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 pub use self::Constructor::*;
  12 use self::Usefulness::*;
  13 use self::WitnessPreference::*;
  14
  15 use rustc::dep_graph::DepNode;
  16 use rustc::middle::const_val::ConstVal;
  17 use ::{eval_const_expr, eval_const_expr_partial, compare_const_vals};
  18 use ::{const_expr_to_pat, lookup_const_by_id};
  19 use ::EvalHint::ExprTypeChecked;
  20 use rustc::hir::def::*;
  21 use rustc::hir::def_id::{DefId};
  22 use rustc::middle::expr_use_visitor::{ConsumeMode, Delegate, ExprUseVisitor};
  23 use rustc::middle::expr_use_visitor::{LoanCause, MutateMode};
  24 use rustc::middle::expr_use_visitor as euv;
  25 use rustc::infer;
  26 use rustc::middle::mem_categorization::{cmt};
  27 use rustc::hir::pat_util::*;
  28 use rustc::traits::ProjectionMode;
  29 use rustc::ty::*;
  30 use rustc::ty;
  31 use std::cmp::Ordering;
  32 use std::fmt;
  33 use std::iter::{FromIterator, IntoIterator, repeat};
  34
  35 use rustc::hir;
  36 use rustc::hir::{Pat, PatKind};
  37 use rustc::hir::intravisit::{self, IdVisitor, IdVisitingOperation, Visitor, FnKind};
  38 use rustc_back::slice;
  39
  40 use syntax::ast::{self, DUMMY_NODE_ID, NodeId};
  41 use syntax::codemap::{Span, Spanned, DUMMY_SP};
  42 use rustc::hir::fold::{Folder, noop_fold_pat};
  43 use rustc::hir::print::pat_to_string;
  44 use syntax::ptr::P;
  45 use rustc::util::nodemap::FnvHashMap;
  46
  47 pub const DUMMY_WILD_PAT: &'static Pat = &Pat {
  48     id: DUMMY_NODE_ID,
  49     node: PatKind::Wild,
  50     span: DUMMY_SP
  51 };
  52
  53 struct Matrix<'a>(Vec<Vec<&'a Pat>>);
  54
  55 /// Pretty-printer for matrices of patterns, example:
  56 /// ++++++++++++++++++++++++++
  57 /// + _     + []             +
  58 /// ++++++++++++++++++++++++++
  59 /// + true  + [First]        +
  60 /// ++++++++++++++++++++++++++
  61 /// + true  + [Second(true)] +
  62 /// ++++++++++++++++++++++++++
  63 /// + false + [_]            +
  64 /// ++++++++++++++++++++++++++
  65 /// + _     + [_, _, ..tail] +
  66 /// ++++++++++++++++++++++++++
  67 impl<'a> fmt::Debug for Matrix<'a> {
  68     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
  69         write!(f, "\n")?;
  70
  71         let &Matrix(ref m) = self;
  72         let pretty_printed_matrix: Vec<Vec<String>> = m.iter().map(|row| {
  73             row.iter()
  74                .map(|&pat| pat_to_string(&pat))
  75                .collect::<Vec<String>>()
  76         }).collect();
  77
  78         let column_count = m.iter().map(|row| row.len()).max().unwrap_or(0);
  79         assert!(m.iter().all(|row| row.len() == column_count));
  80         let column_widths: Vec<usize> = (0..column_count).map(|col| {
  81             pretty_printed_matrix.iter().map(|row| row[col].len()).max().unwrap_or(0)
  82         }).collect();
  83
  84         let total_width = column_widths.iter().cloned().sum::<usize>() + column_count * 3 + 1;
  85         let br = repeat('+').take(total_width).collect::<String>();
  86         write!(f, "{}\n", br)?;
  87         for row in pretty_printed_matrix {
  88             write!(f, "+")?;
  89             for (column, pat_str) in row.into_iter().enumerate() {
  90                 write!(f, " ")?;
  91                 write!(f, "{:1$}", pat_str, column_widths[column])?;
  92                 write!(f, " +")?;
  93             }
  94             write!(f, "\n")?;
  95             write!(f, "{}\n", br)?;
  96         }
  97         Ok(())
  98     }
  99 }
 100
 101 impl<'a> FromIterator<Vec<&'a Pat>> for Matrix<'a> {
 102     fn from_iter<T: IntoIterator<Item=Vec<&'a Pat>>>(iter: T) -> Matrix<'a> {
 103         Matrix(iter.into_iter().collect())
 104     }
 105 }
 106
 107 //NOTE: appears to be the only place other then InferCtxt to contain a ParamEnv
 108 pub struct MatchCheckCtxt<'a, 'tcx: 'a> {
 109     pub tcx: &'a TyCtxt<'tcx>,
 110     pub param_env: ParameterEnvironment<'a, 'tcx>,
 111 }
 112
 113 #[derive(Clone, PartialEq)]
 114 pub enum Constructor {
 115     /// The constructor of all patterns that don't vary by constructor,
 116     /// e.g. struct patterns and fixed-length arrays.
 117     Single,
 118     /// Enum variants.
 119     Variant(DefId),
 120     /// Literal values.
 121     ConstantValue(ConstVal),
 122     /// Ranges of literal values (2..5).
 123     ConstantRange(ConstVal, ConstVal),
 124     /// Array patterns of length n.
 125     Slice(usize),
 126     /// Array patterns with a subslice.
 127     SliceWithSubslice(usize, usize)
 128 }
 129
 130 #[derive(Clone, PartialEq)]
 131 enum Usefulness {
 132     Useful,
 133     UsefulWithWitness(Vec<P<Pat>>),
 134     NotUseful
 135 }
 136
 137 #[derive(Copy, Clone)]
 138 enum WitnessPreference {
 139     ConstructWitness,
 140     LeaveOutWitness
 141 }
 142
 143 impl<'a, 'tcx, 'v> Visitor<'v> for MatchCheckCtxt<'a, 'tcx> {
 144     fn visit_expr(&mut self, ex: &hir::Expr) {
 145         check_expr(self, ex);
 146     }
 147     fn visit_local(&mut self, l: &hir::Local) {
 148         check_local(self, l);
 149     }
 150     fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v hir::FnDecl,
 151                 b: &'v hir::Block, s: Span, n: NodeId) {
 152         check_fn(self, fk, fd, b, s, n);
 153     }
 154 }
 155
 156 pub fn check_crate(tcx: &TyCtxt) {
 157     tcx.visit_all_items_in_krate(DepNode::MatchCheck, &mut MatchCheckCtxt {
 158         tcx: tcx,
 159         param_env: tcx.empty_parameter_environment(),
 160     });
 161     tcx.sess.abort_if_errors();
 162 }
 163
 164 fn check_expr(cx: &mut MatchCheckCtxt, ex: &hir::Expr) {
 165     intravisit::walk_expr(cx, ex);
 166     match ex.node {
 167         hir::ExprMatch(ref scrut, ref arms, source) => {
 168             for arm in arms {
 169                 // First, check legality of move bindings.
