src/librustc/middle/typeck/coherence.rs

   1 // Copyright 2012-2013 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 // Coherence phase
  12 //
  13 // The job of the coherence phase of typechecking is to ensure that each trait
  14 // has at most one implementation for each type. Then we build a mapping from
  15 // each trait in the system to its implementations.
  16
  17
  18 use metadata::csearch::{each_impl, get_impl_trait, each_implementation_for_trait};
  19 use metadata::csearch;
  20 use middle::subst;
  21 use middle::subst::{Substs};
  22 use middle::ty::get;
  23 use middle::ty::{ImplContainer, ImplOrTraitItemId, MethodTraitItemId};
  24 use middle::ty::{lookup_item_type};
  25 use middle::ty::{t, ty_bool, ty_char, ty_bot, ty_box, ty_enum, ty_err};
  26 use middle::ty::{ty_str, ty_vec, ty_float, ty_infer, ty_int, ty_nil};
  27 use middle::ty::{ty_param, Polytype, ty_ptr};
  28 use middle::ty::{ty_rptr, ty_struct, ty_trait, ty_tup};
  29 use middle::ty::{ty_uint, ty_unboxed_closure, ty_uniq, ty_bare_fn};
  30 use middle::ty::{ty_closure};
  31 use middle::ty::type_is_ty_var;
  32 use middle::subst::Subst;
  33 use middle::ty;
  34 use middle::typeck::CrateCtxt;
  35 use middle::typeck::infer::combine::Combine;
  36 use middle::typeck::infer::InferCtxt;
  37 use middle::typeck::infer::{new_infer_ctxt, resolve_ivar, resolve_type};
  38 use middle::typeck::infer;
  39 use util::ppaux::Repr;
  40 use middle::def::{DefStruct, DefTy};
  41 use syntax::ast::{Crate, DefId};
  42 use syntax::ast::{Item, ItemEnum, ItemImpl, ItemMod, ItemStruct};
  43 use syntax::ast::{LOCAL_CRATE, TraitRef, TyPath};
  44 use syntax::ast;
  45 use syntax::ast_map::NodeItem;
  46 use syntax::ast_map;
  47 use syntax::ast_util::{local_def};
  48 use syntax::codemap::{Span, DUMMY_SP};
  49 use syntax::parse::token;
  50 use syntax::visit;
  51
  52 use std::collections::HashSet;
  53 use std::cell::RefCell;
  54 use std::rc::Rc;
  55
  56 struct UniversalQuantificationResult {
  57     monotype: t
  58 }
  59
  60 fn get_base_type(inference_context: &InferCtxt,
  61                  span: Span,
  62                  original_type: t)
  63                  -> Option<t> {
  64     let resolved_type = match resolve_type(inference_context,
  65                                            Some(span),
  66                                            original_type,
  67                                            resolve_ivar) {
  68         Ok(resulting_type) if !type_is_ty_var(resulting_type) => resulting_type,
  69         _ => {
  70             inference_context.tcx.sess.span_fatal(span,
  71                                                   "the type of this value must be known in order \
  72                                                    to determine the base type");
  73         }
  74     };
  75
  76     match get(resolved_type).sty {
  77         ty_enum(..) | ty_struct(..) | ty_unboxed_closure(..) => {
  78             debug!("(getting base type) found base type");
  79             Some(resolved_type)
  80         }
  81
  82         _ if ty::type_is_trait(resolved_type) => {
  83             debug!("(getting base type) found base type (trait)");
  84             Some(resolved_type)
  85         }
  86
  87         ty_nil | ty_bot | ty_bool | ty_char | ty_int(..) | ty_uint(..) | ty_float(..) |
  88         ty_str(..) | ty_vec(..) | ty_bare_fn(..) | ty_closure(..) | ty_tup(..) |
  89         ty_infer(..) | ty_param(..) | ty_err |
  90         ty_box(_) | ty_uniq(_) | ty_ptr(_) | ty_rptr(_, _) => {
  91             debug!("(getting base type) no base type; found {:?}",
  92                    get(original_type).sty);
  93             None
  94         }
  95         ty_trait(..) => fail!("should have been caught")
  96     }
  97 }
  98
  99 fn type_is_defined_in_local_crate(tcx: &ty::ctxt, original_type: t) -> bool {
 100     /*!
