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::{Applicability, 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::ExprKind::Match(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::mir::interpret::struct_error(
 144                         self.tcx.at(pat_span),
 145                         "could not evaluate float literal (see issue #31407)",
 146                     ).emit();
 147                 }
 148                 PatternError::NonConstPath(span) => {
 149                     ::rustc::mir::interpret::struct_error(
 150                         self.tcx.at(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                 if self.tcx.check_for_mutation_in_guard_via_ast_walk() {
 185                     check_for_mutation_in_guard(self, &guard);
 186                 }
 187             }
 188
 189             // Third, perform some lints.
 190             for pat in &arm.pats {
 191                 check_for_bindings_named_the_same_as_variants(self, pat);
 192             }
 193         }
 194
 195         let module = self.tcx.hir.get_module_parent(scrut.id);
 196         MatchCheckCtxt::create_and_enter(self.tcx, module, |ref mut cx| {
 197             let mut have_errors = false;
 198
 199             let inlined_arms : Vec<(Vec<_>, _)> = arms.iter().map(|arm| (
 200                 arm.pats.iter().map(|pat| {
 201                     let mut patcx = PatternContext::new(self.tcx,
 202                                                         self.param_env.and(self.identity_substs),
 203                                                         self.tables);
 204                     let pattern = expand_pattern(cx, patcx.lower_pattern(&pat));
 205                     if !patcx.errors.is_empty() {
 206                         patcx.report_inlining_errors(pat.span);
 207                         have_errors = true;
 208                     }
 209                     (pattern, &**pat)
 210                 }).collect(),
 211                 arm.guard.as_ref().map(|g| match g {
 212                     hir::Guard::If(ref e) => &**e,
 213                 })
 214             )).collect();
 215
 216             // Bail out early if inlining failed.
 217             if have_errors {
 218                 return;
 219             }
 220
 221             // Fourth, check for unreachable arms.
 222             check_arms(cx, &inlined_arms, source);
 223
 224             // Then, if the match has no arms, check whether the scrutinee
 225             // is uninhabited.
 226             let pat_ty = self.tables.node_id_to_type(scrut.hir_id);
 227             let module = self.tcx.hir.get_module_parent(scrut.id);
 228             if inlined_arms.is_empty() {
 229                 let scrutinee_is_uninhabited = if self.tcx.features().exhaustive_patterns {
 230                     self.tcx.is_ty_uninhabited_from(module, pat_ty)
 231                 } else {
 232                     self.conservative_is_uninhabited(pat_ty)
 233                 };
 234                 if !scrutinee_is_uninhabited {
 235                     // We know the type is inhabited, so this must be wrong
 236                     let mut err = create_e0004(self.tcx.sess, scrut.span,
 237                                                format!("non-exhaustive patterns: type {} \
 238                                                         is non-empty",
 239                                                        pat_ty));
 240                     span_help!(&mut err, scrut.span,
 241                                "Please ensure that all possible cases are being handled; \
 242                                 possibly adding wildcards or more match arms.");
 243                     err.emit();
 244                 }
 245                 // If the type *is* uninhabited, it's vacuously exhaustive
 246                 return;
 247             }
 248
 249             let matrix: Matrix = inlined_arms
 250                 .iter()
 251                 .filter(|&&(_, guard)| guard.is_none())
 252                 .flat_map(|arm| &arm.0)
 253                 .map(|pat| vec![pat.0])
 254                 .collect();
 255             let scrut_ty = self.tables.node_id_to_type(scrut.hir_id);
 256             check_exhaustive(cx, scrut_ty, scrut.span, &matrix);
 257         })
 258     }
 259
 260     fn conservative_is_uninhabited(&self, scrutinee_ty: Ty<'tcx>) -> bool {
 261         // "rustc-1.0-style" uncontentious uninhabitableness check
 262         match scrutinee_ty.sty {
 263             ty::Never => true,
 264             ty::Adt(def, _) => def.variants.is_empty(),
 265             _ => false
 266         }
 267     }
 268
 269     fn check_irrefutable(&self, pat: &'tcx Pat, origin: &str) {
 270         let module = self.tcx.hir.get_module_parent(pat.id);
 271         MatchCheckCtxt::create_and_enter(self.tcx, module, |ref mut cx| {
