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::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::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};
  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 span = tcx.hir().span(impl_hir_id);
  74     let param_env = tcx.param_env(impl_did);
  75     assert!(!self_type.has_escaping_bound_vars());
  76
  77     debug!("visit_implementation_of_copy: self_type={:?} (free)", self_type);
  78
  79     let cause = traits::ObligationCause::misc(span, impl_hir_id);
  80     match can_type_implement_copy(tcx, param_env, self_type, cause) {
  81         Ok(()) => {}
  82         Err(CopyImplementationError::InfrigingFields(fields)) => {
  83             let item = tcx.hir().expect_item(impl_did);
  84             let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(ref tr), .. }) = item.kind {
  85                 tr.path.span
  86             } else {
  87                 span
  88             };
  89
  90             let mut err = struct_span_err!(
  91                 tcx.sess,
  92                 span,
  93                 E0204,
  94                 "the trait `Copy` may not be implemented for this type"
  95             );
  96
  97             // We'll try to suggest constraining type parameters to fulfill the requirements of
  98             // their `Copy` implementation.
  99             let mut errors: BTreeMap<_, Vec<_>> = Default::default();
 100             let mut bounds = vec![];
 101
 102             for (field, ty) in fields {
 103                 let field_span = tcx.def_span(field.did);
 104                 let field_ty_span = match tcx.hir().get_if_local(field.did) {
 105                     Some(hir::Node::Field(field_def)) => field_def.ty.span,
 106                     _ => field_span,
 107                 };
 108                 err.span_label(field_span, "this field does not implement `Copy`");
 109                 // Spin up a new FulfillmentContext, so we can get the _precise_ reason
 110                 // why this field does not implement Copy. This is useful because sometimes
 111                 // it is not immediately clear why Copy is not implemented for a field, since
 112                 // all we point at is the field itself.
 113                 tcx.infer_ctxt().ignoring_regions().enter(|infcx| {
 114                     for error in traits::fully_solve_bound(
 115                         &infcx,
 116                         traits::ObligationCause::dummy_with_span(field_ty_span),
 117                         param_env,
 118                         ty,
 119                         tcx.lang_items().copy_trait().unwrap(),
 120                     ) {
 121                         let error_predicate = error.obligation.predicate;
 122                         // Only note if it's not the root obligation, otherwise it's trivial and
 123                         // should be self-explanatory (i.e. a field literally doesn't implement Copy).
 124
 125                         // FIXME: This error could be more descriptive, especially if the error_predicate
 126                         // contains a foreign type or if it's a deeply nested type...
 127                         if error_predicate != error.root_obligation.predicate {
 128                             errors
 129                                 .entry((ty.to_string(), error_predicate.to_string()))
 130                                 .or_default()
 131                                 .push(error.obligation.cause.span);
 132                         }
 133                         if let ty::PredicateKind::Trait(ty::TraitPredicate {
 134                             trait_ref,
 135                             polarity: ty::ImplPolarity::Positive,
 136                             ..
 137                         }) = error_predicate.kind().skip_binder()
 138                         {
 139                             let ty = trait_ref.self_ty();
 140                             if let ty::Param(_) = ty.kind() {
 141                                 bounds.push((
 142                                     format!("{ty}"),
 143                                     trait_ref.print_only_trait_path().to_string(),
 144                                     Some(trait_ref.def_id),
 145                                 ));
 146                             }
 147                         }
 148                     }
 149                 });
 150             }
 151             for ((ty, error_predicate), spans) in errors {
 152                 let span: MultiSpan = spans.into();
 153                 err.span_note(
 154                     span,
 155                     &format!("the `Copy` impl for `{}` requires that `{}`", ty, error_predicate),
 156                 );
 157             }
 158             suggest_constraining_type_params(
 159                 tcx,
 160                 tcx.hir().get_generics(impl_did).expect("impls always have generics"),
 161                 &mut err,
 162                 bounds.iter().map(|(param, constraint, def_id)| {
 163                     (param.as_str(), constraint.as_str(), *def_id)
 164                 }),
 165             );
 166             err.emit();
 167         }
 168         Err(CopyImplementationError::NotAnAdt) => {
 169             let item = tcx.hir().expect_item(impl_did);
 170             let span =
 171                 if let ItemKind::Impl(ref impl_) = item.kind { impl_.self_ty.span } else { span };
 172
 173             tcx.sess.emit_err(CopyImplOnNonAdt { span });
 174         }
 175         Err(CopyImplementationError::HasDestructor) => {
 176             tcx.sess.emit_err(CopyImplOnTypeWithDtor { span });
 177         }
 178     }
 179 }
 180
 181 fn visit_implementation_of_coerce_unsized<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) {
 182     debug!("visit_implementation_of_coerce_unsized: impl_did={:?}", impl_did);
 183
 184     // Just compute this for the side-effects, in particular reporting
 185     // errors; other parts of the code may demand it for the info of
 186     // course.
