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::ty::{self, Ty};
  24 use rustc::ty::util::IntTypeExt;
  25 use rustc::mir::*;
  26 use rustc::hir::{RangeEnd, Mutability};
  27 use syntax_pos::Span;
  28 use std::cmp::Ordering;
  29
  30 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
  31     /// Identifies what test is needed to decide if `match_pair` is applicable.
  32     ///
  33     /// It is a bug to call this with a simplifyable pattern.
  34     pub fn test<'pat>(&mut self, match_pair: &MatchPair<'pat, 'tcx>) -> Test<'tcx> {
  35         match *match_pair.pattern.kind {
  36             PatternKind::Variant { ref adt_def, substs: _, variant_index: _, subpatterns: _ } => {
  37                 Test {
  38                     span: match_pair.pattern.span,
  39                     kind: TestKind::Switch {
  40                         adt_def: adt_def.clone(),
  41                         variants: BitVector::new(adt_def.variants.len()),
  42                     },
  43                 }
  44             }
  45
  46             PatternKind::Constant { .. }
  47             if is_switch_ty(match_pair.pattern.ty) => {
  48                 // for integers, we use a SwitchInt match, which allows
  49                 // us to handle more cases
  50                 Test {
  51                     span: match_pair.pattern.span,
  52                     kind: TestKind::SwitchInt {
  53                         switch_ty: match_pair.pattern.ty,
  54
  55                         // these maps are empty to start; cases are
  56                         // added below in add_cases_to_switch
  57                         options: vec![],
  58                         indices: FxHashMap(),
  59                     }
  60                 }
  61             }
  62
  63             PatternKind::Constant { value } => {
  64                 Test {
  65                     span: match_pair.pattern.span,
  66                     kind: TestKind::Eq {
  67                         value,
  68                         ty: match_pair.pattern.ty.clone()
  69                     }
  70                 }
  71             }
  72
  73             PatternKind::Range { lo, hi, end } => {
  74                 Test {
  75                     span: match_pair.pattern.span,
  76                     kind: TestKind::Range {
  77                         lo: Literal::Value { value: lo },
  78                         hi: Literal::Value { value: hi },
  79                         ty: match_pair.pattern.ty.clone(),
  80                         end,
  81                     },
  82                 }
  83             }
  84
  85             PatternKind::Slice { ref prefix, ref slice, ref suffix }
  86                     if !match_pair.slice_len_checked => {
  87                 let len = prefix.len() + suffix.len();
  88                 let op = if slice.is_some() {
  89                     BinOp::Ge
  90                 } else {
  91                     BinOp::Eq
  92                 };
  93                 Test {
  94                     span: match_pair.pattern.span,
  95                     kind: TestKind::Len { len: len as u64, op: op },
  96                 }
  97             }
  98
  99             PatternKind::Array { .. } |
 100             PatternKind::Slice { .. } |
 101             PatternKind::Wild |
 102             PatternKind::Binding { .. } |
 103             PatternKind::Leaf { .. } |
 104             PatternKind::Deref { .. } => {
 105                 self.error_simplifyable(match_pair)
 106             }
 107         }
 108     }
 109
 110     pub fn add_cases_to_switch<'pat>(&mut self,
 111                                      test_place: &Place<'tcx>,
 112                                      candidate: &Candidate<'pat, 'tcx>,
 113                                      switch_ty: Ty<'tcx>,
 114                                      options: &mut Vec<u128>,
 115                                      indices: &mut FxHashMap<&'tcx ty::Const<'tcx>, usize>)
 116                                      -> bool
 117     {
 118         let match_pair = match candidate.match_pairs.iter().find(|mp| mp.place == *test_place) {
 119             Some(match_pair) => match_pair,
 120             _ => { return false; }
 121         };
 122
 123         match *match_pair.pattern.kind {
 124             PatternKind::Constant { value } => {
 125                 indices.entry(value)
 126                        .or_insert_with(|| {
 127                            options.push(value.unwrap_bits(switch_ty));
 128                            options.len() - 1
 129                        });
 130                 true
 131             }
 132             PatternKind::Variant { .. } => {
 133                 panic!("you should have called add_variants_to_switch instead!");
 134             }
 135             PatternKind::Range { .. } |
 136             PatternKind::Slice { .. } |
 137             PatternKind::Array { .. } |
 138             PatternKind::Wild |
 139             PatternKind::Binding { .. } |
 140             PatternKind::Leaf { .. } |
 141             PatternKind::Deref { .. } => {
 142                 // don't know how to add these patterns to a switch
 143                 false
 144             }
 145         }
 146     }
 147
 148     pub fn add_variants_to_switch<'pat>(&mut self,
 149                                         test_place: &Place<'tcx>,
 150                                         candidate: &Candidate<'pat, 'tcx>,
 151                                         variants: &mut BitVector)
 152                                         -> bool
 153     {
 154         let match_pair = match candidate.match_pairs.iter().find(|mp| mp.place == *test_place) {
 155             Some(match_pair) => match_pair,
 156             _ => { return false; }
 157         };
 158
 159         match *match_pair.pattern.kind {
 160             PatternKind::Variant { adt_def: _ , variant_index,  .. } => {
 161                 // We have a pattern testing for variant `variant_index`
 162                 // set the corresponding index to true
 163                 variants.insert(variant_index);
 164                 true
 165             }
 166             _ => {
 167                 // don't know how to add these patterns to a switch
 168                 false
 169             }
 170         }
 171     }
 172
 173     /// Generates the code to perform a test.
