src/librustc_typeck/coherence/builtin.rs

   1 // Copyright 2016 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 //! Check properties that are required by built-in traits and set
  12 //! up data structures required by type-checking/translation.
  13
  14 use rustc::middle::free_region::FreeRegionMap;
  15 use rustc::middle::region;
  16 use rustc::middle::lang_items::UnsizeTraitLangItem;
  17
  18 use rustc::traits::{self, ObligationCause};
  19 use rustc::ty::{self, Ty, TyCtxt};
  20 use rustc::ty::TypeFoldable;
  21 use rustc::ty::adjustment::CoerceUnsizedInfo;
  22 use rustc::ty::util::CopyImplementationError;
  23 use rustc::infer;
  24
  25 use rustc::hir::def_id::DefId;
  26 use rustc::hir::map as hir_map;
  27 use rustc::hir::{self, ItemImpl};
  28
  29 pub fn check_trait<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, trait_def_id: DefId) {
  30     Checker { tcx, trait_def_id }
  31         .check(tcx.lang_items().drop_trait(), visit_implementation_of_drop)
  32         .check(tcx.lang_items().copy_trait(), visit_implementation_of_copy)
  33         .check(tcx.lang_items().coerce_unsized_trait(),
  34                visit_implementation_of_coerce_unsized);
  35 }
  36
  37 struct Checker<'a, 'tcx: 'a> {
  38     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  39     trait_def_id: DefId
  40 }
  41
  42 impl<'a, 'tcx> Checker<'a, 'tcx> {
  43     fn check<F>(&self, trait_def_id: Option<DefId>, mut f: F) -> &Self
  44         where F: FnMut(TyCtxt<'a, 'tcx, 'tcx>, DefId, DefId)
  45     {
  46         if Some(self.trait_def_id) == trait_def_id {
  47             for &impl_id in self.tcx.hir.trait_impls(self.trait_def_id) {
  48                 let impl_def_id = self.tcx.hir.local_def_id(impl_id);
  49                 f(self.tcx, self.trait_def_id, impl_def_id);
  50             }
  51         }
  52         self
  53     }
  54 }
  55
  56 fn visit_implementation_of_drop<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  57                                           _drop_did: DefId,
  58                                           impl_did: DefId) {
  59     match tcx.type_of(impl_did).sty {
  60         ty::TyAdt(..) => {}
  61         _ => {
  62             // Destructors only work on nominal types.
  63             if let Some(impl_node_id) = tcx.hir.as_local_node_id(impl_did) {
  64                 match tcx.hir.find(impl_node_id) {
  65                     Some(hir_map::NodeItem(item)) => {
  66                         let span = match item.node {
  67                             ItemImpl(.., ref ty, _) => ty.span,
  68                             _ => item.span,
  69                         };
  70                         struct_span_err!(tcx.sess,
  71                                          span,
  72                                          E0120,
  73                                          "the Drop trait may only be implemented on \
  74                                          structures")
  75                             .span_label(span, "implementing Drop requires a struct")
  76                             .emit();
  77                     }
  78                     _ => {
  79                         bug!("didn't find impl in ast map");
  80                     }
  81                 }
  82             } else {
  83                 bug!("found external impl of Drop trait on \
  84                       something other than a struct");
  85             }
  86         }
  87     }
  88 }
  89
  90 fn visit_implementation_of_copy<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  91                                           _copy_did: DefId,
  92                                           impl_did: DefId) {
  93     debug!("visit_implementation_of_copy: impl_did={:?}", impl_did);
  94
  95     let impl_node_id = if let Some(n) = tcx.hir.as_local_node_id(impl_did) {
  96         n
  97     } else {
  98         debug!("visit_implementation_of_copy(): impl not in this \
  99                 crate");
 100         return;
 101     };
 102
 103     let self_type = tcx.type_of(impl_did);
 104     debug!("visit_implementation_of_copy: self_type={:?} (bound)",
 105            self_type);
 106
 107     let span = tcx.hir.span(impl_node_id);
 108     let param_env = tcx.param_env(impl_did);
 109     assert!(!self_type.has_escaping_regions());
 110
 111     debug!("visit_implementation_of_copy: self_type={:?} (free)",
 112            self_type);