 170                 check_legality_of_move_bindings(cx,
 171                                                 arm.guard.is_some(),
 172                                                 &arm.pats);
 173
 174                 // Second, if there is a guard on each arm, make sure it isn't
 175                 // assigning or borrowing anything mutably.
 176                 match arm.guard {
 177                     Some(ref guard) => check_for_mutation_in_guard(cx, &guard),
 178                     None => {}
 179                 }
 180             }
 181
 182             let mut static_inliner = StaticInliner::new(cx.tcx, None);
 183             let inlined_arms = arms.iter().map(|arm| {
 184                 (arm.pats.iter().map(|pat| {
 185                     static_inliner.fold_pat((*pat).clone())
 186                 }).collect(), arm.guard.as_ref().map(|e| &**e))
 187             }).collect::<Vec<(Vec<P<Pat>>, Option<&hir::Expr>)>>();
 188
 189             // Bail out early if inlining failed.
 190             if static_inliner.failed {
 191                 return;
 192             }
 193
 194             for pat in inlined_arms
 195                 .iter()
 196                 .flat_map(|&(ref pats, _)| pats) {
 197                 // Third, check legality of move bindings.
 198                 check_legality_of_bindings_in_at_patterns(cx, &pat);
 199
 200                 // Fourth, check if there are any references to NaN that we should warn about.
 201                 check_for_static_nan(cx, &pat);
 202
 203                 // Fifth, check if for any of the patterns that match an enumerated type
 204                 // are bindings with the same name as one of the variants of said type.
 205                 check_for_bindings_named_the_same_as_variants(cx, &pat);
 206             }
 207
 208             // Fourth, check for unreachable arms.
 209             check_arms(cx, &inlined_arms[..], source);
 210
 211             // Finally, check if the whole match expression is exhaustive.
 212             // Check for empty enum, because is_useful only works on inhabited types.
 213             let pat_ty = cx.tcx.node_id_to_type(scrut.id);
 214             if inlined_arms.is_empty() {
 215                 if !pat_ty.is_empty(cx.tcx) {
 216                     // We know the type is inhabited, so this must be wrong
 217                     let mut err = struct_span_err!(cx.tcx.sess, ex.span, E0002,
 218                                                    "non-exhaustive patterns: type {} is non-empty",
 219                                                    pat_ty);
 220                     span_help!(&mut err, ex.span,
 221                         "Please ensure that all possible cases are being handled; \
 222                          possibly adding wildcards or more match arms.");
 223                     err.emit();
 224                 }
 225                 // If the type *is* empty, it's vacuously exhaustive
 226                 return;
 227             }
 228
 229             let matrix: Matrix = inlined_arms
 230                 .iter()
 231                 .filter(|&&(_, guard)| guard.is_none())
 232                 .flat_map(|arm| &arm.0)
 233                 .map(|pat| vec![&**pat])
 234                 .collect();
 235             check_exhaustive(cx, ex.span, &matrix, source);
 236         },
 237         _ => ()
 238     }
 239 }
 240
 241 fn check_for_bindings_named_the_same_as_variants(cx: &MatchCheckCtxt, pat: &Pat) {
 242     pat.walk(|p| {
 243         match p.node {
 244             PatKind::Ident(hir::BindByValue(hir::MutImmutable), ident, None) => {
 245                 let pat_ty = cx.tcx.pat_ty(p);
 246                 if let ty::TyEnum(edef, _) = pat_ty.sty {
 247                     let def = cx.tcx.def_map.borrow().get(&p.id).map(|d| d.full_def());
 248                     if let Some(Def::Local(..)) = def {
 249                         if edef.variants.iter().any(|variant|
 250                             variant.name == ident.node.unhygienic_name
 251                                 && variant.kind() == VariantKind::Unit
 252                         ) {
 253                             let ty_path = cx.tcx.item_path_str(edef.did);
 254                             let mut err = struct_span_warn!(cx.tcx.sess, p.span, E0170,
 255                                 "pattern binding `{}` is named the same as one \
 256                                  of the variants of the type `{}`",
 257                                 ident.node, ty_path);
 258                             fileline_help!(err, p.span,
 259                                 "if you meant to match on a variant, \
 260                                  consider making the path in the pattern qualified: `{}::{}`",
 261                                 ty_path, ident.node);
 262                             err.emit();
 263                         }
 264                     }
 265                 }
 266             }
 267             _ => ()
 268         }
 269         true
 270     });
 271 }
 272
 273 // Check that we do not match against a static NaN (#6804)
 274 fn check_for_static_nan(cx: &MatchCheckCtxt, pat: &Pat) {
 275     pat.walk(|p| {
 276         if let PatKind::Lit(ref expr) = p.node {
 277             match eval_const_expr_partial(cx.tcx, &expr, ExprTypeChecked, None) {
 278                 Ok(ConstVal::Float(f)) if f.is_nan() => {
 279                     span_warn!(cx.tcx.sess, p.span, E0003,
 280                                "unmatchable NaN in pattern, \
 281                                 use the is_nan method in a guard instead");
 282                 }
 283                 Ok(_) => {}
 284
 285                 Err(err) => {
 286                     let mut diag = struct_span_err!(cx.tcx.sess, err.span, E0471,
 287                                                     "constant evaluation error: {}",
 288                                                     err.description());
 289                     if !p.span.contains(err.span) {
 290                         diag.span_note(p.span, "in pattern here");
 291                     }
 292                     diag.emit();
 293                 }
 294             }
 295         }
 296         true
 297     });
 298 }
 299
 300 // Check for unreachable patterns
 301 fn check_arms(cx: &MatchCheckCtxt,
 302               arms: &[(Vec<P<Pat>>, Option<&hir::Expr>)],
 303               source: hir::MatchSource) {
 304     let mut seen = Matrix(vec![]);
 305     let mut printed_if_let_err = false;
 306     for &(ref pats, guard) in arms {
 307         for pat in pats {
 308             let v = vec![&**pat];
 309
 310             match is_useful(cx, &seen, &v[..], LeaveOutWitness) {
 311                 NotUseful => {
 312                     match source {
 313                         hir::MatchSource::IfLetDesugar { .. } => {
 314                             if printed_if_let_err {
 315                                 // we already printed an irrefutable if-let pattern error.
