compiler/rustc_hir_analysis/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, MultiSpan};
   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::TyCtxtInferExt;
  13 use rustc_middle::ty::adjustment::CoerceUnsizedInfo;
  14 use rustc_middle::ty::{self, suggest_constraining_type_params, Ty, TyCtxt, TypeVisitable};
  15 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt;
  16 use rustc_trait_selection::traits::misc::{
  17     type_allowed_to_implement_copy, CopyImplementationError,
  18 };
  19 use rustc_trait_selection::traits::predicate_for_trait_def;
  20 use rustc_trait_selection::traits::{self, ObligationCause};
  21 use std::collections::BTreeMap;
  22
  23 pub fn check_trait(tcx: TyCtxt<'_>, trait_def_id: DefId) {
  24     let lang_items = tcx.lang_items();
  25     Checker { tcx, trait_def_id }
  26         .check(lang_items.drop_trait(), visit_implementation_of_drop)
  27         .check(lang_items.copy_trait(), visit_implementation_of_copy)
  28         .check(lang_items.coerce_unsized_trait(), visit_implementation_of_coerce_unsized)
  29         .check(lang_items.dispatch_from_dyn_trait(), visit_implementation_of_dispatch_from_dyn);
  30 }
  31
  32 struct Checker<'tcx> {
  33     tcx: TyCtxt<'tcx>,
  34     trait_def_id: DefId,
  35 }
  36
  37 impl<'tcx> Checker<'tcx> {
  38     fn check<F>(&self, trait_def_id: Option<DefId>, mut f: F) -> &Self
  39     where
  40         F: FnMut(TyCtxt<'tcx>, LocalDefId),
  41     {
  42         if Some(self.trait_def_id) == trait_def_id {
  43             for &impl_def_id in self.tcx.hir().trait_impls(self.trait_def_id) {
  44                 f(self.tcx, impl_def_id);
  45             }
  46         }
  47         self
  48     }
  49 }
  50
  51 fn visit_implementation_of_drop(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
  52     // Destructors only work on local ADT types.
  53     match tcx.type_of(impl_did).kind() {
  54         ty::Adt(def, _) if def.did().is_local() => return,
  55         ty::Error(_) => return,
  56         _ => {}
  57     }
  58
  59     let sp = match tcx.hir().expect_item(impl_did).kind {
  60         ItemKind::Impl(ref impl_) => impl_.self_ty.span,
  61         _ => bug!("expected Drop impl item"),
  62     };
  63
  64     tcx.sess.emit_err(DropImplOnWrongItem { span: sp });
  65 }
  66
  67 fn visit_implementation_of_copy(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
  68     debug!("visit_implementation_of_copy: impl_did={:?}", impl_did);
  69
  70     let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
  71
  72     let self_type = tcx.type_of(impl_did);
  73     debug!("visit_implementation_of_copy: self_type={:?} (bound)", self_type);
  74
  75     let param_env = tcx.param_env(impl_did);
  76     assert!(!self_type.has_escaping_bound_vars());
  77
  78     debug!("visit_implementation_of_copy: self_type={:?} (free)", self_type);
  79
  80     let span = match tcx.hir().expect_item(impl_did).kind {
  81         ItemKind::Impl(hir::Impl { polarity: hir::ImplPolarity::Negative(_), .. }) => return,
  82         ItemKind::Impl(impl_) => impl_.self_ty.span,
  83         _ => bug!("expected Copy impl item"),
  84     };
  85
  86     let cause = traits::ObligationCause::misc(span, impl_hir_id);
  87     match type_allowed_to_implement_copy(tcx, param_env, self_type, cause) {
  88         Ok(()) => {}
  89         Err(CopyImplementationError::InfrigingFields(fields)) => {
  90             let mut err = struct_span_err!(
  91                 tcx.sess,
  92                 span,
  93                 E0204,
  94                 "the trait `Copy` may not be implemented for this type"
  95             );
  96
  97             // We'll try to suggest constraining type parameters to fulfill the requirements of
  98             // their `Copy` implementation.
  99             let mut errors: BTreeMap<_, Vec<_>> = Default::default();
 100             let mut bounds = vec![];
 101
 102             for (field, ty) in fields {
 103                 let field_span = tcx.def_span(field.did);
 104                 let field_ty_span = match tcx.hir().get_if_local(field.did) {
 105                     Some(hir::Node::Field(field_def)) => field_def.ty.span,
 106                     _ => field_span,
 107                 };
 108                 err.span_label(field_span, "this field does not implement `Copy`");
 109                 // Spin up a new FulfillmentContext, so we can get the _precise_ reason
 110                 // why this field does not implement Copy. This is useful because sometimes
 111                 // it is not immediately clear why Copy is not implemented for a field, since
 112                 // all we point at is the field itself.
