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;
  22 use rustc::session::Session;
  23 use rustc::ty::{self, Ty, TyCtxt};
  24 use rustc::ty::subst::Substs;
  25 use rustc::lint;
  26 use rustc_errors::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_scope_tree: &self.tcx.region_scope_tree(def_id),
  55             param_env: self.tcx.param_env(def_id),
  56             identity_substs: Substs::identity_for_item(self.tcx, def_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: ty::ParamEnv<'tcx>,
  74     identity_substs: &'tcx Substs<'tcx>,
  75     region_scope_tree: &'a region::ScopeTree,
  76 }
  77
  78 impl<'a, 'tcx> Visitor<'tcx> for MatchVisitor<'a, 'tcx> {
  79     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
  80         NestedVisitorMap::None
  81     }
  82
  83     fn visit_expr(&mut self, ex: &'tcx hir::Expr) {
  84         intravisit::walk_expr(self, ex);
  85
  86         match ex.node {
  87             hir::ExprMatch(ref scrut, ref arms, source) => {
  88                 self.check_match(scrut, arms, source);
  89             }
  90             _ => {}
  91         }
  92     }
  93
  94     fn visit_local(&mut self, loc: &'tcx hir::Local) {
  95         intravisit::walk_local(self, loc);
  96
  97         self.check_irrefutable(&loc.pat, match loc.source {
  98             hir::LocalSource::Normal => "local binding",
  99             hir::LocalSource::ForLoopDesugar => "`for` loop binding",
 100         });
 101
 102         // Check legality of move bindings and `@` patterns.
 103         self.check_patterns(false, slice::ref_slice(&loc.pat));
 104     }
 105
 106     fn visit_body(&mut self, body: &'tcx hir::Body) {
 107         intravisit::walk_body(self, body);
 108
 109         for arg in &body.arguments {
 110             self.check_irrefutable(&arg.pat, "function argument");
 111             self.check_patterns(false, slice::ref_slice(&arg.pat));
 112         }
 113     }
 114 }
 115
 116 impl<'a, 'tcx> PatternContext<'a, 'tcx> {
 117     fn report_inlining_errors(&self, pat_span: Span) {
 118         for error in &self.errors {
 119             match *error {
 120                 PatternError::StaticInPattern(span) => {
 121                     span_err!(self.tcx.sess, span, E0158,
 122                               "statics cannot be referenced in patterns");
 123                 }
 124                 PatternError::ConstEval(ref err) => {
 125                     err.report(self.tcx, pat_span, "pattern");
 126                 }
 127             }
 128         }
 129     }
 130 }
 131
 132 impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
 133     fn check_patterns(&self, has_guard: bool, pats: &[P<Pat>]) {
 134         check_legality_of_move_bindings(self, has_guard, pats);
 135         for pat in pats {
 136             check_legality_of_bindings_in_at_patterns(self, pat);
 137         }
 138     }
 139
 140     fn check_match(
 141         &self,
 142         scrut: &hir::Expr,
 143         arms: &'tcx [hir::Arm],
 144         source: hir::MatchSource)
 145     {
 146         for arm in arms {
 147             // First, check legality of move bindings.
 148             self.check_patterns(arm.guard.is_some(), &arm.pats);
 149
 150             // Second, if there is a guard on each arm, make sure it isn't
 151             // assigning or borrowing anything mutably.
 152             if let Some(ref guard) = arm.guard {
 153                 check_for_mutation_in_guard(self, &guard);
 154             }
 155
 156             // Third, perform some lints.
 157             for pat in &arm.pats {
 158                 check_for_bindings_named_the_same_as_variants(self, pat);
 159             }
 160         }
 161
 162         let module = self.tcx.hir.get_module_parent(scrut.id);
 163         MatchCheckCtxt::create_and_enter(self.tcx, module, |ref mut cx| {
 164             let mut have_errors = false;
 165
 166             let inlined_arms : Vec<(Vec<_>, _)> = arms.iter().map(|arm| (
 167                 arm.pats.iter().map(|pat| {
 168                     let mut patcx = PatternContext::new(self.tcx,
 169                                                         self.param_env.and(self.identity_substs),
 170                                                         self.tables);
 171                     let pattern = expand_pattern(cx, patcx.lower_pattern(&pat));
 172                     if !patcx.errors.is_empty() {
 173                         patcx.report_inlining_errors(pat.span);
 174                         have_errors = true;
 175                     }
 176                     (pattern, &**pat)
 177                 }).collect(),
 178                 arm.guard.as_ref().map(|e| &**e)
 179             )).collect();
 180
 181             // Bail out early if inlining failed.
