src/librustc_typeck/coherence/builtin.rs

   1 //! Check properties that are required by built-in traits and set
   2 //! up data structures required by type-checking/codegen.
   3
   4 use rustc::infer;
   5 use rustc::infer::outlives::env::OutlivesEnvironment;
   6 use rustc::infer::SuppressRegionErrors;
   7 use rustc::middle::lang_items::UnsizeTraitLangItem;
   8 use rustc::middle::region;
   9 use rustc::traits::misc::{can_type_implement_copy, CopyImplementationError};
  10 use rustc::traits::predicate_for_trait_def;
  11 use rustc::traits::{self, ObligationCause, TraitEngine};
  12 use rustc::ty::adjustment::CoerceUnsizedInfo;
  13 use rustc::ty::TypeFoldable;
  14 use rustc::ty::{self, Ty, TyCtxt};
  15 use rustc_errors::struct_span_err;
  16 use rustc_hir as hir;
  17 use rustc_hir::def_id::DefId;
  18 use rustc_hir::ItemKind;
  19
  20 pub fn check_trait(tcx: TyCtxt<'_>, trait_def_id: DefId) {
  21     Checker { tcx, trait_def_id }
  22         .check(tcx.lang_items().drop_trait(), visit_implementation_of_drop)
  23         .check(tcx.lang_items().copy_trait(), visit_implementation_of_copy)
  24         .check(tcx.lang_items().coerce_unsized_trait(), visit_implementation_of_coerce_unsized)
  25         .check(
  26             tcx.lang_items().dispatch_from_dyn_trait(),
  27             visit_implementation_of_dispatch_from_dyn,
  28         );
  29 }
  30
  31 struct Checker<'tcx> {
  32     tcx: TyCtxt<'tcx>,
  33     trait_def_id: DefId,
  34 }
  35
  36 impl<'tcx> Checker<'tcx> {
  37     fn check<F>(&self, trait_def_id: Option<DefId>, mut f: F) -> &Self
  38     where
  39         F: FnMut(TyCtxt<'tcx>, DefId),
  40     {
  41         if Some(self.trait_def_id) == trait_def_id {
  42             for &impl_id in self.tcx.hir().trait_impls(self.trait_def_id) {
  43                 let impl_def_id = self.tcx.hir().local_def_id(impl_id);
  44                 f(self.tcx, impl_def_id);
  45             }
  46         }
  47         self
  48     }
  49 }
  50
  51 fn visit_implementation_of_drop(tcx: TyCtxt<'_>, impl_did: DefId) {
  52     // Destructors only work on nominal types.
  53     if let ty::Adt(..) | ty::Error = tcx.type_of(impl_did).kind {
  54         return;
  55     }
  56
  57     let impl_hir_id = tcx.hir().as_local_hir_id(impl_did).expect("foreign Drop impl on non-ADT");
  58     let sp = match tcx.hir().expect_item(impl_hir_id).kind {
  59         ItemKind::Impl { self_ty, .. } => self_ty.span,
  60         _ => bug!("expected Drop impl item"),
  61     };
  62
  63     struct_span_err!(
  64         tcx.sess,
  65         sp,
  66         E0120,
  67         "the `Drop` trait may only be implemented for structs, enums, and unions",
  68     )
  69     .span_label(sp, "must be a struct, enum, or union")
  70     .emit();
  71 }
  72
  73 fn visit_implementation_of_copy(tcx: TyCtxt<'_>, impl_did: DefId) {
  74     debug!("visit_implementation_of_copy: impl_did={:?}", impl_did);
  75
  76     let impl_hir_id = if let Some(n) = tcx.hir().as_local_hir_id(impl_did) {
  77         n
  78     } else {
  79         debug!("visit_implementation_of_copy(): impl not in this crate");
  80         return;
  81     };
  82
  83     let self_type = tcx.type_of(impl_did);
  84     debug!("visit_implementation_of_copy: self_type={:?} (bound)", self_type);
  85
  86     let span = tcx.hir().span(impl_hir_id);
  87     let param_env = tcx.param_env(impl_did);
  88     assert!(!self_type.has_escaping_bound_vars());
  89
  90     debug!("visit_implementation_of_copy: self_type={:?} (free)", self_type);
  91
  92     match can_type_implement_copy(tcx, param_env, self_type) {
  93         Ok(()) => {}
  94         Err(CopyImplementationError::InfrigingFields(fields)) => {
  95             let item = tcx.hir().expect_item(impl_hir_id);
  96             let span = if let ItemKind::Impl { of_trait: Some(ref tr), .. } = item.kind {
  97                 tr.path.span
  98             } else {
  99                 span
 100             };
 101
 102             let mut err = struct_span_err!(
 103                 tcx.sess,
 104                 span,
 105                 E0204,
 106                 "the trait `Copy` may not be implemented for this type"
 107             );
 108             for span in fields.iter().map(|f| tcx.def_span(f.did)) {
 109                 err.span_label(span, "this field does not implement `Copy`");
 110             }
 111             err.emit()
 112         }
 113         Err(CopyImplementationError::NotAnAdt) => {
 114             let item = tcx.hir().expect_item(impl_hir_id);
