src/librustc_typeck/check/compare_method.rs

   1 use rustc::hir::{self, GenericParamKind, ImplItemKind, TraitItemKind};
   2 use rustc::infer::{self, InferOk};
   3 use rustc::ty::{self, TyCtxt, GenericParamDefKind};
   4 use rustc::ty::util::ExplicitSelf;
   5 use rustc::traits::{self, ObligationCause, ObligationCauseCode, Reveal};
   6 use rustc::ty::error::{ExpectedFound, TypeError};
   7 use rustc::ty::subst::{Subst, Substs};
   8 use rustc::util::common::ErrorReported;
   9 use errors::Applicability;
  10
  11 use syntax_pos::Span;
  12
  13 use super::{Inherited, FnCtxt, potentially_plural_count};
  14
  15 /// Checks that a method from an impl conforms to the signature of
  16 /// the same method as declared in the trait.
  17 ///
  18 /// # Parameters
  19 ///
  20 /// - `impl_m`: type of the method we are checking
  21 /// - `impl_m_span`: span to use for reporting errors
  22 /// - `trait_m`: the method in the trait
  23 /// - `impl_trait_ref`: the TraitRef corresponding to the trait implementation
  24
  25 pub fn compare_impl_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  26                                      impl_m: &ty::AssociatedItem,
  27                                      impl_m_span: Span,
  28                                      trait_m: &ty::AssociatedItem,
  29                                      impl_trait_ref: ty::TraitRef<'tcx>,
  30                                      trait_item_span: Option<Span>) {
  31     debug!("compare_impl_method(impl_trait_ref={:?})",
  32            impl_trait_ref);
  33
  34     let impl_m_span = tcx.sess.source_map().def_span(impl_m_span);
  35
  36     if let Err(ErrorReported) = compare_self_type(tcx,
  37                                                   impl_m,
  38                                                   impl_m_span,
  39                                                   trait_m,
  40                                                   impl_trait_ref) {
  41         return;
  42     }
  43
  44     if let Err(ErrorReported) = compare_number_of_generics(tcx,
  45                                                            impl_m,
  46                                                            impl_m_span,
  47                                                            trait_m,
  48                                                            trait_item_span) {
  49         return;
  50     }
  51
  52     if let Err(ErrorReported) = compare_number_of_method_arguments(tcx,
  53                                                                    impl_m,
  54                                                                    impl_m_span,
  55                                                                    trait_m,
  56                                                                    trait_item_span) {
  57         return;
  58     }
  59
  60     if let Err(ErrorReported) = compare_synthetic_generics(tcx,
  61                                                            impl_m,
  62                                                            trait_m) {
  63         return;
  64     }
  65
  66     if let Err(ErrorReported) = compare_predicate_entailment(tcx,
  67                                                              impl_m,
  68                                                              impl_m_span,
  69                                                              trait_m,
  70                                                              impl_trait_ref) {
  71         return;
  72     }
  73 }
  74
  75 fn compare_predicate_entailment<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  76                                           impl_m: &ty::AssociatedItem,
  77                                           impl_m_span: Span,
  78                                           trait_m: &ty::AssociatedItem,
  79                                           impl_trait_ref: ty::TraitRef<'tcx>)
  80                                           -> Result<(), ErrorReported> {
  81     let trait_to_impl_substs = impl_trait_ref.substs;
  82
  83     // This node-id should be used for the `body_id` field on each
  84     // `ObligationCause` (and the `FnCtxt`). This is what
  85     // `regionck_item` expects.
  86     let impl_m_node_id = tcx.hir().as_local_node_id(impl_m.def_id).unwrap();
  87
  88     let cause = ObligationCause {
  89         span: impl_m_span,
  90         body_id: impl_m_node_id,
  91         code: ObligationCauseCode::CompareImplMethodObligation {
  92             item_name: impl_m.ident.name,
  93             impl_item_def_id: impl_m.def_id,
  94             trait_item_def_id: trait_m.def_id,
  95         },
  96     };
  97
  98     // This code is best explained by example. Consider a trait:
  99     //
 100     //     trait Trait<'t,T> {
 101     //          fn method<'a,M>(t: &'t T, m: &'a M) -> Self;
 102     //     }
 103     //
 104     // And an impl:
 105     //
 106     //     impl<'i, 'j, U> Trait<'j, &'i U> for Foo {
 107     //          fn method<'b,N>(t: &'j &'i U, m: &'b N) -> Foo;
 108     //     }
 109     //
 110     // We wish to decide if those two method types are compatible.
