src/librustc/middle/subst.rs

   1 // Copyright 2012 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 // Type substitutions.
  12
  13 pub use self::ParamSpace::*;
  14 pub use self::RegionSubsts::*;
  15
  16 use middle::ty::{self, Ty};
  17 use middle::ty_fold::{self, TypeFoldable, TypeFolder};
  18
  19 use std::fmt;
  20 use std::iter::IntoIterator;
  21 use std::slice::Iter;
  22 use std::vec::{Vec, IntoIter};
  23 use syntax::codemap::{Span, DUMMY_SP};
  24
  25 ///////////////////////////////////////////////////////////////////////////
  26
  27 /// A substitution mapping type/region parameters to new values. We
  28 /// identify each in-scope parameter by an *index* and a *parameter
  29 /// space* (which indices where the parameter is defined; see
  30 /// `ParamSpace`).
  31 #[derive(Clone, PartialEq, Eq, Hash)]
  32 pub struct Substs<'tcx> {
  33     pub types: VecPerParamSpace<Ty<'tcx>>,
  34     pub regions: RegionSubsts,
  35 }
  36
  37 /// Represents the values to use when substituting lifetime parameters.
  38 /// If the value is `ErasedRegions`, then this subst is occurring during
  39 /// trans, and all region parameters will be replaced with `ty::ReStatic`.
  40 #[derive(Clone, PartialEq, Eq, Hash)]
  41 pub enum RegionSubsts {
  42     ErasedRegions,
  43     NonerasedRegions(VecPerParamSpace<ty::Region>)
  44 }
  45
  46 impl<'tcx> Substs<'tcx> {
  47     pub fn new(t: VecPerParamSpace<Ty<'tcx>>,
  48                r: VecPerParamSpace<ty::Region>)
  49                -> Substs<'tcx>
  50     {
  51         Substs { types: t, regions: NonerasedRegions(r) }
  52     }
  53
  54     pub fn new_type(t: Vec<Ty<'tcx>>,
  55                     r: Vec<ty::Region>)
  56                     -> Substs<'tcx>
  57     {
  58         Substs::new(VecPerParamSpace::new(t, Vec::new(), Vec::new()),
  59                     VecPerParamSpace::new(r, Vec::new(), Vec::new()))
  60     }
  61
  62     pub fn new_trait(t: Vec<Ty<'tcx>>,
  63                      r: Vec<ty::Region>,
  64                      s: Ty<'tcx>)
  65                     -> Substs<'tcx>
  66     {
  67         Substs::new(VecPerParamSpace::new(t, vec!(s), Vec::new()),
  68                     VecPerParamSpace::new(r, Vec::new(), Vec::new()))
  69     }
  70
  71     pub fn erased(t: VecPerParamSpace<Ty<'tcx>>) -> Substs<'tcx>
  72     {
  73         Substs { types: t, regions: ErasedRegions }
  74     }
  75
  76     pub fn empty() -> Substs<'tcx> {
  77         Substs {
  78             types: VecPerParamSpace::empty(),
  79             regions: NonerasedRegions(VecPerParamSpace::empty()),
  80         }
  81     }
  82
  83     pub fn trans_empty() -> Substs<'tcx> {
  84         Substs {
  85             types: VecPerParamSpace::empty(),
  86             regions: ErasedRegions
  87         }
  88     }
  89
  90     pub fn is_noop(&self) -> bool {
  91         let regions_is_noop = match self.regions {
  92             ErasedRegions => false, // may be used to canonicalize
  93             NonerasedRegions(ref regions) => regions.is_empty(),
  94         };
  95
  96         regions_is_noop && self.types.is_empty()
  97     }
  98
  99     pub fn type_for_def(&self, ty_param_def: &ty::TypeParameterDef) -> Ty<'tcx> {
 100         *self.types.get(ty_param_def.space, ty_param_def.index as usize)
 101     }
 102
 103     pub fn has_regions_escaping_depth(&self, depth: u32) -> bool {
 104         self.types.iter().any(|&t| ty::type_escapes_depth(t, depth)) || {
 105             match self.regions {
 106                 ErasedRegions =>
 107                     false,
 108                 NonerasedRegions(ref regions) =>
 109                     regions.iter().any(|r| r.escapes_depth(depth)),
 110             }
 111         }
 112     }
 113
 114     pub fn self_ty(&self) -> Option<Ty<'tcx>> {
 115         self.types.get_self().cloned()
 116     }
 117
 118     pub fn with_self_ty(&self, self_ty: Ty<'tcx>) -> Substs<'tcx> {
 119         assert!(self.self_ty().is_none());
 120         let mut s = (*self).clone();
 121         s.types.push(SelfSpace, self_ty);
 122         s
 123     }
 124
 125     pub fn erase_regions(self) -> Substs<'tcx> {
 126         let Substs { types, regions: _ } = self;
 127         Substs { types: types, regions: ErasedRegions }
 128     }
 129
 130     /// Since ErasedRegions are only to be used in trans, most of the compiler can use this method
 131     /// to easily access the set of region substitutions.