 272             let mut patcx = PatternContext::new(self.tcx,
 273                                                 self.param_env.and(self.identity_substs),
 274                                                 self.tables);
 275             let pattern = patcx.lower_pattern(pat);
 276             let pattern_ty = pattern.ty;
 277             let pats: Matrix = vec![vec![
 278                 expand_pattern(cx, pattern)
 279             ]].into_iter().collect();
 280
 281             let wild_pattern = Pattern {
 282                 ty: pattern_ty,
 283                 span: DUMMY_SP,
 284                 kind: box PatternKind::Wild,
 285             };
 286             let witness = match is_useful(cx, &pats, &[&wild_pattern], ConstructWitness) {
 287                 UsefulWithWitness(witness) => witness,
 288                 NotUseful => return,
 289                 Useful => bug!()
 290             };
 291
 292             let pattern_string = witness[0].single_pattern().to_string();
 293             let mut diag = struct_span_err!(
 294                 self.tcx.sess, pat.span, E0005,
 295                 "refutable pattern in {}: `{}` not covered",
 296                 origin, pattern_string
 297             );
 298             let label_msg = match pat.node {
 299                 PatKind::Path(hir::QPath::Resolved(None, ref path))
 300                         if path.segments.len() == 1 && path.segments[0].args.is_none() => {
 301                     format!("interpreted as a {} pattern, not new variable", path.def.kind_name())
 302                 }
 303                 _ => format!("pattern `{}` not covered", pattern_string),
 304             };
 305             diag.span_label(pat.span, label_msg);
 306             diag.emit();
 307         });
 308     }
 309 }
 310
 311 fn check_for_bindings_named_the_same_as_variants(cx: &MatchVisitor, pat: &Pat) {
 312     pat.walk(|p| {
 313         if let PatKind::Binding(_, _, ident, None) = p.node {
 314             if let Some(&bm) = cx.tables.pat_binding_modes().get(p.hir_id) {
 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::Adt(edef, _) = pat_ty.sty {
 321                     if edef.is_enum() && edef.variants.iter().any(|variant| {
 322                         variant.name == ident.name && 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                             ident, ty_path);
 329                         err.span_suggestion_with_applicability(
 330                             p.span,
 331                             "to match on the variant, qualify the path",
 332                             format!("{}::{}", ty_path, ident),
 333                             Applicability::MachineApplicable
 334                         );
 335                         err.emit();
 336                     }
 337                 }
 338             } else {
 339                 cx.tcx.sess.delay_span_bug(p.span, "missing binding mode");
 340             }
 341         }
 342         true
 343     });
 344 }
 345
 346 /// Checks for common cases of "catchall" patterns that may not be intended as such.
 347 fn pat_is_catchall(pat: &Pat) -> bool {
 348     match pat.node {
 349         PatKind::Binding(.., None) => true,
 350         PatKind::Binding(.., Some(ref s)) => pat_is_catchall(s),
 351         PatKind::Ref(ref s, _) => pat_is_catchall(s),
 352         PatKind::Tuple(ref v, _) => v.iter().all(|p| {
 353             pat_is_catchall(&p)
 354         }),
 355         _ => false
 356     }
 357 }
 358
 359 // Check for unreachable patterns
 360 fn check_arms<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 361                         arms: &[(Vec<(&'a Pattern<'tcx>, &hir::Pat)>, Option<&hir::Expr>)],
 362                         source: hir::MatchSource)
 363 {
 364     let mut seen = Matrix::empty();
 365     let mut catchall = None;
 366     let mut printed_if_let_err = false;
 367     for (arm_index, &(ref pats, guard)) in arms.iter().enumerate() {
 368         for &(pat, hir_pat) in pats {
 369             let v = vec![pat];
 370
 371             match is_useful(cx, &seen, &v, LeaveOutWitness) {
 372                 NotUseful => {
 373                     match source {
 374                         hir::MatchSource::IfLetDesugar { .. } => {
 375                             if cx.tcx.features().irrefutable_let_patterns {
 376                                 cx.tcx.lint_node(
 377                                     lint::builtin::IRREFUTABLE_LET_PATTERNS,
 378                                     hir_pat.id, pat.span,
 379                                     "irrefutable if-let pattern");
 380                             } else {
 381                                 if printed_if_let_err {
 382                                     // we already printed an irrefutable if-let pattern error.
