src/librustc_mir/hair/pattern/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 super::_match::{MatchCheckCtxt, Matrix, expand_pattern, is_useful};
  12 use super::_match::Usefulness::*;
  13 use super::_match::WitnessPreference::*;
  14
  15 use super::{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 use rustc::util::common::ErrorReported;
  28
  29 use rustc::hir::def::*;
  30 use rustc::hir::def_id::DefId;
  31 use rustc::hir::intravisit::{self, Visitor, NestedVisitorMap};
  32 use rustc::hir::{self, Pat, PatKind};
  33
  34 use std::slice;
  35
  36 use syntax::ast;
  37 use syntax::ptr::P;
  38 use syntax_pos::{Span, DUMMY_SP};
  39
  40 struct OuterVisitor<'a, 'tcx: 'a> { tcx: TyCtxt<'a, 'tcx, 'tcx> }
  41
  42 impl<'a, 'tcx> Visitor<'tcx> for OuterVisitor<'a, 'tcx> {
  43     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
  44         NestedVisitorMap::OnlyBodies(&self.tcx.hir)
  45     }
  46
  47     fn visit_body(&mut self, body: &'tcx hir::Body) {
  48         intravisit::walk_body(self, body);
  49         let def_id = self.tcx.hir.body_owner_def_id(body.id());
  50         let _ = self.tcx.check_match(def_id);
  51     }
  52 }
  53
  54 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
  55     tcx.hir.krate().visit_all_item_likes(&mut OuterVisitor { tcx: tcx }.as_deep_visitor());
  56     tcx.sess.abort_if_errors();
  57 }
  58
  59 pub(crate) fn check_match<'a, 'tcx>(
  60     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  61     def_id: DefId,
  62 ) -> Result<(), ErrorReported> {
  63     let body_id = if let Some(id) = tcx.hir.as_local_node_id(def_id) {
  64         tcx.hir.body_owned_by(id)
  65     } else {
  66         return Ok(());
  67     };
  68
  69     tcx.sess.track_errors(|| {
  70         MatchVisitor {
  71             tcx,
  72             tables: tcx.body_tables(body_id),
  73             region_scope_tree: &tcx.region_scope_tree(def_id),
  74             param_env: tcx.param_env(def_id),
  75             identity_substs: Substs::identity_for_item(tcx, def_id),
  76         }.visit_body(tcx.hir.body(body_id));
  77     })
  78 }
  79
  80 fn create_e0004<'a>(sess: &'a Session, sp: Span, error_message: String) -> DiagnosticBuilder<'a> {
  81     struct_span_err!(sess, sp, E0004, "{}", &error_message)
  82 }
  83
  84 struct MatchVisitor<'a, 'tcx: 'a> {
  85     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  86     tables: &'a ty::TypeckTables<'tcx>,
  87     param_env: ty::ParamEnv<'tcx>,
  88     identity_substs: &'tcx Substs<'tcx>,
  89     region_scope_tree: &'a region::ScopeTree,
  90 }
  91
  92 impl<'a, 'tcx> Visitor<'tcx> for MatchVisitor<'a, 'tcx> {
  93     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
  94         NestedVisitorMap::None
  95     }
  96
  97     fn visit_expr(&mut self, ex: &'tcx hir::Expr) {
  98         intravisit::walk_expr(self, ex);
  99
 100         match ex.node {
 101             hir::ExprMatch(ref scrut, ref arms, source) => {
 102                 self.check_match(scrut, arms, source);
 103             }
 104             _ => {}
 105         }
 106     }
 107
 108     fn visit_local(&mut self, loc: &'tcx hir::Local) {
 109         intravisit::walk_local(self, loc);
 110
 111         self.check_irrefutable(&loc.pat, match loc.source {
 112             hir::LocalSource::Normal => "local binding",
 113             hir::LocalSource::ForLoopDesugar => "`for` loop binding",
 114         });
 115
 116         // Check legality of move bindings and `@` patterns.
