src/librustc_mir/build/matches/test.rs

   1 // Testing candidates
   2 //
   3 // After candidates have been simplified, the only match pairs that
   4 // remain are those that require some sort of test. The functions here
   5 // identify what tests are needed, perform the tests, and then filter
   6 // the candidates based on the result.
   7
   8 use crate::build::Builder;
   9 use crate::build::matches::{Candidate, MatchPair, Test, TestKind};
  10 use crate::hair::*;
  11 use crate::hair::pattern::compare_const_vals;
  12 use rustc_data_structures::bit_set::BitSet;
  13 use rustc_data_structures::fx::FxHashMap;
  14 use rustc::ty::{self, Ty};
  15 use rustc::ty::util::IntTypeExt;
  16 use rustc::ty::layout::VariantIdx;
  17 use rustc::mir::*;
  18 use rustc::hir::{RangeEnd, Mutability};
  19 use syntax_pos::Span;
  20 use std::cmp::Ordering;
  21
  22 impl<'a, 'gcx, 'tcx> Builder<'a, 'gcx, 'tcx> {
  23     /// Identifies what test is needed to decide if `match_pair` is applicable.
  24     ///
  25     /// It is a bug to call this with a simplifiable pattern.
  26     pub fn test<'pat>(&mut self, match_pair: &MatchPair<'pat, 'tcx>) -> Test<'tcx> {
  27         match *match_pair.pattern.kind {
  28             PatternKind::Variant { ref adt_def, substs: _, variant_index: _, subpatterns: _ } => {
  29                 Test {
  30                     span: match_pair.pattern.span,
  31                     kind: TestKind::Switch {
  32                         adt_def: adt_def.clone(),
  33                         variants: BitSet::new_empty(adt_def.variants.len()),
  34                     },
  35                 }
  36             }
  37
  38             PatternKind::Constant { .. } if is_switch_ty(match_pair.pattern.ty) => {
  39                 // For integers, we use a `SwitchInt` match, which allows
  40                 // us to handle more cases.
  41                 Test {
  42                     span: match_pair.pattern.span,
  43                     kind: TestKind::SwitchInt {
  44                         switch_ty: match_pair.pattern.ty,
  45
  46                         // these maps are empty to start; cases are
  47                         // added below in add_cases_to_switch
  48                         options: vec![],
  49                         indices: Default::default(),
  50                     }
  51                 }
  52             }
  53
  54             PatternKind::Constant { value } => {
  55                 Test {
  56                     span: match_pair.pattern.span,
  57                     kind: TestKind::Eq {
  58                         value,
  59                         ty: match_pair.pattern.ty.clone()
  60                     }
  61                 }
  62             }
  63
  64             PatternKind::Range(range) => {
  65                 assert!(range.ty == match_pair.pattern.ty);
  66                 Test {
  67                     span: match_pair.pattern.span,
  68                     kind: TestKind::Range(range),
  69                 }
  70             }
  71
  72             PatternKind::Slice { ref prefix, ref slice, ref suffix } => {
  73                 let len = prefix.len() + suffix.len();
  74                 let op = if slice.is_some() {
  75                     BinOp::Ge
  76                 } else {
  77                     BinOp::Eq
  78                 };
  79                 Test {
  80                     span: match_pair.pattern.span,
  81                     kind: TestKind::Len { len: len as u64, op: op },
  82                 }
  83             }
  84
  85             PatternKind::AscribeUserType { .. } |
  86             PatternKind::Array { .. } |
  87             PatternKind::Wild |
  88             PatternKind::Binding { .. } |
  89             PatternKind::Leaf { .. } |
  90             PatternKind::Deref { .. } => {
  91                 self.error_simplifyable(match_pair)
  92             }
  93         }
  94     }
  95
  96     pub fn add_cases_to_switch<'pat>(&mut self,
  97                                      test_place: &Place<'tcx>,
  98                                      candidate: &Candidate<'pat, 'tcx>,
  99                                      switch_ty: Ty<'tcx>,
 100                                      options: &mut Vec<u128>,
 101                                      indices: &mut FxHashMap<ty::Const<'tcx>, usize>)
 102                                      -> bool
 103     {
 104         let match_pair = match candidate.match_pairs.iter().find(|mp| mp.place == *test_place) {
 105             Some(match_pair) => match_pair,
 106             _ => { return false; }
 107         };
 108
 109         match *match_pair.pattern.kind {
 110             PatternKind::Constant { value } => {
 111                 let switch_ty = ty::ParamEnv::empty().and(switch_ty);
 112                 indices.entry(value)
 113                        .or_insert_with(|| {
 114                            options.push(value.unwrap_bits(self.hir.tcx(), switch_ty));
 115                            options.len() - 1
 116                        });
 117                 true
 118             }
 119             PatternKind::Variant { .. } => {
 120                 panic!("you should have called add_variants_to_switch instead!");
 121             }
 122             PatternKind::Range(range) => {
 123                 // Check that none of the switch values are in the range.
