src/librustc_typeck/coherence/mod.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 // Coherence phase
  12 //
  13 // The job of the coherence phase of typechecking is to ensure that
  14 // each trait has at most one implementation for each type. This is
  15 // done by the orphan and overlap modules. Then we build up various
  16 // mappings. That mapping code resides here.
  17
  18
  19 use middle::def_id::DefId;
  20 use middle::lang_items::UnsizeTraitLangItem;
  21 use middle::subst::{self, Subst};
  22 use middle::traits;
  23 use middle::ty::{self, TypeFoldable};
  24 use middle::ty::{ImplOrTraitItemId, ConstTraitItemId};
  25 use middle::ty::{MethodTraitItemId, TypeTraitItemId, ParameterEnvironment};
  26 use middle::ty::{Ty, TyBool, TyChar, TyEnum, TyError};
  27 use middle::ty::{TyParam, TyRawPtr};
  28 use middle::ty::{TyRef, TyStruct, TyTrait, TyTuple};
  29 use middle::ty::{TyStr, TyArray, TySlice, TyFloat, TyInfer, TyInt};
  30 use middle::ty::{TyUint, TyClosure, TyBox, TyBareFn};
  31 use middle::ty::TyProjection;
  32 use middle::ty::util::CopyImplementationError;
  33 use middle::free_region::FreeRegionMap;
  34 use CrateCtxt;
  35 use middle::infer::{self, InferCtxt, TypeOrigin, new_infer_ctxt};
  36 use std::cell::RefCell;
  37 use std::rc::Rc;
  38 use syntax::codemap::Span;
  39 use syntax::parse::token;
  40 use util::nodemap::{DefIdMap, FnvHashMap};
  41 use rustc::dep_graph::DepNode;
  42 use rustc::front::map as hir_map;
  43 use rustc_front::intravisit;
  44 use rustc_front::hir::{Item, ItemImpl};
  45 use rustc_front::hir;
  46
  47 mod orphan;
  48 mod overlap;
  49 mod unsafety;
  50
  51 // Returns the def ID of the base type, if there is one.
  52 fn get_base_type_def_id<'a, 'tcx>(inference_context: &InferCtxt<'a, 'tcx>,
  53                                   span: Span,
  54                                   ty: Ty<'tcx>)
  55                                   -> Option<DefId> {
  56     match ty.sty {
  57         TyEnum(def, _) |
  58         TyStruct(def, _) => {
  59             Some(def.did)
  60         }
  61
  62         TyTrait(ref t) => {
  63             Some(t.principal_def_id())
  64         }
  65
  66         TyBox(_) => {
  67             inference_context.tcx.lang_items.owned_box()
  68         }
  69
  70         TyBool | TyChar | TyInt(..) | TyUint(..) | TyFloat(..) |
  71         TyStr | TyArray(..) | TySlice(..) | TyBareFn(..) | TyTuple(..) |
  72         TyParam(..) | TyError |
  73         TyRawPtr(_) | TyRef(_, _) | TyProjection(..) => {
  74             None
  75         }
  76
  77         TyInfer(..) | TyClosure(..) => {
  78             // `ty` comes from a user declaration so we should only expect types
  79             // that the user can type
  80             inference_context.tcx.sess.span_bug(
  81                 span,
  82                 &format!("coherence encountered unexpected type searching for base type: {}",
  83                          ty));
  84         }
  85     }
  86 }
  87
  88 struct CoherenceChecker<'a, 'tcx: 'a> {
  89     crate_context: &'a CrateCtxt<'a, 'tcx>,
  90     inference_context: InferCtxt<'a, 'tcx>,
  91     inherent_impls: RefCell<DefIdMap<Rc<RefCell<Vec<DefId>>>>>,
  92 }
  93
  94 struct CoherenceCheckVisitor<'a, 'tcx: 'a> {
  95     cc: &'a CoherenceChecker<'a, 'tcx>
  96 }
  97
  98 impl<'a, 'tcx, 'v> intravisit::Visitor<'v> for CoherenceCheckVisitor<'a, 'tcx> {
  99     fn visit_item(&mut self, item: &Item) {
 100         if let ItemImpl(..) = item.node {
 101             self.cc.check_implementation(item)
 102         }
 103     }
 104 }
 105
 106 impl<'a, 'tcx> CoherenceChecker<'a, 'tcx> {
 107     fn check(&self) {
 108         // Check implementations and traits. This populates the tables
 109         // containing the inherent methods and extension methods. It also
 110         // builds up the trait inheritance table.
