src/librustc_mir/hair/pattern/mod.rs

   1 // Copyright 2016 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 //! Code to validate patterns/matches
  12
  13 mod _match;
  14 mod check_match;
  15
  16 pub use self::check_match::check_crate;
  17 pub(crate) use self::check_match::check_match;
  18
  19 use const_eval::{const_field, const_variant_index};
  20
  21 use hair::util::UserAnnotatedTyHelpers;
  22 use hair::constant::*;
  23
  24 use rustc::mir::{fmt_const_val, Field, BorrowKind, Mutability};
  25 use rustc::mir::{ProjectionElem, UserTypeAnnotation, UserTypeProjection, UserTypeProjections};
  26 use rustc::mir::interpret::{Scalar, GlobalId, ConstValue, sign_extend};
  27 use rustc::ty::{self, Region, TyCtxt, AdtDef, Ty};
  28 use rustc::ty::subst::{Substs, Kind};
  29 use rustc::ty::layout::VariantIdx;
  30 use rustc::hir::{self, PatKind, RangeEnd};
  31 use rustc::hir::def::{Def, CtorKind};
  32 use rustc::hir::pat_util::EnumerateAndAdjustIterator;
  33
  34 use rustc_data_structures::indexed_vec::Idx;
  35
  36 use std::cmp::Ordering;
  37 use std::fmt;
  38 use syntax::ast;
  39 use syntax::ptr::P;
  40 use syntax_pos::Span;
  41
  42 #[derive(Clone, Debug)]
  43 pub enum PatternError {
  44     AssociatedConstInPattern(Span),
  45     StaticInPattern(Span),
  46     FloatBug,
  47     NonConstPath(Span),
  48 }
  49
  50 #[derive(Copy, Clone, Debug)]
  51 pub enum BindingMode<'tcx> {
  52     ByValue,
  53     ByRef(Region<'tcx>, BorrowKind),
  54 }
  55
  56 #[derive(Clone, Debug)]
  57 pub struct FieldPattern<'tcx> {
  58     pub field: Field,
  59     pub pattern: Pattern<'tcx>,
  60 }
  61
  62 #[derive(Clone, Debug)]
  63 pub struct Pattern<'tcx> {
  64     pub ty: Ty<'tcx>,
  65     pub span: Span,
  66     pub kind: Box<PatternKind<'tcx>>,
  67 }
  68
  69
  70 #[derive(Clone, Debug)]
  71 pub(crate) struct PatternTypeProjections<'tcx> {
  72     contents: Vec<(PatternTypeProjection<'tcx>, Span)>,
  73 }
  74
  75 impl<'tcx> PatternTypeProjections<'tcx> {
  76     pub(crate) fn user_ty(self) -> UserTypeProjections<'tcx> {
  77         UserTypeProjections::from_projections(
  78             self.contents.into_iter().map(|(pat_ty_proj, span)| (pat_ty_proj.user_ty(), span)))
  79     }
  80
  81     pub(crate) fn none() -> Self {
  82         PatternTypeProjections { contents: vec![] }
  83     }
  84
  85     pub(crate) fn ref_binding(&self) -> Self {
  86         // FIXME(#47184): ignore for now
  87         PatternTypeProjections { contents: vec![] }
  88     }
  89
  90     fn map_projs(&self,
  91                  mut f: impl FnMut(&PatternTypeProjection<'tcx>) -> PatternTypeProjection<'tcx>)
  92                  -> Self
  93     {
  94         PatternTypeProjections {
  95             contents: self.contents
  96                 .iter()
  97                 .map(|(proj, span)| (f(proj), *span))
  98                 .collect(), }
  99     }
 100
 101     pub(crate) fn index(&self) -> Self { self.map_projs(|pat_ty_proj| pat_ty_proj.index()) }
 102
 103     pub(crate) fn subslice(&self, from: u32, to: u32) -> Self {
 104         self.map_projs(|pat_ty_proj| pat_ty_proj.subslice(from, to))
 105     }
 106
 107     pub(crate) fn deref(&self) -> Self { self.map_projs(|pat_ty_proj| pat_ty_proj.deref()) }
 108
 109     pub(crate) fn leaf(&self, field: Field) -> Self {
 110         self.map_projs(|pat_ty_proj| pat_ty_proj.leaf(field))
 111     }
 112
 113     pub(crate) fn variant(&self,
 114                           adt_def: &'tcx AdtDef,
 115                           variant_index: VariantIdx,
 116                           field: Field) -> Self {
 117         self.map_projs(|pat_ty_proj| pat_ty_proj.variant(adt_def, variant_index, field))
 118     }
 119
 120     pub(crate) fn add_user_type(&self, user_ty: &PatternTypeProjection<'tcx>, sp: Span) -> Self {
 121         let mut new = self.clone();
 122         new.contents.push((user_ty.clone(), sp));
 123         new
 124     }
 125 }
 126
 127 #[derive(Clone, Debug)]
 128 pub struct PatternTypeProjection<'tcx>(UserTypeProjection<'tcx>);
 129
 130 impl<'tcx> PatternTypeProjection<'tcx> {
 131     pub(crate) fn index(&self) -> Self {
 132         let mut new = self.clone();
 133         new.0.projs.push(ProjectionElem::Index(()));
 134         new
 135     }
 136
 137     pub(crate) fn subslice(&self, from: u32, to: u32) -> Self {
 138         let mut new = self.clone();
 139         new.0.projs.push(ProjectionElem::Subslice { from, to });
 140         new
 141     }
 142
 143     pub(crate) fn deref(&self) -> Self {
 144         let mut new = self.clone();
 145         new.0.projs.push(ProjectionElem::Deref);
 146         new
 147     }
 148
 149     pub(crate) fn leaf(&self, field: Field) -> Self {
 150         let mut new = self.clone();
 151         new.0.projs.push(ProjectionElem::Field(field, ()));
 152         new
 153     }
 154
 155     pub(crate) fn variant(&self,
 156                           adt_def: &'tcx AdtDef,
 157                           variant_index: VariantIdx,
 158                           field: Field) -> Self {
 159         let mut new = self.clone();
 160         new.0.projs.push(ProjectionElem::Downcast(adt_def, variant_index));
 161         new.0.projs.push(ProjectionElem::Field(field, ()));
 162         new
 163     }
 164
 165     pub(crate) fn from_canonical_ty(c_ty: ty::CanonicalTy<'tcx>) -> Self {
 166         Self::from_user_type(UserTypeAnnotation::Ty(c_ty))
 167     }
 168
 169     pub(crate) fn from_user_type(u_ty: UserTypeAnnotation<'tcx>) -> Self {
 170         Self::from_user_type_proj(UserTypeProjection { base: u_ty, projs: vec![], })
 171     }
 172
 173     pub(crate) fn from_user_type_proj(u_ty: UserTypeProjection<'tcx>) -> Self {
 174         PatternTypeProjection(u_ty)
 175     }
 176
 177     pub(crate) fn user_ty(self) -> UserTypeProjection<'tcx> { self.0 }
 178 }
 179
 180 #[derive(Clone, Debug)]
