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::BitArray;
  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: BitArray::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, ty, end } => {
  74                 assert!(ty == match_pair.pattern.ty);
  75                 Test {
  76                     span: match_pair.pattern.span,
  77                     kind: TestKind::Range {
  78                         lo,
  79                         hi,
  80                         ty,
  81                         end,
  82                     },
  83                 }
  84             }
  85
  86             PatternKind::Slice { ref prefix, ref slice, ref suffix }
  87                     if !match_pair.slice_len_checked => {
  88                 let len = prefix.len() + suffix.len();
  89                 let op = if slice.is_some() {
  90                     BinOp::Ge
  91                 } else {
  92                     BinOp::Eq
  93                 };
  94                 Test {
  95                     span: match_pair.pattern.span,
  96                     kind: TestKind::Len { len: len as u64, op: op },
  97                 }
  98             }
  99
 100             PatternKind::AscribeUserType { .. } |
 101             PatternKind::Array { .. } |
 102             PatternKind::Slice { .. } |
 103             PatternKind::Wild |
 104             PatternKind::Binding { .. } |
 105             PatternKind::Leaf { .. } |
 106             PatternKind::Deref { .. } => {
 107                 self.error_simplifyable(match_pair)
 108             }
 109         }
 110     }
 111
 112     pub fn add_cases_to_switch<'pat>(&mut self,
 113                                      test_place: &Place<'tcx>,
 114                                      candidate: &Candidate<'pat, 'tcx>,
 115                                      switch_ty: Ty<'tcx>,
 116                                      options: &mut Vec<u128>,
 117                                      indices: &mut FxHashMap<&'tcx ty::Const<'tcx>, usize>)
 118                                      -> bool
 119     {
 120         let match_pair = match candidate.match_pairs.iter().find(|mp| mp.place == *test_place) {
 121             Some(match_pair) => match_pair,
 122             _ => { return false; }
 123         };
 124
 125         match *match_pair.pattern.kind {
 126             PatternKind::Constant { value } => {
 127                 let switch_ty = ty::ParamEnv::empty().and(switch_ty);
 128                 indices.entry(value)
 129                        .or_insert_with(|| {
 130                            options.push(value.unwrap_bits(self.hir.tcx(), switch_ty));
 131                            options.len() - 1
 132                        });
 133                 true
 134             }
 135             PatternKind::Variant { .. } => {
 136                 panic!("you should have called add_variants_to_switch instead!");
 137             }
 138             PatternKind::Range { .. } |
 139             PatternKind::Slice { .. } |
 140             PatternKind::Array { .. } |
 141             PatternKind::Wild |
 142             PatternKind::Binding { .. } |
 143             PatternKind::AscribeUserType { .. } |
 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_place: &Place<'tcx>,
 154                                         candidate: &Candidate<'pat, 'tcx>,
 155                                         variants: &mut BitArray<usize>)
 156                                         -> bool
 157     {
 158         let match_pair = match candidate.match_pairs.iter().find(|mp| mp.place == *test_place) {
 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                         place: &Place<'tcx>,
 181                         test: &Test<'tcx>)
 182                         -> Vec<BasicBlock> {
 183         debug!("perform_test({:?}, {:?}: {:?}, {:?})",
 184                block,
 185                place,
 186                place.ty(&self.local_decls, self.hir.tcx()),
 187                test);
 188         let source_info = self.source_info(test.span);
 189         match test.kind {
 190             TestKind::Switch { adt_def, ref variants } => {
 191                 // Variants is a BitVec of indexes into adt_def.variants.