 132     pub fn regions<'a>(&'a self) -> &'a VecPerParamSpace<ty::Region> {
 133         match self.regions {
 134             ErasedRegions => panic!("Erased regions only expected in trans"),
 135             NonerasedRegions(ref r) => r
 136         }
 137     }
 138
 139     /// Since ErasedRegions are only to be used in trans, most of the compiler can use this method
 140     /// to easily access the set of region substitutions.
 141     pub fn mut_regions<'a>(&'a mut self) -> &'a mut VecPerParamSpace<ty::Region> {
 142         match self.regions {
 143             ErasedRegions => panic!("Erased regions only expected in trans"),
 144             NonerasedRegions(ref mut r) => r
 145         }
 146     }
 147
 148     pub fn with_method(self,
 149                        m_types: Vec<Ty<'tcx>>,
 150                        m_regions: Vec<ty::Region>)
 151                        -> Substs<'tcx>
 152     {
 153         let Substs { types, regions } = self;
 154         let types = types.with_vec(FnSpace, m_types);
 155         let regions = regions.map(m_regions,
 156                                   |r, m_regions| r.with_vec(FnSpace, m_regions));
 157         Substs { types: types, regions: regions }
 158     }
 159 }
 160
 161 impl RegionSubsts {
 162     fn map<A, F>(self, a: A, op: F) -> RegionSubsts where
 163         F: FnOnce(VecPerParamSpace<ty::Region>, A) -> VecPerParamSpace<ty::Region>,
 164     {
 165         match self {
 166             ErasedRegions => ErasedRegions,
 167             NonerasedRegions(r) => NonerasedRegions(op(r, a))
 168         }
 169     }
 170
 171     pub fn is_erased(&self) -> bool {
 172         match *self {
 173             ErasedRegions => true,
 174             NonerasedRegions(_) => false,
 175         }
 176     }
 177 }
 178
 179 ///////////////////////////////////////////////////////////////////////////
 180 // ParamSpace
 181
 182 #[derive(PartialOrd, Ord, PartialEq, Eq, Copy,
 183            Clone, Hash, RustcEncodable, RustcDecodable, Debug)]
 184 pub enum ParamSpace {
 185     TypeSpace,  // Type parameters attached to a type definition, trait, or impl
 186     SelfSpace,  // Self parameter on a trait
 187     FnSpace,    // Type parameters attached to a method or fn
 188 }
 189
 190 impl ParamSpace {
 191     pub fn all() -> [ParamSpace; 3] {
 192         [TypeSpace, SelfSpace, FnSpace]
 193     }
 194
 195     pub fn to_uint(self) -> usize {
 196         match self {
 197             TypeSpace => 0,
 198             SelfSpace => 1,
 199             FnSpace => 2,
 200         }
 201     }
 202
 203     pub fn from_uint(u: usize) -> ParamSpace {
 204         match u {
 205             0 => TypeSpace,
 206             1 => SelfSpace,
 207             2 => FnSpace,
 208             _ => panic!("Invalid ParamSpace: {}", u)
 209         }
 210     }
 211 }
 212
 213 /// Vector of things sorted by param space. Used to keep
 214 /// the set of things declared on the type, self, or method
 215 /// distinct.
