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, ConstInt};
  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 { ref value } => {
  65                 Test {
  66                     span: match_pair.pattern.span,
  67                     kind: TestKind::Eq {
  68                         value: value.clone(),
  69                         ty: match_pair.pattern.ty.clone()
  70                     }
  71                 }
  72             }
  73
  74             PatternKind::Range { ref lo, ref hi, ref end } => {
  75                 Test {
  76                     span: match_pair.pattern.span,
  77                     kind: TestKind::Range {
  78                         lo: Literal::Value { value: lo.clone() },
  79                         hi: Literal::Value { value: hi.clone() },
  80                         ty: match_pair.pattern.ty.clone(),
  81                         end: end.clone(),
  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<ConstVal>,
 116                                      indices: &mut FxHashMap<ConstVal, 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 { ref value } => {
 126                 // if the lvalues match, the type should match
 127                 assert_eq!(match_pair.pattern.ty, switch_ty);
 128
 129                 indices.entry(value.clone())
 130                        .or_insert_with(|| {
 131                            options.push(value.clone());
 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, variant) in adt_def.variants.iter().enumerate() {
 195                     target_blocks.place_back() <- if variants.contains(idx) {
 196                         let discr = ConstInt::new_inttype(variant.disr_val, adt_def.discr_ty,
 197                                                           tcx.sess.target.uint_type,
 198                                                           tcx.sess.target.int_type).unwrap();
 199                         values.push(discr);
 200                         *(targets.place_back() <- self.cfg.start_new_block())
 201                     } else {
 202                         if otherwise_block.is_none() {
 203                             otherwise_block = Some(self.cfg.start_new_block());
 204                         }
 205                         otherwise_block.unwrap()
 206                     };
 207                 }
 208                 if let Some(otherwise_block) = otherwise_block {
 209                     targets.push(otherwise_block);
 210                 } else {
 211                     values.pop();
 212                 }
 213                 debug!("num_enum_variants: {}, tested variants: {:?}, variants: {:?}",
 214                        num_enum_variants, values, variants);
 215                 let discr_ty = adt_def.discr_ty.to_ty(tcx);
 216                 let discr = self.temp(discr_ty);
 217                 self.cfg.push_assign(block, source_info, &discr,
 218                                      Rvalue::Discriminant(lvalue.clone()));
 219                 assert_eq!(values.len() + 1, targets.len());
 220                 self.cfg.terminate(block, source_info, TerminatorKind::SwitchInt {
 221                     discr: Operand::Consume(discr),
 222                     switch_ty: discr_ty,
 223                     values: From::from(values),
 224                     targets: targets
 225                 });
 226                 target_blocks
 227             }
 228
 229             TestKind::SwitchInt { switch_ty, ref options, indices: _ } => {
 230                 let (values, targets, ret) = if switch_ty.sty == ty::TyBool {
 231                     assert!(options.len() > 0 && options.len() <= 2);
 232                     let (true_bb, false_bb) = (self.cfg.start_new_block(),
 233                                                self.cfg.start_new_block());
 234                     let ret = match &options[0] {
 235                         &ConstVal::Bool(true) => vec![true_bb, false_bb],
 236                         &ConstVal::Bool(false) => vec![false_bb, true_bb],
 237                         v => span_bug!(test.span, "expected boolean value but got {:?}", v)
 238                     };
 239                     (BOOL_SWITCH_TRUE.clone(), vec![true_bb, false_bb], ret)
 240                 } else {
 241                     // The switch may be inexhaustive so we
 242                     // add a catch all block
 243                     let otherwise = self.cfg.start_new_block();
 244                     let targets: Vec<_> =
 245                         options.iter()
 246                                .map(|_| self.cfg.start_new_block())
 247                                .chain(Some(otherwise))
 248                                .collect();
 249                     let values: Vec<_> = options.iter().map(|v|
 250                         v.to_const_int().expect("switching on integral")
 251                     ).collect();
 252                     (From::from(values), targets.clone(), targets)
 253                 };
 254
 255                 self.cfg.terminate(block, source_info, TerminatorKind::SwitchInt {
 256                     discr: Operand::Consume(lvalue.clone()),
 257                     switch_ty: switch_ty,
 258                     values: values,
 259                     targets: targets.clone(),
 260                 });
 261                 ret
 262             }
 263
 264             TestKind::Eq { ref value, mut ty } => {
 265                 let mut val = Operand::Consume(lvalue.clone());
 266
 267                 // If we're using b"..." as a pattern, we need to insert an
 268                 // unsizing coercion, as the byte string has the type &[u8; N].