 174     pub fn perform_test(&mut self,
 175                         block: BasicBlock,
 176                         place: &Place<'tcx>,
 177                         test: &Test<'tcx>)
 178                         -> Vec<BasicBlock> {
 179         debug!("perform_test({:?}, {:?}: {:?}, {:?})",
 180                block,
 181                place,
 182                place.ty(&self.local_decls, self.hir.tcx()),
 183                test);
 184         let source_info = self.source_info(test.span);
 185         match test.kind {
 186             TestKind::Switch { adt_def, ref variants } => {
 187                 // Variants is a BitVec of indexes into adt_def.variants.
 188                 let num_enum_variants = adt_def.variants.len();
 189                 let used_variants = variants.count();
 190                 let mut otherwise_block = None;
 191                 let mut target_blocks = Vec::with_capacity(num_enum_variants);
 192                 let mut targets = Vec::with_capacity(used_variants + 1);
 193                 let mut values = Vec::with_capacity(used_variants);
 194                 let tcx = self.hir.tcx();
 195                 for (idx, discr) in adt_def.discriminants(tcx).enumerate() {
 196                     target_blocks.push(if variants.contains(idx) {
 197                         values.push(discr.val);
 198                         targets.push(self.cfg.start_new_block());
 199                         *targets.last().unwrap()
 200                     } else {
 201                         if otherwise_block.is_none() {
 202                             otherwise_block = Some(self.cfg.start_new_block());
 203                         }
 204                         otherwise_block.unwrap()
 205                     });
 206                 }
 207                 if let Some(otherwise_block) = otherwise_block {
 208                     targets.push(otherwise_block);
 209                 } else {
 210                     targets.push(self.unreachable_block());
 211                 }
 212                 debug!("num_enum_variants: {}, tested variants: {:?}, variants: {:?}",
 213                        num_enum_variants, values, variants);
 214                 let discr_ty = adt_def.repr.discr_type().to_ty(tcx);
 215                 let discr = self.temp(discr_ty, test.span);
 216                 self.cfg.push_assign(block, source_info, &discr,
 217                                      Rvalue::Discriminant(place.clone()));
 218                 assert_eq!(values.len() + 1, targets.len());
 219                 self.cfg.terminate(block, source_info, TerminatorKind::SwitchInt {
 220                     discr: Operand::Move(discr),
 221                     switch_ty: discr_ty,
 222                     values: From::from(values),
 223                     targets,
 224                 });
 225                 target_blocks
 226             }
 227
 228             TestKind::SwitchInt { switch_ty, ref options, indices: _ } => {
 229                 let (ret, terminator) = if switch_ty.sty == ty::TyBool {
 230                     assert!(options.len() > 0 && options.len() <= 2);
 231                     let (true_bb, false_bb) = (self.cfg.start_new_block(),
 232                                                self.cfg.start_new_block());
 233                     let ret = match options[0] {
 234                         1 => vec![true_bb, false_bb],
 235                         0 => vec![false_bb, true_bb],
 236                         v => span_bug!(test.span, "expected boolean value but got {:?}", v)
 237                     };
 238                     (ret, TerminatorKind::if_(self.hir.tcx(), Operand::Copy(place.clone()),
 239                                               true_bb, false_bb))
 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                     (targets.clone(), TerminatorKind::SwitchInt {
 250                         discr: Operand::Copy(place.clone()),
 251                         switch_ty,
 252                         values: options.clone().into(),
 253                         targets,
 254                     })
 255                 };
 256                 self.cfg.terminate(block, source_info, terminator);
 257                 ret
 258             }
 259
 260             TestKind::Eq { value, mut ty } => {
 261                 let mut val = Operand::Copy(place.clone());
 262                 let mut expect = self.literal_operand(test.span, ty, Literal::Value {
 263                     value
 264                 });
 265                 // Use PartialEq::eq instead of BinOp::Eq
 266                 // (the binop can only handle primitives)
 267                 let fail = self.cfg.start_new_block();
 268                 if !ty.is_scalar() {
 269                     // If we're using b"..." as a pattern, we need to insert an
 270                     // unsizing coercion, as the byte string has the type &[u8; N].