 181 pub enum PatternKind<'tcx> {
 182     Wild,
 183
 184     AscribeUserType {
 185         user_ty: PatternTypeProjection<'tcx>,
 186         subpattern: Pattern<'tcx>,
 187         user_ty_span: Span,
 188     },
 189
 190     /// x, ref x, x @ P, etc
 191     Binding {
 192         mutability: Mutability,
 193         name: ast::Name,
 194         mode: BindingMode<'tcx>,
 195         var: ast::NodeId,
 196         ty: Ty<'tcx>,
 197         subpattern: Option<Pattern<'tcx>>,
 198     },
 199
 200     /// Foo(...) or Foo{...} or Foo, where `Foo` is a variant name from an adt with >1 variants
 201     Variant {
 202         adt_def: &'tcx AdtDef,
 203         substs: &'tcx Substs<'tcx>,
 204         variant_index: VariantIdx,
 205         subpatterns: Vec<FieldPattern<'tcx>>,
 206     },
 207
 208     /// (...), Foo(...), Foo{...}, or Foo, where `Foo` is a variant name from an adt with 1 variant
 209     Leaf {
 210         subpatterns: Vec<FieldPattern<'tcx>>,
 211     },
 212
 213     /// box P, &P, &mut P, etc
 214     Deref {
 215         subpattern: Pattern<'tcx>,
 216     },
 217
 218     Constant {
 219         value: &'tcx ty::Const<'tcx>,
 220     },
 221
 222     Range {
 223         lo: &'tcx ty::Const<'tcx>,
 224         hi: &'tcx ty::Const<'tcx>,
 225         ty: Ty<'tcx>,
 226         end: RangeEnd,
 227     },
 228
 229     /// matches against a slice, checking the length and extracting elements.
 230     /// irrefutable when there is a slice pattern and both `prefix` and `suffix` are empty.
 231     /// e.g. `&[ref xs..]`.
 232     Slice {
 233         prefix: Vec<Pattern<'tcx>>,
 234         slice: Option<Pattern<'tcx>>,
 235         suffix: Vec<Pattern<'tcx>>,
 236     },
 237
 238     /// fixed match against an array, irrefutable
 239     Array {
 240         prefix: Vec<Pattern<'tcx>>,
 241         slice: Option<Pattern<'tcx>>,
 242         suffix: Vec<Pattern<'tcx>>,
 243     },
 244 }
 245
 246 impl<'tcx> fmt::Display for Pattern<'tcx> {
 247     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 248         match *self.kind {
 249             PatternKind::Wild => write!(f, "_"),
 250             PatternKind::AscribeUserType { ref subpattern, .. } =>
 251                 write!(f, "{}: _", subpattern),
 252             PatternKind::Binding { mutability, name, mode, ref subpattern, .. } => {
 253                 let is_mut = match mode {
 254                     BindingMode::ByValue => mutability == Mutability::Mut,
 255                     BindingMode::ByRef(_, bk) => {
 256                         write!(f, "ref ")?;
 257                         match bk { BorrowKind::Mut { .. } => true, _ => false }
 258                     }
 259                 };
 260                 if is_mut {
 261                     write!(f, "mut ")?;
 262                 }
 263                 write!(f, "{}", name)?;
 264                 if let Some(ref subpattern) = *subpattern {
 265                     write!(f, " @ {}", subpattern)?;
 266                 }
 267                 Ok(())
 268             }
 269             PatternKind::Variant { ref subpatterns, .. } |
 270             PatternKind::Leaf { ref subpatterns } => {
 271                 let variant = match *self.kind {
 272                     PatternKind::Variant { adt_def, variant_index, .. } => {
 273                         Some(&adt_def.variants[variant_index])
 274                     }
 275                     _ => if let ty::Adt(adt, _) = self.ty.sty {
 276                         if !adt.is_enum() {
 277                             Some(&adt.variants[VariantIdx::new(0)])
 278                         } else {
 279                             None
 280                         }
 281                     } else {
 282                         None
 283                     }
 284                 };
 285
 286                 let mut first = true;
 287                 let mut start_or_continue = || if first { first = false; "" } else { ", " };
 288
 289                 if let Some(variant) = variant {
 290                     write!(f, "{}", variant.name)?;
 291
 292                     // Only for Adt we can have `S {...}`,
 293                     // which we handle separately here.
 294                     if variant.ctor_kind == CtorKind::Fictive {
 295                         write!(f, " {{ ")?;
 296
 297                         let mut printed = 0;
 298                         for p in subpatterns {
 299                             if let PatternKind::Wild = *p.pattern.kind {
 300                                 continue;
 301                             }
 302                             let name = variant.fields[p.field.index()].ident;
 303                             write!(f, "{}{}: {}", start_or_continue(), name, p.pattern)?;
 304                             printed += 1;
 305                         }
 306
 307                         if printed < variant.fields.len() {
 308                             write!(f, "{}..", start_or_continue())?;
 309                         }
 310
 311                         return write!(f, " }}");
 312                     }
 313                 }
 314
 315                 let num_fields = variant.map_or(subpatterns.len(), |v| v.fields.len());
 316                 if num_fields != 0 || variant.is_none() {
 317                     write!(f, "(")?;
 318                     for i in 0..num_fields {
 319                         write!(f, "{}", start_or_continue())?;
 320
 321                         // Common case: the field is where we expect it.
 322                         if let Some(p) = subpatterns.get(i) {
 323                             if p.field.index() == i {
 324                                 write!(f, "{}", p.pattern)?;
 325                                 continue;
 326                             }
 327                         }
 328
 329                         // Otherwise, we have to go looking for it.