 216 #[derive(PartialEq, Eq, Clone, Hash, RustcEncodable, RustcDecodable)]
 217 pub struct VecPerParamSpace<T> {
 218     // This was originally represented as a tuple with one Vec<T> for
 219     // each variant of ParamSpace, and that remains the abstraction
 220     // that it provides to its clients.
 221     //
 222     // Here is how the representation corresponds to the abstraction
 223     // i.e. the "abstraction function" AF:
 224     //
 225     // AF(self) = (self.content[..self.type_limit],
 226     //             self.content[self.type_limit..self.self_limit],
 227     //             self.content[self.self_limit..])
 228     type_limit: usize,
 229     self_limit: usize,
 230     content: Vec<T>,
 231 }
 232
 233 /// The `split` function converts one `VecPerParamSpace` into this
 234 /// `SeparateVecsPerParamSpace` structure.
 235 pub struct SeparateVecsPerParamSpace<T> {
 236     pub types: Vec<T>,
 237     pub selfs: Vec<T>,
 238     pub fns: Vec<T>,
 239 }
 240
 241 impl<T: fmt::Debug> fmt::Debug for VecPerParamSpace<T> {
 242     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
 243         write!(f, "[{:?};{:?};{:?}]",
 244                self.get_slice(TypeSpace),
 245                self.get_slice(SelfSpace),
 246                self.get_slice(FnSpace))
 247     }
 248 }
 249
 250 impl<T> VecPerParamSpace<T> {
 251     fn limits(&self, space: ParamSpace) -> (usize, usize) {
 252         match space {
 253             TypeSpace => (0, self.type_limit),
 254             SelfSpace => (self.type_limit, self.self_limit),
 255             FnSpace => (self.self_limit, self.content.len()),
 256         }
 257     }
 258
 259     pub fn empty() -> VecPerParamSpace<T> {
 260         VecPerParamSpace {
 261             type_limit: 0,
 262             self_limit: 0,
 263             content: Vec::new()
 264         }
 265     }
 266
 267     pub fn params_from_type(types: Vec<T>) -> VecPerParamSpace<T> {
 268         VecPerParamSpace::empty().with_vec(TypeSpace, types)
 269     }
 270
 271     /// `t` is the type space.
 272     /// `s` is the self space.
 273     /// `f` is the fn space.
 274     pub fn new(t: Vec<T>, s: Vec<T>, f: Vec<T>) -> VecPerParamSpace<T> {
 275         let type_limit = t.len();
 276         let self_limit = type_limit + s.len();
 277
 278         let mut content = t;
 279         content.extend(s);
 280         content.extend(f);
 281
 282         VecPerParamSpace {
 283             type_limit: type_limit,
 284             self_limit: self_limit,
 285             content: content,
 286         }
 287     }
 288
 289     fn new_internal(content: Vec<T>, type_limit: usize, self_limit: usize)
 290                     -> VecPerParamSpace<T>
 291     {
 292         VecPerParamSpace {
 293             type_limit: type_limit,
 294             self_limit: self_limit,
 295             content: content,
 296         }
 297     }
 298
 299     /// Appends `value` to the vector associated with `space`.
 300     ///
 301     /// Unlike the `push` method in `Vec`, this should not be assumed
 302     /// to be a cheap operation (even when amortized over many calls).
 303     pub fn push(&mut self, space: ParamSpace, value: T) {
 304         let (_, limit) = self.limits(space);
 305         match space {
 306             TypeSpace => { self.type_limit += 1; self.self_limit += 1; }
 307             SelfSpace => { self.self_limit += 1; }
 308             FnSpace => { }
 309         }
 310         self.content.insert(limit, value);
 311     }
 312
 313     /// Appends `values` to the vector associated with `space`.
 314     ///
 315     /// Unlike the `extend` method in `Vec`, this should not be assumed
 316     /// to be a cheap operation (even when amortized over many calls).
 317     pub fn extend<I:Iterator<Item=T>>(&mut self, space: ParamSpace, values: I) {
 318         // This could be made more efficient, obviously.
