{Span, DUMMY_SP};
  40
  41 struct OuterVisitor<'a, 'tcx: 'a> { tcx: TyCtxt<'a, 'tcx, 'tcx> }
  42
  43 impl<'a, 'tcx> Visitor<'tcx> for OuterVisitor<'a, 'tcx> {
  44     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
  45         NestedVisitorMap::OnlyBodies(&self.tcx.hir)
  46     }
  47
  48     fn visit_fn(&mut self, fk: FnKind<'tcx>, fd: &'tcx hir::FnDecl,
  49                 b: hir::BodyId, s: Span, id: ast::NodeId) {
  50         intravisit::walk_fn(self, fk, fd, b, s, id);
  51
  52         MatchVisitor {
  53             tcx: self.tcx,
  54             tables: self.tcx.body_tables(b),
  55             param_env: &ty::ParameterEnvironment::for_item(self.tcx, id)
  56         }.visit_body(self.tcx.hir.body(b));
  57     }
  58 }
  59
  60 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
  61     tcx.visit_all_item_likes_in_krate(DepNode::MatchCheck,
  62                                       &mut OuterVisitor { tcx: tcx }.as_deep_visitor());
  63     tcx.sess.abort_if_errors();
  64 }
  65
  66 fn create_e0004<'a>(sess: &'a Session, sp: Span, error_message: String) -> DiagnosticBuilder<'a> {
  67     struct_span_err!(sess, sp, E0004, "{}", &error_message)
  68 }
  69
  70 struct MatchVisitor<'a, 'tcx: 'a> {
  71     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  72     tables: &'a ty::TypeckTables<'tcx>,
  73     param_env: &'a ty::ParameterEnvironment<'tcx>
  74 }
  75
  76 impl<'a, 'tcx> Visitor<'tcx> for MatchVisitor<'a, 'tcx> {
  77     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
  78         NestedVisitorMap::None
  79     }
  80
  81     fn visit_expr(&mut self, ex: &'tcx hir::Expr) {
  82         intravisit::walk_expr(self, ex);
  83
  84         match ex.node {
  85             hir::ExprMatch(ref scrut, ref arms, source) => {
  86                 self.check_match(scrut, arms, source);
  87             }
  88             _ => {}
  89         }
  90     }
  91
  92     fn visit_local(&mut self, loc: &'tcx hir::Local) {
  93         intravisit::walk_local(self, loc);
  94
  95         self.check_irrefutable(&loc.pat, false);
  96
  97         // Check legality of move bindings and `@` patterns.
  98         self.check_patterns(false, slice::ref_slice(&loc.pat));
  99     }
 100
 101     fn visit_body(&mut self, body: &'tcx hir::Body) {
 102         intravisit::walk_body(self, body);
 103
 104         for arg in &body.arguments {
 105             self.check_irrefutable(&arg.pat, true);
 106             self.check_patterns(false, slice::ref_slice(&arg.pat));
 107         }
 108     }
 109 }
 110
 111 impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
 112     fn check_patterns(&self, has_guard: bool, pats: &[P<Pat>]) {
 113         check_legality_of_move_bindings(self, has_guard, pats);
 114         for pat in pats {
 115             check_legality_of_bindings_in_at_patterns(self, pat);
 116         }
 117     }
 118
 119     fn report_inlining_errors(&self, patcx: PatternContext, pat_span: Span) {
 120         for error in patcx.errors {
 121             match error {
 122                 PatternError::StaticInPattern(span) => {
 123                     span_err!(self.tcx.sess, span, E0158,
 124                               "statics cannot be referenced in patterns");
 125                 }
 126                 PatternError::ConstEval(err) => {
 127                     report_const_eval_err(self.tcx, &err, pat_span, "pattern").emit();
 128                 }
 129             }
 130         }
 131     }
 132
 133     fn check_match(
 134         &self,
 135         scrut: &hir::Expr,
 136         arms: &[hir::Arm],
 137         source: hir::MatchSource)
 138     {
 139         for arm in arms {
 140             // First, check legality of move bindings.
