src/librustc_typeck/check/compare_method.rs

   1 use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticId, ErrorReported};
   2 use rustc_hir as hir;
   3 use rustc_hir::def::{DefKind, Res};
   4 use rustc_hir::intravisit;
   5 use rustc_hir::{GenericParamKind, ImplItemKind, TraitItemKind};
   6 use rustc_infer::infer::{self, InferOk, TyCtxtInferExt};
   7 use rustc_middle::ty;
   8 use rustc_middle::ty::error::{ExpectedFound, TypeError};
   9 use rustc_middle::ty::subst::{InternalSubsts, Subst};
  10 use rustc_middle::ty::util::ExplicitSelf;
  11 use rustc_middle::ty::{GenericParamDefKind, ToPredicate, TyCtxt};
  12 use rustc_span::Span;
  13 use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
  14 use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode, Reveal};
  15
  16 use super::{potentially_plural_count, FnCtxt, Inherited};
  17
  18 /// Checks that a method from an impl conforms to the signature of
  19 /// the same method as declared in the trait.
  20 ///
  21 /// # Parameters
  22 ///
  23 /// - `impl_m`: type of the method we are checking
  24 /// - `impl_m_span`: span to use for reporting errors
  25 /// - `trait_m`: the method in the trait
  26 /// - `impl_trait_ref`: the TraitRef corresponding to the trait implementation
  27
  28 crate fn compare_impl_method<'tcx>(
  29     tcx: TyCtxt<'tcx>,
  30     impl_m: &ty::AssocItem,
  31     impl_m_span: Span,
  32     trait_m: &ty::AssocItem,
  33     impl_trait_ref: ty::TraitRef<'tcx>,
  34     trait_item_span: Option<Span>,
  35 ) {
  36     debug!("compare_impl_method(impl_trait_ref={:?})", impl_trait_ref);
  37
  38     let impl_m_span = tcx.sess.source_map().guess_head_span(impl_m_span);
  39
  40     if let Err(ErrorReported) = compare_self_type(tcx, impl_m, impl_m_span, trait_m, impl_trait_ref)
  41     {
  42         return;
  43     }
  44
  45     if let Err(ErrorReported) =
  46         compare_number_of_generics(tcx, impl_m, impl_m_span, trait_m, trait_item_span)
  47     {
  48         return;
  49     }
  50
  51     if let Err(ErrorReported) =
  52         compare_number_of_method_arguments(tcx, impl_m, impl_m_span, trait_m, trait_item_span)
  53     {
  54         return;
  55     }
  56
  57     if let Err(ErrorReported) = compare_synthetic_generics(tcx, impl_m, trait_m) {
  58         return;
  59     }
  60
  61     if let Err(ErrorReported) =
  62         compare_predicate_entailment(tcx, impl_m, impl_m_span, trait_m, impl_trait_ref)
  63     {
  64         return;
  65     }
  66 }
  67
  68 fn compare_predicate_entailment<'tcx>(
  69     tcx: TyCtxt<'tcx>,
  70     impl_m: &ty::AssocItem,
  71     impl_m_span: Span,
  72     trait_m: &ty::AssocItem,
  73     impl_trait_ref: ty::TraitRef<'tcx>,
  74 ) -> Result<(), ErrorReported> {
  75     let trait_to_impl_substs = impl_trait_ref.substs;
  76
  77     // This node-id should be used for the `body_id` field on each
  78     // `ObligationCause` (and the `FnCtxt`). This is what
  79     // `regionck_item` expects.
  80     let impl_m_hir_id = tcx.hir().local_def_id_to_hir_id(impl_m.def_id.expect_local());
  81
  82     // We sometimes modify the span further down.
  83     let mut cause = ObligationCause::new(
  84         impl_m_span,
  85         impl_m_hir_id,
  86         ObligationCauseCode::CompareImplMethodObligation {
  87             item_name: impl_m.ident.name,
  88             impl_item_def_id: impl_m.def_id,
  89             trait_item_def_id: trait_m.def_id,
  90         },
  91     );
  92
  93     // This code is best explained by example. Consider a trait:
  94     //
  95     //     trait Trait<'t, T> {
  96     //         fn method<'a, M>(t: &'t T, m: &'a M) -> Self;
  97     //     }
  98     //
  99     // And an impl:
 100     //
 101     //     impl<'i, 'j, U> Trait<'j, &'i U> for Foo {
 102     //          fn method<'b, N>(t: &'j &'i U, m: &'b N) -> Foo;
 103     //     }
 104     //
 105     // We wish to decide if those two method types are compatible.
 106     //
 107     // We start out with trait_to_impl_substs, that maps the trait
 108     // type parameters to impl type parameters. This is taken from the
 109     // impl trait reference:
 110     //
 111     //     trait_to_impl_substs = {'t => 'j, T => &'i U, Self => Foo}
 112     //
 113     // We create a mapping `dummy_substs` that maps from the impl type
 114     // parameters to fresh types and regions. For type parameters,
 115     // this is the identity transform, but we could as well use any
 116     // placeholder types. For regions, we convert from bound to free
 117     // regions (Note: but only early-bound regions, i.e., those
 118     // declared on the impl or used in type parameter bounds).
 119     //
 120     //     impl_to_placeholder_substs = {'i => 'i0, U => U0, N => N0 }
 121     //
 122     // Now we can apply placeholder_substs to the type of the impl method
 123     // to yield a new function type in terms of our fresh, placeholder
 124     // types:
 125     //
 126     //     <'b> fn(t: &'i0 U0, m: &'b) -> Foo
 127     //
 128     // We now want to extract and substitute the type of the *trait*
 129     // method and compare it. To do so, we must create a compound
 130     // substitution by combining trait_to_impl_substs and
 131     // impl_to_placeholder_substs, and also adding a mapping for the method
 132     // type parameters. We extend the mapping to also include
 133     // the method parameters.
 134     //
 135     //     trait_to_placeholder_substs = { T => &'i0 U0, Self => Foo, M => N0 }
 136     //
 137     // Applying this to the trait method type yields:
 138     //
 139     //     <'a> fn(t: &'i0 U0, m: &'a) -> Foo
 140     //
 141     // This type is also the same but the name of the bound region ('a
 142     // vs 'b).  However, the normal subtyping rules on fn types handle
 143     // this kind of equivalency just fine.
 144     //
 145     // We now use these substitutions to ensure that all declared bounds are
 146     // satisfied by the implementation's method.
 147     //
 148     // We do this by creating a parameter environment which contains a
 149     // substitution corresponding to impl_to_placeholder_substs. We then build
 150     // trait_to_placeholder_substs and use it to convert the predicates contained
 151     // in the trait_m.generics to the placeholder form.
 152     //
 153     // Finally we register each of these predicates as an obligation in
 154     // a fresh FulfillmentCtxt, and invoke select_all_or_error.
 155
 156     // Create mapping from impl to placeholder.
 157     let impl_to_placeholder_substs = InternalSubsts::identity_for_item(tcx, impl_m.def_id);
 158
 159     // Create mapping from trait to placeholder.
