compiler/rustc_hir_analysis/src/coherence/builtin.rs

   1 //! Check properties that are required by built-in traits and set
   2 //! up data structures required by type-checking/codegen.
   3
   4 use crate::errors::{CopyImplOnNonAdt, CopyImplOnTypeWithDtor, DropImplOnWrongItem};
   5 use rustc_errors::{struct_span_err, MultiSpan};
   6 use rustc_hir as hir;
   7 use rustc_hir::def_id::{DefId, LocalDefId};
   8 use rustc_hir::lang_items::LangItem;
   9 use rustc_hir::ItemKind;
  10 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
  11 use rustc_infer::infer::TyCtxtInferExt;
  12 use rustc_infer::infer::{self, RegionResolutionError};
  13 use rustc_middle::ty::adjustment::CoerceUnsizedInfo;
  14 use rustc_middle::ty::{self, suggest_constraining_type_params, Ty, TyCtxt, TypeVisitable};
  15 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt;
  16 use rustc_trait_selection::traits::misc::{
  17     type_allowed_to_implement_copy, CopyImplementationError, InfringingFieldsReason,
  18 };
  19 use rustc_trait_selection::traits::predicate_for_trait_def;
  20 use rustc_trait_selection::traits::{self, ObligationCause};
  21 use std::collections::BTreeMap;
  22
  23 pub fn check_trait(tcx: TyCtxt<'_>, trait_def_id: DefId) {
  24     let lang_items = tcx.lang_items();
  25     Checker { tcx, trait_def_id }
  26         .check(lang_items.drop_trait(), visit_implementation_of_drop)
  27         .check(lang_items.copy_trait(), visit_implementation_of_copy)
  28         .check(lang_items.coerce_unsized_trait(), visit_implementation_of_coerce_unsized)
  29         .check(lang_items.dispatch_from_dyn_trait(), visit_implementation_of_dispatch_from_dyn);
  30 }
  31
  32 struct Checker<'tcx> {
  33     tcx: TyCtxt<'tcx>,
  34     trait_def_id: DefId,
  35 }
  36
  37 impl<'tcx> Checker<'tcx> {
  38     fn check<F>(&self, trait_def_id: Option<DefId>, mut f: F) -> &Self
  39     where
  40         F: FnMut(TyCtxt<'tcx>, LocalDefId),
  41     {
  42         if Some(self.trait_def_id) == trait_def_id {
  43             for &impl_def_id in self.tcx.hir().trait_impls(self.trait_def_id) {
  44                 f(self.tcx, impl_def_id);
  45             }
  46         }
  47         self
  48     }
  49 }
  50
  51 fn visit_implementation_of_drop(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
  52     // Destructors only work on local ADT types.
  53     match tcx.type_of(impl_did).kind() {
  54         ty::Adt(def, _) if def.did().is_local() => return,
  55         ty::Error(_) => return,
  56         _ => {}
  57     }
  58
  59     let impl_ = tcx.hir().expect_item(impl_did).expect_impl();
  60
  61     tcx.sess.emit_err(DropImplOnWrongItem { span: impl_.self_ty.span });
  62 }
  63
  64 fn visit_implementation_of_copy(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
  65     debug!("visit_implementation_of_copy: impl_did={:?}", impl_did);
  66
  67     let self_type = tcx.type_of(impl_did);
  68     debug!("visit_implementation_of_copy: self_type={:?} (bound)", self_type);
  69
  70     let param_env = tcx.param_env(impl_did);
  71     assert!(!self_type.has_escaping_bound_vars());
  72
  73     debug!("visit_implementation_of_copy: self_type={:?} (free)", self_type);
  74
  75     let span = match tcx.hir().expect_item(impl_did).kind {
  76         ItemKind::Impl(hir::Impl { polarity: hir::ImplPolarity::Negative(_), .. }) => return,
  77         ItemKind::Impl(impl_) => impl_.self_ty.span,
  78         _ => bug!("expected Copy impl item"),
  79     };
  80
  81     let cause = traits::ObligationCause::misc(span, impl_did);
  82     match type_allowed_to_implement_copy(tcx, param_env, self_type, cause) {
  83         Ok(()) => {}
  84         Err(CopyImplementationError::InfrigingFields(fields)) => {
  85             let mut err = struct_span_err!(
  86                 tcx.sess,
  87                 span,
  88                 E0204,
  89                 "the trait `Copy` may not be implemented for this type"
  90             );
  91
  92             // We'll try to suggest constraining type parameters to fulfill the requirements of
  93             // their `Copy` implementation.