 111     //
 112     // We start out with trait_to_impl_substs, that maps the trait
 113     // type parameters to impl type parameters. This is taken from the
 114     // impl trait reference:
 115     //
 116     //     trait_to_impl_substs = {'t => 'j, T => &'i U, Self => Foo}
 117     //
 118     // We create a mapping `dummy_substs` that maps from the impl type
 119     // parameters to fresh types and regions. For type parameters,
 120     // this is the identity transform, but we could as well use any
 121     // placeholder types. For regions, we convert from bound to free
 122     // regions (Note: but only early-bound regions, i.e., those
 123     // declared on the impl or used in type parameter bounds).
 124     //
 125     //     impl_to_skol_substs = {'i => 'i0, U => U0, N => N0 }
 126     //
 127     // Now we can apply skol_substs to the type of the impl method
 128     // to yield a new function type in terms of our fresh, placeholder
 129     // types:
 130     //
 131     //     <'b> fn(t: &'i0 U0, m: &'b) -> Foo
 132     //
 133     // We now want to extract and substitute the type of the *trait*
 134     // method and compare it. To do so, we must create a compound
 135     // substitution by combining trait_to_impl_substs and
 136     // impl_to_skol_substs, and also adding a mapping for the method
 137     // type parameters. We extend the mapping to also include
 138     // the method parameters.
 139     //
 140     //     trait_to_skol_substs = { T => &'i0 U0, Self => Foo, M => N0 }
 141     //
 142     // Applying this to the trait method type yields:
 143     //
 144     //     <'a> fn(t: &'i0 U0, m: &'a) -> Foo
 145     //
 146     // This type is also the same but the name of the bound region ('a
 147     // vs 'b).  However, the normal subtyping rules on fn types handle
 148     // this kind of equivalency just fine.
 149     //
 150     // We now use these substitutions to ensure that all declared bounds are
 151     // satisfied by the implementation's method.
 152     //
 153     // We do this by creating a parameter environment which contains a
 154     // substitution corresponding to impl_to_skol_substs. We then build
 155     // trait_to_skol_substs and use it to convert the predicates contained
 156     // in the trait_m.generics to the placeholder form.
 157     //
 158     // Finally we register each of these predicates as an obligation in
 159     // a fresh FulfillmentCtxt, and invoke select_all_or_error.
 160
 161     // Create mapping from impl to placeholder.
 162     let impl_to_skol_substs = Substs::identity_for_item(tcx, impl_m.def_id);
 163
 164     // Create mapping from trait to placeholder.
 165     let trait_to_skol_substs = impl_to_skol_substs.rebase_onto(tcx,
 166                                                                impl_m.container.id(),
 167                                                                trait_to_impl_substs);
 168     debug!("compare_impl_method: trait_to_skol_substs={:?}",
 169            trait_to_skol_substs);
 170
 171     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 172     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 173     let impl_m_predicates = tcx.predicates_of(impl_m.def_id);
 174     let trait_m_predicates = tcx.predicates_of(trait_m.def_id);
 175
 176     // Check region bounds.
 177     check_region_bounds_on_impl_method(tcx,
 178                                        impl_m_span,
 179                                        impl_m,
 180                                        trait_m,
 181                                        &trait_m_generics,
 182                                        &impl_m_generics,
 183                                        trait_to_skol_substs)?;
 184
 185     // Create obligations for each predicate declared by the impl
 186     // definition in the context of the trait's parameter
 187     // environment. We can't just use `impl_env.caller_bounds`,
 188     // however, because we want to replace all late-bound regions with
 189     // region variables.
 190     let impl_predicates = tcx.predicates_of(impl_m_predicates.parent.unwrap());
 191     let mut hybrid_preds = impl_predicates.instantiate_identity(tcx);
 192
 193     debug!("compare_impl_method: impl_bounds={:?}", hybrid_preds);
 194
 195     // This is the only tricky bit of the new way we check implementation methods
 196     // We need to build a set of predicates where only the method-level bounds
 197     // are from the trait and we assume all other bounds from the implementation
 198     // to be previously satisfied.