 383                                     // We don't want two, that's just confusing.
 384                                 } else {
 385                                     // find the first arm pattern so we can use its span
 386                                     let &(ref first_arm_pats, _) = &arms[0];
 387                                     let first_pat = &first_arm_pats[0];
 388                                     let span = first_pat.0.span;
 389                                     struct_span_err!(cx.tcx.sess, span, E0162,
 390                                                     "irrefutable if-let pattern")
 391                                         .span_label(span, "irrefutable pattern")
 392                                         .emit();
 393                                     printed_if_let_err = true;
 394                                 }
 395                             }
 396                         }
 397
 398                         hir::MatchSource::WhileLetDesugar => {
 399                             // check which arm we're on.
 400                             match arm_index {
 401                                 // The arm with the user-specified pattern.
 402                                 0 => {
 403                                     cx.tcx.lint_node(
 404                                         lint::builtin::UNREACHABLE_PATTERNS,
 405                                         hir_pat.id, pat.span,
 406                                         "unreachable pattern");
 407                                 },
 408                                 // The arm with the wildcard pattern.
 409                                 1 => {
 410                                     if cx.tcx.features().irrefutable_let_patterns {
 411                                         cx.tcx.lint_node(
 412                                             lint::builtin::IRREFUTABLE_LET_PATTERNS,
 413                                             hir_pat.id, pat.span,
 414                                             "irrefutable while-let pattern");
 415                                     } else {
 416                                         // find the first arm pattern so we can use its span
 417                                         let &(ref first_arm_pats, _) = &arms[0];
 418                                         let first_pat = &first_arm_pats[0];
 419                                         let span = first_pat.0.span;
 420                                         struct_span_err!(cx.tcx.sess, span, E0165,
 421                                                          "irrefutable while-let pattern")
 422                                             .span_label(span, "irrefutable pattern")
 423                                             .emit();
 424                                     }
 425                                 },
 426                                 _ => bug!(),
 427                             }
 428                         },
 429
 430                         hir::MatchSource::ForLoopDesugar |
 431                         hir::MatchSource::Normal => {
 432                             let mut err = cx.tcx.struct_span_lint_node(
 433                                 lint::builtin::UNREACHABLE_PATTERNS,
 434                                 hir_pat.id,
 435                                 pat.span,
 436                                 "unreachable pattern",
 437                             );
 438                             // if we had a catchall pattern, hint at that
 439                             if let Some(catchall) = catchall {
 440                                 err.span_label(pat.span, "unreachable pattern");
 441                                 err.span_label(catchall, "matches any value");
 442                             }
 443                             err.emit();
 444                         },
 445
 446                         // Unreachable patterns in try expressions occur when one of the arms
 447                         // are an uninhabited type. Which is OK.
