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