src/librustc/ty/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 use hir::def_id::DefId;
  14 use ty::{self, Ty, TyCtxt};
  15 use ty::fold::{TypeFoldable, TypeFolder, TypeVisitor};
  16
  17 use serialize::{self, Encodable, Encoder, Decodable, Decoder};
  18 use syntax_pos::{Span, DUMMY_SP};
  19
  20 use core::nonzero::NonZero;
  21 use std::fmt;
  22 use std::iter;
  23 use std::marker::PhantomData;
  24 use std::mem;
  25
  26 /// An entity in the Rust typesystem, which can be one of
  27 /// several kinds (only types and lifetimes for now).
  28 /// To reduce memory usage, a `Kind` is a interned pointer,
  29 /// with the lowest 2 bits being reserved for a tag to
  30 /// indicate the type (`Ty` or `Region`) it points to.
  31 #[derive(Copy, Clone, PartialEq, Eq, Hash)]
  32 pub struct Kind<'tcx> {
  33     ptr: NonZero<usize>,
  34     marker: PhantomData<(Ty<'tcx>, &'tcx ty::Region)>
  35 }
  36
  37 const TAG_MASK: usize = 0b11;
  38 const TYPE_TAG: usize = 0b00;
  39 const REGION_TAG: usize = 0b01;
  40
  41 impl<'tcx> From<Ty<'tcx>> for Kind<'tcx> {
  42     fn from(ty: Ty<'tcx>) -> Kind<'tcx> {
  43         // Ensure we can use the tag bits.
  44         assert_eq!(mem::align_of_val(ty) & TAG_MASK, 0);
  45
  46         let ptr = ty as *const _ as usize;
  47         Kind {
  48             ptr: unsafe {
  49                 NonZero::new(ptr | TYPE_TAG)
  50             },
  51             marker: PhantomData
  52         }
  53     }
  54 }
  55
  56 impl<'tcx> From<&'tcx ty::Region> for Kind<'tcx> {
  57     fn from(r: &'tcx ty::Region) -> Kind<'tcx> {
  58         // Ensure we can use the tag bits.
  59         assert_eq!(mem::align_of_val(r) & TAG_MASK, 0);
  60
  61         let ptr = r as *const _ as usize;
  62         Kind {
  63             ptr: unsafe {
  64                 NonZero::new(ptr | REGION_TAG)
  65             },
  66             marker: PhantomData
  67         }
  68     }
  69 }
  70
  71 impl<'tcx> Kind<'tcx> {
  72     #[inline]
  73     unsafe fn downcast<T>(self, tag: usize) -> Option<&'tcx T> {
  74         let ptr = *self.ptr;
  75         if ptr & TAG_MASK == tag {
  76             Some(&*((ptr & !TAG_MASK) as *const _))
  77         } else {
  78             None
  79         }
  80     }
  81
  82     #[inline]
  83     pub fn as_type(self) -> Option<Ty<'tcx>> {
  84         unsafe {
  85             self.downcast(TYPE_TAG)
  86         }
  87     }
  88
  89     #[inline]
  90     pub fn as_region(self) -> Option<&'tcx ty::Region> {
  91         unsafe {
  92             self.downcast(REGION_TAG)
  93         }
  94     }
  95 }
  96
  97 impl<'tcx> fmt::Debug for Kind<'tcx> {
  98     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
  99         if let Some(ty) = self.as_type() {
 100             write!(f, "{:?}", ty)
 101         } else if let Some(r) = self.as_region() {
 102             write!(f, "{:?}", r)
 103         } else {
 104             write!(f, "<unknwon @ {:p}>", *self.ptr as *const ())
 105         }
 106     }
 107 }
 108
 109 impl<'tcx> TypeFoldable<'tcx> for Kind<'tcx> {
 110     fn super_fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, folder: &mut F) -> Self {
 111         if let Some(ty) = self.as_type() {
 112             Kind::from(ty.fold_with(folder))
 113         } else if let Some(r) = self.as_region() {