 187     let span = tcx.def_span(impl_did);
 188     tcx.at(span).coerce_unsized_info(impl_did);
 189 }
 190
 191 fn visit_implementation_of_dispatch_from_dyn<'tcx>(tcx: TyCtxt<'tcx>, impl_did: LocalDefId) {
 192     debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);
 193
 194     let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
 195     let span = tcx.hir().span(impl_hir_id);
 196
 197     let dispatch_from_dyn_trait = tcx.require_lang_item(LangItem::DispatchFromDyn, Some(span));
 198
 199     let source = tcx.type_of(impl_did);
 200     assert!(!source.has_escaping_bound_vars());
 201     let target = {
 202         let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
 203         assert_eq!(trait_ref.def_id, dispatch_from_dyn_trait);
 204
 205         trait_ref.substs.type_at(1)
 206     };
 207
 208     debug!("visit_implementation_of_dispatch_from_dyn: {:?} -> {:?}", source, target);
 209
 210     let param_env = tcx.param_env(impl_did);
 211
 212     let create_err = |msg: &str| struct_span_err!(tcx.sess, span, E0378, "{}", msg);
 213
 214     tcx.infer_ctxt().enter(|infcx| {
 215         let cause = ObligationCause::misc(span, impl_hir_id);
 216
 217         use rustc_type_ir::sty::TyKind::*;
 218         match (source.kind(), target.kind()) {
 219             (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))
 220                 if infcx.at(&cause, param_env).eq(r_a, *r_b).is_ok() && mutbl_a == *mutbl_b => {}
 221             (&RawPtr(tm_a), &RawPtr(tm_b)) if tm_a.mutbl == tm_b.mutbl => (),
 222             (&Adt(def_a, substs_a), &Adt(def_b, substs_b))
 223                 if def_a.is_struct() && def_b.is_struct() =>
 224             {
 225                 if def_a != def_b {
 226                     let source_path = tcx.def_path_str(def_a.did());
 227                     let target_path = tcx.def_path_str(def_b.did());
 228
 229                     create_err(&format!(
 230                         "the trait `DispatchFromDyn` may only be implemented \
 231                                 for a coercion between structures with the same \
 232                                 definition; expected `{}`, found `{}`",
 233                         source_path, target_path,
 234                     ))
 235                     .emit();
 236
 237                     return;
 238                 }
 239
 240                 if def_a.repr().c() || def_a.repr().packed() {
 241                     create_err(
 242                         "structs implementing `DispatchFromDyn` may not have \
 243                              `#[repr(packed)]` or `#[repr(C)]`",
 244                     )
 245                     .emit();
 246                 }
 247
 248                 let fields = &def_a.non_enum_variant().fields;
 249
 250                 let coerced_fields = fields
 251                     .iter()
 252                     .filter(|field| {
 253                         let ty_a = field.ty(tcx, substs_a);
 254                         let ty_b = field.ty(tcx, substs_b);
 255
 256                         if let Ok(layout) = tcx.layout_of(param_env.and(ty_a)) {
 257                             if layout.is_zst() && layout.align.abi.bytes() == 1 {
 258                                 // ignore ZST fields with alignment of 1 byte
 259                                 return false;
 260                             }
 261                         }
 262
 263                         if let Ok(ok) = infcx.at(&cause, param_env).eq(ty_a, ty_b) {
 264                             if ok.obligations.is_empty() {
 265                                 create_err(
 266                                     "the trait `DispatchFromDyn` may only be implemented \
 267                                      for structs containing the field being coerced, \
 268                                      ZST fields with 1 byte alignment, and nothing else",
 269                                 )
 270                                 .note(&format!(
 271                                     "extra field `{}` of type `{}` is not allowed",
 272                                     field.name, ty_a,
 273                                 ))
 274                                 .emit();
 275
 276                                 return false;
 277                             }
 278                         }
 279
 280                         return true;
 281                     })
 282                     .collect::<Vec<_>>();
 283
 284                 if coerced_fields.is_empty() {
 285                     create_err(
 286                         "the trait `DispatchFromDyn` may only be implemented \
