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_errors::struct_span_err;
   5 use rustc_hir as hir;
   6 use rustc_hir::def_id::DefId;
   7 use rustc_hir::ItemKind;
   8 use rustc_infer::infer;
   9 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
  10 use rustc_infer::infer::{RegionckMode, TyCtxtInferExt};
  11 use rustc_middle::middle::lang_items::UnsizeTraitLangItem;
  12 use rustc_middle::middle::region;
  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>, DefId),
  39     {
  40         if Some(self.trait_def_id) == trait_def_id {
  41             for &impl_id in self.tcx.hir().trait_impls(self.trait_def_id) {
  42                 let impl_def_id = self.tcx.hir().local_def_id(impl_id);
  43                 f(self.tcx, impl_def_id);
  44             }
  45         }
  46         self
  47     }
  48 }
  49
  50 fn visit_implementation_of_drop(tcx: TyCtxt<'_>, impl_did: DefId) {
  51     // Destructors only work on nominal types.
  52     if let ty::Adt(..) | ty::Error = tcx.type_of(impl_did).kind {
  53         return;
  54     }
  55
  56     let impl_hir_id = tcx.hir().as_local_hir_id(impl_did).expect("foreign Drop impl on non-ADT");
  57     let sp = match tcx.hir().expect_item(impl_hir_id).kind {
  58         ItemKind::Impl { self_ty, .. } => self_ty.span,
  59         _ => bug!("expected Drop impl item"),
  60     };
  61
  62     struct_span_err!(
  63         tcx.sess,
  64         sp,
  65         E0120,
  66         "the `Drop` trait may only be implemented for structs, enums, and unions",
  67     )
  68     .span_label(sp, "must be a struct, enum, or union")
  69     .emit();
  70 }
  71
  72 fn visit_implementation_of_copy(tcx: TyCtxt<'_>, impl_did: DefId) {
  73     debug!("visit_implementation_of_copy: impl_did={:?}", impl_did);
  74
  75     let impl_hir_id = if let Some(n) = tcx.hir().as_local_hir_id(impl_did) {
  76         n
  77     } else {
  78         debug!("visit_implementation_of_copy(): impl not in this crate");
  79         return;
  80     };
  81
  82     let self_type = tcx.type_of(impl_did);
  83     debug!("visit_implementation_of_copy: self_type={:?} (bound)", self_type);
  84
  85     let span = tcx.hir().span(impl_hir_id);
  86     let param_env = tcx.param_env(impl_did);
  87     assert!(!self_type.has_escaping_bound_vars());
  88
  89     debug!("visit_implementation_of_copy: self_type={:?} (free)", self_type);
  90
  91     match can_type_implement_copy(tcx, param_env, self_type) {
  92         Ok(()) => {}
  93         Err(CopyImplementationError::InfrigingFields(fields)) => {
  94             let item = tcx.hir().expect_item(impl_hir_id);
  95             let span = if let ItemKind::Impl { of_trait: Some(ref tr), .. } = item.kind {
  96                 tr.path.span
  97             } else {
  98                 span
  99             };
 100
 101             let mut err = struct_span_err!(
 102                 tcx.sess,
 103                 span,
 104                 E0204,
 105                 "the trait `Copy` may not be implemented for this type"
 106             );
 107             for span in fields.iter().map(|f| tcx.def_span(f.did)) {
 108                 err.span_label(span, "this field does not implement `Copy`");
 109             }
 110             err.emit()
 111         }
 112         Err(CopyImplementationError::NotAnAdt) => {
 113             let item = tcx.hir().expect_item(impl_hir_id);
 114             let span =
 115                 if let ItemKind::Impl { self_ty, .. } = item.kind { self_ty.span } else { span };
 116
 117             struct_span_err!(
 118                 tcx.sess,
 119                 span,
 120                 E0206,
 121                 "the trait `Copy` may not be implemented for this type"
 122             )
 123             .span_label(span, "type is not a structure or enumeration")
 124             .emit();
 125         }
 126         Err(CopyImplementationError::HasDestructor) => {
 127             struct_span_err!(
 128                 tcx.sess,
 129                 span,
 130                 E0184,
 131                 "the trait `Copy` may not be implemented for this type; the \
 132                               type has a destructor"
 133             )
 134             .span_label(span, "Copy not allowed on types with destructors")
 135             .emit();
 136         }
 137     }
 138 }
 139
 140 fn visit_implementation_of_coerce_unsized(tcx: TyCtxt<'tcx>, impl_did: DefId) {
 141     debug!("visit_implementation_of_coerce_unsized: impl_did={:?}", impl_did);
 142
 143     // Just compute this for the side-effects, in particular reporting
 144     // errors; other parts of the code may demand it for the info of
 145     // course.