 101      *
 102      * For coherence, when we have `impl Trait for Type`, we need to
 103      * guarantee that `Type` is "local" to the
 104      * crate.  For our purposes, this means that it must contain
 105      * some nominal type defined in this crate.
 106      */
 107
 108     let mut found_nominal = false;
 109     ty::walk_ty(original_type, |t| {
 110         match get(t).sty {
 111             ty_enum(def_id, _) |
 112             ty_struct(def_id, _) |
 113             ty_unboxed_closure(def_id, _) => {
 114                 if def_id.krate == ast::LOCAL_CRATE {
 115                     found_nominal = true;
 116                 }
 117             }
 118             ty_trait(box ty::TyTrait { def_id, .. }) => {
 119                 if def_id.krate == ast::LOCAL_CRATE {
 120                     found_nominal = true;
 121                 }
 122             }
 123             ty_uniq(..) => {
 124                 match tcx.lang_items.owned_box() {
 125                     Some(did) if did.krate == ast::LOCAL_CRATE => {
 126                         found_nominal = true;
 127                     }
 128                     _ => {}
 129                 }
 130             }
 131             ty_box(..) => {
 132                 match tcx.lang_items.gc() {
 133                     Some(did) if did.krate == ast::LOCAL_CRATE => {
 134                         found_nominal = true;
 135                     }
 136                     _ => {}
 137                 }
 138             }
 139
 140             _ => { }
 141         }
 142     });
 143     return found_nominal;
 144 }
 145
 146 // Returns the def ID of the base type, if there is one.
 147 fn get_base_type_def_id(inference_context: &InferCtxt,
 148                         span: Span,
 149                         original_type: t)
 150                         -> Option<DefId> {
 151     match get_base_type(inference_context, span, original_type) {
 152         None => None,
 153         Some(base_type) => {
 154             match get(base_type).sty {
 155                 ty_enum(def_id, _) |
 156                 ty_struct(def_id, _) |
 157                 ty_unboxed_closure(def_id, _) => {
 158                     Some(def_id)
 159                 }
 160                 ty_rptr(_, ty::mt {ty, ..}) | ty_uniq(ty) => match ty::get(ty).sty {
 161                     ty_trait(box ty::TyTrait { def_id, .. }) => {
 162                         Some(def_id)
 163                     }
 164                     _ => {
 165                         fail!("get_base_type() returned a type that wasn't an \
 166                                enum, struct, or trait");
 167                     }
 168                 },
 169                 _ => {
 170                     fail!("get_base_type() returned a type that wasn't an \
 171                            enum, struct, or trait");
 172                 }
 173             }
 174         }
 175     }
 176 }
 177
 178 struct CoherenceChecker<'a> {
 179     crate_context: &'a CrateCtxt<'a>,
 180     inference_context: InferCtxt<'a>,
 181 }
 182
 183 struct CoherenceCheckVisitor<'a> {
 184     cc: &'a CoherenceChecker<'a>
 185 }
 186
 187 impl<'a> visit::Visitor<()> for CoherenceCheckVisitor<'a> {
 188     fn visit_item(&mut self, item: &Item, _: ()) {
 189
 190         //debug!("(checking coherence) item '{}'", token::get_ident(item.ident));
 191
 192         match item.node {
 193             ItemImpl(_, ref opt_trait, _, _) => {
 194                 match opt_trait.clone() {
 195                     Some(opt_trait) => {
 196                         self.cc.check_implementation(item, [opt_trait]);
 197                     }
 198                     None => self.cc.check_implementation(item, [])
 199                 }
 200             }
 201             _ => {
 202                 // Nothing to do.
