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 ItemKind::Impl(impl_) = tcx.hir().expect_item(impl_did).kind else { bug!("expected Drop impl item") };
  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 impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
  68
  69     let self_type = tcx.type_of(impl_did);
  70     debug!("visit_implementation_of_copy: self_type={:?} (bound)", self_type);
  71
  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 span = match tcx.hir().expect_item(impl_did).kind {
  78         ItemKind::Impl(hir::Impl { polarity: hir::ImplPolarity::Negative(_), .. }) => return,
  79         ItemKind::Impl(impl_) => impl_.self_ty.span,
  80         _ => bug!("expected Copy impl item"),
  81     };
  82
  83     let cause = traits::ObligationCause::misc(span, impl_hir_id);
  84     match type_allowed_to_implement_copy(tcx, param_env, self_type, cause) {
  85         Ok(()) => {}
  86         Err(CopyImplementationError::InfrigingFields(fields)) => {
  87             let mut err = struct_span_err!(
  88                 tcx.sess,
  89                 span,
  90                 E0204,
  91                 "the trait `Copy` may not be implemented for this type"
  92             );
  93
  94             // We'll try to suggest constraining type parameters to fulfill the requirements of
  95             // their `Copy` implementation.
  96             let mut errors: BTreeMap<_, Vec<_>> = Default::default();
  97             let mut bounds = vec![];
  98
  99             for (field, ty, reason) in fields {
 100                 let field_span = tcx.def_span(field.did);
 101                 err.span_label(field_span, "this field does not implement `Copy`");
 102
 103                 match reason {
 104                     InfringingFieldsReason::Fulfill(fulfillment_errors) => {
 105                         for error in fulfillment_errors {
 106                             let error_predicate = error.obligation.predicate;
 107                             // Only note if it's not the root obligation, otherwise it's trivial and
 108                             // should be self-explanatory (i.e. a field literally doesn't implement Copy).
 109
 110                             // FIXME: This error could be more descriptive, especially if the error_predicate
 111                             // contains a foreign type or if it's a deeply nested type...
 112                             if error_predicate != error.root_obligation.predicate {
 113                                 errors
 114                                     .entry((ty.to_string(), error_predicate.to_string()))
 115                                     .or_default()
 116                                     .push(error.obligation.cause.span);
 117                             }
 118                             if let ty::PredicateKind::Clause(ty::Clause::Trait(
 119                                 ty::TraitPredicate {
 120                                     trait_ref,
 121                                     polarity: ty::ImplPolarity::Positive,
 122                                     ..
 123                                 },
 124                             )) = error_predicate.kind().skip_binder()
 125                             {
 126                                 let ty = trait_ref.self_ty();
 127                                 if let ty::Param(_) = ty.kind() {
 128                                     bounds.push((
 129                                         format!("{ty}"),
 130                                         trait_ref.print_only_trait_path().to_string(),
 131                                         Some(trait_ref.def_id),
 132                                     ));
 133                                 }
 134                             }
 135                         }
 136                     }
 137                     InfringingFieldsReason::Regions(region_errors) => {
 138                         for error in region_errors {
 139                             let ty = ty.to_string();
 140                             match error {
 141                                 RegionResolutionError::ConcreteFailure(origin, a, b) => {
 142                                     let predicate = format!("{b}: {a}");
 143                                     errors
 144                                         .entry((ty.clone(), predicate.clone()))
 145                                         .or_default()
 146                                         .push(origin.span());
 147                                     if let ty::RegionKind::ReEarlyBound(ebr) = *b && ebr.has_name() {
 148                                         bounds.push((b.to_string(), a.to_string(), None));
 149                                     }
