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 region_context = self.tcx.hir.local_def_id(id);
  50         let region_maps = self.tcx.region_maps(region_context);
  51
  52         MatchVisitor {
  53             tcx: self.tcx,
  54             tables: self.tcx.body_tables(b),
  55             region_maps: &region_maps,
  56             param_env: &ty::ParameterEnvironment::for_item(self.tcx, id)
  57         }.visit_body(self.tcx.hir.body(b));
  58     }
  59 }
  60
  61 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
  62     tcx.hir.krate().visit_all_item_likes(&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     region_maps: &'a RegionMaps<'tcx>,
  75 }
  76
  77 impl<'a, 'tcx> Visitor<'tcx> for MatchVisitor<'a, 'tcx> {
  78     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
  79         NestedVisitorMap::None
  80     }
  81
  82     fn visit_expr(&mut self, ex: &'tcx hir::Expr) {
  83         intravisit::walk_expr(self, ex);
  84
  85         match ex.node {
  86             hir::ExprMatch(ref scrut, ref arms, source) => {
  87                 self.check_match(scrut, arms, source);
  88             }
  89             _ => {}
  90         }
  91     }
  92
  93     fn visit_local(&mut self, loc: &'tcx hir::Local) {
  94         intravisit::walk_local(self, loc);
  95
  96         self.check_irrefutable(&loc.pat, false);
  97
  98         // Check legality of move bindings and `@` patterns.
  99         self.check_patterns(false, slice::ref_slice(&loc.pat));
 100     }
 101
 102     fn visit_body(&mut self, body: &'tcx hir::Body) {
 103         intravisit::walk_body(self, body);
 104
 105         for arg in &body.arguments {
 106             self.check_irrefutable(&arg.pat, true);
 107             self.check_patterns(false, slice::ref_slice(&arg.pat));
 108         }
 109     }
 110 }
 111
 112 impl<'a, 'gcx, 'tcx> PatternContext<'a, 'gcx, 'tcx> {
 113     fn report_inlining_errors(&self, pat_span: Span) {
 114         for error in &self.errors {
 115             match *error {
 116                 PatternError::StaticInPattern(span) => {
 117                     span_err!(self.tcx.sess, span, E0158,
 118                               "statics cannot be referenced in patterns");
 119                 }
 120                 PatternError::ConstEval(ref err) => {
 121                     err.report(self.tcx, pat_span, "pattern");
 122                 }
 123             }
 124         }
 125     }
 126 }
 127
 128 impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
 129     fn check_patterns(&self, has_guard: bool, pats: &[P<Pat>]) {
 130         check_legality_of_move_bindings(self, has_guard, pats);
 131         for pat in pats {
 132             check_legality_of_bindings_in_at_patterns(self, pat);
 133         }
 134     }
 135
 136     fn check_match(
 137         &self,
 138         scrut: &hir::Expr,
 139         arms: &[hir::Arm],
 140         source: hir::MatchSource)
 141     {
 142         for arm in arms {
 143             // First, check legality of move bindings.
 144             self.check_patterns(arm.guard.is_some(), &arm.pats);
 145
 146             // Second, if there is a guard on each arm, make sure it isn't
 147             // assigning or borrowing anything mutably.
 148             if let Some(ref guard) = arm.guard {
 149                 check_for_mutation_in_guard(self, &guard);
 150             }
 151
 152             // Third, perform some lints.
