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     match can_type_implement_copy(tcx, param_env, self_type) {
  78         Ok(()) => {}
  79         Err(CopyImplementationError::InfrigingFields(fields)) => {
  80             let item = tcx.hir().expect_item(impl_did);
  81             let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(ref tr), .. }) = item.kind {
  82                 tr.path.span
  83             } else {
  84                 span
  85             };
  86
  87             let mut err = struct_span_err!(
  88                 tcx.sess,
  89                 span,
  90                 E0204,
  91                 "the trait `Copy` may not be implemented for this type"
  92             );
  93             for span in fields.iter().map(|f| tcx.def_span(f.did)) {
  94                 err.span_label(span, "this field does not implement `Copy`");
  95             }
  96             err.emit()
  97         }
  98         Err(CopyImplementationError::NotAnAdt) => {
  99             let item = tcx.hir().expect_item(impl_did);
 100             let span =
 101                 if let ItemKind::Impl(ref impl_) = item.kind { impl_.self_ty.span } else { span };
 102
 103             tcx.sess.emit_err(CopyImplOnNonAdt { span });
 104         }
 105         Err(CopyImplementationError::HasDestructor) => {
 106             tcx.sess.emit_err(CopyImplOnTypeWithDtor { span });
 107         }
 108     }
 109 }
 110
 111 fn visit_implementation_of_coerce_unsized<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) {
 112     debug!("visit_implementation_of_coerce_unsized: impl_did={:?}", impl_did);
 113
 114     // Just compute this for the side-effects, in particular reporting
 115     // errors; other parts of the code may demand it for the info of
 116     // course.
 117     let span = tcx.def_span(impl_did);
 118     tcx.at(span).coerce_unsized_info(impl_did);
 119 }
 120
 121 fn visit_implementation_of_dispatch_from_dyn<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) {
 122     debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);
 123
 124     let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
 125     let span = tcx.hir().span(impl_hir_id);
 126
 127     let dispatch_from_dyn_trait = tcx.require_lang_item(LangItem::DispatchFromDyn, Some(span));
 128
 129     let source = tcx.type_of(impl_did);
 130     assert!(!source.has_escaping_bound_vars());
 131     let target = {
 132         let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
 133         assert_eq!(trait_ref.def_id, dispatch_from_dyn_trait);
 134
 135         trait_ref.substs.type_at(1)
 136     };
 137
 138     debug!("visit_implementation_of_dispatch_from_dyn: {:?} -> {:?}", source, target);
 139
 140     let param_env = tcx.param_env(impl_did);
 141
 142     let create_err = |msg: &str| struct_span_err!(tcx.sess, span, E0378, "{}", msg);
 143
 144     tcx.infer_ctxt().enter(|infcx| {
 145         let cause = ObligationCause::misc(span, impl_hir_id);
 146
 147         use ty::TyKind::*;
 148         match (source.kind(), target.kind()) {
 149             (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))
 150                 if infcx.at(&cause, param_env).eq(r_a, *r_b).is_ok() && mutbl_a == *mutbl_b => {}
 151             (&RawPtr(tm_a), &RawPtr(tm_b)) if tm_a.mutbl == tm_b.mutbl => (),
 152             (&Adt(def_a, substs_a), &Adt(def_b, substs_b))
 153                 if def_a.is_struct() && def_b.is_struct() =>
 154             {
 155                 if def_a != def_b {
 156                     let source_path = tcx.def_path_str(def_a.did);
 157                     let target_path = tcx.def_path_str(def_b.did);
 158
 159                     create_err(&format!(
 160                         "the trait `DispatchFromDyn` may only be implemented \
 161                                 for a coercion between structures with the same \
 162                                 definition; expected `{}`, found `{}`",
 163                         source_path, target_path,
 164                     ))
 165                     .emit();
 166
 167                     return;
 168                 }
 169
 170                 if def_a.repr.c() || def_a.repr.packed() {
 171                     create_err(
 172                         "structs implementing `DispatchFromDyn` may not have \
 173                              `#[repr(packed)]` or `#[repr(C)]`",
 174                     )
 175                     .emit();
 176                 }
 177
 178                 let fields = &def_a.non_enum_variant().fields;
 179
 180                 let coerced_fields = fields
 181                     .iter()
 182                     .filter(|field| {
 183                         let ty_a = field.ty(tcx, substs_a);
 184                         let ty_b = field.ty(tcx, substs_b);
 185
 186                         if let Ok(layout) = tcx.layout_of(param_env.and(ty_a)) {
 187                             if layout.is_zst() && layout.align.abi.bytes() == 1 {
 188                                 // ignore ZST fields with alignment of 1 byte
 189                                 return false;
 190                             }
 191                         }
 192
 193                         if let Ok(ok) = infcx.at(&cause, param_env).eq(ty_a, ty_b) {
 194                             if ok.obligations.is_empty() {
 195                                 create_err(
 196                                     "the trait `DispatchFromDyn` may only be implemented \
 197                                      for structs containing the field being coerced, \
 198                                      ZST fields with 1 byte alignment, and nothing else",
 199                                 )
 200                                 .note(&format!(
 201                                     "extra field `{}` of type `{}` is not allowed",
 202                                     field.name, ty_a,
 203                                 ))
 204                                 .emit();
 205
 206                                 return false;
 207                             }
 208                         }
 209
 210                         return true;
 211                     })
 212                     .collect::<Vec<_>>();
 213
 214                 if coerced_fields.is_empty() {
 215                     create_err(
 216                         "the trait `DispatchFromDyn` may only be implemented \
 217                             for a coercion between structures with a single field \
 218                             being coerced, none found",
 219                     )
 220                     .emit();
 221                 } else if coerced_fields.len() > 1 {
 222                     create_err(
 223                         "implementing the `DispatchFromDyn` trait requires multiple coercions",
 224                     )
 225                     .note(
 226                         "the trait `DispatchFromDyn` may only be implemented \
 227                                 for a coercion between structures with a single field \
 228                                 being coerced",
 229                     )