 330                         if let Some(p) = subpatterns.iter().find(|p| p.field.index() == i) {
 331                             write!(f, "{}", p.pattern)?;
 332                         } else {
 333                             write!(f, "_")?;
 334                         }
 335                     }
 336                     write!(f, ")")?;
 337                 }
 338
 339                 Ok(())
 340             }
 341             PatternKind::Deref { ref subpattern } => {
 342                 match self.ty.sty {
 343                     ty::Adt(def, _) if def.is_box() => write!(f, "box ")?,
 344                     ty::Ref(_, _, mutbl) => {
 345                         write!(f, "&")?;
 346                         if mutbl == hir::MutMutable {
 347                             write!(f, "mut ")?;
 348                         }
 349                     }
 350                     _ => bug!("{} is a bad Deref pattern type", self.ty)
 351                 }
 352                 write!(f, "{}", subpattern)
 353             }
 354             PatternKind::Constant { value } => {
 355                 fmt_const_val(f, value)
 356             }
 357             PatternKind::Range { lo, hi, ty: _, end } => {
 358                 fmt_const_val(f, lo)?;
 359                 match end {
 360                     RangeEnd::Included => write!(f, "..=")?,
 361                     RangeEnd::Excluded => write!(f, "..")?,
 362                 }
 363                 fmt_const_val(f, hi)
 364             }
 365             PatternKind::Slice { ref prefix, ref slice, ref suffix } |
 366             PatternKind::Array { ref prefix, ref slice, ref suffix } => {
 367                 let mut first = true;
 368                 let mut start_or_continue = || if first { first = false; "" } else { ", " };
 369                 write!(f, "[")?;
 370                 for p in prefix {
 371                     write!(f, "{}{}", start_or_continue(), p)?;
 372                 }
 373                 if let Some(ref slice) = *slice {
 374                     write!(f, "{}", start_or_continue())?;
 375                     match *slice.kind {
 376                         PatternKind::Wild => {}
 377                         _ => write!(f, "{}", slice)?
 378                     }
 379                     write!(f, "..")?;
 380                 }
 381                 for p in suffix {
 382                     write!(f, "{}{}", start_or_continue(), p)?;
 383                 }
 384                 write!(f, "]")
 385             }
 386         }
 387     }
 388 }
 389
 390 pub struct PatternContext<'a, 'tcx: 'a> {
 391     pub tcx: TyCtxt<'a, 'tcx, 'tcx>,
 392     pub param_env: ty::ParamEnv<'tcx>,
 393     pub tables: &'a ty::TypeckTables<'tcx>,
 394     pub substs: &'tcx Substs<'tcx>,
 395     pub errors: Vec<PatternError>,
 396 }
 397
 398 impl<'a, 'tcx> Pattern<'tcx> {
 399     pub fn from_hir(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 400                     param_env_and_substs: ty::ParamEnvAnd<'tcx, &'tcx Substs<'tcx>>,
 401                     tables: &'a ty::TypeckTables<'tcx>,
 402                     pat: &'tcx hir::Pat) -> Self {
 403         let mut pcx = PatternContext::new(tcx, param_env_and_substs, tables);
 404         let result = pcx.lower_pattern(pat);
 405         if !pcx.errors.is_empty() {
 406             let msg = format!("encountered errors lowering pattern: {:?}", pcx.errors);
 407             tcx.sess.delay_span_bug(pat.span, &msg);
 408         }
 409         debug!("Pattern::from_hir({:?}) = {:?}", pat, result);
 410         result
 411     }
 412 }
 413
 414 impl<'a, 'tcx> PatternContext<'a, 'tcx> {
 415     pub fn new(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 416                param_env_and_substs: ty::ParamEnvAnd<'tcx, &'tcx Substs<'tcx>>,
 417                tables: &'a ty::TypeckTables<'tcx>) -> Self {
 418         PatternContext {
 419             tcx,
 420             param_env: param_env_and_substs.param_env,
 421             tables,
 422             substs: param_env_and_substs.value,
 423             errors: vec![]
 424         }
 425     }
 426
 427     pub fn lower_pattern(&mut self, pat: &'tcx hir::Pat) -> Pattern<'tcx> {
 428         // When implicit dereferences have been inserted in this pattern, the unadjusted lowered
 429         // pattern has the type that results *after* dereferencing. For example, in this code:
 430         //
 431         // ```
 432         // match &&Some(0i32) {
 433         //     Some(n) => { ... },
 434         //     _ => { ... },
 435         // }
 436         // ```
 437         //
 438         // the type assigned to `Some(n)` in `unadjusted_pat` would be `Option<i32>` (this is
 439         // determined in rustc_typeck::check::match). The adjustments would be
 440         //
 441         // `vec![&&Option<i32>, &Option<i32>]`.
 442         //
 443         // Applying the adjustments, we want to instead output `&&Some(n)` (as a HAIR pattern). So
 444         // we wrap the unadjusted pattern in `PatternKind::Deref` repeatedly, consuming the
 445         // adjustments in *reverse order* (last-in-first-out, so that the last `Deref` inserted
 446         // gets the least-dereferenced type).