 141             self.check_patterns(arm.guard.is_some(), &arm.pats);
 142
 143             // Second, if there is a guard on each arm, make sure it isn't
 144             // assigning or borrowing anything mutably.
 145             if let Some(ref guard) = arm.guard {
 146                 check_for_mutation_in_guard(self, &guard);
 147             }
 148
 149             // Third, perform some lints.
 150             for pat in &arm.pats {
 151                 check_for_bindings_named_the_same_as_variants(self, pat);
 152             }
 153         }
 154
 155         let module = self.tcx.hir.local_def_id(self.tcx.hir.get_module_parent(scrut.id));
 156         MatchCheckCtxt::create_and_enter(self.tcx, module, |ref mut cx| {
 157             let mut have_errors = false;
 158
 159             let inlined_arms : Vec<(Vec<_>, _)> = arms.iter().map(|arm| (
 160                 arm.pats.iter().map(|pat| {
 161                     let mut patcx = PatternContext::new(self.tcx, self.tables);
 162                     let pattern = expand_pattern(cx, patcx.lower_pattern(&pat));
 163                     if !patcx.errors.is_empty() {
 164                         self.report_inlining_errors(patcx, pat.span);
 165                         have_errors = true;
 166                     }
 167                     (pattern, &**pat)
 168                 }).collect(),
 169                 arm.guard.as_ref().map(|e| &**e)
 170             )).collect();
 171
 172             // Bail out early if inlining failed.
 173             if have_errors {
 174                 return;
 175             }
 176
 177             // Fourth, check for unreachable arms.
 178             check_arms(cx, &inlined_arms, source);
 179
 180             // Then, if the match has no arms, check whether the scrutinee
 181             // is uninhabited.
 182             let pat_ty = self.tables.node_id_to_type(scrut.id);
 183             let module = self.tcx.hir.local_def_id(self.tcx.hir.get_module_parent(scrut.id));
 184             if inlined_arms.is_empty() {
 185                 let scrutinee_is_uninhabited = if self.tcx.sess.features.borrow().never_type {
 186                     pat_ty.is_uninhabited_from(module, self.tcx)
 187                 } else {
 188                     self.conservative_is_uninhabited(pat_ty)
 189                 };
 190                 if !scrutinee_is_uninhabited {
 191                     // We know the type is inhabited, so this must be wrong
 192                     let mut err = create_e0004(self.tcx.sess, scrut.span,
 193                                                format!("non-exhaustive patterns: type {} \
 194                                                         is non-empty",
 195                                                        pat_ty));
 196                     span_help!(&mut err, scrut.span,
 197                                "Please ensure that all possible cases are being handled; \
 198                                 possibly adding wildcards or more match arms.");
 199                     err.emit();
 200                 }
 201                 // If the type *is* uninhabited, it's vacuously exhaustive
 202                 return;
 203             }
 204
 205             let matrix: Matrix = inlined_arms
 206                 .iter()
 207                 .filter(|&&(_, guard)| guard.is_none())
 208                 .flat_map(|arm| &arm.0)
 209                 .map(|pat| vec![pat.0])
 210                 .collect();
 211             let scrut_ty = self.tables.node_id_to_type(scrut.id);
 212             check_exhaustive(cx, scrut_ty, scrut.span, &matrix, source);
 213         })
 214     }
 215
 216     fn conservative_is_uninhabited(&self, scrutinee_ty: Ty<'tcx>) -> bool {
 217         // "rustc-1.0-style" uncontentious uninhabitableness check
 218         match scrutinee_ty.sty {
 219             ty::TyNever => true,
 220             ty::TyAdt(def, _) => def.variants.is_empty(),
 221             _ => false
 222         }
 223     }
 224
 225     fn check_irrefutable(&self, pat: &Pat, is_fn_arg: bool) {
 226         let origin = if is_fn_arg {
 227             "function argument"
 228         } else {
 229             "local binding"