 230                     .note(&format!(
 231                         "currently, {} fields need coercions: {}",
 232                         coerced_fields.len(),
 233                         coerced_fields
 234                             .iter()
 235                             .map(|field| {
 236                                 format!(
 237                                     "`{}` (`{}` to `{}`)",
 238                                     field.name,
 239                                     field.ty(tcx, substs_a),
 240                                     field.ty(tcx, substs_b),
 241                                 )
 242                             })
 243                             .collect::<Vec<_>>()
 244                             .join(", ")
 245                     ))
 246                     .emit();
 247                 } else {
 248                     let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
 249
 250                     for field in coerced_fields {
 251                         let predicate = predicate_for_trait_def(
 252                             tcx,
 253                             param_env,
 254                             cause.clone(),
 255                             dispatch_from_dyn_trait,
 256                             0,
 257                             field.ty(tcx, substs_a),
 258                             &[field.ty(tcx, substs_b).into()],
 259                         );
 260
 261                         fulfill_cx.register_predicate_obligation(&infcx, predicate);
 262                     }
 263
 264                     // Check that all transitive obligations are satisfied.
 265                     let errors = fulfill_cx.select_all_or_error(&infcx);
 266                     if !errors.is_empty() {
 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>(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_did = impl_did.expect_local();
 295     let span = tcx.def_span(impl_did);
 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 impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
 318         let cause = ObligationCause::misc(span, impl_hir_id);
 319         let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
 320                            mt_b: ty::TypeAndMut<'tcx>,
 321                            mk_ptr: &dyn Fn(Ty<'tcx>) -> Ty<'tcx>| {
 322             if (mt_a.mutbl, mt_b.mutbl) == (hir::Mutability::Not, hir::Mutability::Mut) {
 323                 infcx
 324                     .report_mismatched_types(
 325                         &cause,
 326                         mk_ptr(mt_b.ty),
 327                         target,
 328                         ty::error::TypeError::Mutability,
 329                     )
 330                     .emit();
 331             }
 332             (mt_a.ty, mt_b.ty, unsize_trait, None)
 333         };
 334         let (source, target, trait_def_id, kind) = match (source.kind(), target.kind()) {
 335             (&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b)) => {
 336                 infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
 337                 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 338                 let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };
 339                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ref(r_b, ty))
 340             }
 341
 342             (&ty::Ref(_, ty_a, mutbl_a), &ty::RawPtr(mt_b)) => {
 343                 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 344                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 345             }
 346
 347             (&ty::RawPtr(mt_a), &ty::RawPtr(mt_b)) => {
 348                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 349             }
 350
 351             (&ty::Adt(def_a, substs_a), &ty::Adt(def_b, substs_b))
 352                 if def_a.is_struct() && def_b.is_struct() =>
 353             {
 354                 if def_a != def_b {
 355                     let source_path = tcx.def_path_str(def_a.did);
 356                     let target_path = tcx.def_path_str(def_b.did);
 357                     struct_span_err!(
 358                         tcx.sess,
 359                         span,
 360                         E0377,
 361                         "the trait `CoerceUnsized` may only be implemented \
 362                                for a coercion between structures with the same \
 363                                definition; expected `{}`, found `{}`",
 364                         source_path,
 365                         target_path
 366                     )
 367                     .emit();
 368                     return err_info;
 369                 }
 370
 371                 // Here we are considering a case of converting
 372                 // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
 373                 // which acts like a pointer to `U`, but carries along some extra data of type `T`:
 374                 //
 375                 //     struct Foo<T, U> {
 376                 //         extra: T,
 377                 //         ptr: *mut U,
 378                 //     }
 379                 //
 380                 // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
 381                 // to `Foo<T, [i32]>`. That impl would look like:
 382                 //
 383                 //   impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
 384                 //
 385                 // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
 386                 // when this coercion occurs, we would be changing the
 387                 // field `ptr` from a thin pointer of type `*mut [i32;
 388                 // 3]` to a fat pointer of type `*mut [i32]` (with
 389                 // extra data `3`).  **The purpose of this check is to
 390                 // make sure that we know how to do this conversion.**
 391                 //
 392                 // To check if this impl is legal, we would walk down
 393                 // the fields of `Foo` and consider their types with
 394                 // both substitutes. We are looking to find that
 395                 // exactly one (non-phantom) field has changed its
 396                 // type, which we will expect to be the pointer that
 397                 // is becoming fat (we could probably generalize this
 398                 // to multiple thin pointers of the same type becoming
 399                 // fat, but we don't). In this case:
 400                 //
 401                 // - `extra` has type `T` before and type `T` after
 402                 // - `ptr` has type `*mut U` before and type `*mut V` after
 403                 //
 404                 // Since just one field changed, we would then check
 405                 // that `*mut U: CoerceUnsized<*mut V>` is implemented
 406                 // (in other words, that we know how to do this
 407                 // conversion). This will work out because `U:
 408                 // Unsize<V>`, and we have a builtin rule that `*mut
 409                 // U` can be coerced to `*mut V` if `U: Unsize<V>`.