 447         let unadjusted_pat = self.lower_pattern_unadjusted(pat);
 448         self.tables
 449             .pat_adjustments()
 450             .get(pat.hir_id)
 451             .unwrap_or(&vec![])
 452             .iter()
 453             .rev()
 454             .fold(unadjusted_pat, |pat, ref_ty| {
 455                     debug!("{:?}: wrapping pattern with type {:?}", pat, ref_ty);
 456                     Pattern {
 457                         span: pat.span,
 458                         ty: ref_ty,
 459                         kind: Box::new(PatternKind::Deref { subpattern: pat }),
 460                     }
 461                 },
 462             )
 463     }
 464
 465     fn lower_pattern_unadjusted(&mut self, pat: &'tcx hir::Pat) -> Pattern<'tcx> {
 466         let mut ty = self.tables.node_id_to_type(pat.hir_id);
 467
 468         let kind = match pat.node {
 469             PatKind::Wild => PatternKind::Wild,
 470
 471             PatKind::Lit(ref value) => self.lower_lit(value),
 472
 473             PatKind::Range(ref lo_expr, ref hi_expr, end) => {
 474                 match (self.lower_lit(lo_expr), self.lower_lit(hi_expr)) {
 475                     (PatternKind::Constant { value: lo },
 476                      PatternKind::Constant { value: hi }) => {
 477                         use std::cmp::Ordering;
 478                         let cmp = compare_const_vals(
 479                             self.tcx,
 480                             lo,
 481                             hi,
 482                             self.param_env.and(ty),
 483                         );
 484                         match (end, cmp) {
 485                             (RangeEnd::Excluded, Some(Ordering::Less)) =>
 486                                 PatternKind::Range { lo, hi, ty, end },
 487                             (RangeEnd::Excluded, _) => {
 488                                 span_err!(
 489                                     self.tcx.sess,
 490                                     lo_expr.span,
 491                                     E0579,
 492                                     "lower range bound must be less than upper",
 493                                 );
 494                                 PatternKind::Wild
 495                             }
 496                             (RangeEnd::Included, Some(Ordering::Equal)) => {
 497                                 PatternKind::Constant { value: lo }
 498                             }
 499                             (RangeEnd::Included, Some(Ordering::Less)) => {
 500                                 PatternKind::Range { lo, hi, ty, end }
 501                             }
 502                             (RangeEnd::Included, _) => {
 503                                 let mut err = struct_span_err!(
 504                                     self.tcx.sess,
 505                                     lo_expr.span,
 506                                     E0030,
 507                                     "lower range bound must be less than or equal to upper"
 508                                 );
 509                                 err.span_label(
 510                                     lo_expr.span,
 511                                     "lower bound larger than upper bound",
 512                                 );
 513                                 if self.tcx.sess.teach(&err.get_code().unwrap()) {
 514                                     err.note("When matching against a range, the compiler \
 515                                               verifies that the range is non-empty. Range \
 516                                               patterns include both end-points, so this is \
 517                                               equivalent to requiring the start of the range \
 518                                               to be less than or equal to the end of the range.");
 519                                 }
 520                                 err.emit();
 521                                 PatternKind::Wild
 522                             }
 523                         }
 524                     }
 525                     _ => PatternKind::Wild
 526                 }
 527             }
 528
 529             PatKind::Path(ref qpath) => {
 530                 return self.lower_path(qpath, pat.hir_id, pat.span);
 531             }
 532
 533             PatKind::Ref(ref subpattern, _) |
 534             PatKind::Box(ref subpattern) => {
 535                 PatternKind::Deref { subpattern: self.lower_pattern(subpattern) }
 536             }
 537
 538             PatKind::Slice(ref prefix, ref slice, ref suffix) => {
 539                 match ty.sty {
 540                     ty::Ref(_, ty, _) =>
 541                         PatternKind::Deref {
 542                             subpattern: Pattern {
 543                                 ty,
 544                                 span: pat.span,
 545                                 kind: Box::new(self.slice_or_array_pattern(
 546                                     pat.span, ty, prefix, slice, suffix))
 547                             },
 548                         },
 549                     ty::Slice(..) |
 550                     ty::Array(..) =>
 551                         self.slice_or_array_pattern(pat.span, ty, prefix, slice, suffix),
 552                     ty::Error => { // Avoid ICE
 553                         return Pattern { span: pat.span, ty, kind: Box::new(PatternKind::Wild) };
 554                     }
 555                     ref sty =>
 556                         span_bug!(
 557                             pat.span,
 558                             "unexpanded type for vector pattern: {:?}",
 559                             sty),
 560                 }
 561             }
 562
 563             PatKind::Tuple(ref subpatterns, ddpos) => {
 564                 match ty.sty {
 565                     ty::Tuple(ref tys) => {
 566                         let subpatterns =
 567                             subpatterns.iter()
 568                                        .enumerate_and_adjust(tys.len(), ddpos)
 569                                        .map(|(i, subpattern)| FieldPattern {
 570                                             field: Field::new(i),
 571                                             pattern: self.lower_pattern(subpattern)
 572                                        })
 573                                        .collect();
 574
 575                         PatternKind::Leaf { subpatterns }
 576                     }
 577                     ty::Error => { // Avoid ICE (#50577)
 578                         return Pattern { span: pat.span, ty, kind: Box::new(PatternKind::Wild) };
 579                     }
 580                     ref sty => span_bug!(pat.span, "unexpected type for tuple pattern: {:?}", sty),
 581                 }
 582             }
 583
 584             PatKind::Binding(_, id, ident, ref sub) => {
 585                 let var_ty = self.tables.node_id_to_type(pat.hir_id);
 586                 let region = match var_ty.sty {
 587                     ty::Ref(r, _, _) => Some(r),
 588                     ty::Error => { // Avoid ICE
 589                         return Pattern { span: pat.span, ty, kind: Box::new(PatternKind::Wild) };
 590                     }
 591                     _ => None,
 592                 };
 593                 let bm = *self.tables.pat_binding_modes().get(pat.hir_id)
 594                                                          .expect("missing binding mode");
 595                 let (mutability, mode) = match bm {
 596                     ty::BindByValue(hir::MutMutable) =>
 597                         (Mutability::Mut, BindingMode::ByValue),
 598                     ty::BindByValue(hir::MutImmutable) =>
 599                         (Mutability::Not, BindingMode::ByValue),
 600                     ty::BindByReference(hir::MutMutable) =>
 601                         (Mutability::Not, BindingMode::ByRef(
 602                             region.unwrap(), BorrowKind::Mut { allow_two_phase_borrow: false })),
 603                     ty::BindByReference(hir::MutImmutable) =>
 604                         (Mutability::Not, BindingMode::ByRef(
 605                             region.unwrap(), BorrowKind::Shared)),
 606                 };
 607
 608                 // A ref x pattern is the same node used for x, and as such it has
 609                 // x's type, which is &T, where we want T (the type being matched).