 271                     //
 272                     // We want to do this even when the scrutinee is a reference to an
 273                     // array, so we can call `<[u8]>::eq` rather than having to find an
 274                     // `<[u8; N]>::eq`.
 275                     let unsize = |ty: Ty<'tcx>| match ty.sty {
 276                         ty::TyRef(region, rty, _) => match rty.sty {
 277                             ty::TyArray(inner_ty, n) => Some((region, inner_ty, n)),
 278                             _ => None,
 279                         },
 280                         _ => None,
 281                     };
 282                     let opt_ref_ty = unsize(ty);
 283                     let opt_ref_test_ty = unsize(value.ty);
 284                     let mut place = place.clone();
 285                     match (opt_ref_ty, opt_ref_test_ty) {
 286                         // nothing to do, neither is an array
 287                         (None, None) => {},
 288                         (Some((region, elem_ty, _)), _) |
 289                         (None, Some((region, elem_ty, _))) => {
 290                             let tcx = self.hir.tcx();
 291                             // make both a slice
 292                             ty = tcx.mk_imm_ref(region, tcx.mk_slice(elem_ty));
 293                             if opt_ref_ty.is_some() {
 294                                 place = self.temp(ty, test.span);
 295                                 self.cfg.push_assign(block, source_info, &place,
 296                                                     Rvalue::Cast(CastKind::Unsize, val, ty));
 297                             }
 298                             if opt_ref_test_ty.is_some() {
 299                                 let array = self.literal_operand(
 300                                     test.span,
 301                                     value.ty,
 302                                     Literal::Value {
 303                                         value
 304                                     },
 305                                 );
 306
 307                                 let slice = self.temp(ty, test.span);
 308                                 self.cfg.push_assign(block, source_info, &slice,
 309                                                     Rvalue::Cast(CastKind::Unsize, array, ty));
 310                                 expect = Operand::Move(slice);
 311                             }
 312                         },
 313                     }
 314                     let eq_def_id = self.hir.tcx().lang_items().eq_trait().unwrap();
 315                     let (mty, method) = self.hir.trait_method(eq_def_id, "eq", ty, &[ty.into()]);
 316
 317                     // take the argument by reference
 318                     let region_scope = self.topmost_scope();
 319                     let region = self.hir.tcx().mk_region(ty::ReScope(region_scope));
 320                     let tam = ty::TypeAndMut {
 321                         ty,
 322                         mutbl: Mutability::MutImmutable,
 323                     };
 324                     let ref_ty = self.hir.tcx().mk_ref(region, tam);
 325
 326                     // let lhs_ref_place = &lhs;
 327                     let ref_rvalue = Rvalue::Ref(region, BorrowKind::Shared, place.clone());
 328                     let lhs_ref_place = self.temp(ref_ty, test.span);
 329                     self.cfg.push_assign(block, source_info, &lhs_ref_place, ref_rvalue);
 330                     let val = Operand::Move(lhs_ref_place);
 331
 332                     // let rhs_place = rhs;
 333                     let rhs_place = self.temp(ty, test.span);
 334                     self.cfg.push_assign(block, source_info, &rhs_place, Rvalue::Use(expect));
 335
 336                     // let rhs_ref_place = &rhs_place;
 337                     let ref_rvalue = Rvalue::Ref(region, BorrowKind::Shared, rhs_place);
 338                     let rhs_ref_place = self.temp(ref_ty, test.span);
 339                     self.cfg.push_assign(block, source_info, &rhs_ref_place, ref_rvalue);
 340                     let expect = Operand::Move(rhs_ref_place);
 341
 342                     let bool_ty = self.hir.bool_ty();
 343                     let eq_result = self.temp(bool_ty, test.span);
 344                     let eq_block = self.cfg.start_new_block();
 345                     let cleanup = self.diverge_cleanup();
 346                     self.cfg.terminate(block, source_info, TerminatorKind::Call {
 347                         func: Operand::Constant(box Constant {
 348                             span: test.span,
 349                             ty: mty,
 350                             literal: method
 351                         }),