 124                 self.values_not_contained_in_range(range, indices)
 125                     .unwrap_or(false)
 126             }
 127             PatternKind::Slice { .. } |
 128             PatternKind::Array { .. } |
 129             PatternKind::Wild |
 130             PatternKind::Binding { .. } |
 131             PatternKind::AscribeUserType { .. } |
 132             PatternKind::Leaf { .. } |
 133             PatternKind::Deref { .. } => {
 134                 // don't know how to add these patterns to a switch
 135                 false
 136             }
 137         }
 138     }
 139
 140     pub fn add_variants_to_switch<'pat>(&mut self,
 141                                         test_place: &Place<'tcx>,
 142                                         candidate: &Candidate<'pat, 'tcx>,
 143                                         variants: &mut BitSet<VariantIdx>)
 144                                         -> bool
 145     {
 146         let match_pair = match candidate.match_pairs.iter().find(|mp| mp.place == *test_place) {
 147             Some(match_pair) => match_pair,
 148             _ => { return false; }
 149         };
 150
 151         match *match_pair.pattern.kind {
 152             PatternKind::Variant { adt_def: _ , variant_index,  .. } => {
 153                 // We have a pattern testing for variant `variant_index`
 154                 // set the corresponding index to true
 155                 variants.insert(variant_index);
 156                 true
 157             }
 158             _ => {
 159                 // don't know how to add these patterns to a switch
 160                 false
 161             }
 162         }
 163     }
 164
 165     /// Generates the code to perform a test.
 166     pub fn perform_test(&mut self,
 167                         block: BasicBlock,
 168                         place: &Place<'tcx>,
 169                         test: &Test<'tcx>)
 170                         -> Vec<BasicBlock> {
 171         debug!("perform_test({:?}, {:?}: {:?}, {:?})",
 172                block,
 173                place,
 174                place.ty(&self.local_decls, self.hir.tcx()),
 175                test);
 176         let source_info = self.source_info(test.span);
 177         match test.kind {
 178             TestKind::Switch { adt_def, ref variants } => {
 179                 // Variants is a BitVec of indexes into adt_def.variants.