 203             }
 204         };
 205
 206         visit::walk_item(self, item, ());
 207     }
 208 }
 209
 210 struct PrivilegedScopeVisitor<'a> { cc: &'a CoherenceChecker<'a> }
 211
 212 impl<'a> visit::Visitor<()> for PrivilegedScopeVisitor<'a> {
 213     fn visit_item(&mut self, item: &Item, _: ()) {
 214
 215         match item.node {
 216             ItemMod(ref module_) => {
 217                 // Then visit the module items.
 218                 visit::walk_mod(self, module_, ());
 219             }
 220             ItemImpl(_, None, ref ast_ty, _) => {
 221                 if !self.cc.ast_type_is_defined_in_local_crate(&**ast_ty) {
 222                     // This is an error.
 223                     let session = &self.cc.crate_context.tcx.sess;
 224                     span_err!(session, item.span, E0116,
 225                               "cannot associate methods with a type outside the \
 226                                crate the type is defined in; define and implement \
 227                                a trait or new type instead");
 228                 }
 229             }
 230             ItemImpl(_, Some(ref trait_ref), _, _) => {
 231                 let tcx = self.cc.crate_context.tcx;
 232                 // `for_ty` is `Type` in `impl Trait for Type`
 233                 let for_ty = ty::node_id_to_type(tcx, item.id);
 234                 if !type_is_defined_in_local_crate(tcx, for_ty) {
 235                     // This implementation is not in scope of its base
 236                     // type. This still might be OK if the trait is
 237                     // defined in the same crate.
 238
 239                     let trait_def_id =
 240                         self.cc.trait_ref_to_trait_def_id(trait_ref);
 241
 242                     if trait_def_id.krate != LOCAL_CRATE {
 243                         let session = &self.cc.crate_context.tcx.sess;
 244                         span_err!(session, item.span, E0117,
 245                                   "cannot provide an extension implementation \
 246                                    where both trait and type are not defined in this crate");
 247                     }
 248                 }
 249
 250                 visit::walk_item(self, item, ());
 251             }
 252             _ => {
 253                 visit::walk_item(self, item, ());
 254             }
 255         }
 256     }
 257 }
 258
 259 impl<'a> CoherenceChecker<'a> {
 260     fn check(&self, krate: &Crate) {
 261         // Check implementations and traits. This populates the tables
 262         // containing the inherent methods and extension methods. It also
 263         // builds up the trait inheritance table.
 264         let mut visitor = CoherenceCheckVisitor { cc: self };
 265         visit::walk_crate(&mut visitor, krate, ());
 266
 267         // Check that there are no overlapping trait instances
 268         self.check_implementation_coherence();
 269
 270         // Check whether traits with base types are in privileged scopes.
 271         self.check_privileged_scopes(krate);
 272
 273         // Bring in external crates. It's fine for this to happen after the
 274         // coherence checks, because we ensure by construction that no errors
 275         // can happen at link time.
 276         self.add_external_crates();
 277
 278         // Populate the table of destructors. It might seem a bit strange to
 279         // do this here, but it's actually the most convenient place, since
 280         // the coherence tables contain the trait -> type mappings.
 281         self.populate_destructor_table();
 282     }
 283
 284     fn check_implementation(&self, item: &Item,
 285                             associated_traits: &[TraitRef]) {
 286         let tcx = self.crate_context.tcx;
 287         let impl_did = local_def(item.id);
 288         let self_type = ty::lookup_item_type(tcx, impl_did);
 289
 290         // If there are no traits, then this implementation must have a
 291         // base type.
 292
 293         if associated_traits.len() == 0 {
 294             debug!("(checking implementation) no associated traits for item '{}'",
 295                    token::get_ident(item.ident));
 296
 297             match get_base_type_def_id(&self.inference_context,
 298                                        item.span,
 299                                        self_type.ty) {
 300                 None => {
 301                     let session = &self.crate_context.tcx.sess;
 302                     span_err!(session, item.span, E0118,
 303                               "no base type found for inherent implementation; \
 304                                implement a trait or new type instead");
 305                 }
 306                 Some(_) => {
 307                     // Nothing to do.