 117         self.check_patterns(false, slice::from_ref(&loc.pat));
 118     }
 119
 120     fn visit_body(&mut self, body: &'tcx hir::Body) {
 121         intravisit::walk_body(self, body);
 122
 123         for arg in &body.arguments {
 124             self.check_irrefutable(&arg.pat, "function argument");
 125             self.check_patterns(false, slice::from_ref(&arg.pat));
 126         }
 127     }
 128 }
 129
 130
 131 impl<'a, 'tcx> PatternContext<'a, 'tcx> {
 132     fn report_inlining_errors(&self, pat_span: Span) {
 133         for error in &self.errors {
 134             match *error {
 135                 PatternError::StaticInPattern(span) => {
 136                     self.span_e0158(span, "statics cannot be referenced in patterns")
 137                 }
 138                 PatternError::AssociatedConstInPattern(span) => {
 139                     self.span_e0158(span, "associated consts cannot be referenced in patterns")
 140                 }
 141                 PatternError::FloatBug => {
 142                     // FIXME(#31407) this is only necessary because float parsing is buggy
 143                     ::rustc::middle::const_val::struct_error(
 144                         self.tcx, pat_span,
 145                         "could not evaluate float literal (see issue #31407)",
 146                     ).emit();
 147                 }
 148                 PatternError::NonConstPath(span) => {
 149                     ::rustc::middle::const_val::struct_error(
 150                         self.tcx, span,
 151                         "runtime values cannot be referenced in patterns",
 152                     ).emit();
 153                 }
 154             }
 155         }
 156     }
 157
 158     fn span_e0158(&self, span: Span, text: &str) {
 159         span_err!(self.tcx.sess, span, E0158, "{}", text)
 160     }
 161 }
 162
 163 impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
 164     fn check_patterns(&self, has_guard: bool, pats: &[P<Pat>]) {
 165         check_legality_of_move_bindings(self, has_guard, pats);
 166         for pat in pats {
 167             check_legality_of_bindings_in_at_patterns(self, pat);
 168         }
 169     }
 170
 171     fn check_match(
 172         &self,
 173         scrut: &hir::Expr,
 174         arms: &'tcx [hir::Arm],
 175         source: hir::MatchSource)
 176     {
 177         for arm in arms {
 178             // First, check legality of move bindings.
 179             self.check_patterns(arm.guard.is_some(), &arm.pats);
 180
 181             // Second, if there is a guard on each arm, make sure it isn't
 182             // assigning or borrowing anything mutably.
 183             if let Some(ref guard) = arm.guard {
 184                 check_for_mutation_in_guard(self, &guard);
 185             }
 186
 187             // Third, perform some lints.
 188             for pat in &arm.pats {
 189                 check_for_bindings_named_the_same_as_variants(self, pat);
 190             }
 191         }
 192
 193         let module = self.tcx.hir.get_module_parent(scrut.id);
 194         MatchCheckCtxt::create_and_enter(self.tcx, module, |ref mut cx| {
 195             let mut have_errors = false;
 196
 197             let inlined_arms : Vec<(Vec<_>, _)> = arms.iter().map(|arm| (
 198                 arm.pats.iter().map(|pat| {
 199                     let mut patcx = PatternContext::new(self.tcx,
 200                                                         self.param_env.and(self.identity_substs),
 201                                                         self.tables);
 202                     let pattern = expand_pattern(cx, patcx.lower_pattern(&pat));
 203                     if !patcx.errors.is_empty() {
 204                         patcx.report_inlining_errors(pat.span);
 205                         have_errors = true;
 206                     }
 207                     (pattern, &**pat)
 208                 }).collect(),
 209                 arm.guard.as_ref().map(|e| &**e)
 210             )).collect();
 211
 212             // Bail out early if inlining failed.
 213             if have_errors {
 214                 return;
 215             }
 216
 217             // Fourth, check for unreachable arms.
 218             check_arms(cx, &inlined_arms, source);
 219
 220             // Then, if the match has no arms, check whether the scrutinee
 221             // is uninhabited.