 146     if impl_did.is_local() {
 147         let span = tcx.def_span(impl_did);
 148         tcx.at(span).coerce_unsized_info(impl_did);
 149     }
 150 }
 151
 152 fn visit_implementation_of_dispatch_from_dyn(tcx: TyCtxt<'_>, impl_did: DefId) {
 153     debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);
 154     if impl_did.is_local() {
 155         let dispatch_from_dyn_trait = tcx.lang_items().dispatch_from_dyn_trait().unwrap();
 156
 157         let impl_hir_id = tcx.hir().as_local_hir_id(impl_did).unwrap();
 158         let span = tcx.hir().span(impl_hir_id);
 159
 160         let source = tcx.type_of(impl_did);
 161         assert!(!source.has_escaping_bound_vars());
 162         let target = {
 163             let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
 164             assert_eq!(trait_ref.def_id, dispatch_from_dyn_trait);
 165
 166             trait_ref.substs.type_at(1)
 167         };
 168
 169         debug!("visit_implementation_of_dispatch_from_dyn: {:?} -> {:?}", source, target);
 170
 171         let param_env = tcx.param_env(impl_did);
 172
 173         let create_err = |msg: &str| struct_span_err!(tcx.sess, span, E0378, "{}", msg);
 174
 175         tcx.infer_ctxt().enter(|infcx| {
 176             let cause = ObligationCause::misc(span, impl_hir_id);
 177
 178             use ty::TyKind::*;
 179             match (&source.kind, &target.kind) {
 180                 (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))
 181                     if infcx.at(&cause, param_env).eq(r_a, r_b).is_ok() && mutbl_a == *mutbl_b => {}
 182                 (&RawPtr(tm_a), &RawPtr(tm_b)) if tm_a.mutbl == tm_b.mutbl => (),
 183                 (&Adt(def_a, substs_a), &Adt(def_b, substs_b))
 184                     if def_a.is_struct() && def_b.is_struct() =>
 185                 {
 186                     if def_a != def_b {
 187                         let source_path = tcx.def_path_str(def_a.did);
 188                         let target_path = tcx.def_path_str(def_b.did);
 189
 190                         create_err(&format!(
 191                             "the trait `DispatchFromDyn` may only be implemented \
 192                                 for a coercion between structures with the same \
 193                                 definition; expected `{}`, found `{}`",
 194                             source_path, target_path,
 195                         ))
 196                         .emit();
 197
 198                         return;
 199                     }
 200
 201                     if def_a.repr.c() || def_a.repr.packed() {
 202                         create_err(
 203                             "structs implementing `DispatchFromDyn` may not have \
 204                              `#[repr(packed)]` or `#[repr(C)]`",
 205                         )
 206                         .emit();
 207                     }
 208
 209                     let fields = &def_a.non_enum_variant().fields;
 210
 211                     let coerced_fields = fields
 212                         .iter()
 213                         .filter_map(|field| {
 214                             let ty_a = field.ty(tcx, substs_a);
 215                             let ty_b = field.ty(tcx, substs_b);
 216
 217                             if let Ok(layout) = tcx.layout_of(param_env.and(ty_a)) {
 218                                 if layout.is_zst() && layout.align.abi.bytes() == 1 {
 219                                     // ignore ZST fields with alignment of 1 byte
 220                                     return None;
 221                                 }
 222                             }
 223
 224                             if let Ok(ok) = infcx.at(&cause, param_env).eq(ty_a, ty_b) {
 225                                 if ok.obligations.is_empty() {
 226                                     create_err(
 227                                         "the trait `DispatchFromDyn` may only be implemented \
 228                                      for structs containing the field being coerced, \
 229                                      ZST fields with 1 byte alignment, and nothing else",
 230                                     )
 231                                     .note(&format!(
 232                                         "extra field `{}` of type `{}` is not allowed",
 233                                         field.ident, ty_a,
 234                                     ))
 235                                     .emit();
 236
 237                                     return None;
 238                                 }
 239                             }
 240
 241                             Some(field)
 242                         })
 243                         .collect::<Vec<_>>();
 244
 245                     if coerced_fields.is_empty() {
 246                         create_err(
 247                             "the trait `DispatchFromDyn` may only be implemented \
 248                             for a coercion between structures with a single field \
 249                             being coerced, none found",
 250                         )
 251                         .emit();
 252                     } else if coerced_fields.len() > 1 {
 253                         create_err(
 254                             "implementing the `DispatchFromDyn` trait requires multiple coercions",
 255                         )
 256                         .note(
 257                             "the trait `DispatchFromDyn` may only be implemented \
 258                                 for a coercion between structures with a single field \
 259                                 being coerced",
 260                         )
 261                         .note(&format!(
 262                             "currently, {} fields need coercions: {}",
 263                             coerced_fields.len(),
 264                             coerced_fields
 265                                 .iter()
 266                                 .map(|field| {
 267                                     format!(
 268                                         "`{}` (`{}` to `{}`)",
 269                                         field.ident,
 270                                         field.ty(tcx, substs_a),
 271                                         field.ty(tcx, substs_b),
 272                                     )
 273                                 })
 274                                 .collect::<Vec<_>>()
 275                                 .join(", ")
 276                         ))
 277                         .emit();
 278                     } else {
 279                         let mut fulfill_cx = TraitEngine::new(infcx.tcx);
 280
 281                         for field in coerced_fields {
 282                             let predicate = predicate_for_trait_def(
 283                                 tcx,
 284                                 param_env,
 285                                 cause.clone(),
 286                                 dispatch_from_dyn_trait,
 287                                 0,
 288                                 field.ty(tcx, substs_a),
 289                                 &[field.ty(tcx, substs_b).into()],
 290                             );
 291
 292                             fulfill_cx.register_predicate_obligation(&infcx, predicate);
 293                         }
 294
 295                         // Check that all transitive obligations are satisfied.
