src/librustc_mir/build/matches/test.rs

   1 // Copyright 2015 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 // Testing candidates
  12 //
  13 // After candidates have been simplified, the only match pairs that
  14 // remain are those that require some sort of test. The functions here
  15 // identify what tests are needed, perform the tests, and then filter
  16 // the candidates based on the result.
  17
  18 use build::Builder;
  19 use build::matches::{Candidate, MatchPair, Test, TestKind};
  20 use hair::*;
  21 use rustc_data_structures::fx::FxHashMap;
  22 use rustc_data_structures::bitvec::BitVector;
  23 use rustc::middle::const_val::ConstVal;
  24 use rustc::ty::{self, Ty};
  25 use rustc::mir::*;
  26 use rustc::hir::RangeEnd;
  27 use syntax_pos::Span;
  28 use std::cmp::Ordering;
  29
  30 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
  31     /// Identifies what test is needed to decide if `match_pair` is applicable.
  32     ///
  33     /// It is a bug to call this with a simplifyable pattern.
  34     pub fn test<'pat>(&mut self, match_pair: &MatchPair<'pat, 'tcx>) -> Test<'tcx> {
  35         match *match_pair.pattern.kind {
  36             PatternKind::Variant { ref adt_def, substs: _, variant_index: _, subpatterns: _ } => {
  37                 Test {
  38                     span: match_pair.pattern.span,
  39                     kind: TestKind::Switch {
  40                         adt_def: adt_def.clone(),
  41                         variants: BitVector::new(self.hir.num_variants(adt_def)),
  42                     },
  43                 }
  44             }
  45
  46             PatternKind::Constant { .. }
  47             if is_switch_ty(match_pair.pattern.ty) => {
  48                 // for integers, we use a SwitchInt match, which allows
  49                 // us to handle more cases
  50                 Test {
  51                     span: match_pair.pattern.span,
  52                     kind: TestKind::SwitchInt {
  53                         switch_ty: match_pair.pattern.ty,
  54
  55                         // these maps are empty to start; cases are
  56                         // added below in add_cases_to_switch
  57                         options: vec![],
  58                         indices: FxHashMap(),
  59                     }
  60                 }
  61             }
  62
  63             PatternKind::Constant { ref value } => {
  64                 Test {
  65                     span: match_pair.pattern.span,
  66                     kind: TestKind::Eq {
  67                         value: value.clone(),
  68                         ty: match_pair.pattern.ty.clone()
  69                     }
  70                 }
  71             }
  72
  73             PatternKind::Range { ref lo, ref hi, ref end } => {
  74                 Test {
  75                     span: match_pair.pattern.span,
  76                     kind: TestKind::Range {
  77                         lo: Literal::Value { value: lo.clone() },
  78                         hi: Literal::Value { value: hi.clone() },
  79                         ty: match_pair.pattern.ty.clone(),
  80                         end: end.clone(),
  81                     },
  82                 }
  83             }
  84
  85             PatternKind::Slice { ref prefix, ref slice, ref suffix }
  86                     if !match_pair.slice_len_checked => {
  87                 let len = prefix.len() + suffix.len();
  88                 let op = if slice.is_some() {
  89                     BinOp::Ge
  90                 } else {
  91                     BinOp::Eq
  92                 };
  93                 Test {
  94                     span: match_pair.pattern.span,
  95                     kind: TestKind::Len { len: len as u64, op: op },
  96                 }
  97             }
  98
  99             PatternKind::Array { .. } |
 100             PatternKind::Slice { .. } |
 101             PatternKind::Wild |
 102             PatternKind::Binding { .. } |
 103             PatternKind::Leaf { .. } |
 104             PatternKind::Deref { .. } => {
 105                 self.error_simplifyable(match_pair)
 106             }
 107         }
 108     }
 109
 110     pub fn add_cases_to_switch<'pat>(&mut self,
 111                                      test_lvalue: &Lvalue<'tcx>,
