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, ty_open};
  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 | ty_open(..) |
  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                 ty_trait(box ty::TyTrait { def_id, .. }) => {
 170                     Some(def_id)
 171                 }
 172                 _ => {
 173                     fail!("get_base_type() returned a type that wasn't an \
 174                            enum, struct, or trait");
 175                 }
 176             }
 177         }
 178     }
 179 }
 180
 181 struct CoherenceChecker<'a> {
 182     crate_context: &'a CrateCtxt<'a>,
 183     inference_context: InferCtxt<'a>,
 184 }
 185
 186 struct CoherenceCheckVisitor<'a> {
 187     cc: &'a CoherenceChecker<'a>
 188 }
 189
 190 impl<'a> visit::Visitor<()> for CoherenceCheckVisitor<'a> {
 191     fn visit_item(&mut self, item: &Item, _: ()) {
 192
 193         //debug!("(checking coherence) item '{}'", token::get_ident(item.ident));
 194
 195         match item.node {
 196             ItemImpl(_, ref opt_trait, _, _) => {
 197                 match opt_trait.clone() {
 198                     Some(opt_trait) => {
 199                         self.cc.check_implementation(item, [opt_trait]);
 200                     }
 201                     None => self.cc.check_implementation(item, [])
 202                 }
 203             }
 204             _ => {
 205                 // Nothing to do.
 206             }
 207         };
 208
 209         visit::walk_item(self, item, ());
 210     }
 211 }
 212
 213 struct PrivilegedScopeVisitor<'a> { cc: &'a CoherenceChecker<'a> }
 214
 215 impl<'a> visit::Visitor<()> for PrivilegedScopeVisitor<'a> {
 216     fn visit_item(&mut self, item: &Item, _: ()) {
 217
 218         match item.node {
 219             ItemMod(ref module_) => {
 220                 // Then visit the module items.
 221                 visit::walk_mod(self, module_, ());
 222             }
 223             ItemImpl(_, None, ref ast_ty, _) => {
 224                 if !self.cc.ast_type_is_defined_in_local_crate(&**ast_ty) {
 225                     // This is an error.
 226                     let session = &self.cc.crate_context.tcx.sess;
 227                     span_err!(session, item.span, E0116,
 228                               "cannot associate methods with a type outside the \
 229                                crate the type is defined in; define and implement \
 230                                a trait or new type instead");
 231                 }
 232             }
 233             ItemImpl(_, Some(ref trait_ref), _, _) => {
 234                 let tcx = self.cc.crate_context.tcx;
 235                 // `for_ty` is `Type` in `impl Trait for Type`
 236                 let for_ty = ty::node_id_to_type(tcx, item.id);
 237                 if !type_is_defined_in_local_crate(tcx, for_ty) {
 238                     // This implementation is not in scope of its base
 239                     // type. This still might be OK if the trait is
 240                     // defined in the same crate.
 241
 242                     let trait_def_id =
 243                         self.cc.trait_ref_to_trait_def_id(trait_ref);
 244
 245                     if trait_def_id.krate != LOCAL_CRATE {
 246                         let session = &self.cc.crate_context.tcx.sess;
 247                         span_err!(session, item.span, E0117,
 248                                   "cannot provide an extension implementation \
 249                                    where both trait and type are not defined in this crate");
 250                     }
 251                 }
 252
 253                 visit::walk_item(self, item, ());
 254             }
 255             _ => {
 256                 visit::walk_item(self, item, ());
 257             }
 258         }
 259     }
 260 }
 261
 262 impl<'a> CoherenceChecker<'a> {
 263     fn check(&self, krate: &Crate) {
 264         // Check implementations and traits. This populates the tables
 265         // containing the inherent methods and extension methods. It also
 266         // builds up the trait inheritance table.
 267         let mut visitor = CoherenceCheckVisitor { cc: self };
 268         visit::walk_crate(&mut visitor, krate, ());
 269
 270         // Check that there are no overlapping trait instances
 271         self.check_implementation_coherence();
 272
 273         // Check whether traits with base types are in privileged scopes.
 274         self.check_privileged_scopes(krate);
 275
 276         // Bring in external crates. It's fine for this to happen after the
 277         // coherence checks, because we ensure by construction that no errors
 278         // can happen at link time.