 160     let trait_to_placeholder_substs =
 161         impl_to_placeholder_substs.rebase_onto(tcx, impl_m.container.id(), trait_to_impl_substs);
 162     debug!("compare_impl_method: trait_to_placeholder_substs={:?}", trait_to_placeholder_substs);
 163
 164     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 165     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 166     let impl_m_predicates = tcx.predicates_of(impl_m.def_id);
 167     let trait_m_predicates = tcx.predicates_of(trait_m.def_id);
 168
 169     // Check region bounds.
 170     check_region_bounds_on_impl_item(
 171         tcx,
 172         impl_m_span,
 173         impl_m,
 174         trait_m,
 175         &trait_m_generics,
 176         &impl_m_generics,
 177     )?;
 178
 179     // Create obligations for each predicate declared by the impl
 180     // definition in the context of the trait's parameter
 181     // environment. We can't just use `impl_env.caller_bounds`,
 182     // however, because we want to replace all late-bound regions with
 183     // region variables.
 184     let impl_predicates = tcx.predicates_of(impl_m_predicates.parent.unwrap());
 185     let mut hybrid_preds = impl_predicates.instantiate_identity(tcx);
 186
 187     debug!("compare_impl_method: impl_bounds={:?}", hybrid_preds);
 188
 189     // This is the only tricky bit of the new way we check implementation methods
 190     // We need to build a set of predicates where only the method-level bounds
 191     // are from the trait and we assume all other bounds from the implementation
 192     // to be previously satisfied.
 193     //
 194     // We then register the obligations from the impl_m and check to see
 195     // if all constraints hold.
 196     hybrid_preds
 197         .predicates
 198         .extend(trait_m_predicates.instantiate_own(tcx, trait_to_placeholder_substs).predicates);
 199
 200     // Construct trait parameter environment and then shift it into the placeholder viewpoint.
 201     // The key step here is to update the caller_bounds's predicates to be
 202     // the new hybrid bounds we computed.
 203     let normalize_cause = traits::ObligationCause::misc(impl_m_span, impl_m_hir_id);
 204     let param_env = ty::ParamEnv::new(
 205         tcx.intern_predicates(&hybrid_preds.predicates),
 206         Reveal::UserFacing,
 207         None,
 208     );
 209     let param_env = traits::normalize_param_env_or_error(
 210         tcx,
 211         impl_m.def_id,
 212         param_env,
 213         normalize_cause.clone(),
 214     );
 215
 216     tcx.infer_ctxt().enter(|infcx| {
 217         let inh = Inherited::new(infcx, impl_m.def_id.expect_local());
 218         let infcx = &inh.infcx;
 219
 220         debug!("compare_impl_method: caller_bounds={:?}", param_env.caller_bounds());
 221
 222         let mut selcx = traits::SelectionContext::new(&infcx);
 223
 224         let impl_m_own_bounds = impl_m_predicates.instantiate_own(tcx, impl_to_placeholder_substs);
 225         let (impl_m_own_bounds, _) = infcx.replace_bound_vars_with_fresh_vars(
 226             impl_m_span,
 227             infer::HigherRankedType,
 228             &ty::Binder::bind(impl_m_own_bounds.predicates),
 229         );
 230         for predicate in impl_m_own_bounds {
 231             let traits::Normalized { value: predicate, obligations } =
 232                 traits::normalize(&mut selcx, param_env, normalize_cause.clone(), &predicate);
 233
 234             inh.register_predicates(obligations);
 235             inh.register_predicate(traits::Obligation::new(cause.clone(), param_env, predicate));
 236         }
 237
 238         // We now need to check that the signature of the impl method is
 239         // compatible with that of the trait method. We do this by
 240         // checking that `impl_fty <: trait_fty`.
 241         //
 242         // FIXME. Unfortunately, this doesn't quite work right now because
 243         // associated type normalization is not integrated into subtype
 244         // checks. For the comparison to be valid, we need to
 245         // normalize the associated types in the impl/trait methods
 246         // first. However, because function types bind regions, just
 247         // calling `normalize_associated_types_in` would have no effect on
 248         // any associated types appearing in the fn arguments or return
 249         // type.
 250
 251         // Compute placeholder form of impl and trait method tys.
 252         let tcx = infcx.tcx;
 253
 254         let (impl_sig, _) = infcx.replace_bound_vars_with_fresh_vars(
 255             impl_m_span,
 256             infer::HigherRankedType,
 257             &tcx.fn_sig(impl_m.def_id),
 258         );
 259         let impl_sig =
 260             inh.normalize_associated_types_in(impl_m_span, impl_m_hir_id, param_env, &impl_sig);
 261         let impl_fty = tcx.mk_fn_ptr(ty::Binder::bind(impl_sig));
 262         debug!("compare_impl_method: impl_fty={:?}", impl_fty);
 263
 264         let trait_sig = tcx.liberate_late_bound_regions(impl_m.def_id, &tcx.fn_sig(trait_m.def_id));
 265         let trait_sig = trait_sig.subst(tcx, trait_to_placeholder_substs);
 266         let trait_sig =
 267             inh.normalize_associated_types_in(impl_m_span, impl_m_hir_id, param_env, &trait_sig);
 268         let trait_fty = tcx.mk_fn_ptr(ty::Binder::bind(trait_sig));
 269
 270         debug!("compare_impl_method: trait_fty={:?}", trait_fty);
 271
 272         let sub_result = infcx.at(&cause, param_env).sup(trait_fty, impl_fty).map(
 273             |InferOk { obligations, .. }| {
 274                 inh.register_predicates(obligations);
 275             },
 276         );
 277
 278         if let Err(terr) = sub_result {
 279             debug!("sub_types failed: impl ty {:?}, trait ty {:?}", impl_fty, trait_fty);
 280
 281             let (impl_err_span, trait_err_span) = extract_spans_for_error_reporting(
 282                 &infcx, param_env, &terr, &cause, impl_m, impl_sig, trait_m, trait_sig,
 283             );
 284
 285             cause.make_mut().span = impl_err_span;
 286
 287             let mut diag = struct_span_err!(
 288                 tcx.sess,
 289                 cause.span(tcx),
 290                 E0053,
 291                 "method `{}` has an incompatible type for trait",
 292                 trait_m.ident
 293             );
 294             if let TypeError::Mutability = terr {
 295                 if let Some(trait_err_span) = trait_err_span {
 296                     if let Ok(trait_err_str) = tcx.sess.source_map().span_to_snippet(trait_err_span)
 297                     {
 298                         diag.span_suggestion(
 299                             impl_err_span,
 300                             "consider change the type to match the mutability in trait",
 301                             trait_err_str,
 302                             Applicability::MachineApplicable,
 303                         );
 304                     }
 305                 }
 306             }
 307
 308             infcx.note_type_err(
 309                 &mut diag,
 310                 &cause,
 311                 trait_err_span.map(|sp| (sp, "type in trait".to_owned())),
 312                 Some(infer::ValuePairs::Types(ExpectedFound {
 313                     expected: trait_fty,
 314                     found: impl_fty,
 315                 })),
 316                 &terr,
 317             );
 318             diag.emit();
 319             return Err(ErrorReported);
 320         }
 321
 322         // Check that all obligations are satisfied by the implementation's
 323         // version.