 316                                 // We don't want two, that's just confusing.
 317                             } else {
 318                                 // find the first arm pattern so we can use its span
 319                                 let &(ref first_arm_pats, _) = &arms[0];
 320                                 let first_pat = &first_arm_pats[0];
 321                                 let span = first_pat.span;
 322                                 span_err!(cx.tcx.sess, span, E0162, "irrefutable if-let pattern");
 323                                 printed_if_let_err = true;
 324                             }
 325                         },
 326
 327                         hir::MatchSource::WhileLetDesugar => {
 328                             // find the first arm pattern so we can use its span
 329                             let &(ref first_arm_pats, _) = &arms[0];
 330                             let first_pat = &first_arm_pats[0];
 331                             let span = first_pat.span;
 332                             span_err!(cx.tcx.sess, span, E0165, "irrefutable while-let pattern");
 333                         },
 334
 335                         hir::MatchSource::ForLoopDesugar => {
 336                             // this is a bug, because on `match iter.next()` we cover
 337                             // `Some(<head>)` and `None`. It's impossible to have an unreachable
 338                             // pattern
 339                             // (see libsyntax/ext/expand.rs for the full expansion of a for loop)
 340                             span_bug!(pat.span, "unreachable for-loop pattern")
 341                         },
 342
 343                         hir::MatchSource::Normal => {
 344                             span_err!(cx.tcx.sess, pat.span, E0001, "unreachable pattern")
 345                         },
 346
 347                         hir::MatchSource::TryDesugar => {
 348                             span_bug!(pat.span, "unreachable try pattern")
 349                         },
 350                     }
 351                 }
 352                 Useful => (),
 353                 UsefulWithWitness(_) => bug!()
 354             }
 355             if guard.is_none() {
 356                 let Matrix(mut rows) = seen;
 357                 rows.push(v);
 358                 seen = Matrix(rows);
 359             }
 360         }
 361     }
 362 }
 363
 364 fn raw_pat<'a>(p: &'a Pat) -> &'a Pat {
 365     match p.node {
 366         PatKind::Ident(_, _, Some(ref s)) => raw_pat(&s),
 367         _ => p
 368     }
 369 }
 370
 371 fn check_exhaustive(cx: &MatchCheckCtxt, sp: Span, matrix: &Matrix, source: hir::MatchSource) {
 372     match is_useful(cx, matrix, &[DUMMY_WILD_PAT], ConstructWitness) {
 373         UsefulWithWitness(pats) => {
 374             let witnesses = if pats.is_empty() {
 375                 vec![DUMMY_WILD_PAT]
 376             } else {
 377                 pats.iter().map(|w| &**w ).collect()
 378             };
 379             match source {
 380                 hir::MatchSource::ForLoopDesugar => {
 381                     // `witnesses[0]` has the form `Some(<head>)`, peel off the `Some`
 382                     let witness = match witnesses[0].node {
 383                         PatKind::TupleStruct(_, Some(ref pats)) => match &pats[..] {
 384                             [ref pat] => &**pat,
 385                             _ => bug!(),
 386                         },
 387                         _ => bug!(),
 388                     };
 389                     span_err!(cx.tcx.sess, sp, E0297,
 390                         "refutable pattern in `for` loop binding: \
 391                                 `{}` not covered",
 392                                 pat_to_string(witness));
 393                 },
 394                 _ => {
 395                     let pattern_strings: Vec<_> = witnesses.iter().map(|w| {
 396                         pat_to_string(w)
 397                     }).collect();
 398                     const LIMIT: usize = 3;
 399                     let joined_patterns = match pattern_strings.len() {
 400                         0 => bug!(),
 401                         1 => format!("`{}`", pattern_strings[0]),
 402                         2...LIMIT => {
 403                             let (tail, head) = pattern_strings.split_last().unwrap();
 404                             format!("`{}`", head.join("`, `") + "` and `" + tail)
 405                         },
 406                         _ => {
 407                             let (head, tail) = pattern_strings.split_at(LIMIT);
 408                             format!("`{}` and {} more", head.join("`, `"), tail.len())
 409                         }
 410                     };
 411                     span_err!(cx.tcx.sess, sp, E0004,
 412                         "non-exhaustive patterns: {} not covered",
 413                         joined_patterns
 414                     );
 415                 },
 416             }
 417         }
 418         NotUseful => {
 419             // This is good, wildcard pattern isn't reachable
 420         },
 421         _ => bug!()
 422     }
 423 }
 424
 425 fn const_val_to_expr(value: &ConstVal) -> P<hir::Expr> {
 426     let node = match value {
 427         &ConstVal::Bool(b) => ast::LitKind::Bool(b),
 428         _ => bug!()
 429     };
 430     P(hir::Expr {
 431         id: 0,
 432         node: hir::ExprLit(P(Spanned { node: node, span: DUMMY_SP })),
 433         span: DUMMY_SP,
 434         attrs: None,
 435     })
 436 }
 437
 438 pub struct StaticInliner<'a, 'tcx: 'a> {
 439     pub tcx: &'a TyCtxt<'tcx>,
 440     pub failed: bool,
 441     pub renaming_map: Option<&'a mut FnvHashMap<(NodeId, Span), NodeId>>,
 442 }
 443
 444 impl<'a, 'tcx> StaticInliner<'a, 'tcx> {
 445     pub fn new<'b>(tcx: &'b TyCtxt<'tcx>,
 446                    renaming_map: Option<&'b mut FnvHashMap<(NodeId, Span), NodeId>>)
 447                    -> StaticInliner<'b, 'tcx> {
 448         StaticInliner {
 449             tcx: tcx,
 450             failed: false,
 451             renaming_map: renaming_map
 452         }
 453     }
 454 }
 455
 456 struct RenamingRecorder<'map> {
 457     substituted_node_id: NodeId,
 458     origin_span: Span,
 459     renaming_map: &'map mut FnvHashMap<(NodeId, Span), NodeId>
 460 }
 461
 462 impl<'map> IdVisitingOperation for RenamingRecorder<'map> {
 463     fn visit_id(&mut self, node_id: NodeId) {
 464         let key = (node_id, self.origin_span);
 465         self.renaming_map.insert(key, self.substituted_node_id);
 466     }
 467 }
 468
 469 impl<'a, 'tcx> Folder for StaticInliner<'a, 'tcx> {
 470     fn fold_pat(&mut self, pat: P<Pat>) -> P<Pat> {
 471         return match pat.node {
 472             PatKind::Ident(..) | PatKind::Path(..) | PatKind::QPath(..) => {
 473                 let def = self.tcx.def_map.borrow().get(&pat.id).map(|d| d.full_def());
 474                 match def {
 475                     Some(Def::AssociatedConst(did)) |
 476                     Some(Def::Const(did)) => {
 477                         let substs = Some(self.tcx.node_id_item_substs(pat.id).substs);
 478                         if let Some((const_expr, _)) = lookup_const_by_id(self.tcx, did, substs) {
 479                             match const_expr_to_pat(self.tcx, const_expr, pat.id, pat.span) {