 112                                      candidate: &Candidate<'pat, 'tcx>,
 113                                      switch_ty: Ty<'tcx>,
 114                                      options: &mut Vec<ConstVal>,
 115                                      indices: &mut FxHashMap<ConstVal, usize>)
 116                                      -> bool
 117     {
 118         let match_pair = match candidate.match_pairs.iter().find(|mp| mp.lvalue == *test_lvalue) {
 119             Some(match_pair) => match_pair,
 120             _ => { return false; }
 121         };
 122
 123         match *match_pair.pattern.kind {
 124             PatternKind::Constant { ref value } => {
 125                 // if the lvalues match, the type should match
 126                 assert_eq!(match_pair.pattern.ty, switch_ty);
 127
 128                 indices.entry(value.clone())
 129                        .or_insert_with(|| {
 130                            options.push(value.clone());
 131                            options.len() - 1
 132                        });
 133                 true
 134             }
 135             PatternKind::Variant { .. } => {
 136                 panic!("you should have called add_variants_to_switch instead!");
 137             }
 138             PatternKind::Range { .. } |
 139             PatternKind::Slice { .. } |
 140             PatternKind::Array { .. } |
 141             PatternKind::Wild |
 142             PatternKind::Binding { .. } |
 143             PatternKind::Leaf { .. } |
 144             PatternKind::Deref { .. } => {
 145                 // don't know how to add these patterns to a switch
 146                 false
 147             }
 148         }
 149     }
 150
 151     pub fn add_variants_to_switch<'pat>(&mut self,
 152                                         test_lvalue: &Lvalue<'tcx>,
 153                                         candidate: &Candidate<'pat, 'tcx>,
 154                                         variants: &mut BitVector)
 155                                         -> bool
 156     {
 157         let match_pair = match candidate.match_pairs.iter().find(|mp| mp.lvalue == *test_lvalue) {
 158             Some(match_pair) => match_pair,
 159             _ => { return false; }
 160         };
 161
 162         match *match_pair.pattern.kind {
 163             PatternKind::Variant { adt_def: _ , variant_index,  .. } => {
 164                 // We have a pattern testing for variant `variant_index`
 165                 // set the corresponding index to true
 166                 variants.insert(variant_index);
 167                 true
 168             }
 169             _ => {
 170                 // don't know how to add these patterns to a switch
 171                 false
 172             }
 173         }
 174     }
 175
 176     /// Generates the code to perform a test.
 177     pub fn perform_test(&mut self,
 178                         block: BasicBlock,
 179                         lvalue: &Lvalue<'tcx>,
 180                         test: &Test<'tcx>)
 181                         -> Vec<BasicBlock> {
 182         let source_info = self.source_info(test.span);
 183         match test.kind {
 184             TestKind::Switch { adt_def, ref variants } => {
 185                 let num_enum_variants = self.hir.num_variants(adt_def);
 186                 let mut otherwise_block = None;
 187                 let target_blocks: Vec<_> = (0..num_enum_variants).map(|i| {
 188                     if variants.contains(i) {
 189                         self.cfg.start_new_block()
 190                     } else {
 191                         if otherwise_block.is_none() {
 192                             otherwise_block = Some(self.cfg.start_new_block());
 193                         }
 194                         otherwise_block.unwrap()
 195                     }
 196                 }).collect();
 197                 debug!("num_enum_variants: {}, num tested variants: {}, variants: {:?}",
 198                        num_enum_variants, variants.iter().count(), variants);
 199                 self.cfg.terminate(block, source_info, TerminatorKind::Switch {
 200                     discr: lvalue.clone(),
 201                     adt_def: adt_def,
 202                     targets: target_blocks.clone()
 203                 });
 204                 target_blocks
 205             }
 206