 180                 let num_enum_variants = adt_def.variants.len();
 181                 let used_variants = variants.count();
 182                 let mut otherwise_block = None;
 183                 let mut target_blocks = Vec::with_capacity(num_enum_variants);
 184                 let mut targets = Vec::with_capacity(used_variants + 1);
 185                 let mut values = Vec::with_capacity(used_variants);
 186                 let tcx = self.hir.tcx();
 187                 for (idx, discr) in adt_def.discriminants(tcx) {
 188                     target_blocks.push(if variants.contains(idx) {
 189                         values.push(discr.val);
 190                         let block = self.cfg.start_new_block();
 191                         targets.push(block);
 192                         block
 193                     } else {
 194                         *otherwise_block
 195                             .get_or_insert_with(|| self.cfg.start_new_block())
 196                     });
 197                 }
 198                 targets.push(
 199                     otherwise_block
 200                         .unwrap_or_else(|| self.unreachable_block()),
 201                 );
 202                 debug!("num_enum_variants: {}, tested variants: {:?}, variants: {:?}",
 203                        num_enum_variants, values, variants);
 204                 let discr_ty = adt_def.repr.discr_type().to_ty(tcx);
 205                 let discr = self.temp(discr_ty, test.span);
 206                 self.cfg.push_assign(block, source_info, &discr,
 207                                      Rvalue::Discriminant(place.clone()));
 208                 assert_eq!(values.len() + 1, targets.len());
 209                 self.cfg.terminate(block, source_info, TerminatorKind::SwitchInt {
 210                     discr: Operand::Move(discr),
 211                     switch_ty: discr_ty,
 212                     values: From::from(values),
 213                     targets,
 214                 });
 215                 target_blocks
 216             }
 217
 218             TestKind::SwitchInt { switch_ty, ref options, indices: _ } => {
 219                 let (ret, terminator) = if switch_ty.sty == ty::Bool {
 220                     assert!(options.len() > 0 && options.len() <= 2);
 221                     let (true_bb, false_bb) = (self.cfg.start_new_block(),
 222                                                self.cfg.start_new_block());
 223                     let ret = match options[0] {
 224                         1 => vec![true_bb, false_bb],
 225                         0 => vec![false_bb, true_bb],
 226                         v => span_bug!(test.span, "expected boolean value but got {:?}", v)
 227                     };
 228                     (ret, TerminatorKind::if_(self.hir.tcx(), Operand::Copy(place.clone()),
 229                                               true_bb, false_bb))
 230                 } else {
 231                     // The switch may be inexhaustive so we
 232                     // add a catch all block
 233                     let otherwise = self.cfg.start_new_block();
 234                     let targets: Vec<_> =
 235                         options.iter()
 236                                .map(|_| self.cfg.start_new_block())
 237                                .chain(Some(otherwise))
 238                                .collect();
 239                     (targets.clone(), TerminatorKind::SwitchInt {
 240                         discr: Operand::Copy(place.clone()),
 241                         switch_ty,
 242                         values: options.clone().into(),
 243                         targets,
 244                     })
 245                 };
 246                 self.cfg.terminate(block, source_info, terminator);
 247                 ret
 248             }
 249
 250             TestKind::Eq { value, mut ty } => {
 251                 let val = Operand::Copy(place.clone());
 252                 let mut expect = self.literal_operand(test.span, ty, value);
 253                 // Use `PartialEq::eq` instead of `BinOp::Eq`
 254                 // (the binop can only handle primitives)
 255                 let fail = self.cfg.start_new_block();
 256                 if !ty.is_scalar() {
 257                     // If we're using `b"..."` as a pattern, we need to insert an
 258                     // unsizing coercion, as the byte string has the type `&[u8; N]`.
 259                     //
 260                     // We want to do this even when the scrutinee is a reference to an
 261                     // array, so we can call `<[u8]>::eq` rather than having to find an
 262                     // `<[u8; N]>::eq`.
