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 smallvec::smallvec;
  35 use std::slice;
  36
  37 use syntax::ast;
  38 use syntax::ptr::P;
  39 use syntax_pos::{Span, DUMMY_SP, MultiSpan};
  40
  41 struct OuterVisitor<'a, 'tcx: 'a> { tcx: TyCtxt<'a, 'tcx, 'tcx> }
  42
  43 impl<'a, 'tcx> Visitor<'tcx> for OuterVisitor<'a, 'tcx> {
  44     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
  45         NestedVisitorMap::OnlyBodies(&self.tcx.hir())
  46     }
  47
  48     fn visit_body(&mut self, body: &'tcx hir::Body) {
  49         intravisit::walk_body(self, body);
  50         let def_id = self.tcx.hir().body_owner_def_id(body.id());
  51         let _ = self.tcx.check_match(def_id);
  52     }
  53 }
  54
  55 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
  56     tcx.hir().krate().visit_all_item_likes(&mut OuterVisitor { tcx }.as_deep_visitor());
  57     tcx.sess.abort_if_errors();
  58 }
  59
  60 pub(crate) fn check_match<'a, 'tcx>(
  61     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  62     def_id: DefId,
  63 ) -> Result<(), ErrorReported> {
  64     let body_id = if let Some(id) = tcx.hir().as_local_node_id(def_id) {
  65         tcx.hir().body_owned_by(id)
  66     } else {
  67         return Ok(());
  68     };
  69
  70     tcx.sess.track_errors(|| {
  71         MatchVisitor {
  72             tcx,
  73             tables: tcx.body_tables(body_id),
  74             region_scope_tree: &tcx.region_scope_tree(def_id),
  75             param_env: tcx.param_env(def_id),
  76             identity_substs: Substs::identity_for_item(tcx, def_id),
  77         }.visit_body(tcx.hir().body(body_id));
  78     })
  79 }
  80
  81 fn create_e0004<'a>(sess: &'a Session, sp: Span, error_message: String) -> DiagnosticBuilder<'a> {
  82     struct_span_err!(sess, sp, E0004, "{}", &error_message)
  83 }
  84
  85 struct MatchVisitor<'a, 'tcx: 'a> {
  86     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  87     tables: &'a ty::TypeckTables<'tcx>,
  88     param_env: ty::ParamEnv<'tcx>,
  89     identity_substs: &'tcx Substs<'tcx>,
  90     region_scope_tree: &'a region::ScopeTree,
  91 }
  92
  93 impl<'a, 'tcx> Visitor<'tcx> for MatchVisitor<'a, 'tcx> {
  94     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
  95         NestedVisitorMap::None
  96     }
  97
  98     fn visit_expr(&mut self, ex: &'tcx hir::Expr) {
  99         intravisit::walk_expr(self, ex);
 100
 101         match ex.node {
 102             hir::ExprKind::Match(ref scrut, ref arms, source) => {
 103                 self.check_match(scrut, arms, source);
 104             }
 105             _ => {}
 106         }
 107     }
 108
 109     fn visit_local(&mut self, loc: &'tcx hir::Local) {
 110         intravisit::walk_local(self, loc);
 111
 112         self.check_irrefutable(&loc.pat, match loc.source {
 113             hir::LocalSource::Normal => "local binding",
 114             hir::LocalSource::ForLoopDesugar => "`for` loop binding",
 115         });
 116
 117         // Check legality of move bindings and `@` patterns.
