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