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 rustc::middle::expr_use_visitor::{ConsumeMode, Delegate, ExprUseVisitor};
  18 use rustc::middle::expr_use_visitor::{LoanCause, MutateMode};
  19 use rustc::middle::expr_use_visitor as euv;
  20 use rustc::middle::mem_categorization::{cmt};
  21 use rustc::middle::region::RegionMaps;
  22 use rustc::session::Session;
  23 use rustc::ty::{self, Ty, TyCtxt};
  24 use rustc::lint;
  25 use rustc_errors::{Diagnostic, Level, DiagnosticBuilder};
  26
  27 use rustc::hir::def::*;
  28 use rustc::hir::intravisit::{self, Visitor, FnKind, NestedVisitorMap};
  29 use rustc::hir::{self, Pat, PatKind};
  30
  31 use rustc_back::slice;
  32
  33 use syntax::ast;
  34 use syntax::ptr::P;
  35 use syntax_pos::{Span, DUMMY_SP};
  36
  37 struct OuterVisitor<'a, 'tcx: 'a> { tcx: TyCtxt<'a, 'tcx, 'tcx> }
  38
  39 impl<'a, 'tcx> Visitor<'tcx> for OuterVisitor<'a, 'tcx> {
  40     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
  41         NestedVisitorMap::OnlyBodies(&self.tcx.hir)
  42     }
  43
  44     fn visit_fn(&mut self, fk: FnKind<'tcx>, fd: &'tcx hir::FnDecl,
  45                 b: hir::BodyId, s: Span, id: ast::NodeId) {
  46         intravisit::walk_fn(self, fk, fd, b, s, id);
  47
  48         let def_id = self.tcx.hir.local_def_id(id);
  49
  50         MatchVisitor {
  51             tcx: self.tcx,
  52             tables: self.tcx.body_tables(b),
  53             region_maps: &self.tcx.region_maps(def_id),
  54             param_env: self.tcx.param_env(def_id)
  55         }.visit_body(self.tcx.hir.body(b));
  56     }
  57 }
  58
  59 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
  60     tcx.hir.krate().visit_all_item_likes(&mut OuterVisitor { tcx: tcx }.as_deep_visitor());
  61     tcx.sess.abort_if_errors();
  62 }
  63
  64 fn create_e0004<'a>(sess: &'a Session, sp: Span, error_message: String) -> DiagnosticBuilder<'a> {
  65     struct_span_err!(sess, sp, E0004, "{}", &error_message)
  66 }
  67
  68 struct MatchVisitor<'a, 'tcx: 'a> {
  69     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  70     tables: &'a ty::TypeckTables<'tcx>,
  71     param_env: ty::ParamEnv<'tcx>,
  72     region_maps: &'a RegionMaps,
  73 }
  74
  75 impl<'a, 'tcx> Visitor<'tcx> for MatchVisitor<'a, 'tcx> {
  76     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
  77         NestedVisitorMap::None
  78     }
  79
  80     fn visit_expr(&mut self, ex: &'tcx hir::Expr) {
  81         intravisit::walk_expr(self, ex);
  82
  83         match ex.node {
  84             hir::ExprMatch(ref scrut, ref arms, source) => {
  85                 self.check_match(scrut, arms, source);
  86             }
  87             _ => {}
  88         }
  89     }
  90
  91     fn visit_local(&mut self, loc: &'tcx hir::Local) {
  92         intravisit::walk_local(self, loc);
  93
  94         self.check_irrefutable(&loc.pat, false);
  95
  96         // Check legality of move bindings and `@` patterns.