 222             let pat_ty = self.tables.node_id_to_type(scrut.hir_id);
 223             let module = self.tcx.hir.get_module_parent(scrut.id);
 224             if inlined_arms.is_empty() {
 225                 let scrutinee_is_uninhabited = if self.tcx.features().exhaustive_patterns {
 226                     self.tcx.is_ty_uninhabited_from(module, pat_ty)
 227                 } else {
 228                     self.conservative_is_uninhabited(pat_ty)
 229                 };
 230                 if !scrutinee_is_uninhabited {
 231                     // We know the type is inhabited, so this must be wrong
 232                     let mut err = create_e0004(self.tcx.sess, scrut.span,
 233                                                format!("non-exhaustive patterns: type {} \
 234                                                         is non-empty",
 235                                                        pat_ty));
 236                     span_help!(&mut err, scrut.span,
 237                                "Please ensure that all possible cases are being handled; \
 238                                 possibly adding wildcards or more match arms.");
 239                     err.emit();
 240                 }
 241                 // If the type *is* uninhabited, it's vacuously exhaustive
 242                 return;
 243             }
 244
 245             let matrix: Matrix = inlined_arms
 246                 .iter()
 247                 .filter(|&&(_, guard)| guard.is_none())
 248                 .flat_map(|arm| &arm.0)
 249                 .map(|pat| vec![pat.0])
 250                 .collect();
 251             let scrut_ty = self.tables.node_id_to_type(scrut.hir_id);
 252             check_exhaustive(cx, scrut_ty, scrut.span, &matrix);
 253         })
 254     }
 255
 256     fn conservative_is_uninhabited(&self, scrutinee_ty: Ty<'tcx>) -> bool {
 257         // "rustc-1.0-style" uncontentious uninhabitableness check
 258         match scrutinee_ty.sty {
 259             ty::TyNever => true,
 260             ty::TyAdt(def, _) => def.variants.is_empty(),
 261             _ => false
 262         }
 263     }
 264
 265     fn check_irrefutable(&self, pat: &'tcx Pat, origin: &str) {
 266         let module = self.tcx.hir.get_module_parent(pat.id);
 267         MatchCheckCtxt::create_and_enter(self.tcx, module, |ref mut cx| {
 268             let mut patcx = PatternContext::new(self.tcx,
 269                                                 self.param_env.and(self.identity_substs),
 270                                                 self.tables);
 271             let pattern = patcx.lower_pattern(pat);
 272             let pattern_ty = pattern.ty;
 273             let pats : Matrix = vec![vec![
 274                 expand_pattern(cx, pattern)
 275             ]].into_iter().collect();
 276
 277             let wild_pattern = Pattern {
 278                 ty: pattern_ty,
 279                 span: DUMMY_SP,
 280                 kind: box PatternKind::Wild,
 281             };
 282             let witness = match is_useful(cx, &pats, &[&wild_pattern], ConstructWitness) {
 283                 UsefulWithWitness(witness) => witness,
 284                 NotUseful => return,
 285                 Useful => bug!()
 286             };
 287
 288             let pattern_string = witness[0].single_pattern().to_string();
 289             let mut diag = struct_span_err!(
 290                 self.tcx.sess, pat.span, E0005,
 291                 "refutable pattern in {}: `{}` not covered",
 292                 origin, pattern_string
 293             );
 294             let label_msg = match pat.node {
 295                 PatKind::Path(hir::QPath::Resolved(None, ref path))
 296                         if path.segments.len() == 1 && path.segments[0].parameters.is_none() => {
 297                     format!("interpreted as a {} pattern, not new variable", path.def.kind_name())
 298                 }
 299                 _ => format!("pattern `{}` not covered", pattern_string),
 300             };
 301             diag.span_label(pat.span, label_msg);
 302             diag.emit();
 303         });
 304     }
 305 }
 306
 307 fn check_for_bindings_named_the_same_as_variants(cx: &MatchVisitor, pat: &Pat) {
 308     pat.walk(|p| {
 309         if let PatKind::Binding(_, _, name, None) = p.node {
 310             let bm = *cx.tables
 311                         .pat_binding_modes()
 312                         .get(p.hir_id)
 313                         .expect("missing binding mode");
 314
 315             if bm != ty::BindByValue(hir::MutImmutable) {
 316                 // Nothing to check.
 317                 return true;
 318             }
 319             let pat_ty = cx.tables.pat_ty(p);
 320             if let ty::TyAdt(edef, _) = pat_ty.sty {
 321                 if edef.is_enum() && edef.variants.iter().any(|variant| {
 322                     variant.name == name.node && variant.ctor_kind == CtorKind::Const
 323                 }) {
 324                     let ty_path = cx.tcx.item_path_str(edef.did);
 325                     let mut err = struct_span_warn!(cx.tcx.sess, p.span, E0170,
 326                         "pattern binding `{}` is named the same as one \
 327                          of the variants of the type `{}`",
 328                         name.node, ty_path);
 329                     help!(err,
 330                         "if you meant to match on a variant, \
 331                         consider making the path in the pattern qualified: `{}::{}`",
 332                         ty_path, name.node);
 333                     err.emit();
 334                 }
 335             }
 336         }
 337         true
 338     });
 339 }
 340
 341 /// Checks for common cases of "catchall" patterns that may not be intended as such.