 480                                 Ok(new_pat) => {
 481                                     if let Some(ref mut map) = self.renaming_map {
 482                                         // Record any renamings we do here
 483                                         record_renamings(const_expr, &pat, map);
 484                                     }
 485                                     new_pat
 486                                 }
 487                                 Err(def_id) => {
 488                                     self.failed = true;
 489                                     self.tcx.sess.span_err(
 490                                         pat.span,
 491                                         &format!("constants of the type `{}` \
 492                                                   cannot be used in patterns",
 493                                                  self.tcx.item_path_str(def_id)));
 494                                     pat
 495                                 }
 496                             }
 497                         } else {
 498                             self.failed = true;
 499                             span_err!(self.tcx.sess, pat.span, E0158,
 500                                 "statics cannot be referenced in patterns");
 501                             pat
 502                         }
 503                     }
 504                     _ => noop_fold_pat(pat, self)
 505                 }
 506             }
 507             _ => noop_fold_pat(pat, self)
 508         };
 509
 510         fn record_renamings(const_expr: &hir::Expr,
 511                             substituted_pat: &hir::Pat,
 512                             renaming_map: &mut FnvHashMap<(NodeId, Span), NodeId>) {
 513             let mut renaming_recorder = RenamingRecorder {
 514                 substituted_node_id: substituted_pat.id,
 515                 origin_span: substituted_pat.span,
 516                 renaming_map: renaming_map,
 517             };
 518
 519             let mut id_visitor = IdVisitor::new(&mut renaming_recorder);
 520
 521             id_visitor.visit_expr(const_expr);
 522         }
 523     }
 524 }
 525
 526 /// Constructs a partial witness for a pattern given a list of
 527 /// patterns expanded by the specialization step.
 528 ///
 529 /// When a pattern P is discovered to be useful, this function is used bottom-up
 530 /// to reconstruct a complete witness, e.g. a pattern P' that covers a subset
 531 /// of values, V, where each value in that set is not covered by any previously
 532 /// used patterns and is covered by the pattern P'. Examples:
 533 ///
 534 /// left_ty: tuple of 3 elements
 535 /// pats: [10, 20, _]           => (10, 20, _)
 536 ///
 537 /// left_ty: struct X { a: (bool, &'static str), b: usize}
 538 /// pats: [(false, "foo"), 42]  => X { a: (false, "foo"), b: 42 }
 539 fn construct_witness<'a,'tcx>(cx: &MatchCheckCtxt<'a,'tcx>, ctor: &Constructor,
 540                               pats: Vec<&Pat>, left_ty: Ty<'tcx>) -> P<Pat> {
 541     let pats_len = pats.len();
 542     let mut pats = pats.into_iter().map(|p| P((*p).clone()));
 543     let pat = match left_ty.sty {
 544         ty::TyTuple(_) => PatKind::Tup(pats.collect()),
 545
 546         ty::TyEnum(adt, _) | ty::TyStruct(adt, _)  => {
 547             let v = ctor.variant_for_adt(adt);
 548             match v.kind() {
 549                 VariantKind::Struct => {
 550                     let field_pats: hir::HirVec<_> = v.fields.iter()
 551                         .zip(pats)
 552                         .filter(|&(_, ref pat)| pat.node != PatKind::Wild)
 553                         .map(|(field, pat)| Spanned {
 554                             span: DUMMY_SP,
 555                             node: hir::FieldPat {
 556                                 name: field.name,
 557                                 pat: pat,
 558                                 is_shorthand: false,
 559                             }
 560                         }).collect();
 561                     let has_more_fields = field_pats.len() < pats_len;
 562                     PatKind::Struct(def_to_path(cx.tcx, v.did), field_pats, has_more_fields)
 563                 }
 564                 VariantKind::Tuple => {
 565                     PatKind::TupleStruct(def_to_path(cx.tcx, v.did), Some(pats.collect()))
 566                 }
 567                 VariantKind::Unit => {
 568                     PatKind::Path(def_to_path(cx.tcx, v.did))
 569                 }
 570             }
 571         }
 572
 573         ty::TyRef(_, ty::TypeAndMut { ty, mutbl }) => {
 574             match ty.sty {
 575                ty::TyArray(_, n) => match ctor {
 576                     &Single => {
 577                         assert_eq!(pats_len, n);
 578                         PatKind::Vec(pats.collect(), None, hir::HirVec::new())
 579                     },
 580                     _ => bug!()
 581                 },
 582                 ty::TySlice(_) => match ctor {
 583                     &Slice(n) => {
 584                         assert_eq!(pats_len, n);
 585                         PatKind::Vec(pats.collect(), None, hir::HirVec::new())
 586                     },
 587                     _ => bug!()
 588                 },
 589                 ty::TyStr => PatKind::Wild,
 590
 591                 _ => {
 592                     assert_eq!(pats_len, 1);
 593                     PatKind::Ref(pats.nth(0).unwrap(), mutbl)
 594                 }
 595             }
 596         }
 597
 598         ty::TyArray(_, len) => {
 599             assert_eq!(pats_len, len);
 600             PatKind::Vec(pats.collect(), None, hir::HirVec::new())
 601         }
 602
 603         _ => {
 604             match *ctor {
 605                 ConstantValue(ref v) => PatKind::Lit(const_val_to_expr(v)),
 606                 _ => PatKind::Wild,
 607             }
 608         }
 609     };
 610
 611     P(hir::Pat {
 612         id: 0,
 613         node: pat,
 614         span: DUMMY_SP
 615     })
 616 }
 617
 618 impl Constructor {
 619     fn variant_for_adt<'tcx, 'container, 'a>(&self,
 620                                              adt: &'a ty::AdtDefData<'tcx, 'container>)
 621                                              -> &'a VariantDefData<'tcx, 'container> {
 622         match self {
 623             &Variant(vid) => adt.variant_with_id(vid),
 624             _ => adt.struct_variant()
 625         }
 626     }
 627 }
 628
 629 fn missing_constructors(cx: &MatchCheckCtxt, &Matrix(ref rows): &Matrix,
 630                        left_ty: Ty, max_slice_length: usize) -> Vec<Constructor> {
 631     let used_constructors: Vec<Constructor> = rows.iter()
 632         .flat_map(|row| pat_constructors(cx, row[0], left_ty, max_slice_length))
 633         .collect();
 634     all_constructors(cx, left_ty, max_slice_length)
 635         .into_iter()
 636         .filter(|c| !used_constructors.contains(c))
 637         .collect()
 638 }
 639
 640 /// This determines the set of all possible constructors of a pattern matching
 641 /// values of type `left_ty`. For vectors, this would normally be an infinite set
 642 /// but is instead bounded by the maximum fixed length of slice patterns in
 643 /// the column of patterns being analyzed.