 279         self.add_external_crates();
 280
 281         // Populate the table of destructors. It might seem a bit strange to
 282         // do this here, but it's actually the most convenient place, since
 283         // the coherence tables contain the trait -> type mappings.
 284         self.populate_destructor_table();
 285     }
 286
 287     fn check_implementation(&self, item: &Item,
 288                             associated_traits: &[TraitRef]) {
 289         let tcx = self.crate_context.tcx;
 290         let impl_did = local_def(item.id);
 291         let self_type = ty::lookup_item_type(tcx, impl_did);
 292
 293         // If there are no traits, then this implementation must have a
 294         // base type.
 295
 296         if associated_traits.len() == 0 {
 297             debug!("(checking implementation) no associated traits for item '{}'",
 298                    token::get_ident(item.ident));
 299
 300             match get_base_type_def_id(&self.inference_context,
 301                                        item.span,
 302                                        self_type.ty) {
 303                 None => {
 304                     let session = &self.crate_context.tcx.sess;
 305                     span_err!(session, item.span, E0118,
 306                               "no base type found for inherent implementation; \
 307                                implement a trait or new type instead");
 308                 }
 309                 Some(_) => {
 310                     // Nothing to do.
 311                 }
 312             }
 313         }
 314
 315         let impl_items = self.create_impl_from_item(item);
 316
 317         for associated_trait in associated_traits.iter() {
 318             let trait_ref = ty::node_id_to_trait_ref(
 319                 self.crate_context.tcx, associated_trait.ref_id);
 320             debug!("(checking implementation) adding impl for trait '{}', item '{}'",
 321                    trait_ref.repr(self.crate_context.tcx),
 322                    token::get_ident(item.ident));
 323
 324             self.add_trait_impl(trait_ref.def_id, impl_did);
 325         }
 326
 327         // Add the implementation to the mapping from implementation to base
 328         // type def ID, if there is a base type for this implementation and
 329         // the implementation does not have any associated traits.
 330         match get_base_type_def_id(&self.inference_context,
 331                                    item.span,
 332                                    self_type.ty) {
 333             None => {
 334                 // Nothing to do.
 335             }
 336             Some(base_type_def_id) => {
 337                 // FIXME: Gather up default methods?
 338                 if associated_traits.len() == 0 {
 339                     self.add_inherent_impl(base_type_def_id, impl_did);
 340                 }
 341             }
 342         }
 343
 344         tcx.impl_items.borrow_mut().insert(impl_did, impl_items);
 345     }
 346
 347     // Creates default method IDs and performs type substitutions for an impl
 348     // and trait pair. Then, for each provided method in the trait, inserts a
 349     // `ProvidedMethodInfo` instance into the `provided_method_sources` map.
 350     fn instantiate_default_methods(
 351             &self,
 352             impl_id: DefId,
 353             trait_ref: &ty::TraitRef,
 354             all_impl_items: &mut Vec<ImplOrTraitItemId>) {
 355         let tcx = self.crate_context.tcx;
 356         debug!("instantiate_default_methods(impl_id={:?}, trait_ref={})",
 357                impl_id, trait_ref.repr(tcx));
 358
 359         let impl_poly_type = ty::lookup_item_type(tcx, impl_id);
 360
 361         let prov = ty::provided_trait_methods(tcx, trait_ref.def_id);
 362         for trait_method in prov.iter() {
 363             // Synthesize an ID.
 364             let new_id = tcx.sess.next_node_id();
 365             let new_did = local_def(new_id);
 366
 367             debug!("new_did={:?} trait_method={}", new_did, trait_method.repr(tcx));
 368
 369             // Create substitutions for the various trait parameters.
 370             let new_method_ty =
 371                 Rc::new(subst_receiver_types_in_method_ty(
 372                     tcx,
 373                     impl_id,
 374                     &impl_poly_type,
 375                     trait_ref,
 376                     new_did,
 377                     &**trait_method,
 378                     Some(trait_method.def_id)));
 379
 380             debug!("new_method_ty={}", new_method_ty.repr(tcx));
 381             all_impl_items.push(MethodTraitItemId(new_did));
 382
 383             // construct the polytype for the method based on the
 384             // method_ty.  it will have all the generics from the
 385             // impl, plus its own.