 610                 if let ty::BindByReference(_) = bm {
 611                     if let ty::Ref(_, rty, _) = ty.sty {
 612                         ty = rty;
 613                     } else {
 614                         bug!("`ref {}` has wrong type {}", ident, ty);
 615                     }
 616                 }
 617
 618                 PatternKind::Binding {
 619                     mutability,
 620                     mode,
 621                     name: ident.name,
 622                     var: id,
 623                     ty: var_ty,
 624                     subpattern: self.lower_opt_pattern(sub),
 625                 }
 626             }
 627
 628             PatKind::TupleStruct(ref qpath, ref subpatterns, ddpos) => {
 629                 let def = self.tables.qpath_def(qpath, pat.hir_id);
 630                 let adt_def = match ty.sty {
 631                     ty::Adt(adt_def, _) => adt_def,
 632                     ty::Error => { // Avoid ICE (#50585)
 633                         return Pattern { span: pat.span, ty, kind: Box::new(PatternKind::Wild) };
 634                     }
 635                     _ => span_bug!(pat.span,
 636                                    "tuple struct pattern not applied to an ADT {:?}",
 637                                    ty.sty),
 638                 };
 639                 let variant_def = adt_def.variant_of_def(def);
 640
 641                 let subpatterns =
 642                         subpatterns.iter()
 643                                    .enumerate_and_adjust(variant_def.fields.len(), ddpos)
 644                                    .map(|(i, field)| FieldPattern {
 645                                        field: Field::new(i),
 646                                        pattern: self.lower_pattern(field),
 647                                    })
 648                     .collect();
 649
 650                 self.lower_variant_or_leaf(def, pat.hir_id, pat.span, ty, subpatterns)
 651             }
 652
 653             PatKind::Struct(ref qpath, ref fields, _) => {
 654                 let def = self.tables.qpath_def(qpath, pat.hir_id);
 655                 let subpatterns =
 656                     fields.iter()
 657                           .map(|field| {
 658                               FieldPattern {
 659                                   field: Field::new(self.tcx.field_index(field.node.id,
 660                                                                          self.tables)),
 661                                   pattern: self.lower_pattern(&field.node.pat),
 662                               }
 663                           })
 664                           .collect();
 665
 666                 self.lower_variant_or_leaf(def, pat.hir_id, pat.span, ty, subpatterns)
 667             }
 668         };
 669
 670         Pattern {
 671             span: pat.span,
 672             ty,
 673             kind: Box::new(kind),
 674         }
 675     }
 676
 677     fn lower_patterns(&mut self, pats: &'tcx [P<hir::Pat>]) -> Vec<Pattern<'tcx>> {
 678         pats.iter().map(|p| self.lower_pattern(p)).collect()
 679     }
 680
 681     fn lower_opt_pattern(&mut self, pat: &'tcx Option<P<hir::Pat>>) -> Option<Pattern<'tcx>>
 682     {
 683         pat.as_ref().map(|p| self.lower_pattern(p))
 684     }
 685
 686     fn flatten_nested_slice_patterns(
 687         &mut self,
 688         prefix: Vec<Pattern<'tcx>>,
 689         slice: Option<Pattern<'tcx>>,
 690         suffix: Vec<Pattern<'tcx>>)
 691         -> (Vec<Pattern<'tcx>>, Option<Pattern<'tcx>>, Vec<Pattern<'tcx>>)
 692     {
 693         let orig_slice = match slice {
 694             Some(orig_slice) => orig_slice,
 695             None => return (prefix, slice, suffix)
 696         };
 697         let orig_prefix = prefix;
 698         let orig_suffix = suffix;
 699
 700         // dance because of intentional borrow-checker stupidity.
 701         let kind = *orig_slice.kind;
 702         match kind {
 703             PatternKind::Slice { prefix, slice, mut suffix } |
 704             PatternKind::Array { prefix, slice, mut suffix } => {
 705                 let mut orig_prefix = orig_prefix;
 706
 707                 orig_prefix.extend(prefix);
 708                 suffix.extend(orig_suffix);
 709
 710                 (orig_prefix, slice, suffix)
 711             }
 712             _ => {
 713                 (orig_prefix, Some(Pattern {
 714                     kind: box kind, ..orig_slice
 715                 }), orig_suffix)
 716             }
 717         }
 718     }
 719
 720     fn slice_or_array_pattern(
 721         &mut self,
 722         span: Span,
 723         ty: Ty<'tcx>,
 724         prefix: &'tcx [P<hir::Pat>],
 725         slice: &'tcx Option<P<hir::Pat>>,
 726         suffix: &'tcx [P<hir::Pat>])
 727         -> PatternKind<'tcx>
 728     {
 729         let prefix = self.lower_patterns(prefix);
 730         let slice = self.lower_opt_pattern(slice);
 731         let suffix = self.lower_patterns(suffix);
 732         let (prefix, slice, suffix) =
 733             self.flatten_nested_slice_patterns(prefix, slice, suffix);
 734
 735         match ty.sty {
 736             ty::Slice(..) => {
 737                 // matching a slice or fixed-length array
 738                 PatternKind::Slice { prefix: prefix, slice: slice, suffix: suffix }
 739             }
 740
 741             ty::Array(_, len) => {
 742                 // fixed-length array
 743                 let len = len.unwrap_usize(self.tcx);
 744                 assert!(len >= prefix.len() as u64 + suffix.len() as u64);
 745                 PatternKind::Array { prefix: prefix, slice: slice, suffix: suffix }
 746             }
 747
 748             _ => {
 749                 span_bug!(span, "bad slice pattern type {:?}", ty);
 750             }
 751         }
 752     }
 753
 754     fn lower_variant_or_leaf(
 755         &mut self,
 756         def: Def,
 757         hir_id: hir::HirId,
 758         span: Span,
 759         ty: Ty<'tcx>,
 760         subpatterns: Vec<FieldPattern<'tcx>>,
 761     ) -> PatternKind<'tcx> {
 762         let mut kind = match def {
 763             Def::Variant(variant_id) | Def::VariantCtor(variant_id, ..) => {
 764                 let enum_id = self.tcx.parent_def_id(variant_id).unwrap();
 765                 let adt_def = self.tcx.adt_def(enum_id);
 766                 if adt_def.is_enum() {
 767                     let substs = match ty.sty {
 768                         ty::Adt(_, substs) |
 769                         ty::FnDef(_, substs) => substs,
 770                         ty::Error => {  // Avoid ICE (#50585)
 771                             return PatternKind::Wild;
 772                         }
 773                         _ => bug!("inappropriate type for def: {:?}", ty.sty),
 774                     };
 775                     PatternKind::Variant {
 776                         adt_def,