 182             if have_errors {
 183                 return;
 184             }
 185
 186             // Fourth, check for unreachable arms.
 187             check_arms(cx, &inlined_arms, source);
 188
 189             // Then, if the match has no arms, check whether the scrutinee
 190             // is uninhabited.
 191             let pat_ty = self.tables.node_id_to_type(scrut.hir_id);
 192             let module = self.tcx.hir.get_module_parent(scrut.id);
 193             if inlined_arms.is_empty() {
 194                 let scrutinee_is_uninhabited = if self.tcx.sess.features.borrow().never_type {
 195                     pat_ty.is_uninhabited_from(module, self.tcx)
 196                 } else {
 197                     self.conservative_is_uninhabited(pat_ty)
 198                 };
 199                 if !scrutinee_is_uninhabited {
 200                     // We know the type is inhabited, so this must be wrong
 201                     let mut err = create_e0004(self.tcx.sess, scrut.span,
 202                                                format!("non-exhaustive patterns: type {} \
 203                                                         is non-empty",
 204                                                        pat_ty));
 205                     span_help!(&mut err, scrut.span,
 206                                "Please ensure that all possible cases are being handled; \
 207                                 possibly adding wildcards or more match arms.");
 208                     err.emit();
 209                 }
 210                 // If the type *is* uninhabited, it's vacuously exhaustive
 211                 return;
 212             }
 213
 214             let matrix: Matrix = inlined_arms
 215                 .iter()
 216                 .filter(|&&(_, guard)| guard.is_none())
 217                 .flat_map(|arm| &arm.0)
 218                 .map(|pat| vec![pat.0])
 219                 .collect();
 220             let scrut_ty = self.tables.node_id_to_type(scrut.hir_id);
 221             check_exhaustive(cx, scrut_ty, scrut.span, &matrix);
 222         })
 223     }
 224
 225     fn conservative_is_uninhabited(&self, scrutinee_ty: Ty<'tcx>) -> bool {
 226         // "rustc-1.0-style" uncontentious uninhabitableness check
 227         match scrutinee_ty.sty {
 228             ty::TyNever => true,
 229             ty::TyAdt(def, _) => def.variants.is_empty(),
 230             _ => false
 231         }
 232     }
 233
 234     fn check_irrefutable(&self, pat: &'tcx Pat, origin: &str) {
 235         let module = 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,
 238                                                 self.param_env.and(self.identity_substs),
 239                                                 self.tables);
 240             let pattern = patcx.lower_pattern(pat);
 241             let pattern_ty = pattern.ty;
 242             let pats : Matrix = vec![vec![
 243                 expand_pattern(cx, pattern)
 244             ]].into_iter().collect();
 245
 246             let wild_pattern = Pattern {
 247                 ty: pattern_ty,
 248                 span: DUMMY_SP,
 249                 kind: box PatternKind::Wild,
 250             };
 251             let witness = match is_useful(cx, &pats, &[&wild_pattern], ConstructWitness) {
 252                 UsefulWithWitness(witness) => witness,
 253                 NotUseful => return,
 254                 Useful => bug!()
 255             };
 256
 257             let pattern_string = witness[0].single_pattern().to_string();
 258             let mut diag = struct_span_err!(
 259                 self.tcx.sess, pat.span, E0005,
 260                 "refutable pattern in {}: `{}` not covered",
 261                 origin, pattern_string
 262             );
 263             diag.span_label(pat.span, format!("pattern `{}` not covered", pattern_string));
 264             diag.emit();
 265         });
 266     }
 267 }
 268
 269 fn check_for_bindings_named_the_same_as_variants(cx: &MatchVisitor, pat: &Pat) {
 270     pat.walk(|p| {
 271         if let PatKind::Binding(_, _, name, None) = p.node {
 272             let bm = *cx.tables
 273                         .pat_binding_modes()
 274                         .get(p.hir_id)
 275                         .expect("missing binding mode");
 276
 277             if bm != ty::BindByValue(hir::MutImmutable) {
 278                 // Nothing to check.
