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;
   5
   6 use rustc_data_structures::fx::FxHashSet;
   7 use rustc_errors::{ErrorGuaranteed, MultiSpan};
   8 use rustc_hir as hir;
   9 use rustc_hir::def_id::{DefId, LocalDefId};
  10 use rustc_hir::lang_items::LangItem;
  11 use rustc_hir::ItemKind;
  12 use rustc_infer::infer::outlives::env::OutlivesEnvironment;
  13 use rustc_infer::infer::TyCtxtInferExt;
  14 use rustc_infer::infer::{self, RegionResolutionError};
  15 use rustc_infer::traits::Obligation;
  16 use rustc_middle::ty::adjustment::CoerceUnsizedInfo;
  17 use rustc_middle::ty::print::PrintTraitRefExt as _;
  18 use rustc_middle::ty::{self, suggest_constraining_type_params, Ty, TyCtxt, TypeVisitableExt};
  19 use rustc_span::{Span, DUMMY_SP};
  20 use rustc_trait_selection::traits::error_reporting::TypeErrCtxtExt;
  21 use rustc_trait_selection::traits::misc::{
  22     type_allowed_to_implement_const_param_ty, type_allowed_to_implement_copy,
  23     ConstParamTyImplementationError, CopyImplementationError, InfringingFieldsReason,
  24 };
  25 use rustc_trait_selection::traits::ObligationCtxt;
  26 use rustc_trait_selection::traits::{self, ObligationCause};
  27 use std::collections::BTreeMap;
  28
  29 pub(super) fn check_trait<'tcx>(
  30     tcx: TyCtxt<'tcx>,
  31     trait_def_id: DefId,
  32     impl_def_id: LocalDefId,
  33     impl_header: ty::ImplTraitHeader<'tcx>,
  34 ) -> Result<(), ErrorGuaranteed> {
  35     let lang_items = tcx.lang_items();
  36     let checker = Checker { tcx, trait_def_id, impl_def_id, impl_header };
  37     let mut res = checker.check(lang_items.drop_trait(), visit_implementation_of_drop);
  38     res = res.and(checker.check(lang_items.copy_trait(), visit_implementation_of_copy));
  39     res = res.and(
  40         checker.check(lang_items.const_param_ty_trait(), visit_implementation_of_const_param_ty),
  41     );
  42     res = res.and(
  43         checker.check(lang_items.coerce_unsized_trait(), visit_implementation_of_coerce_unsized),
  44     );
  45     res.and(
  46         checker
  47             .check(lang_items.dispatch_from_dyn_trait(), visit_implementation_of_dispatch_from_dyn),
  48     )
  49 }
  50
  51 struct Checker<'tcx> {
  52     tcx: TyCtxt<'tcx>,
  53     trait_def_id: DefId,
  54     impl_def_id: LocalDefId,
  55     impl_header: ty::ImplTraitHeader<'tcx>,
  56 }
  57
  58 impl<'tcx> Checker<'tcx> {
  59     fn check(
  60         &self,
  61         trait_def_id: Option<DefId>,
  62         f: impl FnOnce(&Self) -> Result<(), ErrorGuaranteed>,
  63     ) -> Result<(), ErrorGuaranteed> {
  64         if Some(self.trait_def_id) == trait_def_id { f(self) } else { Ok(()) }
  65     }
  66 }
  67
  68 fn visit_implementation_of_drop(checker: &Checker<'_>) -> Result<(), ErrorGuaranteed> {
  69     let tcx = checker.tcx;
  70     let impl_did = checker.impl_def_id;
  71     // Destructors only work on local ADT types.