 448                         hir::MatchSource::TryDesugar => {}
 449                     }
 450                 }
 451                 Useful => (),
 452                 UsefulWithWitness(_) => bug!()
 453             }
 454             if guard.is_none() {
 455                 seen.push(v);
 456                 if catchall.is_none() && pat_is_catchall(hir_pat) {
 457                     catchall = Some(pat.span);
 458                 }
 459             }
 460         }
 461     }
 462 }
 463
 464 fn check_exhaustive<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 465                               scrut_ty: Ty<'tcx>,
 466                               sp: Span,
 467                               matrix: &Matrix<'a, 'tcx>) {
 468     let wild_pattern = Pattern {
 469         ty: scrut_ty,
 470         span: DUMMY_SP,
 471         kind: box PatternKind::Wild,
 472     };
 473     match is_useful(cx, matrix, &[&wild_pattern], ConstructWitness) {
 474         UsefulWithWitness(pats) => {
 475             let witnesses = if pats.is_empty() {
 476                 vec![&wild_pattern]
 477             } else {
 478                 pats.iter().map(|w| w.single_pattern()).collect()
 479             };
 480
 481             const LIMIT: usize = 3;
 482             let joined_patterns = match witnesses.len() {
 483                 0 => bug!(),
 484                 1 => format!("`{}`", witnesses[0]),
 485                 2..=LIMIT => {
 486                     let (tail, head) = witnesses.split_last().unwrap();
 487                     let head: Vec<_> = head.iter().map(|w| w.to_string()).collect();
 488                     format!("`{}` and `{}`", head.join("`, `"), tail)
 489                 },
 490                 _ => {
 491                     let (head, tail) = witnesses.split_at(LIMIT);
 492                     let head: Vec<_> = head.iter().map(|w| w.to_string()).collect();
 493                     format!("`{}` and {} more", head.join("`, `"), tail.len())
 494                 }
 495             };
 496
 497             let label_text = match witnesses.len() {
 498                 1 => format!("pattern {} not covered", joined_patterns),
 499                 _ => format!("patterns {} not covered", joined_patterns)
 500             };
 501             create_e0004(cx.tcx.sess, sp,
 502                             format!("non-exhaustive patterns: {} not covered",
 503                                     joined_patterns))
 504                 .span_label(sp, label_text)
 505                 .emit();
 506         }
 507         NotUseful => {
 508             // This is good, wildcard pattern isn't reachable
 509         },
 510         _ => bug!()
 511     }
 512 }
 513
 514 // Legality of move bindings checking
 515 fn check_legality_of_move_bindings(cx: &MatchVisitor,
 516                                    has_guard: bool,
 517                                    pats: &[P<Pat>]) {
 518     let mut by_ref_span = None;
 519     for pat in pats {
 520         pat.each_binding(|_, hir_id, span, _path| {
 521             if let Some(&bm) = cx.tables.pat_binding_modes().get(hir_id) {
 522                 if let ty::BindByReference(..) = bm {
 523                     by_ref_span = Some(span);
 524                 }
 525             } else {
 526                 cx.tcx.sess.delay_span_bug(pat.span, "missing binding mode");
 527             }
 528         })
 529     }
 530
 531     let check_move = |p: &Pat, sub: Option<&Pat>| {
 532         // check legality of moving out of the enum
 533
 534         // x @ Foo(..) is legal, but x @ Foo(y) isn't.
 535         if sub.map_or(false, |p| p.contains_bindings()) {
 536             struct_span_err!(cx.tcx.sess, p.span, E0007,
 537                              "cannot bind by-move with sub-bindings")
 538                 .span_label(p.span, "binds an already bound by-move value by moving it")
 539                 .emit();
 540         } else if has_guard {
 541             struct_span_err!(cx.tcx.sess, p.span, E0008,
 542                       "cannot bind by-move into a pattern guard")
 543                 .span_label(p.span, "moves value into pattern guard")
 544                 .emit();
 545         } else if by_ref_span.is_some() {
 546             struct_span_err!(cx.tcx.sess, p.span, E0009,
 547                             "cannot bind by-move and by-ref in the same pattern")
 548                     .span_label(p.span, "by-move pattern here")
 549                     .span_label(by_ref_span.unwrap(), "both by-ref and by-move used")
 550                     .emit();
 551         }
 552     };
 553
 554     for pat in pats {
 555         pat.walk(|p| {
 556             if let PatKind::Binding(_, _, _, ref sub) = p.node {
 557                 if let Some(&bm) = cx.tables.pat_binding_modes().get(p.hir_id) {
 558                     match bm {
 559                         ty::BindByValue(..) => {
 560                             let pat_ty = cx.tables.node_id_to_type(p.hir_id);
 561                             if pat_ty.moves_by_default(cx.tcx, cx.param_env, pat.span) {
 562                                 check_move(p, sub.as_ref().map(|p| &**p));
 563                             }
 564                         }
 565                         _ => {}
 566                     }
 567                 } else {
 568                     cx.tcx.sess.delay_span_bug(pat.span, "missing binding mode");
 569                 }
 570             }
 571             true
 572         });
 573     }
 574 }
 575
 576 /// Ensures that a pattern guard doesn't borrow by mutable reference or
 577 /// assign.