 410                 let fields = &def_a.non_enum_variant().fields;
 411                 let diff_fields = fields
 412                     .iter()
 413                     .enumerate()
 414                     .filter_map(|(i, f)| {
 415                         let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));
 416
 417                         if tcx.type_of(f.did).is_phantom_data() {
 418                             // Ignore PhantomData fields
 419                             return None;
 420                         }
 421
 422                         // Ignore fields that aren't changed; it may
 423                         // be that we could get away with subtyping or
 424                         // something more accepting, but we use
 425                         // equality because we want to be able to
 426                         // perform this check without computing
 427                         // variance where possible. (This is because
 428                         // we may have to evaluate constraint
 429                         // expressions in the course of execution.)
 430                         // See e.g., #41936.
 431                         if let Ok(ok) = infcx.at(&cause, param_env).eq(a, b) {
 432                             if ok.obligations.is_empty() {
 433                                 return None;
 434                             }
 435                         }
 436
 437                         // Collect up all fields that were significantly changed
 438                         // i.e., those that contain T in coerce_unsized T -> U
 439                         Some((i, a, b))
 440                     })
 441                     .collect::<Vec<_>>();
 442
 443                 if diff_fields.is_empty() {
 444                     struct_span_err!(
 445                         tcx.sess,
 446                         span,
 447                         E0374,
 448                         "the trait `CoerceUnsized` may only be implemented \
 449                                for a coercion between structures with one field \
 450                                being coerced, none found"
 451                     )
 452                     .emit();
 453                     return err_info;
 454                 } else if diff_fields.len() > 1 {
 455                     let item = tcx.hir().expect_item(impl_did);
 456                     let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(ref t), .. }) =
 457                         item.kind
 458                     {
 459                         t.path.span
 460                     } else {
 461                         tcx.def_span(impl_did)
 462                     };
 463
 464                     struct_span_err!(
 465                         tcx.sess,
 466                         span,
 467                         E0375,
 468                         "implementing the trait \
 469                                                     `CoerceUnsized` requires multiple \
 470                                                     coercions"
 471                     )
 472                     .note(
 473                         "`CoerceUnsized` may only be implemented for \
 474                               a coercion between structures with one field being coerced",
 475                     )
 476                     .note(&format!(
 477                         "currently, {} fields need coercions: {}",
 478                         diff_fields.len(),
 479                         diff_fields
 480                             .iter()
 481                             .map(|&(i, a, b)| {
 482                                 format!("`{}` (`{}` to `{}`)", fields[i].name, a, b)
 483                             })
 484                             .collect::<Vec<_>>()
 485                             .join(", ")
 486                     ))
 487                     .span_label(span, "requires multiple coercions")
 488                     .emit();
 489                     return err_info;
 490                 }
 491
 492                 let (i, a, b) = diff_fields[0];
 493                 let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
 494                 (a, b, coerce_unsized_trait, Some(kind))
 495             }
 496
 497             _ => {
 498                 struct_span_err!(
 499                     tcx.sess,
 500                     span,
 501                     E0376,
 502                     "the trait `CoerceUnsized` may only be implemented \
 503                            for a coercion between structures"
 504                 )
 505                 .emit();
 506                 return err_info;
 507             }
 508         };
 509
 510         let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
 511
 512         // Register an obligation for `A: Trait<B>`.
 513         let cause = traits::ObligationCause::misc(span, impl_hir_id);
 514         let predicate = predicate_for_trait_def(
 515             tcx,
 516             param_env,
 517             cause,
 518             trait_def_id,
 519             0,
 520             source,
 521             &[target.into()],
 522         );
 523         fulfill_cx.register_predicate_obligation(&infcx, predicate);
 524
 525         // Check that all transitive obligations are satisfied.
 526         let errors = fulfill_cx.select_all_or_error(&infcx);
 527         if !errors.is_empty() {
 528             infcx.report_fulfillment_errors(&errors, None, false);
 529         }
 530
 531         // Finally, resolve all regions.
 532         let outlives_env = OutlivesEnvironment::new(param_env);
 533         infcx.resolve_regions_and_report_errors(
 534             impl_did.to_def_id(),
 535             &outlives_env,
 536             RegionckMode::default(),
 537         );
 538
 539         CoerceUnsizedInfo { custom_kind: kind }
 540     })
 541 }