 150                                 }
 151                                 RegionResolutionError::GenericBoundFailure(origin, a, b) => {
 152                                     let predicate = format!("{a}: {b}");
 153                                     errors
 154                                         .entry((ty.clone(), predicate.clone()))
 155                                         .or_default()
 156                                         .push(origin.span());
 157                                     if let infer::region_constraints::GenericKind::Param(_) = a {
 158                                         bounds.push((a.to_string(), b.to_string(), None));
 159                                     }
 160                                 }
 161                                 _ => continue,
 162                             }
 163                         }
 164                     }
 165                 }
 166             }
 167             for ((ty, error_predicate), spans) in errors {
 168                 let span: MultiSpan = spans.into();
 169                 err.span_note(
 170                     span,
 171                     &format!("the `Copy` impl for `{}` requires that `{}`", ty, error_predicate),
 172                 );
 173             }
 174             suggest_constraining_type_params(
 175                 tcx,
 176                 tcx.hir().get_generics(impl_did).expect("impls always have generics"),
 177                 &mut err,
 178                 bounds.iter().map(|(param, constraint, def_id)| {
 179                     (param.as_str(), constraint.as_str(), *def_id)
 180                 }),
 181             );
 182             err.emit();
 183         }
 184         Err(CopyImplementationError::NotAnAdt) => {
 185             tcx.sess.emit_err(CopyImplOnNonAdt { span });
 186         }
 187         Err(CopyImplementationError::HasDestructor) => {
 188             tcx.sess.emit_err(CopyImplOnTypeWithDtor { span });
 189         }
 190     }
 191 }
 192
 193 fn visit_implementation_of_coerce_unsized(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
 194     debug!("visit_implementation_of_coerce_unsized: impl_did={:?}", impl_did);
 195
 196     // Just compute this for the side-effects, in particular reporting
 197     // errors; other parts of the code may demand it for the info of
 198     // course.
 199     let span = tcx.def_span(impl_did);
 200     tcx.at(span).coerce_unsized_info(impl_did);
 201 }
 202
 203 fn visit_implementation_of_dispatch_from_dyn(tcx: TyCtxt<'_>, impl_did: LocalDefId) {
 204     debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);
 205
 206     let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
 207     let span = tcx.hir().span(impl_hir_id);
 208
 209     let dispatch_from_dyn_trait = tcx.require_lang_item(LangItem::DispatchFromDyn, Some(span));
 210
 211     let source = tcx.type_of(impl_did);
 212     assert!(!source.has_escaping_bound_vars());
 213     let target = {
 214         let trait_ref = tcx.impl_trait_ref(impl_did).unwrap().subst_identity();
 215         assert_eq!(trait_ref.def_id, dispatch_from_dyn_trait);
 216
 217         trait_ref.substs.type_at(1)
 218     };
 219
 220     debug!("visit_implementation_of_dispatch_from_dyn: {:?} -> {:?}", source, target);
 221
 222     let param_env = tcx.param_env(impl_did);
 223
 224     let create_err = |msg: &str| struct_span_err!(tcx.sess, span, E0378, "{}", msg);
 225
 226     let infcx = tcx.infer_ctxt().build();
 227     let cause = ObligationCause::misc(span, impl_hir_id);
 228
 229     use rustc_type_ir::sty::TyKind::*;
 230     match (source.kind(), target.kind()) {
 231         (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b))
 232             if infcx.at(&cause, param_env).eq(r_a, *r_b).is_ok() && mutbl_a == *mutbl_b => {}
 233         (&RawPtr(tm_a), &RawPtr(tm_b)) if tm_a.mutbl == tm_b.mutbl => (),
 234         (&Adt(def_a, substs_a), &Adt(def_b, substs_b))
 235             if def_a.is_struct() && def_b.is_struct() =>
 236         {
 237             if def_a != def_b {
 238                 let source_path = tcx.def_path_str(def_a.did());
 239                 let target_path = tcx.def_path_str(def_b.did());
 240
 241                 create_err(&format!(
 242                     "the trait `DispatchFromDyn` may only be implemented \
 243                             for a coercion between structures with the same \
 244                             definition; expected `{}`, found `{}`",
 245                     source_path, target_path,
 246                 ))
 247                 .emit();
 248
 249                 return;
 250             }
 251
 252             if def_a.repr().c() || def_a.repr().packed() {
 253                 create_err(