 308                 }
 309             }
 310         }
 311
 312         let impl_items = self.create_impl_from_item(item);
 313
 314         for associated_trait in associated_traits.iter() {
 315             let trait_ref = ty::node_id_to_trait_ref(
 316                 self.crate_context.tcx, associated_trait.ref_id);
 317             debug!("(checking implementation) adding impl for trait '{}', item '{}'",
 318                    trait_ref.repr(self.crate_context.tcx),
 319                    token::get_ident(item.ident));
 320
 321             self.add_trait_impl(trait_ref.def_id, impl_did);
 322         }
 323
 324         // Add the implementation to the mapping from implementation to base
 325         // type def ID, if there is a base type for this implementation and
 326         // the implementation does not have any associated traits.
 327         match get_base_type_def_id(&self.inference_context,
 328                                    item.span,
 329                                    self_type.ty) {
 330             None => {
 331                 // Nothing to do.
 332             }
 333             Some(base_type_def_id) => {
 334                 // FIXME: Gather up default methods?
 335                 if associated_traits.len() == 0 {
 336                     self.add_inherent_impl(base_type_def_id, impl_did);
 337                 }
 338             }
 339         }
 340
 341         tcx.impl_items.borrow_mut().insert(impl_did, impl_items);
 342     }
 343
 344     // Creates default method IDs and performs type substitutions for an impl
 345     // and trait pair. Then, for each provided method in the trait, inserts a
 346     // `ProvidedMethodInfo` instance into the `provided_method_sources` map.
 347     fn instantiate_default_methods(
 348             &self,
 349             impl_id: DefId,
 350             trait_ref: &ty::TraitRef,
 351             all_impl_items: &mut Vec<ImplOrTraitItemId>) {
 352         let tcx = self.crate_context.tcx;
 353         debug!("instantiate_default_methods(impl_id={:?}, trait_ref={})",
 354                impl_id, trait_ref.repr(tcx));
 355
 356         let impl_poly_type = ty::lookup_item_type(tcx, impl_id);
 357
 358         let prov = ty::provided_trait_methods(tcx, trait_ref.def_id);
 359         for trait_method in prov.iter() {
 360             // Synthesize an ID.
 361             let new_id = tcx.sess.next_node_id();
 362             let new_did = local_def(new_id);
 363
 364             debug!("new_did={:?} trait_method={}", new_did, trait_method.repr(tcx));
 365
 366             // Create substitutions for the various trait parameters.
 367             let new_method_ty =
 368                 Rc::new(subst_receiver_types_in_method_ty(
 369                     tcx,
 370                     impl_id,
 371                     &impl_poly_type,
 372                     trait_ref,
 373                     new_did,
 374                     &**trait_method,
 375                     Some(trait_method.def_id)));
 376
 377             debug!("new_method_ty={}", new_method_ty.repr(tcx));
 378             all_impl_items.push(MethodTraitItemId(new_did));
 379
 380             // construct the polytype for the method based on the
 381             // method_ty.  it will have all the generics from the
 382             // impl, plus its own.
 383             let new_polytype = ty::Polytype {
 384                 generics: new_method_ty.generics.clone(),
 385                 ty: ty::mk_bare_fn(tcx, new_method_ty.fty.clone())
 386             };
 387             debug!("new_polytype={}", new_polytype.repr(tcx));
 388
 389             tcx.tcache.borrow_mut().insert(new_did, new_polytype);
 390             tcx.impl_or_trait_items
 391                .borrow_mut()
 392                .insert(new_did, ty::MethodTraitItem(new_method_ty));
 393
 394             // Pair the new synthesized ID up with the
 395             // ID of the method.