 578 ///
 579 /// FIXME: this should be done by borrowck.
 580 fn check_for_mutation_in_guard(cx: &MatchVisitor, guard: &hir::Guard) {
 581     let mut checker = MutationChecker {
 582         cx,
 583     };
 584     match guard {
 585         hir::Guard::If(expr) =>
 586             ExprUseVisitor::new(&mut checker,
 587                                 cx.tcx,
 588                                 cx.param_env,
 589                                 cx.region_scope_tree,
 590                                 cx.tables,
 591                                 None).walk_expr(expr),
 592     };
 593 }
 594
 595 struct MutationChecker<'a, 'tcx: 'a> {
 596     cx: &'a MatchVisitor<'a, 'tcx>,
 597 }
 598
 599 impl<'a, 'tcx> Delegate<'tcx> for MutationChecker<'a, 'tcx> {
 600     fn matched_pat(&mut self, _: &Pat, _: &cmt_, _: euv::MatchMode) {}
 601     fn consume(&mut self, _: ast::NodeId, _: Span, _: &cmt_, _: ConsumeMode) {}
 602     fn consume_pat(&mut self, _: &Pat, _: &cmt_, _: ConsumeMode) {}
 603     fn borrow(&mut self,
 604               _: ast::NodeId,
 605               span: Span,
 606               _: &cmt_,
 607               _: ty::Region<'tcx>,
 608               kind:ty:: BorrowKind,
 609               _: LoanCause) {
 610         match kind {
 611             ty::MutBorrow => {
 612                 struct_span_err!(self.cx.tcx.sess, span, E0301,
 613                           "cannot mutably borrow in a pattern guard")
 614                     .span_label(span, "borrowed mutably in pattern guard")
 615                     .emit();
 616             }
 617             ty::ImmBorrow | ty::UniqueImmBorrow => {}
 618         }
 619     }
 620     fn decl_without_init(&mut self, _: ast::NodeId, _: Span) {}
 621     fn mutate(&mut self, _: ast::NodeId, span: Span, _: &cmt_, mode: MutateMode) {
 622         match mode {
 623             MutateMode::JustWrite | MutateMode::WriteAndRead => {
 624                 struct_span_err!(self.cx.tcx.sess, span, E0302, "cannot assign in a pattern guard")
 625                     .span_label(span, "assignment in pattern guard")
 626                     .emit();
 627             }
 628             MutateMode::Init => {}
 629         }
 630     }
 631 }
 632
 633 /// Forbids bindings in `@` patterns. This is necessary for memory safety,
 634 /// because of the way rvalues are handled in the borrow check. (See issue
 635 /// #14587.)
 636 fn check_legality_of_bindings_in_at_patterns(cx: &MatchVisitor, pat: &Pat) {
 637     AtBindingPatternVisitor { cx: cx, bindings_allowed: true }.visit_pat(pat);
 638 }
 639
 640 struct AtBindingPatternVisitor<'a, 'b:'a, 'tcx:'b> {
 641     cx: &'a MatchVisitor<'b, 'tcx>,
 642     bindings_allowed: bool
 643 }
 644
 645 impl<'a, 'b, 'tcx, 'v> Visitor<'v> for AtBindingPatternVisitor<'a, 'b, 'tcx> {
 646     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
 647         NestedVisitorMap::None
 648     }
 649
 650     fn visit_pat(&mut self, pat: &Pat) {
 651         match pat.node {
 652             PatKind::Binding(.., ref subpat) => {
 653                 if !self.bindings_allowed {
 654                     struct_span_err!(self.cx.tcx.sess, pat.span, E0303,
 655                                      "pattern bindings are not allowed after an `@`")
 656                         .span_label(pat.span,  "not allowed after `@`")
 657                         .emit();
 658                 }
 659
 660                 if subpat.is_some() {
 661                     let bindings_were_allowed = self.bindings_allowed;
 662                     self.bindings_allowed = false;
 663                     intravisit::walk_pat(self, pat);
 664                     self.bindings_allowed = bindings_were_allowed;
 665                 }
 666             }
 667             _ => intravisit::walk_pat(self, pat),
 668         }
 669     }
 670 }