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                 let switch_ty = ty::ParamEnv::empty().and(switch_ty);
 126                 indices.entry(value)
 127                        .or_insert_with(|| {
 128                            options.push(value.unwrap_bits(self.hir.tcx(), switch_ty));
 129                            options.len() - 1
 130                        });
 131                 true
 132             }
 133             PatternKind::Variant { .. } => {
 134                 panic!("you should have called add_variants_to_switch instead!");
 135             }
 136             PatternKind::Range { .. } |
 137             PatternKind::Slice { .. } |
 138             PatternKind::Array { .. } |
 139             PatternKind::Wild |
 140             PatternKind::Binding { .. } |
 141             PatternKind::Leaf { .. } |
 142             PatternKind::Deref { .. } => {
 143                 // don't know how to add these patterns to a switch
 144                 false
 145             }
 146         }
 147     }
 148
 149     pub fn add_variants_to_switch<'pat>(&mut self,
 150                                         test_place: &Place<'tcx>,
 151                                         candidate: &Candidate<'pat, 'tcx>,
 152                                         variants: &mut BitVector)
 153                                         -> bool
 154     {
 155         let match_pair = match candidate.match_pairs.iter().find(|mp| mp.place == *test_place) {
 156             Some(match_pair) => match_pair,
 157             _ => { return false; }
 158         };
 159
 160         match *match_pair.pattern.kind {
 161             PatternKind::Variant { adt_def: _ , variant_index,  .. } => {
 162                 // We have a pattern testing for variant `variant_index`
 163                 // set the corresponding index to true
 164                 variants.insert(variant_index);
 165                 true
 166             }
 167             _ => {
 168                 // don't know how to add these patterns to a switch
 169                 false
 170             }
 171         }
 172     }
 173
 174     /// Generates the code to perform a test.
 175     pub fn perform_test(&mut self,
 176                         block: BasicBlock,
 177                         place: &Place<'tcx>,
 178                         test: &Test<'tcx>)
 179                         -> Vec<BasicBlock> {
 180         debug!("perform_test({:?}, {:?}: {:?}, {:?})",
 181                block,
 182                place,
 183                place.ty(&self.local_decls, self.hir.tcx()),
 184                test);
 185         let source_info = self.source_info(test.span);
 186         match test.kind {
 187             TestKind::Switch { adt_def, ref variants } => {
 188                 // Variants is a BitVec of indexes into adt_def.variants.
 189                 let num_enum_variants = adt_def.variants.len();
 190                 let used_variants = variants.count();
 191                 let mut otherwise_block = None;
 192                 let mut target_blocks = Vec::with_capacity(num_enum_variants);
 193                 let mut targets = Vec::with_capacity(used_variants + 1);
 194                 let mut values = Vec::with_capacity(used_variants);
 195                 let tcx = self.hir.tcx();
 196                 for (idx, discr) in adt_def.discriminants(tcx).enumerate() {
 197                     target_blocks.push(if variants.contains(idx) {
 198                         values.push(discr.val);
 199                         targets.push(self.cfg.start_new_block());
 200                         *targets.last().unwrap()
 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                     targets.push(self.unreachable_block());
 212                 }
 213                 debug!("num_enum_variants: {}, tested variants: {:?}, variants: {:?}",
 214                        num_enum_variants, values, variants);
 215                 let discr_ty = adt_def.repr.discr_type().to_ty(tcx);
 216                 let discr = self.temp(discr_ty, test.span);
 217                 self.cfg.push_assign(block, source_info, &discr,
 218                                      Rvalue::Discriminant(place.clone()));
 219                 assert_eq!(values.len() + 1, targets.len());
 220                 self.cfg.terminate(block, source_info, TerminatorKind::SwitchInt {
 221                     discr: Operand::Move(discr),
 222                     switch_ty: discr_ty,
 223                     values: From::from(values),
 224                     targets,
 225                 });
 226                 target_blocks
 227             }
 228
 229             TestKind::SwitchInt { switch_ty, ref options, indices: _ } => {
 230                 let (ret, terminator) = if switch_ty.sty == ty::TyBool {
 231                     assert!(options.len() > 0 && options.len() <= 2);
 232                     let (true_bb, false_bb) = (self.cfg.start_new_block(),
 233                                                self.cfg.start_new_block());
 234                     let ret = match options[0] {
 235                         1 => vec![true_bb, false_bb],
 236                         0 => vec![false_bb, true_bb],
 237                         v => span_bug!(test.span, "expected boolean value but got {:?}", v)
 238                     };
 239                     (ret, TerminatorKind::if_(self.hir.tcx(), Operand::Copy(place.clone()),
