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