 324         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
 325             infcx.report_fulfillment_errors(errors, None, false);
 326             return Err(ErrorReported);
 327         }
 328
 329         // Finally, resolve all regions. This catches wily misuses of
 330         // lifetime parameters.
 331         let fcx = FnCtxt::new(&inh, param_env, impl_m_hir_id);
 332         fcx.regionck_item(impl_m_hir_id, impl_m_span, &[]);
 333
 334         Ok(())
 335     })
 336 }
 337
 338 fn check_region_bounds_on_impl_item<'tcx>(
 339     tcx: TyCtxt<'tcx>,
 340     span: Span,
 341     impl_m: &ty::AssocItem,
 342     trait_m: &ty::AssocItem,
 343     trait_generics: &ty::Generics,
 344     impl_generics: &ty::Generics,
 345 ) -> Result<(), ErrorReported> {
 346     let trait_params = trait_generics.own_counts().lifetimes;
 347     let impl_params = impl_generics.own_counts().lifetimes;
 348
 349     debug!(
 350         "check_region_bounds_on_impl_item: \
 351             trait_generics={:?} \
 352             impl_generics={:?}",
 353         trait_generics, impl_generics
 354     );
 355
 356     // Must have same number of early-bound lifetime parameters.
 357     // Unfortunately, if the user screws up the bounds, then this
 358     // will change classification between early and late.  E.g.,
 359     // if in trait we have `<'a,'b:'a>`, and in impl we just have
 360     // `<'a,'b>`, then we have 2 early-bound lifetime parameters
 361     // in trait but 0 in the impl. But if we report "expected 2
 362     // but found 0" it's confusing, because it looks like there
 363     // are zero. Since I don't quite know how to phrase things at
 364     // the moment, give a kind of vague error message.
 365     if trait_params != impl_params {
 366         let item_kind = assoc_item_kind_str(impl_m);
 367         let def_span = tcx.sess.source_map().guess_head_span(span);
 368         let span = tcx.hir().get_generics(impl_m.def_id).map(|g| g.span).unwrap_or(def_span);
 369         let mut err = struct_span_err!(
 370             tcx.sess,
 371             span,
 372             E0195,
 373             "lifetime parameters or bounds on {} `{}` do not match the trait declaration",
 374             item_kind,
 375             impl_m.ident,
 376         );
 377         err.span_label(span, &format!("lifetimes do not match {} in trait", item_kind));
 378         if let Some(sp) = tcx.hir().span_if_local(trait_m.def_id) {
 379             let def_sp = tcx.sess.source_map().guess_head_span(sp);
 380             let sp = tcx.hir().get_generics(trait_m.def_id).map(|g| g.span).unwrap_or(def_sp);
 381             err.span_label(
 382                 sp,
 383                 &format!("lifetimes in impl do not match this {} in trait", item_kind),
 384             );
 385         }
 386         err.emit();
 387         return Err(ErrorReported);
 388     }
 389
 390     Ok(())
 391 }
 392
 393 fn extract_spans_for_error_reporting<'a, 'tcx>(
 394     infcx: &infer::InferCtxt<'a, 'tcx>,
 395     param_env: ty::ParamEnv<'tcx>,
 396     terr: &TypeError<'_>,
 397     cause: &ObligationCause<'tcx>,
 398     impl_m: &ty::AssocItem,
 399     impl_sig: ty::FnSig<'tcx>,
 400     trait_m: &ty::AssocItem,
 401     trait_sig: ty::FnSig<'tcx>,
 402 ) -> (Span, Option<Span>) {
 403     let tcx = infcx.tcx;
 404     let impl_m_hir_id = tcx.hir().local_def_id_to_hir_id(impl_m.def_id.expect_local());
 405     let (impl_m_output, impl_m_iter) = match tcx.hir().expect_impl_item(impl_m_hir_id).kind {
 406         ImplItemKind::Fn(ref impl_m_sig, _) => {
 407             (&impl_m_sig.decl.output, impl_m_sig.decl.inputs.iter())
 408         }
 409         _ => bug!("{:?} is not a method", impl_m),
 410     };
 411
 412     match *terr {
 413         TypeError::Mutability => {
 414             if let Some(def_id) = trait_m.def_id.as_local() {
 415                 let trait_m_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 416                 let trait_m_iter = match tcx.hir().expect_trait_item(trait_m_hir_id).kind {
 417                     TraitItemKind::Fn(ref trait_m_sig, _) => trait_m_sig.decl.inputs.iter(),
 418                     _ => bug!("{:?} is not a TraitItemKind::Fn", trait_m),
 419                 };
 420
 421                 impl_m_iter
 422                     .zip(trait_m_iter)
 423                     .find(|&(ref impl_arg, ref trait_arg)| {
 424                         match (&impl_arg.kind, &trait_arg.kind) {
 425                             (
 426                                 &hir::TyKind::Rptr(_, ref impl_mt),
 427                                 &hir::TyKind::Rptr(_, ref trait_mt),
 428                             )
 429                             | (&hir::TyKind::Ptr(ref impl_mt), &hir::TyKind::Ptr(ref trait_mt)) => {
 430                                 impl_mt.mutbl != trait_mt.mutbl
 431                             }
 432                             _ => false,
 433                         }
 434                     })
 435                     .map(|(ref impl_arg, ref trait_arg)| (impl_arg.span, Some(trait_arg.span)))
 436                     .unwrap_or_else(|| (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id)))
 437             } else {
 438                 (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id))
 439             }
 440         }
 441         TypeError::Sorts(ExpectedFound { .. }) => {
 442             if let Some(def_id) = trait_m.def_id.as_local() {
 443                 let trait_m_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 444                 let (trait_m_output, trait_m_iter) =
 445                     match tcx.hir().expect_trait_item(trait_m_hir_id).kind {
 446                         TraitItemKind::Fn(ref trait_m_sig, _) => {
 447                             (&trait_m_sig.decl.output, trait_m_sig.decl.inputs.iter())
 448                         }
 449                         _ => bug!("{:?} is not a TraitItemKind::Fn", trait_m),
 450                     };
 451
 452                 let impl_iter = impl_sig.inputs().iter();
 453                 let trait_iter = trait_sig.inputs().iter();
 454                 impl_iter
 455                     .zip(trait_iter)
 456                     .zip(impl_m_iter)
 457                     .zip(trait_m_iter)
 458                     .find_map(|(((&impl_arg_ty, &trait_arg_ty), impl_arg), trait_arg)| match infcx
 459                         .at(&cause, param_env)
 460                         .sub(trait_arg_ty, impl_arg_ty)
 461                     {
 462                         Ok(_) => None,
 463                         Err(_) => Some((impl_arg.span, Some(trait_arg.span))),
 464                     })
 465                     .unwrap_or_else(|| {
 466                         if infcx
 467                             .at(&cause, param_env)
 468                             .sup(trait_sig.output(), impl_sig.output())
 469                             .is_err()
 470                         {
 471                             (impl_m_output.span(), Some(trait_m_output.span()))
 472                         } else {
 473                             (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id))
 474                         }
 475                     })
 476             } else {
 477                 (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id))
 478             }
 479         }
 480         _ => (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id)),
 481     }
 482 }
 483
 484 fn compare_self_type<'tcx>(
 485     tcx: TyCtxt<'tcx>,
 486     impl_m: &ty::AssocItem,
 487     impl_m_span: Span,
 488     trait_m: &ty::AssocItem,
 489     impl_trait_ref: ty::TraitRef<'tcx>,
 490 ) -> Result<(), ErrorReported> {
 491     // Try to give more informative error messages about self typing
 492     // mismatches.  Note that any mismatch will also be detected
 493     // below, where we construct a canonical function type that
 494     // includes the self parameter as a normal parameter.  It's just
 495     // that the error messages you get out of this code are a bit more
 496     // inscrutable, particularly for cases where one method has no
 497     // self.
