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