src/librustc/middle/check_match.rs

   1 // Copyright 2012-2014 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 dep_graph::DepNode;
  16 use middle::const_eval::{compare_const_vals, ConstVal};
  17 use middle::const_eval::{eval_const_expr, eval_const_expr_partial};
  18 use middle::const_eval::{const_expr_to_pat, lookup_const_by_id};
  19 use middle::const_eval::EvalHint::ExprTypeChecked;
  20 use middle::def::*;
  21 use middle::def_id::{DefId};
  22 use middle::expr_use_visitor::{ConsumeMode, Delegate, ExprUseVisitor};
  23 use middle::expr_use_visitor::{LoanCause, MutateMode};
  24 use middle::expr_use_visitor as euv;
  25 use middle::infer;
  26 use middle::mem_categorization::{cmt};
  27 use middle::pat_util::*;
  28 use middle::ty::*;
  29 use middle::ty;
  30 use std::cmp::Ordering;
  31 use std::fmt;
  32 use std::iter::{FromIterator, IntoIterator, repeat};
  33
  34 use rustc_front::hir;
  35 use rustc_front::hir::{Pat, PatKind};
  36 use rustc_front::intravisit::{self, Visitor, FnKind};
  37 use rustc_front::util as front_util;
  38 use rustc_back::slice;
  39
  40 use syntax::ast::{self, DUMMY_NODE_ID, NodeId};
  41 use syntax::ast_util;
  42 use syntax::codemap::{Span, Spanned, DUMMY_SP};
  43 use rustc_front::fold::{Folder, noop_fold_pat};
  44 use rustc_front::print::pprust::pat_to_string;
  45 use syntax::ptr::P;
  46 use util::nodemap::FnvHashMap;
  47
  48 pub const DUMMY_WILD_PAT: &'static Pat = &Pat {
  49     id: DUMMY_NODE_ID,
  50     node: PatKind::Wild,
  51     span: DUMMY_SP
  52 };
  53
  54 struct Matrix<'a>(Vec<Vec<&'a Pat>>);
  55
  56 /// Pretty-printer for matrices of patterns, example:
  57 /// ++++++++++++++++++++++++++
  58 /// + _     + []             +
  59 /// ++++++++++++++++++++++++++
  60 /// + true  + [First]        +
  61 /// ++++++++++++++++++++++++++
  62 /// + true  + [Second(true)] +
  63 /// ++++++++++++++++++++++++++
  64 /// + false + [_]            +
  65 /// ++++++++++++++++++++++++++
  66 /// + _     + [_, _, ..tail] +
  67 /// ++++++++++++++++++++++++++
  68 impl<'a> fmt::Debug for Matrix<'a> {
  69     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
  70         try!(write!(f, "\n"));
  71
  72         let &Matrix(ref m) = self;
  73         let pretty_printed_matrix: Vec<Vec<String>> = m.iter().map(|row| {
  74             row.iter()
  75                .map(|&pat| pat_to_string(&pat))
  76                .collect::<Vec<String>>()
  77         }).collect();
  78
  79         let column_count = m.iter().map(|row| row.len()).max().unwrap_or(0);
  80         assert!(m.iter().all(|row| row.len() == column_count));
  81         let column_widths: Vec<usize> = (0..column_count).map(|col| {
  82             pretty_printed_matrix.iter().map(|row| row[col].len()).max().unwrap_or(0)
  83         }).collect();
  84
  85         let total_width = column_widths.iter().cloned().sum::<usize>() + column_count * 3 + 1;
  86         let br = repeat('+').take(total_width).collect::<String>();
  87         try!(write!(f, "{}\n", br));
  88         for row in pretty_printed_matrix {
  89             try!(write!(f, "+"));
  90             for (column, pat_str) in row.into_iter().enumerate() {
  91                 try!(write!(f, " "));
  92                 try!(write!(f, "{:1$}", pat_str, column_widths[column]));
  93                 try!(write!(f, " +"));
  94             }
  95             try!(write!(f, "\n"));
  96             try!(write!(f, "{}\n", br));
  97         }
  98         Ok(())
  99     }
 100 }
 101
 102 impl<'a> FromIterator<Vec<&'a Pat>> for Matrix<'a> {
 103     fn from_iter<T: IntoIterator<Item=Vec<&'a Pat>>>(iter: T) -> Matrix<'a> {
 104         Matrix(iter.into_iter().collect())
 105     }
 106 }
 107
 108 //NOTE: appears to be the only place other then InferCtxt to contain a ParamEnv
 109 pub struct MatchCheckCtxt<'a, 'tcx: 'a> {
 110     pub tcx: &'a ty::ctxt<'tcx>,
 111     pub param_env: ParameterEnvironment<'a, 'tcx>,
 112 }
 113
 114 #[derive(Clone, PartialEq)]
 115 pub enum Constructor {
 116     /// The constructor of all patterns that don't vary by constructor,
 117     /// e.g. struct patterns and fixed-length arrays.
 118     Single,
 119     /// Enum variants.
 120     Variant(DefId),
 121     /// Literal values.
 122     ConstantValue(ConstVal),
 123     /// Ranges of literal values (2..5).
 124     ConstantRange(ConstVal, ConstVal),
 125     /// Array patterns of length n.
 126     Slice(usize),
 127     /// Array patterns with a subslice.
 128     SliceWithSubslice(usize, usize)
 129 }
 130
 131 #[derive(Clone, PartialEq)]
 132 enum Usefulness {
 133     Useful,
 134     UsefulWithWitness(Vec<P<Pat>>),
 135     NotUseful
 136 }
 137
 138 #[derive(Copy, Clone)]
 139 enum WitnessPreference {
 140     ConstructWitness,
 141     LeaveOutWitness
 142 }
 143
 144 impl<'a, 'tcx, 'v> Visitor<'v> for MatchCheckCtxt<'a, 'tcx> {
 145     fn visit_expr(&mut self, ex: &hir::Expr) {
 146         check_expr(self, ex);
 147     }
 148     fn visit_local(&mut self, l: &hir::Local) {
 149         check_local(self, l);
 150     }
 151     fn visit_fn(&mut self, fk: FnKind<'v>, fd: &'v hir::FnDecl,
 152                 b: &'v hir::Block, s: Span, n: NodeId) {
 153         check_fn(self, fk, fd, b, s, n);
 154     }
 155 }
 156
 157 pub fn check_crate(tcx: &ty::ctxt) {
 158     tcx.visit_all_items_in_krate(DepNode::MatchCheck, &mut MatchCheckCtxt {
 159         tcx: tcx,
 160         param_env: tcx.empty_parameter_environment(),
 161     });
 162     tcx.sess.abort_if_errors();
 163 }
 164
 165 fn check_expr(cx: &mut MatchCheckCtxt, ex: &hir::Expr) {
 166     intravisit::walk_expr(cx, ex);
 167     match ex.node {
 168         hir::ExprMatch(ref scrut, ref arms, source) => {
 169             for arm in arms {
 170                 // First, check legality of move bindings.
