compiler/rustc_typeck/src/coherence/builtin.rs

   1 //! Check properties that are required by built-in traits and set
   2 //! up data structures required by type-checking/codegen.
   3
   4 use crate::errors::{CopyImplOnNonAdt, CopyImplOnTypeWithDtor, DropImplOnWrongItem};
   5 use rustc_errors::{struct_span_err, 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::{RegionckMode, TyCtxtInferExt};
  13 use rustc_middle::ty::adjustment::CoerceUnsizedInfo;
  14 use rustc_middle::ty::{self, suggest_constraining_type_params, Ty, TyCtxt, TypeFoldable};
  15 use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
  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, TraitEngine, TraitEngineExt};
  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 nominal types.
  51     if let ty::Adt(..) | ty::Error(_) = tcx.type_of(impl_did).kind() {
  52         return;
  53     }
  54
  55     let sp = match tcx.hir().expect_item(impl_did).kind {
  56         ItemKind::Impl(ref impl_) => impl_.self_ty.span,
  57         _ => bug!("expected Drop impl item"),
  58     };
  59
  60     tcx.sess.emit_err(DropImplOnWrongItem { span: sp });
  61 }
  62
  63 fn visit_implementation_of_copy(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
  64     debug!("visit_implementation_of_copy: impl_did={:?}", impl_did);
  65
  66     let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
  67
  68     let self_type = tcx.type_of(impl_did);
  69     debug!("visit_implementation_of_copy: self_type={:?} (bound)", self_type);
  70
  71     let span = tcx.hir().span(impl_hir_id);
  72     let param_env = tcx.param_env(impl_did);
  73     assert!(!self_type.has_escaping_bound_vars());
  74
  75     debug!("visit_implementation_of_copy: self_type={:?} (free)", self_type);
  76
  77     let cause = traits::ObligationCause::misc(span, impl_hir_id);
  78     match can_type_implement_copy(tcx, param_env, self_type, cause) {
  79         Ok(()) => {}
  80         Err(CopyImplementationError::InfrigingFields(fields)) => {
  81             let item = tcx.hir().expect_item(impl_did);
  82             let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(ref tr), .. }) = item.kind {
  83                 tr.path.span
  84             } else {
  85                 span
  86             };
  87
  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 generics = None;
  98             if let ty::Adt(def, _substs) = self_type.kind() {
  99                 let self_def_id = def.did();
 100                 if let Some(local) = self_def_id.as_local() {
 101                     let self_item = tcx.hir().expect_item(local);
 102                     generics = self_item.kind.generics();
 103                 }
 104             }
 105             let mut errors: BTreeMap<_, Vec<_>> = Default::default();
 106             let mut bounds = vec![];
 107
 108             for (field, ty) in fields {
 109                 let field_span = tcx.def_span(field.did);
 110                 err.span_label(field_span, "this field does not implement `Copy`");
 111                 // Spin up a new FulfillmentContext, so we can get the _precise_ reason
 112                 // why this field does not implement Copy. This is useful because sometimes
 113                 // it is not immediately clear why Copy is not implemented for a field, since
 114                 // all we point at is the field itself.
 115                 tcx.infer_ctxt().enter(|infcx| {
 116                     let mut fulfill_cx = traits::FulfillmentContext::new_ignoring_regions();
 117                     fulfill_cx.register_bound(
 118                         &infcx,
 119                         param_env,
 120                         ty,
 121                         tcx.lang_items().copy_trait().unwrap(),
 122                         traits::ObligationCause::dummy_with_span(field_span),
 123                     );
 124                     for error in fulfill_cx.select_all_or_error(&infcx) {
 125                         let error_predicate = error.obligation.predicate;
 126                         // Only note if it's not the root obligation, otherwise it's trivial and
 127                         // should be self-explanatory (i.e. a field literally doesn't implement Copy).
 128
 129                         // FIXME: This error could be more descriptive, especially if the error_predicate
 130                         // contains a foreign type or if it's a deeply nested type...
 131                         if error_predicate != error.root_obligation.predicate {
 132                             errors
 133                                 .entry((ty.to_string(), error_predicate.to_string()))
 134                                 .or_default()
 135                                 .push(error.obligation.cause.span);
 136                         }
 137                         if let ty::PredicateKind::Trait(ty::TraitPredicate {
 138                             trait_ref,
 139                             polarity: ty::ImplPolarity::Positive,
 140                             ..