  94             let mut errors: BTreeMap<_, Vec<_>> = Default::default();
  95             let mut bounds = vec![];
  96
  97             for (field, ty, reason) in fields {
  98                 let field_span = tcx.def_span(field.did);
  99                 err.span_label(field_span, "this field does not implement `Copy`");
 100
 101                 match reason {
 102                     InfringingFieldsReason::Fulfill(fulfillment_errors) => {
 103                         for error in fulfillment_errors {
 104                             let error_predicate = error.obligation.predicate;
 105                             // Only note if it's not the root obligation, otherwise it's trivial and
 106                             // should be self-explanatory (i.e. a field literally doesn't implement Copy).
 107
 108                             // FIXME: This error could be more descriptive, especially if the error_predicate
 109                             // contains a foreign type or if it's a deeply nested type...
 110                             if error_predicate != error.root_obligation.predicate {
 111                                 errors
 112                                     .entry((ty.to_string(), error_predicate.to_string()))
 113                                     .or_default()
 114                                     .push(error.obligation.cause.span);
 115                             }
 116                             if let ty::PredicateKind::Clause(ty::Clause::Trait(
 117                                 ty::TraitPredicate {
 118                                     trait_ref,
 119                                     polarity: ty::ImplPolarity::Positive,
 120                                     ..
 121                                 },
 122                             )) = error_predicate.kind().skip_binder()
 123                             {
 124                                 let ty = trait_ref.self_ty();
 125                                 if let ty::Param(_) = ty.kind() {
 126                                     bounds.push((
 127                                         format!("{ty}"),
 128                                         trait_ref.print_only_trait_path().to_string(),
 129                                         Some(trait_ref.def_id),
 130                                     ));
 131                                 }
 132                             }
 133                         }
 134                     }
 135                     InfringingFieldsReason::Regions(region_errors) => {
 136                         for error in region_errors {
 137                             let ty = ty.to_string();
 138                             match error {
 139                                 RegionResolutionError::ConcreteFailure(origin, a, b) => {
 140                                     let predicate = format!("{b}: {a}");
 141                                     errors
 142                                         .entry((ty.clone(), predicate.clone()))
 143                                         .or_default()
 144                                         .push(origin.span());
 145                                     if let ty::RegionKind::ReEarlyBound(ebr) = *b && ebr.has_name() {
 146                                         bounds.push((b.to_string(), a.to_string(), None));
 147                                     }
 148                                 }
 149                                 RegionResolutionError::GenericBoundFailure(origin, a, b) => {
 150                                     let predicate = format!("{a}: {b}");
 151                                     errors
 152                                         .entry((ty.clone(), predicate.clone()))
 153                                         .or_default()
 154                                         .push(origin.span());
 155                                     if let infer::region_constraints::GenericKind::Param(_) = a {
 156                                         bounds.push((a.to_string(), b.to_string(), None));
 157                                     }
 158                                 }
 159                                 _ => continue,
 160                             }
 161                         }
 162                     }
 163                 }
 164             }
 165             for ((ty, error_predicate), spans) in errors {
 166                 let span: MultiSpan = spans.into();
 167                 err.span_note(
 168                     span,
 169                     &format!("the `Copy` impl for `{}` requires that `{}`", ty, error_predicate),
 170                 );
 171             }
 172             suggest_constraining_type_params(
 173                 tcx,
 174                 tcx.hir().get_generics(impl_did).expect("impls always have generics"),
 175                 &mut err,
 176                 bounds.iter().map(|(param, constraint, def_id)| {
 177                     (param.as_str(), constraint.as_str(), *def_id)
 178                 }),
 179             );
 180             err.emit();
 181         }
 182         Err(CopyImplementationError::NotAnAdt) => {
 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: TyCtxt<'_>, 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: TyCtxt<'_>, 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().subst_identity();
 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     let infcx = tcx.infer_ctxt().build();
 225     let cause = ObligationCause::misc(span, impl_did);
 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("implementing the `DispatchFromDyn` trait requires multiple coercions")
 303                     .note(
 304                         "the trait `DispatchFromDyn` may only be implemented \
 305                             for a coercion between structures with a single field \
 306                             being coerced",
 307                     )
 308                     .note(&format!(
 309                         "currently, {} fields need coercions: {}",
 310                         coerced_fields.len(),
 311                         coerced_fields
 312                             .iter()
 313                             .map(|field| {
 314                                 format!(
 315                                     "`{}` (`{}` to `{}`)",
 316                                     field.name,
 317                                     field.ty(tcx, substs_a),
 318                                     field.ty(tcx, substs_b),
 319                                 )
 320                             })
 321                             .collect::<Vec<_>>()
 322                             .join(", ")
 323                     ))
 324                     .emit();
 325             } else {
 326                 let errors = traits::fully_solve_obligations(
 327                     &infcx,
 328                     coerced_fields.into_iter().map(|field| {
 329                         predicate_for_trait_def(
 330                             tcx,
 331                             param_env,
 332                             cause.clone(),
 333                             dispatch_from_dyn_trait,
 334                             0,
 335                             [field.ty(tcx, substs_a), field.ty(tcx, substs_b)],
 336                         )
 337                     }),
 338                 );
 339                 if !errors.is_empty() {
 340                     infcx.err_ctxt().report_fulfillment_errors(&errors, None);
 341                 }
 342
 343                 // Finally, resolve all regions.