 207             TestKind::SwitchInt { switch_ty, ref options, indices: _ } => {
 208                 let (targets, term) = match switch_ty.sty {
 209                     // If we're matching on boolean we can
 210                     // use the If TerminatorKind instead
 211                     ty::TyBool => {
 212                         assert!(options.len() > 0 && options.len() <= 2);
 213
 214                         let (true_bb, else_bb) =
 215                             (self.cfg.start_new_block(),
 216                              self.cfg.start_new_block());
 217
 218                         let targets = match &options[0] {
 219                             &ConstVal::Bool(true) => vec![true_bb, else_bb],
 220                             &ConstVal::Bool(false) => vec![else_bb, true_bb],
 221                             v => span_bug!(test.span, "expected boolean value but got {:?}", v)
 222                         };
 223
 224                         (targets,
 225                          TerminatorKind::If {
 226                              cond: Operand::Consume(lvalue.clone()),
 227                              targets: (true_bb, else_bb)
 228                          })
 229
 230                     }
 231                     _ => {
 232                         // The switch may be inexhaustive so we
 233                         // add a catch all block
 234                         let otherwise = self.cfg.start_new_block();
 235                         let targets: Vec<_> =
 236                             options.iter()
 237                                    .map(|_| self.cfg.start_new_block())
 238                                    .chain(Some(otherwise))
 239                                    .collect();
 240
 241                         (targets.clone(),
 242                          TerminatorKind::SwitchInt {
 243                              discr: lvalue.clone(),
 244                              switch_ty: switch_ty,
 245                              values: options.clone(),
 246                              targets: targets
 247                          })
 248                     }
 249                 };
 250
 251                 self.cfg.terminate(block, source_info, term);
 252                 targets
 253             }
 254
 255             TestKind::Eq { ref value, mut ty } => {
 256                 let mut val = Operand::Consume(lvalue.clone());
 257
 258                 // If we're using b"..." as a pattern, we need to insert an
 259                 // unsizing coercion, as the byte string has the type &[u8; N].
 260                 let expect = if let ConstVal::ByteStr(ref bytes) = *value {
 261                     let tcx = self.hir.tcx();
 262
 263                     // Unsize the lvalue to &[u8], too, if necessary.
 264                     if let ty::TyRef(region, mt) = ty.sty {
 265                         if let ty::TyArray(_, _) = mt.ty.sty {
 266                             ty = tcx.mk_imm_ref(region, tcx.mk_slice(tcx.types.u8));
 267                             let val_slice = self.temp(ty);
 268                             self.cfg.push_assign(block, source_info, &val_slice,
 269                                                  Rvalue::Cast(CastKind::Unsize, val, ty));
 270                             val = Operand::Consume(val_slice);
 271                         }
 272                     }
 273
 274                     assert!(ty.is_slice());
 275
 276                     let array_ty = tcx.mk_array(tcx.types.u8, bytes.len());
 277                     let array_ref = tcx.mk_imm_ref(tcx.mk_region(ty::ReStatic), array_ty);
 278                     let array = self.literal_operand(test.span, array_ref, Literal::Value {
 279                         value: value.clone()
 280                     });
 281
 282                     let slice = self.temp(ty);
 283                     self.cfg.push_assign(block, source_info, &slice,
 284                                          Rvalue::Cast(CastKind::Unsize, array, ty));
 285                     Operand::Consume(slice)
 286                 } else {
 287                     self.literal_operand(test.span, ty, Literal::Value {
 288                         value: value.clone()
 289                     })
 290                 };
 291
 292                 // Use PartialEq::eq for &str and &[u8] slices, instead of BinOp::Eq.
