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                 let field_ty_span = match tcx.hir().get_if_local(field.did) {
 111                     Some(hir::Node::Field(field_def)) => field_def.ty.span,
 112                     _ => field_span,
 113                 };
 114                 err.span_label(field_span, "this field does not implement `Copy`");
 115                 // Spin up a new FulfillmentContext, so we can get the _precise_ reason
 116                 // why this field does not implement Copy. This is useful because sometimes
 117                 // it is not immediately clear why Copy is not implemented for a field, since
 118                 // all we point at is the field itself.
 119                 tcx.infer_ctxt().enter(|infcx| {
 120                     let mut fulfill_cx = traits::FulfillmentContext::new_ignoring_regions();
 121                     fulfill_cx.register_bound(
 122                         &infcx,
 123                         param_env,
 124                         ty,
 125                         tcx.lang_items().copy_trait().unwrap(),
 126                         traits::ObligationCause::dummy_with_span(field_ty_span),
 127                     );
 128                     for error in fulfill_cx.select_all_or_error(&infcx) {
 129                         let error_predicate = error.obligation.predicate;
 130                         // Only note if it's not the root obligation, otherwise it's trivial and
 131                         // should be self-explanatory (i.e. a field literally doesn't implement Copy).
 132
 133                         // FIXME: This error could be more descriptive, especially if the error_predicate
 134                         // contains a foreign type or if it's a deeply nested type...
 135                         if error_predicate != error.root_obligation.predicate {
 136                             errors
 137                                 .entry((ty.to_string(), error_predicate.to_string()))
 138                                 .or_default()
 139                                 .push(error.obligation.cause.span);
 140                         }
 141                         if let ty::PredicateKind::Trait(ty::TraitPredicate {
 142                             trait_ref,
 143                             polarity: ty::ImplPolarity::Positive,
 144                             ..
 145                         }) = error_predicate.kind().skip_binder()
 146                         {
 147                             let ty = trait_ref.self_ty();
 148                             if let ty::Param(_) = ty.kind() {
 149                                 bounds.push((
 150                                     format!("{ty}"),
 151                                     trait_ref.print_only_trait_path().to_string(),
 152                                     Some(trait_ref.def_id),
 153                                 ));
 154                             }
 155                         }
 156                     }
 157                 });
 158             }
 159             for ((ty, error_predicate), spans) in errors {
 160                 let span: MultiSpan = spans.into();
 161                 err.span_note(
 162                     span,
 163                     &format!("the `Copy` impl for `{}` requires that `{}`", ty, error_predicate),
 164                 );
 165             }
 166             if let Some(generics) = generics {
 167                 suggest_constraining_type_params(
 168                     tcx,
 169                     generics,
 170                     &mut err,
 171                     bounds.iter().map(|(param, constraint, def_id)| {
 172                         (param.as_str(), constraint.as_str(), *def_id)
 173                     }),
 174                 );
 175             }
 176             err.emit();
 177         }
 178         Err(CopyImplementationError::NotAnAdt) => {
 179             let item = tcx.hir().expect_item(impl_did);
 180             let span =
 181                 if let ItemKind::Impl(ref impl_) = item.kind { impl_.self_ty.span } else { span };
 182
 183             tcx.sess.emit_err(CopyImplOnNonAdt { span });
 184         }
 185         Err(CopyImplementationError::HasDestructor) => {
 186             tcx.sess.emit_err(CopyImplOnTypeWithDtor { span });
 187         }
 188     }
 189 }
 190
 191 fn visit_implementation_of_coerce_unsized<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) {
 192     debug!("visit_implementation_of_coerce_unsized: impl_did={:?}", impl_did);
 193
 194     // Just compute this for the side-effects, in particular reporting
 195     // errors; other parts of the code may demand it for the info of
 196     // course.