 644 fn all_constructors(_cx: &MatchCheckCtxt, left_ty: Ty,
 645                     max_slice_length: usize) -> Vec<Constructor> {
 646     match left_ty.sty {
 647         ty::TyBool =>
 648             [true, false].iter().map(|b| ConstantValue(ConstVal::Bool(*b))).collect(),
 649
 650         ty::TyRef(_, ty::TypeAndMut { ty, .. }) => match ty.sty {
 651             ty::TySlice(_) =>
 652                 (0..max_slice_length+1).map(|length| Slice(length)).collect(),
 653             _ => vec![Single]
 654         },
 655
 656         ty::TyEnum(def, _) => def.variants.iter().map(|v| Variant(v.did)).collect(),
 657         _ => vec![Single]
 658     }
 659 }
 660
 661 // Algorithm from http://moscova.inria.fr/~maranget/papers/warn/index.html
 662 //
 663 // Whether a vector `v` of patterns is 'useful' in relation to a set of such
 664 // vectors `m` is defined as there being a set of inputs that will match `v`
 665 // but not any of the sets in `m`.
 666 //
 667 // This is used both for reachability checking (if a pattern isn't useful in
 668 // relation to preceding patterns, it is not reachable) and exhaustiveness
 669 // checking (if a wildcard pattern is useful in relation to a matrix, the
 670 // matrix isn't exhaustive).
 671
 672 // Note: is_useful doesn't work on empty types, as the paper notes.
 673 // So it assumes that v is non-empty.
 674 fn is_useful(cx: &MatchCheckCtxt,
 675              matrix: &Matrix,
 676              v: &[&Pat],
 677              witness: WitnessPreference)
 678              -> Usefulness {
 679     let &Matrix(ref rows) = matrix;
 680     debug!("{:?}", matrix);
 681     if rows.is_empty() {
 682         return match witness {
 683             ConstructWitness => UsefulWithWitness(vec!()),
 684             LeaveOutWitness => Useful
 685         };
 686     }
 687     if rows[0].is_empty() {
 688         return NotUseful;
 689     }
 690     assert!(rows.iter().all(|r| r.len() == v.len()));
 691     let real_pat = match rows.iter().find(|r| (*r)[0].id != DUMMY_NODE_ID) {
 692         Some(r) => raw_pat(r[0]),
 693         None if v.is_empty() => return NotUseful,
 694         None => v[0]
 695     };
 696     let left_ty = if real_pat.id == DUMMY_NODE_ID {
 697         cx.tcx.mk_nil()
 698     } else {
 699         let left_ty = cx.tcx.pat_ty(&real_pat);
 700
 701         match real_pat.node {
 702             PatKind::Ident(hir::BindByRef(..), _, _) => {
 703                 left_ty.builtin_deref(false, NoPreference).unwrap().ty
 704             }
 705             _ => left_ty,
 706         }
 707     };
 708
 709     let max_slice_length = rows.iter().filter_map(|row| match row[0].node {
 710         PatKind::Vec(ref before, _, ref after) => Some(before.len() + after.len()),
 711         _ => None
 712     }).max().map_or(0, |v| v + 1);
 713
 714     let constructors = pat_constructors(cx, v[0], left_ty, max_slice_length);
 715     if constructors.is_empty() {
 716         let constructors = missing_constructors(cx, matrix, left_ty, max_slice_length);
 717         if constructors.is_empty() {
 718             all_constructors(cx, left_ty, max_slice_length).into_iter().map(|c| {
 719                 match is_useful_specialized(cx, matrix, v, c.clone(), left_ty, witness) {
 720                     UsefulWithWitness(pats) => UsefulWithWitness({
 721                         let arity = constructor_arity(cx, &c, left_ty);
 722                         let mut result = {
 723                             let pat_slice = &pats[..];
 724                             let subpats: Vec<_> = (0..arity).map(|i| {
 725                                 pat_slice.get(i).map_or(DUMMY_WILD_PAT, |p| &**p)
 726                             }).collect();
 727                             vec![construct_witness(cx, &c, subpats, left_ty)]
 728                         };
 729                         result.extend(pats.into_iter().skip(arity));
 730                         result
 731                     }),
 732                     result => result
 733                 }
 734             }).find(|result| result != &NotUseful).unwrap_or(NotUseful)
 735         } else {
 736             let matrix = rows.iter().filter_map(|r| {
 737                 if pat_is_binding_or_wild(&cx.tcx.def_map.borrow(), raw_pat(r[0])) {
 738                     Some(r[1..].to_vec())
 739                 } else {
 740                     None
 741                 }
 742             }).collect();
 743             match is_useful(cx, &matrix, &v[1..], witness) {
 744                 UsefulWithWitness(pats) => {
 745                     let mut new_pats: Vec<_> = constructors.into_iter().map(|constructor| {
 746                         let arity = constructor_arity(cx, &constructor, left_ty);
 747                         let wild_pats = vec![DUMMY_WILD_PAT; arity];
 748                         construct_witness(cx, &constructor, wild_pats, left_ty)
 749                     }).collect();
 750                     new_pats.extend(pats);
 751                     UsefulWithWitness(new_pats)
 752                 },
 753                 result => result
 754             }
 755         }
 756     } else {
 757         constructors.into_iter().map(|c|
 758             is_useful_specialized(cx, matrix, v, c.clone(), left_ty, witness)
 759         ).find(|result| result != &NotUseful).unwrap_or(NotUseful)
 760     }
 761 }
 762
 763 fn is_useful_specialized(cx: &MatchCheckCtxt, &Matrix(ref m): &Matrix,
 764                          v: &[&Pat], ctor: Constructor, lty: Ty,
 765                          witness: WitnessPreference) -> Usefulness {
 766     let arity = constructor_arity(cx, &ctor, lty);
 767     let matrix = Matrix(m.iter().filter_map(|r| {
 768         specialize(cx, &r[..], &ctor, 0, arity)
 769     }).collect());
 770     match specialize(cx, v, &ctor, 0, arity) {
 771         Some(v) => is_useful(cx, &matrix, &v[..], witness),
 772         None => NotUseful
 773     }
 774 }
 775
 776 /// Determines the constructors that the given pattern can be specialized to.