 114             Kind::from(r.fold_with(folder))
 115         } else {
 116             bug!()
 117         }
 118     }
 119
 120     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
 121         if let Some(ty) = self.as_type() {
 122             ty.visit_with(visitor)
 123         } else if let Some(r) = self.as_region() {
 124             r.visit_with(visitor)
 125         } else {
 126             bug!()
 127         }
 128     }
 129 }
 130
 131 impl<'tcx> Encodable for Kind<'tcx> {
 132     fn encode<E: Encoder>(&self, e: &mut E) -> Result<(), E::Error> {
 133         e.emit_enum("Kind", |e| {
 134             if let Some(ty) = self.as_type() {
 135                 e.emit_enum_variant("Ty", TYPE_TAG, 1, |e| {
 136                     e.emit_enum_variant_arg(0, |e| ty.encode(e))
 137                 })
 138             } else if let Some(r) = self.as_region() {
 139                 e.emit_enum_variant("Region", REGION_TAG, 1, |e| {
 140                     e.emit_enum_variant_arg(0, |e| r.encode(e))
 141                 })
 142             } else {
 143                 bug!()
 144             }
 145         })
 146     }
 147 }
 148
 149 impl<'tcx> Decodable for Kind<'tcx> {
 150     fn decode<D: Decoder>(d: &mut D) -> Result<Kind<'tcx>, D::Error> {
 151         d.read_enum("Kind", |d| {
 152             d.read_enum_variant(&["Ty", "Region"], |d, tag| {
 153                 match tag {
 154                     TYPE_TAG => Ty::decode(d).map(Kind::from),
 155                     REGION_TAG => <&ty::Region>::decode(d).map(Kind::from),
 156                     _ => Err(d.error("invalid Kind tag"))
 157                 }
 158             })
 159         })
 160     }
 161 }
 162
 163 /// A substitution mapping type/region parameters to new values.
 164 #[derive(Clone, PartialEq, Eq, Debug, Hash, RustcEncodable, RustcDecodable)]
 165 pub struct Substs<'tcx> {
 166     params: Vec<Kind<'tcx>>
 167 }
 168
 169 impl<'a, 'gcx, 'tcx> Substs<'tcx> {
 170     pub fn new<I>(tcx: TyCtxt<'a, 'gcx, 'tcx>, params: I)
 171                   -> &'tcx Substs<'tcx>
 172     where I: IntoIterator<Item=Kind<'tcx>> {
 173         tcx.mk_substs(Substs {
 174             params: params.into_iter().collect()
 175         })
 176     }
 177
 178     pub fn maybe_new<I, E>(tcx: TyCtxt<'a, 'gcx, 'tcx>, params: I)
 179                            -> Result<&'tcx Substs<'tcx>, E>
 180     where I: IntoIterator<Item=Result<Kind<'tcx>, E>> {
 181         Ok(tcx.mk_substs(Substs {
 182             params: params.into_iter().collect::<Result<_, _>>()?
 183         }))
 184     }
 185
 186     pub fn new_trait(tcx: TyCtxt<'a, 'gcx, 'tcx>,
 187                      s: Ty<'tcx>,
 188                      t: &[Ty<'tcx>])
 189                     -> &'tcx Substs<'tcx>
 190     {
 191         let t = iter::once(s).chain(t.iter().cloned());
 192         Substs::new(tcx, t.map(Kind::from))
 193     }
 194
 195     pub fn empty(tcx: TyCtxt<'a, 'gcx, 'tcx>) -> &'tcx Substs<'tcx> {
 196         Substs::new(tcx, vec![])
 197     }
 198
 199     /// Creates a Substs for generic parameter definitions,
 200     /// by calling closures to obtain each region and type.
 201     /// The closures get to observe the Substs as they're
 202     /// being built, which can be used to correctly
 203     /// substitute defaults of type parameters.