 113                 let infcx = tcx.infer_ctxt().ignoring_regions().build();
 114                 for error in traits::fully_solve_bound(
 115                     &infcx,
 116                     traits::ObligationCause::dummy_with_span(field_ty_span),
 117                     param_env,
 118                     ty,
 119                     tcx.require_lang_item(LangItem::Copy, Some(span)),
 120                 ) {
 121                     let error_predicate = error.obligation.predicate;
 122                     // Only note if it's not the root obligation, otherwise it's trivial and
 123                     // should be self-explanatory (i.e. a field literally doesn't implement Copy).
 124
 125                     // FIXME: This error could be more descriptive, especially if the error_predicate
 126                     // contains a foreign type or if it's a deeply nested type...
 127                     if error_predicate != error.root_obligation.predicate {
 128                         errors
 129                             .entry((ty.to_string(), error_predicate.to_string()))
 130                             .or_default()
 131                             .push(error.obligation.cause.span);
 132                     }
 133                     if let ty::PredicateKind::Clause(ty::Clause::Trait(ty::TraitPredicate {
 134                         trait_ref,
 135                         polarity: ty::ImplPolarity::Positive,
 136                         ..
 137                     })) = error_predicate.kind().skip_binder()
 138                     {
 139                         let ty = trait_ref.self_ty();
 140                         if let ty::Param(_) = ty.kind() {
 141                             bounds.push((
 142                                 format!("{ty}"),
 143                                 trait_ref.print_only_trait_path().to_string(),
 144                                 Some(trait_ref.def_id),
 145                             ));
 146                         }
 147                     }
 148                 }
 149             }
 150             for ((ty, error_predicate), spans) in errors {
 151                 let span: MultiSpan = spans.into();
 152                 err.span_note(
 153                     span,
 154                     &format!("the `Copy` impl for `{}` requires that `{}`", ty, error_predicate),
 155                 );
 156             }
 157             suggest_constraining_type_params(
 158                 tcx,
 159                 tcx.hir().get_generics(impl_did).expect("impls always have generics"),
 160                 &mut err,
 161                 bounds.iter().map(|(param, constraint, def_id)| {
 162                     (param.as_str(), constraint.as_str(), *def_id)
 163                 }),
 164             );
 165             err.emit();
 166         }
 167         Err(CopyImplementationError::NotAnAdt) => {
 168             tcx.sess.emit_err(CopyImplOnNonAdt { span });
 169         }
 170         Err(CopyImplementationError::HasDestructor) => {
 171             tcx.sess.emit_err(CopyImplOnTypeWithDtor { span });
 172         }
 173     }
 174 }
 175
 176 fn visit_implementation_of_coerce_unsized(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
 177     debug!("visit_implementation_of_coerce_unsized: impl_did={:?}", impl_did);
 178
 179     // Just compute this for the side-effects, in particular reporting
 180     // errors; other parts of the code may demand it for the info of
 181     // course.