 118         self.check_patterns(false, slice::from_ref(&loc.pat));
 119     }
 120
 121     fn visit_body(&mut self, body: &'tcx hir::Body) {
 122         intravisit::walk_body(self, body);
 123
 124         for arg in &body.arguments {
 125             self.check_irrefutable(&arg.pat, "function argument");
 126             self.check_patterns(false, slice::from_ref(&arg.pat));
 127         }
 128     }
 129 }
 130
 131
 132 impl<'a, 'tcx> PatternContext<'a, 'tcx> {
 133     fn report_inlining_errors(&self, pat_span: Span) {
 134         for error in &self.errors {
 135             match *error {
 136                 PatternError::StaticInPattern(span) => {
 137                     self.span_e0158(span, "statics cannot be referenced in patterns")
 138                 }
 139                 PatternError::AssociatedConstInPattern(span) => {
 140                     self.span_e0158(span, "associated consts cannot be referenced in patterns")
 141                 }
 142                 PatternError::FloatBug => {
 143                     // FIXME(#31407) this is only necessary because float parsing is buggy
 144                     ::rustc::mir::interpret::struct_error(
 145                         self.tcx.at(pat_span),
 146                         "could not evaluate float literal (see issue #31407)",
 147                     ).emit();
 148                 }
 149                 PatternError::NonConstPath(span) => {
 150                     ::rustc::mir::interpret::struct_error(
 151                         self.tcx.at(span),
 152                         "runtime values cannot be referenced in patterns",
 153                     ).emit();
 154                 }
 155             }
 156         }
 157     }
 158
 159     fn span_e0158(&self, span: Span, text: &str) {
 160         span_err!(self.tcx.sess, span, E0158, "{}", text)
 161     }
 162 }
 163
 164 impl<'a, 'tcx> MatchVisitor<'a, 'tcx> {
 165     fn check_patterns(&self, has_guard: bool, pats: &[P<Pat>]) {
 166         check_legality_of_move_bindings(self, has_guard, pats);
 167         for pat in pats {
 168             check_legality_of_bindings_in_at_patterns(self, pat);
 169         }
 170     }
 171
 172     fn check_match(
 173         &self,
 174         scrut: &hir::Expr,
 175         arms: &'tcx [hir::Arm],
 176         source: hir::MatchSource)
 177     {
 178         for arm in arms {
 179             // First, check legality of move bindings.
 180             self.check_patterns(arm.guard.is_some(), &arm.pats);
 181
 182             // Second, if there is a guard on each arm, make sure it isn't
 183             // assigning or borrowing anything mutably.
 184             if let Some(ref guard) = arm.guard {
 185                 if self.tcx.check_for_mutation_in_guard_via_ast_walk() {
 186                     check_for_mutation_in_guard(self, &guard);
 187                 }
 188             }
 189
 190             // Third, perform some lints.
 191             for pat in &arm.pats {
 192                 check_for_bindings_named_same_as_variants(self, pat);
 193             }
 194         }
 195
 196         let module = self.tcx.hir().get_module_parent(scrut.id);
 197         MatchCheckCtxt::create_and_enter(self.tcx, self.param_env, module, |ref mut cx| {
 198             let mut have_errors = false;
 199
 200             let inlined_arms : Vec<(Vec<_>, _)> = arms.iter().map(|arm| (
 201                 arm.pats.iter().map(|pat| {
 202                     let mut patcx = PatternContext::new(self.tcx,
 203                                                         self.param_env.and(self.identity_substs),
 204                                                         self.tables);
 205                     let pattern = expand_pattern(cx, patcx.lower_pattern(&pat));
 206                     if !patcx.errors.is_empty() {
 207                         patcx.report_inlining_errors(pat.span);
 208                         have_errors = true;
 209                     }
 210                     (pattern, &**pat)
 211                 }).collect(),
 212                 arm.guard.as_ref().map(|g| match g {
 213                     hir::Guard::If(ref e) => &**e,
 214                 })
 215             )).collect();
 216
 217             // Bail out early if inlining failed.
 218             if have_errors {
 219                 return;
 220             }
 221
 222             // Fourth, check for unreachable arms.
 223             check_arms(cx, &inlined_arms, source);
 224
 225             // Then, if the match has no arms, check whether the scrutinee
 226             // is uninhabited.