 386             let new_polytype = ty::Polytype {
 387                 generics: new_method_ty.generics.clone(),
 388                 ty: ty::mk_bare_fn(tcx, new_method_ty.fty.clone())
 389             };
 390             debug!("new_polytype={}", new_polytype.repr(tcx));
 391
 392             tcx.tcache.borrow_mut().insert(new_did, new_polytype);
 393             tcx.impl_or_trait_items
 394                .borrow_mut()
 395                .insert(new_did, ty::MethodTraitItem(new_method_ty));
 396
 397             // Pair the new synthesized ID up with the
 398             // ID of the method.
 399             self.crate_context.tcx.provided_method_sources.borrow_mut()
 400                 .insert(new_did, trait_method.def_id);
 401         }
 402     }
 403
 404     fn add_inherent_impl(&self, base_def_id: DefId, impl_def_id: DefId) {
 405         let tcx = self.crate_context.tcx;
 406         match tcx.inherent_impls.borrow().find(&base_def_id) {
 407             Some(implementation_list) => {
 408                 implementation_list.borrow_mut().push(impl_def_id);
 409                 return;
 410             }
 411             None => {}
 412         }
 413
 414         tcx.inherent_impls.borrow_mut().insert(base_def_id,
 415                                                Rc::new(RefCell::new(vec!(impl_def_id))));
 416     }
 417
 418     fn add_trait_impl(&self, base_def_id: DefId, impl_def_id: DefId) {
 419         ty::record_trait_implementation(self.crate_context.tcx,
 420                                         base_def_id,
 421                                         impl_def_id);
 422     }
 423
 424     fn check_implementation_coherence(&self) {
 425         for trait_id in self.crate_context.tcx.trait_impls.borrow().keys() {
 426             self.check_implementation_coherence_of(*trait_id);
 427         }
 428     }
 429
 430     fn check_implementation_coherence_of(&self, trait_def_id: DefId) {
 431         // Unify pairs of polytypes.
 432         self.iter_impls_of_trait_local(trait_def_id, |impl_a| {
 433             let polytype_a =
 434                 self.get_self_type_for_implementation(impl_a);
 435
 436             // "We have an impl of trait <trait_def_id> for type <polytype_a>,
 437             // and that impl is <impl_a>"
 438             self.iter_impls_of_trait(trait_def_id, |impl_b| {
 439
 440                 // An impl is coherent with itself
 441                 if impl_a != impl_b {
 442                     let polytype_b = self.get_self_type_for_implementation(
 443                             impl_b);
 444
 445                     if self.polytypes_unify(polytype_a.clone(), polytype_b) {
 446                         let session = &self.crate_context.tcx.sess;
 447                         span_err!(session, self.span_of_impl(impl_a), E0119,
 448                                   "conflicting implementations for trait `{}`",
 449                                   ty::item_path_str(self.crate_context.tcx, trait_def_id));
 450                         if impl_b.krate == LOCAL_CRATE {
 451                             span_note!(session, self.span_of_impl(impl_b),
 452                                        "note conflicting implementation here");
 453                         } else {
 454                             let crate_store = &self.crate_context.tcx.sess.cstore;
 455                             let cdata = crate_store.get_crate_data(impl_b.krate);
 456                             span_note!(session, self.span_of_impl(impl_a),
 457                                        "conflicting implementation in crate `{}`",
 458                                        cdata.name);
 459                         }
 460                     }
 461                 }
 462             })
 463         })
 464     }
 465
 466     fn iter_impls_of_trait(&self, trait_def_id: DefId, f: |DefId|) {
 467         self.iter_impls_of_trait_local(trait_def_id, |x| f(x));
 468
 469         if trait_def_id.krate == LOCAL_CRATE {
 470             return;
 471         }
 472
 473         let crate_store = &self.crate_context.tcx.sess.cstore;
 474         csearch::each_implementation_for_trait(crate_store, trait_def_id, |impl_def_id| {
 475             // Is this actually necessary?