 269                 let expect = if let ConstVal::ByteStr(ref bytes) = *value {
 270                     let tcx = self.hir.tcx();
 271
 272                     // Unsize the lvalue to &[u8], too, if necessary.
 273                     if let ty::TyRef(region, mt) = ty.sty {
 274                         if let ty::TyArray(_, _) = mt.ty.sty {
 275                             ty = tcx.mk_imm_ref(region, tcx.mk_slice(tcx.types.u8));
 276                             let val_slice = self.temp(ty);
 277                             self.cfg.push_assign(block, source_info, &val_slice,
 278                                                  Rvalue::Cast(CastKind::Unsize, val, ty));
 279                             val = Operand::Consume(val_slice);
 280                         }
 281                     }
 282
 283                     assert!(ty.is_slice());
 284
 285                     let array_ty = tcx.mk_array(tcx.types.u8, bytes.len());
 286                     let array_ref = tcx.mk_imm_ref(tcx.mk_region(ty::ReStatic), array_ty);
 287                     let array = self.literal_operand(test.span, array_ref, Literal::Value {
 288                         value: value.clone()
 289                     });
 290
 291                     let slice = self.temp(ty);
 292                     self.cfg.push_assign(block, source_info, &slice,
 293                                          Rvalue::Cast(CastKind::Unsize, array, ty));
 294                     Operand::Consume(slice)
 295                 } else {
 296                     self.literal_operand(test.span, ty, Literal::Value {
 297                         value: value.clone()
 298                     })
 299                 };
 300
 301                 // Use PartialEq::eq for &str and &[u8] slices, instead of BinOp::Eq.
 302                 let fail = self.cfg.start_new_block();
 303                 if let ty::TyRef(_, mt) = ty.sty {
 304                     assert!(ty.is_slice());
 305                     let eq_def_id = self.hir.tcx().lang_items.eq_trait().unwrap();
 306                     let ty = mt.ty;
 307                     let (mty, method) = self.hir.trait_method(eq_def_id, "eq", ty, &[ty]);
 308
 309                     let bool_ty = self.hir.bool_ty();
 310                     let eq_result = self.temp(bool_ty);
 311                     let eq_block = self.cfg.start_new_block();
 312                     let cleanup = self.diverge_cleanup();
 313                     self.cfg.terminate(block, source_info, TerminatorKind::Call {
 314                         func: Operand::Constant(Constant {
 315                             span: test.span,
 316                             ty: mty,
 317                             literal: method
 318                         }),
 319                         args: vec![val, expect],
 320                         destination: Some((eq_result.clone(), eq_block)),
 321                         cleanup: cleanup,
 322                     });
 323
 324                     // check the result
 325                     let block = self.cfg.start_new_block();
 326                     self.cfg.terminate(eq_block, source_info, TerminatorKind::SwitchInt {
 327                         discr: Operand::Consume(eq_result),
 328                         switch_ty: self.hir.bool_ty(),
 329                         values: BOOL_SWITCH_TRUE.clone(),
 330                         targets: vec![block, fail],
 331                     });
 332                     vec![block, fail]
 333                 } else {
 334                     let block = self.compare(block, fail, test.span, BinOp::Eq, expect, val);
 335                     vec![block, fail]
 336                 }
 337             }
 338
 339             TestKind::Range { ref lo, ref hi, ty, ref end } => {
 340                 // Test `val` by computing `lo <= val && val <= hi`, using primitive comparisons.
