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