 476             let _ = lookup_item_type(self.crate_context.tcx, impl_def_id);
 477             f(impl_def_id);
 478         });
 479     }
 480
 481     fn iter_impls_of_trait_local(&self, trait_def_id: DefId, f: |DefId|) {
 482         match self.crate_context.tcx.trait_impls.borrow().find(&trait_def_id) {
 483             Some(impls) => {
 484                 for &impl_did in impls.borrow().iter() {
 485                     f(impl_did);
 486                 }
 487             }
 488             None => { /* no impls? */ }
 489         }
 490     }
 491
 492     fn polytypes_unify(&self,
 493                        polytype_a: Polytype,
 494                        polytype_b: Polytype)
 495                        -> bool {
 496         let universally_quantified_a =
 497             self.universally_quantify_polytype(polytype_a);
 498         let universally_quantified_b =
 499             self.universally_quantify_polytype(polytype_b);
 500
 501         return self.can_unify_universally_quantified(
 502             &universally_quantified_a, &universally_quantified_b) ||
 503             self.can_unify_universally_quantified(
 504             &universally_quantified_b, &universally_quantified_a);
 505     }
 506
 507     // Converts a polytype to a monotype by replacing all parameters with
 508     // type variables. Returns the monotype and the type variables created.
 509     fn universally_quantify_polytype(&self, polytype: Polytype)
 510                                      -> UniversalQuantificationResult
 511     {
 512         let substitutions =
 513             self.inference_context.fresh_substs_for_type(DUMMY_SP,
 514                                                          &polytype.generics);
 515         let monotype = polytype.ty.subst(self.crate_context.tcx, &substitutions);
 516
 517         UniversalQuantificationResult {
 518             monotype: monotype
 519         }
 520     }
 521
 522     fn can_unify_universally_quantified<'a>(&self,
 523                                             a: &'a UniversalQuantificationResult,
 524                                             b: &'a UniversalQuantificationResult)
 525                                             -> bool
 526     {
 527         infer::can_mk_subty(&self.inference_context,
 528                             a.monotype,
 529                             b.monotype).is_ok()
 530     }
 531
 532     fn get_self_type_for_implementation(&self, impl_did: DefId)
 533                                         -> Polytype {
 534         self.crate_context.tcx.tcache.borrow().get_copy(&impl_did)
 535     }
 536
 537     // Privileged scope checking
 538     fn check_privileged_scopes(&self, krate: &Crate) {
 539         let mut visitor = PrivilegedScopeVisitor{ cc: self };
 540         visit::walk_crate(&mut visitor, krate, ());
 541     }
 542
 543     fn trait_ref_to_trait_def_id(&self, trait_ref: &TraitRef) -> DefId {
 544         let def_map = &self.crate_context.tcx.def_map;
 545         let trait_def = def_map.borrow().get_copy(&trait_ref.ref_id);
 546         let trait_id = trait_def.def_id();
 547         return trait_id;
 548     }
 549
 550     /// For coherence, when we have `impl Type`, we need to guarantee that
 551     /// `Type` is "local" to the crate. For our purposes, this means that it
 552     /// must precisely name some nominal type defined in this crate.
 553     fn ast_type_is_defined_in_local_crate(&self, original_type: &ast::Ty) -> bool {
 554         match original_type.node {
 555             TyPath(_, _, path_id) => {
 556                 match self.crate_context.tcx.def_map.borrow().get_copy(&path_id) {
 557                     DefTy(def_id) | DefStruct(def_id) => {
 558                         if def_id.krate != LOCAL_CRATE {
 559                             return false;
 560                         }
 561
 562                         // Make sure that this type precisely names a nominal
 563                         // type.
 564                         match self.crate_context.tcx.map.find(def_id.node) {
 565                             None => {
 566                                 self.crate_context.tcx.sess.span_bug(
 567                                     original_type.span,
 568                                     "resolve didn't resolve this type?!");
 569                             }
 570                             Some(NodeItem(item)) => {
 571                                 match item.node {
 572                                     ItemStruct(..) | ItemEnum(..) => true,
 573                                     _ => false,
 574                                 }
 575                             }
 576                             Some(_) => false,
 577                         }
 578                     }
 579                     _ => false
 580                 }
 581             }
 582             _ => false
 583         }
 584     }
 585
 586     // Converts an implementation in the AST to a vector of items.