 171                 check_legality_of_move_bindings(cx,
 172                                                 arm.guard.is_some(),
 173                                                 &arm.pats);
 174
 175                 // Second, if there is a guard on each arm, make sure it isn't
 176                 // assigning or borrowing anything mutably.
 177                 match arm.guard {
 178                     Some(ref guard) => check_for_mutation_in_guard(cx, &guard),
 179                     None => {}
 180                 }
 181             }
 182
 183             let mut static_inliner = StaticInliner::new(cx.tcx, None);
 184             let inlined_arms = arms.iter().map(|arm| {
 185                 (arm.pats.iter().map(|pat| {
 186                     static_inliner.fold_pat((*pat).clone())
 187                 }).collect(), arm.guard.as_ref().map(|e| &**e))
 188             }).collect::<Vec<(Vec<P<Pat>>, Option<&hir::Expr>)>>();
 189
 190             // Bail out early if inlining failed.
 191             if static_inliner.failed {
 192                 return;
 193             }
 194
 195             for pat in inlined_arms
 196                 .iter()
 197                 .flat_map(|&(ref pats, _)| pats) {
 198                 // Third, check legality of move bindings.
 199                 check_legality_of_bindings_in_at_patterns(cx, &pat);
 200
 201                 // Fourth, check if there are any references to NaN that we should warn about.
 202                 check_for_static_nan(cx, &pat);
 203
 204                 // Fifth, check if for any of the patterns that match an enumerated type
 205                 // are bindings with the same name as one of the variants of said type.
 206                 check_for_bindings_named_the_same_as_variants(cx, &pat);
 207             }
 208
 209             // Fourth, check for unreachable arms.
 210             check_arms(cx, &inlined_arms[..], source);
 211
 212             // Finally, check if the whole match expression is exhaustive.
 213             // Check for empty enum, because is_useful only works on inhabited types.
 214             let pat_ty = cx.tcx.node_id_to_type(scrut.id);
 215             if inlined_arms.is_empty() {
 216                 if !pat_ty.is_empty(cx.tcx) {
 217                     // We know the type is inhabited, so this must be wrong
 218                     let mut err = struct_span_err!(cx.tcx.sess, ex.span, E0002,
 219                                                    "non-exhaustive patterns: type {} is non-empty",
 220                                                    pat_ty);
 221                     span_help!(&mut err, ex.span,
 222                         "Please ensure that all possible cases are being handled; \
 223                          possibly adding wildcards or more match arms.");
 224                     err.emit();
 225                 }
 226                 // If the type *is* empty, it's vacuously exhaustive
 227                 return;
 228             }
 229
 230             let matrix: Matrix = inlined_arms
 231                 .iter()
 232                 .filter(|&&(_, guard)| guard.is_none())
 233                 .flat_map(|arm| &arm.0)
 234                 .map(|pat| vec![&**pat])
 235                 .collect();
 236             check_exhaustive(cx, ex.span, &matrix, source);
 237         },
 238         _ => ()
 239     }
 240 }
 241
 242 fn check_for_bindings_named_the_same_as_variants(cx: &MatchCheckCtxt, pat: &Pat) {
 243     front_util::walk_pat(pat, |p| {
 244         match p.node {
 245             PatKind::Ident(hir::BindByValue(hir::MutImmutable), ident, None) => {
 246                 let pat_ty = cx.tcx.pat_ty(p);
 247                 if let ty::TyEnum(edef, _) = pat_ty.sty {
 248                     let def = cx.tcx.def_map.borrow().get(&p.id).map(|d| d.full_def());
 249                     if let Some(Def::Local(..)) = def {
 250                         if edef.variants.iter().any(|variant|
 251                             variant.name == ident.node.unhygienic_name
 252                                 && variant.kind() == VariantKind::Unit
 253                         ) {
 254                             let ty_path = cx.tcx.item_path_str(edef.did);
 255                             let mut err = struct_span_warn!(cx.tcx.sess, p.span, E0170,
 256                                 "pattern binding `{}` is named the same as one \
 257                                  of the variants of the type `{}`",
 258                                 ident.node, ty_path);
 259                             fileline_help!(err, p.span,
 260                                 "if you meant to match on a variant, \
 261                                  consider making the path in the pattern qualified: `{}::{}`",
 262                                 ty_path, ident.node);
 263                             err.emit();
 264                         }
 265                     }
 266                 }
 267             }
 268             _ => ()
 269         }
 270         true
 271     });
 272 }
 273
 274 // Check that we do not match against a static NaN (#6804)
 275 fn check_for_static_nan(cx: &MatchCheckCtxt, pat: &Pat) {
 276     front_util::walk_pat(pat, |p| {
 277         if let PatKind::Lit(ref expr) = p.node {
 278             match eval_const_expr_partial(cx.tcx, &expr, ExprTypeChecked, None) {
 279                 Ok(ConstVal::Float(f)) if f.is_nan() => {
 280                     span_warn!(cx.tcx.sess, p.span, E0003,
 281                                "unmatchable NaN in pattern, \
 282                                 use the is_nan method in a guard instead");
 283                 }
 284                 Ok(_) => {}
 285
 286                 Err(err) => {
 287                     let mut diag = struct_span_err!(cx.tcx.sess, err.span, E0471,
 288                                                     "constant evaluation error: {}",
 289                                                     err.description());
 290                     if !p.span.contains(err.span) {
 291                         diag.span_note(p.span, "in pattern here");
 292                     }
 293                     diag.emit();
 294                 }
 295             }
 296         }
 297         true
 298     });
 299 }
 300
 301 // Check for unreachable patterns
 302 fn check_arms(cx: &MatchCheckCtxt,
 303               arms: &[(Vec<P<Pat>>, Option<&hir::Expr>)],
 304               source: hir::MatchSource) {
 305     let mut seen = Matrix(vec![]);
 306     let mut printed_if_let_err = false;
 307     for &(ref pats, guard) in arms {
 308         for pat in pats {
 309             let v = vec![&**pat];
 310
 311             match is_useful(cx, &seen, &v[..], LeaveOutWitness) {
 312                 NotUseful => {
 313                     match source {
 314                         hir::MatchSource::IfLetDesugar { .. } => {
 315                             if printed_if_let_err {
 316                                 // we already printed an irrefutable if-let pattern error.