 230         };
 231
 232         let module = self.tcx.hir.local_def_id(self.tcx.hir.get_module_parent(pat.id));
 233         MatchCheckCtxt::create_and_enter(self.tcx, module, |ref mut cx| {
 234             let mut patcx = PatternContext::new(self.tcx, self.tables);
 235             let pattern = patcx.lower_pattern(pat);
 236             let pattern_ty = pattern.ty;
 237             let pats : Matrix = vec![vec![
 238                 expand_pattern(cx, pattern)
 239             ]].into_iter().collect();
 240
 241             let wild_pattern = Pattern {
 242                 ty: pattern_ty,
 243                 span: DUMMY_SP,
 244                 kind: box PatternKind::Wild,
 245             };
 246             let witness = match is_useful(cx, &pats, &[&wild_pattern], ConstructWitness) {
 247                 UsefulWithWitness(witness) => witness,
 248                 NotUseful => return,
 249                 Useful => bug!()
 250             };
 251
 252             let pattern_string = witness[0].single_pattern().to_string();
 253             let mut diag = struct_span_err!(
 254                 self.tcx.sess, pat.span, E0005,
 255                 "refutable pattern in {}: `{}` not covered",
 256                 origin, pattern_string
 257             );
 258             diag.span_label(pat.span, &format!("pattern `{}` not covered", pattern_string));
 259             diag.emit();
 260         });
 261     }
 262 }
 263
 264 fn check_for_bindings_named_the_same_as_variants(cx: &MatchVisitor, pat: &Pat) {
 265     pat.walk(|p| {
 266         if let PatKind::Binding(hir::BindByValue(hir::MutImmutable), _, name, None) = p.node {
 267             let pat_ty = cx.tables.pat_ty(p);
 268             if let ty::TyAdt(edef, _) = pat_ty.sty {
 269                 if edef.is_enum() && edef.variants.iter().any(|variant| {
 270                     variant.name == name.node && variant.ctor_kind == CtorKind::Const
 271                 }) {
 272                     let ty_path = cx.tcx.item_path_str(edef.did);
 273                     let mut err = struct_span_warn!(cx.tcx.sess, p.span, E0170,
 274                         "pattern binding `{}` is named the same as one \
 275                          of the variants of the type `{}`",
 276                         name.node, ty_path);
 277                     help!(err,
 278                         "if you meant to match on a variant, \
 279                         consider making the path in the pattern qualified: `{}::{}`",
 280                         ty_path, name.node);
 281                     err.emit();
 282                 }
 283             }
 284         }
 285         true
 286     });
 287 }
 288
 289 /// Checks for common cases of "catchall" patterns that may not be intended as such.
 290 fn pat_is_catchall(pat: &Pat) -> bool {
 291     match pat.node {
 292         PatKind::Binding(.., None) => true,
 293         PatKind::Binding(.., Some(ref s)) => pat_is_catchall(s),
 294         PatKind::Ref(ref s, _) => pat_is_catchall(s),
 295         PatKind::Tuple(ref v, _) => v.iter().all(|p| {
 296             pat_is_catchall(&p)
 297         }),
 298         _ => false
 299     }
 300 }
 301
 302 // Check for unreachable patterns
 303 fn check_arms<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 304                         arms: &[(Vec<(&'a Pattern<'tcx>, &hir::Pat)>, Option<&hir::Expr>)],
 305                         source: hir::MatchSource)
 306 {
 307     let mut seen = Matrix::empty();
 308     let mut catchall = None;
 309     let mut printed_if_let_err = false;
 310     for (arm_index, &(ref pats, guard)) in arms.iter().enumerate() {
 311         for &(pat, hir_pat) in pats {
 312             let v = vec![pat];
 313
 314             match is_useful(cx, &seen, &v[..], LeaveOutWitness) {
 315                 NotUseful => {
 316                     match source {
 317                         hir::MatchSource::IfLetDesugar { .. } => {
 318                             if printed_if_let_err {
 319                                 // we already printed an irrefutable if-let pattern error.
 320                                 // We don't want two, that's just confusing.