  72     match checker.impl_header.trait_ref.instantiate_identity().self_ty().kind() {
  73         ty::Adt(def, _) if def.did().is_local() => return Ok(()),
  74         ty::Error(_) => return Ok(()),
  75         _ => {}
  76     }
  77
  78     let impl_ = tcx.hir().expect_item(impl_did).expect_impl();
  79
  80     Err(tcx.dcx().emit_err(errors::DropImplOnWrongItem { span: impl_.self_ty.span }))
  81 }
  82
  83 fn visit_implementation_of_copy(checker: &Checker<'_>) -> Result<(), ErrorGuaranteed> {
  84     let tcx = checker.tcx;
  85     let impl_header = checker.impl_header;
  86     let impl_did = checker.impl_def_id;
  87     debug!("visit_implementation_of_copy: impl_did={:?}", impl_did);
  88
  89     let self_type = impl_header.trait_ref.instantiate_identity().self_ty();
  90     debug!("visit_implementation_of_copy: self_type={:?} (bound)", self_type);
  91
  92     let param_env = tcx.param_env(impl_did);
  93     assert!(!self_type.has_escaping_bound_vars());
  94
  95     debug!("visit_implementation_of_copy: self_type={:?} (free)", self_type);
  96
  97     if let ty::ImplPolarity::Negative = impl_header.polarity {
  98         return Ok(());
  99     }
 100
 101     let cause = traits::ObligationCause::misc(DUMMY_SP, impl_did);
 102     match type_allowed_to_implement_copy(tcx, param_env, self_type, cause) {
 103         Ok(()) => Ok(()),
 104         Err(CopyImplementationError::InfringingFields(fields)) => {
 105             let span = tcx.hir().expect_item(impl_did).expect_impl().self_ty.span;
 106             Err(infringing_fields_error(tcx, fields, LangItem::Copy, impl_did, span))
 107         }
 108         Err(CopyImplementationError::NotAnAdt) => {
 109             let span = tcx.hir().expect_item(impl_did).expect_impl().self_ty.span;
 110             Err(tcx.dcx().emit_err(errors::CopyImplOnNonAdt { span }))
 111         }
 112         Err(CopyImplementationError::HasDestructor) => {
 113             let span = tcx.hir().expect_item(impl_did).expect_impl().self_ty.span;
 114             Err(tcx.dcx().emit_err(errors::CopyImplOnTypeWithDtor { span }))
 115         }
 116     }
 117 }
 118
 119 fn visit_implementation_of_const_param_ty(checker: &Checker<'_>) -> Result<(), ErrorGuaranteed> {
 120     let tcx = checker.tcx;
 121     let header = checker.impl_header;
 122     let impl_did = checker.impl_def_id;
 123     let self_type = header.trait_ref.instantiate_identity().self_ty();
 124     assert!(!self_type.has_escaping_bound_vars());
 125
 126     let param_env = tcx.param_env(impl_did);
 127
 128     if let ty::ImplPolarity::Negative = header.polarity {
 129         return Ok(());
 130     }
 131
 132     let cause = traits::ObligationCause::misc(DUMMY_SP, impl_did);
 133     match type_allowed_to_implement_const_param_ty(tcx, param_env, self_type, cause) {
 134         Ok(()) => Ok(()),
 135         Err(ConstParamTyImplementationError::InfrigingFields(fields)) => {
 136             let span = tcx.hir().expect_item(impl_did).expect_impl().self_ty.span;
 137             Err(infringing_fields_error(tcx, fields, LangItem::ConstParamTy, impl_did, span))
 138         }
 139         Err(ConstParamTyImplementationError::NotAnAdtOrBuiltinAllowed) => {
 140             let span = tcx.hir().expect_item(impl_did).expect_impl().self_ty.span;
 141             Err(tcx.dcx().emit_err(errors::ConstParamTyImplOnNonAdt { span }))
 142         }
 143     }
 144 }
 145
 146 fn visit_implementation_of_coerce_unsized(checker: &Checker<'_>) -> Result<(), ErrorGuaranteed> {
 147     let tcx = checker.tcx;
 148     let impl_did = checker.impl_def_id;
 149     debug!("visit_implementation_of_coerce_unsized: impl_did={:?}", impl_did);
 150
 151     // Just compute this for the side-effects, in particular reporting
 152     // errors; other parts of the code may demand it for the info of
 153     // course.