 254                     "structs implementing `DispatchFromDyn` may not have \
 255                          `#[repr(packed)]` or `#[repr(C)]`",
 256                 )
 257                 .emit();
 258             }
 259
 260             let fields = &def_a.non_enum_variant().fields;
 261
 262             let coerced_fields = fields
 263                 .iter()
 264                 .filter(|field| {
 265                     let ty_a = field.ty(tcx, substs_a);
 266                     let ty_b = field.ty(tcx, substs_b);
 267
 268                     if let Ok(layout) = tcx.layout_of(param_env.and(ty_a)) {
 269                         if layout.is_zst() && layout.align.abi.bytes() == 1 {
 270                             // ignore ZST fields with alignment of 1 byte
 271                             return false;
 272                         }
 273                     }
 274
 275                     if let Ok(ok) = infcx.at(&cause, param_env).eq(ty_a, ty_b) {
 276                         if ok.obligations.is_empty() {
 277                             create_err(
 278                                 "the trait `DispatchFromDyn` may only be implemented \
 279                                  for structs containing the field being coerced, \
 280                                  ZST fields with 1 byte alignment, and nothing else",
 281                             )
 282                             .note(&format!(
 283                                 "extra field `{}` of type `{}` is not allowed",
 284                                 field.name, ty_a,
 285                             ))
 286                             .emit();
 287
 288                             return false;
 289                         }
 290                     }
 291
 292                     return true;
 293                 })
 294                 .collect::<Vec<_>>();
 295
 296             if coerced_fields.is_empty() {
 297                 create_err(
 298                     "the trait `DispatchFromDyn` may only be implemented \
 299                         for a coercion between structures with a single field \
 300                         being coerced, none found",
 301                 )
 302                 .emit();
 303             } else if coerced_fields.len() > 1 {
 304                 create_err("implementing the `DispatchFromDyn` trait requires multiple coercions")
 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 errors = traits::fully_solve_obligations(
 329                     &infcx,
 330                     coerced_fields.into_iter().map(|field| {
 331                         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), field.ty(tcx, substs_b)],
 338                         )
 339                     }),
 340                 );
 341                 if !errors.is_empty() {
 342                     infcx.err_ctxt().report_fulfillment_errors(&errors, None);
 343                 }
 344
 345                 // Finally, resolve all regions.
 346                 let outlives_env = OutlivesEnvironment::new(param_env);
 347                 let _ = infcx
 348                     .err_ctxt()
 349                     .check_region_obligations_and_report_errors(impl_did, &outlives_env);
 350             }
 351         }
 352         _ => {
 353             create_err(
 354                 "the trait `DispatchFromDyn` may only be implemented \
 355                     for a coercion between structures",
 356             )
 357             .emit();
 358         }
 359     }
 360 }
 361
 362 pub fn coerce_unsized_info<'tcx>(tcx: TyCtxt<'tcx>, impl_did: DefId) -> CoerceUnsizedInfo {
 363     debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
 364
 365     // this provider should only get invoked for local def-ids
 366     let impl_did = impl_did.expect_local();
 367     let span = tcx.def_span(impl_did);
 368
 369     let coerce_unsized_trait = tcx.require_lang_item(LangItem::CoerceUnsized, Some(span));
 370
 371     let unsize_trait = tcx.lang_items().require(LangItem::Unsize).unwrap_or_else(|err| {
 372         tcx.sess.fatal(&format!("`CoerceUnsized` implementation {}", err.to_string()));
 373     });
 374
 375     let source = tcx.type_of(impl_did);
 376     let trait_ref = tcx.impl_trait_ref(impl_did).unwrap().subst_identity();
 377     assert_eq!(trait_ref.def_id, coerce_unsized_trait);
 378     let target = trait_ref.substs.type_at(1);
 379     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)", source, target);
 380
 381     let param_env = tcx.param_env(impl_did);
 382     assert!(!source.has_escaping_bound_vars());
 383
 384     let err_info = CoerceUnsizedInfo { custom_kind: None };
 385
 386     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);
 387
 388     let infcx = tcx.infer_ctxt().build();