 352                         args: vec![val, expect],
 353                         destination: Some((eq_result.clone(), eq_block)),
 354                         cleanup: Some(cleanup),
 355                     });
 356
 357                     // check the result
 358                     let block = self.cfg.start_new_block();
 359                     self.cfg.terminate(eq_block, source_info,
 360                                        TerminatorKind::if_(self.hir.tcx(),
 361                                                            Operand::Move(eq_result),
 362                                                            block, fail));
 363                     vec![block, fail]
 364                 } else {
 365                     let block = self.compare(block, fail, test.span, BinOp::Eq, expect, val);
 366                     vec![block, fail]
 367                 }
 368             }
 369
 370             TestKind::Range { ref lo, ref hi, ty, ref end } => {
 371                 // Test `val` by computing `lo <= val && val <= hi`, using primitive comparisons.
 372                 let lo = self.literal_operand(test.span, ty.clone(), lo.clone());
 373                 let hi = self.literal_operand(test.span, ty.clone(), hi.clone());
 374                 let val = Operand::Copy(place.clone());
 375
 376                 let fail = self.cfg.start_new_block();
 377                 let block = self.compare(block, fail, test.span, BinOp::Le, lo, val.clone());
 378                 let block = match *end {
 379                     RangeEnd::Included => self.compare(block, fail, test.span, BinOp::Le, val, hi),
 380                     RangeEnd::Excluded => self.compare(block, fail, test.span, BinOp::Lt, val, hi),
 381                 };
 382
 383                 vec![block, fail]
 384             }
 385
 386             TestKind::Len { len, op } => {
 387                 let (usize_ty, bool_ty) = (self.hir.usize_ty(), self.hir.bool_ty());
 388                 let (actual, result) = (self.temp(usize_ty, test.span),
 389                                         self.temp(bool_ty, test.span));
 390
 391                 // actual = len(place)
 392                 self.cfg.push_assign(block, source_info,
 393                                      &actual, Rvalue::Len(place.clone()));
 394
 395                 // expected = <N>
 396                 let expected = self.push_usize(block, source_info, len);
 397
 398                 // result = actual == expected OR result = actual < expected
 399                 self.cfg.push_assign(block, source_info, &result,
 400                                      Rvalue::BinaryOp(op,
 401                                                       Operand::Move(actual),
 402                                                       Operand::Move(expected)));
 403
 404                 // branch based on result
 405                 let (false_bb, true_bb) = (self.cfg.start_new_block(),
 406                                            self.cfg.start_new_block());
 407                 self.cfg.terminate(block, source_info,
 408                                    TerminatorKind::if_(self.hir.tcx(), Operand::Move(result),
 409                                                        true_bb, false_bb));
 410                 vec![true_bb, false_bb]
 411             }
 412         }
 413     }
 414
 415     fn compare(&mut self,
 416                block: BasicBlock,
 417                fail_block: BasicBlock,
 418                span: Span,
 419                op: BinOp,
 420                left: Operand<'tcx>,
 421                right: Operand<'tcx>) -> BasicBlock {
 422         let bool_ty = self.hir.bool_ty();
 423         let result = self.temp(bool_ty, span);
 424
 425         // result = op(left, right)
 426         let source_info = self.source_info(span);
 427         self.cfg.push_assign(block, source_info, &result,
 428                              Rvalue::BinaryOp(op, left, right));
 429
 430         // branch based on result
 431         let target_block = self.cfg.start_new_block();
 432         self.cfg.terminate(block, source_info,
 433                            TerminatorKind::if_(self.hir.tcx(), Operand::Move(result),
 434                                                target_block, fail_block));
 435         target_block
 436     }
 437
 438     /// Given that we are performing `test` against `test_place`,
 439     /// this job sorts out what the status of `candidate` will be
 440     /// after the test. The `resulting_candidates` vector stores, for
 441     /// each possible outcome of `test`, a vector of the candidates
 442     /// that will result. This fn should add a (possibly modified)
 443     /// clone of candidate into `resulting_candidates` wherever
 444     /// appropriate.