 777 ///
 778 /// In most cases, there's only one constructor that a specific pattern
 779 /// represents, such as a specific enum variant or a specific literal value.
 780 /// Slice patterns, however, can match slices of different lengths. For instance,
 781 /// `[a, b, ..tail]` can match a slice of length 2, 3, 4 and so on.
 782 ///
 783 /// On the other hand, a wild pattern and an identifier pattern cannot be
 784 /// specialized in any way.
 785 fn pat_constructors(cx: &MatchCheckCtxt, p: &Pat,
 786                     left_ty: Ty, max_slice_length: usize) -> Vec<Constructor> {
 787     let pat = raw_pat(p);
 788     match pat.node {
 789         PatKind::Struct(..) | PatKind::TupleStruct(..) | PatKind::Path(..) | PatKind::Ident(..) =>
 790             match cx.tcx.def_map.borrow().get(&pat.id).unwrap().full_def() {
 791                 Def::Const(..) | Def::AssociatedConst(..) =>
 792                     span_bug!(pat.span, "const pattern should've \
 793                                          been rewritten"),
 794                 Def::Struct(..) | Def::TyAlias(..) => vec![Single],
 795                 Def::Variant(_, id) => vec![Variant(id)],
 796                 Def::Local(..) => vec![],
 797                 def => span_bug!(pat.span, "pat_constructors: unexpected \
 798                                             definition {:?}", def),
 799             },
 800         PatKind::QPath(..) =>
 801             span_bug!(pat.span, "const pattern should've been rewritten"),
 802         PatKind::Lit(ref expr) =>
 803             vec!(ConstantValue(eval_const_expr(cx.tcx, &expr))),
 804         PatKind::Range(ref lo, ref hi) =>
 805             vec!(ConstantRange(eval_const_expr(cx.tcx, &lo), eval_const_expr(cx.tcx, &hi))),
 806         PatKind::Vec(ref before, ref slice, ref after) =>
 807             match left_ty.sty {
 808                 ty::TyArray(_, _) => vec!(Single),
 809                 _                      => if slice.is_some() {
 810                     (before.len() + after.len()..max_slice_length+1)
 811                         .map(|length| Slice(length))
 812                         .collect()
 813                 } else {
 814                     vec!(Slice(before.len() + after.len()))
 815                 }
 816             },
 817         PatKind::Box(_) | PatKind::Tup(_) | PatKind::Ref(..) =>
 818             vec!(Single),
 819         PatKind::Wild =>
 820             vec!(),
 821     }
 822 }
 823
 824 /// This computes the arity of a constructor. The arity of a constructor
 825 /// is how many subpattern patterns of that constructor should be expanded to.
 826 ///
 827 /// For instance, a tuple pattern (_, 42, Some([])) has the arity of 3.
 828 /// A struct pattern's arity is the number of fields it contains, etc.
 829 pub fn constructor_arity(_cx: &MatchCheckCtxt, ctor: &Constructor, ty: Ty) -> usize {
 830     match ty.sty {
 831         ty::TyTuple(ref fs) => fs.len(),
 832         ty::TyBox(_) => 1,
 833         ty::TyRef(_, ty::TypeAndMut { ty, .. }) => match ty.sty {
 834             ty::TySlice(_) => match *ctor {
 835                 Slice(length) => length,
 836                 ConstantValue(_) => 0,
 837                 _ => bug!()
 838             },
 839             ty::TyStr => 0,
 840             _ => 1
 841         },
 842         ty::TyEnum(adt, _) | ty::TyStruct(adt, _) => {
 843             ctor.variant_for_adt(adt).fields.len()
 844         }
 845         ty::TyArray(_, n) => n,
 846         _ => 0
 847     }
 848 }
 849
 850 fn range_covered_by_constructor(ctor: &Constructor,
 851                                 from: &ConstVal, to: &ConstVal) -> Option<bool> {
 852     let (c_from, c_to) = match *ctor {
 853         ConstantValue(ref value)        => (value, value),
 854         ConstantRange(ref from, ref to) => (from, to),
 855         Single                          => return Some(true),
 856         _                               => bug!()
 857     };
 858     let cmp_from = compare_const_vals(c_from, from);
 859     let cmp_to = compare_const_vals(c_to, to);
 860     match (cmp_from, cmp_to) {
 861         (Some(cmp_from), Some(cmp_to)) => {
 862             Some(cmp_from != Ordering::Less && cmp_to != Ordering::Greater)
 863         }
 864         _ => None
 865     }
 866 }
 867
 868 /// This is the main specialization step. It expands the first pattern in the given row
 869 /// into `arity` patterns based on the constructor. For most patterns, the step is trivial,
 870 /// for instance tuple patterns are flattened and box patterns expand into their inner pattern.
 871 ///
 872 /// OTOH, slice patterns with a subslice pattern (..tail) can be expanded into multiple
 873 /// different patterns.
 874 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
 875 /// fields filled with wild patterns.