 154     let span = tcx.def_span(impl_did);
 155     tcx.at(span).ensure().coerce_unsized_info(impl_did)
 156 }
 157
 158 fn visit_implementation_of_dispatch_from_dyn(checker: &Checker<'_>) -> Result<(), ErrorGuaranteed> {
 159     let tcx = checker.tcx;
 160     let impl_did = checker.impl_def_id;
 161     let trait_ref = checker.impl_header.trait_ref.instantiate_identity();
 162     debug!("visit_implementation_of_dispatch_from_dyn: impl_did={:?}", impl_did);
 163
 164     let span = tcx.def_span(impl_did);
 165
 166     let dispatch_from_dyn_trait = tcx.require_lang_item(LangItem::DispatchFromDyn, Some(span));
 167
 168     let source = trait_ref.self_ty();
 169     assert!(!source.has_escaping_bound_vars());
 170     let target = {
 171         assert_eq!(trait_ref.def_id, dispatch_from_dyn_trait);
 172
 173         trait_ref.args.type_at(1)
 174     };
 175
 176     debug!("visit_implementation_of_dispatch_from_dyn: {:?} -> {:?}", source, target);
 177
 178     let param_env = tcx.param_env(impl_did);
 179
 180     let infcx = tcx.infer_ctxt().build();
 181     let cause = ObligationCause::misc(span, impl_did);
 182
 183     // Later parts of the compiler rely on all DispatchFromDyn types to be ABI-compatible with raw
 184     // pointers. This is enforced here: we only allow impls for references, raw pointers, and things
 185     // that are effectively repr(transparent) newtypes around types that already hav a
 186     // DispatchedFromDyn impl. We cannot literally use repr(transparent) on those tpyes since some
 187     // of them support an allocator, but we ensure that for the cases where the type implements this
 188     // trait, they *do* satisfy the repr(transparent) rules, and then we assume that everything else
 189     // in the compiler (in particular, all the call ABI logic) will treat them as repr(transparent)
 190     // even if they do not carry that attribute.
 191     use rustc_type_ir::TyKind::*;
 192     match (source.kind(), target.kind()) {
 193         (&Ref(r_a, _, mutbl_a), Ref(r_b, _, mutbl_b)) if r_a == *r_b && mutbl_a == *mutbl_b => {
 194             Ok(())
 195         }
 196         (&RawPtr(_, a_mutbl), &RawPtr(_, b_mutbl)) if a_mutbl == b_mutbl => Ok(()),
 197         (&Adt(def_a, args_a), &Adt(def_b, args_b)) if def_a.is_struct() && def_b.is_struct() => {
 198             if def_a != def_b {
 199                 let source_path = tcx.def_path_str(def_a.did());
 200                 let target_path = tcx.def_path_str(def_b.did());
 201
 202                 return Err(tcx.dcx().emit_err(errors::DispatchFromDynCoercion {
 203                     span,
 204                     trait_name: "DispatchFromDyn",
 205                     note: true,
 206                     source_path,
 207                     target_path,
 208                 }));
 209             }
 210
 211             let mut res = Ok(());
 212             if def_a.repr().c() || def_a.repr().packed() {
 213                 res = Err(tcx.dcx().emit_err(errors::DispatchFromDynRepr { span }));
 214             }
 215
 216             let fields = &def_a.non_enum_variant().fields;
 217
 218             let coerced_fields = fields
 219                 .iter()
 220                 .filter(|field| {
 221                     let ty_a = field.ty(tcx, args_a);
 222                     let ty_b = field.ty(tcx, args_b);
 223
 224                     if let Ok(layout) = tcx.layout_of(param_env.and(ty_a)) {
 225                         if layout.is_1zst() {
 226                             // ignore 1-ZST fields
 227                             return false;
 228                         }
 229                     }
 230
 231                     if ty_a == ty_b {
 232                         res = Err(tcx.dcx().emit_err(errors::DispatchFromDynZST {
 233                             span,
 234                             name: field.name,
 235                             ty: ty_a,
 236                         }));
 237
 238                         return false;
 239                     }
 240
 241                     return true;
 242                 })
 243                 .collect::<Vec<_>>();