 498
 499     let self_string = |method: &ty::AssocItem| {
 500         let untransformed_self_ty = match method.container {
 501             ty::ImplContainer(_) => impl_trait_ref.self_ty(),
 502             ty::TraitContainer(_) => tcx.types.self_param,
 503         };
 504         let self_arg_ty = tcx.fn_sig(method.def_id).input(0).skip_binder();
 505         let param_env = ty::ParamEnv::reveal_all();
 506
 507         tcx.infer_ctxt().enter(|infcx| {
 508             let self_arg_ty =
 509                 tcx.liberate_late_bound_regions(method.def_id, &ty::Binder::bind(self_arg_ty));
 510             let can_eq_self = |ty| infcx.can_eq(param_env, untransformed_self_ty, ty).is_ok();
 511             match ExplicitSelf::determine(self_arg_ty, can_eq_self) {
 512                 ExplicitSelf::ByValue => "self".to_owned(),
 513                 ExplicitSelf::ByReference(_, hir::Mutability::Not) => "&self".to_owned(),
 514                 ExplicitSelf::ByReference(_, hir::Mutability::Mut) => "&mut self".to_owned(),
 515                 _ => format!("self: {}", self_arg_ty),
 516             }
 517         })
 518     };
 519
 520     match (trait_m.fn_has_self_parameter, impl_m.fn_has_self_parameter) {
 521         (false, false) | (true, true) => {}
 522
 523         (false, true) => {
 524             let self_descr = self_string(impl_m);
 525             let mut err = struct_span_err!(
 526                 tcx.sess,
 527                 impl_m_span,
 528                 E0185,
 529                 "method `{}` has a `{}` declaration in the impl, but \
 530                                             not in the trait",
 531                 trait_m.ident,
 532                 self_descr
 533             );
 534             err.span_label(impl_m_span, format!("`{}` used in impl", self_descr));
 535             if let Some(span) = tcx.hir().span_if_local(trait_m.def_id) {
 536                 err.span_label(span, format!("trait method declared without `{}`", self_descr));
 537             } else {
 538                 err.note_trait_signature(trait_m.ident.to_string(), trait_m.signature(tcx));
 539             }
 540             err.emit();
 541             return Err(ErrorReported);
 542         }
 543
 544         (true, false) => {
 545             let self_descr = self_string(trait_m);
 546             let mut err = struct_span_err!(
 547                 tcx.sess,
 548                 impl_m_span,
 549                 E0186,
 550                 "method `{}` has a `{}` declaration in the trait, but \
 551                                             not in the impl",
 552                 trait_m.ident,
 553                 self_descr
 554             );
 555             err.span_label(impl_m_span, format!("expected `{}` in impl", self_descr));
 556             if let Some(span) = tcx.hir().span_if_local(trait_m.def_id) {
 557                 err.span_label(span, format!("`{}` used in trait", self_descr));
 558             } else {
 559                 err.note_trait_signature(trait_m.ident.to_string(), trait_m.signature(tcx));
 560             }
 561             err.emit();
 562             return Err(ErrorReported);
 563         }
 564     }
 565
 566     Ok(())
 567 }
 568
 569 fn compare_number_of_generics<'tcx>(
 570     tcx: TyCtxt<'tcx>,
 571     impl_: &ty::AssocItem,
 572     _impl_span: Span,
 573     trait_: &ty::AssocItem,
 574     trait_span: Option<Span>,
 575 ) -> Result<(), ErrorReported> {
 576     let trait_own_counts = tcx.generics_of(trait_.def_id).own_counts();
 577     let impl_own_counts = tcx.generics_of(impl_.def_id).own_counts();
 578
 579     let matchings = [
 580         ("type", trait_own_counts.types, impl_own_counts.types),
 581         ("const", trait_own_counts.consts, impl_own_counts.consts),
 582     ];
 583
 584     let item_kind = assoc_item_kind_str(impl_);
 585
 586     let mut err_occurred = false;
 587     for &(kind, trait_count, impl_count) in &matchings {
 588         if impl_count != trait_count {
 589             err_occurred = true;
 590
 591             let (trait_spans, impl_trait_spans) = if let Some(def_id) = trait_.def_id.as_local() {
 592                 let trait_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 593                 let trait_item = tcx.hir().expect_trait_item(trait_hir_id);
 594                 if trait_item.generics.params.is_empty() {
 595                     (Some(vec![trait_item.generics.span]), vec![])
 596                 } else {
 597                     let arg_spans: Vec<Span> =
 598                         trait_item.generics.params.iter().map(|p| p.span).collect();
 599                     let impl_trait_spans: Vec<Span> = trait_item
 600                         .generics
 601                         .params
 602                         .iter()
 603                         .filter_map(|p| match p.kind {
 604                             GenericParamKind::Type {
 605                                 synthetic: Some(hir::SyntheticTyParamKind::ImplTrait),
 606                                 ..
 607                             } => Some(p.span),
 608                             _ => None,
 609                         })
 610                         .collect();
 611                     (Some(arg_spans), impl_trait_spans)
 612                 }
 613             } else {
 614                 (trait_span.map(|s| vec![s]), vec![])
 615             };
 616
 617             let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_.def_id.expect_local());
 618             let impl_item = tcx.hir().expect_impl_item(impl_hir_id);
 619             let impl_item_impl_trait_spans: Vec<Span> = impl_item
 620                 .generics
 621                 .params
 622                 .iter()
 623                 .filter_map(|p| match p.kind {
 624                     GenericParamKind::Type {
 625                         synthetic: Some(hir::SyntheticTyParamKind::ImplTrait),
 626                         ..