 240                                               true_bb, false_bb))
 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                     (targets.clone(), TerminatorKind::SwitchInt {
 251                         discr: Operand::Copy(place.clone()),
 252                         switch_ty,
 253                         values: options.clone().into(),
 254                         targets,
 255                     })
 256                 };
 257                 self.cfg.terminate(block, source_info, terminator);
 258                 ret
 259             }
 260
 261             TestKind::Eq { value, mut ty } => {
 262                 let mut val = Operand::Copy(place.clone());
 263                 let mut expect = self.literal_operand(test.span, ty, Literal::Value {
 264                     value
 265                 });
 266                 // Use PartialEq::eq instead of BinOp::Eq
 267                 // (the binop can only handle primitives)
 268                 let fail = self.cfg.start_new_block();
 269                 if !ty.is_scalar() {
 270                     // If we're using b"..." as a pattern, we need to insert an
 271                     // unsizing coercion, as the byte string has the type &[u8; N].
 272                     //
 273                     // We want to do this even when the scrutinee is a reference to an
 274                     // array, so we can call `<[u8]>::eq` rather than having to find an
 275                     // `<[u8; N]>::eq`.
 276                     let unsize = |ty: Ty<'tcx>| match ty.sty {
 277                         ty::TyRef(region, rty, _) => match rty.sty {
 278                             ty::TyArray(inner_ty, n) => Some((region, inner_ty, n)),
 279                             _ => None,
 280                         },
 281                         _ => None,
 282                     };
 283                     let opt_ref_ty = unsize(ty);
 284                     let opt_ref_test_ty = unsize(value.ty);
 285                     let mut place = place.clone();
 286                     match (opt_ref_ty, opt_ref_test_ty) {
 287                         // nothing to do, neither is an array
 288                         (None, None) => {},
 289                         (Some((region, elem_ty, _)), _) |
 290                         (None, Some((region, elem_ty, _))) => {
 291                             let tcx = self.hir.tcx();
 292                             // make both a slice
 293                             ty = tcx.mk_imm_ref(region, tcx.mk_slice(elem_ty));
 294                             if opt_ref_ty.is_some() {
 295                                 place = self.temp(ty, test.span);
 296                                 self.cfg.push_assign(block, source_info, &place,
 297                                                     Rvalue::Cast(CastKind::Unsize, val, ty));
 298                             }
 299                             if opt_ref_test_ty.is_some() {
 300                                 let array = self.literal_operand(
 301                                     test.span,
 302                                     value.ty,
 303                                     Literal::Value {
 304                                         value
 305                                     },
 306                                 );
 307
 308                                 let slice = self.temp(ty, test.span);
 309                                 self.cfg.push_assign(block, source_info, &slice,
 310                                                     Rvalue::Cast(CastKind::Unsize, array, ty));
 311                                 expect = Operand::Move(slice);
 312                             }
 313                         },
 314                     }
 315                     let eq_def_id = self.hir.tcx().lang_items().eq_trait().unwrap();
 316                     let (mty, method) = self.hir.trait_method(eq_def_id, "eq", ty, &[ty.into()]);
 317
 318                     // take the argument by reference
 319                     let region_scope = self.topmost_scope();
 320                     let region = self.hir.tcx().mk_region(ty::ReScope(region_scope));
 321                     let tam = ty::TypeAndMut {
 322                         ty,
 323                         mutbl: Mutability::MutImmutable,
 324                     };
 325                     let ref_ty = self.hir.tcx().mk_ref(region, tam);
 326
 327                     // let lhs_ref_place = &lhs;
 328                     let ref_rvalue = Rvalue::Ref(region, BorrowKind::Shared, place.clone());
 329                     let lhs_ref_place = self.temp(ref_ty, test.span);
 330                     self.cfg.push_assign(block, source_info, &lhs_ref_place, ref_rvalue);
 331                     let val = Operand::Move(lhs_ref_place);
 332
 333                     // let rhs_place = rhs;
 334                     let rhs_place = self.temp(ty, test.span);
 335                     self.cfg.push_assign(block, source_info, &rhs_place, Rvalue::Use(expect));
 336
 337                     // let rhs_ref_place = &rhs_place;
 338                     let ref_rvalue = Rvalue::Ref(region, BorrowKind::Shared, rhs_place);
 339                     let rhs_ref_place = self.temp(ref_ty, test.span);
 340                     self.cfg.push_assign(block, source_info, &rhs_ref_place, ref_rvalue);