 153             for pat in &arm.pats {
 154                 check_for_bindings_named_the_same_as_variants(self, pat);
 155             }
 156         }
 157
 158         let module = self.tcx.hir.local_def_id(self.tcx.hir.get_module_parent(scrut.id));
 159         MatchCheckCtxt::create_and_enter(self.tcx, module, |ref mut cx| {
 160             let mut have_errors = false;
 161
 162             let inlined_arms : Vec<(Vec<_>, _)> = arms.iter().map(|arm| (
 163                 arm.pats.iter().map(|pat| {
 164                     let mut patcx = PatternContext::new(self.tcx, self.tables);
 165                     let pattern = expand_pattern(cx, patcx.lower_pattern(&pat));
 166                     if !patcx.errors.is_empty() {
 167                         patcx.report_inlining_errors(pat.span);
 168                         have_errors = true;
 169                     }
 170                     (pattern, &**pat)
 171                 }).collect(),
 172                 arm.guard.as_ref().map(|e| &**e)
 173             )).collect();
 174
 175             // Bail out early if inlining failed.
 176             if have_errors {
 177                 return;
 178             }
 179
 180             // Fourth, check for unreachable arms.
 181             check_arms(cx, &inlined_arms, source);
 182
 183             // Then, if the match has no arms, check whether the scrutinee
 184             // is uninhabited.
 185             let pat_ty = self.tables.node_id_to_type(scrut.id);
 186             let module = self.tcx.hir.local_def_id(self.tcx.hir.get_module_parent(scrut.id));
 187             if inlined_arms.is_empty() {
 188                 let scrutinee_is_uninhabited = if self.tcx.sess.features.borrow().never_type {
 189                     pat_ty.is_uninhabited_from(module, self.tcx)
 190                 } else {
 191                     self.conservative_is_uninhabited(pat_ty)
 192                 };
 193                 if !scrutinee_is_uninhabited {
 194                     // We know the type is inhabited, so this must be wrong
 195                     let mut err = create_e0004(self.tcx.sess, scrut.span,
 196                                                format!("non-exhaustive patterns: type {} \
 197                                                         is non-empty",
 198                                                        pat_ty));
 199                     span_help!(&mut err, scrut.span,
 200                                "Please ensure that all possible cases are being handled; \
 201                                 possibly adding wildcards or more match arms.");
 202                     err.emit();
 203                 }
 204                 // If the type *is* uninhabited, it's vacuously exhaustive
 205                 return;
 206             }
 207
 208             let matrix: Matrix = inlined_arms
 209                 .iter()
 210                 .filter(|&&(_, guard)| guard.is_none())
 211                 .flat_map(|arm| &arm.0)
 212                 .map(|pat| vec![pat.0])
 213                 .collect();
 214             let scrut_ty = self.tables.node_id_to_type(scrut.id);
 215             check_exhaustive(cx, scrut_ty, scrut.span, &matrix, source);
 216         })
 217     }
 218
 219     fn conservative_is_uninhabited(&self, scrutinee_ty: Ty<'tcx>) -> bool {
 220         // "rustc-1.0-style" uncontentious uninhabitableness check
 221         match scrutinee_ty.sty {
 222             ty::TyNever => true,
 223             ty::TyAdt(def, _) => def.variants.is_empty(),
 224             _ => false
 225         }
 226     }
 227
 228     fn check_irrefutable(&self, pat: &Pat, is_fn_arg: bool) {
 229         let origin = if is_fn_arg {
 230             "function argument"
 231         } else {
 232             "local binding"
 233         };
 234
 235         let module = self.tcx.hir.local_def_id(self.tcx.hir.get_module_parent(pat.id));
 236         MatchCheckCtxt::create_and_enter(self.tcx, module, |ref mut cx| {
 237             let mut patcx = PatternContext::new(self.tcx, self.tables);
 238             let pattern = patcx.lower_pattern(pat);
 239             let pattern_ty = pattern.ty;
 240             let pats : Matrix = vec![vec![
 241                 expand_pattern(cx, pattern)
 242             ]].into_iter().collect();
 243
 244             let wild_pattern = Pattern {
 245                 ty: pattern_ty,
 246                 span: DUMMY_SP,
 247                 kind: box PatternKind::Wild,
 248             };
 249             let witness = match is_useful(cx, &pats, &[&wild_pattern], ConstructWitness) {