 115             let span =
 116                 if let ItemKind::Impl { self_ty, .. } = item.kind { self_ty.span } else { span };
 117
 118             struct_span_err!(
 119                 tcx.sess,
 120                 span,
 121                 E0206,
 122                 "the trait `Copy` may not be implemented for this type"
 123             )
 124             .span_label(span, "type is not a structure or enumeration")
 125             .emit();
 126         }
 127         Err(CopyImplementationError::HasDestructor) => {
 128             struct_span_err!(
 129                 tcx.sess,
 130                 span,
 131                 E0184,
 132                 "the trait `Copy` may not be implemented for this type; the \
 133                               type has a destructor"
 134             )
 135             .span_label(span, "Copy not allowed on types with destructors")
 136             .emit();
 137         }
 138     }
 139 }
 140
 141 fn visit_implementation_of_coerce_unsized(tcx: TyCtxt<'tcx>, impl_did: DefId) {
 142     debug!("visit_implementation_of_coerce_unsized: impl_did={:?}", impl_did);
 143
 144     // Just compute this for the side-effects, in particular reporting
 145     // errors; other parts of the code may demand it for the info of
 146     // course.
 147     if impl_did.is_local() {
 148         let span = tcx.def_span(impl_did);
 149         tcx.at(span).coerce_unsized_info(impl_did);
 150     }
 151 }
 152
 153 fn visit_implementation_of_dispatch_from_dyn(tcx: TyCtxt<'_>, impl_did: DefId) {
 154     debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);
 155     if impl_did.is_local() {
 156         let dispatch_from_dyn_trait = tcx.lang_items().dispatch_from_dyn_trait().unwrap();
 157
 158         let impl_hir_id = tcx.hir().as_local_hir_id(impl_did).unwrap();
 159         let span = tcx.hir().span(impl_hir_id);
 160
 161         let source = tcx.type_of(impl_did);
 162         assert!(!source.has_escaping_bound_vars());
 163         let target = {
 164             let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
 165             assert_eq!(trait_ref.def_id, dispatch_from_dyn_trait);
 166
 167             trait_ref.substs.type_at(1)
 168         };
 169
 170         debug!("visit_implementation_of_dispatch_from_dyn: {:?} -> {:?}", source, target);
 171
 172         let param_env = tcx.param_env(impl_did);
 173
 174         let create_err = |msg: &str| struct_span_err!(tcx.sess, span, E0378, "{}", msg);
 175
 176         tcx.infer_ctxt().enter(|infcx| {
 177             let cause = ObligationCause::misc(span, impl_hir_id);
 178
 179             use ty::TyKind::*;
 180             match (&source.kind, &target.kind) {
 181                 (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))
 182                     if infcx.at(&cause, param_env).eq(r_a, r_b).is_ok() && mutbl_a == *mutbl_b =>
 183                 {
 184                     ()
 185                 }
 186                 (&RawPtr(tm_a), &RawPtr(tm_b)) if tm_a.mutbl == tm_b.mutbl => (),
 187                 (&Adt(def_a, substs_a), &Adt(def_b, substs_b))
 188                     if def_a.is_struct() && def_b.is_struct() =>
 189                 {
 190                     if def_a != def_b {
 191                         let source_path = tcx.def_path_str(def_a.did);
 192                         let target_path = tcx.def_path_str(def_b.did);
 193
 194                         create_err(&format!(
 195                             "the trait `DispatchFromDyn` may only be implemented \
 196                                 for a coercion between structures with the same \
 197                                 definition; expected `{}`, found `{}`",
 198                             source_path, target_path,
 199                         ))
 200                         .emit();
 201
 202                         return;
 203                     }
 204
 205                     if def_a.repr.c() || def_a.repr.packed() {
 206                         create_err(
 207                             "structs implementing `DispatchFromDyn` may not have \
 208                              `#[repr(packed)]` or `#[repr(C)]`",
 209                         )
 210                         .emit();
 211                     }
 212
 213                     let fields = &def_a.non_enum_variant().fields;
 214
 215                     let coerced_fields = fields
 216                         .iter()
 217                         .filter_map(|field| {
 218                             let ty_a = field.ty(tcx, substs_a);