 287                             for a coercion between structures with a single field \
 288                             being coerced, none found",
 289                     )
 290                     .emit();
 291                 } else if coerced_fields.len() > 1 {
 292                     create_err(
 293                         "implementing the `DispatchFromDyn` trait requires multiple coercions",
 294                     )
 295                     .note(
 296                         "the trait `DispatchFromDyn` may only be implemented \
 297                                 for a coercion between structures with a single field \
 298                                 being coerced",
 299                     )
 300                     .note(&format!(
 301                         "currently, {} fields need coercions: {}",
 302                         coerced_fields.len(),
 303                         coerced_fields
 304                             .iter()
 305                             .map(|field| {
 306                                 format!(
 307                                     "`{}` (`{}` to `{}`)",
 308                                     field.name,
 309                                     field.ty(tcx, substs_a),
 310                                     field.ty(tcx, substs_b),
 311                                 )
 312                             })
 313                             .collect::<Vec<_>>()
 314                             .join(", ")
 315                     ))
 316                     .emit();
 317                 } else {
 318                     let errors = traits::fully_solve_obligations(
 319                         &infcx,
 320                         coerced_fields.into_iter().map(|field| {
 321                             predicate_for_trait_def(
 322                                 tcx,
 323                                 param_env,
 324                                 cause.clone(),
 325                                 dispatch_from_dyn_trait,
 326                                 0,
 327                                 field.ty(tcx, substs_a),
 328                                 &[field.ty(tcx, substs_b).into()],
 329                             )
 330                         }),
 331                     );
 332                     if !errors.is_empty() {
 333                         infcx.report_fulfillment_errors(&errors, None, false);
 334                     }
 335
 336                     // Finally, resolve all regions.
 337                     let outlives_env = OutlivesEnvironment::new(param_env);
 338                     infcx.check_region_obligations_and_report_errors(impl_did, &outlives_env);
 339                 }
 340             }
 341             _ => {
 342                 create_err(
 343                     "the trait `DispatchFromDyn` may only be implemented \
 344                         for a coercion between structures",
 345                 )
 346                 .emit();
 347             }
 348         }
 349     })
 350 }
 351
 352 pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUnsizedInfo {
 353     debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
 354
 355     // this provider should only get invoked for local def-ids
 356     let impl_did = impl_did.expect_local();
 357     let span = tcx.def_span(impl_did);
 358
 359     let coerce_unsized_trait = tcx.require_lang_item(LangItem::CoerceUnsized, Some(span));
 360
 361     let unsize_trait = tcx.lang_items().require(LangItem::Unsize).unwrap_or_else(|err| {
 362         tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err.to_string()));
 363     });
 364
 365     let source = tcx.type_of(impl_did);
 366     let trait_ref = tcx.impl_trait_ref(impl_did).unwrap();
 367     assert_eq!(trait_ref.def_id, coerce_unsized_trait);
 368     let target = trait_ref.substs.type_at(1);
 369     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)", source, target);
 370
 371     let param_env = tcx.param_env(impl_did);
 372     assert!(!source.has_escaping_bound_vars());
 373
 374     let err_info = CoerceUnsizedInfo { custom_kind: None };
 375
 376     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);
 377
 378     tcx.infer_ctxt().enter(|infcx| {
 379         let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
 380         let cause = ObligationCause::misc(span, impl_hir_id);
 381         let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
 382                            mt_b: ty::TypeAndMut<'tcx>,
 383                            mk_ptr: &dyn Fn(Ty<'tcx>) -> Ty<'tcx>| {
 384             if (mt_a.mutbl, mt_b.mutbl) == (hir::Mutability::Not, hir::Mutability::Mut) {
 385                 infcx