 319         for item in values {
 320             self.push(space, item);
 321         }
 322     }
 323
 324     pub fn pop(&mut self, space: ParamSpace) -> Option<T> {
 325         let (start, limit) = self.limits(space);
 326         if start == limit {
 327             None
 328         } else {
 329             match space {
 330                 TypeSpace => { self.type_limit -= 1; self.self_limit -= 1; }
 331                 SelfSpace => { self.self_limit -= 1; }
 332                 FnSpace => {}
 333             }
 334             if self.content.is_empty() {
 335                 None
 336             } else {
 337                 Some(self.content.remove(limit - 1))
 338             }
 339         }
 340     }
 341
 342     pub fn truncate(&mut self, space: ParamSpace, len: usize) {
 343         // FIXME (#15435): slow; O(n^2); could enhance vec to make it O(n).
 344         while self.len(space) > len {
 345             self.pop(space);
 346         }
 347     }
 348
 349     pub fn replace(&mut self, space: ParamSpace, elems: Vec<T>) {
 350         // FIXME (#15435): slow; O(n^2); could enhance vec to make it O(n).
 351         self.truncate(space, 0);
 352         for t in elems {
 353             self.push(space, t);
 354         }
 355     }
 356
 357     pub fn get_self<'a>(&'a self) -> Option<&'a T> {
 358         let v = self.get_slice(SelfSpace);
 359         assert!(v.len() <= 1);
 360         if v.is_empty() { None } else { Some(&v[0]) }
 361     }
 362
 363     pub fn len(&self, space: ParamSpace) -> usize {
 364         self.get_slice(space).len()
 365     }
 366
 367     pub fn is_empty_in(&self, space: ParamSpace) -> bool {
 368         self.len(space) == 0
 369     }
 370
 371     pub fn get_slice<'a>(&'a self, space: ParamSpace) -> &'a [T] {
 372         let (start, limit) = self.limits(space);
 373         &self.content[start.. limit]
 374     }
 375
 376     pub fn get_mut_slice<'a>(&'a mut self, space: ParamSpace) -> &'a mut [T] {
 377         let (start, limit) = self.limits(space);
 378         &mut self.content[start.. limit]
 379     }
 380
 381     pub fn opt_get<'a>(&'a self,
 382                        space: ParamSpace,
 383                        index: usize)
 384                        -> Option<&'a T> {
 385         let v = self.get_slice(space);
 386         if index < v.len() { Some(&v[index]) } else { None }
 387     }
 388
 389     pub fn get<'a>(&'a self, space: ParamSpace, index: usize) -> &'a T {
 390         &self.get_slice(space)[index]
 391     }
 392
 393     pub fn iter<'a>(&'a self) -> Iter<'a,T> {
 394         self.content.iter()
 395     }
 396
 397     pub fn into_iter(self) -> IntoIter<T> {
 398         self.content.into_iter()
 399     }
 400
 401     pub fn iter_enumerated<'a>(&'a self) -> EnumeratedItems<'a,T> {
 402         EnumeratedItems::new(self)
 403     }
 404
 405     pub fn as_slice(&self) -> &[T] {
 406         &self.content
 407     }
 408
 409     pub fn into_vec(self) -> Vec<T> {
 410         self.content
 411     }
 412
 413     pub fn all_vecs<P>(&self, mut pred: P) -> bool where
 414         P: FnMut(&[T]) -> bool,
 415     {
 416         let spaces = [TypeSpace, SelfSpace, FnSpace];
 417         spaces.iter().all(|&space| { pred(self.get_slice(space)) })
 418     }
 419
 420     pub fn all<P>(&self, pred: P) -> bool where P: FnMut(&T) -> bool {
 421         self.iter().all(pred)
 422     }
 423
 424     pub fn any<P>(&self, pred: P) -> bool where P: FnMut(&T) -> bool {
 425         self.iter().any(pred)
 426     }
 427
 428     pub fn is_empty(&self) -> bool {
 429         self.all_vecs(|v| v.is_empty())
 430     }
 431