 199     //
 200     // We then register the obligations from the impl_m and check to see
 201     // if all constraints hold.
 202     hybrid_preds.predicates.extend(
 203         trait_m_predicates.instantiate_own(tcx, trait_to_skol_substs).predicates);
 204
 205     // Construct trait parameter environment and then shift it into the placeholder viewpoint.
 206     // The key step here is to update the caller_bounds's predicates to be
 207     // the new hybrid bounds we computed.
 208     let normalize_cause = traits::ObligationCause::misc(impl_m_span, impl_m_node_id);
 209     let param_env = ty::ParamEnv::new(
 210         tcx.intern_predicates(&hybrid_preds.predicates),
 211         Reveal::UserFacing,
 212         None
 213     );
 214     let param_env = traits::normalize_param_env_or_error(tcx,
 215                                                          impl_m.def_id,
 216                                                          param_env,
 217                                                          normalize_cause.clone());
 218
 219     tcx.infer_ctxt().enter(|infcx| {
 220         let inh = Inherited::new(infcx, impl_m.def_id);
 221         let infcx = &inh.infcx;
 222
 223         debug!("compare_impl_method: caller_bounds={:?}",
 224                param_env.caller_bounds);
 225
 226         let mut selcx = traits::SelectionContext::new(&infcx);
 227
 228         let impl_m_own_bounds = impl_m_predicates.instantiate_own(tcx, impl_to_skol_substs);
 229         let (impl_m_own_bounds, _) = infcx.replace_bound_vars_with_fresh_vars(
 230             impl_m_span,
 231             infer::HigherRankedType,
 232             &ty::Binder::bind(impl_m_own_bounds.predicates)
 233         );
 234         for predicate in impl_m_own_bounds {
 235             let traits::Normalized { value: predicate, obligations } =
 236                 traits::normalize(&mut selcx, param_env, normalize_cause.clone(), &predicate);
 237
 238             inh.register_predicates(obligations);
 239             inh.register_predicate(traits::Obligation::new(cause.clone(), param_env, predicate));
 240         }
 241
 242         // We now need to check that the signature of the impl method is
 243         // compatible with that of the trait method. We do this by
 244         // checking that `impl_fty <: trait_fty`.
 245         //
 246         // FIXME. Unfortunately, this doesn't quite work right now because
 247         // associated type normalization is not integrated into subtype
 248         // checks. For the comparison to be valid, we need to
 249         // normalize the associated types in the impl/trait methods
 250         // first. However, because function types bind regions, just
 251         // calling `normalize_associated_types_in` would have no effect on
 252         // any associated types appearing in the fn arguments or return
 253         // type.
 254
 255         // Compute placeholder form of impl and trait method tys.
 256         let tcx = infcx.tcx;
 257
 258         let (impl_sig, _) = infcx.replace_bound_vars_with_fresh_vars(
 259             impl_m_span,
 260             infer::HigherRankedType,
 261             &tcx.fn_sig(impl_m.def_id)
 262         );
 263         let impl_sig =
 264             inh.normalize_associated_types_in(impl_m_span,
 265                                               impl_m_node_id,
 266                                               param_env,
 267                                               &impl_sig);
 268         let impl_fty = tcx.mk_fn_ptr(ty::Binder::bind(impl_sig));
 269         debug!("compare_impl_method: impl_fty={:?}", impl_fty);
 270
 271         let trait_sig = tcx.liberate_late_bound_regions(
 272             impl_m.def_id,
 273             &tcx.fn_sig(trait_m.def_id));
 274         let trait_sig =
 275             trait_sig.subst(tcx, trait_to_skol_substs);
 276         let trait_sig =
 277             inh.normalize_associated_types_in(impl_m_span,
 278                                               impl_m_node_id,
 279                                               param_env,
 280                                               &trait_sig);
 281         let trait_fty = tcx.mk_fn_ptr(ty::Binder::bind(trait_sig));
 282
 283         debug!("compare_impl_method: trait_fty={:?}", trait_fty);
 284
 285         let sub_result = infcx.at(&cause, param_env)
 286                               .sup(trait_fty, impl_fty)
 287                               .map(|InferOk { obligations, .. }| {
 288                                   inh.register_predicates(obligations);
 289                               });
 290
 291         if let Err(terr) = sub_result {
 292             debug!("sub_types failed: impl ty {:?}, trait ty {:?}",
 293                    impl_fty,
 294                    trait_fty);
 295