 141                         }) = error_predicate.kind().skip_binder()
 142                         {
 143                             let ty = trait_ref.self_ty();
 144                             if let ty::Param(_) = ty.kind() {
 145                                 bounds.push((
 146                                     format!("{ty}"),
 147                                     trait_ref.print_only_trait_path().to_string(),
 148                                     Some(trait_ref.def_id),
 149                                 ));
 150                             }
 151                         }
 152                     }
 153                 });
 154             }
 155             for ((ty, error_predicate), spans) in errors {
 156                 let span: MultiSpan = spans.into();
 157                 err.span_note(
 158                     span,
 159                     &format!("the `Copy` impl for `{}` requires that `{}`", ty, error_predicate),
 160                 );
 161             }
 162             if let Some(generics) = generics {
 163                 suggest_constraining_type_params(
 164                     tcx,
 165                     generics,
 166                     &mut err,
 167                     bounds.iter().map(|(param, constraint, def_id)| {
 168                         (param.as_str(), constraint.as_str(), *def_id)
 169                     }),
 170                 );
 171             }
 172             err.emit();
 173         }
 174         Err(CopyImplementationError::NotAnAdt) => {
 175             let item = tcx.hir().expect_item(impl_did);
 176             let span =
 177                 if let ItemKind::Impl(ref impl_) = item.kind { impl_.self_ty.span } else { span };
 178
 179             tcx.sess.emit_err(CopyImplOnNonAdt { span });
 180         }
 181         Err(CopyImplementationError::HasDestructor) => {
 182             tcx.sess.emit_err(CopyImplOnTypeWithDtor { span });
 183         }
 184     }
 185 }
 186
 187 fn visit_implementation_of_coerce_unsized<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) {
 188     debug!("visit_implementation_of_coerce_unsized: impl_did={:?}", impl_did);
 189
 190     // Just compute this for the side-effects, in particular reporting
 191     // errors; other parts of the code may demand it for the info of
 192     // course.
 193     let span = tcx.def_span(impl_did);
 194     tcx.at(span).coerce_unsized_info(impl_did);
 195 }
 196
 197 fn visit_implementation_of_dispatch_from_dyn<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) {
 198     debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);
 199
 200     let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
 201     let span = tcx.hir().span(impl_hir_id);
 202
 203     let dispatch_from_dyn_trait = tcx.require_lang_item(LangItem::DispatchFromDyn, Some(span));
 204
 205     let source = tcx.type_of(impl_did);
 206     assert!(!source.has_escaping_bound_vars());
 207     let target = {
 208         let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
 209         assert_eq!(trait_ref.def_id, dispatch_from_dyn_trait);
 210
 211         trait_ref.substs.type_at(1)
 212     };
 213
 214     debug!("visit_implementation_of_dispatch_from_dyn: {:?} -> {:?}", source, target);
 215
 216     let param_env = tcx.param_env(impl_did);
 217
 218     let create_err = |msg: &str| struct_span_err!(tcx.sess, span, E0378, "{}", msg);
 219
 220     tcx.infer_ctxt().enter(|infcx| {
 221         let cause = ObligationCause::misc(span, impl_hir_id);
 222
 223         use rustc_type_ir::sty::TyKind::*;
 224         match (source.kind(), target.kind()) {
 225             (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))
 226                 if infcx.at(&cause, param_env).eq(r_a, *r_b).is_ok() && mutbl_a == *mutbl_b => {}
 227             (&RawPtr(tm_a), &RawPtr(tm_b)) if tm_a.mutbl == tm_b.mutbl => (),
 228             (&Adt(def_a, substs_a), &Adt(def_b, substs_b))
 229                 if def_a.is_struct() && def_b.is_struct() =>
 230             {
 231                 if def_a != def_b {
 232                     let source_path = tcx.def_path_str(def_a.did());
 233                     let target_path = tcx.def_path_str(def_b.did());
 234
 235                     create_err(&format!(
 236                         "the trait `DispatchFromDyn` may only be implemented \
 237                                 for a coercion between structures with the same \
 238                                 definition; expected `{}`, found `{}`",
 239                         source_path, target_path,
 240                     ))
 241                     .emit();
 242
 243                     return;
 244                 }
 245
 246                 if def_a.repr().c() || def_a.repr().packed() {
 247                     create_err(
