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