 344                 let outlives_env = OutlivesEnvironment::new(param_env);
 345                 let _ = infcx
 346                     .err_ctxt()
 347                     .check_region_obligations_and_report_errors(impl_did, &outlives_env);
 348             }
 349         }
 350         _ => {
 351             create_err(
 352                 "the trait `DispatchFromDyn` may only be implemented \
 353                     for a coercion between structures",
 354             )
 355             .emit();
 356         }
 357     }
 358 }
 359
 360 pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUnsizedInfo {
 361     debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
 362
 363     // this provider should only get invoked for local def-ids
 364     let impl_did = impl_did.expect_local();
 365     let span = tcx.def_span(impl_did);
 366
 367     let coerce_unsized_trait = tcx.require_lang_item(LangItem::CoerceUnsized, Some(span));
 368
 369     let unsize_trait = tcx.lang_items().require(LangItem::Unsize).unwrap_or_else(|err| {
 370         tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err.to_string()));
 371     });
 372
 373     let source = tcx.type_of(impl_did);
 374     let trait_ref = tcx.impl_trait_ref(impl_did).unwrap().subst_identity();
 375     assert_eq!(trait_ref.def_id, coerce_unsized_trait);
 376     let target = trait_ref.substs.type_at(1);
 377     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)", source, target);
 378
 379     let param_env = tcx.param_env(impl_did);
 380     assert!(!source.has_escaping_bound_vars());
 381
 382     let err_info = CoerceUnsizedInfo { custom_kind: None };
 383
 384     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);
 385
 386     let infcx = tcx.infer_ctxt().build();
 387     let cause = ObligationCause::misc(span, impl_did);
 388     let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
 389                        mt_b: ty::TypeAndMut<'tcx>,
 390                        mk_ptr: &dyn Fn(Ty<'tcx>) -> Ty<'tcx>| {
 391         if mt_a.mutbl < mt_b.mutbl {
 392             infcx
 393                 .err_ctxt()
 394                 .report_mismatched_types(
 395                     &cause,
 396                     mk_ptr(mt_b.ty),
 397                     target,
 398                     ty::error::TypeError::Mutability,
 399                 )
 400                 .emit();
 401         }
 402         (mt_a.ty, mt_b.ty, unsize_trait, None)
 403     };
 404     let (source, target, trait_def_id, kind) = match (source.kind(), target.kind()) {
 405         (&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b)) => {
 406             infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
 407             let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 408             let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };
 409             check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ref(r_b, ty))
 410         }
 411
 412         (&ty::Ref(_, ty_a, mutbl_a), &ty::RawPtr(mt_b)) => {
 413             let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 414             check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 415         }
 416
 417         (&ty::RawPtr(mt_a), &ty::RawPtr(mt_b)) => check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty)),
 418
 419         (&ty::Adt(def_a, substs_a), &ty::Adt(def_b, substs_b))
 420             if def_a.is_struct() && def_b.is_struct() =>
 421         {
 422             if def_a != def_b {
 423                 let source_path = tcx.def_path_str(def_a.did());
 424                 let target_path = tcx.def_path_str(def_b.did());
 425                 struct_span_err!(
 426                     tcx.sess,
 427                     span,
 428                     E0377,
 429                     "the trait `CoerceUnsized` may only be implemented \
 430                            for a coercion between structures with the same \
 431                            definition; expected `{}`, found `{}`",
 432                     source_path,
 433                     target_path
 434                 )
 435                 .emit();
 436                 return err_info;
 437             }
 438
 439             // Here we are considering a case of converting
 440             // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
 441             // which acts like a pointer to `U`, but carries along some extra data of type `T`:
 442             //
 443             //     struct Foo<T, U> {
 444             //         extra: T,
 445             //         ptr: *mut U,
 446             //     }
 447             //
 448             // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
 449             // to `Foo<T, [i32]>`. That impl would look like:
 450             //
 451             //   impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
 452             //
 453             // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
 454             // when this coercion occurs, we would be changing the
 455             // field `ptr` from a thin pointer of type `*mut [i32;
 456             // 3]` to a fat pointer of type `*mut [i32]` (with
 457             // extra data `3`). **The purpose of this check is to
 458             // make sure that we know how to do this conversion.**
 459             //
 460             // To check if this impl is legal, we would walk down
 461             // the fields of `Foo` and consider their types with
 462             // both substitutes. We are looking to find that
 463             // exactly one (non-phantom) field has changed its
 464             // type, which we will expect to be the pointer that
 465             // is becoming fat (we could probably generalize this
 466             // to multiple thin pointers of the same type becoming
 467             // fat, but we don't). In this case:
 468             //
 469             // - `extra` has type `T` before and type `T` after
 470             // - `ptr` has type `*mut U` before and type `*mut V` after
 471             //
 472             // Since just one field changed, we would then check
 473             // that `*mut U: CoerceUnsized<*mut V>` is implemented
 474             // (in other words, that we know how to do this
 475             // conversion). This will work out because `U:
 476             // Unsize<V>`, and we have a builtin rule that `*mut
 477             // U` can be coerced to `*mut V` if `U: Unsize<V>`.