 389     let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_did);
 390     let cause = ObligationCause::misc(span, impl_hir_id);
 391     let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
 392                        mt_b: ty::TypeAndMut<'tcx>,
 393                        mk_ptr: &dyn Fn(Ty<'tcx>) -> Ty<'tcx>| {
 394         if mt_a.mutbl < mt_b.mutbl {
 395             infcx
 396                 .err_ctxt()
 397                 .report_mismatched_types(
 398                     &cause,
 399                     mk_ptr(mt_b.ty),
 400                     target,
 401                     ty::error::TypeError::Mutability,
 402                 )
 403                 .emit();
 404         }
 405         (mt_a.ty, mt_b.ty, unsize_trait, None)
 406     };
 407     let (source, target, trait_def_id, kind) = match (source.kind(), target.kind()) {
 408         (&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b)) => {
 409             infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
 410             let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 411             let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };
 412             check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ref(r_b, ty))
 413         }
 414
 415         (&ty::Ref(_, ty_a, mutbl_a), &ty::RawPtr(mt_b)) => {
 416             let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 417             check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty))
 418         }
 419
 420         (&ty::RawPtr(mt_a), &ty::RawPtr(mt_b)) => check_mutbl(mt_a, mt_b, &|ty| tcx.mk_imm_ptr(ty)),
 421
 422         (&ty::Adt(def_a, substs_a), &ty::Adt(def_b, substs_b))
 423             if def_a.is_struct() && def_b.is_struct() =>
 424         {
 425             if def_a != def_b {
 426                 let source_path = tcx.def_path_str(def_a.did());
 427                 let target_path = tcx.def_path_str(def_b.did());
 428                 struct_span_err!(
 429                     tcx.sess,
 430                     span,
 431                     E0377,
 432                     "the trait `CoerceUnsized` may only be implemented \
 433                            for a coercion between structures with the same \
 434                            definition; expected `{}`, found `{}`",
 435                     source_path,
 436                     target_path
 437                 )
 438                 .emit();
 439                 return err_info;
 440             }
 441
 442             // Here we are considering a case of converting
 443             // `S<P0...Pn>` to S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
 444             // which acts like a pointer to `U`, but carries along some extra data of type `T`:
 445             //
 446             //     struct Foo<T, U> {
 447             //         extra: T,
 448             //         ptr: *mut U,
 449             //     }
 450             //
 451             // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
 452             // to `Foo<T, [i32]>`. That impl would look like:
 453             //
 454             //   impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
 455             //
 456             // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
 457             // when this coercion occurs, we would be changing the
 458             // field `ptr` from a thin pointer of type `*mut [i32;
 459             // 3]` to a fat pointer of type `*mut [i32]` (with
 460             // extra data `3`). **The purpose of this check is to
 461             // make sure that we know how to do this conversion.**
 462             //
 463             // To check if this impl is legal, we would walk down
 464             // the fields of `Foo` and consider their types with
 465             // both substitutes. We are looking to find that
 466             // exactly one (non-phantom) field has changed its
 467             // type, which we will expect to be the pointer that
 468             // is becoming fat (we could probably generalize this
 469             // to multiple thin pointers of the same type becoming
 470             // fat, but we don't). In this case:
 471             //
 472             // - `extra` has type `T` before and type `T` after
 473             // - `ptr` has type `*mut U` before and type `*mut V` after
 474             //
 475             // Since just one field changed, we would then check
 476             // that `*mut U: CoerceUnsized<*mut V>` is implemented
 477             // (in other words, that we know how to do this
 478             // conversion). This will work out because `U:
 479             // Unsize<V>`, and we have a builtin rule that `*mut
 480             // U` can be coerced to `*mut V` if `U: Unsize<V>`.