 627                     } => Some(p.span),
 628                     _ => None,
 629                 })
 630                 .collect();
 631             let spans = impl_item.generics.spans();
 632             let span = spans.primary_span();
 633
 634             let mut err = tcx.sess.struct_span_err_with_code(
 635                 spans,
 636                 &format!(
 637                     "{} `{}` has {} {kind} parameter{} but its trait \
 638                      declaration has {} {kind} parameter{}",
 639                     item_kind,
 640                     trait_.ident,
 641                     impl_count,
 642                     pluralize!(impl_count),
 643                     trait_count,
 644                     pluralize!(trait_count),
 645                     kind = kind,
 646                 ),
 647                 DiagnosticId::Error("E0049".into()),
 648             );
 649
 650             let mut suffix = None;
 651
 652             if let Some(spans) = trait_spans {
 653                 let mut spans = spans.iter();
 654                 if let Some(span) = spans.next() {
 655                     err.span_label(
 656                         *span,
 657                         format!(
 658                             "expected {} {} parameter{}",
 659                             trait_count,
 660                             kind,
 661                             pluralize!(trait_count),
 662                         ),
 663                     );
 664                 }
 665                 for span in spans {
 666                     err.span_label(*span, "");
 667                 }
 668             } else {
 669                 suffix = Some(format!(", expected {}", trait_count));
 670             }
 671
 672             if let Some(span) = span {
 673                 err.span_label(
 674                     span,
 675                     format!(
 676                         "found {} {} parameter{}{}",
 677                         impl_count,
 678                         kind,
 679                         pluralize!(impl_count),
 680                         suffix.unwrap_or_else(String::new),
 681                     ),
 682                 );
 683             }
 684
 685             for span in impl_trait_spans.iter().chain(impl_item_impl_trait_spans.iter()) {
 686                 err.span_label(*span, "`impl Trait` introduces an implicit type parameter");
 687             }
 688
 689             err.emit();
 690         }
 691     }
 692
 693     if err_occurred { Err(ErrorReported) } else { Ok(()) }
 694 }
 695
 696 fn compare_number_of_method_arguments<'tcx>(
 697     tcx: TyCtxt<'tcx>,
 698     impl_m: &ty::AssocItem,
 699     impl_m_span: Span,
 700     trait_m: &ty::AssocItem,
 701     trait_item_span: Option<Span>,
 702 ) -> Result<(), ErrorReported> {
 703     let impl_m_fty = tcx.fn_sig(impl_m.def_id);
 704     let trait_m_fty = tcx.fn_sig(trait_m.def_id);
 705     let trait_number_args = trait_m_fty.inputs().skip_binder().len();
 706     let impl_number_args = impl_m_fty.inputs().skip_binder().len();
 707     if trait_number_args != impl_number_args {
 708         let trait_span = if let Some(def_id) = trait_m.def_id.as_local() {
 709             let trait_id = tcx.hir().local_def_id_to_hir_id(def_id);
 710             match tcx.hir().expect_trait_item(trait_id).kind {
 711                 TraitItemKind::Fn(ref trait_m_sig, _) => {
 712                     let pos = if trait_number_args > 0 { trait_number_args - 1 } else { 0 };
 713                     if let Some(arg) = trait_m_sig.decl.inputs.get(pos) {
 714                         Some(if pos == 0 {
 715                             arg.span
 716                         } else {
 717                             Span::new(
 718                                 trait_m_sig.decl.inputs[0].span.lo(),
 719                                 arg.span.hi(),
 720                                 arg.span.ctxt(),
 721                             )
 722                         })
 723                     } else {
 724                         trait_item_span
 725                     }
 726                 }
 727                 _ => bug!("{:?} is not a method", impl_m),
 728             }
 729         } else {
 730             trait_item_span
 731         };
 732         let impl_m_hir_id = tcx.hir().local_def_id_to_hir_id(impl_m.def_id.expect_local());
 733         let impl_span = match tcx.hir().expect_impl_item(impl_m_hir_id).kind {
 734             ImplItemKind::Fn(ref impl_m_sig, _) => {
 735                 let pos = if impl_number_args > 0 { impl_number_args - 1 } else { 0 };
 736                 if let Some(arg) = impl_m_sig.decl.inputs.get(pos) {
 737                     if pos == 0 {
 738                         arg.span
 739                     } else {
 740                         Span::new(
 741                             impl_m_sig.decl.inputs[0].span.lo(),
 742                             arg.span.hi(),
 743                             arg.span.ctxt(),
 744                         )
 745                     }
 746                 } else {
 747                     impl_m_span
 748                 }
 749             }
 750             _ => bug!("{:?} is not a method", impl_m),
 751         };
 752         let mut err = struct_span_err!(
 753             tcx.sess,
 754             impl_span,
 755             E0050,
 756             "method `{}` has {} but the declaration in \
 757                                         trait `{}` has {}",
 758             trait_m.ident,
 759             potentially_plural_count(impl_number_args, "parameter"),
 760             tcx.def_path_str(trait_m.def_id),
 761             trait_number_args
 762         );
 763         if let Some(trait_span) = trait_span {
 764             err.span_label(
 765                 trait_span,
 766                 format!(
 767                     "trait requires {}",
 768                     potentially_plural_count(trait_number_args, "parameter")
 769                 ),
 770             );
 771         } else {
 772             err.note_trait_signature(trait_m.ident.to_string(), trait_m.signature(tcx));
 773         }
 774         err.span_label(
 775             impl_span,
 776             format!(
 777                 "expected {}, found {}",
 778                 potentially_plural_count(trait_number_args, "parameter"),
 779                 impl_number_args
 780             ),
 781         );
 782         err.emit();
 783         return Err(ErrorReported);
 784     }
 785
 786     Ok(())
 787 }
 788
 789 fn compare_synthetic_generics<'tcx>(
 790     tcx: TyCtxt<'tcx>,
 791     impl_m: &ty::AssocItem,
 792     trait_m: &ty::AssocItem,
 793 ) -> Result<(), ErrorReported> {
 794     // FIXME(chrisvittal) Clean up this function, list of FIXME items:
 795     //     1. Better messages for the span labels
 796     //     2. Explanation as to what is going on
 797     // If we get here, we already have the same number of generics, so the zip will
 798     // be okay.