 396             self.crate_context.tcx.provided_method_sources.borrow_mut()
 397                 .insert(new_did, trait_method.def_id);
 398         }
 399     }
 400
 401     fn add_inherent_impl(&self, base_def_id: DefId, impl_def_id: DefId) {
 402         let tcx = self.crate_context.tcx;
 403         match tcx.inherent_impls.borrow().find(&base_def_id) {
 404             Some(implementation_list) => {
 405                 implementation_list.borrow_mut().push(impl_def_id);
 406                 return;
 407             }
 408             None => {}
 409         }
 410
 411         tcx.inherent_impls.borrow_mut().insert(base_def_id,
 412                                                Rc::new(RefCell::new(vec!(impl_def_id))));
 413     }
 414
 415     fn add_trait_impl(&self, base_def_id: DefId, impl_def_id: DefId) {
 416         ty::record_trait_implementation(self.crate_context.tcx,
 417                                         base_def_id,
 418                                         impl_def_id);
 419     }
 420
 421     fn check_implementation_coherence(&self) {
 422         for trait_id in self.crate_context.tcx.trait_impls.borrow().keys() {
 423             self.check_implementation_coherence_of(*trait_id);
 424         }
 425     }
 426
 427     fn check_implementation_coherence_of(&self, trait_def_id: DefId) {
 428         // Unify pairs of polytypes.
 429         self.iter_impls_of_trait_local(trait_def_id, |impl_a| {
 430             let polytype_a =
 431                 self.get_self_type_for_implementation(impl_a);
 432
 433             // "We have an impl of trait <trait_def_id> for type <polytype_a>,
 434             // and that impl is <impl_a>"
 435             self.iter_impls_of_trait(trait_def_id, |impl_b| {
 436
 437                 // An impl is coherent with itself
 438                 if impl_a != impl_b {
 439                     let polytype_b = self.get_self_type_for_implementation(
 440                             impl_b);
 441
 442                     if self.polytypes_unify(polytype_a.clone(), polytype_b) {
 443                         let session = &self.crate_context.tcx.sess;
 444                         span_err!(session, self.span_of_impl(impl_a), E0119,
 445                                   "conflicting implementations for trait `{}`",
 446                                   ty::item_path_str(self.crate_context.tcx, trait_def_id));
 447                         if impl_b.krate == LOCAL_CRATE {
 448                             span_note!(session, self.span_of_impl(impl_b),
 449                                        "note conflicting implementation here");
 450                         } else {
 451                             let crate_store = &self.crate_context.tcx.sess.cstore;
 452                             let cdata = crate_store.get_crate_data(impl_b.krate);
 453                             span_note!(session, self.span_of_impl(impl_a),
 454                                        "conflicting implementation in crate `{}`",
 455                                        cdata.name);
 456                         }
 457                     }
 458                 }
 459             })
 460         })
 461     }
 462
 463     fn iter_impls_of_trait(&self, trait_def_id: DefId, f: |DefId|) {
 464         self.iter_impls_of_trait_local(trait_def_id, |x| f(x));
 465
 466         if trait_def_id.krate == LOCAL_CRATE {
 467             return;
 468         }
 469
 470         let crate_store = &self.crate_context.tcx.sess.cstore;
 471         csearch::each_implementation_for_trait(crate_store, trait_def_id, |impl_def_id| {
 472             // Is this actually necessary?
 473             let _ = lookup_item_type(self.crate_context.tcx, impl_def_id);
 474             f(impl_def_id);
 475         });
 476     }
 477
 478     fn iter_impls_of_trait_local(&self, trait_def_id: DefId, f: |DefId|) {
 479         match self.crate_context.tcx.trait_impls.borrow().find(&trait_def_id) {
 480             Some(impls) => {
 481                 for &impl_did in impls.borrow().iter() {
 482                     f(impl_did);
 483                 }
 484             }
 485             None => { /* no impls? */ }
 486         }
 487     }
 488
 489     fn polytypes_unify(&self,
 490                        polytype_a: Polytype,
 491                        polytype_b: Polytype)
 492                        -> bool {
 493         let universally_quantified_a =
 494             self.universally_quantify_polytype(polytype_a);
 495         let universally_quantified_b =
 496             self.universally_quantify_polytype(polytype_b);
 497
 498         return self.can_unify_universally_quantified(
 499             &universally_quantified_a, &universally_quantified_b) ||
 500             self.can_unify_universally_quantified(
 501             &universally_quantified_b, &universally_quantified_a);
 502     }
 503
 504     // Converts a polytype to a monotype by replacing all parameters with
 505     // type variables. Returns the monotype and the type variables created.
