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