 445     ///
 446     /// So, for example, if this candidate is `x @ Some(P0)` and the
 447     /// test is a variant test, then we would add `(x as Option).0 @
 448     /// P0` to the `resulting_candidates` entry corresponding to the
 449     /// variant `Some`.
 450     ///
 451     /// However, in some cases, the test may just not be relevant to
 452     /// candidate. For example, suppose we are testing whether `foo.x == 22`,
 453     /// but in one match arm we have `Foo { x: _, ... }`... in that case,
 454     /// the test for what value `x` has has no particular relevance
 455     /// to this candidate. In such cases, this function just returns false
 456     /// without doing anything. This is used by the overall `match_candidates`
 457     /// algorithm to structure the match as a whole. See `match_candidates` for
 458     /// more details.
 459     ///
 460     /// FIXME(#29623). In some cases, we have some tricky choices to
 461     /// make.  for example, if we are testing that `x == 22`, but the
 462     /// candidate is `x @ 13..55`, what should we do? In the event
 463     /// that the test is true, we know that the candidate applies, but
 464     /// in the event of false, we don't know that it *doesn't*
 465     /// apply. For now, we return false, indicate that the test does
 466     /// not apply to this candidate, but it might be we can get
 467     /// tighter match code if we do something a bit different.
 468     pub fn sort_candidate<'pat>(&mut self,
 469                                 test_place: &Place<'tcx>,
 470                                 test: &Test<'tcx>,
 471                                 candidate: &Candidate<'pat, 'tcx>,
 472                                 resulting_candidates: &mut [Vec<Candidate<'pat, 'tcx>>])
 473                                 -> bool {
 474         // Find the match_pair for this place (if any). At present,
 475         // afaik, there can be at most one. (In the future, if we
 476         // adopted a more general `@` operator, there might be more
 477         // than one, but it'd be very unusual to have two sides that
 478         // both require tests; you'd expect one side to be simplified
 479         // away.)
 480         let tested_match_pair = candidate.match_pairs.iter()
 481                                                      .enumerate()
 482                                                      .filter(|&(_, mp)| mp.place == *test_place)
 483                                                      .next();
 484         let (match_pair_index, match_pair) = match tested_match_pair {
 485             Some(pair) => pair,
 486             None => {
 487                 // We are not testing this place. Therefore, this
 488                 // candidate applies to ALL outcomes.
 489                 return false;
 490             }
 491         };
 492
 493         match (&test.kind, &*match_pair.pattern.kind) {
 494             // If we are performing a variant switch, then this
 495             // informs variant patterns, but nothing else.
 496             (&TestKind::Switch { adt_def: tested_adt_def, .. },
 497              &PatternKind::Variant { adt_def, variant_index, ref subpatterns, .. }) => {
 498                 assert_eq!(adt_def, tested_adt_def);
 499                 let new_candidate =
 500                     self.candidate_after_variant_switch(match_pair_index,
 501                                                         adt_def,
 502                                                         variant_index,
 503                                                         subpatterns,
 504                                                         candidate);
 505                 resulting_candidates[variant_index].push(new_candidate);
 506                 true
 507             }
 508             (&TestKind::Switch { .. }, _) => false,
 509
 510             // If we are performing a switch over integers, then this informs integer
 511             // equality, but nothing else.
 512             //
 513             // FIXME(#29623) we could use PatternKind::Range to rule
 514             // things out here, in some cases.