 204     pub fn for_item<FR, FT>(tcx: TyCtxt<'a, 'gcx, 'tcx>,
 205                             def_id: DefId,
 206                             mut mk_region: FR,
 207                             mut mk_type: FT)
 208                             -> &'tcx Substs<'tcx>
 209     where FR: FnMut(&ty::RegionParameterDef, &Substs<'tcx>) -> &'tcx ty::Region,
 210           FT: FnMut(&ty::TypeParameterDef<'tcx>, &Substs<'tcx>) -> Ty<'tcx> {
 211         let defs = tcx.lookup_generics(def_id);
 212         let mut substs = Substs {
 213             params: Vec::with_capacity(defs.count())
 214         };
 215
 216         substs.fill_item(tcx, defs, &mut mk_region, &mut mk_type);
 217
 218         tcx.mk_substs(substs)
 219     }
 220
 221     fn fill_item<FR, FT>(&mut self,
 222                          tcx: TyCtxt<'a, 'gcx, 'tcx>,
 223                          defs: &ty::Generics<'tcx>,
 224                          mk_region: &mut FR,
 225                          mk_type: &mut FT)
 226     where FR: FnMut(&ty::RegionParameterDef, &Substs<'tcx>) -> &'tcx ty::Region,
 227           FT: FnMut(&ty::TypeParameterDef<'tcx>, &Substs<'tcx>) -> Ty<'tcx> {
 228         if let Some(def_id) = defs.parent {
 229             let parent_defs = tcx.lookup_generics(def_id);
 230             self.fill_item(tcx, parent_defs, mk_region, mk_type);
 231         }
 232
 233         // Handle Self first, before all regions.
 234         let mut types = defs.types.iter();
 235         if defs.parent.is_none() && defs.has_self {
 236             let def = types.next().unwrap();
 237             let ty = mk_type(def, self);
 238             assert_eq!(def.index as usize, self.params.len());
 239             self.params.push(Kind::from(ty));
 240         }
 241
 242         for def in &defs.regions {
 243             let region = mk_region(def, self);
 244             assert_eq!(def.index as usize, self.params.len());
 245             self.params.push(Kind::from(region));
 246         }
 247
 248         for def in types {
 249             let ty = mk_type(def, self);
 250             assert_eq!(def.index as usize, self.params.len());
 251             self.params.push(Kind::from(ty));
 252         }
 253     }
 254
 255     pub fn is_noop(&self) -> bool {
 256         self.params.is_empty()
 257     }
 258
 259     #[inline]
 260     pub fn params(&self) -> &[Kind<'tcx>] {
 261         &self.params
 262     }
 263
 264     #[inline]
 265     pub fn types(&'a self) -> impl DoubleEndedIterator<Item=Ty<'tcx>> + 'a {
 266         self.params.iter().filter_map(|k| k.as_type())
 267     }
 268
 269     #[inline]
 270     pub fn regions(&'a self) -> impl DoubleEndedIterator<Item=&'tcx ty::Region> + 'a {
 271         self.params.iter().filter_map(|k| k.as_region())
 272     }
 273
 274     #[inline]
 275     pub fn type_at(&self, i: usize) -> Ty<'tcx> {
 276         self.params[i].as_type().unwrap_or_else(|| {
 277             bug!("expected type for param #{} in {:?}", i, self.params);
 278         })
 279     }
 280
 281     #[inline]
 282     pub fn region_at(&self, i: usize) -> &'tcx ty::Region {
 283         self.params[i].as_region().unwrap_or_else(|| {
 284             bug!("expected region for param #{} in {:?}", i, self.params);
 285         })
 286     }
 287
 288     #[inline]
 289     pub fn type_for_def(&self, ty_param_def: &ty::TypeParameterDef) -> Ty<'tcx> {
 290         self.type_at(ty_param_def.index as usize)
 291     }
 292
 293     #[inline]
 294     pub fn region_for_def(&self, def: &ty::RegionParameterDef) -> &'tcx ty::Region {
 295         self.region_at(def.index as usize)
 296     }
 297
 298     /// Transform from substitutions for a child of `source_ancestor`
 299     /// (e.g. a trait or impl) to substitutions for the same child
 300     /// in a different item, with `target_substs` as the base for
 301     /// the target impl/trait, with the source child-specific
 302     /// parameters (e.g. method parameters) on top of that base.
