src/librustc_typeck/check/wf.rs

   1 // Copyright 2014 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 use astconv::AstConv;
  12 use check::{FnCtxt, Inherited, blank_fn_ctxt, regionck, wfcheck};
  13 use constrained_type_params::{identify_constrained_type_params, Parameter};
  14 use CrateCtxt;
  15 use middle::def_id::DefId;
  16 use middle::region;
  17 use middle::subst::{self, TypeSpace, FnSpace, ParamSpace, SelfSpace};
  18 use middle::traits;
  19 use middle::ty::{self, Ty};
  20 use middle::ty_fold::{TypeFolder, TypeFoldable, super_fold_ty};
  21
  22 use std::cell::RefCell;
  23 use std::collections::HashSet;
  24 use syntax::ast;
  25 use syntax::codemap::{DUMMY_SP, Span};
  26 use syntax::parse::token::{special_idents};
  27 use syntax::visit;
  28 use syntax::visit::{FnKind, Visitor};
  29
  30 pub struct CheckTypeWellFormedVisitor<'ccx, 'tcx:'ccx> {
  31     ccx: &'ccx CrateCtxt<'ccx, 'tcx>,
  32     cache: HashSet<Ty<'tcx>>
  33 }
  34
  35 impl<'ccx, 'tcx> CheckTypeWellFormedVisitor<'ccx, 'tcx> {
  36     pub fn new(ccx: &'ccx CrateCtxt<'ccx, 'tcx>) -> CheckTypeWellFormedVisitor<'ccx, 'tcx> {
  37         CheckTypeWellFormedVisitor { ccx: ccx, cache: HashSet::new() }
  38     }
  39
  40     fn tcx(&self) -> &ty::ctxt<'tcx> {
  41         self.ccx.tcx
  42     }
  43
  44     /// Checks that the field types (in a struct def'n) or argument types (in an enum def'n) are
  45     /// well-formed, meaning that they do not require any constraints not declared in the struct
  46     /// definition itself. For example, this definition would be illegal:
  47     ///
  48     ///     struct Ref<'a, T> { x: &'a T }
  49     ///
  50     /// because the type did not declare that `T:'a`.
  51     ///
  52     /// We do this check as a pre-pass before checking fn bodies because if these constraints are
  53     /// not included it frequently leads to confusing errors in fn bodies. So it's better to check
  54     /// the types first.
  55     fn check_item_well_formed(&mut self, item: &ast::Item) {
  56         let ccx = self.ccx;
  57         debug!("check_item_well_formed(it.id={}, it.ident={})",
  58                item.id,
  59                ccx.tcx.item_path_str(DefId::local(item.id)));
  60
  61         match item.node {
  62             /// Right now we check that every default trait implementation
  63             /// has an implementation of itself. Basically, a case like:
  64             ///
  65             /// `impl Trait for T {}`
  66             ///
  67             /// has a requirement of `T: Trait` which was required for default
  68             /// method implementations. Although this could be improved now that
  69             /// there's a better infrastructure in place for this, it's being left
  70             /// for a follow-up work.
