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 sp = match tcx.hir().expect_item(impl_did).kind {
  56         ItemKind::Impl(ref impl_) => impl_.self_ty.span,
  57         _ => bug!("expected Drop impl item"),
  58     };
  59
  60     tcx.sess.emit_err(DropImplOnWrongItem { span: sp });
  61 }
  62
  63 fn visit_implementation_of_copy(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
  64     debug!("visit_implementation_of_copy: impl_did={:?}", impl_did);
  65
  66     let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
  67
  68     let self_type = tcx.type_of(impl_did);
  69     debug!("visit_implementation_of_copy: self_type={:?} (bound)", self_type);
  70
  71     let span = tcx.hir().span(impl_hir_id);
  72     let param_env = tcx.param_env(impl_did);
  73     assert!(!self_type.has_escaping_bound_vars());
  74
  75     debug!("visit_implementation_of_copy: self_type={:?} (free)", self_type);
  76
  77     let cause = traits::ObligationCause::misc(span, impl_hir_id);
  78     match can_type_implement_copy(tcx, param_env, self_type, cause) {
  79         Ok(()) => {}
  80         Err(CopyImplementationError::InfrigingFields(fields)) => {
  81             let item = tcx.hir().expect_item(impl_did);
  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 (field, ty) in fields {
  95                 let field_span = tcx.def_span(field.did);
  96                 err.span_label(field_span, "this field does not implement `Copy`");
  97                 // Spin up a new FulfillmentContext, so we can get the _precise_ reason
  98                 // why this field does not implement Copy. This is useful because sometimes
  99                 // it is not immediately clear why Copy is not implemented for a field, since
 100                 // all we point at is the field itself.
 101                 tcx.infer_ctxt().enter(|infcx| {
 102                     let mut fulfill_cx = traits::FulfillmentContext::new_ignoring_regions();
 103                     fulfill_cx.register_bound(
 104                         &infcx,
 105                         param_env,
 106                         ty,
 107                         tcx.lang_items().copy_trait().unwrap(),
 108                         traits::ObligationCause::dummy_with_span(field_span),
 109                     );
 110                     for error in fulfill_cx.select_all_or_error(&infcx) {
 111                         let error_predicate = error.obligation.predicate;
 112                         // Only note if it's not the root obligation, otherwise it's trivial and
 113                         // should be self-explanatory (i.e. a field literally doesn't implement Copy).
 114
 115                         // FIXME: This error could be more descriptive, especially if the error_predicate
 116                         // contains a foreign type or if it's a deeply nested type...
 117                         if error_predicate != error.root_obligation.predicate {
 118                             err.span_note(
 119                                 error.obligation.cause.span,
 120                                 &format!(
 121                                     "the `Copy` impl for `{}` requires that `{}`",
 122                                     ty, error_predicate
 123                                 ),
 124                             );
 125                         }
 126                     }
 127                 });
 128             }
 129             err.emit();
 130         }
 131         Err(CopyImplementationError::NotAnAdt) => {
 132             let item = tcx.hir().expect_item(impl_did);
 133             let span =
 134                 if let ItemKind::Impl(ref impl_) = item.kind { impl_.self_ty.span } else { span };
 135
 136             tcx.sess.emit_err(CopyImplOnNonAdt { span });
 137         }
 138         Err(CopyImplementationError::HasDestructor) => {
 139             tcx.sess.emit_err(CopyImplOnTypeWithDtor { span });
 140         }
 141     }
 142 }
 143
 144 fn visit_implementation_of_coerce_unsized<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) {
 145     debug!("visit_implementation_of_coerce_unsized: impl_did={:?}", impl_did);
 146
 147     // Just compute this for the side-effects, in particular reporting
 148     // errors; other parts of the code may demand it for the info of
 149     // course.