 799     let mut error_found = false;
 800     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 801     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 802     let impl_m_type_params = impl_m_generics.params.iter().filter_map(|param| match param.kind {
 803         GenericParamDefKind::Type { synthetic, .. } => Some((param.def_id, synthetic)),
 804         GenericParamDefKind::Lifetime | GenericParamDefKind::Const => None,
 805     });
 806     let trait_m_type_params = trait_m_generics.params.iter().filter_map(|param| match param.kind {
 807         GenericParamDefKind::Type { synthetic, .. } => Some((param.def_id, synthetic)),
 808         GenericParamDefKind::Lifetime | GenericParamDefKind::Const => None,
 809     });
 810     for ((impl_def_id, impl_synthetic), (trait_def_id, trait_synthetic)) in
 811         impl_m_type_params.zip(trait_m_type_params)
 812     {
 813         if impl_synthetic != trait_synthetic {
 814             let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_def_id.expect_local());
 815             let impl_span = tcx.hir().span(impl_hir_id);
 816             let trait_span = tcx.def_span(trait_def_id);
 817             let mut err = struct_span_err!(
 818                 tcx.sess,
 819                 impl_span,
 820                 E0643,
 821                 "method `{}` has incompatible signature for trait",
 822                 trait_m.ident
 823             );
 824             err.span_label(trait_span, "declaration in trait here");
 825             match (impl_synthetic, trait_synthetic) {
 826                 // The case where the impl method uses `impl Trait` but the trait method uses
 827                 // explicit generics
 828                 (Some(hir::SyntheticTyParamKind::ImplTrait), None) => {
 829                     err.span_label(impl_span, "expected generic parameter, found `impl Trait`");
 830                     (|| {
 831                         // try taking the name from the trait impl
 832                         // FIXME: this is obviously suboptimal since the name can already be used
 833                         // as another generic argument
 834                         let new_name = tcx.sess.source_map().span_to_snippet(trait_span).ok()?;
 835                         let trait_m = tcx.hir().local_def_id_to_hir_id(trait_m.def_id.as_local()?);
 836                         let trait_m = tcx.hir().trait_item(hir::TraitItemId { hir_id: trait_m });
 837
 838                         let impl_m = tcx.hir().local_def_id_to_hir_id(impl_m.def_id.as_local()?);
 839                         let impl_m = tcx.hir().impl_item(hir::ImplItemId { hir_id: impl_m });
 840
 841                         // in case there are no generics, take the spot between the function name
 842                         // and the opening paren of the argument list
 843                         let new_generics_span =
 844                             tcx.sess.source_map().generate_fn_name_span(impl_span)?.shrink_to_hi();
 845                         // in case there are generics, just replace them
 846                         let generics_span =
 847                             impl_m.generics.span.substitute_dummy(new_generics_span);
 848                         // replace with the generics from the trait
 849                         let new_generics =
 850                             tcx.sess.source_map().span_to_snippet(trait_m.generics.span).ok()?;
 851
 852                         err.multipart_suggestion(
 853                             "try changing the `impl Trait` argument to a generic parameter",
 854                             vec![
 855                                 // replace `impl Trait` with `T`
 856                                 (impl_span, new_name),
 857                                 // replace impl method generics with trait method generics
 858                                 // This isn't quite right, as users might have changed the names
 859                                 // of the generics, but it works for the common case
 860                                 (generics_span, new_generics),
 861                             ],
 862                             Applicability::MaybeIncorrect,
 863                         );
 864                         Some(())
 865                     })();
 866                 }
 867                 // The case where the trait method uses `impl Trait`, but the impl method uses
 868                 // explicit generics.
 869                 (None, Some(hir::SyntheticTyParamKind::ImplTrait)) => {
 870                     err.span_label(impl_span, "expected `impl Trait`, found generic parameter");
 871                     (|| {
 872                         let impl_m = tcx.hir().local_def_id_to_hir_id(impl_m.def_id.as_local()?);
 873                         let impl_m = tcx.hir().impl_item(hir::ImplItemId { hir_id: impl_m });
 874                         let input_tys = match impl_m.kind {
 875                             hir::ImplItemKind::Fn(ref sig, _) => sig.decl.inputs,
 876                             _ => unreachable!(),
 877                         };
 878                         struct Visitor(Option<Span>, hir::def_id::DefId);
 879                         impl<'v> intravisit::Visitor<'v> for Visitor {
 880                             fn visit_ty(&mut self, ty: &'v hir::Ty<'v>) {
 881                                 intravisit::walk_ty(self, ty);
 882                                 if let hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) =
 883                                     ty.kind
 884                                 {
 885                                     if let Res::Def(DefKind::TyParam, def_id) = path.res {
 886                                         if def_id == self.1 {
 887                                             self.0 = Some(ty.span);
 888                                         }
 889                                     }
 890                                 }
 891                             }
 892                             type Map = intravisit::ErasedMap<'v>;
 893                             fn nested_visit_map(
 894                                 &mut self,
 895                             ) -> intravisit::NestedVisitorMap<Self::Map>
 896                             {
 897                                 intravisit::NestedVisitorMap::None
 898                             }
 899                         }
 900                         let mut visitor = Visitor(None, impl_def_id);
 901                         for ty in input_tys {
 902                             intravisit::Visitor::visit_ty(&mut visitor, ty);
 903                         }
 904                         let span = visitor.0?;
 905
 906                         let bounds =
 907                             impl_m.generics.params.iter().find_map(|param| match param.kind {
 908                                 GenericParamKind::Lifetime { .. } => None,
 909                                 GenericParamKind::Type { .. } | GenericParamKind::Const { .. } => {
 910                                     if param.hir_id == impl_hir_id {
 911                                         Some(&param.bounds)
 912                                     } else {
 913                                         None
 914                                     }
 915                                 }
 916                             })?;
 917                         let bounds = bounds.first()?.span().to(bounds.last()?.span());
 918                         let bounds = tcx.sess.source_map().span_to_snippet(bounds).ok()?;
 919
 920                         err.multipart_suggestion(
 921                             "try removing the generic parameter and using `impl Trait` instead",
 922                             vec![
 923                                 // delete generic parameters
 924                                 (impl_m.generics.span, String::new()),
 925                                 // replace param usage with `impl Trait`
 926                                 (span, format!("impl {}", bounds)),
 927                             ],
 928                             Applicability::MaybeIncorrect,
 929                         );
 930                         Some(())
 931                     })();
 932                 }
 933                 _ => unreachable!(),
 934             }
 935             err.emit();
 936             error_found = true;
 937         }
 938     }
 939     if error_found { Err(ErrorReported) } else { Ok(()) }
 940 }
 941
 942 crate fn compare_const_impl<'tcx>(
 943     tcx: TyCtxt<'tcx>,
 944     impl_c: &ty::AssocItem,
 945     impl_c_span: Span,
 946     trait_c: &ty::AssocItem,
 947     impl_trait_ref: ty::TraitRef<'tcx>,
 948 ) {
 949     debug!("compare_const_impl(impl_trait_ref={:?})", impl_trait_ref);
 950
 951     tcx.infer_ctxt().enter(|infcx| {
 952         let param_env = tcx.param_env(impl_c.def_id);
 953         let inh = Inherited::new(infcx, impl_c.def_id.expect_local());
 954         let infcx = &inh.infcx;
 955
 956         // The below is for the most part highly similar to the procedure
 957         // for methods above. It is simpler in many respects, especially
 958         // because we shouldn't really have to deal with lifetimes or
 959         // predicates. In fact some of this should probably be put into
 960         // shared functions because of DRY violations...
 961         let trait_to_impl_substs = impl_trait_ref.substs;
 962
 963         // Create a parameter environment that represents the implementation's
 964         // method.
 965         let impl_c_hir_id = tcx.hir().local_def_id_to_hir_id(impl_c.def_id.expect_local());
 966
 967         // Compute placeholder form of impl and trait const tys.
 968         let impl_ty = tcx.type_of(impl_c.def_id);
 969         let trait_ty = tcx.type_of(trait_c.def_id).subst(tcx, trait_to_impl_substs);
 970         let mut cause = ObligationCause::new(
 971             impl_c_span,
 972             impl_c_hir_id,
 973             ObligationCauseCode::CompareImplConstObligation,
 974         );
 975
 976         // There is no "body" here, so just pass dummy id.