 341                 let lo = self.literal_operand(test.span, ty.clone(), lo.clone());
 342                 let hi = self.literal_operand(test.span, ty.clone(), hi.clone());
 343                 let val = Operand::Consume(lvalue.clone());
 344
 345                 let fail = self.cfg.start_new_block();
 346                 let block = self.compare(block, fail, test.span, BinOp::Le, lo, val.clone());
 347                 let block = match *end {
 348                     RangeEnd::Included => self.compare(block, fail, test.span, BinOp::Le, val, hi),
 349                     RangeEnd::Excluded => self.compare(block, fail, test.span, BinOp::Lt, val, hi),
 350                 };
 351
 352                 vec![block, fail]
 353             }
 354
 355             TestKind::Len { len, op } => {
 356                 let (usize_ty, bool_ty) = (self.hir.usize_ty(), self.hir.bool_ty());
 357                 let (actual, result) = (self.temp(usize_ty), self.temp(bool_ty));
 358
 359                 // actual = len(lvalue)
 360                 self.cfg.push_assign(block, source_info,
 361                                      &actual, Rvalue::Len(lvalue.clone()));
 362
 363                 // expected = <N>
 364                 let expected = self.push_usize(block, source_info, len);
 365
 366                 // result = actual == expected OR result = actual < expected
 367                 self.cfg.push_assign(block, source_info, &result,
 368                                      Rvalue::BinaryOp(op,
 369                                                       Operand::Consume(actual),
 370                                                       Operand::Consume(expected)));
 371
 372                 // branch based on result
 373                 let (false_bb, true_bb) = (self.cfg.start_new_block(),
 374                                            self.cfg.start_new_block());
 375                 self.cfg.terminate(block, source_info, TerminatorKind::SwitchInt {
 376                     discr: Operand::Consume(result),
 377                     switch_ty: self.hir.bool_ty(),
 378                     values: BOOL_SWITCH_TRUE.clone(),
 379                     targets: vec![true_bb, false_bb],
 380                 });
 381                 vec![true_bb, false_bb]
 382             }
 383         }
 384     }
 385
 386     fn compare(&mut self,
 387                block: BasicBlock,
 388                fail_block: BasicBlock,
 389                span: Span,
 390                op: BinOp,
 391                left: Operand<'tcx>,
 392                right: Operand<'tcx>) -> BasicBlock {
 393         let bool_ty = self.hir.bool_ty();
 394         let result = self.temp(bool_ty);
 395
 396         // result = op(left, right)
 397         let source_info = self.source_info(span);
 398         self.cfg.push_assign(block, source_info, &result,
 399                              Rvalue::BinaryOp(op, left, right));
 400
 401         // branch based on result
 402         let target_block = self.cfg.start_new_block();
 403         self.cfg.terminate(block, source_info, TerminatorKind::SwitchInt {
 404             discr: Operand::Consume(result),
 405             switch_ty: self.hir.bool_ty(),
 406             values: BOOL_SWITCH_TRUE.clone(),
 407             targets: vec![target_block, fail_block]
 408         });
 409         target_block
 410     }
 411
 412     /// Given that we are performing `test` against `test_lvalue`,
 413     /// this job sorts out what the status of `candidate` will be
 414     /// after the test. The `resulting_candidates` vector stores, for
 415     /// each possible outcome of `test`, a vector of the candidates
 416     /// that will result. This fn should add a (possibly modified)
 417     /// clone of candidate into `resulting_candidates` wherever
 418     /// appropriate.
 419     ///
 420     /// So, for example, if this candidate is `x @ Some(P0)` and the
 421     /// test is a variant test, then we would add `(x as Option).0 @
 422     /// P0` to the `resulting_candidates` entry corresponding to the
 423     /// variant `Some`.