 515             (&TestKind::SwitchInt { switch_ty: _, options: _, ref indices },
 516              &PatternKind::Constant { ref value })
 517             if is_switch_ty(match_pair.pattern.ty) => {
 518                 let index = indices[value];
 519                 let new_candidate = self.candidate_without_match_pair(match_pair_index,
 520                                                                       candidate);
 521                 resulting_candidates[index].push(new_candidate);
 522                 true
 523             }
 524             (&TestKind::SwitchInt { .. }, _) => false,
 525
 526
 527             (&TestKind::Len { len: test_len, op: BinOp::Eq },
 528              &PatternKind::Slice { ref prefix, ref slice, ref suffix }) => {
 529                 let pat_len = (prefix.len() + suffix.len()) as u64;
 530                 match (test_len.cmp(&pat_len), slice) {
 531                     (Ordering::Equal, &None) => {
 532                         // on true, min_len = len = $actual_length,
 533                         // on false, len != $actual_length
 534                         resulting_candidates[0].push(
 535                             self.candidate_after_slice_test(match_pair_index,
 536                                                             candidate,
 537                                                             prefix,
 538                                                             slice.as_ref(),
 539                                                             suffix)
 540                         );
 541                         true
 542                     }
 543                     (Ordering::Less, _) => {
 544                         // test_len < pat_len. If $actual_len = test_len,
 545                         // then $actual_len < pat_len and we don't have
 546                         // enough elements.
 547                         resulting_candidates[1].push(candidate.clone());
 548                         true
 549                     }
 550                     (Ordering::Equal, &Some(_)) | (Ordering::Greater, &Some(_)) => {
 551                         // This can match both if $actual_len = test_len >= pat_len,
 552                         // and if $actual_len > test_len. We can't advance.
 553                         false
 554                     }
 555                     (Ordering::Greater, &None) => {
 556                         // test_len != pat_len, so if $actual_len = test_len, then
 557                         // $actual_len != pat_len.
 558                         resulting_candidates[1].push(candidate.clone());
 559                         true
 560                     }
 561                 }
 562             }
 563
 564             (&TestKind::Len { len: test_len, op: BinOp::Ge },
 565              &PatternKind::Slice { ref prefix, ref slice, ref suffix }) => {
 566                 // the test is `$actual_len >= test_len`
 567                 let pat_len = (prefix.len() + suffix.len()) as u64;
 568                 match (test_len.cmp(&pat_len), slice) {
 569                     (Ordering::Equal, &Some(_))  => {
 570                         // $actual_len >= test_len = pat_len,
 571                         // so we can match.
 572                         resulting_candidates[0].push(
 573                             self.candidate_after_slice_test(match_pair_index,
 574                                                             candidate,
 575                                                             prefix,
 576                                                             slice.as_ref(),
 577                                                             suffix)
 578                         );
 579                         true
 580                     }
 581                     (Ordering::Less, _) | (Ordering::Equal, &None) => {
 582                         // test_len <= pat_len. If $actual_len < test_len,
 583                         // then it is also < pat_len, so the test passing is
 584                         // necessary (but insufficient).
 585                         resulting_candidates[0].push(candidate.clone());
 586                         true
 587                     }
 588                     (Ordering::Greater, &None) => {
 589                         // test_len > pat_len. If $actual_len >= test_len > pat_len,
 590                         // then we know we won't have a match.
 591                         resulting_candidates[1].push(candidate.clone());
 592                         true
 593                     }
 594                     (Ordering::Greater, &Some(_)) => {
 595                         // test_len < pat_len, and is therefore less
 596                         // strict. This can still go both ways.
 597                         false
 598                     }
 599                 }
 600             }
 601
 602             (&TestKind::Eq { .. }, _) |
 603             (&TestKind::Range { .. }, _) |
 604             (&TestKind::Len { .. }, _) => {
 605                 // These are all binary tests.