 279                 return true;
 280             }
 281             let pat_ty = cx.tables.pat_ty(p);
 282             if let ty::TyAdt(edef, _) = pat_ty.sty {
 283                 if edef.is_enum() && edef.variants.iter().any(|variant| {
 284                     variant.name == name.node && variant.ctor_kind == CtorKind::Const
 285                 }) {
 286                     let ty_path = cx.tcx.item_path_str(edef.did);
 287                     let mut err = struct_span_warn!(cx.tcx.sess, p.span, E0170,
 288                         "pattern binding `{}` is named the same as one \
 289                          of the variants of the type `{}`",
 290                         name.node, ty_path);
 291                     help!(err,
 292                         "if you meant to match on a variant, \
 293                         consider making the path in the pattern qualified: `{}::{}`",
 294                         ty_path, name.node);
 295                     err.emit();
 296                 }
 297             }
 298         }
 299         true
 300     });
 301 }
 302
 303 /// Checks for common cases of "catchall" patterns that may not be intended as such.
 304 fn pat_is_catchall(pat: &Pat) -> bool {
 305     match pat.node {
 306         PatKind::Binding(.., None) => true,
 307         PatKind::Binding(.., Some(ref s)) => pat_is_catchall(s),
 308         PatKind::Ref(ref s, _) => pat_is_catchall(s),
 309         PatKind::Tuple(ref v, _) => v.iter().all(|p| {
 310             pat_is_catchall(&p)
 311         }),
 312         _ => false
 313     }
 314 }
 315
 316 // Check for unreachable patterns
 317 fn check_arms<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 318                         arms: &[(Vec<(&'a Pattern<'tcx>, &hir::Pat)>, Option<&hir::Expr>)],
 319                         source: hir::MatchSource)
 320 {
 321     let mut seen = Matrix::empty();
 322     let mut catchall = None;
 323     let mut printed_if_let_err = false;
 324     for (arm_index, &(ref pats, guard)) in arms.iter().enumerate() {
 325         for &(pat, hir_pat) in pats {
 326             let v = vec![pat];
 327
 328             match is_useful(cx, &seen, &v, LeaveOutWitness) {
 329                 NotUseful => {
 330                     match source {
 331                         hir::MatchSource::IfLetDesugar { .. } => {
 332                             if printed_if_let_err {
 333                                 // we already printed an irrefutable if-let pattern error.
 334                                 // We don't want two, that's just confusing.
 335                             } else {
 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                                 struct_span_err!(cx.tcx.sess, span, E0162,
 341                                                 "irrefutable if-let pattern")
 342                                     .span_label(span, "irrefutable pattern")
 343                                     .emit();
 344                                 printed_if_let_err = true;
 345                             }
 346                         },
 347
 348                         hir::MatchSource::WhileLetDesugar => {
 349                             // find the first arm pattern so we can use its span
 350                             let &(ref first_arm_pats, _) = &arms[0];
 351                             let first_pat = &first_arm_pats[0];
 352                             let span = first_pat.0.span;
 353
 354                             // check which arm we're on.
 355                             match arm_index {
 356                                 // The arm with the user-specified pattern.
 357                                 0 => {
 358                                     cx.tcx.lint_node(
 359                                             lint::builtin::UNREACHABLE_PATTERNS,
 360                                         hir_pat.id, pat.span,
 361                                         "unreachable pattern");
 362                                 },
 363                                 // The arm with the wildcard pattern.
 364                                 1 => {
 365                                     struct_span_err!(cx.tcx.sess, span, E0165,
 366                                                      "irrefutable while-let pattern")
 367                                         .span_label(span, "irrefutable pattern")
 368                                         .emit();
 369                                 },
 370                                 _ => bug!(),
 371                             }
 372                         },
 373
 374                         hir::MatchSource::ForLoopDesugar |
 375                         hir::MatchSource::Normal => {
 376                             let mut err = cx.tcx.struct_span_lint_node(
 377                                 lint::builtin::UNREACHABLE_PATTERNS,
 378                                 hir_pat.id,
 379                                 pat.span,
 380                                 "unreachable pattern",
 381                             );
 382                             // if we had a catchall pattern, hint at that
 383                             if let Some(catchall) = catchall {
 384                                 err.span_label(pat.span, "this is an unreachable pattern");
 385                                 err.span_note(catchall, "this pattern matches any value");
 386                             }
 387                             err.emit();
 388                         },
 389
 390                         // Unreachable patterns in try expressions occur when one of the arms
 391                         // are an uninhabited type. Which is OK.