 506     fn universally_quantify_polytype(&self, polytype: Polytype)
 507                                      -> UniversalQuantificationResult
 508     {
 509         let substitutions =
 510             self.inference_context.fresh_substs_for_type(DUMMY_SP,
 511                                                          &polytype.generics);
 512         let monotype = polytype.ty.subst(self.crate_context.tcx, &substitutions);
 513
 514         UniversalQuantificationResult {
 515             monotype: monotype
 516         }
 517     }
 518
 519     fn can_unify_universally_quantified<'a>(&self,
 520                                             a: &'a UniversalQuantificationResult,
 521                                             b: &'a UniversalQuantificationResult)
 522                                             -> bool
 523     {
 524         infer::can_mk_subty(&self.inference_context,
 525                             a.monotype,
 526                             b.monotype).is_ok()
 527     }
 528
 529     fn get_self_type_for_implementation(&self, impl_did: DefId)
 530                                         -> Polytype {
 531         self.crate_context.tcx.tcache.borrow().get_copy(&impl_did)
 532     }
 533
 534     // Privileged scope checking
 535     fn check_privileged_scopes(&self, krate: &Crate) {
 536         let mut visitor = PrivilegedScopeVisitor{ cc: self };
 537         visit::walk_crate(&mut visitor, krate, ());
 538     }
 539
 540     fn trait_ref_to_trait_def_id(&self, trait_ref: &TraitRef) -> DefId {
 541         let def_map = &self.crate_context.tcx.def_map;
 542         let trait_def = def_map.borrow().get_copy(&trait_ref.ref_id);
 543         let trait_id = trait_def.def_id();
 544         return trait_id;
 545     }
 546
 547     /// For coherence, when we have `impl Type`, we need to guarantee that
 548     /// `Type` is "local" to the crate. For our purposes, this means that it
 549     /// must precisely name some nominal type defined in this crate.
 550     fn ast_type_is_defined_in_local_crate(&self, original_type: &ast::Ty) -> bool {
 551         match original_type.node {
 552             TyPath(_, _, path_id) => {
 553                 match self.crate_context.tcx.def_map.borrow().get_copy(&path_id) {
 554                     DefTy(def_id) | DefStruct(def_id) => {
 555                         if def_id.krate != LOCAL_CRATE {
 556                             return false;
 557                         }
 558
 559                         // Make sure that this type precisely names a nominal
 560                         // type.
 561                         match self.crate_context.tcx.map.find(def_id.node) {
 562                             None => {
 563                                 self.crate_context.tcx.sess.span_bug(
 564                                     original_type.span,
 565                                     "resolve didn't resolve this type?!");
 566                             }
 567                             Some(NodeItem(item)) => {
 568                                 match item.node {
 569                                     ItemStruct(..) | ItemEnum(..) => true,
 570                                     _ => false,
 571                                 }
 572                             }
 573                             Some(_) => false,
 574                         }
 575                     }
 576                     _ => false
 577                 }
 578             }
 579             _ => false
 580         }
 581     }
 582
 583     // Converts an implementation in the AST to a vector of items.