  71             ///
  72             /// Since there's such a requirement, we need to check *just* positive
  73             /// implementations, otherwise things like:
  74             ///
  75             /// impl !Send for T {}
  76             ///
  77             /// won't be allowed unless there's an *explicit* implementation of `Send`
  78             /// for `T`
  79             ast::ItemImpl(_, ast::ImplPolarity::Positive, _, _, _, _) => {
  80                 self.check_impl(item);
  81             }
  82             ast::ItemImpl(_, ast::ImplPolarity::Negative, _, Some(_), _, _) => {
  83                 let trait_ref = ccx.tcx.impl_trait_ref(DefId::local(item.id)).unwrap();
  84                 ccx.tcx.populate_implementations_for_trait_if_necessary(trait_ref.def_id);
  85                 match ccx.tcx.lang_items.to_builtin_kind(trait_ref.def_id) {
  86                     Some(ty::BoundSend) | Some(ty::BoundSync) => {}
  87                     Some(_) | None => {
  88                         if !ccx.tcx.trait_has_default_impl(trait_ref.def_id) {
  89                             wfcheck::error_192(ccx, item.span);
  90                         }
  91                     }
  92                 }
  93             }
  94             ast::ItemFn(..) => {
  95                 self.check_item_type(item);
  96             }
  97             ast::ItemStatic(..) => {
  98                 self.check_item_type(item);
  99             }
 100             ast::ItemConst(..) => {
 101                 self.check_item_type(item);
 102             }
 103             ast::ItemStruct(ref struct_def, ref ast_generics) => {
 104                 self.check_type_defn(item, |fcx| {
 105                     vec![struct_variant(fcx, &**struct_def)]
 106                 });
 107
 108                 self.check_variances_for_type_defn(item, ast_generics);
 109             }
 110             ast::ItemEnum(ref enum_def, ref ast_generics) => {
 111                 self.check_type_defn(item, |fcx| {
 112                     enum_variants(fcx, enum_def)
 113                 });
 114
 115                 self.check_variances_for_type_defn(item, ast_generics);
 116             }
 117             ast::ItemTrait(_, _, _, ref items) => {
 118                 let trait_predicates =
 119                     ccx.tcx.lookup_predicates(DefId::local(item.id));
 120                 reject_non_type_param_bounds(ccx.tcx, item.span, &trait_predicates);
 121                 if ccx.tcx.trait_has_default_impl(DefId::local(item.id)) {
 122                     if !items.is_empty() {
 123                         wfcheck::error_380(ccx, item.span);
 124                     }
 125                 }
 126             }
 127             _ => {}
 128         }
 129     }
 130
 131     fn with_fcx<F>(&mut self, item: &ast::Item, mut f: F) where
 132         F: for<'fcx> FnMut(&mut CheckTypeWellFormedVisitor<'ccx, 'tcx>, &FnCtxt<'fcx, 'tcx>),
 133     {
 134         let ccx = self.ccx;
 135         let item_def_id = DefId::local(item.id);
 136         let type_scheme = ccx.tcx.lookup_item_type(item_def_id);
 137         let type_predicates = ccx.tcx.lookup_predicates(item_def_id);
 138         reject_non_type_param_bounds(ccx.tcx, item.span, &type_predicates);
 139         let param_env = ccx.tcx.construct_parameter_environment(item.span,
 140                                                                 &type_scheme.generics,
 141                                                                 &type_predicates,
 142                                                                 item.id);
 143         let tables = RefCell::new(ty::Tables::empty());
 144         let inh = Inherited::new(ccx.tcx, &tables, param_env);
 145         let fcx = blank_fn_ctxt(ccx, &inh, ty::FnConverging(type_scheme.ty), item.id);
 146         f(self, &fcx);
 147         fcx.select_all_obligations_or_error();
 148         regionck::regionck_item(&fcx, item.id, item.span, &[]);
 149     }
 150
 151     /// In a type definition, we check that to ensure that the types of the fields are well-formed.
 152     fn check_type_defn<F>(&mut self, item: &ast::Item, mut lookup_fields: F) where
 153         F: for<'fcx> FnMut(&FnCtxt<'fcx, 'tcx>) -> Vec<AdtVariant<'tcx>>,
 154     {
 155         self.with_fcx(item, |this, fcx| {
 156             let variants = lookup_fields(fcx);
 157             let mut bounds_checker = BoundsChecker::new(fcx,
 158                                                         item.id,
 159                                                         Some(&mut this.cache));
 160             debug!("check_type_defn at bounds_checker.scope: {:?}", bounds_checker.scope);
 161
 162             for variant in &variants {
 163                 for field in &variant.fields {
 164                     // Regions are checked below.
 165                     bounds_checker.check_traits_in_ty(field.ty, field.span);
 166                 }
 167
 168                 // For DST, all intermediate types must be sized.