 317                                 // We don't want two, that's just confusing.
 318                             } else {
 319                                 // find the first arm pattern so we can use its span
 320                                 let &(ref first_arm_pats, _) = &arms[0];
 321                                 let first_pat = &first_arm_pats[0];
 322                                 let span = first_pat.span;
 323                                 span_err!(cx.tcx.sess, span, E0162, "irrefutable if-let pattern");
 324                                 printed_if_let_err = true;
 325                             }
 326                         },
 327
 328                         hir::MatchSource::WhileLetDesugar => {
 329                             // find the first arm pattern so we can use its span
 330                             let &(ref first_arm_pats, _) = &arms[0];
 331                             let first_pat = &first_arm_pats[0];
 332                             let span = first_pat.span;
 333                             span_err!(cx.tcx.sess, span, E0165, "irrefutable while-let pattern");
 334                         },
 335
 336                         hir::MatchSource::ForLoopDesugar => {
 337                             // this is a bug, because on `match iter.next()` we cover
 338                             // `Some(<head>)` and `None`. It's impossible to have an unreachable
 339                             // pattern
 340                             // (see libsyntax/ext/expand.rs for the full expansion of a for loop)
 341                             cx.tcx.sess.span_bug(pat.span, "unreachable for-loop pattern")
 342                         },
 343
 344                         hir::MatchSource::Normal => {
 345                             span_err!(cx.tcx.sess, pat.span, E0001, "unreachable pattern")
 346                         },
 347                     }
 348                 }
 349                 Useful => (),
 350                 UsefulWithWitness(_) => unreachable!()
 351             }
 352             if guard.is_none() {
 353                 let Matrix(mut rows) = seen;
 354                 rows.push(v);
 355                 seen = Matrix(rows);
 356             }
 357         }
 358     }
 359 }
 360
 361 fn raw_pat<'a>(p: &'a Pat) -> &'a Pat {
 362     match p.node {
 363         PatKind::Ident(_, _, Some(ref s)) => raw_pat(&s),
 364         _ => p
 365     }
 366 }
 367
 368 fn check_exhaustive(cx: &MatchCheckCtxt, sp: Span, matrix: &Matrix, source: hir::MatchSource) {
 369     match is_useful(cx, matrix, &[DUMMY_WILD_PAT], ConstructWitness) {
 370         UsefulWithWitness(pats) => {
 371             let witnesses = if pats.is_empty() {
 372                 vec![DUMMY_WILD_PAT]
 373             } else {
 374                 pats.iter().map(|w| &**w ).collect()
 375             };
 376             match source {
 377                 hir::MatchSource::ForLoopDesugar => {
 378                     // `witnesses[0]` has the form `Some(<head>)`, peel off the `Some`
 379                     let witness = match witnesses[0].node {
 380                         PatKind::TupleStruct(_, Some(ref pats)) => match &pats[..] {
 381                             [ref pat] => &**pat,
 382                             _ => unreachable!(),
 383                         },
 384                         _ => unreachable!(),
 385                     };
 386                     span_err!(cx.tcx.sess, sp, E0297,
 387                         "refutable pattern in `for` loop binding: \
 388                                 `{}` not covered",
 389                                 pat_to_string(witness));
 390                 },
 391                 _ => {
 392                     let pattern_strings: Vec<_> = witnesses.iter().map(|w| {
 393                         pat_to_string(w)
 394                     }).collect();
 395                     const LIMIT: usize = 3;
 396                     let joined_patterns = match pattern_strings.len() {
 397                         0 => unreachable!(),
 398                         1 => format!("`{}`", pattern_strings[0]),
 399                         2...LIMIT => {
 400                             let (tail, head) = pattern_strings.split_last().unwrap();
 401                             format!("`{}`", head.join("`, `") + "` and `" + tail)
 402                         },
 403                         _ => {
 404                             let (head, tail) = pattern_strings.split_at(LIMIT);
 405                             format!("`{}` and {} more", head.join("`, `"), tail.len())
 406                         }
 407                     };
 408                     span_err!(cx.tcx.sess, sp, E0004,
 409                         "non-exhaustive patterns: {} not covered",
 410                         joined_patterns
 411                     );
 412                 },
 413             }
 414         }
 415         NotUseful => {
 416             // This is good, wildcard pattern isn't reachable
 417         },
 418         _ => unreachable!()
 419     }
 420 }
 421
 422 fn const_val_to_expr(value: &ConstVal) -> P<hir::Expr> {
 423     let node = match value {
 424         &ConstVal::Bool(b) => ast::LitKind::Bool(b),
 425         _ => unreachable!()
 426     };
 427     P(hir::Expr {
 428         id: 0,
 429         node: hir::ExprLit(P(Spanned { node: node, span: DUMMY_SP })),
 430         span: DUMMY_SP,
 431         attrs: None,
 432     })
 433 }
 434
 435 pub struct StaticInliner<'a, 'tcx: 'a> {
 436     pub tcx: &'a ty::ctxt<'tcx>,
 437     pub failed: bool,
 438     pub renaming_map: Option<&'a mut FnvHashMap<(NodeId, Span), NodeId>>,
 439 }
 440
 441 impl<'a, 'tcx> StaticInliner<'a, 'tcx> {
 442     pub fn new<'b>(tcx: &'b ty::ctxt<'tcx>,
 443                    renaming_map: Option<&'b mut FnvHashMap<(NodeId, Span), NodeId>>)
 444                    -> StaticInliner<'b, 'tcx> {
 445         StaticInliner {
 446             tcx: tcx,
 447             failed: false,
 448             renaming_map: renaming_map
 449         }
 450     }
 451 }
 452
 453 struct RenamingRecorder<'map> {
 454     substituted_node_id: NodeId,
 455     origin_span: Span,
 456     renaming_map: &'map mut FnvHashMap<(NodeId, Span), NodeId>
 457 }
 458
 459 impl<'map> ast_util::IdVisitingOperation for RenamingRecorder<'map> {
 460     fn visit_id(&mut self, node_id: NodeId) {
 461         let key = (node_id, self.origin_span);
 462         self.renaming_map.insert(key, self.substituted_node_id);
 463     }
 464 }
 465
 466 impl<'a, 'tcx> Folder for StaticInliner<'a, 'tcx> {
 467     fn fold_pat(&mut self, pat: P<Pat>) -> P<Pat> {
 468         return match pat.node {
 469             PatKind::Ident(..) | PatKind::Path(..) | PatKind::QPath(..) => {
 470                 let def = self.tcx.def_map.borrow().get(&pat.id).map(|d| d.full_def());
 471                 match def {
 472                     Some(Def::AssociatedConst(did)) |
 473                     Some(Def::Const(did)) => match lookup_const_by_id(self.tcx, did,
 474                                                                     Some(pat.id), None) {
 475                         Some(const_expr) => {
 476                             const_expr_to_pat(self.tcx, const_expr, pat.span).map(|new_pat| {
 477
 478                                 if let Some(ref mut renaming_map) = self.renaming_map {
 479                                     // Record any renamings we do here
 480                                     record_renamings(const_expr, &pat, renaming_map);
 481                                 }
 482
 483                                 new_pat
 484                             })
 485                         }
 486                         None => {
 487                             self.failed = true;
 488                             span_err!(self.tcx.sess, pat.span, E0158,
 489                                 "statics cannot be referenced in patterns");
 490                             pat
 491                         }
 492                     },
 493                     _ => noop_fold_pat(pat, self)
 494                 }
 495             }
 496             _ => noop_fold_pat(pat, self)
 497         };
 498
 499         fn record_renamings(const_expr: &hir::Expr,
 500                             substituted_pat: &hir::Pat,
 501                             renaming_map: &mut FnvHashMap<(NodeId, Span), NodeId>) {
 502             let mut renaming_recorder = RenamingRecorder {
 503                 substituted_node_id: substituted_pat.id,
 504                 origin_span: substituted_pat.span,
 505                 renaming_map: renaming_map,
 506             };
 507
 508             let mut id_visitor = front_util::IdVisitor::new(&mut renaming_recorder);
 509
 510             id_visitor.visit_expr(const_expr);
 511         }
 512     }
 513 }
 514
 515 /// Constructs a partial witness for a pattern given a list of
 516 /// patterns expanded by the specialization step.