 481             let fields = &def_a.non_enum_variant().fields;
 482             let diff_fields = fields
 483                 .iter()
 484                 .enumerate()
 485                 .filter_map(|(i, f)| {
 486                     let (a, b) = (f.ty(tcx, substs_a), f.ty(tcx, substs_b));
 487
 488                     if tcx.type_of(f.did).is_phantom_data() {
 489                         // Ignore PhantomData fields
 490                         return None;
 491                     }
 492
 493                     // Ignore fields that aren't changed; it may
 494                     // be that we could get away with subtyping or
 495                     // something more accepting, but we use
 496                     // equality because we want to be able to
 497                     // perform this check without computing
 498                     // variance where possible. (This is because
 499                     // we may have to evaluate constraint
 500                     // expressions in the course of execution.)
 501                     // See e.g., #41936.
 502                     if let Ok(ok) = infcx.at(&cause, param_env).eq(a, b) {
 503                         if ok.obligations.is_empty() {
 504                             return None;
 505                         }
 506                     }
 507
 508                     // Collect up all fields that were significantly changed
 509                     // i.e., those that contain T in coerce_unsized T -> U
 510                     Some((i, a, b))
 511                 })
 512                 .collect::<Vec<_>>();
 513
 514             if diff_fields.is_empty() {
 515                 struct_span_err!(
 516                     tcx.sess,
 517                     span,
 518                     E0374,
 519                     "the trait `CoerceUnsized` may only be implemented \
 520                            for a coercion between structures with one field \
 521                            being coerced, none found"
 522                 )
 523                 .emit();
 524                 return err_info;
 525             } else if diff_fields.len() > 1 {
 526                 let item = tcx.hir().expect_item(impl_did);
 527                 let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(t), .. }) = &item.kind {
 528                     t.path.span
 529                 } else {
 530                     tcx.def_span(impl_did)
 531                 };
 532
 533                 struct_span_err!(
 534                     tcx.sess,
 535                     span,
 536                     E0375,
 537                     "implementing the trait \
 538                                                 `CoerceUnsized` requires multiple \
 539                                                 coercions"
 540                 )
 541                 .note(
 542                     "`CoerceUnsized` may only be implemented for \
 543                           a coercion between structures with one field being coerced",
 544                 )
 545                 .note(&format!(
 546                     "currently, {} fields need coercions: {}",
 547                     diff_fields.len(),
 548                     diff_fields
 549                         .iter()
 550                         .map(|&(i, a, b)| { format!("`{}` (`{}` to `{}`)", fields[i].name, a, b) })
 551                         .collect::<Vec<_>>()
 552                         .join(", ")
 553                 ))
 554                 .span_label(span, "requires multiple coercions")
 555                 .emit();
 556                 return err_info;
 557             }
 558
 559             let (i, a, b) = diff_fields[0];
 560             let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
 561             (a, b, coerce_unsized_trait, Some(kind))
 562         }
 563
 564         _ => {
 565             struct_span_err!(
 566                 tcx.sess,
 567                 span,
 568                 E0376,
 569                 "the trait `CoerceUnsized` may only be implemented \
 570                        for a coercion between structures"
 571             )
 572             .emit();
 573             return err_info;
 574         }
 575     };
 576
 577     // Register an obligation for `A: Trait<B>`.
 578     let cause = traits::ObligationCause::misc(span, impl_hir_id);
 579     let predicate =
 580         predicate_for_trait_def(tcx, param_env, cause, trait_def_id, 0, [source, target]);
 581     let errors = traits::fully_solve_obligation(&infcx, predicate);
 582     if !errors.is_empty() {
 583         infcx.err_ctxt().report_fulfillment_errors(&errors, None);
 584     }
 585
 586     // Finally, resolve all regions.
 587     let outlives_env = OutlivesEnvironment::new(param_env);
 588     let _ = infcx.err_ctxt().check_region_obligations_and_report_errors(impl_did, &outlives_env);
 589
 590     CoerceUnsizedInfo { custom_kind: kind }
 591 }