 169                 if let Some((_, fields)) = variant.fields.split_last() {
 170                     for field in fields {
 171                         fcx.register_builtin_bound(
 172                             field.ty,
 173                             ty::BoundSized,
 174                             traits::ObligationCause::new(field.span,
 175                                                          fcx.body_id,
 176                                                          traits::FieldSized));
 177                     }
 178                 }
 179             }
 180
 181             for field in variants.iter().flat_map(|v| v.fields.iter()) {
 182                 fcx.register_old_wf_obligation(field.ty, field.span, traits::MiscObligation);
 183             }
 184         });
 185     }
 186
 187     fn check_item_type(&mut self,
 188                        item: &ast::Item)
 189     {
 190         self.with_fcx(item, |this, fcx| {
 191             let mut bounds_checker = BoundsChecker::new(fcx,
 192                                                         item.id,
 193                                                         Some(&mut this.cache));
 194             debug!("check_item_type at bounds_checker.scope: {:?}", bounds_checker.scope);
 195
 196             let type_scheme = fcx.tcx().lookup_item_type(DefId::local(item.id));
 197             let item_ty = fcx.instantiate_type_scheme(item.span,
 198                                                       &fcx.inh
 199                                                           .infcx
 200                                                           .parameter_environment
 201                                                           .free_substs,
 202                                                       &type_scheme.ty);
 203
 204             bounds_checker.check_traits_in_ty(item_ty, item.span);
 205         });
 206     }
 207
 208     fn check_impl(&mut self,
 209                   item: &ast::Item)
 210     {
 211         self.with_fcx(item, |this, fcx| {
 212             let mut bounds_checker = BoundsChecker::new(fcx,
 213                                                         item.id,
 214                                                         Some(&mut this.cache));
 215             debug!("check_impl at bounds_checker.scope: {:?}", bounds_checker.scope);
 216
 217             // Find the impl self type as seen from the "inside" --
 218             // that is, with all type parameters converted from bound
 219             // to free.
 220             let self_ty = fcx.tcx().node_id_to_type(item.id);
 221             let self_ty = fcx.instantiate_type_scheme(item.span,
 222                                                       &fcx.inh
 223                                                           .infcx
 224                                                           .parameter_environment
 225                                                           .free_substs,
 226                                                       &self_ty);
 227
 228             bounds_checker.check_traits_in_ty(self_ty, item.span);
 229
 230             // Similarly, obtain an "inside" reference to the trait
 231             // that the impl implements.
 232             let trait_ref = match fcx.tcx().impl_trait_ref(DefId::local(item.id)) {
 233                 None => { return; }
 234                 Some(t) => { t }
 235             };
 236
 237             let trait_ref = fcx.instantiate_type_scheme(item.span,
 238                                                         &fcx.inh
 239                                                             .infcx
 240                                                             .parameter_environment
 241                                                             .free_substs,
 242                                                         &trait_ref);
 243
 244             // We are stricter on the trait-ref in an impl than the
 245             // self-type.  In particular, we enforce region
 246             // relationships. The reason for this is that (at least
 247             // presently) "applying" an impl does not require that the
 248             // application site check the well-formedness constraints on the
 249             // trait reference. Instead, this is done at the impl site.
 250             // Arguably this is wrong and we should treat the trait-reference
 251             // the same way as we treat the self-type.
 252             bounds_checker.check_trait_ref(&trait_ref, item.span);
 253
 254             let cause =
 255                 traits::ObligationCause::new(
 256                     item.span,
 257                     fcx.body_id,
 258                     traits::ItemObligation(trait_ref.def_id));
 259
 260             // Find the supertrait bounds. This will add `int:Bar`.