 113
 114     match param_env.can_type_implement_copy(tcx, self_type, span) {
 115         Ok(()) => {}
 116         Err(CopyImplementationError::InfrigingField(field)) => {
 117             let item = tcx.hir.expect_item(impl_node_id);
 118             let span = if let ItemImpl(.., Some(ref tr), _, _) = item.node {
 119                 tr.path.span
 120             } else {
 121                 span
 122             };
 123
 124             struct_span_err!(tcx.sess,
 125                              span,
 126                              E0204,
 127                              "the trait `Copy` may not be implemented for this type")
 128                 .span_label(
 129                     tcx.def_span(field.did),
 130                     "this field does not implement `Copy`")
 131                 .emit()
 132         }
 133         Err(CopyImplementationError::NotAnAdt) => {
 134             let item = tcx.hir.expect_item(impl_node_id);
 135             let span = if let ItemImpl(.., ref ty, _) = item.node {
 136                 ty.span
 137             } else {
 138                 span
 139             };
 140
 141             struct_span_err!(tcx.sess,
 142                              span,
 143                              E0206,
 144                              "the trait `Copy` may not be implemented for this type")
 145                 .span_label(span, "type is not a structure or enumeration")
 146                 .emit();
 147         }
 148         Err(CopyImplementationError::HasDestructor) => {
 149             struct_span_err!(tcx.sess,
 150                              span,
 151                              E0184,
 152                              "the trait `Copy` may not be implemented for this type; the \
 153                               type has a destructor")
 154                 .span_label(span, "Copy not allowed on types with destructors")
 155                 .emit();
 156         }
 157     }
 158 }
 159
 160 fn visit_implementation_of_coerce_unsized<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 161                                                     _: DefId,
 162                                                     impl_did: DefId) {
 163     debug!("visit_implementation_of_coerce_unsized: impl_did={:?}",
 164            impl_did);
 165
 166     // Just compute this for the side-effects, in particular reporting
 167     // errors; other parts of the code may demand it for the info of
 168     // course.
 169     if impl_did.is_local() {
 170         let span = tcx.def_span(impl_did);
 171         tcx.at(span).coerce_unsized_info(impl_did);
 172     }
 173 }
 174
 175 pub fn coerce_unsized_info<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 176                                      impl_did: DefId)
 177                                      -> CoerceUnsizedInfo {
 178     debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
 179     let coerce_unsized_trait = tcx.lang_items().coerce_unsized_trait().unwrap();
 180
 181     let unsize_trait = match tcx.lang_items().require(UnsizeTraitLangItem) {
 182         Ok(id) => id,
 183         Err(err) => {
 184             tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err));
 185         }
 186     };
 187
 188     // this provider should only get invoked for local def-ids
 189     let impl_node_id = tcx.hir.as_local_node_id(impl_did).unwrap_or_else(|| {
 190         bug!("coerce_unsized_info: invoked for non-local def-id {:?}", impl_did)
 191     });
 192
 193     let source = tcx.type_of(impl_did);
 194     let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
 195     assert_eq!(trait_ref.def_id, coerce_unsized_trait);
 196     let target = trait_ref.substs.type_at(1);
 197     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)",
 198            source,
 199            target);
 200
 201     let span = tcx.hir.span(impl_node_id);
 202     let param_env = tcx.param_env(impl_did);
 203     assert!(!source.has_escaping_regions());
 204
 205     let err_info = CoerceUnsizedInfo { custom_kind: None };
 206
 207     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)",
 208            source,
 209            target);
 210
 211     tcx.infer_ctxt().enter(|infcx| {
 212         let cause = ObligationCause::misc(span, impl_node_id);
 213         let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
 214                            mt_b: ty::TypeAndMut<'tcx>,