  97         self.check_patterns(false, slice::ref_slice(&loc.pat));
  98     }
  99
 100     fn visit_body(&mut self, body: &'tcx hir::Body) {
 101         intravisit::walk_body(self, body);
 102
 103         for arg in &body.arguments {
 104             self.check_irrefutable(&arg.pat, true);
 105             self.check_patterns(false, slice::ref_slice(&arg.pat));
 106         }
 107     }
 108 }
 109
 110 impl<'a, 'gcx, 'tcx> PatternContext<'a, 'gcx, 'tcx> {
 111     fn report_inlining_errors(&self, pat_span: Span) {
 112         for error in &self.errors {
 113             match *error {
 114                 PatternError::StaticInPattern(span) => {
 115                     span_err!(self.tcx.sess, span, E0158,
 116                               "statics cannot be referenced in patterns");
 117                 }
 118                 PatternError::ConstEval(ref err) => {
 119                     err.report(self.tcx, pat_span, "pattern");
 120                 }
 121             }
 122         }
 123     }
 124 }
 125
 126 impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
 127     fn check_patterns(&self, has_guard: bool, pats: &[P<Pat>]) {
 128         check_legality_of_move_bindings(self, has_guard, pats);
 129         for pat in pats {
 130             check_legality_of_bindings_in_at_patterns(self, pat);
 131         }
 132     }
 133
 134     fn check_match(
 135         &self,
 136         scrut: &hir::Expr,
 137         arms: &[hir::Arm],
 138         source: hir::MatchSource)
 139     {
 140         for arm in arms {
 141             // First, check legality of move bindings.
 142             self.check_patterns(arm.guard.is_some(), &arm.pats);
 143
 144             // Second, if there is a guard on each arm, make sure it isn't
 145             // assigning or borrowing anything mutably.
 146             if let Some(ref guard) = arm.guard {
 147                 check_for_mutation_in_guard(self, &guard);
 148             }
 149
 150             // Third, perform some lints.
 151             for pat in &arm.pats {
 152                 check_for_bindings_named_the_same_as_variants(self, pat);
 153             }
 154         }
 155
 156         let module = self.tcx.hir.get_module_parent(scrut.id);
 157         MatchCheckCtxt::create_and_enter(self.tcx, module, |ref mut cx| {
 158             let mut have_errors = false;
 159
 160             let inlined_arms : Vec<(Vec<_>, _)> = arms.iter().map(|arm| (
 161                 arm.pats.iter().map(|pat| {
 162                     let mut patcx = PatternContext::new(self.tcx, self.tables);
 163                     let pattern = expand_pattern(cx, patcx.lower_pattern(&pat));
 164                     if !patcx.errors.is_empty() {
 165                         patcx.report_inlining_errors(pat.span);
 166                         have_errors = true;
 167                     }
 168                     (pattern, &**pat)
 169                 }).collect(),
 170                 arm.guard.as_ref().map(|e| &**e)
 171             )).collect();
 172
 173             // Bail out early if inlining failed.
 174             if have_errors {
 175                 return;
 176             }
 177
 178             // Fourth, check for unreachable arms.
 179             check_arms(cx, &inlined_arms, source);
 180
 181             // Then, if the match has no arms, check whether the scrutinee
 182             // is uninhabited.
 183             let pat_ty = self.tables.node_id_to_type(scrut.id);
 184             let module = self.tcx.hir.get_module_parent(scrut.id);
 185             if inlined_arms.is_empty() {
 186                 let scrutinee_is_uninhabited = if self.tcx.sess.features.borrow().never_type {
 187                     pat_ty.is_uninhabited_from(module, self.tcx)
 188                 } else {
 189                     self.conservative_is_uninhabited(pat_ty)
 190                 };
 191                 if !scrutinee_is_uninhabited {
 192                     // We know the type is inhabited, so this must be wrong
 193                     let mut err = create_e0004(self.tcx.sess, scrut.span,
 194                                                format!("non-exhaustive patterns: type {} \
 195                                                         is non-empty",
 196                                                        pat_ty));
 197                     span_help!(&mut err, scrut.span,
 198                                "Please ensure that all possible cases are being handled; \
 199                                 possibly adding wildcards or more match arms.");
 200                     err.emit();
 201                 }
 202                 // If the type *is* uninhabited, it's vacuously exhaustive
 203                 return;
 204             }
 205
 206             let matrix: Matrix = inlined_arms
 207                 .iter()
 208                 .filter(|&&(_, guard)| guard.is_none())