 192                 let num_enum_variants = adt_def.variants.len();
 193                 let used_variants = variants.count();
 194                 let mut otherwise_block = None;
 195                 let mut target_blocks = Vec::with_capacity(num_enum_variants);
 196                 let mut targets = Vec::with_capacity(used_variants + 1);
 197                 let mut values = Vec::with_capacity(used_variants);
 198                 let tcx = self.hir.tcx();
 199                 for (idx, discr) in adt_def.discriminants(tcx).enumerate() {
 200                     target_blocks.push(if variants.contains(idx) {
 201                         values.push(discr.val);
 202                         targets.push(self.cfg.start_new_block());
 203                         *targets.last().unwrap()
 204                     } else {
 205                         if otherwise_block.is_none() {
 206                             otherwise_block = Some(self.cfg.start_new_block());
 207                         }
 208                         otherwise_block.unwrap()
 209                     });
 210                 }
 211                 if let Some(otherwise_block) = otherwise_block {
 212                     targets.push(otherwise_block);
 213                 } else {
 214                     targets.push(self.unreachable_block());
 215                 }
 216                 debug!("num_enum_variants: {}, tested variants: {:?}, variants: {:?}",
 217                        num_enum_variants, values, variants);
 218                 let discr_ty = adt_def.repr.discr_type().to_ty(tcx);
 219                 let discr = self.temp(discr_ty, test.span);
 220                 self.cfg.push_assign(block, source_info, &discr,
 221                                      Rvalue::Discriminant(place.clone()));
 222                 assert_eq!(values.len() + 1, targets.len());
 223                 self.cfg.terminate(block, source_info, TerminatorKind::SwitchInt {
 224                     discr: Operand::Move(discr),
 225                     switch_ty: discr_ty,
 226                     values: From::from(values),
 227                     targets,
 228                 });
 229                 target_blocks
 230             }
 231
 232             TestKind::SwitchInt { switch_ty, ref options, indices: _ } => {
 233                 let (ret, terminator) = if switch_ty.sty == ty::Bool {
 234                     assert!(options.len() > 0 && options.len() <= 2);
 235                     let (true_bb, false_bb) = (self.cfg.start_new_block(),
 236                                                self.cfg.start_new_block());
 237                     let ret = match options[0] {
 238                         1 => vec![true_bb, false_bb],
 239                         0 => vec![false_bb, true_bb],
 240                         v => span_bug!(test.span, "expected boolean value but got {:?}", v)
 241                     };
 242                     (ret, TerminatorKind::if_(self.hir.tcx(), Operand::Copy(place.clone()),
 243                                               true_bb, false_bb))
 244                 } else {
 245                     // The switch may be inexhaustive so we
 246                     // add a catch all block
 247                     let otherwise = self.cfg.start_new_block();
 248                     let targets: Vec<_> =
 249                         options.iter()
 250                                .map(|_| self.cfg.start_new_block())
 251                                .chain(Some(otherwise))
 252                                .collect();
 253                     (targets.clone(), TerminatorKind::SwitchInt {
 254                         discr: Operand::Copy(place.clone()),
 255                         switch_ty,
 256                         values: options.clone().into(),
 257                         targets,
 258                     })
 259                 };
 260                 self.cfg.terminate(block, source_info, terminator);
 261                 ret
 262             }
 263
 264             TestKind::Eq { value, mut ty } => {
 265                 let val = Operand::Copy(place.clone());
 266                 let mut expect = self.literal_operand(test.span, ty, value);
 267                 // Use PartialEq::eq instead of BinOp::Eq
 268                 // (the binop can only handle primitives)
 269                 let fail = self.cfg.start_new_block();
 270                 if !ty.is_scalar() {
 271                     // If we're using b"..." as a pattern, we need to insert an
 272                     // unsizing coercion, as the byte string has the type &[u8; N].
 273                     //
 274                     // We want to do this even when the scrutinee is a reference to an
 275                     // array, so we can call `<[u8]>::eq` rather than having to find an
 276                     // `<[u8; N]>::eq`.
 277                     let unsize = |ty: Ty<'tcx>| match ty.sty {
 278                         ty::Ref(region, rty, _) => match rty.sty {
 279                             ty::Array(inner_ty, n) => Some((region, inner_ty, n)),
 280                             _ => None,
 281                         },
 282                         _ => None,
 283                     };
 284                     let opt_ref_ty = unsize(ty);
 285                     let opt_ref_test_ty = unsize(value.ty);
 286                     let mut place = place.clone();
 287                     match (opt_ref_ty, opt_ref_test_ty) {
 288                         // nothing to do, neither is an array
 289                         (None, None) => {},
 290                         (Some((region, elem_ty, _)), _) |
 291                         (None, Some((region, elem_ty, _))) => {
 292                             let tcx = self.hir.tcx();
 293                             // make both a slice
 294                             ty = tcx.mk_imm_ref(region, tcx.mk_slice(elem_ty));
 295                             if opt_ref_ty.is_some() {
 296                                 place = self.temp(ty, test.span);
 297                                 self.cfg.push_assign(block, source_info, &place,
 298                                                     Rvalue::Cast(CastKind::Unsize, val, ty));
 299                             }
 300                             if opt_ref_test_ty.is_some() {
 301                                 let array = self.literal_operand(
 302                                     test.span,
 303                                     value.ty,
 304                                     value,
 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
 351                             // FIXME(#47184): This constant comes from user
 352                             // input (a constant in a pattern).  Are
 353                             // there forms where users can add type
 354                             // annotations here?  For example, an
 355                             // associated constant? Need to
 356                             // experiment.