 215                            mk_ptr: &Fn(Ty<'tcx>) -> Ty<'tcx>| {
 216             if (mt_a.mutbl, mt_b.mutbl) == (hir::MutImmutable, hir::MutMutable) {
 217                 infcx.report_mismatched_types(&cause,
 218                                              mk_ptr(mt_b.ty),
 219                                              target,
 220                                              ty::error::TypeError::Mutability)
 221                     .emit();
 222             }
 223             (mt_a.ty, mt_b.ty, unsize_trait, None)
 224         };
 225         let (source, target, trait_def_id, kind) = match (&source.sty, &target.sty) {
 226             (&ty::TyRef(r_a, mt_a), &ty::TyRef(r_b, mt_b)) => {
 227                 infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
 228                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ref(r_b, ty))
 229             }
 230
 231             (&ty::TyRef(_, mt_a), &ty::TyRawPtr(mt_b)) |
 232             (&ty::TyRawPtr(mt_a), &ty::TyRawPtr(mt_b)) => {
 233                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 234             }
 235
 236             (&ty::TyAdt(def_a, substs_a), &ty::TyAdt(def_b, substs_b)) if def_a.is_struct() &&
 237                                                                           def_b.is_struct() => {
 238                 if def_a != def_b {
 239                     let source_path = tcx.item_path_str(def_a.did);
 240                     let target_path = tcx.item_path_str(def_b.did);
 241                     span_err!(tcx.sess,
 242                               span,
 243                               E0377,
 244                               "the trait `CoerceUnsized` may only be implemented \
 245                                for a coercion between structures with the same \
 246                                definition; expected {}, found {}",
 247                               source_path,
 248                               target_path);
 249                     return err_info;
 250                 }
 251
 252                 // Here we are considering a case of converting
 253                 // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
 254                 // which acts like a pointer to `U`, but carries along some extra data of type `T`:
 255                 //
 256                 //     struct Foo<T, U> {
 257                 //         extra: T,
 258                 //         ptr: *mut U,
 259                 //     }
 260                 //
 261                 // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
 262                 // to `Foo<T, [i32]>`. That impl would look like:
 263                 //
 264                 //   impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
 265                 //
 266                 // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
 267                 // when this coercion occurs, we would be changing the
 268                 // field `ptr` from a thin pointer of type `*mut [i32;
 269                 // 3]` to a fat pointer of type `*mut [i32]` (with
 270                 // extra data `3`).  **The purpose of this check is to
 271                 // make sure that we know how to do this conversion.**
 272                 //
 273                 // To check if this impl is legal, we would walk down
 274                 // the fields of `Foo` and consider their types with
 275                 // both substitutes. We are looking to find that
 276                 // exactly one (non-phantom) field has changed its
 277                 // type, which we will expect to be the pointer that
 278                 // is becoming fat (we could probably generalize this
 279                 // to mutiple thin pointers of the same type becoming
 280                 // fat, but we don't). In this case:
 281                 //
 282                 // - `extra` has type `T` before and type `T` after
 283                 // - `ptr` has type `*mut U` before and type `*mut V` after
 284                 //
 285                 // Since just one field changed, we would then check
 286                 // that `*mut U: CoerceUnsized<*mut V>` is implemented
 287                 // (in other words, that we know how to do this
 288                 // conversion). This will work out because `U:
 289                 // Unsize<V>`, and we have a builtin rule that `*mut
 290                 // U` can be coerced to `*mut V` if `U: Unsize<V>`.