 606                 //
 607                 // FIXME(#29623) we can be more clever here
 608                 let pattern_test = self.test(&match_pair);
 609                 if pattern_test.kind == test.kind {
 610                     let new_candidate = self.candidate_without_match_pair(match_pair_index,
 611                                                                           candidate);
 612                     resulting_candidates[0].push(new_candidate);
 613                     true
 614                 } else {
 615                     false
 616                 }
 617             }
 618         }
 619     }
 620
 621     fn candidate_without_match_pair<'pat>(&mut self,
 622                                           match_pair_index: usize,
 623                                           candidate: &Candidate<'pat, 'tcx>)
 624                                           -> Candidate<'pat, 'tcx> {
 625         let other_match_pairs =
 626             candidate.match_pairs.iter()
 627                                  .enumerate()
 628                                  .filter(|&(index, _)| index != match_pair_index)
 629                                  .map(|(_, mp)| mp.clone())
 630                                  .collect();
 631         Candidate {
 632             span: candidate.span,
 633             match_pairs: other_match_pairs,
 634             bindings: candidate.bindings.clone(),
 635             guard: candidate.guard.clone(),
 636             arm_index: candidate.arm_index,
 637             pre_binding_block: candidate.pre_binding_block,
 638             next_candidate_pre_binding_block: candidate.next_candidate_pre_binding_block,
 639         }
 640     }
 641
 642     fn candidate_after_slice_test<'pat>(&mut self,
 643                                         match_pair_index: usize,
 644                                         candidate: &Candidate<'pat, 'tcx>,
 645                                         prefix: &'pat [Pattern<'tcx>],
 646                                         opt_slice: Option<&'pat Pattern<'tcx>>,
 647                                         suffix: &'pat [Pattern<'tcx>])
 648                                         -> Candidate<'pat, 'tcx> {
 649         let mut new_candidate =
 650             self.candidate_without_match_pair(match_pair_index, candidate);
 651         self.prefix_slice_suffix(
 652             &mut new_candidate.match_pairs,
 653             &candidate.match_pairs[match_pair_index].place,
 654             prefix,
 655             opt_slice,
 656             suffix);
 657
 658         new_candidate
 659     }
 660
 661     fn candidate_after_variant_switch<'pat>(&mut self,
 662                                             match_pair_index: usize,
 663                                             adt_def: &'tcx ty::AdtDef,
 664                                             variant_index: usize,
 665                                             subpatterns: &'pat [FieldPattern<'tcx>],
 666                                             candidate: &Candidate<'pat, 'tcx>)
 667                                             -> Candidate<'pat, 'tcx> {
 668         let match_pair = &candidate.match_pairs[match_pair_index];
 669
 670         // So, if we have a match-pattern like `x @ Enum::Variant(P1, P2)`,
 671         // we want to create a set of derived match-patterns like
 672         // `(x as Variant).0 @ P1` and `(x as Variant).1 @ P1`.
 673         let elem = ProjectionElem::Downcast(adt_def, variant_index);
 674         let downcast_place = match_pair.place.clone().elem(elem); // `(x as Variant)`
 675         let consequent_match_pairs =
 676             subpatterns.iter()
 677                        .map(|subpattern| {
 678                            // e.g., `(x as Variant).0`
 679                            let place = downcast_place.clone().field(subpattern.field,
 680                                                                       subpattern.pattern.ty);
 681                            // e.g., `(x as Variant).0 @ P1`
 682                            MatchPair::new(place, &subpattern.pattern)
 683                        });
 684
 685         // In addition, we need all the other match pairs from the old candidate.
 686         let other_match_pairs =
 687             candidate.match_pairs.iter()
 688                                  .enumerate()
 689                                  .filter(|&(index, _)| index != match_pair_index)
 690                                  .map(|(_, mp)| mp.clone());
 691
 692         let all_match_pairs = consequent_match_pairs.chain(other_match_pairs).collect();
 693
 694         Candidate {
 695             span: candidate.span,
 696             match_pairs: all_match_pairs,
 697             bindings: candidate.bindings.clone(),
 698             guard: candidate.guard.clone(),
 699             arm_index: candidate.arm_index,
 700             pre_binding_block: candidate.pre_binding_block,
 701             next_candidate_pre_binding_block: candidate.next_candidate_pre_binding_block,
 702         }
 703     }
 704
 705     fn error_simplifyable<'pat>(&mut self, match_pair: &MatchPair<'pat, 'tcx>) -> ! {
 706         span_bug!(match_pair.pattern.span,
 707                   "simplifyable pattern found: {:?}",
 708                   match_pair.pattern)
 709     }
 710 }
 711
 712 fn is_switch_ty<'tcx>(ty: Ty<'tcx>) -> bool {
 713     ty.is_integral() || ty.is_char() || ty.is_bool()
 714 }