 263                     let unsize = |ty: Ty<'tcx>| match ty.sty {
 264                         ty::Ref(region, rty, _) => match rty.sty {
 265                             ty::Array(inner_ty, n) => Some((region, inner_ty, n)),
 266                             _ => None,
 267                         },
 268                         _ => None,
 269                     };
 270                     let opt_ref_ty = unsize(ty);
 271                     let opt_ref_test_ty = unsize(value.ty);
 272                     let mut place = place.clone();
 273                     match (opt_ref_ty, opt_ref_test_ty) {
 274                         // nothing to do, neither is an array
 275                         (None, None) => {},
 276                         (Some((region, elem_ty, _)), _) |
 277                         (None, Some((region, elem_ty, _))) => {
 278                             let tcx = self.hir.tcx();
 279                             // make both a slice
 280                             ty = tcx.mk_imm_ref(region, tcx.mk_slice(elem_ty));
 281                             if opt_ref_ty.is_some() {
 282                                 place = self.temp(ty, test.span);
 283                                 self.cfg.push_assign(block, source_info, &place,
 284                                                     Rvalue::Cast(CastKind::Unsize, val, ty));
 285                             }
 286                             if opt_ref_test_ty.is_some() {
 287                                 let array = self.literal_operand(
 288                                     test.span,
 289                                     value.ty,
 290                                     value,
 291                                 );
 292
 293                                 let slice = self.temp(ty, test.span);
 294                                 self.cfg.push_assign(block, source_info, &slice,
 295                                                     Rvalue::Cast(CastKind::Unsize, array, ty));
 296                                 expect = Operand::Move(slice);
 297                             }
 298                         },
 299                     }
 300                     let eq_def_id = self.hir.tcx().lang_items().eq_trait().unwrap();
 301                     let (mty, method) = self.hir.trait_method(eq_def_id, "eq", ty, &[ty.into()]);
 302                     let method = self.hir.tcx().mk_lazy_const(ty::LazyConst::Evaluated(method));
 303
 304                     let re_erased = self.hir.tcx().types.re_erased;
 305                     // take the argument by reference
 306                     let tam = ty::TypeAndMut {
 307                         ty,
 308                         mutbl: Mutability::MutImmutable,
 309                     };
 310                     let ref_ty = self.hir.tcx().mk_ref(re_erased, tam);
 311
 312                     // let lhs_ref_place = &lhs;
 313                     let ref_rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, place);
 314                     let lhs_ref_place = self.temp(ref_ty, test.span);
 315                     self.cfg.push_assign(block, source_info, &lhs_ref_place, ref_rvalue);
 316                     let val = Operand::Move(lhs_ref_place);
 317
 318                     // let rhs_place = rhs;
 319                     let rhs_place = self.temp(ty, test.span);
 320                     self.cfg.push_assign(block, source_info, &rhs_place, Rvalue::Use(expect));
 321
 322                     // let rhs_ref_place = &rhs_place;
 323                     let ref_rvalue = Rvalue::Ref(re_erased, BorrowKind::Shared, rhs_place);
 324                     let rhs_ref_place = self.temp(ref_ty, test.span);
 325                     self.cfg.push_assign(block, source_info, &rhs_ref_place, ref_rvalue);
 326                     let expect = Operand::Move(rhs_ref_place);
 327
 328                     let bool_ty = self.hir.bool_ty();
 329                     let eq_result = self.temp(bool_ty, test.span);
 330                     let eq_block = self.cfg.start_new_block();
 331                     let cleanup = self.diverge_cleanup();
 332                     self.cfg.terminate(block, source_info, TerminatorKind::Call {
 333                         func: Operand::Constant(box Constant {
 334                             span: test.span,
 335                             ty: mty,
 336
 337                             // FIXME(#54571): This constant comes from user
 338                             // input (a constant in a pattern).  Are
 339                             // there forms where users can add type
 340                             // annotations here?  For example, an
 341                             // associated constant? Need to
 342                             // experiment.
 343                             user_ty: None,
 344
 345                             literal: method,
 346                         }),
 347                         args: vec![val, expect],
 348                         destination: Some((eq_result.clone(), eq_block)),
 349                         cleanup: Some(cleanup),
 350                         from_hir_call: false,
 351                     });
 352
 353                     // check the result
 354                     let block = self.cfg.start_new_block();
 355                     self.cfg.terminate(eq_block, source_info,
 356                                        TerminatorKind::if_(self.hir.tcx(),
 357                                                            Operand::Move(eq_result),
 358                                                            block, fail));
 359                     vec![block, fail]
 360                 } else {
 361                     let block = self.compare(block, fail, test.span, BinOp::Eq, expect, val);
 362                     vec![block, fail]
 363                 }
 364             }
 365
 366             TestKind::Range(PatternRange { ref lo, ref hi, ty, ref end }) => {
 367                 // Test `val` by computing `lo <= val && val <= hi`, using primitive comparisons.