 250                 UsefulWithWitness(witness) => witness,
 251                 NotUseful => return,
 252                 Useful => bug!()
 253             };
 254
 255             let pattern_string = witness[0].single_pattern().to_string();
 256             let mut diag = struct_span_err!(
 257                 self.tcx.sess, pat.span, E0005,
 258                 "refutable pattern in {}: `{}` not covered",
 259                 origin, pattern_string
 260             );
 261             diag.span_label(pat.span, &format!("pattern `{}` not covered", pattern_string));
 262             diag.emit();
 263         });
 264     }
 265 }
 266
 267 fn check_for_bindings_named_the_same_as_variants(cx: &MatchVisitor, pat: &Pat) {
 268     pat.walk(|p| {
 269         if let PatKind::Binding(hir::BindByValue(hir::MutImmutable), _, name, None) = p.node {
 270             let pat_ty = cx.tables.pat_ty(p);
 271             if let ty::TyAdt(edef, _) = pat_ty.sty {
 272                 if edef.is_enum() && edef.variants.iter().any(|variant| {
 273                     variant.name == name.node && variant.ctor_kind == CtorKind::Const
 274                 }) {
 275                     let ty_path = cx.tcx.item_path_str(edef.did);
 276                     let mut err = struct_span_warn!(cx.tcx.sess, p.span, E0170,
 277                         "pattern binding `{}` is named the same as one \
 278                          of the variants of the type `{}`",
 279                         name.node, ty_path);
 280                     help!(err,
 281                         "if you meant to match on a variant, \
 282                         consider making the path in the pattern qualified: `{}::{}`",
 283                         ty_path, name.node);
 284                     err.emit();
 285                 }
 286             }
 287         }
 288         true
 289     });
 290 }
 291
 292 /// Checks for common cases of "catchall" patterns that may not be intended as such.
 293 fn pat_is_catchall(pat: &Pat) -> bool {
 294     match pat.node {
 295         PatKind::Binding(.., None) => true,
 296         PatKind::Binding(.., Some(ref s)) => pat_is_catchall(s),
 297         PatKind::Ref(ref s, _) => pat_is_catchall(s),
 298         PatKind::Tuple(ref v, _) => v.iter().all(|p| {
 299             pat_is_catchall(&p)
 300         }),
 301         _ => false
 302     }
 303 }
 304
 305 // Check for unreachable patterns
 306 fn check_arms<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 307                         arms: &[(Vec<(&'a Pattern<'tcx>, &hir::Pat)>, Option<&hir::Expr>)],
 308                         source: hir::MatchSource)
 309 {
 310     let mut seen = Matrix::empty();
 311     let mut catchall = None;
 312     let mut printed_if_let_err = false;
 313     for (arm_index, &(ref pats, guard)) in arms.iter().enumerate() {
 314         for &(pat, hir_pat) in pats {
 315             let v = vec![pat];
 316
 317             match is_useful(cx, &seen, &v, LeaveOutWitness) {
 318                 NotUseful => {
 319                     match source {
 320                         hir::MatchSource::IfLetDesugar { .. } => {
 321                             if printed_if_let_err {
 322                                 // we already printed an irrefutable if-let pattern error.
 323                                 // We don't want two, that's just confusing.
 324                             } else {
 325                                 // find the first arm pattern so we can use its span
 326                                 let &(ref first_arm_pats, _) = &arms[0];
 327                                 let first_pat = &first_arm_pats[0];
 328                                 let span = first_pat.0.span;
 329                                 struct_span_err!(cx.tcx.sess, span, E0162,
 330                                                 "irrefutable if-let pattern")
 331                                     .span_label(span, &format!("irrefutable pattern"))
 332                                     .emit();
 333                                 printed_if_let_err = true;
 334                             }
 335                         },
 336
 337                         hir::MatchSource::WhileLetDesugar => {
 338                             // find the first arm pattern so we can use its span
 339                             let &(ref first_arm_pats, _) = &arms[0];
 340                             let first_pat = &first_arm_pats[0];
 341                             let span = first_pat.0.span;
 342
 343                             // check which arm we're on.