 321                             } else {
 322                                 // find the first arm pattern so we can use its span
 323                                 let &(ref first_arm_pats, _) = &arms[0];
 324                                 let first_pat = &first_arm_pats[0];
 325                                 let span = first_pat.0.span;
 326                                 struct_span_err!(cx.tcx.sess, span, E0162,
 327                                                 "irrefutable if-let pattern")
 328                                     .span_label(span, &format!("irrefutable pattern"))
 329                                     .emit();
 330                                 printed_if_let_err = true;
 331                             }
 332                         },
 333
 334                         hir::MatchSource::WhileLetDesugar => {
 335                             // find the first arm pattern so we can use its span
 336                             let &(ref first_arm_pats, _) = &arms[0];
 337                             let first_pat = &first_arm_pats[0];
 338                             let span = first_pat.0.span;
 339
 340                             // check which arm we're on.
 341                             match arm_index {
 342                                 // The arm with the user-specified pattern.
 343                                 0 => {
 344                                     let mut diagnostic = Diagnostic::new(Level::Warning,
 345                                                                          "unreachable pattern");
 346                                     diagnostic.set_span(pat.span);
 347                                     cx.tcx.sess.add_lint_diagnostic(
 348                                             lint::builtin::UNREACHABLE_PATTERNS,
 349                                             hir_pat.id, diagnostic);
 350                                 },
 351                                 // The arm with the wildcard pattern.
 352                                 1 => {
 353                                     struct_span_err!(cx.tcx.sess, span, E0165,
 354                                                      "irrefutable while-let pattern")
 355                                         .span_label(span, &format!("irrefutable pattern"))
 356                                         .emit();
 357                                 },
 358                                 _ => bug!(),
 359                             }
 360                         },
 361
 362                         hir::MatchSource::ForLoopDesugar |
 363                         hir::MatchSource::Normal => {
 364                             let mut diagnostic = Diagnostic::new(Level::Warning,
 365                                                                  "unreachable pattern");
 366                             diagnostic.set_span(pat.span);
 367                             // if we had a catchall pattern, hint at that
 368                             if let Some(catchall) = catchall {
 369                                 diagnostic.span_label(pat.span, &"this is an unreachable pattern");
 370                                 diagnostic.span_note(catchall, "this pattern matches any value");
 371                             }
 372                             cx.tcx.sess.add_lint_diagnostic(lint::builtin::UNREACHABLE_PATTERNS,
 373                                                             hir_pat.id, diagnostic);
 374                         },
 375
 376                         // Unreachable patterns in try expressions occur when one of the arms
 377                         // are an uninhabited type. Which is OK.
 378                         hir::MatchSource::TryDesugar => {}
 379                     }
 380                 }
 381                 Useful => (),
 382                 UsefulWithWitness(_) => bug!()
 383             }
 384             if guard.is_none() {
 385                 seen.push(v);
 386                 if catchall.is_none() && pat_is_catchall(hir_pat) {
 387                     catchall = Some(pat.span);
 388                 }
 389             }
 390         }
 391     }
 392 }
 393
 394 fn check_exhaustive<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 395                               scrut_ty: Ty<'tcx>,
 396                               sp: Span,
 397                               matrix: &Matrix<'a, 'tcx>,
 398                               source: hir::MatchSource) {
 399     let wild_pattern = Pattern {
 400         ty: scrut_ty,
 401         span: DUMMY_SP,
 402         kind: box PatternKind::Wild,
 403     };
 404     match is_useful(cx, matrix, &[&wild_pattern], ConstructWitness) {
 405         UsefulWithWitness(pats) => {
 406             let witnesses = if pats.is_empty() {
 407                 vec![&wild_pattern]
 408             } else {
 409                 pats.iter().map(|w| w.single_pattern()).collect()
 410             };
 411             match source {
 412                 hir::MatchSource::ForLoopDesugar => {