 209                 .flat_map(|arm| &arm.0)
 210                 .map(|pat| vec![pat.0])
 211                 .collect();
 212             let scrut_ty = self.tables.node_id_to_type(scrut.id);
 213             check_exhaustive(cx, scrut_ty, scrut.span, &matrix, source);
 214         })
 215     }
 216
 217     fn conservative_is_uninhabited(&self, scrutinee_ty: Ty<'tcx>) -> bool {
 218         // "rustc-1.0-style" uncontentious uninhabitableness check
 219         match scrutinee_ty.sty {
 220             ty::TyNever => true,
 221             ty::TyAdt(def, _) => def.variants.is_empty(),
 222             _ => false
 223         }
 224     }
 225
 226     fn check_irrefutable(&self, pat: &Pat, is_fn_arg: bool) {
 227         let origin = if is_fn_arg {
 228             "function argument"
 229         } else {
 230             "local binding"
 231         };
 232
 233         let module = self.tcx.hir.get_module_parent(pat.id);
 234         MatchCheckCtxt::create_and_enter(self.tcx, module, |ref mut cx| {
 235             let mut patcx = PatternContext::new(self.tcx, self.tables);
 236             let pattern = patcx.lower_pattern(pat);
 237             let pattern_ty = pattern.ty;
 238             let pats : Matrix = vec![vec![
 239                 expand_pattern(cx, pattern)
 240             ]].into_iter().collect();
 241
 242             let wild_pattern = Pattern {
 243                 ty: pattern_ty,
 244                 span: DUMMY_SP,
 245                 kind: box PatternKind::Wild,
 246             };
 247             let witness = match is_useful(cx, &pats, &[&wild_pattern], ConstructWitness) {
 248                 UsefulWithWitness(witness) => witness,
 249                 NotUseful => return,
 250                 Useful => bug!()
 251             };
 252
 253             let pattern_string = witness[0].single_pattern().to_string();
 254             let mut diag = struct_span_err!(
 255                 self.tcx.sess, pat.span, E0005,
 256                 "refutable pattern in {}: `{}` not covered",
 257                 origin, pattern_string
 258             );
 259             diag.span_label(pat.span, format!("pattern `{}` not covered", pattern_string));
 260             diag.emit();
 261         });
 262     }
 263 }
 264
 265 fn check_for_bindings_named_the_same_as_variants(cx: &MatchVisitor, pat: &Pat) {
 266     pat.walk(|p| {
 267         if let PatKind::Binding(hir::BindByValue(hir::MutImmutable), _, name, None) = p.node {
 268             let pat_ty = cx.tables.pat_ty(p);
 269             if let ty::TyAdt(edef, _) = pat_ty.sty {
 270                 if edef.is_enum() && edef.variants.iter().any(|variant| {
 271                     variant.name == name.node && variant.ctor_kind == CtorKind::Const
 272                 }) {
 273                     let ty_path = cx.tcx.item_path_str(edef.did);
 274                     let mut err = struct_span_warn!(cx.tcx.sess, p.span, E0170,
 275                         "pattern binding `{}` is named the same as one \
 276                          of the variants of the type `{}`",
 277                         name.node, ty_path);
 278                     help!(err,
 279                         "if you meant to match on a variant, \
 280                         consider making the path in the pattern qualified: `{}::{}`",
 281                         ty_path, name.node);
 282                     err.emit();
 283                 }
 284             }
 285         }
 286         true
 287     });
 288 }
 289
 290 /// Checks for common cases of "catchall" patterns that may not be intended as such.
 291 fn pat_is_catchall(pat: &Pat) -> bool {
 292     match pat.node {
 293         PatKind::Binding(.., None) => true,
 294         PatKind::Binding(.., Some(ref s)) => pat_is_catchall(s),
 295         PatKind::Ref(ref s, _) => pat_is_catchall(s),
 296         PatKind::Tuple(ref v, _) => v.iter().all(|p| {
 297             pat_is_catchall(&p)
 298         }),
 299         _ => false
 300     }
 301 }
 302
 303 // Check for unreachable patterns
 304 fn check_arms<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 305                         arms: &[(Vec<(&'a Pattern<'tcx>, &hir::Pat)>, Option<&hir::Expr>)],
 306                         source: hir::MatchSource)
 307 {
 308     let mut seen = Matrix::empty();
 309     let mut catchall = None;
 310     let mut printed_if_let_err = false;
 311     for (arm_index, &(ref pats, guard)) in arms.iter().enumerate() {
 312         for &(pat, hir_pat) in pats {
 313             let v = vec![pat];
 314
 315             match is_useful(cx, &seen, &v, LeaveOutWitness) {
 316                 NotUseful => {
 317                     match source {
 318                         hir::MatchSource::IfLetDesugar { .. } => {
 319                             if printed_if_let_err {
 320                                 // we already printed an irrefutable if-let pattern error.