 111         self.crate_context.tcx.visit_all_items_in_krate(
 112             DepNode::CoherenceCheckImpl,
 113             &mut CoherenceCheckVisitor { cc: self });
 114
 115         // Copy over the inherent impls we gathered up during the walk into
 116         // the tcx.
 117         let mut tcx_inherent_impls =
 118             self.crate_context.tcx.inherent_impls.borrow_mut();
 119         for (k, v) in self.inherent_impls.borrow().iter() {
 120             tcx_inherent_impls.insert((*k).clone(),
 121                                       Rc::new((*v.borrow()).clone()));
 122         }
 123
 124         // Populate the table of destructors. It might seem a bit strange to
 125         // do this here, but it's actually the most convenient place, since
 126         // the coherence tables contain the trait -> type mappings.
 127         self.populate_destructors();
 128
 129         // Check to make sure implementations of `Copy` are legal.
 130         self.check_implementations_of_copy();
 131
 132         // Check to make sure implementations of `CoerceUnsized` are legal
 133         // and collect the necessary information from them.
 134         self.check_implementations_of_coerce_unsized();
 135     }
 136
 137     fn check_implementation(&self, item: &Item) {
 138         let tcx = self.crate_context.tcx;
 139         let impl_did = tcx.map.local_def_id(item.id);
 140         let self_type = tcx.lookup_item_type(impl_did);
 141
 142         // If there are no traits, then this implementation must have a
 143         // base type.
 144
 145         let impl_items = self.create_impl_from_item(item);
 146
 147         if let Some(trait_ref) = self.crate_context.tcx.impl_trait_ref(impl_did) {
 148             debug!("(checking implementation) adding impl for trait '{:?}', item '{}'",
 149                    trait_ref,
 150                    item.name);
 151
 152             // Skip impls where one of the self type is an error type.
 153             // This occurs with e.g. resolve failures (#30589).
 154             if trait_ref.references_error() {
 155                 return;
 156             }
 157
 158             enforce_trait_manually_implementable(self.crate_context.tcx,
 159                                                  item.span,
 160                                                  trait_ref.def_id);
 161             self.add_trait_impl(trait_ref, impl_did);
 162         } else {
 163             // Skip inherent impls where the self type is an error
 164             // type. This occurs with e.g. resolve failures (#30589).
 165             if self_type.ty.references_error() {
 166                 return;
 167             }
 168
 169             // Add the implementation to the mapping from implementation to base
 170             // type def ID, if there is a base type for this implementation and
 171             // the implementation does not have any associated traits.
 172             if let Some(base_type_def_id) = get_base_type_def_id(
 173                     &self.inference_context, item.span, self_type.ty) {
 174                 self.add_inherent_impl(base_type_def_id, impl_did);
 175             }
 176         }
 177
 178         tcx.impl_items.borrow_mut().insert(impl_did, impl_items);
 179     }
 180
 181     fn add_inherent_impl(&self, base_def_id: DefId, impl_def_id: DefId) {
 182         match self.inherent_impls.borrow().get(&base_def_id) {
 183             Some(implementation_list) => {
 184                 implementation_list.borrow_mut().push(impl_def_id);
 185                 return;
 186             }
 187             None => {}
 188         }
 189
 190         self.inherent_impls.borrow_mut().insert(
 191             base_def_id,
 192             Rc::new(RefCell::new(vec!(impl_def_id))));
 193     }
 194
 195     fn add_trait_impl(&self, impl_trait_ref: ty::TraitRef<'tcx>, impl_def_id: DefId) {
 196         debug!("add_trait_impl: impl_trait_ref={:?} impl_def_id={:?}",
 197                impl_trait_ref, impl_def_id);
 198         let trait_def = self.crate_context.tcx.lookup_trait_def(impl_trait_ref.def_id);
 199         trait_def.record_impl(self.crate_context.tcx, impl_def_id, impl_trait_ref);
 200     }
 201
 202     // Converts an implementation in the AST to a vector of items.