 219                             let ty_b = field.ty(tcx, substs_b);
 220
 221                             if let Ok(layout) = tcx.layout_of(param_env.and(ty_a)) {
 222                                 if layout.is_zst() && layout.details.align.abi.bytes() == 1 {
 223                                     // ignore ZST fields with alignment of 1 byte
 224                                     return None;
 225                                 }
 226                             }
 227
 228                             if let Ok(ok) = infcx.at(&cause, param_env).eq(ty_a, ty_b) {
 229                                 if ok.obligations.is_empty() {
 230                                     create_err(
 231                                         "the trait `DispatchFromDyn` may only be implemented \
 232                                      for structs containing the field being coerced, \
 233                                      ZST fields with 1 byte alignment, and nothing else",
 234                                     )
 235                                     .note(&format!(
 236                                         "extra field `{}` of type `{}` is not allowed",
 237                                         field.ident, ty_a,
 238                                     ))
 239                                     .emit();
 240
 241                                     return None;
 242                                 }
 243                             }
 244
 245                             Some(field)
 246                         })
 247                         .collect::<Vec<_>>();
 248
 249                     if coerced_fields.is_empty() {
 250                         create_err(
 251                             "the trait `DispatchFromDyn` may only be implemented \
 252                             for a coercion between structures with a single field \
 253                             being coerced, none found",
 254                         )
 255                         .emit();
 256                     } else if coerced_fields.len() > 1 {
 257                         create_err(
 258                             "implementing the `DispatchFromDyn` trait requires multiple coercions",
 259                         )
 260                         .note(
 261                             "the trait `DispatchFromDyn` may only be implemented \
 262                                 for a coercion between structures with a single field \
 263                                 being coerced",
 264                         )
 265                         .note(&format!(
 266                             "currently, {} fields need coercions: {}",
 267                             coerced_fields.len(),
 268                             coerced_fields
 269                                 .iter()
 270                                 .map(|field| {
 271                                     format!(
 272                                         "`{}` (`{}` to `{}`)",
 273                                         field.ident,
 274                                         field.ty(tcx, substs_a),
 275                                         field.ty(tcx, substs_b),
 276                                     )
 277                                 })
 278                                 .collect::<Vec<_>>()
 279                                 .join(", ")
 280                         ))
 281                         .emit();
 282                     } else {
 283                         let mut fulfill_cx = TraitEngine::new(infcx.tcx);
 284
 285                         for field in coerced_fields {
 286                             let predicate = predicate_for_trait_def(
 287                                 tcx,
 288                                 param_env,
 289                                 cause.clone(),
 290                                 dispatch_from_dyn_trait,
 291                                 0,
 292                                 field.ty(tcx, substs_a),
 293                                 &[field.ty(tcx, substs_b).into()],
 294                             );
 295
 296                             fulfill_cx.register_predicate_obligation(&infcx, predicate);
 297                         }
 298
 299                         // Check that all transitive obligations are satisfied.
 300                         if let Err(errors) = fulfill_cx.select_all_or_error(&infcx) {
 301                             infcx.report_fulfillment_errors(&errors, None, false);
 302                         }
 303
 304                         // Finally, resolve all regions.