 368                 let lo = self.literal_operand(test.span, ty.clone(), lo.clone());
 369                 let hi = self.literal_operand(test.span, ty.clone(), hi.clone());
 370                 let val = Operand::Copy(place.clone());
 371
 372                 let fail = self.cfg.start_new_block();
 373                 let block = self.compare(block, fail, test.span, BinOp::Le, lo, val.clone());
 374                 let block = match *end {
 375                     RangeEnd::Included => self.compare(block, fail, test.span, BinOp::Le, val, hi),
 376                     RangeEnd::Excluded => self.compare(block, fail, test.span, BinOp::Lt, val, hi),
 377                 };
 378
 379                 vec![block, fail]
 380             }
 381
 382             TestKind::Len { len, op } => {
 383                 let (usize_ty, bool_ty) = (self.hir.usize_ty(), self.hir.bool_ty());
 384                 let (actual, result) = (self.temp(usize_ty, test.span),
 385                                         self.temp(bool_ty, test.span));
 386
 387                 // actual = len(place)
 388                 self.cfg.push_assign(block, source_info,
 389                                      &actual, Rvalue::Len(place.clone()));
 390
 391                 // expected = <N>
 392                 let expected = self.push_usize(block, source_info, len);
 393
 394                 // result = actual == expected OR result = actual < expected
 395                 self.cfg.push_assign(block, source_info, &result,
 396                                      Rvalue::BinaryOp(op,
 397                                                       Operand::Move(actual),
 398                                                       Operand::Move(expected)));
 399
 400                 // branch based on result
 401                 let (false_bb, true_bb) = (self.cfg.start_new_block(),
 402                                            self.cfg.start_new_block());
 403                 self.cfg.terminate(block, source_info,
 404                                    TerminatorKind::if_(self.hir.tcx(), Operand::Move(result),
 405                                                        true_bb, false_bb));
 406                 vec![true_bb, false_bb]
 407             }
 408         }
 409     }
 410
 411     fn compare(&mut self,
 412                block: BasicBlock,
 413                fail_block: BasicBlock,
 414                span: Span,
 415                op: BinOp,
 416                left: Operand<'tcx>,
 417                right: Operand<'tcx>) -> BasicBlock {
 418         let bool_ty = self.hir.bool_ty();
 419         let result = self.temp(bool_ty, span);
 420
 421         // result = op(left, right)
 422         let source_info = self.source_info(span);
 423         self.cfg.push_assign(block, source_info, &result,
 424                              Rvalue::BinaryOp(op, left, right));
 425
 426         // branch based on result
 427         let target_block = self.cfg.start_new_block();
 428         self.cfg.terminate(block, source_info,
 429                            TerminatorKind::if_(self.hir.tcx(), Operand::Move(result),
 430                                                target_block, fail_block));
 431         target_block
 432     }
 433
 434     /// Given that we are performing `test` against `test_place`, this job
 435     /// sorts out what the status of `candidate` will be after the test. See
 436     /// `test_candidates` for the usage of this function. The returned index is
 437     /// the index that this candiate should be placed in the
 438     /// `target_candidates` vec. The candidate may be modified to update its
 439     /// `match_pairs`.
 440     ///
 441     /// So, for example, if this candidate is `x @ Some(P0)` and the `Test` is
 442     /// a variant test, then we would modify the candidate to be `(x as
 443     /// Option).0 @ P0` and return the index corresponding to the variant
 444     /// `Some`.
 445     ///
 446     /// However, in some cases, the test may just not be relevant to candidate.
 447     /// For example, suppose we are testing whether `foo.x == 22`, but in one
 448     /// match arm we have `Foo { x: _, ... }`... in that case, the test for
 449     /// what value `x` has has no particular relevance to this candidate. In
 450     /// such cases, this function just returns None without doing anything.
 451     /// This is used by the overall `match_candidates` algorithm to structure
 452     /// the match as a whole. See `match_candidates` for more details.