 342 fn pat_is_catchall(pat: &Pat) -> bool {
 343     match pat.node {
 344         PatKind::Binding(.., None) => true,
 345         PatKind::Binding(.., Some(ref s)) => pat_is_catchall(s),
 346         PatKind::Ref(ref s, _) => pat_is_catchall(s),
 347         PatKind::Tuple(ref v, _) => v.iter().all(|p| {
 348             pat_is_catchall(&p)
 349         }),
 350         _ => false
 351     }
 352 }
 353
 354 // Check for unreachable patterns
 355 fn check_arms<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 356                         arms: &[(Vec<(&'a Pattern<'tcx>, &hir::Pat)>, Option<&hir::Expr>)],
 357                         source: hir::MatchSource)
 358 {
 359     let mut seen = Matrix::empty();
 360     let mut catchall = None;
 361     let mut printed_if_let_err = false;
 362     for (arm_index, &(ref pats, guard)) in arms.iter().enumerate() {
 363         for &(pat, hir_pat) in pats {
 364             let v = vec![pat];
 365
 366             match is_useful(cx, &seen, &v, LeaveOutWitness) {
 367                 NotUseful => {
 368                     match source {
 369                         hir::MatchSource::IfLetDesugar { .. } => {
 370                             if printed_if_let_err {
 371                                 // we already printed an irrefutable if-let pattern error.
 372                                 // We don't want two, that's just confusing.
 373                             } else {
 374                                 // find the first arm pattern so we can use its span
 375                                 let &(ref first_arm_pats, _) = &arms[0];
 376                                 let first_pat = &first_arm_pats[0];
 377                                 let span = first_pat.0.span;
 378                                 struct_span_err!(cx.tcx.sess, span, E0162,
 379                                                 "irrefutable if-let pattern")
 380                                     .span_label(span, "irrefutable pattern")
 381                                     .emit();
 382                                 printed_if_let_err = true;
 383                             }
 384                         },
 385
 386                         hir::MatchSource::WhileLetDesugar => {
 387                             // find the first arm pattern so we can use its span
 388                             let &(ref first_arm_pats, _) = &arms[0];
 389                             let first_pat = &first_arm_pats[0];
 390                             let span = first_pat.0.span;
 391
 392                             // check which arm we're on.
 393                             match arm_index {
 394                                 // The arm with the user-specified pattern.
 395                                 0 => {
 396                                     cx.tcx.lint_node(
 397                                             lint::builtin::UNREACHABLE_PATTERNS,
 398                                         hir_pat.id, pat.span,
 399                                         "unreachable pattern");
 400                                 },
 401                                 // The arm with the wildcard pattern.
 402                                 1 => {
 403                                     struct_span_err!(cx.tcx.sess, span, E0165,
 404                                                      "irrefutable while-let pattern")
 405                                         .span_label(span, "irrefutable pattern")
 406                                         .emit();
 407                                 },
 408                                 _ => bug!(),
 409                             }
 410                         },
 411
 412                         hir::MatchSource::ForLoopDesugar |
 413                         hir::MatchSource::Normal => {
 414                             let mut err = cx.tcx.struct_span_lint_node(
 415                                 lint::builtin::UNREACHABLE_PATTERNS,
 416                                 hir_pat.id,
 417                                 pat.span,
 418                                 "unreachable pattern",
 419                             );
 420                             // if we had a catchall pattern, hint at that
 421                             if let Some(catchall) = catchall {
 422                                 err.span_label(pat.span, "unreachable pattern");
 423                                 err.span_label(catchall, "matches any value");
 424                             }
 425                             err.emit();
 426                         },
 427
 428                         // Unreachable patterns in try expressions occur when one of the arms
 429                         // are an uninhabited type. Which is OK.