 341                     let expect = Operand::Move(rhs_ref_place);
 342
 343                     let bool_ty = self.hir.bool_ty();
 344                     let eq_result = self.temp(bool_ty, test.span);
 345                     let eq_block = self.cfg.start_new_block();
 346                     let cleanup = self.diverge_cleanup();
 347                     self.cfg.terminate(block, source_info, TerminatorKind::Call {
 348                         func: Operand::Constant(box Constant {
 349                             span: test.span,
 350                             ty: mty,
 351                             literal: method
 352                         }),
 353                         args: vec![val, expect],
 354                         destination: Some((eq_result.clone(), eq_block)),
 355                         cleanup: Some(cleanup),
 356                     });
 357
 358                     // check the result
 359                     let block = self.cfg.start_new_block();
 360                     self.cfg.terminate(eq_block, source_info,
 361                                        TerminatorKind::if_(self.hir.tcx(),
 362                                                            Operand::Move(eq_result),
 363                                                            block, fail));
 364                     vec![block, fail]
 365                 } else {
 366                     let block = self.compare(block, fail, test.span, BinOp::Eq, expect, val);
 367                     vec![block, fail]
 368                 }
 369             }
 370
 371             TestKind::Range { ref lo, ref hi, ty, ref end } => {
 372                 // Test `val` by computing `lo <= val && val <= hi`, using primitive comparisons.
 373                 let lo = self.literal_operand(test.span, ty.clone(), lo.clone());
 374                 let hi = self.literal_operand(test.span, ty.clone(), hi.clone());
 375                 let val = Operand::Copy(place.clone());
 376
 377                 let fail = self.cfg.start_new_block();
 378                 let block = self.compare(block, fail, test.span, BinOp::Le, lo, val.clone());
 379                 let block = match *end {
 380                     RangeEnd::Included => self.compare(block, fail, test.span, BinOp::Le, val, hi),
 381                     RangeEnd::Excluded => self.compare(block, fail, test.span, BinOp::Lt, val, hi),
 382                 };
 383
 384                 vec![block, fail]
 385             }
 386
 387             TestKind::Len { len, op } => {
 388                 let (usize_ty, bool_ty) = (self.hir.usize_ty(), self.hir.bool_ty());
 389                 let (actual, result) = (self.temp(usize_ty, test.span),
 390                                         self.temp(bool_ty, test.span));
 391
 392                 // actual = len(place)
 393                 self.cfg.push_assign(block, source_info,
 394                                      &actual, Rvalue::Len(place.clone()));
 395
 396                 // expected = <N>
 397                 let expected = self.push_usize(block, source_info, len);
 398
 399                 // result = actual == expected OR result = actual < expected
 400                 self.cfg.push_assign(block, source_info, &result,
 401                                      Rvalue::BinaryOp(op,
 402                                                       Operand::Move(actual),
 403                                                       Operand::Move(expected)));
 404
 405                 // branch based on result
 406                 let (false_bb, true_bb) = (self.cfg.start_new_block(),
 407                                            self.cfg.start_new_block());
 408                 self.cfg.terminate(block, source_info,
 409                                    TerminatorKind::if_(self.hir.tcx(), Operand::Move(result),
 410                                                        true_bb, false_bb));
 411                 vec![true_bb, false_bb]
 412             }
 413         }
 414     }
 415
 416     fn compare(&mut self,
 417                block: BasicBlock,
 418                fail_block: BasicBlock,
 419                span: Span,
 420                op: BinOp,
 421                left: Operand<'tcx>,
 422                right: Operand<'tcx>) -> BasicBlock {
 423         let bool_ty = self.hir.bool_ty();
 424         let result = self.temp(bool_ty, span);
 425
 426         // result = op(left, right)
 427         let source_info = self.source_info(span);
 428         self.cfg.push_assign(block, source_info, &result,
 429                              Rvalue::BinaryOp(op, left, right));
 430
 431         // branch based on result
 432         let target_block = self.cfg.start_new_block();
 433         self.cfg.terminate(block, source_info,
 434                            TerminatorKind::if_(self.hir.tcx(), Operand::Move(result),
 435                                                target_block, fail_block));
 436         target_block
 437     }
 438
 439     /// Given that we are performing `test` against `test_place`,
 440     /// this job sorts out what the status of `candidate` will be
 441     /// after the test. The `resulting_candidates` vector stores, for
 442     /// each possible outcome of `test`, a vector of the candidates
 443     /// that will result. This fn should add a (possibly modified)
 444     /// clone of candidate into `resulting_candidates` wherever
 445     /// appropriate.