 296                         if let Err(errors) = fulfill_cx.select_all_or_error(&infcx) {
 297                             infcx.report_fulfillment_errors(&errors, None, false);
 298                         }
 299
 300                         // Finally, resolve all regions.
 301                         let region_scope_tree = region::ScopeTree::default();
 302                         let outlives_env = OutlivesEnvironment::new(param_env);
 303                         infcx.resolve_regions_and_report_errors(
 304                             impl_did,
 305                             &region_scope_tree,
 306                             &outlives_env,
 307                             RegionckMode::default(),
 308                         );
 309                     }
 310                 }
 311                 _ => {
 312                     create_err(
 313                         "the trait `DispatchFromDyn` may only be implemented \
 314                         for a coercion between structures",
 315                     )
 316                     .emit();
 317                 }
 318             }
 319         })
 320     }
 321 }
 322
 323 pub fn coerce_unsized_info(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUnsizedInfo {
 324     debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
 325     let coerce_unsized_trait = tcx.lang_items().coerce_unsized_trait().unwrap();
 326
 327     let unsize_trait = tcx.lang_items().require(UnsizeTraitLangItem).unwrap_or_else(|err| {
 328         tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err));
 329     });
 330
 331     // this provider should only get invoked for local def-ids
 332     let impl_hir_id = tcx.hir().as_local_hir_id(impl_did).unwrap_or_else(|| {
 333         bug!("coerce_unsized_info: invoked for non-local def-id {:?}", impl_did)
 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 span = tcx.hir().span(impl_hir_id);
 343     let param_env = tcx.param_env(impl_did);
 344     assert!(!source.has_escaping_bound_vars());
 345
 346     let err_info = CoerceUnsizedInfo { custom_kind: None };
 347
 348     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);
 349
 350     tcx.infer_ctxt().enter(|infcx| {
 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_hir_id);
 489                     let span = if let ItemKind::Impl { of_trait: Some(ref t), .. } = item.kind {
 490                         t.path.span
 491                     } else {
 492                         tcx.hir().span(impl_hir_id)
 493                     };
 494
 495                     struct_span_err!(
 496                         tcx.sess,
 497                         span,
 498                         E0375,
 499                         "implementing the trait \
 500                                                     `CoerceUnsized` requires multiple \
 501                                                     coercions"
 502                     )
 503                     .note(
 504                         "`CoerceUnsized` may only be implemented for \
 505                               a coercion between structures with one field being coerced",
 506                     )
 507                     .note(&format!(
 508                         "currently, {} fields need coercions: {}",
 509                         diff_fields.len(),
 510                         diff_fields
 511                             .iter()
 512                             .map(|&(i, a, b)| {
 513                                 format!("`{}` (`{}` to `{}`)", fields[i].ident, a, b)
 514                             })
 515                             .collect::<Vec<_>>()
 516                             .join(", ")
 517                     ))
 518                     .span_label(span, "requires multiple coercions")
 519                     .emit();
 520                     return err_info;
 521                 }
 522
 523                 let (i, a, b) = diff_fields[0];
 524                 let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
 525                 (a, b, coerce_unsized_trait, Some(kind))
 526             }
 527
 528             _ => {
 529                 struct_span_err!(
 530                     tcx.sess,
 531                     span,
 532                     E0376,
 533                     "the trait `CoerceUnsized` may only be implemented \
 534                            for a coercion between structures"
 535                 )
 536                 .emit();
 537                 return err_info;
 538             }
 539         };
 540
 541         let mut fulfill_cx = TraitEngine::new(infcx.tcx);
 542
 543         // Register an obligation for `A: Trait<B>`.
 544         let cause = traits::ObligationCause::misc(span, impl_hir_id);
 545         let predicate = predicate_for_trait_def(
 546             tcx,
 547             param_env,
 548             cause,
 549             trait_def_id,
 550             0,
 551             source,
 552             &[target.into()],
 553         );
 554         fulfill_cx.register_predicate_obligation(&infcx, predicate);
 555
 556         // Check that all transitive obligations are satisfied.
 557         if let Err(errors) = fulfill_cx.select_all_or_error(&infcx) {
 558             infcx.report_fulfillment_errors(&errors, None, false);
 559         }
 560
 561         // Finally, resolve all regions.
 562         let region_scope_tree = region::ScopeTree::default();
 563         let outlives_env = OutlivesEnvironment::new(param_env);
 564         infcx.resolve_regions_and_report_errors(
 565             impl_did,
 566             &region_scope_tree,
 567             &outlives_env,
 568             RegionckMode::default(),
 569         );
 570
 571         CoerceUnsizedInfo { custom_kind: kind }
 572     })
 573 }