 227             let pat_ty = self.tables.node_id_to_type(scrut.hir_id);
 228             let module = self.tcx.hir().get_module_parent(scrut.id);
 229             if inlined_arms.is_empty() {
 230                 let scrutinee_is_uninhabited = if self.tcx.features().exhaustive_patterns {
 231                     self.tcx.is_ty_uninhabited_from(module, pat_ty)
 232                 } else {
 233                     pat_ty.conservative_is_uninhabited(self.tcx)
 234                 };
 235                 if !scrutinee_is_uninhabited {
 236                     // We know the type is inhabited, so this must be wrong
 237                     let mut err = create_e0004(self.tcx.sess, scrut.span,
 238                                                format!("non-exhaustive patterns: type `{}` \
 239                                                         is non-empty",
 240                                                        pat_ty));
 241                     span_help!(&mut err, scrut.span,
 242                                "ensure that all possible cases are being handled, \
 243                                 possibly by adding wildcards or more match arms");
 244                     err.emit();
 245                 }
 246                 // If the type *is* uninhabited, it's vacuously exhaustive
 247                 return;
 248             }
 249
 250             let matrix: Matrix = inlined_arms
 251                 .iter()
 252                 .filter(|&&(_, guard)| guard.is_none())
 253                 .flat_map(|arm| &arm.0)
 254                 .map(|pat| smallvec![pat.0])
 255                 .collect();
 256             let scrut_ty = self.tables.node_id_to_type(scrut.hir_id);
 257             check_exhaustive(cx, scrut_ty, scrut.span, &matrix);
 258         })
 259     }
 260
 261     fn check_irrefutable(&self, pat: &'tcx Pat, origin: &str) {
 262         let module = self.tcx.hir().get_module_parent(pat.id);
 263         MatchCheckCtxt::create_and_enter(self.tcx, self.param_env, module, |ref mut cx| {
 264             let mut patcx = PatternContext::new(self.tcx,
 265                                                 self.param_env.and(self.identity_substs),
 266                                                 self.tables);
 267             let pattern = patcx.lower_pattern(pat);
 268             let pattern_ty = pattern.ty;
 269             let pats: Matrix = vec![smallvec![
 270                 expand_pattern(cx, pattern)
 271             ]].into_iter().collect();
 272
 273             let wild_pattern = Pattern {
 274                 ty: pattern_ty,
 275                 span: DUMMY_SP,
 276                 kind: box PatternKind::Wild,
 277             };
 278             let witness = match is_useful(cx, &pats, &[&wild_pattern], ConstructWitness) {
 279                 UsefulWithWitness(witness) => witness,
 280                 NotUseful => return,
 281                 Useful => bug!()
 282             };
 283
 284             let pattern_string = witness[0].single_pattern().to_string();
 285             let mut diag = struct_span_err!(
 286                 self.tcx.sess, pat.span, E0005,
 287                 "refutable pattern in {}: `{}` not covered",
 288                 origin, pattern_string
 289             );
 290             let label_msg = match pat.node {
 291                 PatKind::Path(hir::QPath::Resolved(None, ref path))
 292                         if path.segments.len() == 1 && path.segments[0].args.is_none() => {
 293                     format!("interpreted as {} {} pattern, not new variable",
 294                             path.def.article(), path.def.kind_name())
 295                 }
 296                 _ => format!("pattern `{}` not covered", pattern_string),
 297             };
 298             diag.span_label(pat.span, label_msg);
 299             diag.emit();
 300         });
 301     }
 302 }
 303
 304 fn check_for_bindings_named_same_as_variants(cx: &MatchVisitor, pat: &Pat) {
 305     pat.walk(|p| {
 306         if let PatKind::Binding(_, _, ident, None) = p.node {
 307             if let Some(&bm) = cx.tables.pat_binding_modes().get(p.hir_id) {
 308                 if bm != ty::BindByValue(hir::MutImmutable) {
 309                     // Nothing to check.