 182     let span = tcx.def_span(impl_did);
 183     tcx.at(span).coerce_unsized_info(impl_did);
 184 }
 185
 186 fn visit_implementation_of_dispatch_from_dyn(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
 187     debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);
 188
 189     let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
 190     let span = tcx.hir().span(impl_hir_id);
 191
 192     let dispatch_from_dyn_trait = tcx.require_lang_item(LangItem::DispatchFromDyn, Some(span));
 193
 194     let source = tcx.type_of(impl_did);
 195     assert!(!source.has_escaping_bound_vars());
 196     let target = {
 197         let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
 198         assert_eq!(trait_ref.def_id, dispatch_from_dyn_trait);
 199
 200         trait_ref.substs.type_at(1)
 201     };
 202
 203     debug!("visit_implementation_of_dispatch_from_dyn: {:?} -> {:?}", source, target);
 204
 205     let param_env = tcx.param_env(impl_did);
 206
 207     let create_err = |msg: &str| struct_span_err!(tcx.sess, span, E0378, "{}", msg);
 208
 209     let infcx = tcx.infer_ctxt().build();
 210     let cause = ObligationCause::misc(span, impl_hir_id);
 211
 212     use rustc_type_ir::sty::TyKind::*;
 213     match (source.kind(), target.kind()) {
 214         (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))
 215             if infcx.at(&cause, param_env).eq(r_a, *r_b).is_ok() && mutbl_a == *mutbl_b => {}
 216         (&RawPtr(tm_a), &RawPtr(tm_b)) if tm_a.mutbl == tm_b.mutbl => (),
 217         (&Adt(def_a, substs_a), &Adt(def_b, substs_b))
 218             if def_a.is_struct() && def_b.is_struct() =>
 219         {
 220             if def_a != def_b {
 221                 let source_path = tcx.def_path_str(def_a.did());
 222                 let target_path = tcx.def_path_str(def_b.did());
 223
 224                 create_err(&format!(
 225                     "the trait `DispatchFromDyn` may only be implemented \
 226                             for a coercion between structures with the same \
 227                             definition; expected `{}`, found `{}`",
 228                     source_path, target_path,
 229                 ))
 230                 .emit();
 231
 232                 return;
 233             }
 234
 235             if def_a.repr().c() || def_a.repr().packed() {
 236                 create_err(
 237                     "structs implementing `DispatchFromDyn` may not have \
 238                          `#[repr(packed)]` or `#[repr(C)]`",
 239                 )
 240                 .emit();
 241             }
 242
 243             let fields = &def_a.non_enum_variant().fields;
 244
 245             let coerced_fields = fields
 246                 .iter()
 247                 .filter(|field| {
 248                     let ty_a = field.ty(tcx, substs_a);
 249                     let ty_b = field.ty(tcx, substs_b);
 250
 251                     if let Ok(layout) = tcx.layout_of(param_env.and(ty_a)) {
 252                         if layout.is_zst() && layout.align.abi.bytes() == 1 {
 253                             // ignore ZST fields with alignment of 1 byte
 254                             return false;
 255                         }
 256                     }
 257
 258                     if let Ok(ok) = infcx.at(&cause, param_env).eq(ty_a, ty_b) {
 259                         if ok.obligations.is_empty() {
 260                             create_err(
 261                                 "the trait `DispatchFromDyn` may only be implemented \
 262                                  for structs containing the field being coerced, \
 263                                  ZST fields with 1 byte alignment, and nothing else",
 264                             )
 265                             .note(&format!(
 266                                 "extra field `{}` of type `{}` is not allowed",
 267                                 field.name, ty_a,
 268                             ))
 269                             .emit();
 270
 271                             return false;
 272                         }
 273                     }
 274
 275                     return true;
 276                 })
 277                 .collect::<Vec<_>>();
 278
 279             if coerced_fields.is_empty() {
 280                 create_err(
 281                     "the trait `DispatchFromDyn` may only be implemented \
 282                         for a coercion between structures with a single field \
 283                         being coerced, none found",
 284                 )
 285                 .emit();
 286             } else if coerced_fields.len() > 1 {
 287                 create_err("implementing the `DispatchFromDyn` trait requires multiple coercions")
 288                     .note(
 289                         "the trait `DispatchFromDyn` may only be implemented \
 290                             for a coercion between structures with a single field \
 291                             being coerced",
 292                     )
 293                     .note(&format!(
 294                         "currently, {} fields need coercions: {}",
 295                         coerced_fields.len(),
 296                         coerced_fields
 297                             .iter()
 298                             .map(|field| {
 299                                 format!(
 300                                     "`{}` (`{}` to `{}`)",
 301                                     field.name,
 302                                     field.ty(tcx, substs_a),
 303                                     field.ty(tcx, substs_b),
 304                                 )
 305                             })
 306                             .collect::<Vec<_>>()
 307                             .join(", ")
 308                     ))
 309                     .emit();
 310             } else {
 311                 let errors = traits::fully_solve_obligations(
 312                     &infcx,
 313                     coerced_fields.into_iter().map(|field| {
 314                         predicate_for_trait_def(
 315                             tcx,
 316                             param_env,
 317                             cause.clone(),
 318                             dispatch_from_dyn_trait,
 319                             0,
 320                             [field.ty(tcx, substs_a), field.ty(tcx, substs_b)],
 321                         )
 322                     }),
 323                 );
 324                 if !errors.is_empty() {
 325                     infcx.err_ctxt().report_fulfillment_errors(&errors, None);
 326                 }
 327
 328                 // Finally, resolve all regions.