 261             let poly_trait_ref = ty::Binder(trait_ref);
 262             let predicates = fcx.tcx().lookup_super_predicates(poly_trait_ref.def_id());
 263             let predicates = predicates.instantiate_supertrait(fcx.tcx(), &poly_trait_ref);
 264             let predicates = {
 265                 let selcx = &mut traits::SelectionContext::new(fcx.infcx());
 266                 traits::normalize(selcx, cause.clone(), &predicates)
 267             };
 268             for predicate in predicates.value.predicates {
 269                 fcx.register_predicate(traits::Obligation::new(cause.clone(), predicate));
 270             }
 271             for obligation in predicates.obligations {
 272                 fcx.register_predicate(obligation);
 273             }
 274         });
 275     }
 276
 277     fn check_variances_for_type_defn(&self,
 278                                      item: &ast::Item,
 279                                      ast_generics: &ast::Generics)
 280     {
 281         let item_def_id = DefId::local(item.id);
 282         let ty_predicates = self.tcx().lookup_predicates(item_def_id);
 283         let variances = self.tcx().item_variances(item_def_id);
 284
 285         let mut constrained_parameters: HashSet<_> =
 286             variances.types
 287                      .iter_enumerated()
 288                      .filter(|&(_, _, &variance)| variance != ty::Bivariant)
 289                      .map(|(space, index, _)| self.param_ty(ast_generics, space, index))
 290                      .map(|p| Parameter::Type(p))
 291                      .collect();
 292
 293         identify_constrained_type_params(self.tcx(),
 294                                          ty_predicates.predicates.as_slice(),
 295                                          None,
 296                                          &mut constrained_parameters);
 297
 298         for (space, index, _) in variances.types.iter_enumerated() {
 299             let param_ty = self.param_ty(ast_generics, space, index);
 300             if constrained_parameters.contains(&Parameter::Type(param_ty)) {
 301                 continue;
 302             }
 303             let span = self.ty_param_span(ast_generics, item, space, index);
 304             self.report_bivariance(span, param_ty.name);
 305         }
 306
 307         for (space, index, &variance) in variances.regions.iter_enumerated() {
 308             if variance != ty::Bivariant {
 309                 continue;
 310             }
 311
 312             assert_eq!(space, TypeSpace);
 313             let span = ast_generics.lifetimes[index].lifetime.span;
 314             let name = ast_generics.lifetimes[index].lifetime.name;
 315             self.report_bivariance(span, name);
 316         }
 317     }
 318
 319     fn param_ty(&self,
 320                 ast_generics: &ast::Generics,
 321                 space: ParamSpace,
 322                 index: usize)
 323                 -> ty::ParamTy
 324     {
 325         let name = match space {
 326             TypeSpace => ast_generics.ty_params[index].ident.name,
 327             SelfSpace => special_idents::type_self.name,
 328             FnSpace => self.tcx().sess.bug("Fn space occupied?"),
 329         };
 330
 331         ty::ParamTy { space: space, idx: index as u32, name: name }
 332     }
 333
 334     fn ty_param_span(&self,
 335                      ast_generics: &ast::Generics,
 336                      item: &ast::Item,
 337                      space: ParamSpace,
 338                      index: usize)
 339                      -> Span
 340     {
 341         match space {
 342             TypeSpace => ast_generics.ty_params[index].span,
 343             SelfSpace => item.span,
 344             FnSpace => self.tcx().sess.span_bug(item.span, "Fn space occupied?"),
 345         }
 346     }
 347
 348     fn report_bivariance(&self,
 349                          span: Span,
 350                          param_name: ast::Name)
 351     {
 352         wfcheck::error_392(self.tcx(), span, param_name);
 353
 354         let suggested_marker_id = self.tcx().lang_items.phantom_data();
 355         match suggested_marker_id {
 356             Some(def_id) => {
 357                 self.tcx().sess.fileline_help(
 358                     span,
 359                     &format!("consider removing `{}` or using a marker such as `{}`",
 360                              param_name,
 361                              self.tcx().item_path_str(def_id)));
 362             }
 363             None => {
 364                 // no lang items, no help!
 365             }
 366         }
 367     }
 368 }
 369
 370 // Reject any predicates that do not involve a type parameter.