 517 ///
 518 /// When a pattern P is discovered to be useful, this function is used bottom-up
 519 /// to reconstruct a complete witness, e.g. a pattern P' that covers a subset
 520 /// of values, V, where each value in that set is not covered by any previously
 521 /// used patterns and is covered by the pattern P'. Examples:
 522 ///
 523 /// left_ty: tuple of 3 elements
 524 /// pats: [10, 20, _]           => (10, 20, _)
 525 ///
 526 /// left_ty: struct X { a: (bool, &'static str), b: usize}
 527 /// pats: [(false, "foo"), 42]  => X { a: (false, "foo"), b: 42 }
 528 fn construct_witness<'a,'tcx>(cx: &MatchCheckCtxt<'a,'tcx>, ctor: &Constructor,
 529                               pats: Vec<&Pat>, left_ty: Ty<'tcx>) -> P<Pat> {
 530     let pats_len = pats.len();
 531     let mut pats = pats.into_iter().map(|p| P((*p).clone()));
 532     let pat = match left_ty.sty {
 533         ty::TyTuple(_) => PatKind::Tup(pats.collect()),
 534
 535         ty::TyEnum(adt, _) | ty::TyStruct(adt, _)  => {
 536             let v = adt.variant_of_ctor(ctor);
 537             match v.kind() {
 538                 VariantKind::Struct => {
 539                     let field_pats: hir::HirVec<_> = v.fields.iter()
 540                         .zip(pats)
 541                         .filter(|&(_, ref pat)| pat.node != PatKind::Wild)
 542                         .map(|(field, pat)| Spanned {
 543                             span: DUMMY_SP,
 544                             node: hir::FieldPat {
 545                                 name: field.name,
 546                                 pat: pat,
 547                                 is_shorthand: false,
 548                             }
 549                         }).collect();
 550                     let has_more_fields = field_pats.len() < pats_len;
 551                     PatKind::Struct(def_to_path(cx.tcx, v.did), field_pats, has_more_fields)
 552                 }
 553                 VariantKind::Tuple => {
 554                     PatKind::TupleStruct(def_to_path(cx.tcx, v.did), Some(pats.collect()))
 555                 }
 556                 VariantKind::Unit => {
 557                     PatKind::Path(def_to_path(cx.tcx, v.did))
 558                 }
 559             }
 560         }
 561
 562         ty::TyRef(_, ty::TypeAndMut { ty, mutbl }) => {
 563             match ty.sty {
 564                ty::TyArray(_, n) => match ctor {
 565                     &Single => {
 566                         assert_eq!(pats_len, n);
 567                         PatKind::Vec(pats.collect(), None, hir::HirVec::new())
 568                     },
 569                     _ => unreachable!()
 570                 },
 571                 ty::TySlice(_) => match ctor {
 572                     &Slice(n) => {
 573                         assert_eq!(pats_len, n);
 574                         PatKind::Vec(pats.collect(), None, hir::HirVec::new())
 575                     },
 576                     _ => unreachable!()
 577                 },
 578                 ty::TyStr => PatKind::Wild,
 579
 580                 _ => {
 581                     assert_eq!(pats_len, 1);
 582                     PatKind::Ref(pats.nth(0).unwrap(), mutbl)
 583                 }
 584             }
 585         }
 586
 587         ty::TyArray(_, len) => {
 588             assert_eq!(pats_len, len);
 589             PatKind::Vec(pats.collect(), None, hir::HirVec::new())
 590         }
 591
 592         _ => {
 593             match *ctor {
 594                 ConstantValue(ref v) => PatKind::Lit(const_val_to_expr(v)),
 595                 _ => PatKind::Wild,
 596             }
 597         }
 598     };
 599
 600     P(hir::Pat {
 601         id: 0,
 602         node: pat,
 603         span: DUMMY_SP
 604     })
 605 }
 606
 607 impl<'tcx, 'container> ty::AdtDefData<'tcx, 'container> {
 608     fn variant_of_ctor(&self,
 609                        ctor: &Constructor)
 610                        -> &VariantDefData<'tcx, 'container> {
 611         match ctor {
 612             &Variant(vid) => self.variant_with_id(vid),
 613             _ => self.struct_variant()
 614         }
 615     }
 616 }
 617
 618 fn missing_constructors(cx: &MatchCheckCtxt, &Matrix(ref rows): &Matrix,
 619                        left_ty: Ty, max_slice_length: usize) -> Vec<Constructor> {
 620     let used_constructors: Vec<Constructor> = rows.iter()
 621         .flat_map(|row| pat_constructors(cx, row[0], left_ty, max_slice_length))
 622         .collect();
 623     all_constructors(cx, left_ty, max_slice_length)
 624         .into_iter()
 625         .filter(|c| !used_constructors.contains(c))
 626         .collect()
 627 }
 628
 629 /// This determines the set of all possible constructors of a pattern matching
 630 /// values of type `left_ty`. For vectors, this would normally be an infinite set
 631 /// but is instead bounded by the maximum fixed length of slice patterns in
 632 /// the column of patterns being analyzed.