 584     fn create_impl_from_item(&self, item: &Item) -> Vec<ImplOrTraitItemId> {
 585         match item.node {
 586             ItemImpl(_, ref trait_refs, _, ref ast_items) => {
 587                 let mut items: Vec<ImplOrTraitItemId> =
 588                         ast_items.iter()
 589                                  .map(|ast_item| {
 590                             match *ast_item {
 591                                 ast::MethodImplItem(ast_method) => {
 592                                     MethodTraitItemId(
 593                                         local_def(ast_method.id))
 594                                 }
 595                             }
 596                         }).collect();
 597
 598                 for trait_ref in trait_refs.iter() {
 599                     let ty_trait_ref = ty::node_id_to_trait_ref(
 600                         self.crate_context.tcx,
 601                         trait_ref.ref_id);
 602
 603                     self.instantiate_default_methods(local_def(item.id),
 604                                                      &*ty_trait_ref,
 605                                                      &mut items);
 606                 }
 607
 608                 items
 609             }
 610             _ => {
 611                 self.crate_context.tcx.sess.span_bug(item.span,
 612                                                      "can't convert a non-impl to an impl");
 613             }
 614         }
 615     }
 616
 617     fn span_of_impl(&self, impl_did: DefId) -> Span {
 618         assert_eq!(impl_did.krate, LOCAL_CRATE);
 619         self.crate_context.tcx.map.span(impl_did.node)
 620     }
 621
 622     // External crate handling
 623
 624     fn add_external_impl(&self,
 625                          impls_seen: &mut HashSet<DefId>,
 626                          impl_def_id: DefId) {
 627         let tcx = self.crate_context.tcx;
 628         let impl_items = csearch::get_impl_items(&tcx.sess.cstore,
 629                                                  impl_def_id);
 630
 631         // Make sure we don't visit the same implementation multiple times.
 632         if !impls_seen.insert(impl_def_id) {
 633             // Skip this one.
 634             return
 635         }
 636         // Good. Continue.
 637
 638         let _ = lookup_item_type(tcx, impl_def_id);
 639         let associated_traits = get_impl_trait(tcx, impl_def_id);
 640
 641         // Do a sanity check.
 642         assert!(associated_traits.is_some());
 643
 644         // Record all the trait items.
 645         for trait_ref in associated_traits.iter() {
 646             self.add_trait_impl(trait_ref.def_id, impl_def_id);
 647         }
 648
 649         // For any methods that use a default implementation, add them to
 650         // the map. This is a bit unfortunate.
 651         for item_def_id in impl_items.iter() {
 652             let impl_item = ty::impl_or_trait_item(tcx, item_def_id.def_id());
 653             match impl_item {
 654                 ty::MethodTraitItem(ref method) => {
 655                     for &source in method.provided_source.iter() {
 656                         tcx.provided_method_sources
 657                            .borrow_mut()
 658                            .insert(item_def_id.def_id(), source);
 659                     }
 660                 }
 661             }
 662         }
 663
 664         tcx.impl_items.borrow_mut().insert(impl_def_id, impl_items);
 665     }
 666
 667     // Adds implementations and traits from external crates to the coherence
 668     // info.
 669     fn add_external_crates(&self) {
 670         let mut impls_seen = HashSet::new();
 671
 672         let crate_store = &self.crate_context.tcx.sess.cstore;
 673         crate_store.iter_crate_data(|crate_number, _crate_metadata| {
 674             each_impl(crate_store, crate_number, |def_id| {
 675                 assert_eq!(crate_number, def_id.krate);
 676                 self.add_external_impl(&mut impls_seen, def_id)
 677             })
 678         })
 679     }
 680
 681     //
 682     // Destructors
 683     //
 684
 685     fn populate_destructor_table(&self) {
 686         let tcx = self.crate_context.tcx;
 687         let drop_trait = match tcx.lang_items.drop_trait() {
 688             Some(id) => id, None => { return }
 689         };
 690
 691         let impl_items = tcx.impl_items.borrow();
 692         let trait_impls = match tcx.trait_impls.borrow().find_copy(&drop_trait) {
 693             None => return, // No types with (new-style) dtors present.
 694             Some(found_impls) => found_impls
 695         };
 696
 697         for &impl_did in trait_impls.borrow().iter() {
 698             let items = impl_items.get(&impl_did);
 699             if items.len() < 1 {
 700                 // We'll error out later. For now, just don't ICE.
 701                 continue;
 702             }
 703             let method_def_id = *items.get(0);
 704
 705             let self_type = self.get_self_type_for_implementation(impl_did);
 706             match ty::get(self_type.ty).sty {
 707                 ty::ty_enum(type_def_id, _) |
 708                 ty::ty_struct(type_def_id, _) |
 709                 ty::ty_unboxed_closure(type_def_id, _) => {
 710                     tcx.destructor_for_type
 711                        .borrow_mut()
 712                        .insert(type_def_id, method_def_id.def_id());
 713                     tcx.destructors
 714                        .borrow_mut()
 715                        .insert(method_def_id.def_id());
 716                 }
 717                 _ => {
 718                     // Destructors only work on nominal types.