 371 fn reject_non_type_param_bounds<'tcx>(tcx: &ty::ctxt<'tcx>,
 372                                       span: Span,
 373                                       predicates: &ty::GenericPredicates<'tcx>) {
 374     for predicate in &predicates.predicates {
 375         match predicate {
 376             &ty::Predicate::Trait(ty::Binder(ref tr)) => {
 377                 let found_param = tr.input_types().iter()
 378                                     .flat_map(|ty| ty.walk())
 379                                     .any(is_ty_param);
 380                 if !found_param { report_bound_error(tcx, span, tr.self_ty() )}
 381             }
 382             &ty::Predicate::TypeOutlives(ty::Binder(ty::OutlivesPredicate(ty, _))) => {
 383                 let found_param = ty.walk().any(|t| is_ty_param(t));
 384                 if !found_param { report_bound_error(tcx, span, ty) }
 385             }
 386             _ => {}
 387         };
 388     }
 389
 390     fn report_bound_error<'t>(tcx: &ty::ctxt<'t>,
 391                           span: Span,
 392                           bounded_ty: ty::Ty<'t>) {
 393         span_err!(tcx.sess, span, E0193,
 394             "cannot bound type `{}`, where clause \
 395                 bounds may only be attached to types involving \
 396                 type parameters",
 397                 bounded_ty)
 398     }
 399
 400     fn is_ty_param(ty: ty::Ty) -> bool {
 401         match &ty.sty {
 402             &ty::TyParam(_) => true,
 403             _ => false
 404         }
 405     }
 406 }
 407
 408 fn reject_shadowing_type_parameters<'tcx>(tcx: &ty::ctxt<'tcx>,
 409                                           span: Span,
 410                                           generics: &ty::Generics<'tcx>) {
 411     let impl_params = generics.types.get_slice(subst::TypeSpace).iter()
 412         .map(|tp| tp.name).collect::<HashSet<_>>();
 413
 414     for method_param in generics.types.get_slice(subst::FnSpace) {
 415         if impl_params.contains(&method_param.name) {
 416             wfcheck::error_194(tcx, span, method_param.name);
 417         }
 418     }
 419 }
 420
 421 impl<'ccx, 'tcx, 'v> Visitor<'v> for CheckTypeWellFormedVisitor<'ccx, 'tcx> {
 422     fn visit_item(&mut self, i: &ast::Item) {
 423         self.check_item_well_formed(i);
 424         visit::walk_item(self, i);
 425     }
 426
 427     fn visit_fn(&mut self,
 428                 fk: FnKind<'v>, fd: &'v ast::FnDecl,
 429                 b: &'v ast::Block, span: Span, id: ast::NodeId) {
 430         match fk {
 431             FnKind::Closure | FnKind::ItemFn(..) => {}
 432             FnKind::Method(..) => {
 433                 match self.tcx().impl_or_trait_item(DefId::local(id)) {
 434                     ty::ImplOrTraitItem::MethodTraitItem(ty_method) => {
 435                         reject_shadowing_type_parameters(self.tcx(), span, &ty_method.generics)
 436                     }
 437                     _ => {}
 438                 }
 439             }
 440         }
 441         visit::walk_fn(self, fk, fd, b, span)
 442     }
 443
 444     fn visit_trait_item(&mut self, trait_item: &'v ast::TraitItem) {
 445         if let ast::MethodTraitItem(_, None) = trait_item.node {
 446             match self.tcx().impl_or_trait_item(DefId::local(trait_item.id)) {
 447                 ty::ImplOrTraitItem::MethodTraitItem(ty_method) => {
 448                     reject_non_type_param_bounds(
 449                         self.tcx(),
 450                         trait_item.span,
 451                         &ty_method.predicates);
 452                     reject_shadowing_type_parameters(
 453                         self.tcx(),
 454                         trait_item.span,
 455                         &ty_method.generics);
 456                 }
 457                 _ => {}
 458             }
 459         }
 460
 461         visit::walk_trait_item(self, trait_item)
 462     }
 463 }
 464
 465 pub struct BoundsChecker<'cx,'tcx:'cx> {
 466     fcx: &'cx FnCtxt<'cx,'tcx>,
 467     span: Span,
 468
 469     scope: region::CodeExtent,
 470
 471     binding_count: usize,
 472     cache: Option<&'cx mut HashSet<Ty<'tcx>>>,
 473 }
 474
 475 impl<'cx,'tcx> BoundsChecker<'cx,'tcx> {
 476     pub fn new(fcx: &'cx FnCtxt<'cx,'tcx>,
 477                scope: ast::NodeId,
 478                cache: Option<&'cx mut HashSet<Ty<'tcx>>>)
 479                -> BoundsChecker<'cx,'tcx> {
 480         let scope = fcx.tcx().region_maps.item_extent(scope);
 481         BoundsChecker { fcx: fcx, span: DUMMY_SP, scope: scope,
 482                         cache: cache, binding_count: 0 }
 483     }
 484
 485     /// Given a trait ref like `A : Trait<B>`, where `Trait` is defined as (say):
 486     ///
 487     ///     trait Trait<B:OtherTrait> : Copy { ... }
 488     ///
 489     /// This routine will check that `B : OtherTrait` and `A : Trait<B>`. It will also recursively
 490     /// check that the types `A` and `B` are well-formed.