 633 fn all_constructors(_cx: &MatchCheckCtxt, left_ty: Ty,
 634                     max_slice_length: usize) -> Vec<Constructor> {
 635     match left_ty.sty {
 636         ty::TyBool =>
 637             [true, false].iter().map(|b| ConstantValue(ConstVal::Bool(*b))).collect(),
 638
 639         ty::TyRef(_, ty::TypeAndMut { ty, .. }) => match ty.sty {
 640             ty::TySlice(_) =>
 641                 (0..max_slice_length+1).map(|length| Slice(length)).collect(),
 642             _ => vec![Single]
 643         },
 644
 645         ty::TyEnum(def, _) => def.variants.iter().map(|v| Variant(v.did)).collect(),
 646         _ => vec![Single]
 647     }
 648 }
 649
 650 // Algorithm from http://moscova.inria.fr/~maranget/papers/warn/index.html
 651 //
 652 // Whether a vector `v` of patterns is 'useful' in relation to a set of such
 653 // vectors `m` is defined as there being a set of inputs that will match `v`
 654 // but not any of the sets in `m`.
 655 //
 656 // This is used both for reachability checking (if a pattern isn't useful in
 657 // relation to preceding patterns, it is not reachable) and exhaustiveness
 658 // checking (if a wildcard pattern is useful in relation to a matrix, the
 659 // matrix isn't exhaustive).
 660
 661 // Note: is_useful doesn't work on empty types, as the paper notes.
 662 // So it assumes that v is non-empty.
 663 fn is_useful(cx: &MatchCheckCtxt,
 664              matrix: &Matrix,
 665              v: &[&Pat],
 666              witness: WitnessPreference)
 667              -> Usefulness {
 668     let &Matrix(ref rows) = matrix;
 669     debug!("{:?}", matrix);
 670     if rows.is_empty() {
 671         return match witness {
 672             ConstructWitness => UsefulWithWitness(vec!()),
 673             LeaveOutWitness => Useful
 674         };
 675     }
 676     if rows[0].is_empty() {
 677         return NotUseful;
 678     }
 679     assert!(rows.iter().all(|r| r.len() == v.len()));
 680     let real_pat = match rows.iter().find(|r| (*r)[0].id != DUMMY_NODE_ID) {
 681         Some(r) => raw_pat(r[0]),
 682         None if v.is_empty() => return NotUseful,
 683         None => v[0]
 684     };
 685     let left_ty = if real_pat.id == DUMMY_NODE_ID {
 686         cx.tcx.mk_nil()
 687     } else {
 688         let left_ty = cx.tcx.pat_ty(&real_pat);
 689
 690         match real_pat.node {
 691             PatKind::Ident(hir::BindByRef(..), _, _) => {
 692                 left_ty.builtin_deref(false, NoPreference).unwrap().ty
 693             }
 694             _ => left_ty,
 695         }
 696     };
 697
 698     let max_slice_length = rows.iter().filter_map(|row| match row[0].node {
 699         PatKind::Vec(ref before, _, ref after) => Some(before.len() + after.len()),
 700         _ => None
 701     }).max().map_or(0, |v| v + 1);
 702
 703     let constructors = pat_constructors(cx, v[0], left_ty, max_slice_length);
 704     if constructors.is_empty() {
 705         let constructors = missing_constructors(cx, matrix, left_ty, max_slice_length);
 706         if constructors.is_empty() {
 707             all_constructors(cx, left_ty, max_slice_length).into_iter().map(|c| {
 708                 match is_useful_specialized(cx, matrix, v, c.clone(), left_ty, witness) {
 709                     UsefulWithWitness(pats) => UsefulWithWitness({
 710                         let arity = constructor_arity(cx, &c, left_ty);
 711                         let mut result = {
 712                             let pat_slice = &pats[..];
 713                             let subpats: Vec<_> = (0..arity).map(|i| {
 714                                 pat_slice.get(i).map_or(DUMMY_WILD_PAT, |p| &**p)
 715                             }).collect();
 716                             vec![construct_witness(cx, &c, subpats, left_ty)]
 717                         };
 718                         result.extend(pats.into_iter().skip(arity));
 719                         result
 720                     }),
 721                     result => result
 722                 }
 723             }).find(|result| result != &NotUseful).unwrap_or(NotUseful)
 724         } else {
 725             let matrix = rows.iter().filter_map(|r| {
 726                 if pat_is_binding_or_wild(&cx.tcx.def_map.borrow(), raw_pat(r[0])) {
 727                     Some(r[1..].to_vec())
 728                 } else {
 729                     None
 730                 }
 731             }).collect();
 732             match is_useful(cx, &matrix, &v[1..], witness) {
 733                 UsefulWithWitness(pats) => {
 734                     let mut new_pats: Vec<_> = constructors.into_iter().map(|constructor| {
 735                         let arity = constructor_arity(cx, &constructor, left_ty);
 736                         let wild_pats = vec![DUMMY_WILD_PAT; arity];
 737                         construct_witness(cx, &constructor, wild_pats, left_ty)
 738                     }).collect();
 739                     new_pats.extend(pats);
 740                     UsefulWithWitness(new_pats)
 741                 },
 742                 result => result
 743             }
 744         }
 745     } else {
 746         constructors.into_iter().map(|c|
 747             is_useful_specialized(cx, matrix, v, c.clone(), left_ty, witness)
 748         ).find(|result| result != &NotUseful).unwrap_or(NotUseful)
 749     }
 750 }
 751
 752 fn is_useful_specialized(cx: &MatchCheckCtxt, &Matrix(ref m): &Matrix,
 753                          v: &[&Pat], ctor: Constructor, lty: Ty,
 754                          witness: WitnessPreference) -> Usefulness {
 755     let arity = constructor_arity(cx, &ctor, lty);
 756     let matrix = Matrix(m.iter().filter_map(|r| {
 757         specialize(cx, &r[..], &ctor, 0, arity)
 758     }).collect());
 759     match specialize(cx, v, &ctor, 0, arity) {
 760         Some(v) => is_useful(cx, &matrix, &v[..], witness),
 761         None => NotUseful
 762     }
 763 }
 764
 765 /// Determines the constructors that the given pattern can be specialized to.
 766 ///
 767 /// In most cases, there's only one constructor that a specific pattern
 768 /// represents, such as a specific enum variant or a specific literal value.
 769 /// Slice patterns, however, can match slices of different lengths. For instance,
 770 /// `[a, b, ..tail]` can match a slice of length 2, 3, 4 and so on.