 453     ///
 454     /// FIXME(#29623). In some cases, we have some tricky choices to make.  for
 455     /// example, if we are testing that `x == 22`, but the candidate is `x @
 456     /// 13..55`, what should we do? In the event that the test is true, we know
 457     /// that the candidate applies, but in the event of false, we don't know
 458     /// that it *doesn't* apply. For now, we return false, indicate that the
 459     /// test does not apply to this candidate, but it might be we can get
 460     /// tighter match code if we do something a bit different.
 461     pub fn sort_candidate<'pat, 'cand>(
 462         &mut self,
 463         test_place: &Place<'tcx>,
 464         test: &Test<'tcx>,
 465         candidate: &mut Candidate<'pat, 'tcx>,
 466     ) -> Option<usize> {
 467         // Find the match_pair for this place (if any). At present,
 468         // afaik, there can be at most one. (In the future, if we
 469         // adopted a more general `@` operator, there might be more
 470         // than one, but it'd be very unusual to have two sides that
 471         // both require tests; you'd expect one side to be simplified
 472         // away.)
 473         let (match_pair_index, match_pair) = candidate.match_pairs
 474             .iter()
 475             .enumerate()
 476             .find(|&(_, mp)| mp.place == *test_place)?;
 477
 478         match (&test.kind, &*match_pair.pattern.kind) {
 479             // If we are performing a variant switch, then this
 480             // informs variant patterns, but nothing else.
 481             (&TestKind::Switch { adt_def: tested_adt_def, .. },
 482              &PatternKind::Variant { adt_def, variant_index, ref subpatterns, .. }) => {
 483                 assert_eq!(adt_def, tested_adt_def);
 484                 self.candidate_after_variant_switch(match_pair_index,
 485                                                     adt_def,
 486                                                     variant_index,
 487                                                     subpatterns,
 488                                                     candidate);
 489                 Some(variant_index.as_usize())
 490             }
 491
 492             (&TestKind::Switch { .. }, _) => None,
 493
 494             // If we are performing a switch over integers, then this informs integer
 495             // equality, but nothing else.
 496             //
 497             // FIXME(#29623) we could use PatternKind::Range to rule
 498             // things out here, in some cases.
 499             (&TestKind::SwitchInt { switch_ty: _, options: _, ref indices },
 500              &PatternKind::Constant { ref value })
 501             if is_switch_ty(match_pair.pattern.ty) => {
 502                 let index = indices[value];
 503                 self.candidate_without_match_pair(match_pair_index, candidate);
 504                 Some(index)
 505             }
 506
 507             (&TestKind::SwitchInt { switch_ty: _, ref options, ref indices },
 508              &PatternKind::Range(range)) => {
 509                 let not_contained = self
 510                     .values_not_contained_in_range(range, indices)
 511                     .unwrap_or(false);
 512
 513                 if not_contained {
 514                     // No switch values are contained in the pattern range,
 515                     // so the pattern can be matched only if this test fails.
 516                     let otherwise = options.len();
 517                     Some(otherwise)
 518                 } else {
 519                     None
 520                 }
 521             }
 522
 523             (&TestKind::SwitchInt { .. }, _) => None,
 524
 525             (&TestKind::Len { len: test_len, op: BinOp::Eq },
 526              &PatternKind::Slice { ref prefix, ref slice, ref suffix }) => {
 527                 let pat_len = (prefix.len() + suffix.len()) as u64;
 528                 match (test_len.cmp(&pat_len), slice) {
 529                     (Ordering::Equal, &None) => {
 530                         // on true, min_len = len = $actual_length,
 531                         // on false, len != $actual_length
 532                         self.candidate_after_slice_test(match_pair_index,
 533                                                         candidate,
 534                                                         prefix,
 535                                                         slice.as_ref(),
 536                                                         suffix);
 537                         Some(0)
 538                     }
 539                     (Ordering::Less, _) => {
 540                         // test_len < pat_len. If $actual_len = test_len,
 541                         // then $actual_len < pat_len and we don't have
 542                         // enough elements.
 543                         Some(1)
 544                     }
 545                     (Ordering::Equal, &Some(_)) | (Ordering::Greater, &Some(_)) => {
 546                         // This can match both if $actual_len = test_len >= pat_len,
 547                         // and if $actual_len > test_len. We can't advance.
