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