 197     let span = tcx.def_span(impl_did);
 198     tcx.at(span).coerce_unsized_info(impl_did);
 199 }
 200
 201 fn visit_implementation_of_dispatch_from_dyn<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) {
 202     debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);
 203
 204     let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
 205     let span = tcx.hir().span(impl_hir_id);
 206
 207     let dispatch_from_dyn_trait = tcx.require_lang_item(LangItem::DispatchFromDyn, Some(span));
 208
 209     let source = tcx.type_of(impl_did);
 210     assert!(!source.has_escaping_bound_vars());
 211     let target = {
 212         let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
 213         assert_eq!(trait_ref.def_id, dispatch_from_dyn_trait);
 214
 215         trait_ref.substs.type_at(1)
 216     };
 217
 218     debug!("visit_implementation_of_dispatch_from_dyn: {:?} -> {:?}", source, target);
 219
 220     let param_env = tcx.param_env(impl_did);
 221
 222     let create_err = |msg: &str| struct_span_err!(tcx.sess, span, E0378, "{}", msg);
 223
 224     tcx.infer_ctxt().enter(|infcx| {
 225         let cause = ObligationCause::misc(span, impl_hir_id);
 226
 227         use rustc_type_ir::sty::TyKind::*;
 228         match (source.kind(), target.kind()) {
 229             (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))
 230                 if infcx.at(&cause, param_env).eq(r_a, *r_b).is_ok() && mutbl_a == *mutbl_b => {}
 231             (&RawPtr(tm_a), &RawPtr(tm_b)) if tm_a.mutbl == tm_b.mutbl => (),
 232             (&Adt(def_a, substs_a), &Adt(def_b, substs_b))
 233                 if def_a.is_struct() && def_b.is_struct() =>
 234             {
 235                 if def_a != def_b {
 236                     let source_path = tcx.def_path_str(def_a.did());
 237                     let target_path = tcx.def_path_str(def_b.did());
 238
 239                     create_err(&format!(
 240                         "the trait `DispatchFromDyn` may only be implemented \
 241                                 for a coercion between structures with the same \
 242                                 definition; expected `{}`, found `{}`",
 243                         source_path, target_path,
 244                     ))
 245                     .emit();
 246
 247                     return;
 248                 }
 249
 250                 if def_a.repr().c() || def_a.repr().packed() {
 251                     create_err(
 252                         "structs implementing `DispatchFromDyn` may not have \
 253                              `#[repr(packed)]` or `#[repr(C)]`",
 254                     )
 255                     .emit();
 256                 }
 257
 258                 let fields = &def_a.non_enum_variant().fields;
 259
 260                 let coerced_fields = fields
 261                     .iter()
 262                     .filter(|field| {
 263                         let ty_a = field.ty(tcx, substs_a);
 264                         let ty_b = field.ty(tcx, substs_b);
 265
 266                         if let Ok(layout) = tcx.layout_of(param_env.and(ty_a)) {
 267                             if layout.is_zst() && layout.align.abi.bytes() == 1 {
 268                                 // ignore ZST fields with alignment of 1 byte
 269                                 return false;
 270                             }
 271                         }
 272
 273                         if let Ok(ok) = infcx.at(&cause, param_env).eq(ty_a, ty_b) {
 274                             if ok.obligations.is_empty() {
 275                                 create_err(
 276                                     "the trait `DispatchFromDyn` may only be implemented \
 277                                      for structs containing the field being coerced, \
 278                                      ZST fields with 1 byte alignment, and nothing else",
 279                                 )
 280                                 .note(&format!(
 281                                     "extra field `{}` of type `{}` is not allowed",
 282                                     field.name, ty_a,
 283                                 ))
 284                                 .emit();
 285
 286                                 return false;
 287                             }
 288                         }
 289
 290                         return true;
 291                     })
 292                     .collect::<Vec<_>>();
 293
 294                 if coerced_fields.is_empty() {
 295                     create_err(
 296                         "the trait `DispatchFromDyn` may only be implemented \
 297                             for a coercion between structures with a single field \
 298                             being coerced, none found",
 299                     )
 300                     .emit();
 301                 } else if coerced_fields.len() > 1 {
 302                     create_err(
 303                         "implementing the `DispatchFromDyn` trait requires multiple coercions",
 304                     )
 305                     .note(
 306                         "the trait `DispatchFromDyn` may only be implemented \
 307                                 for a coercion between structures with a single field \
 308                                 being coerced",
 309                     )
 310                     .note(&format!(
 311                         "currently, {} fields need coercions: {}",
 312                         coerced_fields.len(),
 313                         coerced_fields
 314                             .iter()
 315                             .map(|field| {
 316                                 format!(
 317                                     "`{}` (`{}` to `{}`)",
 318                                     field.name,
 319                                     field.ty(tcx, substs_a),
 320                                     field.ty(tcx, substs_b),
 321                                 )
 322                             })
 323                             .collect::<Vec<_>>()
 324                             .join(", ")
 325                     ))
 326                     .emit();
 327                 } else {
 328                     let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
 329
 330                     for field in coerced_fields {
 331                         let predicate = predicate_for_trait_def(
 332                             tcx,
 333                             param_env,
 334                             cause.clone(),
 335                             dispatch_from_dyn_trait,
 336                             0,
 337                             field.ty(tcx, substs_a),
 338                             &[field.ty(tcx, substs_b).into()],
 339                         );
 340
 341                         fulfill_cx.register_predicate_obligation(&infcx, predicate);
 342                     }
 343
 344                     // Check that all transitive obligations are satisfied.
 345                     let errors = fulfill_cx.select_all_or_error(&infcx);
 346                     if !errors.is_empty() {
 347                         infcx.report_fulfillment_errors(&errors, None, false);
 348                     }
 349
 350                     // Finally, resolve all regions.