 150     let span = tcx.def_span(impl_did);
 151     tcx.at(span).coerce_unsized_info(impl_did);
 152 }
 153
 154 fn visit_implementation_of_dispatch_from_dyn<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) {
 155     debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);
 156
 157     let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
 158     let span = tcx.hir().span(impl_hir_id);
 159
 160     let dispatch_from_dyn_trait = tcx.require_lang_item(LangItem::DispatchFromDyn, Some(span));
 161
 162     let source = tcx.type_of(impl_did);
 163     assert!(!source.has_escaping_bound_vars());
 164     let target = {
 165         let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
 166         assert_eq!(trait_ref.def_id, dispatch_from_dyn_trait);
 167
 168         trait_ref.substs.type_at(1)
 169     };
 170
 171     debug!("visit_implementation_of_dispatch_from_dyn: {:?} -> {:?}", source, target);
 172
 173     let param_env = tcx.param_env(impl_did);
 174
 175     let create_err = |msg: &str| struct_span_err!(tcx.sess, span, E0378, "{}", msg);
 176
 177     tcx.infer_ctxt().enter(|infcx| {
 178         let cause = ObligationCause::misc(span, impl_hir_id);
 179
 180         use rustc_type_ir::sty::TyKind::*;
 181         match (source.kind(), target.kind()) {
 182             (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))
 183                 if infcx.at(&cause, param_env).eq(r_a, *r_b).is_ok() && mutbl_a == *mutbl_b => {}
 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(|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.align.abi.bytes() == 1 {
 221                                 // ignore ZST fields with alignment of 1 byte
 222                                 return false;
 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.name, ty_a,
 236                                 ))
 237                                 .emit();
 238
 239                                 return false;
 240                             }
 241                         }
 242
 243                         return true;
 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.name,
 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 = <dyn 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                     let errors = fulfill_cx.select_all_or_error(&infcx);
 299                     if !errors.is_empty() {
 300                         infcx.report_fulfillment_errors(&errors, None, false);
 301                     }
 302
 303                     // Finally, resolve all regions.
 304                     let outlives_env = OutlivesEnvironment::new(param_env);
 305                     infcx.resolve_regions_and_report_errors(
 306                         impl_did.to_def_id(),
 307                         &outlives_env,
 308                         RegionckMode::default(),
 309                     );
 310                 }
 311             }
 312             _ => {
 313                 create_err(
 314                     "the trait `DispatchFromDyn` may only be implemented \
 315                         for a coercion between structures",
 316                 )
 317                 .emit();
 318             }
 319         }
 320     })
 321 }
 322
 323 pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUnsizedInfo {
 324     debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
 325
 326     // this provider should only get invoked for local def-ids
 327     let impl_did = impl_did.expect_local();
 328     let span = tcx.def_span(impl_did);
 329
 330     let coerce_unsized_trait = tcx.require_lang_item(LangItem::CoerceUnsized, Some(span));
 331
 332     let unsize_trait = tcx.lang_items().require(LangItem::Unsize).unwrap_or_else(|err| {
 333         tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err));
 334     });
 335
 336     let source = tcx.type_of(impl_did);
 337     let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
 338     assert_eq!(trait_ref.def_id, coerce_unsized_trait);
 339     let target = trait_ref.substs.type_at(1);
 340     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)", source, target);
 341
 342     let param_env = tcx.param_env(impl_did);
 343     assert!(!source.has_escaping_bound_vars());
 344
 345     let err_info = CoerceUnsizedInfo { custom_kind: None };
 346
 347     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);
 348
 349     tcx.infer_ctxt().enter(|infcx| {
 350         let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
 351         let cause = ObligationCause::misc(span, impl_hir_id);
 352         let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
 353                            mt_b: ty::TypeAndMut<'tcx>,
 354                            mk_ptr: &dyn Fn(Ty<'tcx>) -> Ty<'tcx>| {
 355             if (mt_a.mutbl, mt_b.mutbl) == (hir::Mutability::Not, hir::Mutability::Mut) {
 356                 infcx
 357                     .report_mismatched_types(
 358                         &cause,
 359                         mk_ptr(mt_b.ty),
 360                         target,
 361                         ty::error::TypeError::Mutability,
 362                     )
 363                     .emit();
 364             }
 365             (mt_a.ty, mt_b.ty, unsize_trait, None)
 366         };
 367         let (source, target, trait_def_id, kind) = match (source.kind(), target.kind()) {
 368             (&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b)) => {
 369                 infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
 370                 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 371                 let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };
 372                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ref(r_b, ty))
 373             }
 374
 375             (&ty::Ref(_, ty_a, mutbl_a), &ty::RawPtr(mt_b)) => {
 376                 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 377                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 378             }
 379
 380             (&ty::RawPtr(mt_a), &ty::RawPtr(mt_b)) => {
 381                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 382             }
 383
 384             (&ty::Adt(def_a, substs_a), &ty::Adt(def_b, substs_b))
 385                 if def_a.is_struct() && def_b.is_struct() =>
 386             {
 387                 if def_a != def_b {
 388                     let source_path = tcx.def_path_str(def_a.did());
 389                     let target_path = tcx.def_path_str(def_b.did());