 719                     if impl_did.krate == ast::LOCAL_CRATE {
 720                         {
 721                             match tcx.map.find(impl_did.node) {
 722                                 Some(ast_map::NodeItem(item)) => {
 723                                     span_err!(tcx.sess, item.span, E0120,
 724                                         "the Drop trait may only be implemented on structures");
 725                                 }
 726                                 _ => {
 727                                     tcx.sess.bug("didn't find impl in ast \
 728                                                   map");
 729                                 }
 730                             }
 731                         }
 732                     } else {
 733                         tcx.sess.bug("found external impl of Drop trait on \
 734                                       something other than a struct");
 735                     }
 736                 }
 737             }
 738         }
 739     }
 740 }
 741
 742 pub fn make_substs_for_receiver_types(tcx: &ty::ctxt,
 743                                       trait_ref: &ty::TraitRef,
 744                                       method: &ty::Method)
 745                                       -> subst::Substs
 746 {
 747     /*!
 748      * Substitutes the values for the receiver's type parameters
 749      * that are found in method, leaving the method's type parameters
 750      * intact.
 751      */
 752
 753     let meth_tps: Vec<ty::t> =
 754         method.generics.types.get_slice(subst::FnSpace)
 755               .iter()
 756               .map(|def| ty::mk_param_from_def(tcx, def))
 757               .collect();
 758     let meth_regions: Vec<ty::Region> =
 759         method.generics.regions.get_slice(subst::FnSpace)
 760               .iter()
 761               .map(|def| ty::ReEarlyBound(def.def_id.node, def.space,
 762                                           def.index, def.name))
 763               .collect();
 764     trait_ref.substs.clone().with_method(meth_tps, meth_regions)
 765 }
 766
 767 fn subst_receiver_types_in_method_ty(tcx: &ty::ctxt,
 768                                      impl_id: ast::DefId,
 769                                      impl_poly_type: &ty::Polytype,
 770                                      trait_ref: &ty::TraitRef,
 771                                      new_def_id: ast::DefId,
 772                                      method: &ty::Method,
 773                                      provided_source: Option<ast::DefId>)
 774                                      -> ty::Method
 775 {
 776     let combined_substs = make_substs_for_receiver_types(tcx, trait_ref, method);
 777
 778     debug!("subst_receiver_types_in_method_ty: combined_substs={}",
 779            combined_substs.repr(tcx));
 780
 781     let mut method_generics = method.generics.subst(tcx, &combined_substs);
 782
 783     // replace the type parameters declared on the trait with those
 784     // from the impl
 785     for &space in [subst::TypeSpace, subst::SelfSpace].iter() {
 786         method_generics.types.replace(
 787             space,
 788             Vec::from_slice(impl_poly_type.generics.types.get_slice(space)));
 789         method_generics.regions.replace(
 790             space,
 791             Vec::from_slice(impl_poly_type.generics.regions.get_slice(space)));
 792     }
 793
 794     debug!("subst_receiver_types_in_method_ty: method_generics={}",
 795            method_generics.repr(tcx));
 796
 797     let method_fty = method.fty.subst(tcx, &combined_substs);
 798
 799     debug!("subst_receiver_types_in_method_ty: method_ty={}",
 800            method.fty.repr(tcx));
 801
 802     ty::Method::new(
 803         method.ident,
 804         method_generics,
 805         method_fty,
 806         method.explicit_self,
 807         method.vis,
 808         new_def_id,
 809         ImplContainer(impl_id),
 810         provided_source
 811     )
 812 }
 813
 814 pub fn check_coherence(crate_context: &CrateCtxt, krate: &Crate) {
 815     CoherenceChecker {
 816         crate_context: crate_context,
 817         inference_context: new_infer_ctxt(crate_context.tcx),
 818     }.check(krate);
 819 }