 293                 let fail = self.cfg.start_new_block();
 294                 if let ty::TyRef(_, mt) = ty.sty {
 295                     assert!(ty.is_slice());
 296                     let eq_def_id = self.hir.tcx().lang_items.eq_trait().unwrap();
 297                     let ty = mt.ty;
 298                     let (mty, method) = self.hir.trait_method(eq_def_id, "eq", ty, &[ty]);
 299
 300                     let bool_ty = self.hir.bool_ty();
 301                     let eq_result = self.temp(bool_ty);
 302                     let eq_block = self.cfg.start_new_block();
 303                     let cleanup = self.diverge_cleanup();
 304                     self.cfg.terminate(block, source_info, TerminatorKind::Call {
 305                         func: Operand::Constant(Constant {
 306                             span: test.span,
 307                             ty: mty,
 308                             literal: method
 309                         }),
 310                         args: vec![val, expect],
 311                         destination: Some((eq_result.clone(), eq_block)),
 312                         cleanup: cleanup,
 313                     });
 314
 315                     // check the result
 316                     let block = self.cfg.start_new_block();
 317                     self.cfg.terminate(eq_block, source_info, TerminatorKind::If {
 318                         cond: Operand::Consume(eq_result),
 319                         targets: (block, fail),
 320                     });
 321
 322                     vec![block, fail]
 323                 } else {
 324                     let block = self.compare(block, fail, test.span, BinOp::Eq, expect, val);
 325                     vec![block, fail]
 326                 }
 327             }
 328
 329             TestKind::Range { ref lo, ref hi, ty, ref end } => {
 330                 // Test `val` by computing `lo <= val && val <= hi`, using primitive comparisons.
 331                 let lo = self.literal_operand(test.span, ty.clone(), lo.clone());
 332                 let hi = self.literal_operand(test.span, ty.clone(), hi.clone());
 333                 let val = Operand::Consume(lvalue.clone());
 334
 335                 let fail = self.cfg.start_new_block();
 336                 let block = self.compare(block, fail, test.span, BinOp::Le, lo, val.clone());
 337                 let block = match *end {
 338                     RangeEnd::Included => self.compare(block, fail, test.span, BinOp::Le, val, hi),
 339                     RangeEnd::Excluded => self.compare(block, fail, test.span, BinOp::Lt, val, hi),
 340                 };
 341
 342                 vec![block, fail]
 343             }
 344
 345             TestKind::Len { len, op } => {
 346                 let (usize_ty, bool_ty) = (self.hir.usize_ty(), self.hir.bool_ty());
 347                 let (actual, result) = (self.temp(usize_ty), self.temp(bool_ty));
 348
 349                 // actual = len(lvalue)
 350                 self.cfg.push_assign(block, source_info,
 351                                      &actual, Rvalue::Len(lvalue.clone()));
 352
 353                 // expected = <N>
 354                 let expected = self.push_usize(block, source_info, len);
 355
 356                 // result = actual == expected OR result = actual < expected
 357                 self.cfg.push_assign(block, source_info, &result,
 358                                      Rvalue::BinaryOp(op,
 359                                                       Operand::Consume(actual),
 360                                                       Operand::Consume(expected)));
 361
 362                 // branch based on result
 363                 let target_blocks: Vec<_> = vec![self.cfg.start_new_block(),
 364                                                  self.cfg.start_new_block()];
 365                 self.cfg.terminate(block, source_info, TerminatorKind::If {
 366                     cond: Operand::Consume(result),
 367                     targets: (target_blocks[0], target_blocks[1])
 368                 });
 369
 370                 target_blocks
 371             }
 372         }
 373     }
 374
 375     fn compare(&mut self,
 376                block: BasicBlock,
 377                fail_block: BasicBlock,
 378                span: Span,
 379                op: BinOp,
 380                left: Operand<'tcx>,
 381                right: Operand<'tcx>) -> BasicBlock {
 382         let bool_ty = self.hir.bool_ty();
 383         let result = self.temp(bool_ty);
 384
 385         // result = op(left, right)
 386         let source_info = self.source_info(span);
 387         self.cfg.push_assign(block, source_info, &result,
 388                              Rvalue::BinaryOp(op, left, right));
 389
 390         // branch based on result
 391         let target_block = self.cfg.start_new_block();
 392         self.cfg.terminate(block, source_info, TerminatorKind::If {
 393             cond: Operand::Consume(result),
 394             targets: (target_block, fail_block)
 395         });
 396
 397         target_block
 398     }
 399
 400     /// Given that we are performing `test` against `test_lvalue`,
 401     /// this job sorts out what the status of `candidate` will be
 402     /// after the test. The `resulting_candidates` vector stores, for
 403     /// each possible outcome of `test`, a vector of the candidates
 404     /// that will result. This fn should add a (possibly modified)
 405     /// clone of candidate into `resulting_candidates` wherever
 406     /// appropriate.