 548                         None
 549                     }
 550                     (Ordering::Greater, &None) => {
 551                         // test_len != pat_len, so if $actual_len = test_len, then
 552                         // $actual_len != pat_len.
 553                         Some(1)
 554                     }
 555                 }
 556             }
 557
 558             (&TestKind::Len { len: test_len, op: BinOp::Ge },
 559              &PatternKind::Slice { ref prefix, ref slice, ref suffix }) => {
 560                 // the test is `$actual_len >= test_len`
 561                 let pat_len = (prefix.len() + suffix.len()) as u64;
 562                 match (test_len.cmp(&pat_len), slice) {
 563                     (Ordering::Equal, &Some(_))  => {
 564                         // $actual_len >= test_len = pat_len,
 565                         // so we can match.
 566                         self.candidate_after_slice_test(match_pair_index,
 567                                                         candidate,
 568                                                         prefix,
 569                                                         slice.as_ref(),
 570                                                         suffix);
 571                         Some(0)
 572                     }
 573                     (Ordering::Less, _) | (Ordering::Equal, &None) => {
 574                         // test_len <= pat_len. If $actual_len < test_len,
 575                         // then it is also < pat_len, so the test passing is
 576                         // necessary (but insufficient).
 577                         Some(0)
 578                     }
 579                     (Ordering::Greater, &None) => {
 580                         // test_len > pat_len. If $actual_len >= test_len > pat_len,
 581                         // then we know we won't have a match.
 582                         Some(1)
 583                     }
 584                     (Ordering::Greater, &Some(_)) => {
 585                         // test_len < pat_len, and is therefore less
 586                         // strict. This can still go both ways.
 587                         None
 588                     }
 589                 }
 590             }
 591
 592             (&TestKind::Range(test),
 593              &PatternKind::Range(pat)) => {
 594                 if test == pat {
 595                     self.candidate_without_match_pair(
 596                         match_pair_index,
 597                         candidate,
 598                     );
 599                     return Some(0);
 600                 }
 601
 602                 let no_overlap = (|| {
 603                     use std::cmp::Ordering::*;
 604                     use rustc::hir::RangeEnd::*;
 605
 606                     let param_env = ty::ParamEnv::empty().and(test.ty);
 607                     let tcx = self.hir.tcx();
 608
 609                     let lo = compare_const_vals(tcx, test.lo, pat.hi, param_env)?;
 610                     let hi = compare_const_vals(tcx, test.hi, pat.lo, param_env)?;
 611
 612                     match (test.end, pat.end, lo, hi) {
 613                         // pat < test
 614                         (_, _, Greater, _) |
 615                         (_, Excluded, Equal, _) |
 616                         // pat > test
 617                         (_, _, _, Less) |
 618                         (Excluded, _, _, Equal) => Some(true),
 619                         _ => Some(false),
 620                     }
 621                 })();
 622
 623                 if no_overlap == Some(true) {
 624                     // Testing range does not overlap with pattern range,
 625                     // so the pattern can be matched only if this test fails.
 626                     Some(1)
 627                 } else {
 628                     None
 629                 }
 630             }
 631
 632             (&TestKind::Range(range), &PatternKind::Constant { value }) => {
 633                 if self.const_range_contains(range, value) == Some(false) {
 634                     // `value` is not contained in the testing range,
 635                     // so `value` can be matched only if this test fails.
 636                     Some(1)
 637                 } else {
 638                     None
 639                 }
 640             }
 641
 642             (&TestKind::Range { .. }, _) => None,
 643
 644             (&TestKind::Eq { .. }, _) |
 645             (&TestKind::Len { .. }, _) => {
 646                 // These are all binary tests.