 413                     // `witnesses[0]` has the form `Some(<head>)`, peel off the `Some`
 414                     let witness = match *witnesses[0].kind {
 415                         PatternKind::Variant { ref subpatterns, .. } => match &subpatterns[..] {
 416                             &[ref pat] => &pat.pattern,
 417                             _ => bug!(),
 418                         },
 419                         _ => bug!(),
 420                     };
 421                     let pattern_string = witness.to_string();
 422                     struct_span_err!(cx.tcx.sess, sp, E0297,
 423                         "refutable pattern in `for` loop binding: \
 424                                 `{}` not covered",
 425                                 pattern_string)
 426                         .span_label(sp, &format!("pattern `{}` not covered", pattern_string))
 427                         .emit();
 428                 },
 429                 _ => {
 430                     const LIMIT: usize = 3;
 431                     let joined_patterns = match witnesses.len() {
 432                         0 => bug!(),
 433                         1 => format!("`{}`", witnesses[0]),
 434                         2...LIMIT => {
 435                             let (tail, head) = witnesses.split_last().unwrap();
 436                             let head: Vec<_> = head.iter().map(|w| w.to_string()).collect();
 437                             format!("`{}` and `{}`", head.join("`, `"), tail)
 438                         },
 439                         _ => {
 440                             let (head, tail) = witnesses.split_at(LIMIT);
 441                             let head: Vec<_> = head.iter().map(|w| w.to_string()).collect();
 442                             format!("`{}` and {} more", head.join("`, `"), tail.len())
 443                         }
 444                     };
 445
 446                     let label_text = match witnesses.len() {
 447                         1 => format!("pattern {} not covered", joined_patterns),
 448                         _ => format!("patterns {} not covered", joined_patterns)
 449                     };
 450                     create_e0004(cx.tcx.sess, sp,
 451                                  format!("non-exhaustive patterns: {} not covered",
 452                                          joined_patterns))
 453                         .span_label(sp, &label_text)
 454                         .emit();
 455                 },
 456             }
 457         }
 458         NotUseful => {
 459             // This is good, wildcard pattern isn't reachable
 460         },
 461         _ => bug!()
 462     }
 463 }
 464
 465 // Legality of move bindings checking
 466 fn check_legality_of_move_bindings(cx: &MatchVisitor,
 467                                    has_guard: bool,
 468                                    pats: &[P<Pat>]) {
 469     let mut by_ref_span = None;
 470     for pat in pats {
 471         pat.each_binding(|bm, _, span, _path| {
 472             if let hir::BindByRef(..) = bm {
 473                 by_ref_span = Some(span);
 474             }
 475         })
 476     }
 477
 478     let check_move = |p: &Pat, sub: Option<&Pat>| {
 479         // check legality of moving out of the enum
 480
 481         // x @ Foo(..) is legal, but x @ Foo(y) isn't.
 482         if sub.map_or(false, |p| p.contains_bindings()) {
 483             struct_span_err!(cx.tcx.sess, p.span, E0007,
 484                              "cannot bind by-move with sub-bindings")
 485                 .span_label(p.span, &format!("binds an already bound by-move value by moving it"))
 486                 .emit();
 487         } else if has_guard {
 488             struct_span_err!(cx.tcx.sess, p.span, E0008,
 489                       "cannot bind by-move into a pattern guard")
 490                 .span_label(p.span, &format!("moves value into pattern guard"))
 491                 .emit();
 492         } else if by_ref_span.is_some() {
 493             struct_span_err!(cx.tcx.sess, p.span, E0009,
 494                             "cannot bind by-move and by-ref in the same pattern")
 495                     .span_label(p.span, &format!("by-move pattern here"))
 496                     .span_label(by_ref_span.unwrap(), &format!("both by-ref and by-move used"))
 497                     .emit();
 498         }
 499     };
 500
 501     for pat in pats {
 502         pat.walk(|p| {
 503             if let PatKind::Binding(hir::BindByValue(..), _, _, ref sub) = p.node {
 504                 let pat_ty = cx.tables.node_id_to_type(p.id);
 505                 if pat_ty.moves_by_default(cx.tcx, cx.param_env, pat.span) {
 506                     check_move(p, sub.as_ref().map(|p| &**p));
 507                 }
 508             }
 509             true
 510         });
 511     }
 512 }
 513
 514 /// Ensures that a pattern guard doesn't borrow by mutable reference or
 515 /// assign.
 516 ///
 517 /// FIXME: this should be done by borrowck.