 244
 245             if coerced_fields.is_empty() {
 246                 res = Err(tcx.dcx().emit_err(errors::DispatchFromDynSingle {
 247                     span,
 248                     trait_name: "DispatchFromDyn",
 249                     note: true,
 250                 }));
 251             } else if coerced_fields.len() > 1 {
 252                 res = Err(tcx.dcx().emit_err(errors::DispatchFromDynMulti {
 253                     span,
 254                     coercions_note: true,
 255                     number: coerced_fields.len(),
 256                     coercions: coerced_fields
 257                         .iter()
 258                         .map(|field| {
 259                             format!(
 260                                 "`{}` (`{}` to `{}`)",
 261                                 field.name,
 262                                 field.ty(tcx, args_a),
 263                                 field.ty(tcx, args_b),
 264                             )
 265                         })
 266                         .collect::<Vec<_>>()
 267                         .join(", "),
 268                 }));
 269             } else {
 270                 let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
 271                 for field in coerced_fields {
 272                     ocx.register_obligation(Obligation::new(
 273                         tcx,
 274                         cause.clone(),
 275                         param_env,
 276                         ty::TraitRef::new(
 277                             tcx,
 278                             dispatch_from_dyn_trait,
 279                             [field.ty(tcx, args_a), field.ty(tcx, args_b)],
 280                         ),
 281                     ));
 282                 }
 283                 let errors = ocx.select_all_or_error();
 284                 if !errors.is_empty() {
 285                     res = Err(infcx.err_ctxt().report_fulfillment_errors(errors));
 286                 }
 287
 288                 // Finally, resolve all regions.
 289                 let outlives_env = OutlivesEnvironment::new(param_env);
 290                 res = res.and(ocx.resolve_regions_and_report_errors(impl_did, &outlives_env));
 291             }
 292             res
 293         }
 294         _ => Err(tcx
 295             .dcx()
 296             .emit_err(errors::CoerceUnsizedMay { span, trait_name: "DispatchFromDyn" })),
 297     }
 298 }
 299
 300 pub fn coerce_unsized_info<'tcx>(
 301     tcx: TyCtxt<'tcx>,
 302     impl_did: LocalDefId,
 303 ) -> Result<CoerceUnsizedInfo, ErrorGuaranteed> {
 304     debug!("compute_coerce_unsized_info(impl_did={:?})", impl_did);
 305     let span = tcx.def_span(impl_did);
 306
 307     let coerce_unsized_trait = tcx.require_lang_item(LangItem::CoerceUnsized, Some(span));
 308
 309     let unsize_trait = tcx.require_lang_item(LangItem::Unsize, Some(span));
 310
 311     let source = tcx.type_of(impl_did).instantiate_identity();
 312     let trait_ref = tcx.impl_trait_ref(impl_did).unwrap().instantiate_identity();
 313     assert_eq!(trait_ref.def_id, coerce_unsized_trait);
 314     let target = trait_ref.args.type_at(1);
 315     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (bound)", source, target);
 316
 317     let param_env = tcx.param_env(impl_did);
 318     assert!(!source.has_escaping_bound_vars());
 319
 320     debug!("visit_implementation_of_coerce_unsized: {:?} -> {:?} (free)", source, target);
 321
 322     let infcx = tcx.infer_ctxt().build();
 323     let cause = ObligationCause::misc(span, impl_did);
 324     let check_mutbl = |mt_a: ty::TypeAndMut<'tcx>,
 325                        mt_b: ty::TypeAndMut<'tcx>,
 326                        mk_ptr: &dyn Fn(Ty<'tcx>) -> Ty<'tcx>| {
 327         if mt_a.mutbl < mt_b.mutbl {
 328             infcx
 329                 .err_ctxt()
 330                 .report_mismatched_types(
 331                     &cause,
 332                     mk_ptr(mt_b.ty),
 333                     target,
 334                     ty::error::TypeError::Mutability,
 335                 )
 336                 .emit();
 337         }
