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