 291                 let fields = &def_a.struct_variant().fields;
 292                 let diff_fields = fields.iter()
 293                     .enumerate()
 294                     .filter_map(|(i, f)| {
 295                         let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));
 296
 297                         if tcx.type_of(f.did).is_phantom_data() {
 298                             // Ignore PhantomData fields
 299                             return None;
 300                         }
 301
 302                         // Ignore fields that aren't changed; it may
 303                         // be that we could get away with subtyping or
 304                         // something more accepting, but we use
 305                         // equality because we want to be able to
 306                         // perform this check without computing
 307                         // variance where possible. (This is because
 308                         // we may have to evaluate constraint
 309                         // expressions in the course of execution.)
 310                         // See e.g. #41936.
 311                         if let Ok(ok) = infcx.at(&cause, param_env).eq(a, b) {
 312                             if ok.obligations.is_empty() {
 313                                 return None;
 314                             }
 315                         }
 316
 317                         // Collect up all fields that were significantly changed
 318                         // i.e. those that contain T in coerce_unsized T -> U
 319                         Some((i, a, b))
 320                     })
 321                     .collect::<Vec<_>>();
 322
 323                 if diff_fields.is_empty() {
 324                     span_err!(tcx.sess,
 325                               span,
 326                               E0374,
 327                               "the trait `CoerceUnsized` may only be implemented \
 328                                for a coercion between structures with one field \
 329                                being coerced, none found");
 330                     return err_info;
 331                 } else if diff_fields.len() > 1 {
 332                     let item = tcx.hir.expect_item(impl_node_id);
 333                     let span = if let ItemImpl(.., Some(ref t), _, _) = item.node {
 334                         t.path.span
 335                     } else {
 336                         tcx.hir.span(impl_node_id)
 337                     };
 338
 339                     let mut err = struct_span_err!(tcx.sess,
 340                                                    span,
 341                                                    E0375,
 342                                                    "implementing the trait \
 343                                                     `CoerceUnsized` requires multiple \
 344                                                     coercions");
 345                     err.note("`CoerceUnsized` may only be implemented for \
 346                               a coercion between structures with one field being coerced");
 347                     err.note(&format!("currently, {} fields need coercions: {}",
 348                                       diff_fields.len(),
 349                                       diff_fields.iter()
 350                                           .map(|&(i, a, b)| {
 351                                               format!("{} ({} to {})", fields[i].name, a, b)
 352                                           })
 353                                           .collect::<Vec<_>>()
 354                                           .join(", ")));
 355                     err.span_label(span, "requires multiple coercions");
 356                     err.emit();
 357                     return err_info;
 358                 }
 359
 360                 let (i, a, b) = diff_fields[0];
 361                 let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
 362                 (a, b, coerce_unsized_trait, Some(kind))
 363             }
 364
 365             _ => {
 366                 span_err!(tcx.sess,
 367                           span,
 368                           E0376,
 369                           "the trait `CoerceUnsized` may only be implemented \
 370                            for a coercion between structures");
 371                 return err_info;
 372             }
 373         };
 374
 375         let mut fulfill_cx = traits::FulfillmentContext::new();
 376
 377         // Register an obligation for `A: Trait<B>`.
 378         let cause = traits::ObligationCause::misc(span, impl_node_id);
 379         let predicate = tcx.predicate_for_trait_def(param_env,
 380                                                     cause,
 381                                                     trait_def_id,
 382                                                     0,
 383                                                     source,
 384                                                     &[target]);
 385         fulfill_cx.register_predicate_obligation(&infcx, predicate);
 386
 387         // Check that all transitive obligations are satisfied.
 388         if let Err(errors) = fulfill_cx.select_all_or_error(&infcx) {
 389             infcx.report_fulfillment_errors(&errors, None);
 390         }
 391
 392         // Finally, resolve all regions.
 393         let region_scope_tree = region::ScopeTree::default();
 394         let mut free_regions = FreeRegionMap::new();
 395         free_regions.relate_free_regions_from_predicates(&param_env.caller_bounds);
 396         infcx.resolve_regions_and_report_errors(impl_did, &region_scope_tree, &free_regions);
 397
 398         CoerceUnsizedInfo {
 399             custom_kind: kind
 400         }
 401     })
 402 }