 446     ///
 447     /// So, for example, if this candidate is `x @ Some(P0)` and the
 448     /// test is a variant test, then we would add `(x as Option).0 @
 449     /// P0` to the `resulting_candidates` entry corresponding to the
 450     /// variant `Some`.
 451     ///
 452     /// However, in some cases, the test may just not be relevant to
 453     /// candidate. For example, suppose we are testing whether `foo.x == 22`,
 454     /// but in one match arm we have `Foo { x: _, ... }`... in that case,
 455     /// the test for what value `x` has has no particular relevance
 456     /// to this candidate. In such cases, this function just returns false
 457     /// without doing anything. This is used by the overall `match_candidates`
 458     /// algorithm to structure the match as a whole. See `match_candidates` for
 459     /// more details.
 460     ///
 461     /// FIXME(#29623). In some cases, we have some tricky choices to
 462     /// make.  for example, if we are testing that `x == 22`, but the
 463     /// candidate is `x @ 13..55`, what should we do? In the event
 464     /// that the test is true, we know that the candidate applies, but
 465     /// in the event of false, we don't know that it *doesn't*
 466     /// apply. For now, we return false, indicate that the test does
 467     /// not apply to this candidate, but it might be we can get
 468     /// tighter match code if we do something a bit different.
 469     pub fn sort_candidate<'pat>(&mut self,
 470                                 test_place: &Place<'tcx>,
 471                                 test: &Test<'tcx>,
 472                                 candidate: &Candidate<'pat, 'tcx>,
 473                                 resulting_candidates: &mut [Vec<Candidate<'pat, 'tcx>>])
 474                                 -> bool {
 475         // Find the match_pair for this place (if any). At present,
 476         // afaik, there can be at most one. (In the future, if we
 477         // adopted a more general `@` operator, there might be more
 478         // than one, but it'd be very unusual to have two sides that
 479         // both require tests; you'd expect one side to be simplified
 480         // away.)
 481         let tested_match_pair = candidate.match_pairs.iter()
 482                                                      .enumerate()
 483                                                      .filter(|&(_, mp)| mp.place == *test_place)
 484                                                      .next();
 485         let (match_pair_index, match_pair) = match tested_match_pair {
 486             Some(pair) => pair,
 487             None => {
 488                 // We are not testing this place. Therefore, this
 489                 // candidate applies to ALL outcomes.
 490                 return false;
 491             }
 492         };
 493
 494         match (&test.kind, &*match_pair.pattern.kind) {
 495             // If we are performing a variant switch, then this
 496             // informs variant patterns, but nothing else.
 497             (&TestKind::Switch { adt_def: tested_adt_def, .. },
 498              &PatternKind::Variant { adt_def, variant_index, ref subpatterns, .. }) => {
 499                 assert_eq!(adt_def, tested_adt_def);
 500                 let new_candidate =
 501                     self.candidate_after_variant_switch(match_pair_index,
 502                                                         adt_def,
 503                                                         variant_index,
 504                                                         subpatterns,
 505                                                         candidate);
 506                 resulting_candidates[variant_index].push(new_candidate);
 507                 true
 508             }
 509             (&TestKind::Switch { .. }, _) => false,
 510
 511             // If we are performing a switch over integers, then this informs integer
 512             // equality, but nothing else.
 513             //
 514             // FIXME(#29623) we could use PatternKind::Range to rule
 515             // things out here, in some cases.