 310                     return true;
 311                 }
 312                 let pat_ty = cx.tables.pat_ty(p);
 313                 if let ty::Adt(edef, _) = pat_ty.sty {
 314                     if edef.is_enum() && edef.variants.iter().any(|variant| {
 315                         variant.name == ident.name && variant.ctor_kind == CtorKind::Const
 316                     }) {
 317                         let ty_path = cx.tcx.item_path_str(edef.did);
 318                         let mut err = struct_span_warn!(cx.tcx.sess, p.span, E0170,
 319                             "pattern binding `{}` is named the same as one \
 320                             of the variants of the type `{}`",
 321                             ident, ty_path);
 322                         err.span_suggestion_with_applicability(
 323                             p.span,
 324                             "to match on the variant, qualify the path",
 325                             format!("{}::{}", ty_path, ident),
 326                             Applicability::MachineApplicable
 327                         );
 328                         err.emit();
 329                     }
 330                 }
 331             } else {
 332                 cx.tcx.sess.delay_span_bug(p.span, "missing binding mode");
 333             }
 334         }
 335         true
 336     });
 337 }
 338
 339 /// Checks for common cases of "catchall" patterns that may not be intended as such.
 340 fn pat_is_catchall(pat: &Pat) -> bool {
 341     match pat.node {
 342         PatKind::Binding(.., None) => true,
 343         PatKind::Binding(.., Some(ref s)) => pat_is_catchall(s),
 344         PatKind::Ref(ref s, _) => pat_is_catchall(s),
 345         PatKind::Tuple(ref v, _) => v.iter().all(|p| {
 346             pat_is_catchall(&p)
 347         }),
 348         _ => false
 349     }
 350 }
 351
 352 // Check for unreachable patterns
 353 fn check_arms<'a, 'tcx>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 354                         arms: &[(Vec<(&'a Pattern<'tcx>, &hir::Pat)>, Option<&hir::Expr>)],
 355                         source: hir::MatchSource)
 356 {
 357     let mut seen = Matrix::empty();
 358     let mut catchall = None;
 359     let mut printed_if_let_err = false;
 360     for (arm_index, &(ref pats, guard)) in arms.iter().enumerate() {
 361         for &(pat, hir_pat) in pats {
 362             let v = smallvec![pat];
 363
 364             match is_useful(cx, &seen, &v, LeaveOutWitness) {
 365                 NotUseful => {
 366                     match source {
 367                         hir::MatchSource::IfLetDesugar { .. } => {
 368                             if cx.tcx.features().irrefutable_let_patterns {
 369                                 cx.tcx.lint_node(
 370                                     lint::builtin::IRREFUTABLE_LET_PATTERNS,
 371                                     hir_pat.id, pat.span,
 372                                     "irrefutable if-let pattern");
 373                             } else {
 374                                 if printed_if_let_err {
 375                                     // we already printed an irrefutable if-let pattern error.
 376                                     // We don't want two, that's just confusing.