 977         let impl_ty =
 978             inh.normalize_associated_types_in(impl_c_span, impl_c_hir_id, param_env, &impl_ty);
 979
 980         debug!("compare_const_impl: impl_ty={:?}", impl_ty);
 981
 982         let trait_ty =
 983             inh.normalize_associated_types_in(impl_c_span, impl_c_hir_id, param_env, &trait_ty);
 984
 985         debug!("compare_const_impl: trait_ty={:?}", trait_ty);
 986
 987         let err = infcx
 988             .at(&cause, param_env)
 989             .sup(trait_ty, impl_ty)
 990             .map(|ok| inh.register_infer_ok_obligations(ok));
 991
 992         if let Err(terr) = err {
 993             debug!(
 994                 "checking associated const for compatibility: impl ty {:?}, trait ty {:?}",
 995                 impl_ty, trait_ty
 996             );
 997
 998             // Locate the Span containing just the type of the offending impl
 999             match tcx.hir().expect_impl_item(impl_c_hir_id).kind {
1000                 ImplItemKind::Const(ref ty, _) => cause.make_mut().span = ty.span,
1001                 _ => bug!("{:?} is not a impl const", impl_c),
1002             }
1003
1004             let mut diag = struct_span_err!(
1005                 tcx.sess,
1006                 cause.span,
1007                 E0326,
1008                 "implemented const `{}` has an incompatible type for \
1009                                              trait",
1010                 trait_c.ident
1011             );
1012
1013             let trait_c_hir_id =
1014                 trait_c.def_id.as_local().map(|def_id| tcx.hir().local_def_id_to_hir_id(def_id));
1015             let trait_c_span = trait_c_hir_id.map(|trait_c_hir_id| {
1016                 // Add a label to the Span containing just the type of the const
1017                 match tcx.hir().expect_trait_item(trait_c_hir_id).kind {
1018                     TraitItemKind::Const(ref ty, _) => ty.span,
1019                     _ => bug!("{:?} is not a trait const", trait_c),
1020                 }
1021             });
1022
1023             infcx.note_type_err(
1024                 &mut diag,
1025                 &cause,
1026                 trait_c_span.map(|span| (span, "type in trait".to_owned())),
1027                 Some(infer::ValuePairs::Types(ExpectedFound {
1028                     expected: trait_ty,
1029                     found: impl_ty,
1030                 })),
1031                 &terr,
1032             );
1033             diag.emit();
1034         }
1035
1036         // Check that all obligations are satisfied by the implementation's
1037         // version.
1038         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
1039             infcx.report_fulfillment_errors(errors, None, false);
1040             return;
1041         }
1042
1043         let fcx = FnCtxt::new(&inh, param_env, impl_c_hir_id);
1044         fcx.regionck_item(impl_c_hir_id, impl_c_span, &[]);
1045     });
1046 }
1047
1048 crate fn compare_ty_impl<'tcx>(
1049     tcx: TyCtxt<'tcx>,
1050     impl_ty: &ty::AssocItem,
1051     impl_ty_span: Span,
1052     trait_ty: &ty::AssocItem,
1053     impl_trait_ref: ty::TraitRef<'tcx>,
1054     trait_item_span: Option<Span>,
1055 ) {
1056     debug!("compare_impl_type(impl_trait_ref={:?})", impl_trait_ref);
1057
1058     let _: Result<(), ErrorReported> = (|| {
1059         compare_number_of_generics(tcx, impl_ty, impl_ty_span, trait_ty, trait_item_span)?;
1060
1061         compare_type_predicate_entailment(tcx, impl_ty, impl_ty_span, trait_ty, impl_trait_ref)?;
1062
1063         compare_projection_bounds(tcx, trait_ty, impl_ty, impl_ty_span, impl_trait_ref)
1064     })();
1065 }
1066
1067 /// The equivalent of [compare_predicate_entailment], but for associated types
1068 /// instead of associated functions.
1069 fn compare_type_predicate_entailment<'tcx>(
1070     tcx: TyCtxt<'tcx>,
1071     impl_ty: &ty::AssocItem,
1072     impl_ty_span: Span,
1073     trait_ty: &ty::AssocItem,
1074     impl_trait_ref: ty::TraitRef<'tcx>,
1075 ) -> Result<(), ErrorReported> {
1076     let impl_substs = InternalSubsts::identity_for_item(tcx, impl_ty.def_id);
1077     let trait_to_impl_substs =
1078         impl_substs.rebase_onto(tcx, impl_ty.container.id(), impl_trait_ref.substs);
1079
1080     let impl_ty_generics = tcx.generics_of(impl_ty.def_id);
1081     let trait_ty_generics = tcx.generics_of(trait_ty.def_id);
1082     let impl_ty_predicates = tcx.predicates_of(impl_ty.def_id);
1083     let trait_ty_predicates = tcx.predicates_of(trait_ty.def_id);
1084
1085     check_region_bounds_on_impl_item(
1086         tcx,
1087         impl_ty_span,
1088         impl_ty,
1089         trait_ty,
1090         &trait_ty_generics,
1091         &impl_ty_generics,
1092     )?;
1093
1094     let impl_ty_own_bounds = impl_ty_predicates.instantiate_own(tcx, impl_substs);
1095
1096     if impl_ty_own_bounds.is_empty() {
1097         // Nothing to check.
1098         return Ok(());
1099     }
1100
1101     // This `HirId` should be used for the `body_id` field on each
1102     // `ObligationCause` (and the `FnCtxt`). This is what
1103     // `regionck_item` expects.
1104     let impl_ty_hir_id = tcx.hir().local_def_id_to_hir_id(impl_ty.def_id.expect_local());
1105     let cause = ObligationCause::new(
1106         impl_ty_span,
1107         impl_ty_hir_id,
1108         ObligationCauseCode::CompareImplTypeObligation {
1109             item_name: impl_ty.ident.name,
1110             impl_item_def_id: impl_ty.def_id,
1111             trait_item_def_id: trait_ty.def_id,
1112         },
1113     );
1114
1115     debug!("compare_type_predicate_entailment: trait_to_impl_substs={:?}", trait_to_impl_substs);
1116
1117     // The predicates declared by the impl definition, the trait and the
1118     // associated type in the trait are assumed.