 771 ///
 772 /// On the other hand, a wild pattern and an identifier pattern cannot be
 773 /// specialized in any way.
 774 fn pat_constructors(cx: &MatchCheckCtxt, p: &Pat,
 775                     left_ty: Ty, max_slice_length: usize) -> Vec<Constructor> {
 776     let pat = raw_pat(p);
 777     match pat.node {
 778         PatKind::Struct(..) | PatKind::TupleStruct(..) | PatKind::Path(..) | PatKind::Ident(..) =>
 779             match cx.tcx.def_map.borrow().get(&pat.id).unwrap().full_def() {
 780                 Def::Const(..) | Def::AssociatedConst(..) =>
 781                     cx.tcx.sess.span_bug(pat.span, "const pattern should've \
 782                                                     been rewritten"),
 783                 Def::Struct(..) | Def::TyAlias(..) => vec![Single],
 784                 Def::Variant(_, id) => vec![Variant(id)],
 785                 Def::Local(..) => vec![],
 786                 def => cx.tcx.sess.span_bug(pat.span, &format!("pat_constructors: unexpected \
 787                                                                 definition {:?}", def)),
 788             },
 789         PatKind::QPath(..) =>
 790             cx.tcx.sess.span_bug(pat.span, "const pattern should've \
 791                                             been rewritten"),
 792         PatKind::Lit(ref expr) =>
 793             vec!(ConstantValue(eval_const_expr(cx.tcx, &expr))),
 794         PatKind::Range(ref lo, ref hi) =>
 795             vec!(ConstantRange(eval_const_expr(cx.tcx, &lo), eval_const_expr(cx.tcx, &hi))),
 796         PatKind::Vec(ref before, ref slice, ref after) =>
 797             match left_ty.sty {
 798                 ty::TyArray(_, _) => vec!(Single),
 799                 _                      => if slice.is_some() {
 800                     (before.len() + after.len()..max_slice_length+1)
 801                         .map(|length| Slice(length))
 802                         .collect()
 803                 } else {
 804                     vec!(Slice(before.len() + after.len()))
 805                 }
 806             },
 807         PatKind::Box(_) | PatKind::Tup(_) | PatKind::Ref(..) =>
 808             vec!(Single),
 809         PatKind::Wild =>
 810             vec!(),
 811     }
 812 }
 813
 814 /// This computes the arity of a constructor. The arity of a constructor
 815 /// is how many subpattern patterns of that constructor should be expanded to.
 816 ///
 817 /// For instance, a tuple pattern (_, 42, Some([])) has the arity of 3.
 818 /// A struct pattern's arity is the number of fields it contains, etc.
 819 pub fn constructor_arity(_cx: &MatchCheckCtxt, ctor: &Constructor, ty: Ty) -> usize {
 820     match ty.sty {
 821         ty::TyTuple(ref fs) => fs.len(),
 822         ty::TyBox(_) => 1,
 823         ty::TyRef(_, ty::TypeAndMut { ty, .. }) => match ty.sty {
 824             ty::TySlice(_) => match *ctor {
 825                 Slice(length) => length,
 826                 ConstantValue(_) => 0,
 827                 _ => unreachable!()
 828             },
 829             ty::TyStr => 0,
 830             _ => 1
 831         },
 832         ty::TyEnum(adt, _) | ty::TyStruct(adt, _) => {
 833             adt.variant_of_ctor(ctor).fields.len()
 834         }
 835         ty::TyArray(_, n) => n,
 836         _ => 0
 837     }
 838 }
 839
 840 fn range_covered_by_constructor(ctor: &Constructor,
 841                                 from: &ConstVal, to: &ConstVal) -> Option<bool> {
 842     let (c_from, c_to) = match *ctor {
 843         ConstantValue(ref value)        => (value, value),
 844         ConstantRange(ref from, ref to) => (from, to),
 845         Single                          => return Some(true),
 846         _                               => unreachable!()
 847     };
 848     let cmp_from = compare_const_vals(c_from, from);
 849     let cmp_to = compare_const_vals(c_to, to);
 850     match (cmp_from, cmp_to) {
 851         (Some(cmp_from), Some(cmp_to)) => {
 852             Some(cmp_from != Ordering::Less && cmp_to != Ordering::Greater)
 853         }
 854         _ => None
 855     }
 856 }
 857
 858 /// This is the main specialization step. It expands the first pattern in the given row
 859 /// into `arity` patterns based on the constructor. For most patterns, the step is trivial,
 860 /// for instance tuple patterns are flattened and box patterns expand into their inner pattern.
 861 ///
 862 /// OTOH, slice patterns with a subslice pattern (..tail) can be expanded into multiple
 863 /// different patterns.
 864 /// Structure patterns with a partial wild pattern (Foo { a: 42, .. }) have their missing
 865 /// fields filled with wild patterns.