 377                                 } else {
 378                                     // find the first arm pattern so we can use its span
 379                                     let &(ref first_arm_pats, _) = &arms[0];
 380                                     let first_pat = &first_arm_pats[0];
 381                                     let span = first_pat.0.span;
 382                                     struct_span_err!(cx.tcx.sess, span, E0162,
 383                                                     "irrefutable if-let pattern")
 384                                         .span_label(span, "irrefutable pattern")
 385                                         .emit();
 386                                     printed_if_let_err = true;
 387                                 }
 388                             }
 389                         }
 390
 391                         hir::MatchSource::WhileLetDesugar => {
 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                                     if cx.tcx.features().irrefutable_let_patterns {
 404                                         cx.tcx.lint_node(
 405                                             lint::builtin::IRREFUTABLE_LET_PATTERNS,
 406                                             hir_pat.id, pat.span,
 407                                             "irrefutable while-let pattern");
 408                                     } else {
 409                                         // find the first arm pattern so we can use its span
 410                                         let &(ref first_arm_pats, _) = &arms[0];
 411                                         let first_pat = &first_arm_pats[0];
 412                                         let span = first_pat.0.span;
 413                                         struct_span_err!(cx.tcx.sess, span, E0165,
 414                                                          "irrefutable while-let pattern")
 415                                             .span_label(span, "irrefutable pattern")
 416                                             .emit();
 417                                     }
 418                                 },
 419                                 _ => bug!(),
 420                             }
 421                         },
 422
 423                         hir::MatchSource::ForLoopDesugar |
 424                         hir::MatchSource::Normal => {
 425                             let mut err = cx.tcx.struct_span_lint_node(
 426                                 lint::builtin::UNREACHABLE_PATTERNS,
 427                                 hir_pat.id,
 428                                 pat.span,
 429                                 "unreachable pattern",
 430                             );
 431                             // if we had a catchall pattern, hint at that
 432                             if let Some(catchall) = catchall {
 433                                 err.span_label(pat.span, "unreachable pattern");
 434                                 err.span_label(catchall, "matches any value");
 435                             }
 436                             err.emit();
 437                         },
 438
 439                         // Unreachable patterns in try expressions occur when one of the arms
 440                         // are an uninhabited type. Which is OK.
 441                         hir::MatchSource::TryDesugar => {}
 442                     }
 443                 }
 444                 Useful => (),
 445                 UsefulWithWitness(_) => bug!()
 446             }
 447             if guard.is_none() {
 448                 seen.push(v);
 449                 if catchall.is_none() && pat_is_catchall(hir_pat) {
 450                     catchall = Some(pat.span);
 451                 }
 452             }
 453         }
 454     }
 455 }
 456
 457 fn check_exhaustive<'p, 'a: 'p, 'tcx: 'a>(cx: &mut MatchCheckCtxt<'a, 'tcx>,
 458                               scrut_ty: Ty<'tcx>,
 459                               sp: Span,
 460                               matrix: &Matrix<'p, 'tcx>) {
 461     let wild_pattern = Pattern {
 462         ty: scrut_ty,
 463         span: DUMMY_SP,
 464         kind: box PatternKind::Wild,
 465     };
 466     match is_useful(cx, matrix, &[&wild_pattern], ConstructWitness) {
 467         UsefulWithWitness(pats) => {
 468             let witnesses = if pats.is_empty() {
 469                 vec![&wild_pattern]
 470             } else {
 471                 pats.iter().map(|w| w.single_pattern()).collect()
 472             };
 473
 474             const LIMIT: usize = 3;
 475             let joined_patterns = match witnesses.len() {
 476                 0 => bug!(),
 477                 1 => format!("`{}`", witnesses[0]),
 478                 2..=LIMIT => {
 479                     let (tail, head) = witnesses.split_last().unwrap();
 480                     let head: Vec<_> = head.iter().map(|w| w.to_string()).collect();
 481                     format!("`{}` and `{}`", head.join("`, `"), tail)
 482                 },
 483                 _ => {
 484                     let (head, tail) = witnesses.split_at(LIMIT);
 485                     let head: Vec<_> = head.iter().map(|w| w.to_string()).collect();
 486                     format!("`{}` and {} more", head.join("`, `"), tail.len())
 487                 }
 488             };
 489
 490             let label_text = match witnesses.len() {
 491                 1 => format!("pattern {} not covered", joined_patterns),
 492                 _ => format!("patterns {} not covered", joined_patterns)
 493             };
 494             create_e0004(cx.tcx.sess, sp,
 495                             format!("non-exhaustive patterns: {} not covered",
 496                                     joined_patterns))
 497                 .span_label(sp, label_text)
 498                 .emit();
 499         }
 500         NotUseful => {
 501             // This is good, wildcard pattern isn't reachable
 502         },
 503         _ => bug!()
 504     }
 505 }
 506
 507 // Legality of move bindings checking
 508 fn check_legality_of_move_bindings(cx: &MatchVisitor,
 509                                    has_guard: bool,
 510                                    pats: &[P<Pat>]) {
 511     let mut by_ref_span = None;
 512     for pat in pats {
 513         pat.each_binding(|_, hir_id, span, _path| {
 514             if let Some(&bm) = cx.tables.pat_binding_modes().get(hir_id) {
 515                 if let ty::BindByReference(..) = bm {
 516                     by_ref_span = Some(span);
 517                 }
 518             } else {
 519                 cx.tcx.sess.delay_span_bug(pat.span, "missing binding mode");
 520             }
 521         })
 522     }
 523     let span_vec = &mut Vec::new();
 524     let check_move = |p: &Pat, sub: Option<&Pat>, span_vec: &mut Vec<Span>| {
 525         // check legality of moving out of the enum
 526
 527         // x @ Foo(..) is legal, but x @ Foo(y) isn't.