 357                             user_ty: None,
 358
 359                             literal: method,
 360                         }),
 361                         args: vec![val, expect],
 362                         destination: Some((eq_result.clone(), eq_block)),
 363                         cleanup: Some(cleanup),
 364                     });
 365
 366                     // check the result
 367                     let block = self.cfg.start_new_block();
 368                     self.cfg.terminate(eq_block, source_info,
 369                                        TerminatorKind::if_(self.hir.tcx(),
 370                                                            Operand::Move(eq_result),
 371                                                            block, fail));
 372                     vec![block, fail]
 373                 } else {
 374                     let block = self.compare(block, fail, test.span, BinOp::Eq, expect, val);
 375                     vec![block, fail]
 376                 }
 377             }
 378
 379             TestKind::Range { ref lo, ref hi, ty, ref end } => {
 380                 // Test `val` by computing `lo <= val && val <= hi`, using primitive comparisons.
 381                 let lo = self.literal_operand(test.span, ty.clone(), lo.clone());
 382                 let hi = self.literal_operand(test.span, ty.clone(), hi.clone());
 383                 let val = Operand::Copy(place.clone());
 384
 385                 let fail = self.cfg.start_new_block();
 386                 let block = self.compare(block, fail, test.span, BinOp::Le, lo, val.clone());
 387                 let block = match *end {
 388                     RangeEnd::Included => self.compare(block, fail, test.span, BinOp::Le, val, hi),
 389                     RangeEnd::Excluded => self.compare(block, fail, test.span, BinOp::Lt, val, hi),
 390                 };
 391
 392                 vec![block, fail]
 393             }
 394
 395             TestKind::Len { len, op } => {
 396                 let (usize_ty, bool_ty) = (self.hir.usize_ty(), self.hir.bool_ty());
 397                 let (actual, result) = (self.temp(usize_ty, test.span),
 398                                         self.temp(bool_ty, test.span));
 399
 400                 // actual = len(place)
 401                 self.cfg.push_assign(block, source_info,
 402                                      &actual, Rvalue::Len(place.clone()));
 403
 404                 // expected = <N>
 405                 let expected = self.push_usize(block, source_info, len);
 406
 407                 // result = actual == expected OR result = actual < expected
 408                 self.cfg.push_assign(block, source_info, &result,
 409                                      Rvalue::BinaryOp(op,
 410                                                       Operand::Move(actual),
 411                                                       Operand::Move(expected)));
 412
 413                 // branch based on result
 414                 let (false_bb, true_bb) = (self.cfg.start_new_block(),
 415                                            self.cfg.start_new_block());
 416                 self.cfg.terminate(block, source_info,
 417                                    TerminatorKind::if_(self.hir.tcx(), Operand::Move(result),
 418                                                        true_bb, false_bb));
 419                 vec![true_bb, false_bb]
 420             }
 421         }
 422     }
 423
 424     fn compare(&mut self,
 425                block: BasicBlock,
 426                fail_block: BasicBlock,
 427                span: Span,
 428                op: BinOp,
 429                left: Operand<'tcx>,
 430                right: Operand<'tcx>) -> BasicBlock {
 431         let bool_ty = self.hir.bool_ty();
 432         let result = self.temp(bool_ty, span);
 433
 434         // result = op(left, right)
 435         let source_info = self.source_info(span);
 436         self.cfg.push_assign(block, source_info, &result,
 437                              Rvalue::BinaryOp(op, left, right));
 438
 439         // branch based on result
 440         let target_block = self.cfg.start_new_block();
 441         self.cfg.terminate(block, source_info,
 442                            TerminatorKind::if_(self.hir.tcx(), Operand::Move(result),
 443                                                target_block, fail_block));
 444         target_block
 445     }
 446
 447     /// Given that we are performing `test` against `test_place`,
 448     /// this job sorts out what the status of `candidate` will be
 449     /// after the test. The `resulting_candidates` vector stores, for
 450     /// each possible outcome of `test`, a vector of the candidates
 451     /// that will result. This fn should add a (possibly modified)
 452     /// clone of candidate into `resulting_candidates` wherever
 453     /// appropriate.