 203     fn create_impl_from_item(&self, item: &Item) -> Vec<ImplOrTraitItemId> {
 204         match item.node {
 205             ItemImpl(_, _, _, _, _, ref impl_items) => {
 206                 impl_items.iter().map(|impl_item| {
 207                     let impl_def_id = self.crate_context.tcx.map.local_def_id(impl_item.id);
 208                     match impl_item.node {
 209                         hir::ImplItemKind::Const(..) => {
 210                             ConstTraitItemId(impl_def_id)
 211                         }
 212                         hir::ImplItemKind::Method(..) => {
 213                             MethodTraitItemId(impl_def_id)
 214                         }
 215                         hir::ImplItemKind::Type(_) => {
 216                             TypeTraitItemId(impl_def_id)
 217                         }
 218                     }
 219                 }).collect()
 220             }
 221             _ => {
 222                 self.crate_context.tcx.sess.span_bug(item.span,
 223                                                      "can't convert a non-impl \
 224                                                       to an impl");
 225             }
 226         }
 227     }
 228
 229     //
 230     // Destructors
 231     //
 232
 233     fn populate_destructors(&self) {
 234         let tcx = self.crate_context.tcx;
 235         let drop_trait = match tcx.lang_items.drop_trait() {
 236             Some(id) => id, None => { return }
 237         };
 238         tcx.populate_implementations_for_trait_if_necessary(drop_trait);
 239         let drop_trait = tcx.lookup_trait_def(drop_trait);
 240
 241         let impl_items = tcx.impl_items.borrow();
 242
 243         drop_trait.for_each_impl(tcx, |impl_did| {
 244             let items = impl_items.get(&impl_did).unwrap();
 245             if items.is_empty() {
 246                 // We'll error out later. For now, just don't ICE.
 247                 return;
 248             }
 249             let method_def_id = items[0];
 250
 251             let self_type = tcx.lookup_item_type(impl_did);
 252             match self_type.ty.sty {
 253                 ty::TyEnum(type_def, _) |
 254                 ty::TyStruct(type_def, _) => {
 255                     type_def.set_destructor(method_def_id.def_id());
 256                 }
 257                 _ => {
 258                     // Destructors only work on nominal types.
 259                     if let Some(impl_node_id) = tcx.map.as_local_node_id(impl_did) {
 260                         match tcx.map.find(impl_node_id) {
 261                             Some(hir_map::NodeItem(item)) => {
 262                                 span_err!(tcx.sess, item.span, E0120,
 263                                           "the Drop trait may only be implemented on structures");
 264                             }
 265                             _ => {
 266                                 tcx.sess.bug("didn't find impl in ast \
 267                                               map");
 268                             }
 269                         }
 270                     } else {
 271                         tcx.sess.bug("found external impl of Drop trait on \
 272                                       something other than a struct");
 273                     }
 274                 }
 275             }
 276         });
 277     }
 278
 279     /// Ensures that implementations of the built-in trait `Copy` are legal.
 280     fn check_implementations_of_copy(&self) {
 281         let tcx = self.crate_context.tcx;
 282         let copy_trait = match tcx.lang_items.copy_trait() {
 283             Some(id) => id,
 284             None => return,
 285         };
 286         tcx.populate_implementations_for_trait_if_necessary(copy_trait);
 287         let copy_trait = tcx.lookup_trait_def(copy_trait);
 288
 289         copy_trait.for_each_impl(tcx, |impl_did| {
 290             debug!("check_implementations_of_copy: impl_did={:?}",
 291                    impl_did);
 292
 293             let impl_node_id = if let Some(n) = tcx.map.as_local_node_id(impl_did) {
 294                 n
 295             } else {
 296                 debug!("check_implementations_of_copy(): impl not in this \
 297                         crate");
 298                 return
 299             };
 300
 301             let self_type = tcx.lookup_item_type(impl_did);
 302             debug!("check_implementations_of_copy: self_type={:?} (bound)",
 303                    self_type);
 304
 305             let span = tcx.map.span(impl_node_id);
 306             let param_env = ParameterEnvironment::for_item(tcx, impl_node_id);
 307             let self_type = self_type.ty.subst(tcx, &param_env.free_substs);
 308             assert!(!self_type.has_escaping_regions());
 309
 310             debug!("check_implementations_of_copy: self_type={:?} (free)",
 311                    self_type);
 312
 313             match param_env.can_type_implement_copy(self_type, span) {
 314                 Ok(()) => {}
 315                 Err(CopyImplementationError::InfrigingField(name)) => {
 316                        span_err!(tcx.sess, span, E0204,
 317                                  "the trait `Copy` may not be \
 318                                           implemented for this type; field \
 319                                           `{}` does not implement `Copy`",
 320                                          name)
 321                 }
 322                 Err(CopyImplementationError::InfrigingVariant(name)) => {
 323                        span_err!(tcx.sess, span, E0205,
 324                                  "the trait `Copy` may not be \
 325                                           implemented for this type; variant \
 326                                           `{}` does not implement `Copy`",
 327                                          name)
 328                 }
 329                 Err(CopyImplementationError::NotAnAdt) => {
 330                        span_err!(tcx.sess, span, E0206,
 331                                  "the trait `Copy` may not be implemented \
 332                                   for this type; type is not a structure or \
 333                                   enumeration")
 334                 }
 335                 Err(CopyImplementationError::HasDestructor) => {
 336                     span_err!(tcx.sess, span, E0184,
 337                               "the trait `Copy` may not be implemented for this type; \
 338                                the type has a destructor");
 339                 }
 340             }
 341         });
 342     }
 343
 344     /// Process implementations of the built-in trait `CoerceUnsized`.