 430                         hir::MatchSource::TryDesugar => {}
 431                     }
 432                 }
 433                 Useful => (),
 434                 UsefulWithWitness(_) => bug!()
 435             }
 436             if guard.is_none() {
 437                 seen.push(v);
 438                 if catchall.is_none() && pat_is_catchall(hir_pat) {
 439                     catchall = Some(pat.span);
 440                 }
 441             }
 442         }
 443     }
 444 }
 445
 446 fn check_exhaustive<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 447                               scrut_ty: Ty<'tcx>,
 448                               sp: Span,
 449                               matrix: &Matrix<'a, 'tcx>) {
 450     let wild_pattern = Pattern {
 451         ty: scrut_ty,
 452         span: DUMMY_SP,
 453         kind: box PatternKind::Wild,
 454     };
 455     match is_useful(cx, matrix, &[&wild_pattern], ConstructWitness) {
 456         UsefulWithWitness(pats) => {
 457             let witnesses = if pats.is_empty() {
 458                 vec![&wild_pattern]
 459             } else {
 460                 pats.iter().map(|w| w.single_pattern()).collect()
 461             };
 462
 463             const LIMIT: usize = 3;
 464             let joined_patterns = match witnesses.len() {
 465                 0 => bug!(),
 466                 1 => format!("`{}`", witnesses[0]),
 467                 2...LIMIT => {
 468                     let (tail, head) = witnesses.split_last().unwrap();
 469                     let head: Vec<_> = head.iter().map(|w| w.to_string()).collect();
 470                     format!("`{}` and `{}`", head.join("`, `"), tail)
 471                 },
 472                 _ => {
 473                     let (head, tail) = witnesses.split_at(LIMIT);
 474                     let head: Vec<_> = head.iter().map(|w| w.to_string()).collect();
 475                     format!("`{}` and {} more", head.join("`, `"), tail.len())
 476                 }
 477             };
 478
 479             let label_text = match witnesses.len() {
 480                 1 => format!("pattern {} not covered", joined_patterns),
 481                 _ => format!("patterns {} not covered", joined_patterns)
 482             };
 483             create_e0004(cx.tcx.sess, sp,
 484                             format!("non-exhaustive patterns: {} not covered",
 485                                     joined_patterns))
 486                 .span_label(sp, label_text)
 487                 .emit();
 488         }
 489         NotUseful => {
 490             // This is good, wildcard pattern isn't reachable
 491         },
 492         _ => bug!()
 493     }
 494 }
 495
 496 // Legality of move bindings checking
 497 fn check_legality_of_move_bindings(cx: &MatchVisitor,
 498                                    has_guard: bool,
 499                                    pats: &[P<Pat>]) {
 500     let mut by_ref_span = None;
 501     for pat in pats {
 502         pat.each_binding(|_, id, span, _path| {
 503             let hir_id = cx.tcx.hir.node_to_hir_id(id);
 504             let bm = *cx.tables
 505                         .pat_binding_modes()
 506                         .get(hir_id)
 507                         .expect("missing binding mode");
 508             if let ty::BindByReference(..) = bm {
 509                 by_ref_span = Some(span);
 510             }
 511         })
 512     }
 513
 514     let check_move = |p: &Pat, sub: Option<&Pat>| {
 515         // check legality of moving out of the enum
 516
 517         // x @ Foo(..) is legal, but x @ Foo(y) isn't.
 518         if sub.map_or(false, |p| p.contains_bindings()) {
 519             struct_span_err!(cx.tcx.sess, p.span, E0007,
 520                              "cannot bind by-move with sub-bindings")
 521                 .span_label(p.span, "binds an already bound by-move value by moving it")
 522                 .emit();
 523         } else if has_guard {
 524             struct_span_err!(cx.tcx.sess, p.span, E0008,
 525                       "cannot bind by-move into a pattern guard")
 526                 .span_label(p.span, "moves value into pattern guard")
 527                 .emit();
 528         } else if by_ref_span.is_some() {
 529             struct_span_err!(cx.tcx.sess, p.span, E0009,
 530                             "cannot bind by-move and by-ref in the same pattern")
 531                     .span_label(p.span, "by-move pattern here")
 532                     .span_label(by_ref_span.unwrap(), "both by-ref and by-move used")
 533                     .emit();
 534         }
 535     };
 536
 537     for pat in pats {
 538         pat.walk(|p| {
 539             if let PatKind::Binding(_, _, _, ref sub) = p.node {
 540                 let bm = *cx.tables
 541                             .pat_binding_modes()
 542                             .get(p.hir_id)
 543                             .expect("missing binding mode");
 544                 match bm {
 545                     ty::BindByValue(..) => {
 546                         let pat_ty = cx.tables.node_id_to_type(p.hir_id);
 547                         if pat_ty.moves_by_default(cx.tcx, cx.param_env, pat.span) {
 548                             check_move(p, sub.as_ref().map(|p| &**p));
 549                         }
 550                     }
 551                     _ => {}
 552                 }
 553             }
 554             true
 555         });
 556     }
 557 }
 558
 559 /// Ensures that a pattern guard doesn't borrow by mutable reference or
 560 /// assign.
