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::ty::util::IntTypeExt;
  26 use rustc::mir::*;
  27 use rustc::hir::RangeEnd;
  28 use syntax_pos::Span;
  29 use std::cmp::Ordering;
  30
  31 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
  32     /// Identifies what test is needed to decide if `match_pair` is applicable.
  33     ///
  34     /// It is a bug to call this with a simplifyable pattern.
  35     pub fn test<'pat>(&mut self, match_pair: &MatchPair<'pat, 'tcx>) -> Test<'tcx> {
  36         match *match_pair.pattern.kind {
  37             PatternKind::Variant { ref adt_def, substs: _, variant_index: _, subpatterns: _ } => {
  38                 Test {
  39                     span: match_pair.pattern.span,
  40                     kind: TestKind::Switch {
  41                         adt_def: adt_def.clone(),
  42                         variants: BitVector::new(self.hir.num_variants(adt_def)),
  43                     },
  44                 }
  45             }
  46
  47             PatternKind::Constant { .. }
  48             if is_switch_ty(match_pair.pattern.ty) => {
  49                 // for integers, we use a SwitchInt match, which allows
  50                 // us to handle more cases
  51                 Test {
  52                     span: match_pair.pattern.span,
  53                     kind: TestKind::SwitchInt {
  54                         switch_ty: match_pair.pattern.ty,
  55
  56                         // these maps are empty to start; cases are
  57                         // added below in add_cases_to_switch
  58                         options: vec![],
  59                         indices: FxHashMap(),
  60                     }
  61                 }
  62             }
  63
  64             PatternKind::Constant { value } => {
  65                 Test {
  66                     span: match_pair.pattern.span,
  67                     kind: TestKind::Eq {
  68                         value,
  69                         ty: match_pair.pattern.ty.clone()
  70                     }
  71                 }
  72             }
  73
  74             PatternKind::Range { lo, hi, end } => {
  75                 Test {
  76                     span: match_pair.pattern.span,
  77                     kind: TestKind::Range {
  78                         lo: Literal::Value { value: lo },
  79                         hi: Literal::Value { value: hi },
  80                         ty: match_pair.pattern.ty.clone(),
  81                         end,
  82                     },
  83                 }
  84             }
  85
  86             PatternKind::Slice { ref prefix, ref slice, ref suffix }
  87                     if !match_pair.slice_len_checked => {
  88                 let len = prefix.len() + suffix.len();
  89                 let op = if slice.is_some() {
  90                     BinOp::Ge
  91                 } else {
  92                     BinOp::Eq
  93                 };
  94                 Test {
  95                     span: match_pair.pattern.span,
  96                     kind: TestKind::Len { len: len as u64, op: op },
  97                 }
  98             }
  99
 100             PatternKind::Array { .. } |
 101             PatternKind::Slice { .. } |
 102             PatternKind::Wild |
 103             PatternKind::Binding { .. } |
 104             PatternKind::Leaf { .. } |
 105             PatternKind::Deref { .. } => {
 106                 self.error_simplifyable(match_pair)
 107             }
 108         }
 109     }
 110
 111     pub fn add_cases_to_switch<'pat>(&mut self,
 112                                      test_lvalue: &Lvalue<'tcx>,
 113                                      candidate: &Candidate<'pat, 'tcx>,
 114                                      switch_ty: Ty<'tcx>,
 115                                      options: &mut Vec<&'tcx ty::Const<'tcx>>,
 116                                      indices: &mut FxHashMap<&'tcx ty::Const<'tcx>, usize>)
 117                                      -> bool
 118     {
 119         let match_pair = match candidate.match_pairs.iter().find(|mp| mp.lvalue == *test_lvalue) {
 120             Some(match_pair) => match_pair,
 121             _ => { return false; }
 122         };
 123
 124         match *match_pair.pattern.kind {
 125             PatternKind::Constant { value } => {
 126                 // if the lvalues match, the type should match
