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