 321                                 // We don't want two, that's just confusing.
 322                             } else {
 323                                 // find the first arm pattern so we can use its span
 324                                 let &(ref first_arm_pats, _) = &arms[0];
 325                                 let first_pat = &first_arm_pats[0];
 326                                 let span = first_pat.0.span;
 327                                 struct_span_err!(cx.tcx.sess, span, E0162,
 328                                                 "irrefutable if-let pattern")
 329                                     .span_label(span, "irrefutable pattern")
 330                                     .emit();
 331                                 printed_if_let_err = true;
 332                             }
 333                         },
 334
 335                         hir::MatchSource::WhileLetDesugar => {
 336                             // find the first arm pattern so we can use its span
 337                             let &(ref first_arm_pats, _) = &arms[0];
 338                             let first_pat = &first_arm_pats[0];
 339                             let span = first_pat.0.span;
 340
 341                             // check which arm we're on.
 342                             match arm_index {
 343                                 // The arm with the user-specified pattern.
 344                                 0 => {
 345                                     let mut diagnostic = Diagnostic::new(Level::Warning,
 346                                                                          "unreachable pattern");
 347                                     diagnostic.set_span(pat.span);
 348                                     cx.tcx.sess.add_lint_diagnostic(
 349                                             lint::builtin::UNREACHABLE_PATTERNS,
 350                                             hir_pat.id, diagnostic);
 351                                 },
 352                                 // The arm with the wildcard pattern.
 353                                 1 => {
 354                                     struct_span_err!(cx.tcx.sess, span, E0165,
 355                                                      "irrefutable while-let pattern")
 356                                         .span_label(span, "irrefutable pattern")
 357                                         .emit();
 358                                 },
 359                                 _ => bug!(),
 360                             }
 361                         },
 362
 363                         hir::MatchSource::ForLoopDesugar |
 364                         hir::MatchSource::Normal => {
 365                             let mut diagnostic = Diagnostic::new(Level::Warning,
 366                                                                  "unreachable pattern");
 367                             diagnostic.set_span(pat.span);
 368                             // if we had a catchall pattern, hint at that
 369                             if let Some(catchall) = catchall {
 370                                 diagnostic.span_label(pat.span, "this is an unreachable pattern");
 371                                 diagnostic.span_note(catchall, "this pattern matches any value");
 372                             }
 373                             cx.tcx.sess.add_lint_diagnostic(lint::builtin::UNREACHABLE_PATTERNS,
 374                                                             hir_pat.id, diagnostic);
 375                         },
 376
 377                         // Unreachable patterns in try expressions occur when one of the arms
 378                         // are an uninhabited type. Which is OK.