 296             let (impl_err_span, trait_err_span) = extract_spans_for_error_reporting(&infcx,
 297                                                                                     param_env,
 298                                                                                     &terr,
 299                                                                                     &cause,
 300                                                                                     impl_m,
 301                                                                                     impl_sig,
 302                                                                                     trait_m,
 303                                                                                     trait_sig);
 304
 305             let cause = ObligationCause {
 306                 span: impl_err_span,
 307                 ..cause.clone()
 308             };
 309
 310             let mut diag = struct_span_err!(tcx.sess,
 311                                             cause.span(&tcx),
 312                                             E0053,
 313                                             "method `{}` has an incompatible type for trait",
 314                                             trait_m.ident);
 315             if let TypeError::Mutability = terr {
 316                 if let Some(trait_err_span) = trait_err_span {
 317                     if let Ok(trait_err_str) = tcx.sess.source_map()
 318                                                        .span_to_snippet(trait_err_span) {
 319                         diag.span_suggestion(
 320                             impl_err_span,
 321                             "consider change the type to match the mutability in trait",
 322                             trait_err_str,
 323                             Applicability::MachineApplicable,
 324                         );
 325                     }
 326                 }
 327             }
 328
 329             infcx.note_type_err(&mut diag,
 330                                 &cause,
 331                                 trait_err_span.map(|sp| (sp, "type in trait".to_owned())),
 332                                 Some(infer::ValuePairs::Types(ExpectedFound {
 333                                     expected: trait_fty,
 334                                     found: impl_fty,
 335                                 })),
 336                                 &terr);
 337             diag.emit();
 338             return Err(ErrorReported);
 339         }
 340
 341         // Check that all obligations are satisfied by the implementation's
 342         // version.
 343         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
 344             infcx.report_fulfillment_errors(errors, None, false);
 345             return Err(ErrorReported);
 346         }
 347
 348         // Finally, resolve all regions. This catches wily misuses of
 349         // lifetime parameters.
 350         let fcx = FnCtxt::new(&inh, param_env, impl_m_node_id);
 351         fcx.regionck_item(impl_m_node_id, impl_m_span, &[]);
 352
 353         Ok(())
 354     })
 355 }
 356
 357 fn check_region_bounds_on_impl_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 358                                                 span: Span,
 359                                                 impl_m: &ty::AssociatedItem,
 360                                                 trait_m: &ty::AssociatedItem,
 361                                                 trait_generics: &ty::Generics,
 362                                                 impl_generics: &ty::Generics,
 363                                                 trait_to_skol_substs: &Substs<'tcx>)
 364                                                 -> Result<(), ErrorReported> {
 365     let trait_params = trait_generics.own_counts().lifetimes;
 366     let impl_params = impl_generics.own_counts().lifetimes;
 367
 368     debug!("check_region_bounds_on_impl_method: \
 369             trait_generics={:?} \
 370             impl_generics={:?} \
 371             trait_to_skol_substs={:?}",
 372            trait_generics,
 373            impl_generics,
 374            trait_to_skol_substs);
 375
 376     // Must have same number of early-bound lifetime parameters.
 377     // Unfortunately, if the user screws up the bounds, then this
 378     // will change classification between early and late.  E.g.,
 379     // if in trait we have `<'a,'b:'a>`, and in impl we just have
 380     // `<'a,'b>`, then we have 2 early-bound lifetime parameters
 381     // in trait but 0 in the impl. But if we report "expected 2
 382     // but found 0" it's confusing, because it looks like there
 383     // are zero. Since I don't quite know how to phrase things at
 384     // the moment, give a kind of vague error message.