 454     ///
 455     /// So, for example, if this candidate is `x @ Some(P0)` and the
 456     /// test is a variant test, then we would add `(x as Option).0 @
 457     /// P0` to the `resulting_candidates` entry corresponding to the
 458     /// variant `Some`.
 459     ///
 460     /// However, in some cases, the test may just not be relevant to
 461     /// candidate. For example, suppose we are testing whether `foo.x == 22`,
 462     /// but in one match arm we have `Foo { x: _, ... }`... in that case,
 463     /// the test for what value `x` has has no particular relevance
 464     /// to this candidate. In such cases, this function just returns false
 465     /// without doing anything. This is used by the overall `match_candidates`
 466     /// algorithm to structure the match as a whole. See `match_candidates` for
 467     /// more details.
 468     ///
 469     /// FIXME(#29623). In some cases, we have some tricky choices to
 470     /// make.  for example, if we are testing that `x == 22`, but the
 471     /// candidate is `x @ 13..55`, what should we do? In the event
 472     /// that the test is true, we know that the candidate applies, but
 473     /// in the event of false, we don't know that it *doesn't*
 474     /// apply. For now, we return false, indicate that the test does
 475     /// not apply to this candidate, but it might be we can get
 476     /// tighter match code if we do something a bit different.
 477     pub fn sort_candidate<'pat>(&mut self,
 478                                 test_place: &Place<'tcx>,
 479                                 test: &Test<'tcx>,
 480                                 candidate: &Candidate<'pat, 'tcx>,
 481                                 resulting_candidates: &mut [Vec<Candidate<'pat, 'tcx>>])
 482                                 -> bool {
 483         // Find the match_pair for this place (if any). At present,
 484         // afaik, there can be at most one. (In the future, if we
 485         // adopted a more general `@` operator, there might be more
 486         // than one, but it'd be very unusual to have two sides that
 487         // both require tests; you'd expect one side to be simplified
 488         // away.)
 489         let tested_match_pair = candidate.match_pairs.iter()
 490                                                      .enumerate()
 491                                                      .filter(|&(_, mp)| mp.place == *test_place)
 492                                                      .next();
 493         let (match_pair_index, match_pair) = match tested_match_pair {
 494             Some(pair) => pair,
 495             None => {
 496                 // We are not testing this place. Therefore, this
 497                 // candidate applies to ALL outcomes.
 498                 return false;
 499             }
 500         };
 501
 502         match (&test.kind, &*match_pair.pattern.kind) {
 503             // If we are performing a variant switch, then this
 504             // informs variant patterns, but nothing else.
 505             (&TestKind::Switch { adt_def: tested_adt_def, .. },
 506              &PatternKind::Variant { adt_def, variant_index, ref subpatterns, .. }) => {
 507                 assert_eq!(adt_def, tested_adt_def);
 508                 let new_candidate =
 509                     self.candidate_after_variant_switch(match_pair_index,
 510                                                         adt_def,
 511                                                         variant_index,
 512                                                         subpatterns,
 513                                                         candidate);
 514                 resulting_candidates[variant_index].push(new_candidate);
 515                 true
 516             }
 517             (&TestKind::Switch { .. }, _) => false,
 518
 519             // If we are performing a switch over integers, then this informs integer
 520             // equality, but nothing else.
 521             //
 522             // FIXME(#29623) we could use PatternKind::Range to rule
 523             // things out here, in some cases.