 379                         hir::MatchSource::TryDesugar => {}
 380                     }
 381                 }
 382                 Useful => (),
 383                 UsefulWithWitness(_) => bug!()
 384             }
 385             if guard.is_none() {
 386                 seen.push(v);
 387                 if catchall.is_none() && pat_is_catchall(hir_pat) {
 388                     catchall = Some(pat.span);
 389                 }
 390             }
 391         }
 392     }
 393 }
 394
 395 fn check_exhaustive<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 396                               scrut_ty: Ty<'tcx>,
 397                               sp: Span,
 398                               matrix: &Matrix<'a, 'tcx>,
 399                               source: hir::MatchSource) {
 400     let wild_pattern = Pattern {
 401         ty: scrut_ty,
 402         span: DUMMY_SP,
 403         kind: box PatternKind::Wild,
 404     };
 405     match is_useful(cx, matrix, &[&wild_pattern], ConstructWitness) {
 406         UsefulWithWitness(pats) => {
 407             let witnesses = if pats.is_empty() {
 408                 vec![&wild_pattern]
 409             } else {
 410                 pats.iter().map(|w| w.single_pattern()).collect()
 411             };
 412             match source {
 413                 hir::MatchSource::ForLoopDesugar => {
 414                     // `witnesses[0]` has the form `Some(<head>)`, peel off the `Some`
 415                     let witness = match *witnesses[0].kind {
 416                         PatternKind::Variant { ref subpatterns, .. } => match &subpatterns[..] {
 417                             &[ref pat] => &pat.pattern,
 418                             _ => bug!(),
 419                         },
 420                         _ => bug!(),
 421                     };
 422                     let pattern_string = witness.to_string();
 423                     struct_span_err!(cx.tcx.sess, sp, E0297,
 424                         "refutable pattern in `for` loop binding: \
 425                                 `{}` not covered",
 426                                 pattern_string)
 427                         .span_label(sp, format!("pattern `{}` not covered", pattern_string))
 428                         .emit();
 429                 },
 430                 _ => {
 431                     const LIMIT: usize = 3;
 432                     let joined_patterns = match witnesses.len() {
 433                         0 => bug!(),
 434                         1 => format!("`{}`", witnesses[0]),
 435                         2...LIMIT => {
 436                             let (tail, head) = witnesses.split_last().unwrap();
 437                             let head: Vec<_> = head.iter().map(|w| w.to_string()).collect();
 438                             format!("`{}` and `{}`", head.join("`, `"), tail)
 439                         },
 440                         _ => {
 441                             let (head, tail) = witnesses.split_at(LIMIT);
 442                             let head: Vec<_> = head.iter().map(|w| w.to_string()).collect();
 443                             format!("`{}` and {} more", head.join("`, `"), tail.len())
 444                         }
 445                     };
 446
 447                     let label_text = match witnesses.len() {
 448                         1 => format!("pattern {} not covered", joined_patterns),
 449                         _ => format!("patterns {} not covered", joined_patterns)
 450                     };
 451                     create_e0004(cx.tcx.sess, sp,
 452                                  format!("non-exhaustive patterns: {} not covered",
 453                                          joined_patterns))
 454                         .span_label(sp, label_text)
 455                         .emit();
 456                 },
 457             }
 458         }
 459         NotUseful => {
 460             // This is good, wildcard pattern isn't reachable
 461         },
 462         _ => bug!()
 463     }
 464 }
 465
 466 // Legality of move bindings checking
 467 fn check_legality_of_move_bindings(cx: &MatchVisitor,
 468                                    has_guard: bool,
 469                                    pats: &[P<Pat>]) {
 470     let mut by_ref_span = None;
 471     for pat in pats {
 472         pat.each_binding(|bm, _, span, _path| {
 473             if let hir::BindByRef(..) = bm {
 474                 by_ref_span = Some(span);
 475             }
 476         })
 477     }
 478
 479     let check_move = |p: &Pat, sub: Option<&Pat>| {
 480         // check legality of moving out of the enum
 481
 482         // x @ Foo(..) is legal, but x @ Foo(y) isn't.
 483         if sub.map_or(false, |p| p.contains_bindings()) {
 484             struct_span_err!(cx.tcx.sess, p.span, E0007,
 485                              "cannot bind by-move with sub-bindings")
 486                 .span_label(p.span, "binds an already bound by-move value by moving it")
 487                 .emit();
 488         } else if has_guard {
 489             struct_span_err!(cx.tcx.sess, p.span, E0008,
 490                       "cannot bind by-move into a pattern guard")
 491                 .span_label(p.span, "moves value into pattern guard")
 492                 .emit();
 493         } else if by_ref_span.is_some() {
 494             struct_span_err!(cx.tcx.sess, p.span, E0009,
 495                             "cannot bind by-move and by-ref in the same pattern")
 496                     .span_label(p.span, "by-move pattern here")
 497                     .span_label(by_ref_span.unwrap(), "both by-ref and by-move used")
 498                     .emit();
 499         }
 500     };
 501
 502     for pat in pats {
 503         pat.walk(|p| {
 504             if let PatKind::Binding(hir::BindByValue(..), _, _, ref sub) = p.node {
 505                 let pat_ty = cx.tables.node_id_to_type(p.id);
 506                 if pat_ty.moves_by_default(cx.tcx, cx.param_env, pat.span) {
 507                     check_move(p, sub.as_ref().map(|p| &**p));
 508                 }
 509             }
 510             true
 511         });
 512     }
 513 }
 514
 515 /// Ensures that a pattern guard doesn't borrow by mutable reference or
 516 /// assign.