 587     fn create_impl_from_item(&self, item: &Item) -> Vec<ImplOrTraitItemId> {
 588         match item.node {
 589             ItemImpl(_, ref trait_refs, _, ref ast_items) => {
 590                 let mut items: Vec<ImplOrTraitItemId> =
 591                         ast_items.iter()
 592                                  .map(|ast_item| {
 593                             match *ast_item {
 594                                 ast::MethodImplItem(ast_method) => {
 595                                     MethodTraitItemId(
 596                                         local_def(ast_method.id))
 597                                 }
 598                             }
 599                         }).collect();
 600
 601                 for trait_ref in trait_refs.iter() {
 602                     let ty_trait_ref = ty::node_id_to_trait_ref(
 603                         self.crate_context.tcx,
 604                         trait_ref.ref_id);
 605
 606                     self.instantiate_default_methods(local_def(item.id),
 607                                                      &*ty_trait_ref,
 608                                                      &mut items);
 609                 }
 610
 611                 items
 612             }
 613             _ => {
 614                 self.crate_context.tcx.sess.span_bug(item.span,
 615                                                      "can't convert a non-impl to an impl");
 616             }
 617         }
 618     }
 619
 620     fn span_of_impl(&self, impl_did: DefId) -> Span {
 621         assert_eq!(impl_did.krate, LOCAL_CRATE);
 622         self.crate_context.tcx.map.span(impl_did.node)
 623     }
 624
 625     // External crate handling
 626
 627     fn add_external_impl(&self,
 628                          impls_seen: &mut HashSet<DefId>,
 629                          impl_def_id: DefId) {
 630         let tcx = self.crate_context.tcx;
 631         let impl_items = csearch::get_impl_items(&tcx.sess.cstore,
 632                                                  impl_def_id);
 633
 634         // Make sure we don't visit the same implementation multiple times.
 635         if !impls_seen.insert(impl_def_id) {
 636             // Skip this one.
 637             return
 638         }
 639         // Good. Continue.
 640
 641         let _ = lookup_item_type(tcx, impl_def_id);
 642         let associated_traits = get_impl_trait(tcx, impl_def_id);
 643
 644         // Do a sanity check.
 645         assert!(associated_traits.is_some());
 646
 647         // Record all the trait items.
 648         for trait_ref in associated_traits.iter() {
 649             self.add_trait_impl(trait_ref.def_id, impl_def_id);
 650         }
 651
 652         // For any methods that use a default implementation, add them to
 653         // the map. This is a bit unfortunate.
 654         for item_def_id in impl_items.iter() {
 655             let impl_item = ty::impl_or_trait_item(tcx, item_def_id.def_id());
 656             match impl_item {
 657                 ty::MethodTraitItem(ref method) => {
 658                     for &source in method.provided_source.iter() {
 659                         tcx.provided_method_sources
 660                            .borrow_mut()
 661                            .insert(item_def_id.def_id(), source);
 662                     }
 663                 }
 664             }
 665         }
 666
 667         tcx.impl_items.borrow_mut().insert(impl_def_id, impl_items);
 668     }
 669
 670     // Adds implementations and traits from external crates to the coherence
 671     // info.
 672     fn add_external_crates(&self) {
 673         let mut impls_seen = HashSet::new();
 674
 675         let crate_store = &self.crate_context.tcx.sess.cstore;
 676         crate_store.iter_crate_data(|crate_number, _crate_metadata| {
 677             each_impl(crate_store, crate_number, |def_id| {
 678                 assert_eq!(crate_number, def_id.krate);
 679                 self.add_external_impl(&mut impls_seen, def_id)
 680             })
 681         })
 682     }
 683
 684     //
 685     // Destructors
 686     //
 687
 688     fn populate_destructor_table(&self) {
 689         let tcx = self.crate_context.tcx;
 690         let drop_trait = match tcx.lang_items.drop_trait() {
 691             Some(id) => id, None => { return }
 692         };
 693
 694         let impl_items = tcx.impl_items.borrow();
 695         let trait_impls = match tcx.trait_impls.borrow().find_copy(&drop_trait) {
 696             None => return, // No types with (new-style) dtors present.
 697             Some(found_impls) => found_impls
 698         };
 699
 700         for &impl_did in trait_impls.borrow().iter() {
 701             let items = impl_items.get(&impl_did);
 702             if items.len() < 1 {
 703                 // We'll error out later. For now, just don't ICE.