 528         if sub.map_or(false, |p| p.contains_bindings()) {
 529             struct_span_err!(cx.tcx.sess, p.span, E0007,
 530                              "cannot bind by-move with sub-bindings")
 531                 .span_label(p.span, "binds an already bound by-move value by moving it")
 532                 .emit();
 533         } else if has_guard && !cx.tcx.allow_bind_by_move_patterns_with_guards() {
 534             let mut err = struct_span_err!(cx.tcx.sess, p.span, E0008,
 535                                            "cannot bind by-move into a pattern guard");
 536             err.span_label(p.span, "moves value into pattern guard");
 537             if cx.tcx.sess.opts.unstable_features.is_nightly_build() && cx.tcx.use_mir_borrowck() {
 538                 err.help("add #![feature(bind_by_move_pattern_guards)] to the \
 539                           crate attributes to enable");
 540             }
 541             err.emit();
 542         } else if let Some(_by_ref_span) = by_ref_span {
 543             span_vec.push(p.span);
 544         }
 545     };
 546
 547     for pat in pats {
 548         pat.walk(|p| {
 549             if let PatKind::Binding(_, _, _, ref sub) = p.node {
 550                 if let Some(&bm) = cx.tables.pat_binding_modes().get(p.hir_id) {
 551                     match bm {
 552                         ty::BindByValue(..) => {
 553                             let pat_ty = cx.tables.node_id_to_type(p.hir_id);
 554                             if pat_ty.moves_by_default(cx.tcx, cx.param_env, pat.span) {
 555                                 check_move(p, sub.as_ref().map(|p| &**p), span_vec);
 556                             }
 557                         }
 558                         _ => {}
 559                     }
 560                 } else {
 561                     cx.tcx.sess.delay_span_bug(pat.span, "missing binding mode");
 562                 }
 563             }
 564             true
 565         });
 566     }
 567     if !span_vec.is_empty(){
 568         let span = MultiSpan::from_spans(span_vec.clone());
 569         let mut err = struct_span_err!(
 570             cx.tcx.sess,
 571             span,
 572             E0009,
 573             "cannot bind by-move and by-ref in the same pattern",
 574         );
 575         err.span_label(by_ref_span.unwrap(), "both by-ref and by-move used");
 576         for span in span_vec.iter(){
 577             err.span_label(*span, "by-move pattern here");
 578         }
 579         err.emit();
 580     }
 581 }
 582
 583 /// Ensures that a pattern guard doesn't borrow by mutable reference or
 584 /// assign.
 585 ///
 586 /// FIXME: this should be done by borrowck.