 127                 assert_eq!(match_pair.pattern.ty, switch_ty);
 128
 129                 indices.entry(value)
 130                        .or_insert_with(|| {
 131                            options.push(value);
 132                            options.len() - 1
 133                        });
 134                 true
 135             }
 136             PatternKind::Variant { .. } => {
 137                 panic!("you should have called add_variants_to_switch instead!");
 138             }
 139             PatternKind::Range { .. } |
 140             PatternKind::Slice { .. } |
 141             PatternKind::Array { .. } |
 142             PatternKind::Wild |
 143             PatternKind::Binding { .. } |
 144             PatternKind::Leaf { .. } |
 145             PatternKind::Deref { .. } => {
 146                 // don't know how to add these patterns to a switch
 147                 false
 148             }
 149         }
 150     }
 151
 152     pub fn add_variants_to_switch<'pat>(&mut self,
 153                                         test_lvalue: &Lvalue<'tcx>,
 154                                         candidate: &Candidate<'pat, 'tcx>,
 155                                         variants: &mut BitVector)
 156                                         -> bool
 157     {
 158         let match_pair = match candidate.match_pairs.iter().find(|mp| mp.lvalue == *test_lvalue) {
 159             Some(match_pair) => match_pair,
 160             _ => { return false; }
 161         };
 162
 163         match *match_pair.pattern.kind {
 164             PatternKind::Variant { adt_def: _ , variant_index,  .. } => {
 165                 // We have a pattern testing for variant `variant_index`
 166                 // set the corresponding index to true
 167                 variants.insert(variant_index);
 168                 true
 169             }
 170             _ => {
 171                 // don't know how to add these patterns to a switch
 172                 false
 173             }
 174         }
 175     }
 176
 177     /// Generates the code to perform a test.
 178     pub fn perform_test(&mut self,
 179                         block: BasicBlock,
 180                         lvalue: &Lvalue<'tcx>,
 181                         test: &Test<'tcx>)
 182                         -> Vec<BasicBlock> {
 183         let source_info = self.source_info(test.span);
 184         match test.kind {
 185             TestKind::Switch { adt_def, ref variants } => {
 186                 // Variants is a BitVec of indexes into adt_def.variants.
 187                 let num_enum_variants = self.hir.num_variants(adt_def);
 188                 let used_variants = variants.count();
 189                 let mut otherwise_block = None;
 190                 let mut target_blocks = Vec::with_capacity(num_enum_variants);
 191                 let mut targets = Vec::with_capacity(used_variants + 1);
 192                 let mut values = Vec::with_capacity(used_variants);
 193                 let tcx = self.hir.tcx();
 194                 for (idx, discr) in adt_def.discriminants(tcx).enumerate() {
 195                     target_blocks.place_back() <- if variants.contains(idx) {
 196                         values.push(discr);
 197                         *(targets.place_back() <- self.cfg.start_new_block())
 198                     } else {
 199                         if otherwise_block.is_none() {
 200                             otherwise_block = Some(self.cfg.start_new_block());
 201                         }
 202                         otherwise_block.unwrap()
 203                     };
 204                 }
 205                 if let Some(otherwise_block) = otherwise_block {
 206                     targets.push(otherwise_block);
 207                 } else {
 208                     targets.push(self.unreachable_block());
 209                 }
 210                 debug!("num_enum_variants: {}, tested variants: {:?}, variants: {:?}",
 211                        num_enum_variants, values, variants);
 212                 let discr_ty = adt_def.repr.discr_type().to_ty(tcx);
 213                 let discr = self.temp(discr_ty, test.span);
 214                 self.cfg.push_assign(block, source_info, &discr,
 215                                      Rvalue::Discriminant(lvalue.clone()));
 216                 assert_eq!(values.len() + 1, targets.len());