 305                         let region_scope_tree = region::ScopeTree::default();
 306                         let outlives_env = OutlivesEnvironment::new(param_env);
 307                         infcx.resolve_regions_and_report_errors(
 308                             impl_did,
 309                             &region_scope_tree,
 310                             &outlives_env,
 311                             SuppressRegionErrors::default(),
 312                         );
 313                     }
 314                 }
 315                 _ => {
 316                     create_err(
 317                         "the trait `DispatchFromDyn` may only be implemented \
 318                         for a coercion between structures",
 319                     )
 320                     .emit();
 321                 }
 322             }
 323         })
 324     }
 325 }
 326
 327 pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUnsizedInfo {
 328     debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
 329     let coerce_unsized_trait = tcx.lang_items().coerce_unsized_trait().unwrap();
 330
 331     let unsize_trait = tcx.lang_items().require(UnsizeTraitLangItem).unwrap_or_else(|err| {
 332         tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err));
 333     });
 334
 335     // this provider should only get invoked for local def-ids
 336     let impl_hir_id = tcx.hir().as_local_hir_id(impl_did).unwrap_or_else(|| {
 337         bug!("coerce_unsized_info: invoked for non-local def-id {:?}", impl_did)
 338     });
 339
 340     let source = tcx.type_of(impl_did);
 341     let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
 342     assert_eq!(trait_ref.def_id, coerce_unsized_trait);
 343     let target = trait_ref.substs.type_at(1);
 344     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)", source, target);
 345
 346     let span = tcx.hir().span(impl_hir_id);
 347     let param_env = tcx.param_env(impl_did);
 348     assert!(!source.has_escaping_bound_vars());
 349
 350     let err_info = CoerceUnsizedInfo { custom_kind: None };
 351
 352     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);
 353
 354     tcx.infer_ctxt().enter(|infcx| {
 355         let cause = ObligationCause::misc(span, impl_hir_id);
 356         let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
 357                            mt_b: ty::TypeAndMut<'tcx>,
 358                            mk_ptr: &dyn Fn(Ty<'tcx>) -> Ty<'tcx>| {
 359             if (mt_a.mutbl, mt_b.mutbl) == (hir::Mutability::Not, hir::Mutability::Mut) {
 360                 infcx
 361                     .report_mismatched_types(
 362                         &cause,
 363                         mk_ptr(mt_b.ty),
 364                         target,
 365                         ty::error::TypeError::Mutability,
 366                     )
 367                     .emit();
 368             }
 369             (mt_a.ty, mt_b.ty, unsize_trait, None)
 370         };
 371         let (source, target, trait_def_id, kind) = match (&source.kind, &target.kind) {
 372             (&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b)) => {
 373                 infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
 374                 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 375                 let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };
 376                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ref(r_b, ty))
 377             }
 378
 379             (&ty::Ref(_, ty_a, mutbl_a), &ty::RawPtr(mt_b)) => {
 380                 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 381                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 382             }
 383
 384             (&ty::RawPtr(mt_a), &ty::RawPtr(mt_b)) => {
 385                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 386             }
 387
 388             (&ty::Adt(def_a, substs_a), &ty::Adt(def_b, substs_b))
 389                 if def_a.is_struct() && def_b.is_struct() =>
 390             {
 391                 if def_a != def_b {
 392                     let source_path = tcx.def_path_str(def_a.did);
 393                     let target_path = tcx.def_path_str(def_b.did);
 394                     struct_span_err!(
 395                         tcx.sess,
 396                         span,
 397                         E0377,
 398                         "the trait `CoerceUnsized` may only be implemented \
 399                                for a coercion between structures with the same \
 400                                definition; expected `{}`, found `{}`",
 401                         source_path,
 402                         target_path
 403                     )
 404                     .emit();
 405                     return err_info;
 406                 }
 407
 408                 // Here we are considering a case of converting
 409                 // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
 410                 // which acts like a pointer to `U`, but carries along some extra data of type `T`:
 411                 //
 412                 //     struct Foo<T, U> {
 413                 //         extra: T,
 414                 //         ptr: *mut U,
 415                 //     }
 416                 //
 417                 // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
 418                 // to `Foo<T, [i32]>`. That impl would look like:
 419                 //
 420                 //   impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
 421                 //
 422                 // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
 423                 // when this coercion occurs, we would be changing the
 424                 // field `ptr` from a thin pointer of type `*mut [i32;
 425                 // 3]` to a fat pointer of type `*mut [i32]` (with
 426                 // extra data `3`).  **The purpose of this check is to
 427                 // make sure that we know how to do this conversion.**
 428                 //
 429                 // To check if this impl is legal, we would walk down
 430                 // the fields of `Foo` and consider their types with
 431                 // both substitutes. We are looking to find that
 432                 // exactly one (non-phantom) field has changed its
 433                 // type, which we will expect to be the pointer that
 434                 // is becoming fat (we could probably generalize this
 435                 // to multiple thin pointers of the same type becoming
 436                 // fat, but we don't). In this case:
 437                 //
 438                 // - `extra` has type `T` before and type `T` after
 439                 // - `ptr` has type `*mut U` before and type `*mut V` after
 440                 //
 441                 // Since just one field changed, we would then check
 442                 // that `*mut U: CoerceUnsized<*mut V>` is implemented
 443                 // (in other words, that we know how to do this
 444                 // conversion). This will work out because `U:
 445                 // Unsize<V>`, and we have a builtin rule that `*mut
 446                 // U` can be coerced to `*mut V` if `U: Unsize<V>`.