 338         (mt_a.ty, mt_b.ty, unsize_trait, None)
 339     };
 340     let (source, target, trait_def_id, kind) = match (source.kind(), target.kind()) {
 341         (&ty::Ref(r_a, ty_a, mutbl_a), &ty::Ref(r_b, ty_b, mutbl_b)) => {
 342             infcx.sub_regions(infer::RelateObjectBound(span), r_b, r_a);
 343             let mt_a = ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a };
 344             let mt_b = ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b };
 345             check_mutbl(mt_a, mt_b, &|ty| Ty::new_imm_ref(tcx, r_b, ty))
 346         }
 347
 348         (&ty::Ref(_, ty_a, mutbl_a), &ty::RawPtr(ty_b, mutbl_b)) => check_mutbl(
 349             ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a },
 350             ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b },
 351             &|ty| Ty::new_imm_ptr(tcx, ty),
 352         ),
 353
 354         (&ty::RawPtr(ty_a, mutbl_a), &ty::RawPtr(ty_b, mutbl_b)) => check_mutbl(
 355             ty::TypeAndMut { ty: ty_a, mutbl: mutbl_a },
 356             ty::TypeAndMut { ty: ty_b, mutbl: mutbl_b },
 357             &|ty| Ty::new_imm_ptr(tcx, ty),
 358         ),
 359
 360         (&ty::Adt(def_a, args_a), &ty::Adt(def_b, args_b))
 361             if def_a.is_struct() && def_b.is_struct() =>
 362         {
 363             if def_a != def_b {
 364                 let source_path = tcx.def_path_str(def_a.did());
 365                 let target_path = tcx.def_path_str(def_b.did());
 366                 return Err(tcx.dcx().emit_err(errors::DispatchFromDynSame {
 367                     span,
 368                     trait_name: "CoerceUnsized",
 369                     note: true,
 370                     source_path,
 371                     target_path,
 372                 }));
 373             }
 374
 375             // Here we are considering a case of converting
 376             // `S<P0...Pn>` to `S<Q0...Qn>`. As an example, let's imagine a struct `Foo<T, U>`,
 377             // which acts like a pointer to `U`, but carries along some extra data of type `T`:
 378             //
 379             //     struct Foo<T, U> {
 380             //         extra: T,
 381             //         ptr: *mut U,
 382             //     }
 383             //
 384             // We might have an impl that allows (e.g.) `Foo<T, [i32; 3]>` to be unsized
 385             // to `Foo<T, [i32]>`. That impl would look like:
 386             //
 387             //   impl<T, U: Unsize<V>, V> CoerceUnsized<Foo<T, V>> for Foo<T, U> {}
 388             //
 389             // Here `U = [i32; 3]` and `V = [i32]`. At runtime,
 390             // when this coercion occurs, we would be changing the
 391             // field `ptr` from a thin pointer of type `*mut [i32;
 392             // 3]` to a fat pointer of type `*mut [i32]` (with
 393             // extra data `3`). **The purpose of this check is to
 394             // make sure that we know how to do this conversion.**
 395             //
 396             // To check if this impl is legal, we would walk down
 397             // the fields of `Foo` and consider their types with
 398             // both generic parameters. We are looking to find that
 399             // exactly one (non-phantom) field has changed its
 400             // type, which we will expect to be the pointer that
 401             // is becoming fat (we could probably generalize this
 402             // to multiple thin pointers of the same type becoming
 403             // fat, but we don't). In this case:
 404             //
 405             // - `extra` has type `T` before and type `T` after
 406             // - `ptr` has type `*mut U` before and type `*mut V` after
 407             //
 408             // Since just one field changed, we would then check
 409             // that `*mut U: CoerceUnsized<*mut V>` is implemented
 410             // (in other words, that we know how to do this
 411             // conversion). This will work out because `U:
 412             // Unsize<V>`, and we have a builtin rule that `*mut
 413             // U` can be coerced to `*mut V` if `U: Unsize<V>`.