 344                             match arm_index {
 345                                 // The arm with the user-specified pattern.
 346                                 0 => {
 347                                     let mut diagnostic = Diagnostic::new(Level::Warning,
 348                                                                          "unreachable pattern");
 349                                     diagnostic.set_span(pat.span);
 350                                     cx.tcx.sess.add_lint_diagnostic(
 351                                             lint::builtin::UNREACHABLE_PATTERNS,
 352                                             hir_pat.id, diagnostic);
 353                                 },
 354                                 // The arm with the wildcard pattern.
 355                                 1 => {
 356                                     struct_span_err!(cx.tcx.sess, span, E0165,
 357                                                      "irrefutable while-let pattern")
 358                                         .span_label(span, &format!("irrefutable pattern"))
 359                                         .emit();
 360                                 },
 361                                 _ => bug!(),
 362                             }
 363                         },
 364
 365                         hir::MatchSource::ForLoopDesugar |
 366                         hir::MatchSource::Normal => {
 367                             let mut diagnostic = Diagnostic::new(Level::Warning,
 368                                                                  "unreachable pattern");
 369                             diagnostic.set_span(pat.span);
 370                             // if we had a catchall pattern, hint at that
 371                             if let Some(catchall) = catchall {
 372                                 diagnostic.span_label(pat.span, &"this is an unreachable pattern");
 373                                 diagnostic.span_note(catchall, "this pattern matches any value");
 374                             }
 375                             cx.tcx.sess.add_lint_diagnostic(lint::builtin::UNREACHABLE_PATTERNS,
 376                                                             hir_pat.id, diagnostic);
 377                         },
 378
 379                         // Unreachable patterns in try expressions occur when one of the arms
 380                         // are an uninhabited type. Which is OK.
 381                         hir::MatchSource::TryDesugar => {}
 382                     }
 383                 }
 384                 Useful => (),
 385                 UsefulWithWitness(_) => bug!()
 386             }
 387             if guard.is_none() {
 388                 seen.push(v);
 389                 if catchall.is_none() && pat_is_catchall(hir_pat) {
 390                     catchall = Some(pat.span);
 391                 }
 392             }
 393         }
 394     }
 395 }
 396
 397 fn check_exhaustive<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 398                               scrut_ty: Ty<'tcx>,
 399                               sp: Span,
 400                               matrix: &Matrix<'a, 'tcx>,
 401                               source: hir::MatchSource) {
 402     let wild_pattern = Pattern {
 403         ty: scrut_ty,
 404         span: DUMMY_SP,
 405         kind: box PatternKind::Wild,
 406     };
 407     match is_useful(cx, matrix, &[&wild_pattern], ConstructWitness) {
 408         UsefulWithWitness(pats) => {
 409             let witnesses = if pats.is_empty() {
 410                 vec![&wild_pattern]
 411             } else {
 412                 pats.iter().map(|w| w.single_pattern()).collect()
 413             };
 414             match source {
 415                 hir::MatchSource::ForLoopDesugar => {
 416                     // `witnesses[0]` has the form `Some(<head>)`, peel off the `Some`
 417                     let witness = match *witnesses[0].kind {
 418                         PatternKind::Variant { ref subpatterns, .. } => match &subpatterns[..] {
 419                             &[ref pat] => &pat.pattern,
 420                             _ => bug!(),
 421                         },
 422                         _ => bug!(),
 423                     };
 424                     let pattern_string = witness.to_string();
 425                     struct_span_err!(cx.tcx.sess, sp, E0297,