 561 ///
 562 /// FIXME: this should be done by borrowck.
 563 fn check_for_mutation_in_guard(cx: &MatchVisitor, guard: &hir::Expr) {
 564     let mut checker = MutationChecker {
 565         cx,
 566     };
 567     ExprUseVisitor::new(&mut checker, cx.tcx, cx.param_env, cx.region_scope_tree, cx.tables, None)
 568         .walk_expr(guard);
 569 }
 570
 571 struct MutationChecker<'a, 'tcx: 'a> {
 572     cx: &'a MatchVisitor<'a, 'tcx>,
 573 }
 574
 575 impl<'a, 'tcx> Delegate<'tcx> for MutationChecker<'a, 'tcx> {
 576     fn matched_pat(&mut self, _: &Pat, _: &cmt_, _: euv::MatchMode) {}
 577     fn consume(&mut self, _: ast::NodeId, _: Span, _: &cmt_, _: ConsumeMode) {}
 578     fn consume_pat(&mut self, _: &Pat, _: &cmt_, _: ConsumeMode) {}
 579     fn borrow(&mut self,
 580               _: ast::NodeId,
 581               span: Span,
 582               _: &cmt_,
 583               _: ty::Region<'tcx>,
 584               kind:ty:: BorrowKind,
 585               _: LoanCause) {
 586         match kind {
 587             ty::MutBorrow => {
 588                 struct_span_err!(self.cx.tcx.sess, span, E0301,
 589                           "cannot mutably borrow in a pattern guard")
 590                     .span_label(span, "borrowed mutably in pattern guard")
 591                     .emit();
 592             }
 593             ty::ImmBorrow | ty::UniqueImmBorrow => {}
 594         }
 595     }
 596     fn decl_without_init(&mut self, _: ast::NodeId, _: Span) {}
 597     fn mutate(&mut self, _: ast::NodeId, span: Span, _: &cmt_, mode: MutateMode) {
 598         match mode {
 599             MutateMode::JustWrite | MutateMode::WriteAndRead => {
 600                 struct_span_err!(self.cx.tcx.sess, span, E0302, "cannot assign in a pattern guard")
 601                     .span_label(span, "assignment in pattern guard")
 602                     .emit();
 603             }
 604             MutateMode::Init => {}
 605         }
 606     }
 607 }
 608
 609 /// Forbids bindings in `@` patterns. This is necessary for memory safety,
 610 /// because of the way rvalues are handled in the borrow check. (See issue
 611 /// #14587.)
 612 fn check_legality_of_bindings_in_at_patterns(cx: &MatchVisitor, pat: &Pat) {
 613     AtBindingPatternVisitor { cx: cx, bindings_allowed: true }.visit_pat(pat);
 614 }
 615
 616 struct AtBindingPatternVisitor<'a, 'b:'a, 'tcx:'b> {
 617     cx: &'a MatchVisitor<'b, 'tcx>,
 618     bindings_allowed: bool
 619 }
 620
 621 impl<'a, 'b, 'tcx, 'v> Visitor<'v> for AtBindingPatternVisitor<'a, 'b, 'tcx> {
 622     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
 623         NestedVisitorMap::None
 624     }
 625
 626     fn visit_pat(&mut self, pat: &Pat) {
 627         match pat.node {
 628             PatKind::Binding(.., ref subpat) => {
 629                 if !self.bindings_allowed {
 630                     struct_span_err!(self.cx.tcx.sess, pat.span, E0303,
 631                                      "pattern bindings are not allowed after an `@`")
 632                         .span_label(pat.span,  "not allowed after `@`")
 633                         .emit();
 634                 }
 635
 636                 if subpat.is_some() {
 637                     let bindings_were_allowed = self.bindings_allowed;
 638                     self.bindings_allowed = false;
 639                     intravisit::walk_pat(self, pat);
 640                     self.bindings_allowed = bindings_were_allowed;
 641                 }
 642             }
 643             _ => intravisit::walk_pat(self, pat),
 644         }
 645     }
 646 }