 345     fn check_implementations_of_coerce_unsized(&self) {
 346         let tcx = self.crate_context.tcx;
 347         let coerce_unsized_trait = match tcx.lang_items.coerce_unsized_trait() {
 348             Some(id) => id,
 349             None => return,
 350         };
 351         let unsize_trait = match tcx.lang_items.require(UnsizeTraitLangItem) {
 352             Ok(id) => id,
 353             Err(err) => {
 354                 tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err));
 355             }
 356         };
 357
 358         let trait_def = tcx.lookup_trait_def(coerce_unsized_trait);
 359
 360         trait_def.for_each_impl(tcx, |impl_did| {
 361             debug!("check_implementations_of_coerce_unsized: impl_did={:?}",
 362                    impl_did);
 363
 364             let impl_node_id = if let Some(n) = tcx.map.as_local_node_id(impl_did) {
 365                 n
 366             } else {
 367                 debug!("check_implementations_of_coerce_unsized(): impl not \
 368                         in this crate");
 369                 return;
 370             };
 371
 372             let source = tcx.lookup_item_type(impl_did).ty;
 373             let trait_ref = self.crate_context.tcx.impl_trait_ref(impl_did).unwrap();
 374             let target = *trait_ref.substs.types.get(subst::TypeSpace, 0);
 375             debug!("check_implementations_of_coerce_unsized: {:?} -> {:?} (bound)",
 376                    source, target);
 377
 378             let span = tcx.map.span(impl_node_id);
 379             let param_env = ParameterEnvironment::for_item(tcx, impl_node_id);
 380             let source = source.subst(tcx, &param_env.free_substs);
 381             let target = target.subst(tcx, &param_env.free_substs);
 382             assert!(!source.has_escaping_regions());
 383
 384             debug!("check_implementations_of_coerce_unsized: {:?} -> {:?} (free)",
 385                    source, target);
 386
 387             let infcx = new_infer_ctxt(tcx, &tcx.tables, Some(param_env));
 388
 389             let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>, mt_b: ty::TypeAndMut<'tcx>,
 390                                mk_ptr: &Fn(Ty<'tcx>) -> Ty<'tcx>| {
 391                 if (mt_a.mutbl, mt_b.mutbl) == (hir::MutImmutable, hir::MutMutable) {
 392                     infcx.report_mismatched_types(span, mk_ptr(mt_b.ty),
 393                                                   target, &ty::error::TypeError::Mutability);
 394                 }
 395                 (mt_a.ty, mt_b.ty, unsize_trait, None)
 396             };
 397             let (source, target, trait_def_id, kind) = match (&source.sty, &target.sty) {
 398                 (&ty::TyBox(a), &ty::TyBox(b)) => (a, b, unsize_trait, None),
 399
 400                 (&ty::TyRef(r_a, mt_a), &ty::TyRef(r_b, mt_b)) => {
 401                     infer::mk_subr(&infcx, infer::RelateObjectBound(span), *r_b, *r_a);
 402                     check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ref(r_b, ty))
 403                 }
 404
 405                 (&ty::TyRef(_, mt_a), &ty::TyRawPtr(mt_b)) |
 406                 (&ty::TyRawPtr(mt_a), &ty::TyRawPtr(mt_b)) => {
 407                     check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 408                 }
 409
 410                 (&ty::TyStruct(def_a, substs_a), &ty::TyStruct(def_b, substs_b)) => {
 411                     if def_a != def_b {
 412                         let source_path = tcx.item_path_str(def_a.did);
 413                         let target_path = tcx.item_path_str(def_b.did);
 414                         span_err!(tcx.sess, span, E0377,
 415                                   "the trait `CoerceUnsized` may only be implemented \
 416                                    for a coercion between structures with the same \
 417                                    definition; expected {}, found {}",
 418                                   source_path, target_path);
 419                         return;
 420                     }
 421
 422                     let origin = TypeOrigin::Misc(span);
 423                     let fields = &def_a.struct_variant().fields;
 424                     let diff_fields = fields.iter().enumerate().filter_map(|(i, f)| {
 425                         let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));
 426
 427                         if f.unsubst_ty().is_phantom_data() {
 428                             // Ignore PhantomData fields
 429                             None
 430                         } else if infcx.sub_types(false, origin, b, a).is_ok() {
 431                             // Ignore fields that aren't significantly changed
 432                             None
 433                         } else {
 434                             // Collect up all fields that were significantly changed
 435                             // i.e. those that contain T in coerce_unsized T -> U
 436                             Some((i, a, b))
 437                         }