 217                 self.cfg.terminate(block, source_info, TerminatorKind::SwitchInt {
 218                     discr: Operand::Consume(discr),
 219                     switch_ty: discr_ty,
 220                     values: From::from(values),
 221                     targets,
 222                 });
 223                 target_blocks
 224             }
 225
 226             TestKind::SwitchInt { switch_ty, ref options, indices: _ } => {
 227                 let (ret, terminator) = if switch_ty.sty == ty::TyBool {
 228                     assert!(options.len() > 0 && options.len() <= 2);
 229                     let (true_bb, false_bb) = (self.cfg.start_new_block(),
 230                                                self.cfg.start_new_block());
 231                     let ret = match options[0].val {
 232                         ConstVal::Bool(true) => vec![true_bb, false_bb],
 233                         ConstVal::Bool(false) => vec![false_bb, true_bb],
 234                         v => span_bug!(test.span, "expected boolean value but got {:?}", v)
 235                     };
 236                     (ret, TerminatorKind::if_(self.hir.tcx(), Operand::Consume(lvalue.clone()),
 237                                               true_bb, false_bb))
 238                 } else {
 239                     // The switch may be inexhaustive so we
 240                     // add a catch all block
 241                     let otherwise = self.cfg.start_new_block();
 242                     let targets: Vec<_> =
 243                         options.iter()
 244                                .map(|_| self.cfg.start_new_block())
 245                                .chain(Some(otherwise))
 246                                .collect();
 247                     let values: Vec<_> = options.iter().map(|v|
 248                         v.val.to_const_int().expect("switching on integral")
 249                     ).collect();
 250                     (targets.clone(), TerminatorKind::SwitchInt {
 251                         discr: Operand::Consume(lvalue.clone()),
 252                         switch_ty,
 253                         values: From::from(values),
 254                         targets,
 255                     })
 256                 };
 257                 self.cfg.terminate(block, source_info, terminator);
 258                 ret
 259             }
 260
 261             TestKind::Eq { value, mut ty } => {
 262                 let mut val = Operand::Consume(lvalue.clone());
 263
 264                 // If we're using b"..." as a pattern, we need to insert an
 265                 // unsizing coercion, as the byte string has the type &[u8; N].
 266                 let expect = if let ConstVal::ByteStr(bytes) = value.val {
 267                     let tcx = self.hir.tcx();
 268
 269                     // Unsize the lvalue to &[u8], too, if necessary.
 270                     if let ty::TyRef(region, mt) = ty.sty {
 271                         if let ty::TyArray(_, _) = mt.ty.sty {
 272                             ty = tcx.mk_imm_ref(region, tcx.mk_slice(tcx.types.u8));
 273                             let val_slice = self.temp(ty, test.span);
 274                             self.cfg.push_assign(block, source_info, &val_slice,
 275                                                  Rvalue::Cast(CastKind::Unsize, val, ty));
 276                             val = Operand::Consume(val_slice);
 277                         }
 278                     }
 279
 280                     assert!(ty.is_slice());
 281
 282                     let array_ty = tcx.mk_array(tcx.types.u8, bytes.data.len() as u64);
 283                     let array_ref = tcx.mk_imm_ref(tcx.types.re_static, array_ty);
 284                     let array = self.literal_operand(test.span, array_ref, Literal::Value {
 285                         value
 286                     });
 287
 288                     let slice = self.temp(ty, test.span);
 289                     self.cfg.push_assign(block, source_info, &slice,
 290                                          Rvalue::Cast(CastKind::Unsize, array, ty));
 291                     Operand::Consume(slice)
 292                 } else {
 293                     self.literal_operand(test.span, ty, Literal::Value {
 294                         value
 295                     })
 296                 };
 297
 298                 // Use PartialEq::eq for &str and &[u8] slices, instead of BinOp::Eq.
