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::traits::Reveal;
  24 use rustc::ty::{self, Ty, TyCtxt};
  25 use rustc::lint;
  26 use rustc_errors::{Diagnostic, Level, DiagnosticBuilder};
  27
  28 use rustc::hir::def::*;
  29 use rustc::hir::intravisit::{self, Visitor, FnKind, NestedVisitorMap};
  30 use rustc::hir::{self, Pat, PatKind};
  31
  32 use rustc_back::slice;
  33
  34 use syntax::ast;
  35 use syntax::ptr::P;
  36 use syntax_pos::{Span, DUMMY_SP};
  37
  38 struct OuterVisitor<'a, 'tcx: 'a> { tcx: TyCtxt<'a, 'tcx, 'tcx> }
  39
  40 impl<'a, 'tcx> Visitor<'tcx> for OuterVisitor<'a, 'tcx> {
  41     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
  42         NestedVisitorMap::OnlyBodies(&self.tcx.hir)
  43     }
  44
  45     fn visit_fn(&mut self, fk: FnKind<'tcx>, fd: &'tcx hir::FnDecl,
  46                 b: hir::BodyId, s: Span, id: ast::NodeId) {
  47         intravisit::walk_fn(self, fk, fd, b, s, id);
  48
  49         let def_id = self.tcx.hir.local_def_id(id);
  50
  51         MatchVisitor {
  52             tcx: self.tcx,
  53             tables: self.tcx.body_tables(b),
  54             region_maps: &self.tcx.region_maps(def_id),
  55             param_env: self.tcx.param_env(def_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.hir.krate().visit_all_item_likes(&mut OuterVisitor { tcx: tcx }.as_deep_visitor());
  62     tcx.sess.abort_if_errors();
  63 }
  64
  65 fn create_e0004<'a>(sess: &'a Session, sp: Span, error_message: String) -> DiagnosticBuilder<'a> {
  66     struct_span_err!(sess, sp, E0004, "{}", &error_message)
  67 }
  68
  69 struct MatchVisitor<'a, 'tcx: 'a> {
  70     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  71     tables: &'a ty::TypeckTables<'tcx>,
  72     param_env: ty::ParamEnv<'tcx>,
  73     region_maps: &'a RegionMaps,
  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, 'gcx, 'tcx> PatternContext<'a, 'gcx, 'tcx> {
 112     fn report_inlining_errors(&self, pat_span: Span) {
 113         for error in &self.errors {
 114             match *error {
 115                 PatternError::StaticInPattern(span) => {
 116                     span_err!(self.tcx.sess, span, E0158,
 117                               "statics cannot be referenced in patterns");
 118                 }
 119                 PatternError::ConstEval(ref err) => {
 120                     err.report(self.tcx, pat_span, "pattern");
 121                 }
 122             }
 123         }
 124     }
 125 }
 126
 127 impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
 128     fn check_patterns(&self, has_guard: bool, pats: &[P<Pat>]) {
 129         check_legality_of_move_bindings(self, has_guard, pats);
 130         for pat in pats {
 131             check_legality_of_bindings_in_at_patterns(self, pat);
 132         }
 133     }
 134
 135     fn check_match(
 136         &self,
 137         scrut: &hir::Expr,
 138         arms: &[hir::Arm],
 139         source: hir::MatchSource)
 140     {
 141         for arm in arms {
 142             // First, check legality of move bindings.
 143             self.check_patterns(arm.guard.is_some(), &arm.pats);
 144
 145             // Second, if there is a guard on each arm, make sure it isn't
 146             // assigning or borrowing anything mutably.
 147             if let Some(ref guard) = arm.guard {
 148                 check_for_mutation_in_guard(self, &guard);
 149             }
 150
 151             // Third, perform some lints.