 248                         "structs implementing `DispatchFromDyn` may not have \
 249                              `#[repr(packed)]` or `#[repr(C)]`",
 250                     )
 251                     .emit();
 252                 }
 253
 254                 let fields = &def_a.non_enum_variant().fields;
 255
 256                 let coerced_fields = fields
 257                     .iter()
 258                     .filter(|field| {
 259                         let ty_a = field.ty(tcx, substs_a);
 260                         let ty_b = field.ty(tcx, substs_b);
 261
 262                         if let Ok(layout) = tcx.layout_of(param_env.and(ty_a)) {
 263                             if layout.is_zst() && layout.align.abi.bytes() == 1 {
 264                                 // ignore ZST fields with alignment of 1 byte
 265                                 return false;
 266                             }
 267                         }
 268
 269                         if let Ok(ok) = infcx.at(&cause, param_env).eq(ty_a, ty_b) {
 270                             if ok.obligations.is_empty() {
 271                                 create_err(
 272                                     "the trait `DispatchFromDyn` may only be implemented \
 273                                      for structs containing the field being coerced, \
 274                                      ZST fields with 1 byte alignment, and nothing else",
 275                                 )
 276                                 .note(&format!(
 277                                     "extra field `{}` of type `{}` is not allowed",
 278                                     field.name, ty_a,
 279                                 ))
 280                                 .emit();
 281
 282                                 return false;
 283                             }
 284                         }
 285
 286                         return true;
 287                     })
 288                     .collect::<Vec<_>>();
 289
 290                 if coerced_fields.is_empty() {
 291                     create_err(
 292                         "the trait `DispatchFromDyn` may only be implemented \
 293                             for a coercion between structures with a single field \
 294                             being coerced, none found",
 295                     )
 296                     .emit();
 297                 } else if coerced_fields.len() > 1 {
 298                     create_err(
 299                         "implementing the `DispatchFromDyn` trait requires multiple coercions",
 300                     )
 301                     .note(
 302                         "the trait `DispatchFromDyn` may only be implemented \
 303                                 for a coercion between structures with a single field \
 304                                 being coerced",
 305                     )
 306                     .note(&format!(
 307                         "currently, {} fields need coercions: {}",
 308                         coerced_fields.len(),
 309                         coerced_fields
 310                             .iter()
 311                             .map(|field| {
 312                                 format!(
 313                                     "`{}` (`{}` to `{}`)",
 314                                     field.name,
 315                                     field.ty(tcx, substs_a),
 316                                     field.ty(tcx, substs_b),
 317                                 )
 318                             })
 319                             .collect::<Vec<_>>()
 320                             .join(", ")
 321                     ))
 322                     .emit();
 323                 } else {
 324                     let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
 325
 326                     for field in coerced_fields {
 327                         let predicate = predicate_for_trait_def(
 328                             tcx,
 329                             param_env,
 330                             cause.clone(),
 331                             dispatch_from_dyn_trait,
 332                             0,
 333                             field.ty(tcx, substs_a),
 334                             &[field.ty(tcx, substs_b).into()],
 335                         );
 336
 337                         fulfill_cx.register_predicate_obligation(&infcx, predicate);
 338                     }
 339
 340                     // Check that all transitive obligations are satisfied.
 341                     let errors = fulfill_cx.select_all_or_error(&infcx);
 342                     if !errors.is_empty() {
 343                         infcx.report_fulfillment_errors(&errors, None, false);
 344                     }
 345
 346                     // Finally, resolve all regions.