 424     ///
 425     /// However, in some cases, the test may just not be relevant to
 426     /// candidate. For example, suppose we are testing whether `foo.x == 22`,
 427     /// but in one match arm we have `Foo { x: _, ... }`... in that case,
 428     /// the test for what value `x` has has no particular relevance
 429     /// to this candidate. In such cases, this function just returns false
 430     /// without doing anything. This is used by the overall `match_candidates`
 431     /// algorithm to structure the match as a whole. See `match_candidates` for
 432     /// more details.
 433     ///
 434     /// FIXME(#29623). In some cases, we have some tricky choices to
 435     /// make.  for example, if we are testing that `x == 22`, but the
 436     /// candidate is `x @ 13..55`, what should we do? In the event
 437     /// that the test is true, we know that the candidate applies, but
 438     /// in the event of false, we don't know that it *doesn't*
 439     /// apply. For now, we return false, indicate that the test does
 440     /// not apply to this candidate, but it might be we can get
 441     /// tighter match code if we do something a bit different.
 442     pub fn sort_candidate<'pat>(&mut self,
 443                                 test_lvalue: &Lvalue<'tcx>,
 444                                 test: &Test<'tcx>,
 445                                 candidate: &Candidate<'pat, 'tcx>,
 446                                 resulting_candidates: &mut [Vec<Candidate<'pat, 'tcx>>])
 447                                 -> bool {
 448         // Find the match_pair for this lvalue (if any). At present,
 449         // afaik, there can be at most one. (In the future, if we
 450         // adopted a more general `@` operator, there might be more
 451         // than one, but it'd be very unusual to have two sides that
 452         // both require tests; you'd expect one side to be simplified
 453         // away.)
 454         let tested_match_pair = candidate.match_pairs.iter()
 455                                                      .enumerate()
 456                                                      .filter(|&(_, mp)| mp.lvalue == *test_lvalue)
 457                                                      .next();
 458         let (match_pair_index, match_pair) = match tested_match_pair {
 459             Some(pair) => pair,
 460             None => {
 461                 // We are not testing this lvalue. Therefore, this
 462                 // candidate applies to ALL outcomes.
 463                 return false;
 464             }
 465         };
 466
 467         match (&test.kind, &*match_pair.pattern.kind) {
 468             // If we are performing a variant switch, then this
 469             // informs variant patterns, but nothing else.
 470             (&TestKind::Switch { adt_def: tested_adt_def, .. },
 471              &PatternKind::Variant { adt_def, variant_index, ref subpatterns, .. }) => {
 472                 assert_eq!(adt_def, tested_adt_def);
 473                 let new_candidate =
 474                     self.candidate_after_variant_switch(match_pair_index,
 475                                                         adt_def,
 476                                                         variant_index,
 477                                                         subpatterns,
 478                                                         candidate);
 479                 resulting_candidates[variant_index].push(new_candidate);
 480                 true
 481             }
 482             (&TestKind::Switch { .. }, _) => false,
 483
 484             // If we are performing a switch over integers, then this informs integer
 485             // equality, but nothing else.
 486             //
 487             // FIXME(#29623) we could use PatternKind::Range to rule
 488             // things out here, in some cases.