 329                 let outlives_env = OutlivesEnvironment::new(param_env);
 330                 let _ = infcx
 331                     .err_ctxt()
 332                     .check_region_obligations_and_report_errors(impl_did, &outlives_env);
 333             }
 334         }
 335         _ => {
 336             create_err(
 337                 "the trait `DispatchFromDyn` may only be implemented \
 338                     for a coercion between structures",
 339             )
 340             .emit();
 341         }
 342     }
 343 }
 344
 345 pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUnsizedInfo {
 346     debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
 347
 348     // this provider should only get invoked for local def-ids
 349     let impl_did = impl_did.expect_local();
 350     let span = tcx.def_span(impl_did);
 351
 352     let coerce_unsized_trait = tcx.require_lang_item(LangItem::CoerceUnsized, Some(span));
 353
 354     let unsize_trait = tcx.lang_items().require(LangItem::Unsize).unwrap_or_else(|err| {
 355         tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err.to_string()));
 356     });
 357
 358     let source = tcx.type_of(impl_did);
 359     let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
 360     assert_eq!(trait_ref.def_id, coerce_unsized_trait);
 361     let target = trait_ref.substs.type_at(1);
 362     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)", source, target);
 363
 364     let param_env = tcx.param_env(impl_did);
 365     assert!(!source.has_escaping_bound_vars());
 366
 367     let err_info = CoerceUnsizedInfo { custom_kind: None };
 368
 369     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);
 370
 371     let infcx = tcx.infer_ctxt().build();
 372     let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
 373     let cause = ObligationCause::misc(span, impl_hir_id);
 374     let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
 375                        mt_b: ty::TypeAndMut<'tcx>,
 376                        mk_ptr: &dyn Fn(Ty<'tcx>) -> Ty<'tcx>| {
 377         if mt_a.mutbl < mt_b.mutbl {
 378             infcx
 379                 .err_ctxt()
 380                 .report_mismatched_types(
 381                     &cause,
 382                     mk_ptr(mt_b.ty),
 383                     target,
 384                     ty::error::TypeError::Mutability,
 385                 )
 386                 .emit();
 387         }
 388         (mt_a.ty, mt_b.ty, unsize_trait, None)
 389     };
 390     let (source, target, trait_def_id, kind) = match (source.kind(), target.kind()) {
 391         (&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b)) => {
 392             infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
 393             let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 394             let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };
 395             check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ref(r_b, ty))
 396         }
 397
 398         (&ty::Ref(_, ty_a, mutbl_a), &ty::RawPtr(mt_b)) => {
 399             let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 400             check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 401         }
 402
 403         (&ty::RawPtr(mt_a), &ty::RawPtr(mt_b)) => check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty)),
 404
 405         (&ty::Adt(def_a, substs_a), &ty::Adt(def_b, substs_b))
 406             if def_a.is_struct() && def_b.is_struct() =>
 407         {
 408             if def_a != def_b {
 409                 let source_path = tcx.def_path_str(def_a.did());
 410                 let target_path = tcx.def_path_str(def_b.did());
 411                 struct_span_err!(
 412                     tcx.sess,
 413                     span,
 414                     E0377,
 415                     "the trait `CoerceUnsized` may only be implemented \
 416                            for a coercion between structures with the same \
 417                            definition; expected `{}`, found `{}`",
 418                     source_path,
 419                     target_path
 420                 )
 421                 .emit();
 422                 return err_info;
 423             }
 424
 425             // Here we are considering a case of converting
 426             // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
 427             // which acts like a pointer to `U`, but carries along some extra data of type `T`:
 428             //
 429             //     struct Foo<T, U> {
 430             //         extra: T,
 431             //         ptr: *mut U,
 432             //     }
 433             //
 434             // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
 435             // to `Foo<T, [i32]>`. That impl would look like:
 436             //
 437             //   impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
 438             //
 439             // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
 440             // when this coercion occurs, we would be changing the
 441             // field `ptr` from a thin pointer of type `*mut [i32;
 442             // 3]` to a fat pointer of type `*mut [i32]` (with
 443             // extra data `3`).  **The purpose of this check is to
 444             // make sure that we know how to do this conversion.**
 445             //
 446             // To check if this impl is legal, we would walk down
 447             // the fields of `Foo` and consider their types with
 448             // both substitutes. We are looking to find that
 449             // exactly one (non-phantom) field has changed its
 450             // type, which we will expect to be the pointer that
 451             // is becoming fat (we could probably generalize this
 452             // to multiple thin pointers of the same type becoming
 453             // fat, but we don't). In this case:
 454             //
 455             // - `extra` has type `T` before and type `T` after
 456             // - `ptr` has type `*mut U` before and type `*mut V` after
 457             //
 458             // Since just one field changed, we would then check
 459             // that `*mut U: CoerceUnsized<*mut V>` is implemented
 460             // (in other words, that we know how to do this
 461             // conversion). This will work out because `U:
 462             // Unsize<V>`, and we have a builtin rule that `*mut
 463             // U` can be coerced to `*mut V` if `U: Unsize<V>`.