 414             let fields = &def_a.non_enum_variant().fields;
 415             let diff_fields = fields
 416                 .iter_enumerated()
 417                 .filter_map(|(i, f)| {
 418                     let (a, b) = (f.ty(tcx, args_a), f.ty(tcx, args_b));
 419
 420                     if tcx.type_of(f.did).instantiate_identity().is_phantom_data() {
 421                         // Ignore PhantomData fields
 422                         return None;
 423                     }
 424
 425                     // Ignore fields that aren't changed; it may
 426                     // be that we could get away with subtyping or
 427                     // something more accepting, but we use
 428                     // equality because we want to be able to
 429                     // perform this check without computing
 430                     // variance or constraining opaque types' hidden types.
 431                     // (This is because we may have to evaluate constraint
 432                     // expressions in the course of execution.)
 433                     // See e.g., #41936.
 434                     if a == b {
 435                         return None;
 436                     }
 437
 438                     // Collect up all fields that were significantly changed
 439                     // i.e., those that contain T in coerce_unsized T -> U
 440                     Some((i, a, b))
 441                 })
 442                 .collect::<Vec<_>>();
 443
 444             if diff_fields.is_empty() {
 445                 return Err(tcx.dcx().emit_err(errors::CoerceUnsizedOneField {
 446                     span,
 447                     trait_name: "CoerceUnsized",
 448                     note: true,
 449                 }));
 450             } else if diff_fields.len() > 1 {
 451                 let item = tcx.hir().expect_item(impl_did);
 452                 let span = if let ItemKind::Impl(hir::Impl { of_trait: Some(t), .. }) = &item.kind {
 453                     t.path.span
 454                 } else {
 455                     tcx.def_span(impl_did)
 456                 };
 457
 458                 return Err(tcx.dcx().emit_err(errors::CoerceUnsizedMulti {
 459                     span,
 460                     coercions_note: true,
 461                     number: diff_fields.len(),
 462                     coercions: diff_fields
 463                         .iter()
 464                         .map(|&(i, a, b)| format!("`{}` (`{}` to `{}`)", fields[i].name, a, b))
 465                         .collect::<Vec<_>>()
 466                         .join(", "),
 467                 }));
 468             }
 469
 470             let (i, a, b) = diff_fields[0];
 471             let kind = ty::adjustment::CustomCoerceUnsized::Struct(i);
 472             (a, b, coerce_unsized_trait, Some(kind))
 473         }
 474
 475         _ => {
 476             return Err(tcx
 477                 .dcx()
 478                 .emit_err(errors::DispatchFromDynStruct { span, trait_name: "CoerceUnsized" }));
 479         }
 480     };
 481
 482     // Register an obligation for `A: Trait<B>`.
 483     let ocx = ObligationCtxt::new_with_diagnostics(&infcx);
 484     let cause = traits::ObligationCause::misc(span, impl_did);
 485     let obligation = Obligation::new(
 486         tcx,
 487         cause,
 488         param_env,
 489         ty::TraitRef::new(tcx, trait_def_id, [source, target]),
 490     );
 491     ocx.register_obligation(obligation);
 492     let errors = ocx.select_all_or_error();
 493     if !errors.is_empty() {
 494         infcx.err_ctxt().report_fulfillment_errors(errors);
 495     }
 496
 497     // Finally, resolve all regions.
 498     let outlives_env = OutlivesEnvironment::new(param_env);
 499     let _ = ocx.resolve_regions_and_report_errors(impl_did, &outlives_env);
 500
 501     Ok(CoerceUnsizedInfo { custom_kind: kind })
 502 }
 503
 504 fn infringing_fields_error(
 505     tcx: TyCtxt<'_>,
 506     fields: Vec<(&ty::FieldDef, Ty<'_>, InfringingFieldsReason<'_>)>,
 507     lang_item: LangItem,
 508     impl_did: LocalDefId,
 509     impl_span: Span,
 510 ) -> ErrorGuaranteed {
 511     let trait_did = tcx.require_lang_item(lang_item, Some(impl_span));
 512
 513     let trait_name = tcx.def_path_str(trait_did);
 514
 515     // We'll try to suggest constraining type parameters to fulfill the requirements of
 516     // their `Copy` implementation.