 524             (&TestKind::SwitchInt { switch_ty: _, options: _, ref indices },
 525              &PatternKind::Constant { ref value })
 526             if is_switch_ty(match_pair.pattern.ty) => {
 527                 let index = indices[value];
 528                 let new_candidate = self.candidate_without_match_pair(match_pair_index,
 529                                                                       candidate);
 530                 resulting_candidates[index].push(new_candidate);
 531                 true
 532             }
 533             (&TestKind::SwitchInt { .. }, _) => false,
 534
 535
 536             (&TestKind::Len { len: test_len, op: BinOp::Eq },
 537              &PatternKind::Slice { ref prefix, ref slice, ref suffix }) => {
 538                 let pat_len = (prefix.len() + suffix.len()) as u64;
 539                 match (test_len.cmp(&pat_len), slice) {
 540                     (Ordering::Equal, &None) => {
 541                         // on true, min_len = len = $actual_length,
 542                         // on false, len != $actual_length
 543                         resulting_candidates[0].push(
 544                             self.candidate_after_slice_test(match_pair_index,
 545                                                             candidate,
 546                                                             prefix,
 547                                                             slice.as_ref(),
 548                                                             suffix)
 549                         );
 550                         true
 551                     }
 552                     (Ordering::Less, _) => {
 553                         // test_len < pat_len. If $actual_len = test_len,
 554                         // then $actual_len < pat_len and we don't have
 555                         // enough elements.
 556                         resulting_candidates[1].push(candidate.clone());
 557                         true
 558                     }
 559                     (Ordering::Equal, &Some(_)) | (Ordering::Greater, &Some(_)) => {
 560                         // This can match both if $actual_len = test_len >= pat_len,
 561                         // and if $actual_len > test_len. We can't advance.
 562                         false
 563                     }
 564                     (Ordering::Greater, &None) => {
 565                         // test_len != pat_len, so if $actual_len = test_len, then
 566                         // $actual_len != pat_len.
 567                         resulting_candidates[1].push(candidate.clone());
 568                         true
 569                     }
 570                 }
 571             }
 572
 573             (&TestKind::Len { len: test_len, op: BinOp::Ge },
 574              &PatternKind::Slice { ref prefix, ref slice, ref suffix }) => {
 575                 // the test is `$actual_len >= test_len`
 576                 let pat_len = (prefix.len() + suffix.len()) as u64;
 577                 match (test_len.cmp(&pat_len), slice) {
 578                     (Ordering::Equal, &Some(_))  => {
 579                         // $actual_len >= test_len = pat_len,
 580                         // so we can match.
 581                         resulting_candidates[0].push(
 582                             self.candidate_after_slice_test(match_pair_index,
 583                                                             candidate,
 584                                                             prefix,
 585                                                             slice.as_ref(),
 586                                                             suffix)
 587                         );
 588                         true
 589                     }
 590                     (Ordering::Less, _) | (Ordering::Equal, &None) => {
 591                         // test_len <= pat_len. If $actual_len < test_len,
 592                         // then it is also < pat_len, so the test passing is
 593                         // necessary (but insufficient).
 594                         resulting_candidates[0].push(candidate.clone());
 595                         true
 596                     }
 597                     (Ordering::Greater, &None) => {
 598                         // test_len > pat_len. If $actual_len >= test_len > pat_len,
 599                         // then we know we won't have a match.
 600                         resulting_candidates[1].push(candidate.clone());
 601                         true
 602                     }
 603                     (Ordering::Greater, &Some(_)) => {
 604                         // test_len < pat_len, and is therefore less
 605                         // strict. This can still go both ways.
 606                         false
 607                     }
 608                 }
 609             }
 610
 611             (&TestKind::Eq { .. }, _) |
 612             (&TestKind::Range { .. }, _) |
 613             (&TestKind::Len { .. }, _) => {
 614                 // These are all binary tests.