 152             for pat in &arm.pats {
 153                 check_for_bindings_named_the_same_as_variants(self, pat);
 154             }
 155         }
 156
 157         let module = self.tcx.hir.get_module_parent(scrut.id);
 158         MatchCheckCtxt::create_and_enter(self.tcx, module, |ref mut cx| {
 159             let mut have_errors = false;
 160
 161             let inlined_arms : Vec<(Vec<_>, _)> = arms.iter().map(|arm| (
 162                 arm.pats.iter().map(|pat| {
 163                     let mut patcx = PatternContext::new(self.tcx, self.tables);
 164                     let pattern = expand_pattern(cx, patcx.lower_pattern(&pat));
 165                     if !patcx.errors.is_empty() {
 166                         patcx.report_inlining_errors(pat.span);
 167                         have_errors = true;
 168                     }
 169                     (pattern, &**pat)
 170                 }).collect(),
 171                 arm.guard.as_ref().map(|e| &**e)
 172             )).collect();
 173
 174             // Bail out early if inlining failed.
 175             if have_errors {
 176                 return;
 177             }
 178
 179             // Fourth, check for unreachable arms.
 180             check_arms(cx, &inlined_arms, source);
 181
 182             // Then, if the match has no arms, check whether the scrutinee
 183             // is uninhabited.
 184             let pat_ty = self.tables.node_id_to_type(scrut.id);
 185             let module = self.tcx.hir.get_module_parent(scrut.id);
 186             if inlined_arms.is_empty() {
 187                 let scrutinee_is_uninhabited = if self.tcx.sess.features.borrow().never_type {
 188                     pat_ty.is_uninhabited_from(module, self.tcx)
 189                 } else {
 190                     self.conservative_is_uninhabited(pat_ty)
 191                 };
 192                 if !scrutinee_is_uninhabited {
 193                     // We know the type is inhabited, so this must be wrong
 194                     let mut err = create_e0004(self.tcx.sess, scrut.span,
 195                                                format!("non-exhaustive patterns: type {} \
 196                                                         is non-empty",
 197                                                        pat_ty));
 198                     span_help!(&mut err, scrut.span,
 199                                "Please ensure that all possible cases are being handled; \
 200                                 possibly adding wildcards or more match arms.");
 201                     err.emit();
 202                 }
 203                 // If the type *is* uninhabited, it's vacuously exhaustive
 204                 return;
 205             }
 206
 207             let matrix: Matrix = inlined_arms
 208                 .iter()
 209                 .filter(|&&(_, guard)| guard.is_none())
 210                 .flat_map(|arm| &arm.0)
 211                 .map(|pat| vec![pat.0])
 212                 .collect();
 213             let scrut_ty = self.tables.node_id_to_type(scrut.id);
 214             check_exhaustive(cx, scrut_ty, scrut.span, &matrix, source);
 215         })
 216     }
 217
 218     fn conservative_is_uninhabited(&self, scrutinee_ty: Ty<'tcx>) -> bool {
 219         // "rustc-1.0-style" uncontentious uninhabitableness check
 220         match scrutinee_ty.sty {
 221             ty::TyNever => true,
 222             ty::TyAdt(def, _) => def.variants.is_empty(),
 223             _ => false
 224         }
 225     }
 226
 227     fn check_irrefutable(&self, pat: &Pat, is_fn_arg: bool) {
 228         let origin = if is_fn_arg {
 229             "function argument"
 230         } else {
 231             "local binding"
 232         };
 233
 234         let module = self.tcx.hir.get_module_parent(pat.id);
 235         MatchCheckCtxt::create_and_enter(self.tcx, module, |ref mut cx| {
 236             let mut patcx = PatternContext::new(self.tcx, self.tables);
 237             let pattern = patcx.lower_pattern(pat);
 238             let pattern_ty = pattern.ty;
 239             let pats : Matrix = vec![vec![
 240                 expand_pattern(cx, pattern)
 241             ]].into_iter().collect();
 242
 243             let wild_pattern = Pattern {
 244                 ty: pattern_ty,
 245                 span: DUMMY_SP,
 246                 kind: box PatternKind::Wild,
 247             };
 248             let witness = match is_useful(cx, &pats, &[&wild_pattern], ConstructWitness) {