 299                 let fail = self.cfg.start_new_block();
 300                 if let ty::TyRef(_, mt) = ty.sty {
 301                     assert!(ty.is_slice());
 302                     let eq_def_id = self.hir.tcx().lang_items().eq_trait().unwrap();
 303                     let ty = mt.ty;
 304                     let (mty, method) = self.hir.trait_method(eq_def_id, "eq", ty, &[ty]);
 305
 306                     let bool_ty = self.hir.bool_ty();
 307                     let eq_result = self.temp(bool_ty, test.span);
 308                     let eq_block = self.cfg.start_new_block();
 309                     let cleanup = self.diverge_cleanup();
 310                     self.cfg.terminate(block, source_info, TerminatorKind::Call {
 311                         func: Operand::Constant(box Constant {
 312                             span: test.span,
 313                             ty: mty,
 314                             literal: method
 315                         }),
 316                         args: vec![val, expect],
 317                         destination: Some((eq_result.clone(), eq_block)),
 318                         cleanup,
 319                     });
 320
 321                     // check the result
 322                     let block = self.cfg.start_new_block();
 323                     self.cfg.terminate(eq_block, source_info,
 324                                        TerminatorKind::if_(self.hir.tcx(),
 325                                                            Operand::Consume(eq_result),
 326                                                            block, fail));
 327                     vec![block, fail]
 328                 } else {
 329                     let block = self.compare(block, fail, test.span, BinOp::Eq, expect, val);
 330                     vec![block, fail]
 331                 }
 332             }
 333
 334             TestKind::Range { ref lo, ref hi, ty, ref end } => {
 335                 // Test `val` by computing `lo <= val && val <= hi`, using primitive comparisons.
 336                 let lo = self.literal_operand(test.span, ty.clone(), lo.clone());
 337                 let hi = self.literal_operand(test.span, ty.clone(), hi.clone());
 338                 let val = Operand::Consume(lvalue.clone());
 339
 340                 let fail = self.cfg.start_new_block();
 341                 let block = self.compare(block, fail, test.span, BinOp::Le, lo, val.clone());
 342                 let block = match *end {
 343                     RangeEnd::Included => self.compare(block, fail, test.span, BinOp::Le, val, hi),
 344                     RangeEnd::Excluded => self.compare(block, fail, test.span, BinOp::Lt, val, hi),
 345                 };
 346
 347                 vec![block, fail]
 348             }
 349
 350             TestKind::Len { len, op } => {
 351                 let (usize_ty, bool_ty) = (self.hir.usize_ty(), self.hir.bool_ty());
 352                 let (actual, result) = (self.temp(usize_ty, test.span),
 353                                         self.temp(bool_ty, test.span));
 354
 355                 // actual = len(lvalue)
 356                 self.cfg.push_assign(block, source_info,
 357                                      &actual, Rvalue::Len(lvalue.clone()));
 358
 359                 // expected = <N>
 360                 let expected = self.push_usize(block, source_info, len);
 361
 362                 // result = actual == expected OR result = actual < expected
 363                 self.cfg.push_assign(block, source_info, &result,
 364                                      Rvalue::BinaryOp(op,
 365                                                       Operand::Consume(actual),
 366                                                       Operand::Consume(expected)));
 367
 368                 // branch based on result
 369                 let (false_bb, true_bb) = (self.cfg.start_new_block(),
 370                                            self.cfg.start_new_block());
 371                 self.cfg.terminate(block, source_info,
 372                                    TerminatorKind::if_(self.hir.tcx(), Operand::Consume(result),
 373                                                        true_bb, false_bb));
 374                 vec![true_bb, false_bb]
 375             }
 376         }
 377     }
 378
 379     fn compare(&mut self,
 380                block: BasicBlock,
 381                fail_block: BasicBlock,
 382                span: Span,
 383                op: BinOp,
 384                left: Operand<'tcx>,
 385                right: Operand<'tcx>) -> BasicBlock {
 386         let bool_ty = self.hir.bool_ty();
 387         let result = self.temp(bool_ty, span);
 388
 389         // result = op(left, right)
 390         let source_info = self.source_info(span);
 391         self.cfg.push_assign(block, source_info, &result,
 392                              Rvalue::BinaryOp(op, left, right));
 393
 394         // branch based on result
 395         let target_block = self.cfg.start_new_block();
 396         self.cfg.terminate(block, source_info,
 397                            TerminatorKind::if_(self.hir.tcx(), Operand::Consume(result),
 398                                                target_block, fail_block));
 399         target_block
 400     }
 401
 402     /// Given that we are performing `test` against `test_lvalue`,
 403     /// this job sorts out what the status of `candidate` will be
 404     /// after the test. The `resulting_candidates` vector stores, for
 405     /// each possible outcome of `test`, a vector of the candidates
 406     /// that will result. This fn should add a (possibly modified)
 407     /// clone of candidate into `resulting_candidates` wherever
 408     /// appropriate.