 426                         "refutable pattern in `for` loop binding: \
 427                                 `{}` not covered",
 428                                 pattern_string)
 429                         .span_label(sp, &format!("pattern `{}` not covered", pattern_string))
 430                         .emit();
 431                 },
 432                 _ => {
 433                     const LIMIT: usize = 3;
 434                     let joined_patterns = match witnesses.len() {
 435                         0 => bug!(),
 436                         1 => format!("`{}`", witnesses[0]),
 437                         2...LIMIT => {
 438                             let (tail, head) = witnesses.split_last().unwrap();
 439                             let head: Vec<_> = head.iter().map(|w| w.to_string()).collect();
 440                             format!("`{}` and `{}`", head.join("`, `"), tail)
 441                         },
 442                         _ => {
 443                             let (head, tail) = witnesses.split_at(LIMIT);
 444                             let head: Vec<_> = head.iter().map(|w| w.to_string()).collect();
 445                             format!("`{}` and {} more", head.join("`, `"), tail.len())
 446                         }
 447                     };
 448
 449                     let label_text = match witnesses.len() {
 450                         1 => format!("pattern {} not covered", joined_patterns),
 451                         _ => format!("patterns {} not covered", joined_patterns)
 452                     };
 453                     create_e0004(cx.tcx.sess, sp,
 454                                  format!("non-exhaustive patterns: {} not covered",
 455                                          joined_patterns))
 456                         .span_label(sp, &label_text)
 457                         .emit();
 458                 },
 459             }
 460         }
 461         NotUseful => {
 462             // This is good, wildcard pattern isn't reachable
 463         },
 464         _ => bug!()
 465     }
 466 }
 467
 468 // Legality of move bindings checking
 469 fn check_legality_of_move_bindings(cx: &MatchVisitor,
 470                                    has_guard: bool,
 471                                    pats: &[P<Pat>]) {
 472     let mut by_ref_span = None;
 473     for pat in pats {
 474         pat.each_binding(|bm, _, span, _path| {
 475             if let hir::BindByRef(..) = bm {
 476                 by_ref_span = Some(span);
 477             }
 478         })
 479     }
 480
 481     let check_move = |p: &Pat, sub: Option<&Pat>| {
 482         // check legality of moving out of the enum
 483
 484         // x @ Foo(..) is legal, but x @ Foo(y) isn't.
 485         if sub.map_or(false, |p| p.contains_bindings()) {
 486             struct_span_err!(cx.tcx.sess, p.span, E0007,
 487                              "cannot bind by-move with sub-bindings")
 488                 .span_label(p.span, &format!("binds an already bound by-move value by moving it"))
 489                 .emit();
 490         } else if has_guard {
 491             struct_span_err!(cx.tcx.sess, p.span, E0008,
 492                       "cannot bind by-move into a pattern guard")
 493                 .span_label(p.span, &format!("moves value into pattern guard"))
 494                 .emit();
 495         } else if by_ref_span.is_some() {
 496             struct_span_err!(cx.tcx.sess, p.span, E0009,
 497                             "cannot bind by-move and by-ref in the same pattern")
 498                     .span_label(p.span, &format!("by-move pattern here"))
 499                     .span_label(by_ref_span.unwrap(), &format!("both by-ref and by-move used"))
 500                     .emit();
 501         }
 502     };
 503
 504     for pat in pats {
 505         pat.walk(|p| {
 506             if let PatKind::Binding(hir::BindByValue(..), _, _, ref sub) = p.node {
 507                 let pat_ty = cx.tables.node_id_to_type(p.id);
 508                 if pat_ty.moves_by_default(cx.tcx, cx.param_env, pat.span) {
 509                     check_move(p, sub.as_ref().map(|p| &**p));
 510                 }
 511             }
 512             true
 513         });
 514     }
 515 }
 516
 517 /// Ensures that a pattern guard doesn't borrow by mutable reference or
 518 /// assign.
 519 ///
 520 /// FIXME: this should be done by borrowck.