 249                 UsefulWithWitness(witness) => witness,
 250                 NotUseful => return,
 251                 Useful => bug!()
 252             };
 253
 254             let pattern_string = witness[0].single_pattern().to_string();
 255             let mut diag = struct_span_err!(
 256                 self.tcx.sess, pat.span, E0005,
 257                 "refutable pattern in {}: `{}` not covered",
 258                 origin, pattern_string
 259             );
 260             diag.span_label(pat.span, format!("pattern `{}` not covered", pattern_string));
 261             diag.emit();
 262         });
 263     }
 264 }
 265
 266 fn check_for_bindings_named_the_same_as_variants(cx: &MatchVisitor, pat: &Pat) {
 267     pat.walk(|p| {
 268         if let PatKind::Binding(hir::BindByValue(hir::MutImmutable), _, name, None) = p.node {
 269             let pat_ty = cx.tables.pat_ty(p);
 270             if let ty::TyAdt(edef, _) = pat_ty.sty {
 271                 if edef.is_enum() && edef.variants.iter().any(|variant| {
 272                     variant.name == name.node && variant.ctor_kind == CtorKind::Const
 273                 }) {
 274                     let ty_path = cx.tcx.item_path_str(edef.did);
 275                     let mut err = struct_span_warn!(cx.tcx.sess, p.span, E0170,
 276                         "pattern binding `{}` is named the same as one \
 277                          of the variants of the type `{}`",
 278                         name.node, ty_path);
 279                     help!(err,
 280                         "if you meant to match on a variant, \
 281                         consider making the path in the pattern qualified: `{}::{}`",
 282                         ty_path, name.node);
 283                     err.emit();
 284                 }
 285             }
 286         }
 287         true
 288     });
 289 }
 290
 291 /// Checks for common cases of "catchall" patterns that may not be intended as such.
 292 fn pat_is_catchall(pat: &Pat) -> bool {
 293     match pat.node {
 294         PatKind::Binding(.., None) => true,
 295         PatKind::Binding(.., Some(ref s)) => pat_is_catchall(s),
 296         PatKind::Ref(ref s, _) => pat_is_catchall(s),
 297         PatKind::Tuple(ref v, _) => v.iter().all(|p| {
 298             pat_is_catchall(&p)
 299         }),
 300         _ => false
 301     }
 302 }
 303
 304 // Check for unreachable patterns
 305 fn check_arms<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 306                         arms: &[(Vec<(&'a Pattern<'tcx>, &hir::Pat)>, Option<&hir::Expr>)],
 307                         source: hir::MatchSource)
 308 {
 309     let mut seen = Matrix::empty();
 310     let mut catchall = None;
 311     let mut printed_if_let_err = false;
 312     for (arm_index, &(ref pats, guard)) in arms.iter().enumerate() {
 313         for &(pat, hir_pat) in pats {
 314             let v = vec![pat];
 315
 316             match is_useful(cx, &seen, &v, LeaveOutWitness) {
 317                 NotUseful => {
 318                     match source {
 319                         hir::MatchSource::IfLetDesugar { .. } => {
 320                             if printed_if_let_err {
 321                                 // we already printed an irrefutable if-let pattern error.
 322                                 // We don't want two, that's just confusing.
 323                             } else {
 324                                 // find the first arm pattern so we can use its span
 325                                 let &(ref first_arm_pats, _) = &arms[0];
 326                                 let first_pat = &first_arm_pats[0];
 327                                 let span = first_pat.0.span;
 328                                 struct_span_err!(cx.tcx.sess, span, E0162,
 329                                                 "irrefutable if-let pattern")
 330                                     .span_label(span, "irrefutable pattern")
 331                                     .emit();
 332                                 printed_if_let_err = true;
 333                             }
 334                         },
 335
 336                         hir::MatchSource::WhileLetDesugar => {
 337                             // find the first arm pattern so we can use its span
 338                             let &(ref first_arm_pats, _) = &arms[0];
 339                             let first_pat = &first_arm_pats[0];
 340                             let span = first_pat.0.span;
 341
 342                             // check which arm we're on.