 303     pub fn rebase_onto(&self, tcx: TyCtxt<'a, 'gcx, 'tcx>,
 304                        source_ancestor: DefId,
 305                        target_substs: &Substs<'tcx>)
 306                        -> &'tcx Substs<'tcx> {
 307         let defs = tcx.lookup_generics(source_ancestor);
 308         tcx.mk_substs(Substs {
 309             params: target_substs.params.iter()
 310                 .chain(&self.params[defs.own_count()..]).cloned().collect()
 311         })
 312     }
 313 }
 314
 315 impl<'tcx> TypeFoldable<'tcx> for &'tcx Substs<'tcx> {
 316     fn super_fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, folder: &mut F) -> Self {
 317         let params = self.params.iter().map(|k| k.fold_with(folder)).collect();
 318         folder.tcx().mk_substs(Substs {
 319             params: params
 320         })
 321     }
 322
 323     fn fold_with<'gcx: 'tcx, F: TypeFolder<'gcx, 'tcx>>(&self, folder: &mut F) -> Self {
 324         folder.fold_substs(self)
 325     }
 326
 327     fn super_visit_with<V: TypeVisitor<'tcx>>(&self, visitor: &mut V) -> bool {
 328         self.params.visit_with(visitor)
 329     }
 330 }
 331
 332 impl<'tcx> serialize::UseSpecializedDecodable for &'tcx Substs<'tcx> {}
 333
 334 ///////////////////////////////////////////////////////////////////////////
 335 // Public trait `Subst`
 336 //
 337 // Just call `foo.subst(tcx, substs)` to perform a substitution across
 338 // `foo`. Or use `foo.subst_spanned(tcx, substs, Some(span))` when
 339 // there is more information available (for better errors).
 340
 341 pub trait Subst<'tcx> : Sized {
 342     fn subst<'a, 'gcx>(&self, tcx: TyCtxt<'a, 'gcx, 'tcx>,
 343                       substs: &Substs<'tcx>) -> Self {
 344         self.subst_spanned(tcx, substs, None)
 345     }
 346
 347     fn subst_spanned<'a, 'gcx>(&self, tcx: TyCtxt<'a, 'gcx, 'tcx>,
 348                                substs: &Substs<'tcx>,
 349                                span: Option<Span>)
 350                                -> Self;
 351 }
 352
 353 impl<'tcx, T:TypeFoldable<'tcx>> Subst<'tcx> for T {
 354     fn subst_spanned<'a, 'gcx>(&self, tcx: TyCtxt<'a, 'gcx, 'tcx>,
 355                                substs: &Substs<'tcx>,
 356                                span: Option<Span>)
 357                                -> T
 358     {
 359         let mut folder = SubstFolder { tcx: tcx,
 360                                        substs: substs,
 361                                        span: span,
 362                                        root_ty: None,
 363                                        ty_stack_depth: 0,
 364                                        region_binders_passed: 0 };
 365         (*self).fold_with(&mut folder)
 366     }
 367 }
 368
 369 ///////////////////////////////////////////////////////////////////////////
 370 // The actual substitution engine itself is a type folder.
 371
 372 struct SubstFolder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
 373     tcx: TyCtxt<'a, 'gcx, 'tcx>,
 374     substs: &'a Substs<'tcx>,
 375
 376     // The location for which the substitution is performed, if available.
 377     span: Option<Span>,
 378
 379     // The root type that is being substituted, if available.
 380     root_ty: Option<Ty<'tcx>>,
 381
 382     // Depth of type stack
 383     ty_stack_depth: usize,
 384
 385     // Number of region binders we have passed through while doing the substitution
 386     region_binders_passed: u32,
 387 }
 388
 389 impl<'a, 'gcx, 'tcx> TypeFolder<'gcx, 'tcx> for SubstFolder<'a, 'gcx, 'tcx> {
 390     fn tcx<'b>(&'b self) -> TyCtxt<'b, 'gcx, 'tcx> { self.tcx }
 391
 392     fn fold_binder<T: TypeFoldable<'tcx>>(&mut self, t: &ty::Binder<T>) -> ty::Binder<T> {
 393         self.region_binders_passed += 1;
 394         let t = t.super_fold_with(self);
 395         self.region_binders_passed -= 1;
 396         t
 397     }
 398
 399     fn fold_region(&mut self, r: &'tcx ty::Region) -> &'tcx ty::Region {
 400         // Note: This routine only handles regions that are bound on
 401         // type declarations and other outer declarations, not those
 402         // bound in *fn types*. Region substitution of the bound
 403         // regions that appear in a function signature is done using
 404         // the specialized routine `ty::replace_late_regions()`.