 409     ///
 410     /// So, for example, if this candidate is `x @ Some(P0)` and the
 411     /// test is a variant test, then we would add `(x as Option).0 @
 412     /// P0` to the `resulting_candidates` entry corresponding to the
 413     /// variant `Some`.
 414     ///
 415     /// However, in some cases, the test may just not be relevant to
 416     /// candidate. For example, suppose we are testing whether `foo.x == 22`,
 417     /// but in one match arm we have `Foo { x: _, ... }`... in that case,
 418     /// the test for what value `x` has has no particular relevance
 419     /// to this candidate. In such cases, this function just returns false
 420     /// without doing anything. This is used by the overall `match_candidates`
 421     /// algorithm to structure the match as a whole. See `match_candidates` for
 422     /// more details.
 423     ///
 424     /// FIXME(#29623). In some cases, we have some tricky choices to
 425     /// make.  for example, if we are testing that `x == 22`, but the
 426     /// candidate is `x @ 13..55`, what should we do? In the event
 427     /// that the test is true, we know that the candidate applies, but
 428     /// in the event of false, we don't know that it *doesn't*
 429     /// apply. For now, we return false, indicate that the test does
 430     /// not apply to this candidate, but it might be we can get
 431     /// tighter match code if we do something a bit different.
 432     pub fn sort_candidate<'pat>(&mut self,
 433                                 test_lvalue: &Lvalue<'tcx>,
 434                                 test: &Test<'tcx>,
 435                                 candidate: &Candidate<'pat, 'tcx>,
 436                                 resulting_candidates: &mut [Vec<Candidate<'pat, 'tcx>>])
 437                                 -> bool {
 438         // Find the match_pair for this lvalue (if any). At present,
 439         // afaik, there can be at most one. (In the future, if we
 440         // adopted a more general `@` operator, there might be more
 441         // than one, but it'd be very unusual to have two sides that
 442         // both require tests; you'd expect one side to be simplified
 443         // away.)
 444         let tested_match_pair = candidate.match_pairs.iter()
 445                                                      .enumerate()
 446                                                      .filter(|&(_, mp)| mp.lvalue == *test_lvalue)
 447                                                      .next();
 448         let (match_pair_index, match_pair) = match tested_match_pair {
 449             Some(pair) => pair,
 450             None => {
 451                 // We are not testing this lvalue. Therefore, this
 452                 // candidate applies to ALL outcomes.
 453                 return false;
 454             }
 455         };
 456
 457         match (&test.kind, &*match_pair.pattern.kind) {
 458             // If we are performing a variant switch, then this
 459             // informs variant patterns, but nothing else.
 460             (&TestKind::Switch { adt_def: tested_adt_def, .. },
 461              &PatternKind::Variant { adt_def, variant_index, ref subpatterns, .. }) => {
 462                 assert_eq!(adt_def, tested_adt_def);
 463                 let new_candidate =
 464                     self.candidate_after_variant_switch(match_pair_index,
 465                                                         adt_def,
 466                                                         variant_index,
 467                                                         subpatterns,
 468                                                         candidate);
 469                 resulting_candidates[variant_index].push(new_candidate);
 470                 true
 471             }
 472             (&TestKind::Switch { .. }, _) => false,
 473
 474             // If we are performing a switch over integers, then this informs integer
 475             // equality, but nothing else.
 476             //
 477             // FIXME(#29623) we could use PatternKind::Range to rule
 478             // things out here, in some cases.