 615                 //
 616                 // FIXME(#29623) we can be more clever here
 617                 let pattern_test = self.test(&match_pair);
 618                 if pattern_test.kind == test.kind {
 619                     let new_candidate = self.candidate_without_match_pair(match_pair_index,
 620                                                                           candidate);
 621                     resulting_candidates[0].push(new_candidate);
 622                     true
 623                 } else {
 624                     false
 625                 }
 626             }
 627         }
 628     }
 629
 630     fn candidate_without_match_pair<'pat>(&mut self,
 631                                           match_pair_index: usize,
 632                                           candidate: &Candidate<'pat, 'tcx>)
 633                                           -> Candidate<'pat, 'tcx> {
 634         let other_match_pairs =
 635             candidate.match_pairs.iter()
 636                                  .enumerate()
 637                                  .filter(|&(index, _)| index != match_pair_index)
 638                                  .map(|(_, mp)| mp.clone())
 639                                  .collect();
 640         Candidate {
 641             span: candidate.span,
 642             match_pairs: other_match_pairs,
 643             bindings: candidate.bindings.clone(),
 644             ascriptions: candidate.ascriptions.clone(),
 645             guard: candidate.guard.clone(),
 646             arm_index: candidate.arm_index,
 647             pat_index: candidate.pat_index,
 648             pre_binding_block: candidate.pre_binding_block,
 649             next_candidate_pre_binding_block: candidate.next_candidate_pre_binding_block,
 650         }
 651     }
 652
 653     fn candidate_after_slice_test<'pat>(&mut self,
 654                                         match_pair_index: usize,
 655                                         candidate: &Candidate<'pat, 'tcx>,
 656                                         prefix: &'pat [Pattern<'tcx>],
 657                                         opt_slice: Option<&'pat Pattern<'tcx>>,
 658                                         suffix: &'pat [Pattern<'tcx>])
 659                                         -> Candidate<'pat, 'tcx> {
 660         let mut new_candidate =
 661             self.candidate_without_match_pair(match_pair_index, candidate);
 662         self.prefix_slice_suffix(
 663             &mut new_candidate.match_pairs,
 664             &candidate.match_pairs[match_pair_index].place,
 665             prefix,
 666             opt_slice,
 667             suffix);
 668
 669         new_candidate
 670     }
 671
 672     fn candidate_after_variant_switch<'pat>(&mut self,
 673                                             match_pair_index: usize,
 674                                             adt_def: &'tcx ty::AdtDef,
 675                                             variant_index: usize,
 676                                             subpatterns: &'pat [FieldPattern<'tcx>],
 677                                             candidate: &Candidate<'pat, 'tcx>)
 678                                             -> Candidate<'pat, 'tcx> {
 679         let match_pair = &candidate.match_pairs[match_pair_index];
 680
 681         // So, if we have a match-pattern like `x @ Enum::Variant(P1, P2)`,
 682         // we want to create a set of derived match-patterns like
 683         // `(x as Variant).0 @ P1` and `(x as Variant).1 @ P1`.
 684         let elem = ProjectionElem::Downcast(adt_def, variant_index);
 685         let downcast_place = match_pair.place.clone().elem(elem); // `(x as Variant)`
 686         let consequent_match_pairs =
 687             subpatterns.iter()
 688                        .map(|subpattern| {
 689                            // e.g., `(x as Variant).0`
 690                            let place = downcast_place.clone().field(subpattern.field,
 691                                                                       subpattern.pattern.ty);
 692                            // e.g., `(x as Variant).0 @ P1`
 693                            MatchPair::new(place, &subpattern.pattern)
 694                        });
 695
 696         // In addition, we need all the other match pairs from the old candidate.
 697         let other_match_pairs =
 698             candidate.match_pairs.iter()
 699                                  .enumerate()
 700                                  .filter(|&(index, _)| index != match_pair_index)
 701                                  .map(|(_, mp)| mp.clone());
 702
 703         let all_match_pairs = consequent_match_pairs.chain(other_match_pairs).collect();
 704
 705         Candidate {
 706             span: candidate.span,
 707             match_pairs: all_match_pairs,
 708             bindings: candidate.bindings.clone(),
 709             ascriptions: candidate.ascriptions.clone(),
 710             guard: candidate.guard.clone(),
 711             arm_index: candidate.arm_index,
 712             pat_index: candidate.pat_index,
 713             pre_binding_block: candidate.pre_binding_block,
 714             next_candidate_pre_binding_block: candidate.next_candidate_pre_binding_block,
 715         }
 716     }
 717
 718     fn error_simplifyable<'pat>(&mut self, match_pair: &MatchPair<'pat, 'tcx>) -> ! {
 719         span_bug!(match_pair.pattern.span,
 720                   "simplifyable pattern found: {:?}",
 721                   match_pair.pattern)
 722     }
 723 }
 724
 725 fn is_switch_ty<'tcx>(ty: Ty<'tcx>) -> bool {
 726     ty.is_integral() || ty.is_char() || ty.is_bool()
 727 }