 866 pub fn specialize<'a>(cx: &MatchCheckCtxt, r: &[&'a Pat],
 867                       constructor: &Constructor, col: usize, arity: usize) -> Option<Vec<&'a Pat>> {
 868     let &Pat {
 869         id: pat_id, ref node, span: pat_span
 870     } = raw_pat(r[col]);
 871     let head: Option<Vec<&Pat>> = match *node {
 872         PatKind::Wild =>
 873             Some(vec![DUMMY_WILD_PAT; arity]),
 874
 875         PatKind::Path(..) | PatKind::Ident(..) => {
 876             let def = cx.tcx.def_map.borrow().get(&pat_id).unwrap().full_def();
 877             match def {
 878                 Def::Const(..) | Def::AssociatedConst(..) =>
 879                     cx.tcx.sess.span_bug(pat_span, "const pattern should've \
 880                                                     been rewritten"),
 881                 Def::Variant(_, id) if *constructor != Variant(id) => None,
 882                 Def::Variant(..) | Def::Struct(..) => Some(Vec::new()),
 883                 Def::Local(..) => Some(vec![DUMMY_WILD_PAT; arity]),
 884                 _ => cx.tcx.sess.span_bug(pat_span, &format!("specialize: unexpected \
 885                                                               definition {:?}", def)),
 886             }
 887         }
 888
 889         PatKind::TupleStruct(_, ref args) => {
 890             let def = cx.tcx.def_map.borrow().get(&pat_id).unwrap().full_def();
 891             match def {
 892                 Def::Const(..) | Def::AssociatedConst(..) =>
 893                     cx.tcx.sess.span_bug(pat_span, "const pattern should've \
 894                                                     been rewritten"),
 895                 Def::Variant(_, id) if *constructor != Variant(id) => None,
 896                 Def::Variant(..) | Def::Struct(..) => {
 897                     Some(match args {
 898                         &Some(ref args) => args.iter().map(|p| &**p).collect(),
 899                         &None => vec![DUMMY_WILD_PAT; arity],
 900                     })
 901                 }
 902                 _ => None
 903             }
 904         }
 905
 906         PatKind::QPath(_, _) => {
 907             cx.tcx.sess.span_bug(pat_span, "const pattern should've \
 908                                             been rewritten")
 909         }
 910
 911         PatKind::Struct(_, ref pattern_fields, _) => {
 912             let def = cx.tcx.def_map.borrow().get(&pat_id).unwrap().full_def();
 913             let adt = cx.tcx.node_id_to_type(pat_id).ty_adt_def().unwrap();
 914             let variant = adt.variant_of_ctor(constructor);
 915             let def_variant = adt.variant_of_def(def);
 916             if variant.did == def_variant.did {
 917                 Some(variant.fields.iter().map(|sf| {
 918                     match pattern_fields.iter().find(|f| f.node.name == sf.name) {
 919                         Some(ref f) => &*f.node.pat,
 920                         _ => DUMMY_WILD_PAT
 921                     }
 922                 }).collect())
 923             } else {
 924                 None
 925             }
 926         }
 927
 928         PatKind::Tup(ref args) =>
 929             Some(args.iter().map(|p| &**p).collect()),
 930
 931         PatKind::Box(ref inner) | PatKind::Ref(ref inner, _) =>
 932             Some(vec![&**inner]),
 933
 934         PatKind::Lit(ref expr) => {
 935             let expr_value = eval_const_expr(cx.tcx, &expr);
 936             match range_covered_by_constructor(constructor, &expr_value, &expr_value) {
 937                 Some(true) => Some(vec![]),
 938                 Some(false) => None,
 939                 None => {
 940                     span_err!(cx.tcx.sess, pat_span, E0298, "mismatched types between arms");
 941                     None
 942                 }
 943             }
 944         }
 945
 946         PatKind::Range(ref from, ref to) => {
 947             let from_value = eval_const_expr(cx.tcx, &from);
 948             let to_value = eval_const_expr(cx.tcx, &to);
 949             match range_covered_by_constructor(constructor, &from_value, &to_value) {
 950                 Some(true) => Some(vec![]),
 951                 Some(false) => None,
 952                 None => {
 953                     span_err!(cx.tcx.sess, pat_span, E0299, "mismatched types between arms");
 954                     None
 955                 }
 956             }
 957         }
 958
 959         PatKind::Vec(ref before, ref slice, ref after) => {
 960             match *constructor {
 961                 // Fixed-length vectors.
 962                 Single => {
 963                     let mut pats: Vec<&Pat> = before.iter().map(|p| &**p).collect();
 964                     pats.extend(repeat(DUMMY_WILD_PAT).take(arity - before.len() - after.len()));
 965                     pats.extend(after.iter().map(|p| &**p));
 966                     Some(pats)
 967                 },
 968                 Slice(length) if before.len() + after.len() <= length && slice.is_some() => {
 969                     let mut pats: Vec<&Pat> = before.iter().map(|p| &**p).collect();
 970                     pats.extend(repeat(DUMMY_WILD_PAT).take(arity - before.len() - after.len()));
 971                     pats.extend(after.iter().map(|p| &**p));
 972                     Some(pats)
 973                 },
 974                 Slice(length) if before.len() + after.len() == length => {
 975                     let mut pats: Vec<&Pat> = before.iter().map(|p| &**p).collect();
 976                     pats.extend(after.iter().map(|p| &**p));
 977                     Some(pats)
 978                 },
 979                 SliceWithSubslice(prefix, suffix)
 980                     if before.len() == prefix
 981                         && after.len() == suffix
 982                         && slice.is_some() => {
 983                     let mut pats: Vec<&Pat> = before.iter().map(|p| &**p).collect();
 984                     pats.extend(after.iter().map(|p| &**p));
 985                     Some(pats)
 986                 }
 987                 _ => None
 988             }
 989         }
 990     };
 991     head.map(|mut head| {
 992         head.extend_from_slice(&r[..col]);
 993         head.extend_from_slice(&r[col + 1..]);
 994         head
 995     })
 996 }
 997
 998 fn check_local(cx: &mut MatchCheckCtxt, loc: &hir::Local) {
 999     intravisit::walk_local(cx, loc);
1000
1001     let pat = StaticInliner::new(cx.tcx, None).fold_pat(loc.pat.clone());
1002     check_irrefutable(cx, &pat, false);
1003
1004     // Check legality of move bindings and `@` patterns.
1005     check_legality_of_move_bindings(cx, false, slice::ref_slice(&loc.pat));
1006     check_legality_of_bindings_in_at_patterns(cx, &loc.pat);
1007 }
1008
1009 fn check_fn(cx: &mut MatchCheckCtxt,
1010             kind: FnKind,
1011             decl: &hir::FnDecl,
1012             body: &hir::Block,
1013             sp: Span,
1014             fn_id: NodeId) {
1015     match kind {
1016         FnKind::Closure => {}
1017         _ => cx.param_env = ParameterEnvironment::for_item(cx.tcx, fn_id),
1018     }
1019
1020     intravisit::walk_fn(cx, kind, decl, body, sp);
1021
1022     for input in &decl.inputs {
1023         check_irrefutable(cx, &input.pat, true);
1024         check_legality_of_move_bindings(cx, false, slice::ref_slice(&input.pat));
1025         check_legality_of_bindings_in_at_patterns(cx, &input.pat);
1026     }
1027 }
1028
1029 fn check_irrefutable(cx: &MatchCheckCtxt, pat: &Pat, is_fn_arg: bool) {
1030     let origin = if is_fn_arg {
1031         "function argument"
1032     } else {
1033         "local binding"
1034     };
1035
1036     is_refutable(cx, pat, |uncovered_pat| {
1037         span_err!(cx.tcx.sess, pat.span, E0005,
1038             "refutable pattern in {}: `{}` not covered",
1039             origin,
1040             pat_to_string(uncovered_pat),
1041         );
1042     });
1043 }
1044
1045 fn is_refutable<A, F>(cx: &MatchCheckCtxt, pat: &Pat, refutable: F) -> Option<A> where
1046     F: FnOnce(&Pat) -> A,
1047 {
1048     let pats = Matrix(vec!(vec!(pat)));
1049     match is_useful(cx, &pats, &[DUMMY_WILD_PAT], ConstructWitness) {
1050         UsefulWithWitness(pats) => Some(refutable(&pats[0])),
1051         NotUseful => None,
1052         Useful => unreachable!()
1053     }
1054 }
1055
1056 // Legality of move bindings checking
1057 fn check_legality_of_move_bindings(cx: &MatchCheckCtxt,
1058                                    has_guard: bool,
1059                                    pats: &[P<Pat>]) {
1060     let tcx = cx.tcx;
1061     let def_map = &tcx.def_map;
1062     let mut by_ref_span = None;
1063     for pat in pats {
1064         pat_bindings(def_map, &pat, |bm, _, span, _path| {
1065             match bm {
1066                 hir::BindByRef(_) => {
1067                     by_ref_span = Some(span);
1068                 }
1069                 hir::BindByValue(_) => {
1070                 }
1071             }
1072         })
1073     }
1074
1075     let check_move = |p: &Pat, sub: Option<&Pat>| {
1076         // check legality of moving out of the enum
1077
1078         // x @ Foo(..) is legal, but x @ Foo(y) isn't.