 479             (&TestKind::SwitchInt { switch_ty: _, options: _, ref indices },
 480              &PatternKind::Constant { ref value })
 481             if is_switch_ty(match_pair.pattern.ty) => {
 482                 let index = indices[value];
 483                 let new_candidate = self.candidate_without_match_pair(match_pair_index,
 484                                                                       candidate);
 485                 resulting_candidates[index].push(new_candidate);
 486                 true
 487             }
 488             (&TestKind::SwitchInt { .. }, _) => false,
 489
 490
 491             (&TestKind::Len { len: test_len, op: BinOp::Eq },
 492              &PatternKind::Slice { ref prefix, ref slice, ref suffix }) => {
 493                 let pat_len = (prefix.len() + suffix.len()) as u64;
 494                 match (test_len.cmp(&pat_len), slice) {
 495                     (Ordering::Equal, &None) => {
 496                         // on true, min_len = len = $actual_length,
 497                         // on false, len != $actual_length
 498                         resulting_candidates[0].push(
 499                             self.candidate_after_slice_test(match_pair_index,
 500                                                             candidate,
 501                                                             prefix,
 502                                                             slice.as_ref(),
 503                                                             suffix)
 504                         );
 505                         true
 506                     }
 507                     (Ordering::Less, _) => {
 508                         // test_len < pat_len. If $actual_len = test_len,
 509                         // then $actual_len < pat_len and we don't have
 510                         // enough elements.
 511                         resulting_candidates[1].push(candidate.clone());
 512                         true
 513                     }
 514                     (Ordering::Equal, &Some(_)) | (Ordering::Greater, &Some(_)) => {
 515                         // This can match both if $actual_len = test_len >= pat_len,
 516                         // and if $actual_len > test_len. We can't advance.
 517                         false
 518                     }
 519                     (Ordering::Greater, &None) => {
 520                         // test_len != pat_len, so if $actual_len = test_len, then
 521                         // $actual_len != pat_len.
 522                         resulting_candidates[1].push(candidate.clone());
 523                         true
 524                     }
 525                 }
 526             }
 527
 528             (&TestKind::Len { len: test_len, op: BinOp::Ge },
 529              &PatternKind::Slice { ref prefix, ref slice, ref suffix }) => {
 530                 // the test is `$actual_len >= test_len`
 531                 let pat_len = (prefix.len() + suffix.len()) as u64;
 532                 match (test_len.cmp(&pat_len), slice) {
 533                     (Ordering::Equal, &Some(_))  => {
 534                         // $actual_len >= test_len = pat_len,
 535                         // so we can match.
 536                         resulting_candidates[0].push(
 537                             self.candidate_after_slice_test(match_pair_index,
 538                                                             candidate,
 539                                                             prefix,
 540                                                             slice.as_ref(),
 541                                                             suffix)
 542                         );
 543                         true
 544                     }
 545                     (Ordering::Less, _) | (Ordering::Equal, &None) => {
 546                         // test_len <= pat_len. If $actual_len < test_len,
 547                         // then it is also < pat_len, so the test passing is
 548                         // necessary (but insufficient).
 549                         resulting_candidates[0].push(candidate.clone());
 550                         true
 551                     }
 552                     (Ordering::Greater, &None) => {
 553                         // test_len > pat_len. If $actual_len >= test_len > pat_len,
 554                         // then we know we won't have a match.
 555                         resulting_candidates[1].push(candidate.clone());
 556                         true
 557                     }
 558                     (Ordering::Greater, &Some(_)) => {
 559                         // test_len < pat_len, and is therefore less
 560                         // strict. This can still go both ways.
 561                         false
 562                     }
 563                 }
 564             }
 565
 566             (&TestKind::Eq { .. }, _) |
 567             (&TestKind::Range { .. }, _) |
 568             (&TestKind::Len { .. }, _) => {
 569                 // These are all binary tests.