 351                     let outlives_env = OutlivesEnvironment::new(param_env);
 352                     infcx.resolve_regions_and_report_errors(
 353                         impl_did.to_def_id(),
 354                         &outlives_env,
 355                         RegionckMode::default(),
 356                     );
 357                 }
 358             }
 359             _ => {
 360                 create_err(
 361                     "the trait `DispatchFromDyn` may only be implemented \
 362                         for a coercion between structures",
 363                 )
 364                 .emit();
 365             }
 366         }
 367     })
 368 }
 369
 370 pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUnsizedInfo {
 371     debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
 372
 373     // this provider should only get invoked for local def-ids
 374     let impl_did = impl_did.expect_local();
 375     let span = tcx.def_span(impl_did);
 376
 377     let coerce_unsized_trait = tcx.require_lang_item(LangItem::CoerceUnsized, Some(span));
 378
 379     let unsize_trait = tcx.lang_items().require(LangItem::Unsize).unwrap_or_else(|err| {
 380         tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err));
 381     });
 382
 383     let source = tcx.type_of(impl_did);
 384     let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
 385     assert_eq!(trait_ref.def_id, coerce_unsized_trait);
 386     let target = trait_ref.substs.type_at(1);
 387     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)", source, target);
 388
 389     let param_env = tcx.param_env(impl_did);
 390     assert!(!source.has_escaping_bound_vars());
 391
 392     let err_info = CoerceUnsizedInfo { custom_kind: None };
 393
 394     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);
 395
 396     tcx.infer_ctxt().enter(|infcx| {
 397         let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
 398         let cause = ObligationCause::misc(span, impl_hir_id);
 399         let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
 400                            mt_b: ty::TypeAndMut<'tcx>,
 401                            mk_ptr: &dyn Fn(Ty<'tcx>) -> Ty<'tcx>| {
 402             if (mt_a.mutbl, mt_b.mutbl) == (hir::Mutability::Not, hir::Mutability::Mut) {
 403                 infcx
 404                     .report_mismatched_types(
 405                         &cause,
 406                         mk_ptr(mt_b.ty),
 407                         target,
 408                         ty::error::TypeError::Mutability,
 409                     )
 410                     .emit();
 411             }
 412             (mt_a.ty, mt_b.ty, unsize_trait, None)
 413         };
 414         let (source, target, trait_def_id, kind) = match (source.kind(), target.kind()) {
 415             (&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b)) => {
 416                 infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
 417                 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 418                 let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };
 419                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ref(r_b, ty))
 420             }
 421
 422             (&ty::Ref(_, ty_a, mutbl_a), &ty::RawPtr(mt_b)) => {
 423                 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 424                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 425             }
 426
 427             (&ty::RawPtr(mt_a), &ty::RawPtr(mt_b)) => {
 428                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 429             }
 430
 431             (&ty::Adt(def_a, substs_a), &ty::Adt(def_b, substs_b))
 432                 if def_a.is_struct() && def_b.is_struct() =>
 433             {
 434                 if def_a != def_b {
 435                     let source_path = tcx.def_path_str(def_a.did());
 436                     let target_path = tcx.def_path_str(def_b.did());
 437                     struct_span_err!(
 438                         tcx.sess,
 439                         span,
 440                         E0377,
 441                         "the trait `CoerceUnsized` may only be implemented \
 442                                for a coercion between structures with the same \
 443                                definition; expected `{}`, found `{}`",
 444                         source_path,
 445                         target_path
 446                     )
 447                     .emit();
 448                     return err_info;
 449                 }
 450
 451                 // Here we are considering a case of converting
 452                 // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
 453                 // which acts like a pointer to `U`, but carries along some extra data of type `T`:
 454                 //
 455                 //     struct Foo<T, U> {
 456                 //         extra: T,
 457                 //         ptr: *mut U,
 458                 //     }
 459                 //
 460                 // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
 461                 // to `Foo<T, [i32]>`. That impl would look like:
 462                 //
 463                 //   impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
 464                 //
 465                 // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
 466                 // when this coercion occurs, we would be changing the
 467                 // field `ptr` from a thin pointer of type `*mut [i32;
 468                 // 3]` to a fat pointer of type `*mut [i32]` (with
 469                 // extra data `3`).  **The purpose of this check is to
 470                 // make sure that we know how to do this conversion.**
 471                 //
 472                 // To check if this impl is legal, we would walk down
 473                 // the fields of `Foo` and consider their types with
 474                 // both substitutes. We are looking to find that
 475                 // exactly one (non-phantom) field has changed its
 476                 // type, which we will expect to be the pointer that
 477                 // is becoming fat (we could probably generalize this
 478                 // to multiple thin pointers of the same type becoming
 479                 // fat, but we don't). In this case:
 480                 //
 481                 // - `extra` has type `T` before and type `T` after
 482                 // - `ptr` has type `*mut U` before and type `*mut V` after
 483                 //
 484                 // Since just one field changed, we would then check
 485                 // that `*mut U: CoerceUnsized<*mut V>` is implemented
 486                 // (in other words, that we know how to do this
 487                 // conversion). This will work out because `U:
 488                 // Unsize<V>`, and we have a builtin rule that `*mut
 489                 // U` can be coerced to `*mut V` if `U: Unsize<V>`.