 876 pub fn specialize<'a>(cx: &MatchCheckCtxt, r: &[&'a Pat],
 877                       constructor: &Constructor, col: usize, arity: usize) -> Option<Vec<&'a Pat>> {
 878     let &Pat {
 879         id: pat_id, ref node, span: pat_span
 880     } = raw_pat(r[col]);
 881     let head: Option<Vec<&Pat>> = match *node {
 882         PatKind::Wild =>
 883             Some(vec![DUMMY_WILD_PAT; arity]),
 884
 885         PatKind::Path(..) | PatKind::Ident(..) => {
 886             let def = cx.tcx.def_map.borrow().get(&pat_id).unwrap().full_def();
 887             match def {
 888                 Def::Const(..) | Def::AssociatedConst(..) =>
 889                     span_bug!(pat_span, "const pattern should've \
 890                                          been rewritten"),
 891                 Def::Variant(_, id) if *constructor != Variant(id) => None,
 892                 Def::Variant(..) | Def::Struct(..) => Some(Vec::new()),
 893                 Def::Local(..) => Some(vec![DUMMY_WILD_PAT; arity]),
 894                 _ => span_bug!(pat_span, "specialize: unexpected \
 895                                           definition {:?}", def),
 896             }
 897         }
 898
 899         PatKind::TupleStruct(_, ref args) => {
 900             let def = cx.tcx.def_map.borrow().get(&pat_id).unwrap().full_def();
 901             match def {
 902                 Def::Const(..) | Def::AssociatedConst(..) =>
 903                     span_bug!(pat_span, "const pattern should've \
 904                                          been rewritten"),
 905                 Def::Variant(_, id) if *constructor != Variant(id) => None,
 906                 Def::Variant(..) | Def::Struct(..) => {
 907                     Some(match args {
 908                         &Some(ref args) => args.iter().map(|p| &**p).collect(),
 909                         &None => vec![DUMMY_WILD_PAT; arity],
 910                     })
 911                 }
 912                 _ => None
 913             }
 914         }
 915
 916         PatKind::QPath(_, _) => {
 917             span_bug!(pat_span, "const pattern should've been rewritten")
 918         }
 919
 920         PatKind::Struct(_, ref pattern_fields, _) => {
 921             let def = cx.tcx.def_map.borrow().get(&pat_id).unwrap().full_def();
 922             let adt = cx.tcx.node_id_to_type(pat_id).ty_adt_def().unwrap();
 923             let variant = constructor.variant_for_adt(adt);
 924             let def_variant = adt.variant_of_def(def);
 925             if variant.did == def_variant.did {
 926                 Some(variant.fields.iter().map(|sf| {
 927                     match pattern_fields.iter().find(|f| f.node.name == sf.name) {
 928                         Some(ref f) => &*f.node.pat,
 929                         _ => DUMMY_WILD_PAT
 930                     }
 931                 }).collect())
 932             } else {
 933                 None
 934             }
 935         }
 936
 937         PatKind::Tup(ref args) =>
 938             Some(args.iter().map(|p| &**p).collect()),
 939
 940         PatKind::Box(ref inner) | PatKind::Ref(ref inner, _) =>
 941             Some(vec![&**inner]),
 942
 943         PatKind::Lit(ref expr) => {
 944             let expr_value = eval_const_expr(cx.tcx, &expr);
 945             match range_covered_by_constructor(constructor, &expr_value, &expr_value) {
 946                 Some(true) => Some(vec![]),
 947                 Some(false) => None,
 948                 None => {
 949                     span_err!(cx.tcx.sess, pat_span, E0298, "mismatched types between arms");
 950                     None
 951                 }
 952             }
 953         }
 954
 955         PatKind::Range(ref from, ref to) => {
 956             let from_value = eval_const_expr(cx.tcx, &from);
 957             let to_value = eval_const_expr(cx.tcx, &to);
 958             match range_covered_by_constructor(constructor, &from_value, &to_value) {
 959                 Some(true) => Some(vec![]),
 960                 Some(false) => None,
 961                 None => {
 962                     span_err!(cx.tcx.sess, pat_span, E0299, "mismatched types between arms");
 963                     None
 964                 }
 965             }
 966         }
 967
 968         PatKind::Vec(ref before, ref slice, ref after) => {
 969             match *constructor {
 970                 // Fixed-length vectors.
 971                 Single => {
 972                     let mut pats: Vec<&Pat> = before.iter().map(|p| &**p).collect();
 973                     pats.extend(repeat(DUMMY_WILD_PAT).take(arity - before.len() - after.len()));
 974                     pats.extend(after.iter().map(|p| &**p));
 975                     Some(pats)
 976                 },
 977                 Slice(length) if before.len() + after.len() <= length && slice.is_some() => {
 978                     let mut pats: Vec<&Pat> = before.iter().map(|p| &**p).collect();
 979                     pats.extend(repeat(DUMMY_WILD_PAT).take(arity - before.len() - after.len()));
 980                     pats.extend(after.iter().map(|p| &**p));
 981                     Some(pats)
 982                 },
 983                 Slice(length) if before.len() + after.len() == length => {
 984                     let mut pats: Vec<&Pat> = before.iter().map(|p| &**p).collect();
 985                     pats.extend(after.iter().map(|p| &**p));
 986                     Some(pats)
 987                 },
 988                 SliceWithSubslice(prefix, suffix)
 989                     if before.len() == prefix
 990                         && after.len() == suffix
 991                         && slice.is_some() => {
 992                     let mut pats: Vec<&Pat> = before.iter().map(|p| &**p).collect();
 993                     pats.extend(after.iter().map(|p| &**p));
 994                     Some(pats)
 995                 }
 996                 _ => None
 997             }
 998         }
 999     };
1000     head.map(|mut head| {
1001         head.extend_from_slice(&r[..col]);
1002         head.extend_from_slice(&r[col + 1..]);
1003         head
1004     })
1005 }
1006
1007 fn check_local(cx: &mut MatchCheckCtxt, loc: &hir::Local) {
1008     intravisit::walk_local(cx, loc);
1009
1010     let pat = StaticInliner::new(cx.tcx, None).fold_pat(loc.pat.clone());
1011     check_irrefutable(cx, &pat, false);
1012
1013     // Check legality of move bindings and `@` patterns.
1014     check_legality_of_move_bindings(cx, false, slice::ref_slice(&loc.pat));
1015     check_legality_of_bindings_in_at_patterns(cx, &loc.pat);
1016 }
1017
1018 fn check_fn(cx: &mut MatchCheckCtxt,
1019             kind: FnKind,
1020             decl: &hir::FnDecl,
1021             body: &hir::Block,
1022             sp: Span,
1023             fn_id: NodeId) {
1024     match kind {
1025         FnKind::Closure(_) => {}
1026         _ => cx.param_env = ParameterEnvironment::for_item(cx.tcx, fn_id),
1027     }
1028
1029     intravisit::walk_fn(cx, kind, decl, body, sp);
1030
1031     for input in &decl.inputs {
1032         check_irrefutable(cx, &input.pat, true);
1033         check_legality_of_move_bindings(cx, false, slice::ref_slice(&input.pat));
1034         check_legality_of_bindings_in_at_patterns(cx, &input.pat);
1035     }
1036 }
1037
1038 fn check_irrefutable(cx: &MatchCheckCtxt, pat: &Pat, is_fn_arg: bool) {
1039     let origin = if is_fn_arg {
1040         "function argument"
1041     } else {
1042         "local binding"
1043     };
1044
1045     is_refutable(cx, pat, |uncovered_pat| {
1046         span_err!(cx.tcx.sess, pat.span, E0005,
1047             "refutable pattern in {}: `{}` not covered",
1048             origin,
1049             pat_to_string(uncovered_pat),
1050         );
1051     });
1052 }
1053
1054 fn is_refutable<A, F>(cx: &MatchCheckCtxt, pat: &Pat, refutable: F) -> Option<A> where
1055     F: FnOnce(&Pat) -> A,
1056 {
1057     let pats = Matrix(vec!(vec!(pat)));
1058     match is_useful(cx, &pats, &[DUMMY_WILD_PAT], ConstructWitness) {
1059         UsefulWithWitness(pats) => Some(refutable(&pats[0])),
1060         NotUseful => None,
1061         Useful => bug!()
1062     }
1063 }
1064
1065 // Legality of move bindings checking
1066 fn check_legality_of_move_bindings(cx: &MatchCheckCtxt,
1067                                    has_guard: bool,
1068                                    pats: &[P<Pat>]) {
1069     let tcx = cx.tcx;
1070     let def_map = &tcx.def_map;
1071     let mut by_ref_span = None;
1072     for pat in pats {
1073         pat_bindings(def_map, &pat, |bm, _, span, _path| {
1074             match bm {
1075                 hir::BindByRef(_) => {
1076                     by_ref_span = Some(span);
1077                 }
1078                 hir::BindByValue(_) => {
1079                 }
1080             }
1081         })
1082     }
1083
1084     let check_move = |p: &Pat, sub: Option<&Pat>| {
1085         // check legality of moving out of the enum
1086
1087         // x @ Foo(..) is legal, but x @ Foo(y) isn't.