 432     pub fn map<U, P>(&self, pred: P) -> VecPerParamSpace<U> where P: FnMut(&T) -> U {
 433         let result = self.iter().map(pred).collect();
 434         VecPerParamSpace::new_internal(result,
 435                                        self.type_limit,
 436                                        self.self_limit)
 437     }
 438
 439     pub fn map_enumerated<U, P>(&self, pred: P) -> VecPerParamSpace<U> where
 440         P: FnMut((ParamSpace, usize, &T)) -> U,
 441     {
 442         let result = self.iter_enumerated().map(pred).collect();
 443         VecPerParamSpace::new_internal(result,
 444                                        self.type_limit,
 445                                        self.self_limit)
 446     }
 447
 448     pub fn split(self) -> SeparateVecsPerParamSpace<T> {
 449         let VecPerParamSpace { type_limit, self_limit, content } = self;
 450
 451         let mut content_iter = content.into_iter();
 452
 453         SeparateVecsPerParamSpace {
 454             types: content_iter.by_ref().take(type_limit).collect(),
 455             selfs: content_iter.by_ref().take(self_limit - type_limit).collect(),
 456             fns: content_iter.collect()
 457         }
 458     }
 459
 460     pub fn with_vec(mut self, space: ParamSpace, vec: Vec<T>)
 461                     -> VecPerParamSpace<T>
 462     {
 463         assert!(self.is_empty_in(space));
 464         self.replace(space, vec);
 465         self
 466     }
 467 }
 468
 469 #[derive(Clone)]
 470 pub struct EnumeratedItems<'a,T:'a> {
 471     vec: &'a VecPerParamSpace<T>,
 472     space_index: usize,
 473     elem_index: usize
 474 }
 475
 476 impl<'a,T> EnumeratedItems<'a,T> {
 477     fn new(v: &'a VecPerParamSpace<T>) -> EnumeratedItems<'a,T> {
 478         let mut result = EnumeratedItems { vec: v, space_index: 0, elem_index: 0 };
 479         result.adjust_space();
 480         result
 481     }
 482
 483     fn adjust_space(&mut self) {
 484         let spaces = ParamSpace::all();
 485         while
 486             self.space_index < spaces.len() &&
 487             self.elem_index >= self.vec.len(spaces[self.space_index])
 488         {
 489             self.space_index += 1;
 490             self.elem_index = 0;
 491         }
 492     }
 493 }
 494
 495 impl<'a,T> Iterator for EnumeratedItems<'a,T> {
 496     type Item = (ParamSpace, usize, &'a T);
 497
 498     fn next(&mut self) -> Option<(ParamSpace, usize, &'a T)> {
 499         let spaces = ParamSpace::all();
 500         if self.space_index < spaces.len() {
 501             let space = spaces[self.space_index];
 502             let index = self.elem_index;
 503             let item = self.vec.get(space, index);
 504
 505             self.elem_index += 1;
 506             self.adjust_space();
 507
 508             Some((space, index, item))
 509         } else {
 510             None
 511         }
 512     }
 513 }
 514
 515 impl<T> IntoIterator for VecPerParamSpace<T> {
 516     type Item = T;
 517     type IntoIter = IntoIter<T>;
 518
 519     fn into_iter(self) -> IntoIter<T> {
 520         self.into_vec().into_iter()
 521     }
 522 }
 523
 524 impl<'a,T> IntoIterator for &'a VecPerParamSpace<T> {
 525     type Item = &'a T;
 526     type IntoIter = Iter<'a, T>;
 527
 528     fn into_iter(self) -> Iter<'a, T> {
 529         self.as_slice().into_iter()
 530     }
 531 }
 532
 533
 534 ///////////////////////////////////////////////////////////////////////////
 535 // Public trait `Subst`
 536 //
 537 // Just call `foo.subst(tcx, substs)` to perform a substitution across
 538 // `foo`. Or use `foo.subst_spanned(tcx, substs, Some(span))` when
 539 // there is more information available (for better errors).