 517 ///
 518 /// FIXME: this should be done by borrowck.
 519 fn check_for_mutation_in_guard(cx: &MatchVisitor, guard: &hir::Expr) {
 520     cx.tcx.infer_ctxt((cx.tables, cx.param_env)).enter(|infcx| {
 521         let mut checker = MutationChecker {
 522             cx: cx,
 523         };
 524         ExprUseVisitor::new(&mut checker, cx.region_maps, &infcx).walk_expr(guard);
 525     });
 526 }
 527
 528 struct MutationChecker<'a, 'gcx: 'a> {
 529     cx: &'a MatchVisitor<'a, 'gcx>,
 530 }
 531
 532 impl<'a, 'gcx, 'tcx> Delegate<'tcx> for MutationChecker<'a, 'gcx> {
 533     fn matched_pat(&mut self, _: &Pat, _: cmt, _: euv::MatchMode) {}
 534     fn consume(&mut self, _: ast::NodeId, _: Span, _: cmt, _: ConsumeMode) {}
 535     fn consume_pat(&mut self, _: &Pat, _: cmt, _: ConsumeMode) {}
 536     fn borrow(&mut self,
 537               _: ast::NodeId,
 538               span: Span,
 539               _: cmt,
 540               _: ty::Region<'tcx>,
 541               kind:ty:: BorrowKind,
 542               _: LoanCause) {
 543         match kind {
 544             ty::MutBorrow => {
 545                 struct_span_err!(self.cx.tcx.sess, span, E0301,
 546                           "cannot mutably borrow in a pattern guard")
 547                     .span_label(span, "borrowed mutably in pattern guard")
 548                     .emit();
 549             }
 550             ty::ImmBorrow | ty::UniqueImmBorrow => {}
 551         }
 552     }
 553     fn decl_without_init(&mut self, _: ast::NodeId, _: Span) {}
 554     fn mutate(&mut self, _: ast::NodeId, span: Span, _: cmt, mode: MutateMode) {
 555         match mode {
 556             MutateMode::JustWrite | MutateMode::WriteAndRead => {
 557                 struct_span_err!(self.cx.tcx.sess, span, E0302, "cannot assign in a pattern guard")
 558                     .span_label(span, "assignment in pattern guard")
 559                     .emit();
 560             }
 561             MutateMode::Init => {}
 562         }
 563     }
 564 }
 565
 566 /// Forbids bindings in `@` patterns. This is necessary for memory safety,
 567 /// because of the way rvalues are handled in the borrow check. (See issue
 568 /// #14587.)
 569 fn check_legality_of_bindings_in_at_patterns(cx: &MatchVisitor, pat: &Pat) {
 570     AtBindingPatternVisitor { cx: cx, bindings_allowed: true }.visit_pat(pat);
 571 }
 572
 573 struct AtBindingPatternVisitor<'a, 'b:'a, 'tcx:'b> {
 574     cx: &'a MatchVisitor<'b, 'tcx>,
 575     bindings_allowed: bool
 576 }
 577
 578 impl<'a, 'b, 'tcx, 'v> Visitor<'v> for AtBindingPatternVisitor<'a, 'b, 'tcx> {
 579     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
 580         NestedVisitorMap::None
 581     }
 582
 583     fn visit_pat(&mut self, pat: &Pat) {
 584         match pat.node {
 585             PatKind::Binding(.., ref subpat) => {
 586                 if !self.bindings_allowed {
 587                     struct_span_err!(self.cx.tcx.sess, pat.span, E0303,
 588                                      "pattern bindings are not allowed after an `@`")
 589                         .span_label(pat.span,  "not allowed after `@`")
 590                         .emit();
 591                 }
 592
 593                 if subpat.is_some() {
 594                     let bindings_were_allowed = self.bindings_allowed;
 595                     self.bindings_allowed = false;
 596                     intravisit::walk_pat(self, pat);
 597                     self.bindings_allowed = bindings_were_allowed;
 598                 }
 599             }
 600             _ => intravisit::walk_pat(self, pat),
 601         }
 602     }
 603 }