 343                             match arm_index {
 344                                 // The arm with the user-specified pattern.
 345                                 0 => {
 346                                     let mut diagnostic = Diagnostic::new(Level::Warning,
 347                                                                          "unreachable pattern");
 348                                     diagnostic.set_span(pat.span);
 349                                     cx.tcx.sess.add_lint_diagnostic(
 350                                             lint::builtin::UNREACHABLE_PATTERNS,
 351                                             hir_pat.id, diagnostic);
 352                                 },
 353                                 // The arm with the wildcard pattern.
 354                                 1 => {
 355                                     struct_span_err!(cx.tcx.sess, span, E0165,
 356                                                      "irrefutable while-let pattern")
 357                                         .span_label(span, "irrefutable pattern")
 358                                         .emit();
 359                                 },
 360                                 _ => bug!(),
 361                             }
 362                         },
 363
 364                         hir::MatchSource::ForLoopDesugar |
 365                         hir::MatchSource::Normal => {
 366                             let mut diagnostic = Diagnostic::new(Level::Warning,
 367                                                                  "unreachable pattern");
 368                             diagnostic.set_span(pat.span);
 369                             // if we had a catchall pattern, hint at that
 370                             if let Some(catchall) = catchall {
 371                                 diagnostic.span_label(pat.span, "this is an unreachable pattern");
 372                                 diagnostic.span_note(catchall, "this pattern matches any value");
 373                             }
 374                             cx.tcx.sess.add_lint_diagnostic(lint::builtin::UNREACHABLE_PATTERNS,
 375                                                             hir_pat.id, diagnostic);
 376                         },
 377
 378                         // Unreachable patterns in try expressions occur when one of the arms
 379                         // are an uninhabited type. Which is OK.
 380                         hir::MatchSource::TryDesugar => {}
 381                     }
 382                 }
 383                 Useful => (),
 384                 UsefulWithWitness(_) => bug!()
 385             }
 386             if guard.is_none() {
 387                 seen.push(v);
 388                 if catchall.is_none() && pat_is_catchall(hir_pat) {
 389                     catchall = Some(pat.span);
 390                 }
 391             }
 392         }
 393     }
 394 }
 395
 396 fn check_exhaustive<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 397                               scrut_ty: Ty<'tcx>,
 398                               sp: Span,
 399                               matrix: &Matrix<'a, 'tcx>,
 400                               source: hir::MatchSource) {
 401     let wild_pattern = Pattern {
 402         ty: scrut_ty,
 403         span: DUMMY_SP,
 404         kind: box PatternKind::Wild,
 405     };
 406     match is_useful(cx, matrix, &[&wild_pattern], ConstructWitness) {
 407         UsefulWithWitness(pats) => {
 408             let witnesses = if pats.is_empty() {
 409                 vec![&wild_pattern]
 410             } else {
 411                 pats.iter().map(|w| w.single_pattern()).collect()
 412             };
 413             match source {
 414                 hir::MatchSource::ForLoopDesugar => {
 415                     // `witnesses[0]` has the form `Some(<head>)`, peel off the `Some`
 416                     let witness = match *witnesses[0].kind {
 417                         PatternKind::Variant { ref subpatterns, .. } => match &subpatterns[..] {
 418                             &[ref pat] => &pat.pattern,
 419                             _ => bug!(),
 420                         },
 421                         _ => bug!(),
 422                     };
 423                     let pattern_string = witness.to_string();
 424                     struct_span_err!(cx.tcx.sess, sp, E0297,