 438                     }).collect::<Vec<_>>();
 439
 440                     if diff_fields.is_empty() {
 441                         span_err!(tcx.sess, span, E0374,
 442                                   "the trait `CoerceUnsized` may only be implemented \
 443                                    for a coercion between structures with one field \
 444                                    being coerced, none found");
 445                         return;
 446                     } else if diff_fields.len() > 1 {
 447                         span_err!(tcx.sess, span, E0375,
 448                                   "the trait `CoerceUnsized` may only be implemented \
 449                                    for a coercion between structures with one field \
 450                                    being coerced, but {} fields need coercions: {}",
 451                                    diff_fields.len(), diff_fields.iter().map(|&(i, a, b)| {
 452                                         let name = fields[i].name;
 453                                         format!("{} ({} to {})",
 454                                                 if name == token::special_names::unnamed_field {
 455                                                     i.to_string()
 456                                                 } else {
 457                                                     name.to_string()
 458                                                 }, a, b)
 459                                    }).collect::<Vec<_>>().join(", "));
 460                         return;
 461                     }
 462
 463                     let (i, a, b) = diff_fields[0];
 464                     let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
 465                     (a, b, coerce_unsized_trait, Some(kind))
 466                 }
 467
 468                 _ => {
 469                     span_err!(tcx.sess, span, E0376,
 470                               "the trait `CoerceUnsized` may only be implemented \
 471                                for a coercion between structures");
 472                     return;
 473                 }
 474             };
 475
 476             let mut fulfill_cx = traits::FulfillmentContext::new();
 477
 478             // Register an obligation for `A: Trait<B>`.
 479             let cause = traits::ObligationCause::misc(span, impl_node_id);
 480             let predicate = traits::predicate_for_trait_def(tcx, cause, trait_def_id,
 481                                                             0, source, vec![target]);
 482             fulfill_cx.register_predicate_obligation(&infcx, predicate);
 483
 484             // Check that all transitive obligations are satisfied.
 485             if let Err(errors) = fulfill_cx.select_all_or_error(&infcx) {
 486                 traits::report_fulfillment_errors(&infcx, &errors);
 487             }
 488
 489             // Finally, resolve all regions.
 490             let mut free_regions = FreeRegionMap::new();
 491             free_regions.relate_free_regions_from_predicates(tcx, &infcx.parameter_environment
 492                                                                         .caller_bounds);
 493             infcx.resolve_regions_and_report_errors(&free_regions, impl_node_id);
 494
 495             if let Some(kind) = kind {
 496                 tcx.custom_coerce_unsized_kinds.borrow_mut().insert(impl_did, kind);
 497             }
 498         });
 499     }
 500 }
 501
 502 fn enforce_trait_manually_implementable(tcx: &ty::ctxt, sp: Span, trait_def_id: DefId) {
 503     if tcx.sess.features.borrow().unboxed_closures {
 504         // the feature gate allows all of them
 505         return
 506     }
 507     let did = Some(trait_def_id);
 508     let li = &tcx.lang_items;
 509
 510     let trait_name = if did == li.fn_trait() {
 511         "Fn"
 512     } else if did == li.fn_mut_trait() {
 513         "FnMut"
 514     } else if did == li.fn_once_trait() {
 515         "FnOnce"
 516     } else {
 517         return // everything OK
 518     };
 519     let mut err = struct_span_err!(tcx.sess,
 520                                    sp,
 521                                    E0183,
 522                                    "manual implementations of `{}` are experimental",
 523                                    trait_name);
 524     fileline_help!(&mut err, sp,
 525                    "add `#![feature(unboxed_closures)]` to the crate attributes to enable");
 526     err.emit();
 527 }
 528
 529 pub fn check_coherence(crate_context: &CrateCtxt) {
 530     let _task = crate_context.tcx.dep_graph.in_task(DepNode::Coherence);
 531     let infcx = new_infer_ctxt(crate_context.tcx, &crate_context.tcx.tables, None);
 532     CoherenceChecker {
 533         crate_context: crate_context,
 534         inference_context: infcx,
 535         inherent_impls: RefCell::new(FnvHashMap()),
 536     }.check();
 537     unsafety::check(crate_context.tcx);
 538     orphan::check(crate_context.tcx);
 539     overlap::check(crate_context.tcx);
 540 }