 521 fn check_for_mutation_in_guard(cx: &MatchVisitor, guard: &hir::Expr) {
 522     cx.tcx.infer_ctxt((cx.tables, cx.param_env.clone()), Reveal::UserFacing).enter(|infcx| {
 523         let mut checker = MutationChecker {
 524             cx: cx,
 525         };
 526         ExprUseVisitor::new(&mut checker, cx.region_maps, &infcx).walk_expr(guard);
 527     });
 528 }
 529
 530 struct MutationChecker<'a, 'gcx: 'a> {
 531     cx: &'a MatchVisitor<'a, 'gcx>,
 532 }
 533
 534 impl<'a, 'gcx, 'tcx> Delegate<'tcx> for MutationChecker<'a, 'gcx> {
 535     fn matched_pat(&mut self, _: &Pat, _: cmt, _: euv::MatchMode) {}
 536     fn consume(&mut self, _: ast::NodeId, _: Span, _: cmt, _: ConsumeMode) {}
 537     fn consume_pat(&mut self, _: &Pat, _: cmt, _: ConsumeMode) {}
 538     fn borrow(&mut self,
 539               _: ast::NodeId,
 540               span: Span,
 541               _: cmt,
 542               _: ty::Region<'tcx>,
 543               kind:ty:: BorrowKind,
 544               _: LoanCause) {
 545         match kind {
 546             ty::MutBorrow => {
 547                 struct_span_err!(self.cx.tcx.sess, span, E0301,
 548                           "cannot mutably borrow in a pattern guard")
 549                     .span_label(span, &format!("borrowed mutably in pattern guard"))
 550                     .emit();
 551             }
 552             ty::ImmBorrow | ty::UniqueImmBorrow => {}
 553         }
 554     }
 555     fn decl_without_init(&mut self, _: ast::NodeId, _: Span) {}
 556     fn mutate(&mut self, _: ast::NodeId, span: Span, _: cmt, mode: MutateMode) {
 557         match mode {
 558             MutateMode::JustWrite | MutateMode::WriteAndRead => {
 559                 struct_span_err!(self.cx.tcx.sess, span, E0302, "cannot assign in a pattern guard")
 560                     .span_label(span, &format!("assignment in pattern guard"))
 561                     .emit();
 562             }
 563             MutateMode::Init => {}
 564         }
 565     }
 566 }
 567
 568 /// Forbids bindings in `@` patterns. This is necessary for memory safety,
 569 /// because of the way rvalues are handled in the borrow check. (See issue
 570 /// #14587.)
 571 fn check_legality_of_bindings_in_at_patterns(cx: &MatchVisitor, pat: &Pat) {
 572     AtBindingPatternVisitor { cx: cx, bindings_allowed: true }.visit_pat(pat);
 573 }
 574
 575 struct AtBindingPatternVisitor<'a, 'b:'a, 'tcx:'b> {
 576     cx: &'a MatchVisitor<'b, 'tcx>,
 577     bindings_allowed: bool
 578 }
 579
 580 impl<'a, 'b, 'tcx, 'v> Visitor<'v> for AtBindingPatternVisitor<'a, 'b, 'tcx> {
 581     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
 582         NestedVisitorMap::None
 583     }
 584
 585     fn visit_pat(&mut self, pat: &Pat) {
 586         match pat.node {
 587             PatKind::Binding(.., ref subpat) => {
 588                 if !self.bindings_allowed {
 589                     struct_span_err!(self.cx.tcx.sess, pat.span, E0303,
 590                                      "pattern bindings are not allowed after an `@`")
 591                         .span_label(pat.span,  &format!("not allowed after `@`"))
 592                         .emit();
 593                 }
 594
 595                 if subpat.is_some() {
 596                     let bindings_were_allowed = self.bindings_allowed;
 597                     self.bindings_allowed = false;
 598                     intravisit::walk_pat(self, pat);
 599                     self.bindings_allowed = bindings_were_allowed;
 600                 }
 601             }
 602             _ => intravisit::walk_pat(self, pat),
 603         }
 604     }
 605 }