 491     ///
 492     /// Note that it does not (currently, at least) check that `A : Copy` (that check is delegated
 493     /// to the point where impl `A : Trait<B>` is implemented).
 494     pub fn check_trait_ref(&mut self, trait_ref: &ty::TraitRef<'tcx>, span: Span) {
 495         let trait_predicates = self.fcx.tcx().lookup_predicates(trait_ref.def_id);
 496
 497         let bounds = self.fcx.instantiate_bounds(span,
 498                                                  trait_ref.substs,
 499                                                  &trait_predicates);
 500
 501         self.fcx.add_obligations_for_parameters(
 502             traits::ObligationCause::new(
 503                 span,
 504                 self.fcx.body_id,
 505                 traits::ItemObligation(trait_ref.def_id)),
 506             &bounds);
 507
 508         for &ty in &trait_ref.substs.types {
 509             self.check_traits_in_ty(ty, span);
 510         }
 511     }
 512
 513     fn check_traits_in_ty(&mut self, ty: Ty<'tcx>, span: Span) {
 514         self.span = span;
 515         // When checking types outside of a type def'n, we ignore
 516         // region obligations. See discussion below in fold_ty().
 517         self.binding_count += 1;
 518         ty.fold_with(self);
 519         self.binding_count -= 1;
 520     }
 521 }
 522
 523 impl<'cx,'tcx> TypeFolder<'tcx> for BoundsChecker<'cx,'tcx> {
 524     fn tcx(&self) -> &ty::ctxt<'tcx> {
 525         self.fcx.tcx()
 526     }
 527
 528     fn fold_binder<T>(&mut self, binder: &ty::Binder<T>) -> ty::Binder<T>
 529         where T : TypeFoldable<'tcx>
 530     {
 531         self.binding_count += 1;
 532         let value = self.fcx.tcx().liberate_late_bound_regions(
 533             self.scope,
 534             binder);
 535         debug!("BoundsChecker::fold_binder: late-bound regions replaced: {:?} at scope: {:?}",
 536                value, self.scope);
 537         let value = value.fold_with(self);
 538         self.binding_count -= 1;
 539         ty::Binder(value)
 540     }
 541
 542     fn fold_ty(&mut self, t: Ty<'tcx>) -> Ty<'tcx> {
 543         debug!("BoundsChecker t={:?}",
 544                t);
 545
 546         match self.cache {
 547             Some(ref mut cache) => {
 548                 if !cache.insert(t) {
 549                     // Already checked this type! Don't check again.
 550                     debug!("cached");
 551                     return t;
 552                 }
 553             }
 554             None => { }
 555         }
 556
 557         match t.sty{
 558             ty::TyStruct(def, substs) |
 559             ty::TyEnum(def, substs) => {
 560                 let type_predicates = def.predicates(self.fcx.tcx());
 561                 let bounds = self.fcx.instantiate_bounds(self.span, substs,
 562                                                          &type_predicates);
 563
 564                 if self.binding_count == 0 {
 565                     self.fcx.add_obligations_for_parameters(
 566                         traits::ObligationCause::new(self.span,
 567                                                      self.fcx.body_id,
 568                                                      traits::ItemObligation(def.did)),
 569                         &bounds);
 570                 } else {
 571                     // There are two circumstances in which we ignore
 572                     // region obligations.
 573                     //
 574                     // The first is when we are inside of a closure
 575                     // type. This is because in that case the region
 576                     // obligations for the parameter types are things
 577                     // that the closure body gets to assume and the
 578                     // caller must prove at the time of call. In other
 579                     // words, if there is a type like `<'a, 'b> | &'a
 580                     // &'b int |`, it is well-formed, and caller will
 581                     // have to show that `'b : 'a` at the time of
 582                     // call.
 583                     //
 584                     // The second is when we are checking for
 585                     // well-formedness outside of a type def'n or fn
 586                     // body. This is for a similar reason: in general,
 587                     // we only do WF checking for regions in the
 588                     // result of expressions and type definitions, so
 589                     // to as allow for implicit where clauses.