 777                         substs,
 778                         variant_index: adt_def.variant_index_with_id(variant_id),
 779                         subpatterns,
 780                     }
 781                 } else {
 782                     PatternKind::Leaf { subpatterns }
 783                 }
 784             }
 785
 786             Def::Struct(..) | Def::StructCtor(..) | Def::Union(..) |
 787             Def::TyAlias(..) | Def::AssociatedTy(..) | Def::SelfTy(..) | Def::SelfCtor(..) => {
 788                 PatternKind::Leaf { subpatterns }
 789             }
 790
 791             _ => {
 792                 self.errors.push(PatternError::NonConstPath(span));
 793                 PatternKind::Wild
 794             }
 795         };
 796
 797         if let Some(user_ty) = self.user_substs_applied_to_ty_of_hir_id(hir_id) {
 798             let subpattern = Pattern {
 799                 span,
 800                 ty,
 801                 kind: Box::new(kind),
 802             };
 803
 804             debug!("pattern user_ty = {:?} for pattern at {:?}", user_ty, span);
 805
 806             let pat_ty = PatternTypeProjection::from_user_type(user_ty);
 807             kind = PatternKind::AscribeUserType {
 808                 subpattern,
 809                 user_ty: pat_ty,
 810                 user_ty_span: span,
 811             };
 812         }
 813
 814         kind
 815     }
 816
 817     /// Takes a HIR Path. If the path is a constant, evaluates it and feeds
 818     /// it to `const_to_pat`. Any other path (like enum variants without fields)
 819     /// is converted to the corresponding pattern via `lower_variant_or_leaf`
 820     fn lower_path(&mut self,
 821                   qpath: &hir::QPath,
 822                   id: hir::HirId,
 823                   span: Span)
 824                   -> Pattern<'tcx> {
 825         let ty = self.tables.node_id_to_type(id);
 826         let def = self.tables.qpath_def(qpath, id);
 827         let is_associated_const = match def {
 828             Def::AssociatedConst(_) => true,
 829             _ => false,
 830         };
 831         let kind = match def {
 832             Def::Const(def_id) | Def::AssociatedConst(def_id) => {
 833                 let substs = self.tables.node_substs(id);
 834                 match ty::Instance::resolve(
 835                     self.tcx,
 836                     self.param_env,
 837                     def_id,
 838                     substs,
 839                 ) {
 840                     Some(instance) => {
 841                         let cid = GlobalId {
 842                             instance,
 843                             promoted: None,
 844                         };
 845                         match self.tcx.at(span).const_eval(self.param_env.and(cid)) {
 846                             Ok(value) => {
 847                                 return self.const_to_pat(instance, value, id, span)
 848                             },
 849                             Err(_) => {
 850                                 self.tcx.sess.span_err(
 851                                     span,
 852                                     "could not evaluate constant pattern",
 853                                 );
 854                                 PatternKind::Wild
 855                             }
 856                         }
 857                     },
 858                     None => {
 859                         self.errors.push(if is_associated_const {
 860                             PatternError::AssociatedConstInPattern(span)
 861                         } else {
 862                             PatternError::StaticInPattern(span)
 863                         });
 864                         PatternKind::Wild
 865                     },
 866                 }
 867             }
 868             _ => self.lower_variant_or_leaf(def, id, span, ty, vec![]),
 869         };
 870
 871         Pattern {
 872             span,
 873             ty,
 874             kind: Box::new(kind),
 875         }
 876     }
 877
 878     /// Converts literals, paths and negation of literals to patterns.
 879     /// The special case for negation exists to allow things like -128i8
 880     /// which would overflow if we tried to evaluate 128i8 and then negate
 881     /// afterwards.
 882     fn lower_lit(&mut self, expr: &'tcx hir::Expr) -> PatternKind<'tcx> {
 883         match expr.node {
 884             hir::ExprKind::Lit(ref lit) => {
 885                 let ty = self.tables.expr_ty(expr);
 886                 match lit_to_const(&lit.node, self.tcx, ty, false) {
 887                     Ok(val) => {
 888                         let instance = ty::Instance::new(
 889                             self.tables.local_id_root.expect("literal outside any scope"),
 890                             self.substs,
 891                         );
 892                         *self.const_to_pat(instance, val, expr.hir_id, lit.span).kind
 893                     },
 894                     Err(LitToConstError::UnparseableFloat) => {
 895                         self.errors.push(PatternError::FloatBug);
 896                         PatternKind::Wild
 897                     },
 898                     Err(LitToConstError::Reported) => PatternKind::Wild,
 899                 }
 900             },
 901             hir::ExprKind::Path(ref qpath) => *self.lower_path(qpath, expr.hir_id, expr.span).kind,
 902             hir::ExprKind::Unary(hir::UnNeg, ref expr) => {
 903                 let ty = self.tables.expr_ty(expr);
 904                 let lit = match expr.node {
 905                     hir::ExprKind::Lit(ref lit) => lit,
 906                     _ => span_bug!(expr.span, "not a literal: {:?}", expr),
 907                 };
 908                 match lit_to_const(&lit.node, self.tcx, ty, true) {
 909                     Ok(val) => {
 910                         let instance = ty::Instance::new(
 911                             self.tables.local_id_root.expect("literal outside any scope"),
 912                             self.substs,
 913                         );
 914                         *self.const_to_pat(instance, val, expr.hir_id, lit.span).kind
 915                     },
 916                     Err(LitToConstError::UnparseableFloat) => {
 917                         self.errors.push(PatternError::FloatBug);
 918                         PatternKind::Wild
 919                     },
 920                     Err(LitToConstError::Reported) => PatternKind::Wild,
 921                 }
 922             }
 923             _ => span_bug!(expr.span, "not a literal: {:?}", expr),
 924         }
 925     }
 926
 927     /// Converts an evaluated constant to a pattern (if possible).
 928     /// This means aggregate values (like structs and enums) are converted
 929     /// to a pattern that matches the value (as if you'd compare via eq).