 405         match *r {
 406             ty::ReEarlyBound(data) => {
 407                 let r = self.substs.params.get(data.index as usize)
 408                             .and_then(|k| k.as_region());
 409                 match r {
 410                     Some(r) => {
 411                         self.shift_region_through_binders(r)
 412                     }
 413                     None => {
 414                         let span = self.span.unwrap_or(DUMMY_SP);
 415                         span_bug!(
 416                             span,
 417                             "Region parameter out of range \
 418                              when substituting in region {} (root type={:?}) \
 419                              (index={})",
 420                             data.name,
 421                             self.root_ty,
 422                             data.index);
 423                     }
 424                 }
 425             }
 426             _ => r
 427         }
 428     }
 429
 430     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 431         if !t.needs_subst() {
 432             return t;
 433         }
 434
 435         // track the root type we were asked to substitute
 436         let depth = self.ty_stack_depth;
 437         if depth == 0 {
 438             self.root_ty = Some(t);
 439         }
 440         self.ty_stack_depth += 1;
 441
 442         let t1 = match t.sty {
 443             ty::TyParam(p) => {
 444                 self.ty_for_param(p, t)
 445             }
 446             _ => {
 447                 t.super_fold_with(self)
 448             }
 449         };
 450
 451         assert_eq!(depth + 1, self.ty_stack_depth);
 452         self.ty_stack_depth -= 1;
 453         if depth == 0 {
 454             self.root_ty = None;
 455         }
 456
 457         return t1;
 458     }
 459 }
 460
 461 impl<'a, 'gcx, 'tcx> SubstFolder<'a, 'gcx, 'tcx> {
 462     fn ty_for_param(&self, p: ty::ParamTy, source_ty: Ty<'tcx>) -> Ty<'tcx> {
 463         // Look up the type in the substitutions. It really should be in there.
 464         let opt_ty = self.substs.params.get(p.idx as usize)
 465                          .and_then(|k| k.as_type());
 466         let ty = match opt_ty {
 467             Some(t) => t,
 468             None => {
 469                 let span = self.span.unwrap_or(DUMMY_SP);
 470                 span_bug!(
 471                     span,
 472                     "Type parameter `{:?}` ({:?}/{}) out of range \
 473                          when substituting (root type={:?}) substs={:?}",
 474                     p,
 475                     source_ty,
 476                     p.idx,
 477                     self.root_ty,
 478                     self.substs.params);
 479             }
 480         };
 481
 482         self.shift_regions_through_binders(ty)
 483     }
 484
 485     /// It is sometimes necessary to adjust the debruijn indices during substitution. This occurs
 486     /// when we are substituting a type with escaping regions into a context where we have passed
 487     /// through region binders. That's quite a mouthful. Let's see an example:
 488     ///
 489     /// ```
 490     /// type Func<A> = fn(A);
 491     /// type MetaFunc = for<'a> fn(Func<&'a int>)
 492     /// ```
 493     ///
 494     /// The type `MetaFunc`, when fully expanded, will be
 495     ///
 496     ///     for<'a> fn(fn(&'a int))
 497     ///             ^~ ^~ ^~~
 498     ///             |  |  |
 499     ///             |  |  DebruijnIndex of 2
 500     ///             Binders
 501     ///
 502     /// Here the `'a` lifetime is bound in the outer function, but appears as an argument of the
 503     /// inner one. Therefore, that appearance will have a DebruijnIndex of 2, because we must skip
 504     /// over the inner binder (remember that we count Debruijn indices from 1). However, in the
 505     /// definition of `MetaFunc`, the binder is not visible, so the type `&'a int` will have a
 506     /// debruijn index of 1. It's only during the substitution that we can see we must increase the
 507     /// depth by 1 to account for the binder that we passed through.