 516             (&TestKind::SwitchInt { switch_ty: _, options: _, ref indices },
 517              &PatternKind::Constant { ref value })
 518             if is_switch_ty(match_pair.pattern.ty) => {
 519                 let index = indices[value];
 520                 let new_candidate = self.candidate_without_match_pair(match_pair_index,
 521                                                                       candidate);
 522                 resulting_candidates[index].push(new_candidate);
 523                 true
 524             }
 525             (&TestKind::SwitchInt { .. }, _) => false,
 526
 527
 528             (&TestKind::Len { len: test_len, op: BinOp::Eq },
 529              &PatternKind::Slice { ref prefix, ref slice, ref suffix }) => {
 530                 let pat_len = (prefix.len() + suffix.len()) as u64;
 531                 match (test_len.cmp(&pat_len), slice) {
 532                     (Ordering::Equal, &None) => {
 533                         // on true, min_len = len = $actual_length,
 534                         // on false, len != $actual_length
 535                         resulting_candidates[0].push(
 536                             self.candidate_after_slice_test(match_pair_index,
 537                                                             candidate,
 538                                                             prefix,
 539                                                             slice.as_ref(),
 540                                                             suffix)
 541                         );
 542                         true
 543                     }
 544                     (Ordering::Less, _) => {
 545                         // test_len < pat_len. If $actual_len = test_len,
 546                         // then $actual_len < pat_len and we don't have
 547                         // enough elements.
 548                         resulting_candidates[1].push(candidate.clone());
 549                         true
 550                     }
 551                     (Ordering::Equal, &Some(_)) | (Ordering::Greater, &Some(_)) => {
 552                         // This can match both if $actual_len = test_len >= pat_len,
 553                         // and if $actual_len > test_len. We can't advance.
 554                         false
 555                     }
 556                     (Ordering::Greater, &None) => {
 557                         // test_len != pat_len, so if $actual_len = test_len, then
 558                         // $actual_len != pat_len.
 559                         resulting_candidates[1].push(candidate.clone());
 560                         true
 561                     }
 562                 }
 563             }
 564
 565             (&TestKind::Len { len: test_len, op: BinOp::Ge },
 566              &PatternKind::Slice { ref prefix, ref slice, ref suffix }) => {
 567                 // the test is `$actual_len >= test_len`
 568                 let pat_len = (prefix.len() + suffix.len()) as u64;
 569                 match (test_len.cmp(&pat_len), slice) {
 570                     (Ordering::Equal, &Some(_))  => {
 571                         // $actual_len >= test_len = pat_len,
 572                         // so we can match.
 573                         resulting_candidates[0].push(
 574                             self.candidate_after_slice_test(match_pair_index,
 575                                                             candidate,
 576                                                             prefix,
 577                                                             slice.as_ref(),
 578                                                             suffix)
 579                         );
 580                         true
 581                     }
 582                     (Ordering::Less, _) | (Ordering::Equal, &None) => {
 583                         // test_len <= pat_len. If $actual_len < test_len,
 584                         // then it is also < pat_len, so the test passing is
 585                         // necessary (but insufficient).
 586                         resulting_candidates[0].push(candidate.clone());
 587                         true
 588                     }
 589                     (Ordering::Greater, &None) => {
 590                         // test_len > pat_len. If $actual_len >= test_len > pat_len,
 591                         // then we know we won't have a match.
 592                         resulting_candidates[1].push(candidate.clone());
 593                         true
 594                     }
 595                     (Ordering::Greater, &Some(_)) => {
 596                         // test_len < pat_len, and is therefore less
 597                         // strict. This can still go both ways.
 598                         false
 599                     }
 600                 }
 601             }
 602
 603             (&TestKind::Eq { .. }, _) |
 604             (&TestKind::Range { .. }, _) |
 605             (&TestKind::Len { .. }, _) => {
 606                 // These are all binary tests.