 489             (&TestKind::SwitchInt { switch_ty: _, options: _, ref indices },
 490              &PatternKind::Constant { ref value })
 491             if is_switch_ty(match_pair.pattern.ty) => {
 492                 let index = indices[value];
 493                 let new_candidate = self.candidate_without_match_pair(match_pair_index,
 494                                                                       candidate);
 495                 resulting_candidates[index].push(new_candidate);
 496                 true
 497             }
 498             (&TestKind::SwitchInt { .. }, _) => false,
 499
 500
 501             (&TestKind::Len { len: test_len, op: BinOp::Eq },
 502              &PatternKind::Slice { ref prefix, ref slice, ref suffix }) => {
 503                 let pat_len = (prefix.len() + suffix.len()) as u64;
 504                 match (test_len.cmp(&pat_len), slice) {
 505                     (Ordering::Equal, &None) => {
 506                         // on true, min_len = len = $actual_length,
 507                         // on false, len != $actual_length
 508                         resulting_candidates[0].push(
 509                             self.candidate_after_slice_test(match_pair_index,
 510                                                             candidate,
 511                                                             prefix,
 512                                                             slice.as_ref(),
 513                                                             suffix)
 514                         );
 515                         true
 516                     }
 517                     (Ordering::Less, _) => {
 518                         // test_len < pat_len. If $actual_len = test_len,
 519                         // then $actual_len < pat_len and we don't have
 520                         // enough elements.
 521                         resulting_candidates[1].push(candidate.clone());
 522                         true
 523                     }
 524                     (Ordering::Equal, &Some(_)) | (Ordering::Greater, &Some(_)) => {
 525                         // This can match both if $actual_len = test_len >= pat_len,
 526                         // and if $actual_len > test_len. We can't advance.
 527                         false
 528                     }
 529                     (Ordering::Greater, &None) => {
 530                         // test_len != pat_len, so if $actual_len = test_len, then
 531                         // $actual_len != pat_len.
 532                         resulting_candidates[1].push(candidate.clone());
 533                         true
 534                     }
 535                 }
 536             }
 537
 538             (&TestKind::Len { len: test_len, op: BinOp::Ge },
 539              &PatternKind::Slice { ref prefix, ref slice, ref suffix }) => {
 540                 // the test is `$actual_len >= test_len`
 541                 let pat_len = (prefix.len() + suffix.len()) as u64;
 542                 match (test_len.cmp(&pat_len), slice) {
 543                     (Ordering::Equal, &Some(_))  => {
 544                         // $actual_len >= test_len = pat_len,
 545                         // so we can match.
 546                         resulting_candidates[0].push(
 547                             self.candidate_after_slice_test(match_pair_index,
 548                                                             candidate,
 549                                                             prefix,
 550                                                             slice.as_ref(),
 551                                                             suffix)
 552                         );
 553                         true
 554                     }
 555                     (Ordering::Less, _) | (Ordering::Equal, &None) => {
 556                         // test_len <= pat_len. If $actual_len < test_len,
 557                         // then it is also < pat_len, so the test passing is
 558                         // necessary (but insufficient).
 559                         resulting_candidates[0].push(candidate.clone());
 560                         true
 561                     }
 562                     (Ordering::Greater, &None) => {
 563                         // test_len > pat_len. If $actual_len >= test_len > pat_len,
 564                         // then we know we won't have a match.
 565                         resulting_candidates[1].push(candidate.clone());
 566                         true
 567                     }
 568                     (Ordering::Greater, &Some(_)) => {
 569                         // test_len < pat_len, and is therefore less
 570                         // strict. This can still go both ways.
 571                         false
 572                     }
 573                 }
 574             }
 575
 576             (&TestKind::Eq { .. }, _) |
 577             (&TestKind::Range { .. }, _) |
 578             (&TestKind::Len { .. }, _) => {
 579                 // These are all binary tests.