 590                     //
 591                     // (I believe we should do the same for traits, but
 592                     // that will require an RFC. -nmatsakis)
 593                     let bounds = filter_to_trait_obligations(bounds);
 594                     self.fcx.add_obligations_for_parameters(
 595                         traits::ObligationCause::new(self.span,
 596                                                      self.fcx.body_id,
 597                                                      traits::ItemObligation(def.did)),
 598                         &bounds);
 599                 }
 600
 601                 self.fold_substs(substs);
 602             }
 603             _ => {
 604                 super_fold_ty(self, t);
 605             }
 606         }
 607
 608         t // we're not folding to produce a new type, so just return `t` here
 609     }
 610 }
 611
 612 ///////////////////////////////////////////////////////////////////////////
 613 // ADT
 614
 615 struct AdtVariant<'tcx> {
 616     fields: Vec<AdtField<'tcx>>,
 617 }
 618
 619 struct AdtField<'tcx> {
 620     ty: Ty<'tcx>,
 621     span: Span,
 622 }
 623
 624 fn struct_variant<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
 625                             struct_def: &ast::StructDef)
 626                             -> AdtVariant<'tcx> {
 627     let fields =
 628         struct_def.fields
 629         .iter()
 630         .map(|field| {
 631             let field_ty = fcx.tcx().node_id_to_type(field.node.id);
 632             let field_ty = fcx.instantiate_type_scheme(field.span,
 633                                                        &fcx.inh
 634                                                            .infcx
 635                                                            .parameter_environment
 636                                                            .free_substs,
 637                                                        &field_ty);
 638             AdtField { ty: field_ty, span: field.span }
 639         })
 640         .collect();
 641     AdtVariant { fields: fields }
 642 }
 643
 644 fn enum_variants<'a, 'tcx>(fcx: &FnCtxt<'a, 'tcx>,
 645                            enum_def: &ast::EnumDef)
 646                            -> Vec<AdtVariant<'tcx>> {
 647     enum_def.variants.iter()
 648         .map(|variant| {
 649             match variant.node.kind {
 650                 ast::TupleVariantKind(ref args) if !args.is_empty() => {
 651                     let ctor_ty = fcx.tcx().node_id_to_type(variant.node.id);
 652
 653                     // the regions in the argument types come from the
 654                     // enum def'n, and hence will all be early bound
 655                     let arg_tys = fcx.tcx().no_late_bound_regions(&ctor_ty.fn_args()).unwrap();
 656                     AdtVariant {
 657                         fields: args.iter().enumerate().map(|(index, arg)| {
 658                             let arg_ty = arg_tys[index];
 659                             let arg_ty =
 660                                 fcx.instantiate_type_scheme(variant.span,
 661                                                             &fcx.inh
 662                                                                 .infcx
 663                                                                 .parameter_environment
 664                                                                 .free_substs,
 665                                                             &arg_ty);
 666                             AdtField {
 667                                 ty: arg_ty,
 668                                 span: arg.ty.span
 669                             }
 670                         }).collect()
 671                     }
 672                 }
 673                 ast::TupleVariantKind(_) => {
 674                     AdtVariant {
 675                         fields: Vec::new()
 676                     }
 677                 }
 678                 ast::StructVariantKind(ref struct_def) => {
 679                     struct_variant(fcx, &**struct_def)
 680                 }
 681             }
 682         })
 683         .collect()
 684 }
 685
 686 fn filter_to_trait_obligations<'tcx>(bounds: ty::InstantiatedPredicates<'tcx>)
 687                                      -> ty::InstantiatedPredicates<'tcx>
 688 {
 689     let mut result = ty::InstantiatedPredicates::empty();
 690     for (space, _, predicate) in bounds.predicates.iter_enumerated() {
 691         match *predicate {
 692             ty::Predicate::Trait(..) |
 693             ty::Predicate::Projection(..) => {
 694                 result.predicates.push(space, predicate.clone())
 695             }
 696             ty::Predicate::WellFormed(..) |
 697             ty::Predicate::ObjectSafe(..) |
 698             ty::Predicate::Equate(..) |
 699             ty::Predicate::TypeOutlives(..) |
 700             ty::Predicate::RegionOutlives(..) => {
 701             }
 702         }
 703     }
 704     result
 705 }