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