 580                 //
 581                 // FIXME(#29623) we can be more clever here
 582                 let pattern_test = self.test(&match_pair);
 583                 if pattern_test.kind == test.kind {
 584                     let new_candidate = self.candidate_without_match_pair(match_pair_index,
 585                                                                           candidate);
 586                     resulting_candidates[0].push(new_candidate);
 587                     true
 588                 } else {
 589                     false
 590                 }
 591             }
 592         }
 593     }
 594
 595     fn candidate_without_match_pair<'pat>(&mut self,
 596                                           match_pair_index: usize,
 597                                           candidate: &Candidate<'pat, 'tcx>)
 598                                           -> Candidate<'pat, 'tcx> {
 599         let other_match_pairs =
 600             candidate.match_pairs.iter()
 601                                  .enumerate()
 602                                  .filter(|&(index, _)| index != match_pair_index)
 603                                  .map(|(_, mp)| mp.clone())
 604                                  .collect();
 605         Candidate {
 606             span: candidate.span,
 607             match_pairs: other_match_pairs,
 608             bindings: candidate.bindings.clone(),
 609             guard: candidate.guard.clone(),
 610             arm_index: candidate.arm_index,
 611         }
 612     }
 613
 614     fn candidate_after_slice_test<'pat>(&mut self,
 615                                         match_pair_index: usize,
 616                                         candidate: &Candidate<'pat, 'tcx>,
 617                                         prefix: &'pat [Pattern<'tcx>],
 618                                         opt_slice: Option<&'pat Pattern<'tcx>>,
 619                                         suffix: &'pat [Pattern<'tcx>])
 620                                         -> Candidate<'pat, 'tcx> {
 621         let mut new_candidate =
 622             self.candidate_without_match_pair(match_pair_index, candidate);
 623         self.prefix_slice_suffix(
 624             &mut new_candidate.match_pairs,
 625             &candidate.match_pairs[match_pair_index].lvalue,
 626             prefix,
 627             opt_slice,
 628             suffix);
 629
 630         new_candidate
 631     }
 632
 633     fn candidate_after_variant_switch<'pat>(&mut self,
 634                                             match_pair_index: usize,
 635                                             adt_def: &'tcx ty::AdtDef,
 636                                             variant_index: usize,
 637                                             subpatterns: &'pat [FieldPattern<'tcx>],
 638                                             candidate: &Candidate<'pat, 'tcx>)
 639                                             -> Candidate<'pat, 'tcx> {
 640         let match_pair = &candidate.match_pairs[match_pair_index];
 641
 642         // So, if we have a match-pattern like `x @ Enum::Variant(P1, P2)`,
 643         // we want to create a set of derived match-patterns like
 644         // `(x as Variant).0 @ P1` and `(x as Variant).1 @ P1`.
 645         let elem = ProjectionElem::Downcast(adt_def, variant_index);
 646         let downcast_lvalue = match_pair.lvalue.clone().elem(elem); // `(x as Variant)`
 647         let consequent_match_pairs =
 648             subpatterns.iter()
 649                        .map(|subpattern| {
 650                            // e.g., `(x as Variant).0`
 651                            let lvalue = downcast_lvalue.clone().field(subpattern.field,
 652                                                                       subpattern.pattern.ty);
 653                            // e.g., `(x as Variant).0 @ P1`
 654                            MatchPair::new(lvalue, &subpattern.pattern)
 655                        });
 656
 657         // In addition, we need all the other match pairs from the old candidate.
 658         let other_match_pairs =
 659             candidate.match_pairs.iter()
 660                                  .enumerate()
 661                                  .filter(|&(index, _)| index != match_pair_index)
 662                                  .map(|(_, mp)| mp.clone());
 663
 664         let all_match_pairs = consequent_match_pairs.chain(other_match_pairs).collect();
 665
 666         Candidate {
 667             span: candidate.span,
 668             match_pairs: all_match_pairs,
 669             bindings: candidate.bindings.clone(),
 670             guard: candidate.guard.clone(),
 671             arm_index: candidate.arm_index,
 672         }
 673     }
 674
 675     fn error_simplifyable<'pat>(&mut self, match_pair: &MatchPair<'pat, 'tcx>) -> ! {
 676         span_bug!(match_pair.pattern.span,
 677                   "simplifyable pattern found: {:?}",
 678                   match_pair.pattern)
 679     }
 680 }
 681
 682 fn is_switch_ty<'tcx>(ty: Ty<'tcx>) -> bool {
 683     ty.is_integral() || ty.is_char() || ty.is_bool()
 684 }