1088         if sub.map_or(false, |p| pat_contains_bindings(&def_map.borrow(), &p)) {
1089             span_err!(cx.tcx.sess, p.span, E0007, "cannot bind by-move with sub-bindings");
1090         } else if has_guard {
1091             span_err!(cx.tcx.sess, p.span, E0008, "cannot bind by-move into a pattern guard");
1092         } else if by_ref_span.is_some() {
1093             let mut err = struct_span_err!(cx.tcx.sess, p.span, E0009,
1094                                            "cannot bind by-move and by-ref in the same pattern");
1095             span_note!(&mut err, by_ref_span.unwrap(), "by-ref binding occurs here");
1096             err.emit();
1097         }
1098     };
1099
1100     for pat in pats {
1101         pat.walk(|p| {
1102             if pat_is_binding(&def_map.borrow(), &p) {
1103                 match p.node {
1104                     PatKind::Ident(hir::BindByValue(_), _, ref sub) => {
1105                         let pat_ty = tcx.node_id_to_type(p.id);
1106                         //FIXME: (@jroesch) this code should be floated up as well
1107                         let infcx = infer::new_infer_ctxt(cx.tcx,
1108                                                           &cx.tcx.tables,
1109                                                           Some(cx.param_env.clone()),
1110                                                           ProjectionMode::AnyFinal);
1111                         if infcx.type_moves_by_default(pat_ty, pat.span) {
1112                             check_move(p, sub.as_ref().map(|p| &**p));
1113                         }
1114                     }
1115                     PatKind::Ident(hir::BindByRef(_), _, _) => {
1116                     }
1117                     _ => {
1118                         span_bug!(
1119                             p.span,
1120                             "binding pattern {} is not an identifier: {:?}",
1121                             p.id,
1122                             p.node);
1123                     }
1124                 }
1125             }
1126             true
1127         });
1128     }
1129 }
1130
1131 /// Ensures that a pattern guard doesn't borrow by mutable reference or
1132 /// assign.
1133 fn check_for_mutation_in_guard<'a, 'tcx>(cx: &'a MatchCheckCtxt<'a, 'tcx>,
1134                                          guard: &hir::Expr) {
1135     let mut checker = MutationChecker {
1136         cx: cx,
1137     };
1138
1139     let infcx = infer::new_infer_ctxt(cx.tcx,
1140                                       &cx.tcx.tables,
1141                                       Some(checker.cx.param_env.clone()),
1142                                       ProjectionMode::AnyFinal);
1143
1144     let mut visitor = ExprUseVisitor::new(&mut checker, &infcx);
1145     visitor.walk_expr(guard);
1146 }
1147
1148 struct MutationChecker<'a, 'tcx: 'a> {
1149     cx: &'a MatchCheckCtxt<'a, 'tcx>,
1150 }
1151
1152 impl<'a, 'tcx> Delegate<'tcx> for MutationChecker<'a, 'tcx> {
1153     fn matched_pat(&mut self, _: &Pat, _: cmt, _: euv::MatchMode) {}
1154     fn consume(&mut self, _: NodeId, _: Span, _: cmt, _: ConsumeMode) {}
1155     fn consume_pat(&mut self, _: &Pat, _: cmt, _: ConsumeMode) {}
1156     fn borrow(&mut self,
1157               _: NodeId,
1158               span: Span,
1159               _: cmt,
1160               _: Region,
1161               kind: BorrowKind,
1162               _: LoanCause) {
1163         match kind {
1164             MutBorrow => {
1165                 span_err!(self.cx.tcx.sess, span, E0301,
1166                           "cannot mutably borrow in a pattern guard")
1167             }
1168             ImmBorrow | UniqueImmBorrow => {}
1169         }
1170     }
1171     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
1172     fn mutate(&mut self, _: NodeId, span: Span, _: cmt, mode: MutateMode) {
1173         match mode {
1174             MutateMode::JustWrite | MutateMode::WriteAndRead => {
1175                 span_err!(self.cx.tcx.sess, span, E0302, "cannot assign in a pattern guard")
1176             }
1177             MutateMode::Init => {}
1178         }
1179     }
1180 }
1181
1182 /// Forbids bindings in `@` patterns. This is necessary for memory safety,
1183 /// because of the way rvalues are handled in the borrow check. (See issue
1184 /// #14587.)
1185 fn check_legality_of_bindings_in_at_patterns(cx: &MatchCheckCtxt, pat: &Pat) {
1186     AtBindingPatternVisitor { cx: cx, bindings_allowed: true }.visit_pat(pat);
1187 }
1188
1189 struct AtBindingPatternVisitor<'a, 'b:'a, 'tcx:'b> {
1190     cx: &'a MatchCheckCtxt<'b, 'tcx>,
1191     bindings_allowed: bool
1192 }
1193
1194 impl<'a, 'b, 'tcx, 'v> Visitor<'v> for AtBindingPatternVisitor<'a, 'b, 'tcx> {
1195     fn visit_pat(&mut self, pat: &Pat) {
1196         if !self.bindings_allowed && pat_is_binding(&self.cx.tcx.def_map.borrow(), pat) {
1197             span_err!(self.cx.tcx.sess, pat.span, E0303,
1198                                       "pattern bindings are not allowed \
1199                                        after an `@`");
1200         }
1201
1202         match pat.node {
1203             PatKind::Ident(_, _, Some(_)) => {
1204                 let bindings_were_allowed = self.bindings_allowed;
1205                 self.bindings_allowed = false;
1206                 intravisit::walk_pat(self, pat);
1207                 self.bindings_allowed = bindings_were_allowed;
1208             }
1209             _ => intravisit::walk_pat(self, pat),
1210         }
1211     }
1212 }