 490                 let fields = &def_a.non_enum_variant().fields;
 491                 let diff_fields = fields
 492                     .iter()
 493                     .enumerate()
 494                     .filter_map(|(i, f)| {
 495                         let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));
 496
 497                         if tcx.type_of(f.did).is_phantom_data() {
 498                             // Ignore PhantomData fields
 499                             return None;
 500                         }
 501
 502                         // Ignore fields that aren't changed; it may
 503                         // be that we could get away with subtyping or
 504                         // something more accepting, but we use
 505                         // equality because we want to be able to
 506                         // perform this check without computing
 507                         // variance where possible. (This is because
 508                         // we may have to evaluate constraint
 509                         // expressions in the course of execution.)
 510                         // See e.g., #41936.
 511                         if let Ok(ok) = infcx.at(&cause, param_env).eq(a, b) {
 512                             if ok.obligations.is_empty() {
 513                                 return None;
 514                             }
 515                         }
 516
 517                         // Collect up all fields that were significantly changed
 518                         // i.e., those that contain T in coerce_unsized T -> U
 519                         Some((i, a, b))
 520                     })
 521                     .collect::<Vec<_>>();
 522
 523                 if diff_fields.is_empty() {
 524                     struct_span_err!(
 525                         tcx.sess,
 526                         span,
 527                         E0374,
 528                         "the trait `CoerceUnsized` may only be implemented \
 529                                for a coercion between structures with one field \
 530                                being coerced, none found"
 531                     )
 532                     .emit();
 533                     return err_info;
 534                 } else if diff_fields.len() > 1 {
 535                     let item = tcx.hir().expect_item(impl_did);
 536                     let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(ref t), .. }) =
 537                         item.kind
 538                     {
 539                         t.path.span
 540                     } else {
 541                         tcx.def_span(impl_did)
 542                     };
 543
 544                     struct_span_err!(
 545                         tcx.sess,
 546                         span,
 547                         E0375,
 548                         "implementing the trait \
 549                                                     `CoerceUnsized` requires multiple \
 550                                                     coercions"
 551                     )
 552                     .note(
 553                         "`CoerceUnsized` may only be implemented for \
 554                               a coercion between structures with one field being coerced",
 555                     )
 556                     .note(&format!(
 557                         "currently, {} fields need coercions: {}",
 558                         diff_fields.len(),
 559                         diff_fields
 560                             .iter()
 561                             .map(|&(i, a, b)| {
 562                                 format!("`{}` (`{}` to `{}`)", fields[i].name, a, b)
 563                             })
 564                             .collect::<Vec<_>>()
 565                             .join(", ")
 566                     ))
 567                     .span_label(span, "requires multiple coercions")
 568                     .emit();
 569                     return err_info;
 570                 }
 571
 572                 let (i, a, b) = diff_fields[0];
 573                 let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
 574                 (a, b, coerce_unsized_trait, Some(kind))
 575             }
 576
 577             _ => {
 578                 struct_span_err!(
 579                     tcx.sess,
 580                     span,
 581                     E0376,
 582                     "the trait `CoerceUnsized` may only be implemented \
 583                            for a coercion between structures"
 584                 )
 585                 .emit();
 586                 return err_info;
 587             }
 588         };
 589
 590         let mut fulfill_cx = <dyn TraitEngine<'_>>::new(infcx.tcx);
 591
 592         // Register an obligation for `A: Trait<B>`.
 593         let cause = traits::ObligationCause::misc(span, impl_hir_id);
 594         let predicate = predicate_for_trait_def(
 595             tcx,
 596             param_env,
 597             cause,
 598             trait_def_id,
 599             0,
 600             source,
 601             &[target.into()],
 602         );
 603         fulfill_cx.register_predicate_obligation(&infcx, predicate);
 604
 605         // Check that all transitive obligations are satisfied.
 606         let errors = fulfill_cx.select_all_or_error(&infcx);
 607         if !errors.is_empty() {
 608             infcx.report_fulfillment_errors(&errors, None, false);
 609         }
 610
 611         // Finally, resolve all regions.
 612         let outlives_env = OutlivesEnvironment::new(param_env);
 613         infcx.resolve_regions_and_report_errors(
 614             impl_did.to_def_id(),
 615             &outlives_env,
 616             RegionckMode::default(),
 617         );
 618
 619         CoerceUnsizedInfo { custom_kind: kind }
 620     })
 621 }