 587 fn check_for_mutation_in_guard(cx: &MatchVisitor, guard: &hir::Guard) {
 588     let mut checker = MutationChecker {
 589         cx,
 590     };
 591     match guard {
 592         hir::Guard::If(expr) =>
 593             ExprUseVisitor::new(&mut checker,
 594                                 cx.tcx,
 595                                 cx.param_env,
 596                                 cx.region_scope_tree,
 597                                 cx.tables,
 598                                 None).walk_expr(expr),
 599     };
 600 }
 601
 602 struct MutationChecker<'a, 'tcx: 'a> {
 603     cx: &'a MatchVisitor<'a, 'tcx>,
 604 }
 605
 606 impl<'a, 'tcx> Delegate<'tcx> for MutationChecker<'a, 'tcx> {
 607     fn matched_pat(&mut self, _: &Pat, _: &cmt_, _: euv::MatchMode) {}
 608     fn consume(&mut self, _: ast::NodeId, _: Span, _: &cmt_, _: ConsumeMode) {}
 609     fn consume_pat(&mut self, _: &Pat, _: &cmt_, _: ConsumeMode) {}
 610     fn borrow(&mut self,
 611               _: ast::NodeId,
 612               span: Span,
 613               _: &cmt_,
 614               _: ty::Region<'tcx>,
 615               kind:ty:: BorrowKind,
 616               _: LoanCause) {
 617         match kind {
 618             ty::MutBorrow => {
 619                 let mut err = struct_span_err!(self.cx.tcx.sess, span, E0301,
 620                           "cannot mutably borrow in a pattern guard");
 621                 err.span_label(span, "borrowed mutably in pattern guard");
 622                 if self.cx.tcx.sess.opts.unstable_features.is_nightly_build() &&
 623                     self.cx.tcx.use_mir_borrowck()
 624                 {
 625                     err.help("add #![feature(bind_by_move_pattern_guards)] to the \
 626                               crate attributes to enable");
 627                 }
 628                 err.emit();
 629             }
 630             ty::ImmBorrow | ty::UniqueImmBorrow => {}
 631         }
 632     }
 633     fn decl_without_init(&mut self, _: ast::NodeId, _: Span) {}
 634     fn mutate(&mut self, _: ast::NodeId, span: Span, _: &cmt_, mode: MutateMode) {
 635         match mode {
 636             MutateMode::JustWrite | MutateMode::WriteAndRead => {
 637                 struct_span_err!(self.cx.tcx.sess, span, E0302, "cannot assign in a pattern guard")
 638                     .span_label(span, "assignment in pattern guard")
 639                     .emit();
 640             }
 641             MutateMode::Init => {}
 642         }
 643     }
 644 }
 645
 646 /// Forbids bindings in `@` patterns. This is necessary for memory safety,
 647 /// because of the way rvalues are handled in the borrow check. (See issue
 648 /// #14587.)
 649 fn check_legality_of_bindings_in_at_patterns(cx: &MatchVisitor, pat: &Pat) {
 650     AtBindingPatternVisitor { cx: cx, bindings_allowed: true }.visit_pat(pat);
 651 }
 652
 653 struct AtBindingPatternVisitor<'a, 'b:'a, 'tcx:'b> {
 654     cx: &'a MatchVisitor<'b, 'tcx>,
 655     bindings_allowed: bool
 656 }
 657
 658 impl<'a, 'b, 'tcx, 'v> Visitor<'v> for AtBindingPatternVisitor<'a, 'b, 'tcx> {
 659     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'v> {
 660         NestedVisitorMap::None
 661     }
 662
 663     fn visit_pat(&mut self, pat: &Pat) {
 664         match pat.node {
 665             PatKind::Binding(.., ref subpat) => {
 666                 if !self.bindings_allowed {
 667                     struct_span_err!(self.cx.tcx.sess, pat.span, E0303,
 668                                      "pattern bindings are not allowed after an `@`")
 669                         .span_label(pat.span,  "not allowed after `@`")
 670                         .emit();
 671                 }
 672
 673                 if subpat.is_some() {
 674                     let bindings_were_allowed = self.bindings_allowed;
 675                     self.bindings_allowed = false;
 676                     intravisit::walk_pat(self, pat);
 677                     self.bindings_allowed = bindings_were_allowed;
 678                 }
 679             }
 680             _ => intravisit::walk_pat(self, pat),
 681         }
 682     }
 683 }