 478             let fields = &def_a.non_enum_variant().fields;
 479             let diff_fields = fields
 480                 .iter()
 481                 .enumerate()
 482                 .filter_map(|(i, f)| {
 483                     let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));
 484
 485                     if tcx.type_of(f.did).is_phantom_data() {
 486                         // Ignore PhantomData fields
 487                         return None;
 488                     }
 489
 490                     // Ignore fields that aren't changed; it may
 491                     // be that we could get away with subtyping or
 492                     // something more accepting, but we use
 493                     // equality because we want to be able to
 494                     // perform this check without computing
 495                     // variance where possible. (This is because
 496                     // we may have to evaluate constraint
 497                     // expressions in the course of execution.)
 498                     // See e.g., #41936.
 499                     if let Ok(ok) = infcx.at(&cause, param_env).eq(a, b) {
 500                         if ok.obligations.is_empty() {
 501                             return None;
 502                         }
 503                     }
 504
 505                     // Collect up all fields that were significantly changed
 506                     // i.e., those that contain T in coerce_unsized T -> U
 507                     Some((i, a, b))
 508                 })
 509                 .collect::<Vec<_>>();
 510
 511             if diff_fields.is_empty() {
 512                 struct_span_err!(
 513                     tcx.sess,
 514                     span,
 515                     E0374,
 516                     "the trait `CoerceUnsized` may only be implemented \
 517                            for a coercion between structures with one field \
 518                            being coerced, none found"
 519                 )
 520                 .emit();
 521                 return err_info;
 522             } else if diff_fields.len() > 1 {
 523                 let item = tcx.hir().expect_item(impl_did);
 524                 let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(t), .. }) = &item.kind {
 525                     t.path.span
 526                 } else {
 527                     tcx.def_span(impl_did)
 528                 };
 529
 530                 struct_span_err!(
 531                     tcx.sess,
 532                     span,
 533                     E0375,
 534                     "implementing the trait \
 535                                                 `CoerceUnsized` requires multiple \
 536                                                 coercions"
 537                 )
 538                 .note(
 539                     "`CoerceUnsized` may only be implemented for \
 540                           a coercion between structures with one field being coerced",
 541                 )
 542                 .note(&format!(
 543                     "currently, {} fields need coercions: {}",
 544                     diff_fields.len(),
 545                     diff_fields
 546                         .iter()
 547                         .map(|&(i, a, b)| { format!("`{}` (`{}` to `{}`)", fields[i].name, a, b) })
 548                         .collect::<Vec<_>>()
 549                         .join(", ")
 550                 ))
 551                 .span_label(span, "requires multiple coercions")
 552                 .emit();
 553                 return err_info;
 554             }
 555
 556             let (i, a, b) = diff_fields[0];
 557             let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
 558             (a, b, coerce_unsized_trait, Some(kind))
 559         }
 560
 561         _ => {
 562             struct_span_err!(
 563                 tcx.sess,
 564                 span,
 565                 E0376,
 566                 "the trait `CoerceUnsized` may only be implemented \
 567                        for a coercion between structures"
 568             )
 569             .emit();
 570             return err_info;
 571         }
 572     };
 573
 574     // Register an obligation for `A: Trait<B>`.
 575     let cause = traits::ObligationCause::misc(span, impl_did);
 576     let predicate =
 577         predicate_for_trait_def(tcx, param_env, cause, trait_def_id, 0, [source, target]);
 578     let errors = traits::fully_solve_obligation(&infcx, predicate);
 579     if !errors.is_empty() {
 580         infcx.err_ctxt().report_fulfillment_errors(&errors, None);
 581     }
 582
 583     // Finally, resolve all regions.
 584     let outlives_env = OutlivesEnvironment::new(param_env);
 585     let _ = infcx.err_ctxt().check_region_obligations_and_report_errors(impl_did, &outlives_env);
 586
 587     CoerceUnsizedInfo { custom_kind: kind }
 588 }