 570                 //
 571                 // FIXME(#29623) we can be more clever here
 572                 let pattern_test = self.test(&match_pair);
 573                 if pattern_test.kind == test.kind {
 574                     let new_candidate = self.candidate_without_match_pair(match_pair_index,
 575                                                                           candidate);
 576                     resulting_candidates[0].push(new_candidate);
 577                     true
 578                 } else {
 579                     false
 580                 }
 581             }
 582         }
 583     }
 584
 585     fn candidate_without_match_pair<'pat>(&mut self,
 586                                           match_pair_index: usize,
 587                                           candidate: &Candidate<'pat, 'tcx>)
 588                                           -> Candidate<'pat, 'tcx> {
 589         let other_match_pairs =
 590             candidate.match_pairs.iter()
 591                                  .enumerate()
 592                                  .filter(|&(index, _)| index != match_pair_index)
 593                                  .map(|(_, mp)| mp.clone())
 594                                  .collect();
 595         Candidate {
 596             span: candidate.span,
 597             match_pairs: other_match_pairs,
 598             bindings: candidate.bindings.clone(),
 599             guard: candidate.guard.clone(),
 600             arm_index: candidate.arm_index,
 601             pre_binding_block: candidate.pre_binding_block,
 602             next_candidate_pre_binding_block: candidate.next_candidate_pre_binding_block,
 603         }
 604     }
 605
 606     fn candidate_after_slice_test<'pat>(&mut self,
 607                                         match_pair_index: usize,
 608                                         candidate: &Candidate<'pat, 'tcx>,
 609                                         prefix: &'pat [Pattern<'tcx>],
 610                                         opt_slice: Option<&'pat Pattern<'tcx>>,
 611                                         suffix: &'pat [Pattern<'tcx>])
 612                                         -> Candidate<'pat, 'tcx> {
 613         let mut new_candidate =
 614             self.candidate_without_match_pair(match_pair_index, candidate);
 615         self.prefix_slice_suffix(
 616             &mut new_candidate.match_pairs,
 617             &candidate.match_pairs[match_pair_index].lvalue,
 618             prefix,
 619             opt_slice,
 620             suffix);
 621
 622         new_candidate
 623     }
 624
 625     fn candidate_after_variant_switch<'pat>(&mut self,
 626                                             match_pair_index: usize,
 627                                             adt_def: &'tcx ty::AdtDef,
 628                                             variant_index: usize,
 629                                             subpatterns: &'pat [FieldPattern<'tcx>],
 630                                             candidate: &Candidate<'pat, 'tcx>)
 631                                             -> Candidate<'pat, 'tcx> {
 632         let match_pair = &candidate.match_pairs[match_pair_index];
 633
 634         // So, if we have a match-pattern like `x @ Enum::Variant(P1, P2)`,
 635         // we want to create a set of derived match-patterns like
 636         // `(x as Variant).0 @ P1` and `(x as Variant).1 @ P1`.
 637         let elem = ProjectionElem::Downcast(adt_def, variant_index);
 638         let downcast_lvalue = match_pair.lvalue.clone().elem(elem); // `(x as Variant)`
 639         let consequent_match_pairs =
 640             subpatterns.iter()
 641                        .map(|subpattern| {
 642                            // e.g., `(x as Variant).0`
 643                            let lvalue = downcast_lvalue.clone().field(subpattern.field,
 644                                                                       subpattern.pattern.ty);
 645                            // e.g., `(x as Variant).0 @ P1`
 646                            MatchPair::new(lvalue, &subpattern.pattern)
 647                        });
 648
 649         // In addition, we need all the other match pairs from the old candidate.
 650         let other_match_pairs =
 651             candidate.match_pairs.iter()
 652                                  .enumerate()
 653                                  .filter(|&(index, _)| index != match_pair_index)
 654                                  .map(|(_, mp)| mp.clone());
 655
 656         let all_match_pairs = consequent_match_pairs.chain(other_match_pairs).collect();
 657
 658         Candidate {
 659             span: candidate.span,
 660             match_pairs: all_match_pairs,
 661             bindings: candidate.bindings.clone(),
 662             guard: candidate.guard.clone(),
 663             arm_index: candidate.arm_index,
 664             pre_binding_block: candidate.pre_binding_block,
 665             next_candidate_pre_binding_block: candidate.next_candidate_pre_binding_block,
 666         }
 667     }
 668
 669     fn error_simplifyable<'pat>(&mut self, match_pair: &MatchPair<'pat, 'tcx>) -> ! {
 670         span_bug!(match_pair.pattern.span,
 671                   "simplifyable pattern found: {:?}",
 672                   match_pair.pattern)
 673     }
 674 }
 675
 676 fn is_switch_ty<'tcx>(ty: Ty<'tcx>) -> bool {
 677     ty.is_integral() || ty.is_char() || ty.is_bool()
 678 }