 385     if trait_params != impl_params {
 386         let def_span = tcx.sess.source_map().def_span(span);
 387         let span = tcx.hir().get_generics_span(impl_m.def_id).unwrap_or(def_span);
 388         let mut err = struct_span_err!(
 389             tcx.sess,
 390             span,
 391             E0195,
 392             "lifetime parameters or bounds on method `{}` do not match the trait declaration",
 393             impl_m.ident,
 394         );
 395         err.span_label(span, "lifetimes do not match method in trait");
 396         if let Some(sp) = tcx.hir().span_if_local(trait_m.def_id) {
 397             let def_sp = tcx.sess.source_map().def_span(sp);
 398             let sp = tcx.hir().get_generics_span(trait_m.def_id).unwrap_or(def_sp);
 399             err.span_label(sp, "lifetimes in impl do not match this method in trait");
 400         }
 401         err.emit();
 402         return Err(ErrorReported);
 403     }
 404
 405     Ok(())
 406 }
 407
 408 fn extract_spans_for_error_reporting<'a, 'gcx, 'tcx>(infcx: &infer::InferCtxt<'a, 'gcx, 'tcx>,
 409                                                      param_env: ty::ParamEnv<'tcx>,
 410                                                      terr: &TypeError,
 411                                                      cause: &ObligationCause<'tcx>,
 412                                                      impl_m: &ty::AssociatedItem,
 413                                                      impl_sig: ty::FnSig<'tcx>,
 414                                                      trait_m: &ty::AssociatedItem,
 415                                                      trait_sig: ty::FnSig<'tcx>)
 416                                                      -> (Span, Option<Span>) {
 417     let tcx = infcx.tcx;
 418     let impl_m_node_id = tcx.hir().as_local_node_id(impl_m.def_id).unwrap();
 419     let (impl_m_output, impl_m_iter) = match tcx.hir().expect_impl_item(impl_m_node_id).node {
 420         ImplItemKind::Method(ref impl_m_sig, _) => {
 421             (&impl_m_sig.decl.output, impl_m_sig.decl.inputs.iter())
 422         }
 423         _ => bug!("{:?} is not a method", impl_m),
 424     };
 425
 426     match *terr {
 427         TypeError::Mutability => {
 428             if let Some(trait_m_node_id) = tcx.hir().as_local_node_id(trait_m.def_id) {
 429                 let trait_m_iter = match tcx.hir().expect_trait_item(trait_m_node_id).node {
 430                     TraitItemKind::Method(ref trait_m_sig, _) => {
 431                         trait_m_sig.decl.inputs.iter()
 432                     }
 433                     _ => bug!("{:?} is not a TraitItemKind::Method", trait_m),
 434                 };
 435
 436                 impl_m_iter.zip(trait_m_iter).find(|&(ref impl_arg, ref trait_arg)| {
 437                     match (&impl_arg.node, &trait_arg.node) {
 438                         (&hir::TyKind::Rptr(_, ref impl_mt), &hir::TyKind::Rptr(_, ref trait_mt)) |
 439                         (&hir::TyKind::Ptr(ref impl_mt), &hir::TyKind::Ptr(ref trait_mt)) => {
 440                             impl_mt.mutbl != trait_mt.mutbl
 441                         }
 442                         _ => false,
 443                     }
 444                 }).map(|(ref impl_arg, ref trait_arg)| {
 445                     (impl_arg.span, Some(trait_arg.span))
 446                 })
 447                 .unwrap_or_else(|| (cause.span(&tcx), tcx.hir().span_if_local(trait_m.def_id)))
 448             } else {
 449                 (cause.span(&tcx), tcx.hir().span_if_local(trait_m.def_id))
 450             }
 451         }
 452         TypeError::Sorts(ExpectedFound { .. }) => {
 453             if let Some(trait_m_node_id) = tcx.hir().as_local_node_id(trait_m.def_id) {
 454                 let (trait_m_output, trait_m_iter) =
 455                     match tcx.hir().expect_trait_item(trait_m_node_id).node {
 456                         TraitItemKind::Method(ref trait_m_sig, _) => {
 457                             (&trait_m_sig.decl.output, trait_m_sig.decl.inputs.iter())
 458                         }
 459                         _ => bug!("{:?} is not a TraitItemKind::Method", trait_m),
 460                     };
 461
 462                 let impl_iter = impl_sig.inputs().iter();
 463                 let trait_iter = trait_sig.inputs().iter();
 464                 impl_iter.zip(trait_iter)
 465                          .zip(impl_m_iter)