 386                     .report_mismatched_types(
 387                         &cause,
 388                         mk_ptr(mt_b.ty),
 389                         target,
 390                         ty::error::TypeError::Mutability,
 391                     )
 392                     .emit();
 393             }
 394             (mt_a.ty, mt_b.ty, unsize_trait, None)
 395         };
 396         let (source, target, trait_def_id, kind) = match (source.kind(), target.kind()) {
 397             (&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b)) => {
 398                 infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
 399                 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 400                 let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };
 401                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ref(r_b, ty))
 402             }
 403
 404             (&ty::Ref(_, ty_a, mutbl_a), &ty::RawPtr(mt_b)) => {
 405                 let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 406                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 407             }
 408
 409             (&ty::RawPtr(mt_a), &ty::RawPtr(mt_b)) => {
 410                 check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 411             }
 412
 413             (&ty::Adt(def_a, substs_a), &ty::Adt(def_b, substs_b))
 414                 if def_a.is_struct() && def_b.is_struct() =>
 415             {
 416                 if def_a != def_b {
 417                     let source_path = tcx.def_path_str(def_a.did());
 418                     let target_path = tcx.def_path_str(def_b.did());
 419                     struct_span_err!(
 420                         tcx.sess,
 421                         span,
 422                         E0377,
 423                         "the trait `CoerceUnsized` may only be implemented \
 424                                for a coercion between structures with the same \
 425                                definition; expected `{}`, found `{}`",
 426                         source_path,
 427                         target_path
 428                     )
 429                     .emit();
 430                     return err_info;
 431                 }
 432
 433                 // Here we are considering a case of converting
 434                 // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
 435                 // which acts like a pointer to `U`, but carries along some extra data of type `T`:
 436                 //
 437                 //     struct Foo<T, U> {
 438                 //         extra: T,
 439                 //         ptr: *mut U,
 440                 //     }
 441                 //
 442                 // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
 443                 // to `Foo<T, [i32]>`. That impl would look like:
 444                 //
 445                 //   impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
 446                 //
 447                 // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
 448                 // when this coercion occurs, we would be changing the
 449                 // field `ptr` from a thin pointer of type `*mut [i32;
 450                 // 3]` to a fat pointer of type `*mut [i32]` (with
 451                 // extra data `3`).  **The purpose of this check is to
 452                 // make sure that we know how to do this conversion.**
 453                 //
 454                 // To check if this impl is legal, we would walk down
 455                 // the fields of `Foo` and consider their types with
 456                 // both substitutes. We are looking to find that
 457                 // exactly one (non-phantom) field has changed its
 458                 // type, which we will expect to be the pointer that
 459                 // is becoming fat (we could probably generalize this
 460                 // to multiple thin pointers of the same type becoming
 461                 // fat, but we don't). In this case:
 462                 //
 463                 // - `extra` has type `T` before and type `T` after
 464                 // - `ptr` has type `*mut U` before and type `*mut V` after
 465                 //
 466                 // Since just one field changed, we would then check
 467                 // that `*mut U: CoerceUnsized<*mut V>` is implemented
 468                 // (in other words, that we know how to do this
 469                 // conversion). This will work out because `U:
 470                 // Unsize<V>`, and we have a builtin rule that `*mut
 471                 // U` can be coerced to `*mut V` if `U: Unsize<V>`.