 347                     let outlives_env = OutlivesEnvironment::new(param_env);
 348                     infcx.resolve_regions_and_report_errors(
 349                         impl_did.to_def_id(),
 350                         &outlives_env,
 351                         RegionckMode::default(),
 352                     );
 353                 }
 354             }
 355             _ => {
 356                 create_err(
 357                     "the trait `DispatchFromDyn` may only be implemented \
 358                         for a coercion between structures",
 359                 )
 360                 .emit();
 361             }
 362         }
 363     })
 364 }
 365
 366 pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUnsizedInfo {
 367     debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
 368
 369     // this provider should only get invoked for local def-ids
 370     let impl_did = impl_did.expect_local();
 371     let span = tcx.def_span(impl_did);
 372
 373     let coerce_unsized_trait = tcx.require_lang_item(LangItem::CoerceUnsized, Some(span));
 374
 375     let unsize_trait = tcx.lang_items().require(LangItem::Unsize).unwrap_or_else(|err| {
 376         tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err));
 377     });
 378
 379     let source = tcx.type_of(impl_did);
 380     let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
 381     assert_eq!(trait_ref.def_id, coerce_unsized_trait);
 382     let target = trait_ref.substs.type_at(1);
 383     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)", source, target);
 384
 385     let param_env = tcx.param_env(impl_did);
 386     assert!(!source.has_escaping_bound_vars());
 387
 388     let err_info = CoerceUnsizedInfo { custom_kind: None };
 389
 390     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);
 391
 392     tcx.infer_ctxt().enter(|infcx| {
 393         let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
 394         let cause = ObligationCause::misc(span, impl_hir_id);
 395         let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
 396                            mt_b: ty::TypeAndMut<'tcx>,
 397                            mk_ptr: &dyn Fn(Ty<'tcx>) -> Ty<'tcx>| {
 398             if (mt_a.mutbl, mt_b.mutbl) == (hir::Mutability::Not, hir::Mutability::Mut) {
 399                 infcx
 400                     .report_mismatched_types(
 401                         &cause,
 402                         mk_ptr(mt_b.ty),
 403                         target,
 404                         ty::error::TypeError::Mutability,
 405                     )
 406                     .emit();
 407             }
 408             (mt_a.ty, mt_b.ty, unsize_trait, None)
 409         };
 410         let (source, target, trait_def_id, kind) = match (source.kind(), target.kind()) {
 411             (&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b)) => {
 412                 infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
 413                 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 414                 let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };
 415                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ref(r_b, ty))
 416             }
 417
 418             (&ty::Ref(_, ty_a, mutbl_a), &ty::RawPtr(mt_b)) => {
 419                 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 420                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 421             }
 422
 423             (&ty::RawPtr(mt_a), &ty::RawPtr(mt_b)) => {
 424                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 425             }
 426
 427             (&ty::Adt(def_a, substs_a), &ty::Adt(def_b, substs_b))
 428                 if def_a.is_struct() && def_b.is_struct() =>
 429             {
 430                 if def_a != def_b {
 431                     let source_path = tcx.def_path_str(def_a.did());
 432                     let target_path = tcx.def_path_str(def_b.did());
 433                     struct_span_err!(
 434                         tcx.sess,
 435                         span,
 436                         E0377,
 437                         "the trait `CoerceUnsized` may only be implemented \
 438                                for a coercion between structures with the same \
 439                                definition; expected `{}`, found `{}`",
 440                         source_path,
 441                         target_path
 442                     )
 443                     .emit();
 444                     return err_info;
 445                 }
 446
 447                 // Here we are considering a case of converting
 448                 // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
 449                 // which acts like a pointer to `U`, but carries along some extra data of type `T`:
 450                 //
 451                 //     struct Foo<T, U> {
 452                 //         extra: T,
 453                 //         ptr: *mut U,
 454                 //     }
 455                 //
 456                 // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
 457                 // to `Foo<T, [i32]>`. That impl would look like:
 458                 //
 459                 //   impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
 460                 //
 461                 // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
 462                 // when this coercion occurs, we would be changing the
 463                 // field `ptr` from a thin pointer of type `*mut [i32;
 464                 // 3]` to a fat pointer of type `*mut [i32]` (with
 465                 // extra data `3`).  **The purpose of this check is to
 466                 // make sure that we know how to do this conversion.**
 467                 //
 468                 // To check if this impl is legal, we would walk down
 469                 // the fields of `Foo` and consider their types with
 470                 // both substitutes. We are looking to find that
 471                 // exactly one (non-phantom) field has changed its
 472                 // type, which we will expect to be the pointer that
 473                 // is becoming fat (we could probably generalize this
 474                 // to multiple thin pointers of the same type becoming
 475                 // fat, but we don't). In this case:
 476                 //
 477                 // - `extra` has type `T` before and type `T` after
 478                 // - `ptr` has type `*mut U` before and type `*mut V` after
 479                 //
 480                 // Since just one field changed, we would then check
 481                 // that `*mut U: CoerceUnsized<*mut V>` is implemented
 482                 // (in other words, that we know how to do this
 483                 // conversion). This will work out because `U:
 484                 // Unsize<V>`, and we have a builtin rule that `*mut
 485                 // U` can be coerced to `*mut V` if `U: Unsize<V>`.