 407     ///
 408     /// So, for example, if this candidate is `x @ Some(P0)` and the
 409     /// test is a variant test, then we would add `(x as Option).0 @
 410     /// P0` to the `resulting_candidates` entry corresponding to the
 411     /// variant `Some`.
 412     ///
 413     /// However, in some cases, the test may just not be relevant to
 414     /// candidate. For example, suppose we are testing whether `foo.x == 22`,
 415     /// but in one match arm we have `Foo { x: _, ... }`... in that case,
 416     /// the test for what value `x` has has no particular relevance
 417     /// to this candidate. In such cases, this function just returns false
 418     /// without doing anything. This is used by the overall `match_candidates`
 419     /// algorithm to structure the match as a whole. See `match_candidates` for
 420     /// more details.
 421     ///
 422     /// FIXME(#29623). In some cases, we have some tricky choices to
 423     /// make.  for example, if we are testing that `x == 22`, but the
 424     /// candidate is `x @ 13..55`, what should we do? In the event
 425     /// that the test is true, we know that the candidate applies, but
 426     /// in the event of false, we don't know that it *doesn't*
 427     /// apply. For now, we return false, indicate that the test does
 428     /// not apply to this candidate, but it might be we can get
 429     /// tighter match code if we do something a bit different.
 430     pub fn sort_candidate<'pat>(&mut self,
 431                                 test_lvalue: &Lvalue<'tcx>,
 432                                 test: &Test<'tcx>,
 433                                 candidate: &Candidate<'pat, 'tcx>,
 434                                 resulting_candidates: &mut [Vec<Candidate<'pat, 'tcx>>])
 435                                 -> bool {
 436         // Find the match_pair for this lvalue (if any). At present,
 437         // afaik, there can be at most one. (In the future, if we
 438         // adopted a more general `@` operator, there might be more
 439         // than one, but it'd be very unusual to have two sides that
 440         // both require tests; you'd expect one side to be simplified
 441         // away.)
 442         let tested_match_pair = candidate.match_pairs.iter()
 443                                                      .enumerate()
 444                                                      .filter(|&(_, mp)| mp.lvalue == *test_lvalue)
 445                                                      .next();
 446         let (match_pair_index, match_pair) = match tested_match_pair {
 447             Some(pair) => pair,
 448             None => {
 449                 // We are not testing this lvalue. Therefore, this
 450                 // candidate applies to ALL outcomes.
 451                 return false;
 452             }
 453         };
 454
 455         match (&test.kind, &*match_pair.pattern.kind) {
 456             // If we are performing a variant switch, then this
 457             // informs variant patterns, but nothing else.
 458             (&TestKind::Switch { adt_def: tested_adt_def, .. },
 459              &PatternKind::Variant { adt_def, variant_index, ref subpatterns, .. }) => {
 460                 assert_eq!(adt_def, tested_adt_def);
 461                 let new_candidate =
 462                     self.candidate_after_variant_switch(match_pair_index,
 463                                                         adt_def,
 464                                                         variant_index,
 465                                                         subpatterns,
 466                                                         candidate);
 467                 resulting_candidates[variant_index].push(new_candidate);
 468                 true
 469             }
 470             (&TestKind::Switch { .. }, _) => false,
 471
 472             // If we are performing a switch over integers, then this informs integer
 473             // equality, but nothing else.
 474             //
 475             // FIXME(#29623) we could use PatternKind::Range to rule
 476             // things out here, in some cases.