 472                 let fields = &def_a.non_enum_variant().fields;
 473                 let diff_fields = fields
 474                     .iter()
 475                     .enumerate()
 476                     .filter_map(|(i, f)| {
 477                         let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));
 478
 479                         if tcx.type_of(f.did).is_phantom_data() {
 480                             // Ignore PhantomData fields
 481                             return None;
 482                         }
 483
 484                         // Ignore fields that aren't changed; it may
 485                         // be that we could get away with subtyping or
 486                         // something more accepting, but we use
 487                         // equality because we want to be able to
 488                         // perform this check without computing
 489                         // variance where possible. (This is because
 490                         // we may have to evaluate constraint
 491                         // expressions in the course of execution.)
 492                         // See e.g., #41936.
 493                         if let Ok(ok) = infcx.at(&cause, param_env).eq(a, b) {
 494                             if ok.obligations.is_empty() {
 495                                 return None;
 496                             }
 497                         }
 498
 499                         // Collect up all fields that were significantly changed
 500                         // i.e., those that contain T in coerce_unsized T -> U
 501                         Some((i, a, b))
 502                     })
 503                     .collect::<Vec<_>>();
 504
 505                 if diff_fields.is_empty() {
 506                     struct_span_err!(
 507                         tcx.sess,
 508                         span,
 509                         E0374,
 510                         "the trait `CoerceUnsized` may only be implemented \
 511                                for a coercion between structures with one field \
 512                                being coerced, none found"
 513                     )
 514                     .emit();
 515                     return err_info;
 516                 } else if diff_fields.len() > 1 {
 517                     let item = tcx.hir().expect_item(impl_did);
 518                     let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(ref t), .. }) =
 519                         item.kind
 520                     {
 521                         t.path.span
 522                     } else {
 523                         tcx.def_span(impl_did)
 524                     };
 525
 526                     struct_span_err!(
 527                         tcx.sess,
 528                         span,
 529                         E0375,
 530                         "implementing the trait \
 531                                                     `CoerceUnsized` requires multiple \
 532                                                     coercions"
 533                     )
 534                     .note(
 535                         "`CoerceUnsized` may only be implemented for \
 536                               a coercion between structures with one field being coerced",
 537                     )
 538                     .note(&format!(
 539                         "currently, {} fields need coercions: {}",
 540                         diff_fields.len(),
 541                         diff_fields
 542                             .iter()
 543                             .map(|&(i, a, b)| {
 544                                 format!("`{}` (`{}` to `{}`)", fields[i].name, a, b)
 545                             })
 546                             .collect::<Vec<_>>()
 547                             .join(", ")
 548                     ))
 549                     .span_label(span, "requires multiple coercions")
 550                     .emit();
 551                     return err_info;
 552                 }
 553
 554                 let (i, a, b) = diff_fields[0];
 555                 let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
 556                 (a, b, coerce_unsized_trait, Some(kind))
 557             }
 558
 559             _ => {
 560                 struct_span_err!(
 561                     tcx.sess,
 562                     span,
 563                     E0376,
 564                     "the trait `CoerceUnsized` may only be implemented \
 565                            for a coercion between structures"
 566                 )
 567                 .emit();
 568                 return err_info;
 569             }
 570         };
 571
 572         // Register an obligation for `A: Trait<B>`.
 573         let cause = traits::ObligationCause::misc(span, impl_hir_id);
 574         let predicate = predicate_for_trait_def(
 575             tcx,
 576             param_env,
 577             cause,
 578             trait_def_id,
 579             0,
 580             source,
 581             &[target.into()],
 582         );
 583         let errors = traits::fully_solve_obligation(&infcx, predicate);
 584         if !errors.is_empty() {
 585             infcx.report_fulfillment_errors(&errors, None, false);
 586         }
 587
 588         // Finally, resolve all regions.
 589         let outlives_env = OutlivesEnvironment::new(param_env);
 590         infcx.check_region_obligations_and_report_errors(impl_did, &outlives_env);
 591
 592         CoerceUnsizedInfo { custom_kind: kind }
 593     })
 594 }