 930     fn const_to_pat(
 931         &self,
 932         instance: ty::Instance<'tcx>,
 933         cv: &'tcx ty::Const<'tcx>,
 934         id: hir::HirId,
 935         span: Span,
 936     ) -> Pattern<'tcx> {
 937         debug!("const_to_pat: cv={:#?}", cv);
 938         let adt_subpattern = |i, variant_opt| {
 939             let field = Field::new(i);
 940             let val = const_field(
 941                 self.tcx, self.param_env, instance,
 942                 variant_opt, field, cv,
 943             ).expect("field access failed");
 944             self.const_to_pat(instance, val, id, span)
 945         };
 946         let adt_subpatterns = |n, variant_opt| {
 947             (0..n).map(|i| {
 948                 let field = Field::new(i);
 949                 FieldPattern {
 950                     field,
 951                     pattern: adt_subpattern(i, variant_opt),
 952                 }
 953             }).collect::<Vec<_>>()
 954         };
 955         let kind = match cv.ty.sty {
 956             ty::Float(_) => {
 957                 let id = self.tcx.hir().hir_to_node_id(id);
 958                 self.tcx.lint_node(
 959                     ::rustc::lint::builtin::ILLEGAL_FLOATING_POINT_LITERAL_PATTERN,
 960                     id,
 961                     span,
 962                     "floating-point types cannot be used in patterns",
 963                 );
 964                 PatternKind::Constant {
 965                     value: cv,
 966                 }
 967             },
 968             ty::Adt(adt_def, _) if adt_def.is_union() => {
 969                 // Matching on union fields is unsafe, we can't hide it in constants
 970                 self.tcx.sess.span_err(span, "cannot use unions in constant patterns");
 971                 PatternKind::Wild
 972             }
 973             ty::Adt(adt_def, _) if !self.tcx.has_attr(adt_def.did, "structural_match") => {
 974                 let msg = format!("to use a constant of type `{}` in a pattern, \
 975                                     `{}` must be annotated with `#[derive(PartialEq, Eq)]`",
 976                                     self.tcx.item_path_str(adt_def.did),
 977                                     self.tcx.item_path_str(adt_def.did));
 978                 self.tcx.sess.span_err(span, &msg);
 979                 PatternKind::Wild
 980             },
 981             ty::Adt(adt_def, substs) if adt_def.is_enum() => {
 982                 let variant_index = const_variant_index(
 983                     self.tcx, self.param_env, instance, cv
 984                 ).expect("const_variant_index failed");
 985                 let subpatterns = adt_subpatterns(
 986                     adt_def.variants[variant_index].fields.len(),
 987                     Some(variant_index),
 988                 );
 989                 PatternKind::Variant {
 990                     adt_def,
 991                     substs,
 992                     variant_index,
 993                     subpatterns,
 994                 }
 995             },
 996             ty::Adt(adt_def, _) => {
 997                 let struct_var = adt_def.non_enum_variant();
 998                 PatternKind::Leaf {
 999                     subpatterns: adt_subpatterns(struct_var.fields.len(), None),
1000                 }
1001             }
1002             ty::Tuple(fields) => {
1003                 PatternKind::Leaf {
1004                     subpatterns: adt_subpatterns(fields.len(), None),
1005                 }
1006             }
1007             ty::Array(_, n) => {
1008                 PatternKind::Array {
1009                     prefix: (0..n.unwrap_usize(self.tcx))
1010                         .map(|i| adt_subpattern(i as usize, None))
1011                         .collect(),
1012                     slice: None,
1013                     suffix: Vec::new(),
1014                 }
1015             }
1016             _ => {
1017                 PatternKind::Constant {
1018                     value: cv,
1019                 }
1020             },
1021         };
1022
1023         Pattern {
1024             span,
1025             ty: cv.ty,
1026             kind: Box::new(kind),
1027         }
1028     }
1029 }
1030
1031 impl UserAnnotatedTyHelpers<'tcx, 'tcx> for PatternContext<'_, 'tcx> {
1032     fn tcx(&self) -> TyCtxt<'_, 'tcx, 'tcx> {
1033         self.tcx
1034     }
1035
1036     fn tables(&self) -> &ty::TypeckTables<'tcx> {
1037         self.tables
1038     }
1039 }
1040
1041
1042 pub trait PatternFoldable<'tcx> : Sized {
1043     fn fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
1044         self.super_fold_with(folder)
1045     }
1046
1047     fn super_fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self;
1048 }
1049
1050 pub trait PatternFolder<'tcx> : Sized {
1051     fn fold_pattern(&mut self, pattern: &Pattern<'tcx>) -> Pattern<'tcx> {
1052         pattern.super_fold_with(self)
1053     }
1054
1055     fn fold_pattern_kind(&mut self, kind: &PatternKind<'tcx>) -> PatternKind<'tcx> {
1056         kind.super_fold_with(self)
1057     }
1058 }
1059
1060
1061 impl<'tcx, T: PatternFoldable<'tcx>> PatternFoldable<'tcx> for Box<T> {
1062     fn super_fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
1063         let content: T = (**self).fold_with(folder);
1064         box content
1065     }
1066 }
1067
1068 impl<'tcx, T: PatternFoldable<'tcx>> PatternFoldable<'tcx> for Vec<T> {
1069     fn super_fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
1070         self.iter().map(|t| t.fold_with(folder)).collect()
1071     }
1072 }
1073
1074 impl<'tcx, T: PatternFoldable<'tcx>> PatternFoldable<'tcx> for Option<T> {
1075     fn super_fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self{
1076         self.as_ref().map(|t| t.fold_with(folder))
1077     }
1078 }
1079
1080 macro_rules! CloneImpls {
1081     (<$lt_tcx:tt> $($ty:ty),+) => {
1082         $(
1083             impl<$lt_tcx> PatternFoldable<$lt_tcx> for $ty {
1084                 fn super_fold_with<F: PatternFolder<$lt_tcx>>(&self, _: &mut F) -> Self {
1085                     Clone::clone(self)
1086                 }
1087             }
1088         )+
1089     }
1090 }
1091
1092 CloneImpls!{ <'tcx>
1093     Span, Field, Mutability, ast::Name, ast::NodeId, usize, &'tcx ty::Const<'tcx>,
1094     Region<'tcx>, Ty<'tcx>, BindingMode<'tcx>, &'tcx AdtDef,
1095     &'tcx Substs<'tcx>, &'tcx Kind<'tcx>, UserTypeAnnotation<'tcx>,
1096     UserTypeProjection<'tcx>, PatternTypeProjection<'tcx>
1097 }
1098
1099 impl<'tcx> PatternFoldable<'tcx> for FieldPattern<'tcx> {
1100     fn super_fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
1101         FieldPattern {
1102             field: self.field.fold_with(folder),
1103             pattern: self.pattern.fold_with(folder)
1104         }
1105     }
1106 }
1107
1108 impl<'tcx> PatternFoldable<'tcx> for Pattern<'tcx> {
1109     fn fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
1110         folder.fold_pattern(self)
1111     }
1112
1113     fn super_fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