 464             let fields = &def_a.non_enum_variant().fields;
 465             let diff_fields = fields
 466                 .iter()
 467                 .enumerate()
 468                 .filter_map(|(i, f)| {
 469                     let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));
 470
 471                     if tcx.type_of(f.did).is_phantom_data() {
 472                         // Ignore PhantomData fields
 473                         return None;
 474                     }
 475
 476                     // Ignore fields that aren't changed; it may
 477                     // be that we could get away with subtyping or
 478                     // something more accepting, but we use
 479                     // equality because we want to be able to
 480                     // perform this check without computing
 481                     // variance where possible. (This is because
 482                     // we may have to evaluate constraint
 483                     // expressions in the course of execution.)
 484                     // See e.g., #41936.
 485                     if let Ok(ok) = infcx.at(&cause, param_env).eq(a, b) {
 486                         if ok.obligations.is_empty() {
 487                             return None;
 488                         }
 489                     }
 490
 491                     // Collect up all fields that were significantly changed
 492                     // i.e., those that contain T in coerce_unsized T -> U
 493                     Some((i, a, b))
 494                 })
 495                 .collect::<Vec<_>>();
 496
 497             if diff_fields.is_empty() {
 498                 struct_span_err!(
 499                     tcx.sess,
 500                     span,
 501                     E0374,
 502                     "the trait `CoerceUnsized` may only be implemented \
 503                            for a coercion between structures with one field \
 504                            being coerced, none found"
 505                 )
 506                 .emit();
 507                 return err_info;
 508             } else if diff_fields.len() > 1 {
 509                 let item = tcx.hir().expect_item(impl_did);
 510                 let span =
 511                     if let ItemKind::Impl(hir::Impl { of_trait: Some(ref t), .. }) = item.kind {
 512                         t.path.span
 513                     } else {
 514                         tcx.def_span(impl_did)
 515                     };
 516
 517                 struct_span_err!(
 518                     tcx.sess,
 519                     span,
 520                     E0375,
 521                     "implementing the trait \
 522                                                 `CoerceUnsized` requires multiple \
 523                                                 coercions"
 524                 )
 525                 .note(
 526                     "`CoerceUnsized` may only be implemented for \
 527                           a coercion between structures with one field being coerced",
 528                 )
 529                 .note(&format!(
 530                     "currently, {} fields need coercions: {}",
 531                     diff_fields.len(),
 532                     diff_fields
 533                         .iter()
 534                         .map(|&(i, a, b)| { format!("`{}` (`{}` to `{}`)", fields[i].name, a, b) })
 535                         .collect::<Vec<_>>()
 536                         .join(", ")
 537                 ))
 538                 .span_label(span, "requires multiple coercions")
 539                 .emit();
 540                 return err_info;
 541             }
 542
 543             let (i, a, b) = diff_fields[0];
 544             let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
 545             (a, b, coerce_unsized_trait, Some(kind))
 546         }
 547
 548         _ => {
 549             struct_span_err!(
 550                 tcx.sess,
 551                 span,
 552                 E0376,
 553                 "the trait `CoerceUnsized` may only be implemented \
 554                        for a coercion between structures"
 555             )
 556             .emit();
 557             return err_info;
 558         }
 559     };
 560
 561     // Register an obligation for `A: Trait<B>`.
 562     let cause = traits::ObligationCause::misc(span, impl_hir_id);
 563     let predicate =
 564         predicate_for_trait_def(tcx, param_env, cause, trait_def_id, 0, [source, target]);
 565     let errors = traits::fully_solve_obligation(&infcx, predicate);
 566     if !errors.is_empty() {
 567         infcx.err_ctxt().report_fulfillment_errors(&errors, None);
 568     }
 569
 570     // Finally, resolve all regions.
 571     let outlives_env = OutlivesEnvironment::new(param_env);
 572     let _ = infcx.err_ctxt().check_region_obligations_and_report_errors(impl_did, &outlives_env);
 573
 574     CoerceUnsizedInfo { custom_kind: kind }
 575 }