 607                 //
 608                 // FIXME(#29623) we can be more clever here
 609                 let pattern_test = self.test(&match_pair);
 610                 if pattern_test.kind == test.kind {
 611                     let new_candidate = self.candidate_without_match_pair(match_pair_index,
 612                                                                           candidate);
 613                     resulting_candidates[0].push(new_candidate);
 614                     true
 615                 } else {
 616                     false
 617                 }
 618             }
 619         }
 620     }
 621
 622     fn candidate_without_match_pair<'pat>(&mut self,
 623                                           match_pair_index: usize,
 624                                           candidate: &Candidate<'pat, 'tcx>)
 625                                           -> Candidate<'pat, 'tcx> {
 626         let other_match_pairs =
 627             candidate.match_pairs.iter()
 628                                  .enumerate()
 629                                  .filter(|&(index, _)| index != match_pair_index)
 630                                  .map(|(_, mp)| mp.clone())
 631                                  .collect();
 632         Candidate {
 633             span: candidate.span,
 634             match_pairs: other_match_pairs,
 635             bindings: candidate.bindings.clone(),
 636             guard: candidate.guard.clone(),
 637             arm_index: candidate.arm_index,
 638             pre_binding_block: candidate.pre_binding_block,
 639             next_candidate_pre_binding_block: candidate.next_candidate_pre_binding_block,
 640         }
 641     }
 642
 643     fn candidate_after_slice_test<'pat>(&mut self,
 644                                         match_pair_index: usize,
 645                                         candidate: &Candidate<'pat, 'tcx>,
 646                                         prefix: &'pat [Pattern<'tcx>],
 647                                         opt_slice: Option<&'pat Pattern<'tcx>>,
 648                                         suffix: &'pat [Pattern<'tcx>])
 649                                         -> Candidate<'pat, 'tcx> {
 650         let mut new_candidate =
 651             self.candidate_without_match_pair(match_pair_index, candidate);
 652         self.prefix_slice_suffix(
 653             &mut new_candidate.match_pairs,
 654             &candidate.match_pairs[match_pair_index].place,
 655             prefix,
 656             opt_slice,
 657             suffix);
 658
 659         new_candidate
 660     }
 661
 662     fn candidate_after_variant_switch<'pat>(&mut self,
 663                                             match_pair_index: usize,
 664                                             adt_def: &'tcx ty::AdtDef,
 665                                             variant_index: usize,
 666                                             subpatterns: &'pat [FieldPattern<'tcx>],
 667                                             candidate: &Candidate<'pat, 'tcx>)
 668                                             -> Candidate<'pat, 'tcx> {
 669         let match_pair = &candidate.match_pairs[match_pair_index];
 670
 671         // So, if we have a match-pattern like `x @ Enum::Variant(P1, P2)`,
 672         // we want to create a set of derived match-patterns like
 673         // `(x as Variant).0 @ P1` and `(x as Variant).1 @ P1`.
 674         let elem = ProjectionElem::Downcast(adt_def, variant_index);
 675         let downcast_place = match_pair.place.clone().elem(elem); // `(x as Variant)`
 676         let consequent_match_pairs =
 677             subpatterns.iter()
 678                        .map(|subpattern| {
 679                            // e.g., `(x as Variant).0`
 680                            let place = downcast_place.clone().field(subpattern.field,
 681                                                                       subpattern.pattern.ty);
 682                            // e.g., `(x as Variant).0 @ P1`
 683                            MatchPair::new(place, &subpattern.pattern)
 684                        });
 685
 686         // In addition, we need all the other match pairs from the old candidate.
 687         let other_match_pairs =
 688             candidate.match_pairs.iter()
 689                                  .enumerate()
 690                                  .filter(|&(index, _)| index != match_pair_index)
 691                                  .map(|(_, mp)| mp.clone());
 692
 693         let all_match_pairs = consequent_match_pairs.chain(other_match_pairs).collect();
 694
 695         Candidate {
 696             span: candidate.span,
 697             match_pairs: all_match_pairs,
 698             bindings: candidate.bindings.clone(),
 699             guard: candidate.guard.clone(),
 700             arm_index: candidate.arm_index,
 701             pre_binding_block: candidate.pre_binding_block,
 702             next_candidate_pre_binding_block: candidate.next_candidate_pre_binding_block,
 703         }
 704     }
 705
 706     fn error_simplifyable<'pat>(&mut self, match_pair: &MatchPair<'pat, 'tcx>) -> ! {
 707         span_bug!(match_pair.pattern.span,
 708                   "simplifyable pattern found: {:?}",
 709                   match_pair.pattern)
 710     }
 711 }
 712
 713 fn is_switch_ty<'tcx>(ty: Ty<'tcx>) -> bool {
 714     ty.is_integral() || ty.is_char() || ty.is_bool()
 715 }