 477             (&TestKind::SwitchInt { switch_ty: _, options: _, ref indices },
 478              &PatternKind::Constant { ref value })
 479             if is_switch_ty(match_pair.pattern.ty) => {
 480                 let index = indices[value];
 481                 let new_candidate = self.candidate_without_match_pair(match_pair_index,
 482                                                                       candidate);
 483                 resulting_candidates[index].push(new_candidate);
 484                 true
 485             }
 486             (&TestKind::SwitchInt { .. }, _) => false,
 487
 488
 489             (&TestKind::Len { len: test_len, op: BinOp::Eq },
 490              &PatternKind::Slice { ref prefix, ref slice, ref suffix }) => {
 491                 let pat_len = (prefix.len() + suffix.len()) as u64;
 492                 match (test_len.cmp(&pat_len), slice) {
 493                     (Ordering::Equal, &None) => {
 494                         // on true, min_len = len = $actual_length,
 495                         // on false, len != $actual_length
 496                         resulting_candidates[0].push(
 497                             self.candidate_after_slice_test(match_pair_index,
 498                                                             candidate,
 499                                                             prefix,
 500                                                             slice.as_ref(),
 501                                                             suffix)
 502                         );
 503                         true
 504                     }
 505                     (Ordering::Less, _) => {
 506                         // test_len < pat_len. If $actual_len = test_len,
 507                         // then $actual_len < pat_len and we don't have
 508                         // enough elements.
 509                         resulting_candidates[1].push(candidate.clone());
 510                         true
 511                     }
 512                     (Ordering::Equal, &Some(_)) | (Ordering::Greater, &Some(_)) => {
 513                         // This can match both if $actual_len = test_len >= pat_len,
 514                         // and if $actual_len > test_len. We can't advance.
 515                         false
 516                     }
 517                     (Ordering::Greater, &None) => {
 518                         // test_len != pat_len, so if $actual_len = test_len, then
 519                         // $actual_len != pat_len.
 520                         resulting_candidates[1].push(candidate.clone());
 521                         true
 522                     }
 523                 }
 524             }
 525
 526             (&TestKind::Len { len: test_len, op: BinOp::Ge },
 527              &PatternKind::Slice { ref prefix, ref slice, ref suffix }) => {
 528                 // the test is `$actual_len >= test_len`
 529                 let pat_len = (prefix.len() + suffix.len()) as u64;
 530                 match (test_len.cmp(&pat_len), slice) {
 531                     (Ordering::Equal, &Some(_))  => {
 532                         // $actual_len >= test_len = pat_len,
 533                         // so we can match.
 534                         resulting_candidates[0].push(
 535                             self.candidate_after_slice_test(match_pair_index,
 536                                                             candidate,
 537                                                             prefix,
 538                                                             slice.as_ref(),
 539                                                             suffix)
 540                         );
 541                         true
 542                     }
 543                     (Ordering::Less, _) | (Ordering::Equal, &None) => {
 544                         // test_len <= pat_len. If $actual_len < test_len,
 545                         // then it is also < pat_len, so the test passing is
 546                         // necessary (but insufficient).
 547                         resulting_candidates[0].push(candidate.clone());
 548                         true
 549                     }
 550                     (Ordering::Greater, &None) => {
 551                         // test_len > pat_len. If $actual_len >= test_len > pat_len,
 552                         // then we know we won't have a match.
 553                         resulting_candidates[1].push(candidate.clone());
 554                         true
 555                     }
 556                     (Ordering::Greater, &Some(_)) => {
 557                         // test_len < pat_len, and is therefore less
 558                         // strict. This can still go both ways.
 559                         false
 560                     }
 561                 }
 562             }
 563
 564             (&TestKind::Eq { .. }, _) |
 565             (&TestKind::Range { .. }, _) |
 566             (&TestKind::Len { .. }, _) => {
 567                 // These are all binary tests.