 508     ///
 509     /// As a second example, consider this twist:
 510     ///
 511     /// ```
 512     /// type FuncTuple<A> = (A,fn(A));
 513     /// type MetaFuncTuple = for<'a> fn(FuncTuple<&'a int>)
 514     /// ```
 515     ///
 516     /// Here the final type will be:
 517     ///
 518     ///     for<'a> fn((&'a int, fn(&'a int)))
 519     ///                 ^~~         ^~~
 520     ///                 |           |
 521     ///          DebruijnIndex of 1 |
 522     ///                      DebruijnIndex of 2
 523     ///
 524     /// As indicated in the diagram, here the same type `&'a int` is substituted once, but in the
 525     /// first case we do not increase the Debruijn index and in the second case we do. The reason
 526     /// is that only in the second case have we passed through a fn binder.
 527     fn shift_regions_through_binders(&self, ty: Ty<'tcx>) -> Ty<'tcx> {
 528         debug!("shift_regions(ty={:?}, region_binders_passed={:?}, has_escaping_regions={:?})",
 529                ty, self.region_binders_passed, ty.has_escaping_regions());
 530
 531         if self.region_binders_passed == 0 || !ty.has_escaping_regions() {
 532             return ty;
 533         }
 534
 535         let result = ty::fold::shift_regions(self.tcx(), self.region_binders_passed, &ty);
 536         debug!("shift_regions: shifted result = {:?}", result);
 537
 538         result
 539     }
 540
 541     fn shift_region_through_binders(&self, region: &'tcx ty::Region) -> &'tcx ty::Region {
 542         self.tcx().mk_region(ty::fold::shift_region(*region, self.region_binders_passed))
 543     }
 544 }
 545
 546 // Helper methods that modify substitutions.
 547
 548 impl<'a, 'gcx, 'tcx> ty::TraitRef<'tcx> {
 549     pub fn from_method(tcx: TyCtxt<'a, 'gcx, 'tcx>,
 550                        trait_id: DefId,
 551                        substs: &Substs<'tcx>)
 552                        -> ty::TraitRef<'tcx> {
 553         let defs = tcx.lookup_generics(trait_id);
 554
 555         let params = substs.params[..defs.own_count()].iter().cloned();
 556         ty::TraitRef {
 557             def_id: trait_id,
 558             substs: Substs::new(tcx, params)
 559         }
 560     }
 561 }
 562
 563 impl<'a, 'gcx, 'tcx> ty::ExistentialTraitRef<'tcx> {
 564     pub fn erase_self_ty(tcx: TyCtxt<'a, 'gcx, 'tcx>,
 565                          trait_ref: ty::TraitRef<'tcx>)
 566                          -> ty::ExistentialTraitRef<'tcx> {
 567         // Assert there is a Self.
 568         trait_ref.substs.type_at(0);
 569
 570         let params = trait_ref.substs.params[1..].iter().cloned();
 571         ty::ExistentialTraitRef {
 572             def_id: trait_ref.def_id,
 573             substs: Substs::new(tcx, params)
 574         }
 575     }
 576 }
 577
 578 impl<'a, 'gcx, 'tcx> ty::PolyExistentialTraitRef<'tcx> {
 579     /// Object types don't have a self-type specified. Therefore, when
 580     /// we convert the principal trait-ref into a normal trait-ref,
 581     /// you must give *some* self-type. A common choice is `mk_err()`
 582     /// or some skolemized type.
 583     pub fn with_self_ty(&self, tcx: TyCtxt<'a, 'gcx, 'tcx>,
 584                         self_ty: Ty<'tcx>)
 585                         -> ty::PolyTraitRef<'tcx>  {
 586         // otherwise the escaping regions would be captured by the binder
 587         assert!(!self_ty.has_escaping_regions());
 588
 589         self.map_bound(|trait_ref| {
 590             let params = trait_ref.substs.params.iter().cloned();
 591             let params = iter::once(Kind::from(self_ty)).chain(params);
 592             ty::TraitRef {
 593                 def_id: trait_ref.def_id,
 594                 substs: Substs::new(tcx, params)
 595             }
 596         })
 597     }
 598 }