 540
 541 pub trait Subst<'tcx> : Sized {
 542     fn subst(&self, tcx: &ty::ctxt<'tcx>, substs: &Substs<'tcx>) -> Self {
 543         self.subst_spanned(tcx, substs, None)
 544     }
 545
 546     fn subst_spanned(&self, tcx: &ty::ctxt<'tcx>,
 547                      substs: &Substs<'tcx>,
 548                      span: Option<Span>)
 549                      -> Self;
 550 }
 551
 552 impl<'tcx, T:TypeFoldable<'tcx>> Subst<'tcx> for T {
 553     fn subst_spanned(&self,
 554                      tcx: &ty::ctxt<'tcx>,
 555                      substs: &Substs<'tcx>,
 556                      span: Option<Span>)
 557                      -> T
 558     {
 559         let mut folder = SubstFolder { tcx: tcx,
 560                                        substs: substs,
 561                                        span: span,
 562                                        root_ty: None,
 563                                        ty_stack_depth: 0,
 564                                        region_binders_passed: 0 };
 565         (*self).fold_with(&mut folder)
 566     }
 567 }
 568
 569 ///////////////////////////////////////////////////////////////////////////
 570 // The actual substitution engine itself is a type folder.
 571
 572 struct SubstFolder<'a, 'tcx: 'a> {
 573     tcx: &'a ty::ctxt<'tcx>,
 574     substs: &'a Substs<'tcx>,
 575
 576     // The location for which the substitution is performed, if available.
 577     span: Option<Span>,
 578
 579     // The root type that is being substituted, if available.
 580     root_ty: Option<Ty<'tcx>>,
 581
 582     // Depth of type stack
 583     ty_stack_depth: usize,
 584
 585     // Number of region binders we have passed through while doing the substitution
 586     region_binders_passed: u32,
 587 }
 588
 589 impl<'a, 'tcx> TypeFolder<'tcx> for SubstFolder<'a, 'tcx> {
 590     fn tcx(&self) -> &ty::ctxt<'tcx> { self.tcx }
 591
 592     fn enter_region_binder(&mut self) {
 593         self.region_binders_passed += 1;
 594     }
 595
 596     fn exit_region_binder(&mut self) {
 597         self.region_binders_passed -= 1;
 598     }
 599
 600     fn fold_region(&mut self, r: ty::Region) -> ty::Region {
 601         // Note: This routine only handles regions that are bound on
 602         // type declarations and other outer declarations, not those
 603         // bound in *fn types*. Region substitution of the bound
 604         // regions that appear in a function signature is done using
 605         // the specialized routine `ty::replace_late_regions()`.
 606         match r {
 607             ty::ReEarlyBound(data) => {
 608                 match self.substs.regions {
 609                     ErasedRegions => ty::ReStatic,
 610                     NonerasedRegions(ref regions) =>
 611                         match regions.opt_get(data.space, data.index as usize) {
 612                             Some(&r) => {
 613                                 self.shift_region_through_binders(r)
 614                             }
 615                             None => {
 616                                 let span = self.span.unwrap_or(DUMMY_SP);
 617                                 self.tcx().sess.span_bug(
 618                                     span,
 619                                     &format!("Type parameter out of range \
 620                                               when substituting in region {} (root type={:?}) \
 621                                               (space={:?}, index={})",
 622                                              data.name,
 623                                              self.root_ty,
 624                                              data.space,
 625                                              data.index));
 626                             }
 627                         }
 628                 }
 629             }
 630             _ => r
 631         }
 632     }
 633
 634     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 635         if !ty::type_needs_subst(t) {
 636             return t;
 637         }
 638
 639         // track the root type we were asked to substitute
 640         let depth = self.ty_stack_depth;
 641         if depth == 0 {
 642             self.root_ty = Some(t);
 643         }
 644         self.ty_stack_depth += 1;
 645
 646         let t1 = match t.sty {
 647             ty::TyParam(p) => {
 648                 self.ty_for_param(p, t)
 649             }
 650             _ => {
 651                 ty_fold::super_fold_ty(self, t)
 652             }
 653         };
 654
 655         assert_eq!(depth + 1, self.ty_stack_depth);
 656         self.ty_stack_depth -= 1;
 657         if depth == 0 {
 658             self.root_ty = None;
 659         }
 660
 661         return t1;
 662     }
 663 }
 664
 665 impl<'a,'tcx> SubstFolder<'a,'tcx> {
 666     fn ty_for_param(&self, p: ty::ParamTy, source_ty: Ty<'tcx>) -> Ty<'tcx> {
 667         // Look up the type in the substitutions. It really should be in there.