 568                 //
 569                 // FIXME(#29623) we can be more clever here
 570                 let pattern_test = self.test(&match_pair);
 571                 if pattern_test.kind == test.kind {
 572                     let new_candidate = self.candidate_without_match_pair(match_pair_index,
 573                                                                           candidate);
 574                     resulting_candidates[0].push(new_candidate);
 575                     true
 576                 } else {
 577                     false
 578                 }
 579             }
 580         }
 581     }
 582
 583     fn candidate_without_match_pair<'pat>(&mut self,
 584                                           match_pair_index: usize,
 585                                           candidate: &Candidate<'pat, 'tcx>)
 586                                           -> Candidate<'pat, 'tcx> {
 587         let other_match_pairs =
 588             candidate.match_pairs.iter()
 589                                  .enumerate()
 590                                  .filter(|&(index, _)| index != match_pair_index)
 591                                  .map(|(_, mp)| mp.clone())
 592                                  .collect();
 593         Candidate {
 594             span: candidate.span,
 595             match_pairs: other_match_pairs,
 596             bindings: candidate.bindings.clone(),
 597             guard: candidate.guard.clone(),
 598             arm_index: candidate.arm_index,
 599         }
 600     }
 601
 602     fn candidate_after_slice_test<'pat>(&mut self,
 603                                         match_pair_index: usize,
 604                                         candidate: &Candidate<'pat, 'tcx>,
 605                                         prefix: &'pat [Pattern<'tcx>],
 606                                         opt_slice: Option<&'pat Pattern<'tcx>>,
 607                                         suffix: &'pat [Pattern<'tcx>])
 608                                         -> Candidate<'pat, 'tcx> {
 609         let mut new_candidate =
 610             self.candidate_without_match_pair(match_pair_index, candidate);
 611         self.prefix_slice_suffix(
 612             &mut new_candidate.match_pairs,
 613             &candidate.match_pairs[match_pair_index].lvalue,
 614             prefix,
 615             opt_slice,
 616             suffix);
 617
 618         new_candidate
 619     }
 620
 621     fn candidate_after_variant_switch<'pat>(&mut self,
 622                                             match_pair_index: usize,
 623                                             adt_def: &'tcx ty::AdtDef,
 624                                             variant_index: usize,
 625                                             subpatterns: &'pat [FieldPattern<'tcx>],
 626                                             candidate: &Candidate<'pat, 'tcx>)
 627                                             -> Candidate<'pat, 'tcx> {
 628         let match_pair = &candidate.match_pairs[match_pair_index];
 629
 630         // So, if we have a match-pattern like `x @ Enum::Variant(P1, P2)`,
 631         // we want to create a set of derived match-patterns like
 632         // `(x as Variant).0 @ P1` and `(x as Variant).1 @ P1`.
 633         let elem = ProjectionElem::Downcast(adt_def, variant_index);
 634         let downcast_lvalue = match_pair.lvalue.clone().elem(elem); // `(x as Variant)`
 635         let consequent_match_pairs =
 636             subpatterns.iter()
 637                        .map(|subpattern| {
 638                            // e.g., `(x as Variant).0`
 639                            let lvalue = downcast_lvalue.clone().field(subpattern.field,
 640                                                                       subpattern.pattern.ty);
 641                            // e.g., `(x as Variant).0 @ P1`
 642                            MatchPair::new(lvalue, &subpattern.pattern)
 643                        });
 644
 645         // In addition, we need all the other match pairs from the old candidate.
 646         let other_match_pairs =
 647             candidate.match_pairs.iter()
 648                                  .enumerate()
 649                                  .filter(|&(index, _)| index != match_pair_index)
 650                                  .map(|(_, mp)| mp.clone());
 651
 652         let all_match_pairs = consequent_match_pairs.chain(other_match_pairs).collect();
 653
 654         Candidate {
 655             span: candidate.span,
 656             match_pairs: all_match_pairs,
 657             bindings: candidate.bindings.clone(),
 658             guard: candidate.guard.clone(),
 659             arm_index: candidate.arm_index,
 660         }
 661     }
 662
 663     fn error_simplifyable<'pat>(&mut self, match_pair: &MatchPair<'pat, 'tcx>) -> ! {
 664         span_bug!(match_pair.pattern.span,
 665                   "simplifyable pattern found: {:?}",
 666                   match_pair.pattern)
 667     }
 668 }
 669
 670 fn is_switch_ty<'tcx>(ty: Ty<'tcx>) -> bool {
 671     ty.is_integral() || ty.is_char() || ty.is_bool()
 672 }