 392                         hir::MatchSource::TryDesugar => {}
 393                     }
 394                 }
 395                 Useful => (),
 396                 UsefulWithWitness(_) => bug!()
 397             }
 398             if guard.is_none() {
 399                 seen.push(v);
 400                 if catchall.is_none() && pat_is_catchall(hir_pat) {
 401                     catchall = Some(pat.span);
 402                 }
 403             }
 404         }
 405     }
 406 }
 407
 408 fn check_exhaustive<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 409                               scrut_ty: Ty<'tcx>,
 410                               sp: Span,
 411                               matrix: &Matrix<'a, 'tcx>) {
 412     let wild_pattern = Pattern {
 413         ty: scrut_ty,
 414         span: DUMMY_SP,
 415         kind: box PatternKind::Wild,
 416     };
 417     match is_useful(cx, matrix, &[&wild_pattern], ConstructWitness) {
 418         UsefulWithWitness(pats) => {
 419             let witnesses = if pats.is_empty() {
 420                 vec![&wild_pattern]
 421             } else {
 422                 pats.iter().map(|w| w.single_pattern()).collect()
 423             };
 424
 425             const LIMIT: usize = 3;
 426             let joined_patterns = match witnesses.len() {
 427                 0 => bug!(),
 428                 1 => format!("`{}`", witnesses[0]),
 429                 2...LIMIT => {
 430                     let (tail, head) = witnesses.split_last().unwrap();
 431                     let head: Vec<_> = head.iter().map(|w| w.to_string()).collect();
 432                     format!("`{}` and `{}`", head.join("`, `"), tail)
 433                 },
 434                 _ => {
 435                     let (head, tail) = witnesses.split_at(LIMIT);
 436                     let head: Vec<_> = head.iter().map(|w| w.to_string()).collect();
 437                     format!("`{}` and {} more", head.join("`, `"), tail.len())
 438                 }
 439             };
 440
 441             let label_text = match witnesses.len() {
 442                 1 => format!("pattern {} not covered", joined_patterns),
 443                 _ => format!("patterns {} not covered", joined_patterns)
 444             };
 445             create_e0004(cx.tcx.sess, sp,
 446                             format!("non-exhaustive patterns: {} not covered",
 447                                     joined_patterns))
 448                 .span_label(sp, label_text)
 449                 .emit();
 450         }
 451         NotUseful => {
 452             // This is good, wildcard pattern isn't reachable
 453         },
 454         _ => bug!()
 455     }
 456 }
 457
 458 // Legality of move bindings checking
 459 fn check_legality_of_move_bindings(cx: &MatchVisitor,
 460                                    has_guard: bool,
 461                                    pats: &[P<Pat>]) {
 462     let mut by_ref_span = None;
 463     for pat in pats {
 464         pat.each_binding(|_, id, span, _path| {
 465             let hir_id = cx.tcx.hir.node_to_hir_id(id);
 466             let bm = *cx.tables
 467                         .pat_binding_modes()
 468                         .get(hir_id)
 469                         .expect("missing binding mode");
 470             if let ty::BindByReference(..) = bm {
 471                 by_ref_span = Some(span);
 472             }
 473         })
 474     }
 475
 476     let check_move = |p: &Pat, sub: Option<&Pat>| {
 477         // check legality of moving out of the enum
 478
 479         // x @ Foo(..) is legal, but x @ Foo(y) isn't.
 480         if sub.map_or(false, |p| p.contains_bindings()) {
 481             struct_span_err!(cx.tcx.sess, p.span, E0007,
 482                              "cannot bind by-move with sub-bindings")
 483                 .span_label(p.span, "binds an already bound by-move value by moving it")
 484                 .emit();
 485         } else if has_guard {
 486             struct_span_err!(cx.tcx.sess, p.span, E0008,
 487                       "cannot bind by-move into a pattern guard")
 488                 .span_label(p.span, "moves value into pattern guard")
 489                 .emit();
 490         } else if by_ref_span.is_some() {
 491             struct_span_err!(cx.tcx.sess, p.span, E0009,
 492                             "cannot bind by-move and by-ref in the same pattern")
 493                     .span_label(p.span, "by-move pattern here")
 494                     .span_label(by_ref_span.unwrap(), "both by-ref and by-move used")
 495                     .emit();
 496         }
 497     };
 498
 499     for pat in pats {
 500         pat.walk(|p| {
 501             if let PatKind::Binding(_, _, _, ref sub) = p.node {
 502                 let bm = *cx.tables
 503                             .pat_binding_modes()
 504                             .get(p.hir_id)
 505                             .expect("missing binding mode");
 506                 match bm {
 507                     ty::BindByValue(..) => {
 508                         let pat_ty = cx.tables.node_id_to_type(p.hir_id);
 509                         if pat_ty.moves_by_default(cx.tcx, cx.param_env, pat.span) {
 510                             check_move(p, sub.as_ref().map(|p| &**p));
 511                         }
 512                     }
 513                     _ => {}
 514                 }
 515             }
 516             true
 517         });
 518     }
 519 }
 520
 521 /// Ensures that a pattern guard doesn't borrow by mutable reference or
 522 /// assign.