1119     let impl_predicates = tcx.predicates_of(impl_ty_predicates.parent.unwrap());
1120     let mut hybrid_preds = impl_predicates.instantiate_identity(tcx);
1121     hybrid_preds
1122         .predicates
1123         .extend(trait_ty_predicates.instantiate_own(tcx, trait_to_impl_substs).predicates);
1124
1125     debug!("compare_type_predicate_entailment: bounds={:?}", hybrid_preds);
1126
1127     let normalize_cause = traits::ObligationCause::misc(impl_ty_span, impl_ty_hir_id);
1128     let param_env = ty::ParamEnv::new(
1129         tcx.intern_predicates(&hybrid_preds.predicates),
1130         Reveal::UserFacing,
1131         None,
1132     );
1133     let param_env = traits::normalize_param_env_or_error(
1134         tcx,
1135         impl_ty.def_id,
1136         param_env,
1137         normalize_cause.clone(),
1138     );
1139     tcx.infer_ctxt().enter(|infcx| {
1140         let inh = Inherited::new(infcx, impl_ty.def_id.expect_local());
1141         let infcx = &inh.infcx;
1142
1143         debug!("compare_type_predicate_entailment: caller_bounds={:?}", param_env.caller_bounds());
1144
1145         let mut selcx = traits::SelectionContext::new(&infcx);
1146
1147         for predicate in impl_ty_own_bounds.predicates {
1148             let traits::Normalized { value: predicate, obligations } =
1149                 traits::normalize(&mut selcx, param_env, normalize_cause.clone(), &predicate);
1150
1151             inh.register_predicates(obligations);
1152             inh.register_predicate(traits::Obligation::new(cause.clone(), param_env, predicate));
1153         }
1154
1155         // Check that all obligations are satisfied by the implementation's
1156         // version.
1157         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
1158             infcx.report_fulfillment_errors(errors, None, false);
1159             return Err(ErrorReported);
1160         }
1161
1162         // Finally, resolve all regions. This catches wily misuses of
1163         // lifetime parameters.
1164         let fcx = FnCtxt::new(&inh, param_env, impl_ty_hir_id);
1165         fcx.regionck_item(impl_ty_hir_id, impl_ty_span, &[]);
1166
1167         Ok(())
1168     })
1169 }
1170
1171 /// Validate that `ProjectionCandidate`s created for this associated type will
1172 /// be valid.
1173 ///
1174 /// Usually given
1175 ///
1176 /// trait X { type Y: Copy } impl X for T { type Y = S; }
1177 ///
1178 /// We are able to normalize `<T as X>::U` to `S`, and so when we check the
1179 /// impl is well-formed we have to prove `S: Copy`.
1180 ///
1181 /// For default associated types the normalization is not possible (the value
1182 /// from the impl could be overridden). We also can't normalize generic
1183 /// associated types (yet) because they contain bound parameters.
1184 fn compare_projection_bounds<'tcx>(
1185     tcx: TyCtxt<'tcx>,
1186     trait_ty: &ty::AssocItem,
1187     impl_ty: &ty::AssocItem,
1188     impl_ty_span: Span,
1189     impl_trait_ref: ty::TraitRef<'tcx>,
1190 ) -> Result<(), ErrorReported> {
1191     let have_gats = tcx.features().generic_associated_types;
1192     if impl_ty.defaultness.is_final() && !have_gats {
1193         // For "final", non-generic associate type implementations, we
1194         // don't need this as described above.
1195         return Ok(());
1196     }
1197
1198     // Given
1199     //
1200     // impl<A, B> Foo<u32> for (A, B) {
1201     //     type Bar<C> =...
1202     // }
1203     //
1204     // - `impl_substs` would be `[A, B, C]`
1205     // - `rebased_substs` would be `[(A, B), u32, C]`, combining the substs from
1206     //    the *trait* with the generic associated type parameters.
1207     let impl_ty_substs = InternalSubsts::identity_for_item(tcx, impl_ty.def_id);
1208     let rebased_substs =
1209         impl_ty_substs.rebase_onto(tcx, impl_ty.container.id(), impl_trait_ref.substs);
1210     let impl_ty_value = tcx.type_of(impl_ty.def_id);
1211
1212     let param_env = tcx.param_env(impl_ty.def_id);
1213
1214     // When checking something like
1215     //
1216     // trait X { type Y: PartialEq<<Self as X>::Y> }
1217     // impl X for T { default type Y = S; }
1218     //
1219     // We will have to prove the bound S: PartialEq<<T as X>::Y>. In this case
1220     // we want <T as X>::Y to normalize to S. This is valid because we are
1221     // checking the default value specifically here. Add this equality to the
1222     // ParamEnv for normalization specifically.
1223     let normalize_param_env = {
1224         let mut predicates = param_env.caller_bounds().iter().collect::<Vec<_>>();
1225         predicates.push(
1226             ty::Binder::dummy(ty::ProjectionPredicate {
1227                 projection_ty: ty::ProjectionTy {
1228                     item_def_id: trait_ty.def_id,
1229                     substs: rebased_substs,
1230                 },
1231                 ty: impl_ty_value,
1232             })
1233             .to_predicate(tcx),
1234         );
1235         ty::ParamEnv::new(tcx.intern_predicates(&predicates), Reveal::UserFacing, None)
1236     };
1237
1238     tcx.infer_ctxt().enter(move |infcx| {
1239         let inh = Inherited::new(infcx, impl_ty.def_id.expect_local());
1240         let infcx = &inh.infcx;
1241         let mut selcx = traits::SelectionContext::new(&infcx);
1242
1243         let impl_ty_hir_id = tcx.hir().local_def_id_to_hir_id(impl_ty.def_id.expect_local());
1244         let normalize_cause = traits::ObligationCause::misc(impl_ty_span, impl_ty_hir_id);
1245         let cause = ObligationCause::new(
1246             impl_ty_span,
1247             impl_ty_hir_id,
1248             ObligationCauseCode::ItemObligation(trait_ty.def_id),
1249         );
1250
1251         let predicates = tcx.projection_predicates(trait_ty.def_id);
1252         debug!("compare_projection_bounds: projection_predicates={:?}", predicates);
1253
1254         for predicate in predicates {
1255             let concrete_ty_predicate = predicate.subst(tcx, rebased_substs);
1256             debug!("compare_projection_bounds: concrete predicate = {:?}", concrete_ty_predicate);
1257
1258             let traits::Normalized { value: normalized_predicate, obligations } = traits::normalize(
1259                 &mut selcx,
1260                 normalize_param_env,
1261                 normalize_cause.clone(),
1262                 &concrete_ty_predicate,
1263             );
1264             debug!("compare_projection_bounds: normalized predicate = {:?}", normalized_predicate);
1265
1266             inh.register_predicates(obligations);
1267             inh.register_predicate(traits::Obligation::new(
1268                 cause.clone(),
1269                 param_env,
1270                 normalized_predicate,
1271             ));
1272         }
1273
1274         // Check that all obligations are satisfied by the implementation's
1275         // version.
1276         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
1277             infcx.report_fulfillment_errors(errors, None, false);
1278             return Err(ErrorReported);
1279         }
1280
1281         // Finally, resolve all regions. This catches wily misuses of
1282         // lifetime parameters.
1283         let fcx = FnCtxt::new(&inh, param_env, impl_ty_hir_id);
1284         fcx.regionck_item(impl_ty_hir_id, impl_ty_span, &[]);
1285
1286         Ok(())
1287     })
1288 }
1289
1290 fn assoc_item_kind_str(impl_item: &ty::AssocItem) -> &'static str {
1291     match impl_item.kind {
1292         ty::AssocKind::Const => "const",
1293         ty::AssocKind::Fn => "method",
1294         ty::AssocKind::Type => "type",
1295     }
1296 }