 704                 continue;
 705             }
 706             let method_def_id = *items.get(0);
 707
 708             let self_type = self.get_self_type_for_implementation(impl_did);
 709             match ty::get(self_type.ty).sty {
 710                 ty::ty_enum(type_def_id, _) |
 711                 ty::ty_struct(type_def_id, _) |
 712                 ty::ty_unboxed_closure(type_def_id, _) => {
 713                     tcx.destructor_for_type
 714                        .borrow_mut()
 715                        .insert(type_def_id, method_def_id.def_id());
 716                     tcx.destructors
 717                        .borrow_mut()
 718                        .insert(method_def_id.def_id());
 719                 }
 720                 _ => {
 721                     // Destructors only work on nominal types.
 722                     if impl_did.krate == ast::LOCAL_CRATE {
 723                         {
 724                             match tcx.map.find(impl_did.node) {
 725                                 Some(ast_map::NodeItem(item)) => {
 726                                     span_err!(tcx.sess, item.span, E0120,
 727                                         "the Drop trait may only be implemented on structures");
 728                                 }
 729                                 _ => {
 730                                     tcx.sess.bug("didn't find impl in ast \
 731                                                   map");
 732                                 }
 733                             }
 734                         }
 735                     } else {
 736                         tcx.sess.bug("found external impl of Drop trait on \
 737                                       something other than a struct");
 738                     }
 739                 }
 740             }
 741         }
 742     }
 743 }
 744
 745 pub fn make_substs_for_receiver_types(tcx: &ty::ctxt,
 746                                       trait_ref: &ty::TraitRef,
 747                                       method: &ty::Method)
 748                                       -> subst::Substs
 749 {
 750     /*!
 751      * Substitutes the values for the receiver's type parameters
 752      * that are found in method, leaving the method's type parameters
 753      * intact.
 754      */
 755
 756     let meth_tps: Vec<ty::t> =
 757         method.generics.types.get_slice(subst::FnSpace)
 758               .iter()
 759               .map(|def| ty::mk_param_from_def(tcx, def))
 760               .collect();
 761     let meth_regions: Vec<ty::Region> =
 762         method.generics.regions.get_slice(subst::FnSpace)
 763               .iter()
 764               .map(|def| ty::ReEarlyBound(def.def_id.node, def.space,
 765                                           def.index, def.name))
 766               .collect();
 767     trait_ref.substs.clone().with_method(meth_tps, meth_regions)
 768 }
 769
 770 fn subst_receiver_types_in_method_ty(tcx: &ty::ctxt,
 771                                      impl_id: ast::DefId,
 772                                      impl_poly_type: &ty::Polytype,
 773                                      trait_ref: &ty::TraitRef,
 774                                      new_def_id: ast::DefId,
 775                                      method: &ty::Method,
 776                                      provided_source: Option<ast::DefId>)
 777                                      -> ty::Method
 778 {
 779     let combined_substs = make_substs_for_receiver_types(tcx, trait_ref, method);
 780
 781     debug!("subst_receiver_types_in_method_ty: combined_substs={}",
 782            combined_substs.repr(tcx));
 783
 784     let mut method_generics = method.generics.subst(tcx, &combined_substs);
 785
 786     // replace the type parameters declared on the trait with those
 787     // from the impl
 788     for &space in [subst::TypeSpace, subst::SelfSpace].iter() {
 789         method_generics.types.replace(
 790             space,
 791             Vec::from_slice(impl_poly_type.generics.types.get_slice(space)));
 792         method_generics.regions.replace(
 793             space,
 794             Vec::from_slice(impl_poly_type.generics.regions.get_slice(space)));
 795     }
 796
 797     debug!("subst_receiver_types_in_method_ty: method_generics={}",
 798            method_generics.repr(tcx));
 799
 800     let method_fty = method.fty.subst(tcx, &combined_substs);
 801
 802     debug!("subst_receiver_types_in_method_ty: method_ty={}",
 803            method.fty.repr(tcx));
 804
 805     ty::Method::new(
 806         method.ident,
 807         method_generics,
 808         method_fty,
 809         method.explicit_self,
 810         method.vis,
 811         new_def_id,
 812         ImplContainer(impl_id),
 813         provided_source
 814     )
 815 }
 816
 817 pub fn check_coherence(crate_context: &CrateCtxt, krate: &Crate) {
 818     CoherenceChecker {
 819         crate_context: crate_context,
 820         inference_context: new_infer_ctxt(crate_context.tcx),
 821     }.check(krate);
 822 }