 647                 //
 648                 // FIXME(#29623) we can be more clever here
 649                 let pattern_test = self.test(&match_pair);
 650                 if pattern_test.kind == test.kind {
 651                     self.candidate_without_match_pair(match_pair_index, candidate);
 652                     Some(0)
 653                 } else {
 654                     None
 655                 }
 656             }
 657         }
 658     }
 659
 660     fn candidate_without_match_pair(
 661         &mut self,
 662         match_pair_index: usize,
 663         candidate: &mut Candidate<'_, 'tcx>,
 664     ) {
 665         candidate.match_pairs.remove(match_pair_index);
 666     }
 667
 668     fn candidate_after_slice_test<'pat>(&mut self,
 669                                         match_pair_index: usize,
 670                                         candidate: &mut Candidate<'pat, 'tcx>,
 671                                         prefix: &'pat [Pattern<'tcx>],
 672                                         opt_slice: Option<&'pat Pattern<'tcx>>,
 673                                         suffix: &'pat [Pattern<'tcx>]) {
 674         let removed_place = candidate.match_pairs.remove(match_pair_index).place;
 675         self.prefix_slice_suffix(
 676             &mut candidate.match_pairs,
 677             &removed_place,
 678             prefix,
 679             opt_slice,
 680             suffix);
 681     }
 682
 683     fn candidate_after_variant_switch<'pat>(
 684         &mut self,
 685         match_pair_index: usize,
 686         adt_def: &'tcx ty::AdtDef,
 687         variant_index: VariantIdx,
 688         subpatterns: &'pat [FieldPattern<'tcx>],
 689         candidate: &mut Candidate<'pat, 'tcx>,
 690     ) {
 691         let match_pair = candidate.match_pairs.remove(match_pair_index);
 692
 693         // So, if we have a match-pattern like `x @ Enum::Variant(P1, P2)`,
 694         // we want to create a set of derived match-patterns like
 695         // `(x as Variant).0 @ P1` and `(x as Variant).1 @ P1`.
 696         let elem = ProjectionElem::Downcast(adt_def, variant_index);
 697         let downcast_place = match_pair.place.elem(elem); // `(x as Variant)`
 698         let consequent_match_pairs =
 699             subpatterns.iter()
 700                        .map(|subpattern| {
 701                            // e.g., `(x as Variant).0`
 702                            let place = downcast_place.clone().field(subpattern.field,
 703                                                                       subpattern.pattern.ty);
 704                            // e.g., `(x as Variant).0 @ P1`
 705                            MatchPair::new(place, &subpattern.pattern)
 706                        });
 707
 708         candidate.match_pairs.extend(consequent_match_pairs);
 709     }
 710
 711     fn error_simplifyable<'pat>(&mut self, match_pair: &MatchPair<'pat, 'tcx>) -> ! {
 712         span_bug!(match_pair.pattern.span,
 713                   "simplifyable pattern found: {:?}",
 714                   match_pair.pattern)
 715     }
 716
 717     fn const_range_contains(
 718         &self,
 719         range: PatternRange<'tcx>,
 720         value: ty::Const<'tcx>,
 721     ) -> Option<bool> {
 722         use std::cmp::Ordering::*;
 723
 724         let param_env = ty::ParamEnv::empty().and(range.ty);
 725         let tcx = self.hir.tcx();
 726
 727         let a = compare_const_vals(tcx, range.lo, value, param_env)?;
 728         let b = compare_const_vals(tcx, value, range.hi, param_env)?;
 729
 730         match (b, range.end) {
 731             (Less, _) |
 732             (Equal, RangeEnd::Included) if a != Greater => Some(true),
 733             _ => Some(false),
 734         }
 735     }
 736
 737     fn values_not_contained_in_range(
 738         &self,
 739         range: PatternRange<'tcx>,
 740         indices: &FxHashMap<ty::Const<'tcx>, usize>,
 741     ) -> Option<bool> {
 742         for &val in indices.keys() {
 743             if self.const_range_contains(range, val)? {
 744                 return Some(false);
 745             }
 746         }
 747
 748         Some(true)
 749     }
 750 }
 751
 752 fn is_switch_ty<'tcx>(ty: Ty<'tcx>) -> bool {
 753     ty.is_integral() || ty.is_char() || ty.is_bool()
 754 }