 390                     struct_span_err!(
 391                         tcx.sess,
 392                         span,
 393                         E0377,
 394                         "the trait `CoerceUnsized` may only be implemented \
 395                                for a coercion between structures with the same \
 396                                definition; expected `{}`, found `{}`",
 397                         source_path,
 398                         target_path
 399                     )
 400                     .emit();
 401                     return err_info;
 402                 }
 403
 404                 // Here we are considering a case of converting
 405                 // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
 406                 // which acts like a pointer to `U`, but carries along some extra data of type `T`:
 407                 //
 408                 //     struct Foo<T, U> {
 409                 //         extra: T,
 410                 //         ptr: *mut U,
 411                 //     }
 412                 //
 413                 // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
 414                 // to `Foo<T, [i32]>`. That impl would look like:
 415                 //
 416                 //   impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
 417                 //
 418                 // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
 419                 // when this coercion occurs, we would be changing the
 420                 // field `ptr` from a thin pointer of type `*mut [i32;
 421                 // 3]` to a fat pointer of type `*mut [i32]` (with
 422                 // extra data `3`).  **The purpose of this check is to
 423                 // make sure that we know how to do this conversion.**
 424                 //
 425                 // To check if this impl is legal, we would walk down
 426                 // the fields of `Foo` and consider their types with
 427                 // both substitutes. We are looking to find that
 428                 // exactly one (non-phantom) field has changed its
 429                 // type, which we will expect to be the pointer that
 430                 // is becoming fat (we could probably generalize this
 431                 // to multiple thin pointers of the same type becoming
 432                 // fat, but we don't). In this case:
 433                 //
 434                 // - `extra` has type `T` before and type `T` after
 435                 // - `ptr` has type `*mut U` before and type `*mut V` after
 436                 //
 437                 // Since just one field changed, we would then check
 438                 // that `*mut U: CoerceUnsized<*mut V>` is implemented
 439                 // (in other words, that we know how to do this
 440                 // conversion). This will work out because `U:
 441                 // Unsize<V>`, and we have a builtin rule that `*mut
 442                 // U` can be coerced to `*mut V` if `U: Unsize<V>`.
 443                 let fields = &def_a.non_enum_variant().fields;
 444                 let diff_fields = fields
 445                     .iter()
 446                     .enumerate()
 447                     .filter_map(|(i, f)| {
 448                         let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));
 449
 450                         if tcx.type_of(f.did).is_phantom_data() {
 451                             // Ignore PhantomData fields
 452                             return None;
 453                         }
 454
 455                         // Ignore fields that aren't changed; it may
 456                         // be that we could get away with subtyping or
 457                         // something more accepting, but we use
 458                         // equality because we want to be able to
 459                         // perform this check without computing
 460                         // variance where possible. (This is because
 461                         // we may have to evaluate constraint
 462                         // expressions in the course of execution.)
 463                         // See e.g., #41936.
 464                         if let Ok(ok) = infcx.at(&cause, param_env).eq(a, b) {
 465                             if ok.obligations.is_empty() {
 466                                 return None;
 467                             }
 468                         }
 469
 470                         // Collect up all fields that were significantly changed
 471                         // i.e., those that contain T in coerce_unsized T -> U
 472                         Some((i, a, b))
 473                     })
 474                     .collect::<Vec<_>>();
 475
 476                 if diff_fields.is_empty() {
 477                     struct_span_err!(
 478                         tcx.sess,
 479                         span,
 480                         E0374,
 481                         "the trait `CoerceUnsized` may only be implemented \
 482                                for a coercion between structures with one field \
 483                                being coerced, none found"
 484                     )
 485                     .emit();
 486                     return err_info;
 487                 } else if diff_fields.len() > 1 {
 488                     let item = tcx.hir().expect_item(impl_did);
 489                     let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(ref t), .. }) =
 490                         item.kind
 491                     {
 492                         t.path.span
 493                     } else {
 494                         tcx.def_span(impl_did)
 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].name, 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 = <dyn 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         let errors = fulfill_cx.select_all_or_error(&infcx);
 560         if !errors.is_empty() {
 561             infcx.report_fulfillment_errors(&errors, None, false);
 562         }
 563
 564         // Finally, resolve all regions.
 565         let outlives_env = OutlivesEnvironment::new(param_env);
 566         infcx.resolve_regions_and_report_errors(
 567             impl_did.to_def_id(),
 568             &outlives_env,
 569             RegionckMode::default(),
 570         );
 571
 572         CoerceUnsizedInfo { custom_kind: kind }
 573     })
 574 }