 518 fn check_for_mutation_in_guard(cx: &MatchVisitor, guard: &hir::Expr) {
 519     cx.tcx.infer_ctxt((cx.tables, cx.param_env.clone()), Reveal::NotSpecializable).enter(|infcx| {
 520         let mut checker = MutationChecker {
 521             cx: cx,
 522         };
 523         ExprUseVisitor::new(&mut checker, &infcx).walk_expr(guard);
 524     });
 525 }
 526
 527 struct MutationChecker<'a, 'gcx: 'a> {
 528     cx: &'a MatchVisitor<'a, 'gcx>,
 529 }
 530
 531 impl<'a, 'gcx, 'tcx> Delegate<'tcx> for MutationChecker<'a, 'gcx> {
 532     fn matched_pat(&mut self, _: &Pat, _: cmt, _: euv::MatchMode) {}
 533     fn consume(&mut self, _: ast::NodeId, _: Span, _: cmt, _: ConsumeMode) {}
 534     fn consume_pat(&mut self, _: &Pat, _: cmt, _: ConsumeMode) {}
 535     fn borrow(&mut self,
 536               _: ast::NodeId,
 537               span: Span,
 538               _: cmt,
 539               _: &'tcx ty::Region,
 540               kind:ty:: BorrowKind,
 541               _: LoanCause) {
 542         match kind {
 543             ty::MutBorrow => {
 544                 struct_span_err!(self.cx.tcx.sess, span, E0301,
 545                           "cannot mutably borrow in a pattern guard")
 546                     .span_label(span, &format!("borrowed mutably in pattern guard"))
 547                     .emit();
 548             }
 549             ty::ImmBorrow | ty::UniqueImmBorrow => {}
 550         }
 551     }
 552     fn decl_without_init(&mut self, _: ast::NodeId, _: Span) {}
 553     fn mutate(&mut self, _: ast::NodeId, span: Span, _: cmt, mode: MutateMode) {
 554         match mode {
 555             MutateMode::JustWrite | MutateMode::WriteAndRead => {
 556                 struct_span_err!(self.cx.tcx.sess, span, E0302, "cannot assign in a pattern guard")
 557                     .span_label(span, &format!("assignment in pattern guard"))
 558                     .emit();
 559             }
 560             MutateMode::Init => {}
 561         }
 562     }
 563 }
 564
 565 /// Forbids bindings in `@` patterns. This is necessary for memory safety,
 566 /// because of the way rvalues are handled in the borrow check. (See issue
 567 /// #14587.)
 568 fn check_legality_of_bindings_in_at_patterns(cx: &MatchVisitor, pat: &Pat) {
 569     AtBindingPatternVisitor { cx: cx, bindings_allowed: true }.visit_pat(pat);
 570 }
 571
 572 struct AtBindingPatternVisitor<'a, 'b:'a, 'tcx:'b> {
 573     cx: &'a MatchVisitor<'b, 'tcx>,
 574     bindings_allowed: bool
 575 }
 576
 577 impl<'a, 'b, 'tcx, 'v> Visitor<'v> for AtBindingPatternVisitor<'a, 'b, 'tcx> {
 578     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
 579         NestedVisitorMap::None
 580     }
 581
 582     fn visit_pat(&mut self, pat: &Pat) {
 583         match pat.node {
 584             PatKind::Binding(.., ref subpat) => {
 585                 if !self.bindings_allowed {
 586                     struct_span_err!(self.cx.tcx.sess, pat.span, E0303,
 587                                      "pattern bindings are not allowed after an `@`")
 588                         .span_label(pat.span,  &format!("not allowed after `@`"))
 589                         .emit();
 590                 }
 591
 592                 if subpat.is_some() {
 593                     let bindings_were_allowed = self.bindings_allowed;
 594                     self.bindings_allowed = false;
 595                     intravisit::walk_pat(self, pat);
 596                     self.bindings_allowed = bindings_were_allowed;
 597                 }
 598             }
 599             _ => intravisit::walk_pat(self, pat),
 600         }
 601     }
 602 }