1114         Pattern {
1115             ty: self.ty.fold_with(folder),
1116             span: self.span.fold_with(folder),
1117             kind: self.kind.fold_with(folder)
1118         }
1119     }
1120 }
1121
1122 impl<'tcx> PatternFoldable<'tcx> for PatternKind<'tcx> {
1123     fn fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
1124         folder.fold_pattern_kind(self)
1125     }
1126
1127     fn super_fold_with<F: PatternFolder<'tcx>>(&self, folder: &mut F) -> Self {
1128         match *self {
1129             PatternKind::Wild => PatternKind::Wild,
1130             PatternKind::AscribeUserType {
1131                 ref subpattern,
1132                 ref user_ty,
1133                 user_ty_span,
1134             } => PatternKind::AscribeUserType {
1135                 subpattern: subpattern.fold_with(folder),
1136                 user_ty: user_ty.fold_with(folder),
1137                 user_ty_span,
1138             },
1139             PatternKind::Binding {
1140                 mutability,
1141                 name,
1142                 mode,
1143                 var,
1144                 ty,
1145                 ref subpattern,
1146             } => PatternKind::Binding {
1147                 mutability: mutability.fold_with(folder),
1148                 name: name.fold_with(folder),
1149                 mode: mode.fold_with(folder),
1150                 var: var.fold_with(folder),
1151                 ty: ty.fold_with(folder),
1152                 subpattern: subpattern.fold_with(folder),
1153             },
1154             PatternKind::Variant {
1155                 adt_def,
1156                 substs,
1157                 variant_index,
1158                 ref subpatterns,
1159             } => PatternKind::Variant {
1160                 adt_def: adt_def.fold_with(folder),
1161                 substs: substs.fold_with(folder),
1162                 variant_index,
1163                 subpatterns: subpatterns.fold_with(folder)
1164             },
1165             PatternKind::Leaf {
1166                 ref subpatterns,
1167             } => PatternKind::Leaf {
1168                 subpatterns: subpatterns.fold_with(folder),
1169             },
1170             PatternKind::Deref {
1171                 ref subpattern,
1172             } => PatternKind::Deref {
1173                 subpattern: subpattern.fold_with(folder),
1174             },
1175             PatternKind::Constant {
1176                 value
1177             } => PatternKind::Constant {
1178                 value: value.fold_with(folder)
1179             },
1180             PatternKind::Range {
1181                 lo,
1182                 hi,
1183                 ty,
1184                 end,
1185             } => PatternKind::Range {
1186                 lo: lo.fold_with(folder),
1187                 hi: hi.fold_with(folder),
1188                 ty: ty.fold_with(folder),
1189                 end,
1190             },
1191             PatternKind::Slice {
1192                 ref prefix,
1193                 ref slice,
1194                 ref suffix,
1195             } => PatternKind::Slice {
1196                 prefix: prefix.fold_with(folder),
1197                 slice: slice.fold_with(folder),
1198                 suffix: suffix.fold_with(folder)
1199             },
1200             PatternKind::Array {
1201                 ref prefix,
1202                 ref slice,
1203                 ref suffix
1204             } => PatternKind::Array {
1205                 prefix: prefix.fold_with(folder),
1206                 slice: slice.fold_with(folder),
1207                 suffix: suffix.fold_with(folder)
1208             },
1209         }
1210     }
1211 }
1212
1213 pub fn compare_const_vals<'a, 'tcx>(
1214     tcx: TyCtxt<'a, 'tcx, 'tcx>,
1215     a: &'tcx ty::Const<'tcx>,
1216     b: &'tcx ty::Const<'tcx>,
1217     ty: ty::ParamEnvAnd<'tcx, Ty<'tcx>>,
1218 ) -> Option<Ordering> {
1219     trace!("compare_const_vals: {:?}, {:?}", a, b);
1220
1221     let from_bool = |v: bool| {
1222         if v {
1223             Some(Ordering::Equal)
1224         } else {
1225             None
1226         }
1227     };
1228
1229     let fallback = || from_bool(a == b);
1230
1231     // Use the fallback if any type differs
1232     if a.ty != b.ty || a.ty != ty.value {
1233         return fallback();
1234     }
1235
1236     // FIXME: This should use assert_bits(ty) instead of use_bits
1237     // but triggers possibly bugs due to mismatching of arrays and slices
1238     if let (Some(a), Some(b)) = (a.to_bits(tcx, ty), b.to_bits(tcx, ty)) {
1239         use ::rustc_apfloat::Float;
1240         return match ty.value.sty {
1241             ty::Float(ast::FloatTy::F32) => {
1242                 let l = ::rustc_apfloat::ieee::Single::from_bits(a);
1243                 let r = ::rustc_apfloat::ieee::Single::from_bits(b);
1244                 l.partial_cmp(&r)
1245             },
1246             ty::Float(ast::FloatTy::F64) => {
1247                 let l = ::rustc_apfloat::ieee::Double::from_bits(a);
1248                 let r = ::rustc_apfloat::ieee::Double::from_bits(b);
1249                 l.partial_cmp(&r)
1250             },
1251             ty::Int(_) => {
1252                 let layout = tcx.layout_of(ty).ok()?;
1253                 assert!(layout.abi.is_signed());
1254                 let a = sign_extend(a, layout.size);
1255                 let b = sign_extend(b, layout.size);
1256                 Some((a as i128).cmp(&(b as i128)))
1257             },
1258             _ => Some(a.cmp(&b)),
1259         }
1260     }
1261
1262     if let ty::Ref(_, rty, _) = ty.value.sty {
1263         if let ty::Str = rty.sty {
1264             match (a.val, b.val) {
1265                 (
1266                     ConstValue::ScalarPair(
1267                         Scalar::Ptr(ptr_a),
1268                         len_a,
1269                     ),
1270                     ConstValue::ScalarPair(
1271                         Scalar::Ptr(ptr_b),
1272                         len_b,
1273                     ),
1274                 ) if ptr_a.offset.bytes() == 0 && ptr_b.offset.bytes() == 0 => {
1275                     if let Ok(len_a) = len_a.to_bits(tcx.data_layout.pointer_size) {
1276                         if let Ok(len_b) = len_b.to_bits(tcx.data_layout.pointer_size) {
1277                             if len_a == len_b {
1278                                 let map = tcx.alloc_map.lock();
1279                                 let alloc_a = map.unwrap_memory(ptr_a.alloc_id);
1280                                 let alloc_b = map.unwrap_memory(ptr_b.alloc_id);
1281                                 if alloc_a.bytes.len() as u128 == len_a {
1282                                     return from_bool(alloc_a == alloc_b);
1283                                 }
1284                             }
1285                         }
1286                     }
1287                 }
1288                 _ => (),
1289             }
1290         }
1291     }
1292
1293     fallback()
1294 }