 447                 let fields = &def_a.non_enum_variant().fields;
 448                 let diff_fields = fields
 449                     .iter()
 450                     .enumerate()
 451                     .filter_map(|(i, f)| {
 452                         let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));
 453
 454                         if tcx.type_of(f.did).is_phantom_data() {
 455                             // Ignore PhantomData fields
 456                             return None;
 457                         }
 458
 459                         // Ignore fields that aren't changed; it may
 460                         // be that we could get away with subtyping or
 461                         // something more accepting, but we use
 462                         // equality because we want to be able to
 463                         // perform this check without computing
 464                         // variance where possible. (This is because
 465                         // we may have to evaluate constraint
 466                         // expressions in the course of execution.)
 467                         // See e.g., #41936.
 468                         if let Ok(ok) = infcx.at(&cause, param_env).eq(a, b) {
 469                             if ok.obligations.is_empty() {
 470                                 return None;
 471                             }
 472                         }
 473
 474                         // Collect up all fields that were significantly changed
 475                         // i.e., those that contain T in coerce_unsized T -> U
 476                         Some((i, a, b))
 477                     })
 478                     .collect::<Vec<_>>();
 479
 480                 if diff_fields.is_empty() {
 481                     struct_span_err!(
 482                         tcx.sess,
 483                         span,
 484                         E0374,
 485                         "the trait `CoerceUnsized` may only be implemented \
 486                                for a coercion between structures with one field \
 487                                being coerced, none found"
 488                     )
 489                     .emit();
 490                     return err_info;
 491                 } else if diff_fields.len() > 1 {
 492                     let item = tcx.hir().expect_item(impl_hir_id);
 493                     let span = if let ItemKind::Impl { of_trait: Some(ref t), .. } = item.kind {
 494                         t.path.span
 495                     } else {
 496                         tcx.hir().span(impl_hir_id)
 497                     };
 498
 499                     struct_span_err!(
 500                         tcx.sess,
 501                         span,
 502                         E0375,
 503                         "implementing the trait \
 504                                                     `CoerceUnsized` requires multiple \
 505                                                     coercions"
 506                     )
 507                     .note(
 508                         "`CoerceUnsized` may only be implemented for \
 509                               a coercion between structures with one field being coerced",
 510                     )
 511                     .note(&format!(
 512                         "currently, {} fields need coercions: {}",
 513                         diff_fields.len(),
 514                         diff_fields
 515                             .iter()
 516                             .map(|&(i, a, b)| {
 517                                 format!("`{}` (`{}` to `{}`)", fields[i].ident, a, b)
 518                             })
 519                             .collect::<Vec<_>>()
 520                             .join(", ")
 521                     ))
 522                     .span_label(span, "requires multiple coercions")
 523                     .emit();
 524                     return err_info;
 525                 }
 526
 527                 let (i, a, b) = diff_fields[0];
 528                 let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
 529                 (a, b, coerce_unsized_trait, Some(kind))
 530             }
 531
 532             _ => {
 533                 struct_span_err!(
 534                     tcx.sess,
 535                     span,
 536                     E0376,
 537                     "the trait `CoerceUnsized` may only be implemented \
 538                            for a coercion between structures"
 539                 )
 540                 .emit();
 541                 return err_info;
 542             }
 543         };
 544
 545         let mut fulfill_cx = TraitEngine::new(infcx.tcx);
 546
 547         // Register an obligation for `A: Trait<B>`.
 548         let cause = traits::ObligationCause::misc(span, impl_hir_id);
 549         let predicate = predicate_for_trait_def(
 550             tcx,
 551             param_env,
 552             cause,
 553             trait_def_id,
 554             0,
 555             source,
 556             &[target.into()],
 557         );
 558         fulfill_cx.register_predicate_obligation(&infcx, predicate);
 559
 560         // Check that all transitive obligations are satisfied.
 561         if let Err(errors) = fulfill_cx.select_all_or_error(&infcx) {
 562             infcx.report_fulfillment_errors(&errors, None, false);
 563         }
 564
 565         // Finally, resolve all regions.
 566         let region_scope_tree = region::ScopeTree::default();
 567         let outlives_env = OutlivesEnvironment::new(param_env);
 568         infcx.resolve_regions_and_report_errors(
 569             impl_did,
 570             &region_scope_tree,
 571             &outlives_env,
 572             SuppressRegionErrors::default(),
 573         );
 574
 575         CoerceUnsizedInfo { custom_kind: kind }
 576     })
 577 }