 668         let opt_ty = self.substs.types.opt_get(p.space, p.idx as usize);
 669         let ty = match opt_ty {
 670             Some(t) => *t,
 671             None => {
 672                 let span = self.span.unwrap_or(DUMMY_SP);
 673                 self.tcx().sess.span_bug(
 674                     span,
 675                     &format!("Type parameter `{:?}` ({:?}/{:?}/{}) out of range \
 676                                  when substituting (root type={:?}) substs={:?}",
 677                             p,
 678                             source_ty,
 679                             p.space,
 680                             p.idx,
 681                             self.root_ty,
 682                             self.substs));
 683             }
 684         };
 685
 686         self.shift_regions_through_binders(ty)
 687     }
 688
 689     /// It is sometimes necessary to adjust the debruijn indices during substitution. This occurs
 690     /// when we are substituting a type with escaping regions into a context where we have passed
 691     /// through region binders. That's quite a mouthful. Let's see an example:
 692     ///
 693     /// ```
 694     /// type Func<A> = fn(A);
 695     /// type MetaFunc = for<'a> fn(Func<&'a int>)
 696     /// ```
 697     ///
 698     /// The type `MetaFunc`, when fully expanded, will be
 699     ///
 700     ///     for<'a> fn(fn(&'a int))
 701     ///             ^~ ^~ ^~~
 702     ///             |  |  |
 703     ///             |  |  DebruijnIndex of 2
 704     ///             Binders
 705     ///
 706     /// Here the `'a` lifetime is bound in the outer function, but appears as an argument of the
 707     /// inner one. Therefore, that appearance will have a DebruijnIndex of 2, because we must skip
 708     /// over the inner binder (remember that we count Debruijn indices from 1). However, in the
 709     /// definition of `MetaFunc`, the binder is not visible, so the type `&'a int` will have a
 710     /// debruijn index of 1. It's only during the substitution that we can see we must increase the
 711     /// depth by 1 to account for the binder that we passed through.
 712     ///
 713     /// As a second example, consider this twist:
 714     ///
 715     /// ```
 716     /// type FuncTuple<A> = (A,fn(A));
 717     /// type MetaFuncTuple = for<'a> fn(FuncTuple<&'a int>)
 718     /// ```
 719     ///
 720     /// Here the final type will be:
 721     ///
 722     ///     for<'a> fn((&'a int, fn(&'a int)))
 723     ///                 ^~~         ^~~
 724     ///                 |           |
 725     ///          DebruijnIndex of 1 |
 726     ///                      DebruijnIndex of 2
 727     ///
 728     /// As indicated in the diagram, here the same type `&'a int` is substituted once, but in the
 729     /// first case we do not increase the Debruijn index and in the second case we do. The reason
 730     /// is that only in the second case have we passed through a fn binder.
 731     fn shift_regions_through_binders(&self, ty: Ty<'tcx>) -> Ty<'tcx> {
 732         debug!("shift_regions(ty={:?}, region_binders_passed={:?}, type_has_escaping_regions={:?})",
 733                ty, self.region_binders_passed, ty::type_has_escaping_regions(ty));
 734
 735         if self.region_binders_passed == 0 || !ty::type_has_escaping_regions(ty) {
 736             return ty;
 737         }
 738
 739         let result = ty_fold::shift_regions(self.tcx(), self.region_binders_passed, &ty);
 740         debug!("shift_regions: shifted result = {:?}", result);
 741
 742         result
 743     }
 744
 745     fn shift_region_through_binders(&self, region: ty::Region) -> ty::Region {
 746         ty_fold::shift_region(region, self.region_binders_passed)
 747     }
 748 }