 486                 let fields = &def_a.non_enum_variant().fields;
 487                 let diff_fields = fields
 488                     .iter()
 489                     .enumerate()
 490                     .filter_map(|(i, f)| {
 491                         let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));
 492
 493                         if tcx.type_of(f.did).is_phantom_data() {
 494                             // Ignore PhantomData fields
 495                             return None;
 496                         }
 497
 498                         // Ignore fields that aren't changed; it may
 499                         // be that we could get away with subtyping or
 500                         // something more accepting, but we use
 501                         // equality because we want to be able to
 502                         // perform this check without computing
 503                         // variance where possible. (This is because
 504                         // we may have to evaluate constraint
 505                         // expressions in the course of execution.)
 506                         // See e.g., #41936.
 507                         if let Ok(ok) = infcx.at(&cause, param_env).eq(a, b) {
 508                             if ok.obligations.is_empty() {
 509                                 return None;
 510                             }
 511                         }
 512
 513                         // Collect up all fields that were significantly changed
 514                         // i.e., those that contain T in coerce_unsized T -> U
 515                         Some((i, a, b))
 516                     })
 517                     .collect::<Vec<_>>();
 518
 519                 if diff_fields.is_empty() {
 520                     struct_span_err!(
 521                         tcx.sess,
 522                         span,
 523                         E0374,
 524                         "the trait `CoerceUnsized` may only be implemented \
 525                                for a coercion between structures with one field \
 526                                being coerced, none found"
 527                     )
 528                     .emit();
 529                     return err_info;
 530                 } else if diff_fields.len() > 1 {
 531                     let item = tcx.hir().expect_item(impl_did);
 532                     let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(ref t), .. }) =
 533                         item.kind
 534                     {
 535                         t.path.span
 536                     } else {
 537                         tcx.def_span(impl_did)
 538                     };
 539
 540                     struct_span_err!(
 541                         tcx.sess,
 542                         span,
 543                         E0375,
 544                         "implementing the trait \
 545                                                     `CoerceUnsized` requires multiple \
 546                                                     coercions"
 547                     )
 548                     .note(
 549                         "`CoerceUnsized` may only be implemented for \
 550                               a coercion between structures with one field being coerced",
 551                     )
 552                     .note(&format!(
 553                         "currently, {} fields need coercions: {}",
 554                         diff_fields.len(),
 555                         diff_fields
 556                             .iter()
 557                             .map(|&(i, a, b)| {
 558                                 format!("`{}` (`{}` to `{}`)", fields[i].name, a, b)
 559                             })
 560                             .collect::<Vec<_>>()
 561                             .join(", ")
 562                     ))
 563                     .span_label(span, "requires multiple coercions")
 564                     .emit();
 565                     return err_info;
 566                 }
 567
 568                 let (i, a, b) = diff_fields[0];
 569                 let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
 570                 (a, b, coerce_unsized_trait, Some(kind))
 571             }
 572
 573             _ => {
 574                 struct_span_err!(
 575                     tcx.sess,
 576                     span,
 577                     E0376,
 578                     "the trait `CoerceUnsized` may only be implemented \
 579                            for a coercion between structures"
 580                 )
 581                 .emit();
 582                 return err_info;
 583             }
 584         };
 585
 586         let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
 587
 588         // Register an obligation for `A: Trait<B>`.
 589         let cause = traits::ObligationCause::misc(span, impl_hir_id);
 590         let predicate = predicate_for_trait_def(
 591             tcx,
 592             param_env,
 593             cause,
 594             trait_def_id,
 595             0,
 596             source,
 597             &[target.into()],
 598         );
 599         fulfill_cx.register_predicate_obligation(&infcx, predicate);
 600
 601         // Check that all transitive obligations are satisfied.
 602         let errors = fulfill_cx.select_all_or_error(&infcx);
 603         if !errors.is_empty() {
 604             infcx.report_fulfillment_errors(&errors, None, false);
 605         }
 606
 607         // Finally, resolve all regions.
 608         let outlives_env = OutlivesEnvironment::new(param_env);
 609         infcx.resolve_regions_and_report_errors(
 610             impl_did.to_def_id(),
 611             &outlives_env,
 612             RegionckMode::default(),
 613         );
 614
 615         CoerceUnsizedInfo { custom_kind: kind }
 616     })
 617 }