 523 ///
 524 /// FIXME: this should be done by borrowck.
 525 fn check_for_mutation_in_guard(cx: &MatchVisitor, guard: &hir::Expr) {
 526     let mut checker = MutationChecker {
 527         cx,
 528     };
 529     ExprUseVisitor::new(&mut checker, cx.tcx, cx.param_env, cx.region_scope_tree, cx.tables)
 530         .walk_expr(guard);
 531 }
 532
 533 struct MutationChecker<'a, 'tcx: 'a> {
 534     cx: &'a MatchVisitor<'a, 'tcx>,
 535 }
 536
 537 impl<'a, 'tcx> Delegate<'tcx> for MutationChecker<'a, 'tcx> {
 538     fn matched_pat(&mut self, _: &Pat, _: cmt, _: euv::MatchMode) {}
 539     fn consume(&mut self, _: ast::NodeId, _: Span, _: cmt, _: ConsumeMode) {}
 540     fn consume_pat(&mut self, _: &Pat, _: cmt, _: ConsumeMode) {}
 541     fn borrow(&mut self,
 542               _: ast::NodeId,
 543               span: Span,
 544               _: cmt,
 545               _: ty::Region<'tcx>,
 546               kind:ty:: BorrowKind,
 547               _: LoanCause) {
 548         match kind {
 549             ty::MutBorrow => {
 550                 struct_span_err!(self.cx.tcx.sess, span, E0301,
 551                           "cannot mutably borrow in a pattern guard")
 552                     .span_label(span, "borrowed mutably in pattern guard")
 553                     .emit();
 554             }
 555             ty::ImmBorrow | ty::UniqueImmBorrow => {}
 556         }
 557     }
 558     fn decl_without_init(&mut self, _: ast::NodeId, _: Span) {}
 559     fn mutate(&mut self, _: ast::NodeId, span: Span, _: cmt, mode: MutateMode) {
 560         match mode {
 561             MutateMode::JustWrite | MutateMode::WriteAndRead => {
 562                 struct_span_err!(self.cx.tcx.sess, span, E0302, "cannot assign in a pattern guard")
 563                     .span_label(span, "assignment in pattern guard")
 564                     .emit();
 565             }
 566             MutateMode::Init => {}
 567         }
 568     }
 569 }
 570
 571 /// Forbids bindings in `@` patterns. This is necessary for memory safety,
 572 /// because of the way rvalues are handled in the borrow check. (See issue
 573 /// #14587.)
 574 fn check_legality_of_bindings_in_at_patterns(cx: &MatchVisitor, pat: &Pat) {
 575     AtBindingPatternVisitor { cx: cx, bindings_allowed: true }.visit_pat(pat);
 576 }
 577
 578 struct AtBindingPatternVisitor<'a, 'b:'a, 'tcx:'b> {
 579     cx: &'a MatchVisitor<'b, 'tcx>,
 580     bindings_allowed: bool
 581 }
 582
 583 impl<'a, 'b, 'tcx, 'v> Visitor<'v> for AtBindingPatternVisitor<'a, 'b, 'tcx> {
 584     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
 585         NestedVisitorMap::None
 586     }
 587
 588     fn visit_pat(&mut self, pat: &Pat) {
 589         match pat.node {
 590             PatKind::Binding(.., ref subpat) => {
 591                 if !self.bindings_allowed {
 592                     struct_span_err!(self.cx.tcx.sess, pat.span, E0303,
 593                                      "pattern bindings are not allowed after an `@`")
 594                         .span_label(pat.span,  "not allowed after `@`")
 595                         .emit();
 596                 }
 597
 598                 if subpat.is_some() {
 599                     let bindings_were_allowed = self.bindings_allowed;
 600                     self.bindings_allowed = false;
 601                     intravisit::walk_pat(self, pat);
 602                     self.bindings_allowed = bindings_were_allowed;
 603                 }
 604             }
 605             _ => intravisit::walk_pat(self, pat),
 606         }
 607     }
 608 }