 425                         "refutable pattern in `for` loop binding: \
 426                                 `{}` not covered",
 427                                 pattern_string)
 428                         .span_label(sp, format!("pattern `{}` not covered", pattern_string))
 429                         .emit();
 430                 },
 431                 _ => {
 432                     const LIMIT: usize = 3;
 433                     let joined_patterns = match witnesses.len() {
 434                         0 => bug!(),
 435                         1 => format!("`{}`", witnesses[0]),
 436                         2...LIMIT => {
 437                             let (tail, head) = witnesses.split_last().unwrap();
 438                             let head: Vec<_> = head.iter().map(|w| w.to_string()).collect();
 439                             format!("`{}` and `{}`", head.join("`, `"), tail)
 440                         },
 441                         _ => {
 442                             let (head, tail) = witnesses.split_at(LIMIT);
 443                             let head: Vec<_> = head.iter().map(|w| w.to_string()).collect();
 444                             format!("`{}` and {} more", head.join("`, `"), tail.len())
 445                         }
 446                     };
 447
 448                     let label_text = match witnesses.len() {
 449                         1 => format!("pattern {} not covered", joined_patterns),
 450                         _ => format!("patterns {} not covered", joined_patterns)
 451                     };
 452                     create_e0004(cx.tcx.sess, sp,
 453                                  format!("non-exhaustive patterns: {} not covered",
 454                                          joined_patterns))
 455                         .span_label(sp, label_text)
 456                         .emit();
 457                 },
 458             }
 459         }
 460         NotUseful => {
 461             // This is good, wildcard pattern isn't reachable
 462         },
 463         _ => bug!()
 464     }
 465 }
 466
 467 // Legality of move bindings checking
 468 fn check_legality_of_move_bindings(cx: &MatchVisitor,
 469                                    has_guard: bool,
 470                                    pats: &[P<Pat>]) {
 471     let mut by_ref_span = None;
 472     for pat in pats {
 473         pat.each_binding(|bm, _, span, _path| {
 474             if let hir::BindByRef(..) = bm {
 475                 by_ref_span = Some(span);
 476             }
 477         })
 478     }
 479
 480     let check_move = |p: &Pat, sub: Option<&Pat>| {
 481         // check legality of moving out of the enum
 482
 483         // x @ Foo(..) is legal, but x @ Foo(y) isn't.
 484         if sub.map_or(false, |p| p.contains_bindings()) {
 485             struct_span_err!(cx.tcx.sess, p.span, E0007,
 486                              "cannot bind by-move with sub-bindings")
 487                 .span_label(p.span, "binds an already bound by-move value by moving it")
 488                 .emit();
 489         } else if has_guard {
 490             struct_span_err!(cx.tcx.sess, p.span, E0008,
 491                       "cannot bind by-move into a pattern guard")
 492                 .span_label(p.span, "moves value into pattern guard")
 493                 .emit();
 494         } else if by_ref_span.is_some() {
 495             struct_span_err!(cx.tcx.sess, p.span, E0009,
 496                             "cannot bind by-move and by-ref in the same pattern")
 497                     .span_label(p.span, "by-move pattern here")
 498                     .span_label(by_ref_span.unwrap(), "both by-ref and by-move used")
 499                     .emit();
 500         }
 501     };
 502
 503     for pat in pats {
 504         pat.walk(|p| {
 505             if let PatKind::Binding(hir::BindByValue(..), _, _, ref sub) = p.node {
 506                 let pat_ty = cx.tables.node_id_to_type(p.id);
 507                 if pat_ty.moves_by_default(cx.tcx, cx.param_env, pat.span) {
 508                     check_move(p, sub.as_ref().map(|p| &**p));
 509                 }
 510             }
 511             true
 512         });
 513     }
 514 }
 515
 516 /// Ensures that a pattern guard doesn't borrow by mutable reference or
 517 /// assign.
 518 ///
 519 /// FIXME: this should be done by borrowck.