 517     let mut errors: BTreeMap<_, Vec<_>> = Default::default();
 518     let mut bounds = vec![];
 519
 520     let mut seen_tys = FxHashSet::default();
 521
 522     let mut label_spans = Vec::new();
 523
 524     for (field, ty, reason) in fields {
 525         // Only report an error once per type.
 526         if !seen_tys.insert(ty) {
 527             continue;
 528         }
 529
 530         label_spans.push(tcx.def_span(field.did));
 531
 532         match reason {
 533             InfringingFieldsReason::Fulfill(fulfillment_errors) => {
 534                 for error in fulfillment_errors {
 535                     let error_predicate = error.obligation.predicate;
 536                     // Only note if it's not the root obligation, otherwise it's trivial and
 537                     // should be self-explanatory (i.e. a field literally doesn't implement Copy).
 538
 539                     // FIXME: This error could be more descriptive, especially if the error_predicate
 540                     // contains a foreign type or if it's a deeply nested type...
 541                     if error_predicate != error.root_obligation.predicate {
 542                         errors
 543                             .entry((ty.to_string(), error_predicate.to_string()))
 544                             .or_default()
 545                             .push(error.obligation.cause.span);
 546                     }
 547                     if let ty::PredicateKind::Clause(ty::ClauseKind::Trait(ty::TraitPredicate {
 548                         trait_ref,
 549                         polarity: ty::PredicatePolarity::Positive,
 550                         ..
 551                     })) = error_predicate.kind().skip_binder()
 552                     {
 553                         let ty = trait_ref.self_ty();
 554                         if let ty::Param(_) = ty.kind() {
 555                             bounds.push((
 556                                 format!("{ty}"),
 557                                 trait_ref.print_trait_sugared().to_string(),
 558                                 Some(trait_ref.def_id),
 559                             ));
 560                         }
 561                     }
 562                 }
 563             }
 564             InfringingFieldsReason::Regions(region_errors) => {
 565                 for error in region_errors {
 566                     let ty = ty.to_string();
 567                     match error {
 568                         RegionResolutionError::ConcreteFailure(origin, a, b) => {
 569                             let predicate = format!("{b}: {a}");
 570                             errors
 571                                 .entry((ty.clone(), predicate.clone()))
 572                                 .or_default()
 573                                 .push(origin.span());
 574                             if let ty::RegionKind::ReEarlyParam(ebr) = *b
 575                                 && ebr.has_name()
 576                             {
 577                                 bounds.push((b.to_string(), a.to_string(), None));
 578                             }
 579                         }
 580                         RegionResolutionError::GenericBoundFailure(origin, a, b) => {
 581                             let predicate = format!("{a}: {b}");
 582                             errors
 583                                 .entry((ty.clone(), predicate.clone()))
 584                                 .or_default()
 585                                 .push(origin.span());
 586                             if let infer::region_constraints::GenericKind::Param(_) = a {
 587                                 bounds.push((a.to_string(), b.to_string(), None));
 588                             }
 589                         }
 590                         _ => continue,
 591                     }
 592                 }
 593             }
 594         }
 595     }
 596     let mut notes = Vec::new();
 597     for ((ty, error_predicate), spans) in errors {
 598         let span: MultiSpan = spans.into();
 599         notes.push(errors::ImplForTyRequires {
 600             span,
 601             error_predicate,
 602             trait_name: trait_name.clone(),
 603             ty,
 604         });
 605     }
 606
 607     let mut err = tcx.dcx().create_err(errors::TraitCannotImplForTy {
 608         span: impl_span,
 609         trait_name,
 610         label_spans,
 611         notes,
 612     });
 613
 614     suggest_constraining_type_params(
 615         tcx,
 616         tcx.hir().get_generics(impl_did).expect("impls always have generics"),
 617         &mut err,
 618         bounds
 619             .iter()
 620             .map(|(param, constraint, def_id)| (param.as_str(), constraint.as_str(), *def_id)),
 621         None,
 622     );
 623
 624     err.emit()
 625 }