1079         if sub.map_or(false, |p| pat_contains_bindings(&def_map.borrow(), &p)) {
1080             span_err!(cx.tcx.sess, p.span, E0007, "cannot bind by-move with sub-bindings");
1081         } else if has_guard {
1082             span_err!(cx.tcx.sess, p.span, E0008, "cannot bind by-move into a pattern guard");
1083         } else if by_ref_span.is_some() {
1084             let mut err = struct_span_err!(cx.tcx.sess, p.span, E0009,
1085                                            "cannot bind by-move and by-ref in the same pattern");
1086             span_note!(&mut err, by_ref_span.unwrap(), "by-ref binding occurs here");
1087             err.emit();
1088         }
1089     };
1090
1091     for pat in pats {
1092         front_util::walk_pat(&pat, |p| {
1093             if pat_is_binding(&def_map.borrow(), &p) {
1094                 match p.node {
1095                     PatKind::Ident(hir::BindByValue(_), _, ref sub) => {
1096                         let pat_ty = tcx.node_id_to_type(p.id);
1097                         //FIXME: (@jroesch) this code should be floated up as well
1098                         let infcx = infer::new_infer_ctxt(cx.tcx,
1099                                                           &cx.tcx.tables,
1100                                                           Some(cx.param_env.clone()));
1101                         if infcx.type_moves_by_default(pat_ty, pat.span) {
1102                             check_move(p, sub.as_ref().map(|p| &**p));
1103                         }
1104                     }
1105                     PatKind::Ident(hir::BindByRef(_), _, _) => {
1106                     }
1107                     _ => {
1108                         cx.tcx.sess.span_bug(
1109                             p.span,
1110                             &format!("binding pattern {} is not an \
1111                                      identifier: {:?}",
1112                                     p.id,
1113                                     p.node));
1114                     }
1115                 }
1116             }
1117             true
1118         });
1119     }
1120 }
1121
1122 /// Ensures that a pattern guard doesn't borrow by mutable reference or
1123 /// assign.
1124 fn check_for_mutation_in_guard<'a, 'tcx>(cx: &'a MatchCheckCtxt<'a, 'tcx>,
1125                                          guard: &hir::Expr) {
1126     let mut checker = MutationChecker {
1127         cx: cx,
1128     };
1129
1130     let infcx = infer::new_infer_ctxt(cx.tcx,
1131                                       &cx.tcx.tables,
1132                                       Some(checker.cx.param_env.clone()));
1133
1134     let mut visitor = ExprUseVisitor::new(&mut checker, &infcx);
1135     visitor.walk_expr(guard);
1136 }
1137
1138 struct MutationChecker<'a, 'tcx: 'a> {
1139     cx: &'a MatchCheckCtxt<'a, 'tcx>,
1140 }
1141
1142 impl<'a, 'tcx> Delegate<'tcx> for MutationChecker<'a, 'tcx> {
1143     fn matched_pat(&mut self, _: &Pat, _: cmt, _: euv::MatchMode) {}
1144     fn consume(&mut self, _: NodeId, _: Span, _: cmt, _: ConsumeMode) {}
1145     fn consume_pat(&mut self, _: &Pat, _: cmt, _: ConsumeMode) {}
1146     fn borrow(&mut self,
1147               _: NodeId,
1148               span: Span,
1149               _: cmt,
1150               _: Region,
1151               kind: BorrowKind,
1152               _: LoanCause) {
1153         match kind {
1154             MutBorrow => {
1155                 span_err!(self.cx.tcx.sess, span, E0301,
1156                           "cannot mutably borrow in a pattern guard")
1157             }
1158             ImmBorrow | UniqueImmBorrow => {}
1159         }
1160     }
1161     fn decl_without_init(&mut self, _: NodeId, _: Span) {}
1162     fn mutate(&mut self, _: NodeId, span: Span, _: cmt, mode: MutateMode) {
1163         match mode {
1164             MutateMode::JustWrite | MutateMode::WriteAndRead => {
1165                 span_err!(self.cx.tcx.sess, span, E0302, "cannot assign in a pattern guard")
1166             }
1167             MutateMode::Init => {}
1168         }
1169     }
1170 }
1171
1172 /// Forbids bindings in `@` patterns. This is necessary for memory safety,
1173 /// because of the way rvalues are handled in the borrow check. (See issue
1174 /// #14587.)
1175 fn check_legality_of_bindings_in_at_patterns(cx: &MatchCheckCtxt, pat: &Pat) {
1176     AtBindingPatternVisitor { cx: cx, bindings_allowed: true }.visit_pat(pat);
1177 }
1178
1179 struct AtBindingPatternVisitor<'a, 'b:'a, 'tcx:'b> {
1180     cx: &'a MatchCheckCtxt<'b, 'tcx>,
1181     bindings_allowed: bool
1182 }
1183
1184 impl<'a, 'b, 'tcx, 'v> Visitor<'v> for AtBindingPatternVisitor<'a, 'b, 'tcx> {
1185     fn visit_pat(&mut self, pat: &Pat) {
1186         if !self.bindings_allowed && pat_is_binding(&self.cx.tcx.def_map.borrow(), pat) {
1187             span_err!(self.cx.tcx.sess, pat.span, E0303,
1188                                       "pattern bindings are not allowed \
1189                                        after an `@`");
1190         }
1191
1192         match pat.node {
1193             PatKind::Ident(_, _, Some(_)) => {
1194                 let bindings_were_allowed = self.bindings_allowed;
1195                 self.bindings_allowed = false;
1196                 intravisit::walk_pat(self, pat);
1197                 self.bindings_allowed = bindings_were_allowed;
1198             }
1199             _ => intravisit::walk_pat(self, pat),
1200         }
1201     }
1202 }