 520 fn check_for_mutation_in_guard(cx: &MatchVisitor, guard: &hir::Expr) {
 521     cx.tcx.infer_ctxt((cx.tables, cx.param_env), Reveal::UserFacing).enter(|infcx| {
 522         let mut checker = MutationChecker {
 523             cx: cx,
 524         };
 525         ExprUseVisitor::new(&mut checker, cx.region_maps, &infcx).walk_expr(guard);
 526     });
 527 }
 528
 529 struct MutationChecker<'a, 'gcx: 'a> {
 530     cx: &'a MatchVisitor<'a, 'gcx>,
 531 }
 532
 533 impl<'a, 'gcx, 'tcx> Delegate<'tcx> for MutationChecker<'a, 'gcx> {
 534     fn matched_pat(&mut self, _: &Pat, _: cmt, _: euv::MatchMode) {}
 535     fn consume(&mut self, _: ast::NodeId, _: Span, _: cmt, _: ConsumeMode) {}
 536     fn consume_pat(&mut self, _: &Pat, _: cmt, _: ConsumeMode) {}
 537     fn borrow(&mut self,
 538               _: ast::NodeId,
 539               span: Span,
 540               _: cmt,
 541               _: ty::Region<'tcx>,
 542               kind:ty:: BorrowKind,
 543               _: LoanCause) {
 544         match kind {
 545             ty::MutBorrow => {
 546                 struct_span_err!(self.cx.tcx.sess, span, E0301,
 547                           "cannot mutably borrow in a pattern guard")
 548                     .span_label(span, "borrowed mutably in pattern guard")
 549                     .emit();
 550             }
 551             ty::ImmBorrow | ty::UniqueImmBorrow => {}
 552         }
 553     }
 554     fn decl_without_init(&mut self, _: ast::NodeId, _: Span) {}
 555     fn mutate(&mut self, _: ast::NodeId, span: Span, _: cmt, mode: MutateMode) {
 556         match mode {
 557             MutateMode::JustWrite | MutateMode::WriteAndRead => {
 558                 struct_span_err!(self.cx.tcx.sess, span, E0302, "cannot assign in a pattern guard")
 559                     .span_label(span, "assignment in pattern guard")
 560                     .emit();
 561             }
 562             MutateMode::Init => {}
 563         }
 564     }
 565 }
 566
 567 /// Forbids bindings in `@` patterns. This is necessary for memory safety,
 568 /// because of the way rvalues are handled in the borrow check. (See issue
 569 /// #14587.)
 570 fn check_legality_of_bindings_in_at_patterns(cx: &MatchVisitor, pat: &Pat) {
 571     AtBindingPatternVisitor { cx: cx, bindings_allowed: true }.visit_pat(pat);
 572 }
 573
 574 struct AtBindingPatternVisitor<'a, 'b:'a, 'tcx:'b> {
 575     cx: &'a MatchVisitor<'b, 'tcx>,
 576     bindings_allowed: bool
 577 }
 578
 579 impl<'a, 'b, 'tcx, 'v> Visitor<'v> for AtBindingPatternVisitor<'a, 'b, 'tcx> {
 580     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
 581         NestedVisitorMap::None
 582     }
 583
 584     fn visit_pat(&mut self, pat: &Pat) {
 585         match pat.node {
 586             PatKind::Binding(.., ref subpat) => {
 587                 if !self.bindings_allowed {
 588                     struct_span_err!(self.cx.tcx.sess, pat.span, E0303,
 589                                      "pattern bindings are not allowed after an `@`")
 590                         .span_label(pat.span,  "not allowed after `@`")
 591                         .emit();
 592                 }
 593
 594                 if subpat.is_some() {
 595                     let bindings_were_allowed = self.bindings_allowed;
 596                     self.bindings_allowed = false;
 597                     intravisit::walk_pat(self, pat);
 598                     self.bindings_allowed = bindings_were_allowed;
 599                 }
 600             }
 601             _ => intravisit::walk_pat(self, pat),
 602         }
 603     }
 604 }