 466                          .zip(trait_m_iter)
 467                          .filter_map(|(((&impl_arg_ty, &trait_arg_ty), impl_arg), trait_arg)|
 468                              match infcx.at(&cause, param_env).sub(trait_arg_ty, impl_arg_ty) {
 469                                  Ok(_) => None,
 470                                  Err(_) => Some((impl_arg.span, Some(trait_arg.span))),
 471                              }
 472                          )
 473                          .next()
 474                          .unwrap_or_else(||
 475                              if
 476                                  infcx.at(&cause, param_env)
 477                                       .sup(trait_sig.output(), impl_sig.output())
 478                                       .is_err()
 479                              {
 480                                  (impl_m_output.span(), Some(trait_m_output.span()))
 481                              } else {
 482                                  (cause.span(&tcx), tcx.hir().span_if_local(trait_m.def_id))
 483                              }
 484                          )
 485             } else {
 486                 (cause.span(&tcx), tcx.hir().span_if_local(trait_m.def_id))
 487             }
 488         }
 489         _ => (cause.span(&tcx), tcx.hir().span_if_local(trait_m.def_id)),
 490     }
 491 }
 492
 493 fn compare_self_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 494                                impl_m: &ty::AssociatedItem,
 495                                impl_m_span: Span,
 496                                trait_m: &ty::AssociatedItem,
 497                                impl_trait_ref: ty::TraitRef<'tcx>)
 498                                -> Result<(), ErrorReported>
 499 {
 500     // Try to give more informative error messages about self typing
 501     // mismatches.  Note that any mismatch will also be detected
 502     // below, where we construct a canonical function type that
 503     // includes the self parameter as a normal parameter.  It's just
 504     // that the error messages you get out of this code are a bit more
 505     // inscrutable, particularly for cases where one method has no
 506     // self.
 507
 508     let self_string = |method: &ty::AssociatedItem| {
 509         let untransformed_self_ty = match method.container {
 510             ty::ImplContainer(_) => impl_trait_ref.self_ty(),
 511             ty::TraitContainer(_) => tcx.mk_self_type()
 512         };
 513         let self_arg_ty = *tcx.fn_sig(method.def_id).input(0).skip_binder();
 514         let param_env = ty::ParamEnv::reveal_all();
 515
 516         tcx.infer_ctxt().enter(|infcx| {
 517             let self_arg_ty = tcx.liberate_late_bound_regions(
 518                 method.def_id,
 519                 &ty::Binder::bind(self_arg_ty)
 520             );
 521             let can_eq_self = |ty| infcx.can_eq(param_env, untransformed_self_ty, ty).is_ok();
 522             match ExplicitSelf::determine(self_arg_ty, can_eq_self) {
 523                 ExplicitSelf::ByValue => "self".to_owned(),
 524                 ExplicitSelf::ByReference(_, hir::MutImmutable) => "&self".to_owned(),
 525                 ExplicitSelf::ByReference(_, hir::MutMutable) => "&mut self".to_owned(),
 526                 _ => format!("self: {}", self_arg_ty)
 527             }
 528         })
 529     };
 530
 531     match (trait_m.method_has_self_argument, impl_m.method_has_self_argument) {
 532         (false, false) | (true, true) => {}
 533
 534         (false, true) => {
 535             let self_descr = self_string(impl_m);
 536             let mut err = struct_span_err!(tcx.sess,
 537                                            impl_m_span,
 538                                            E0185,
 539                                            "method `{}` has a `{}` declaration in the impl, but \
 540                                             not in the trait",
 541                                            trait_m.ident,
 542                                            self_descr);
 543             err.span_label(impl_m_span, format!("`{}` used in impl", self_descr));
 544             if let Some(span) = tcx.hir().span_if_local(trait_m.def_id) {
 545                 err.span_label(span, format!("trait method declared without `{}`", self_descr));
 546             } else {
 547                 err.note_trait_signature(trait_m.ident.to_string(),
 548                                          trait_m.signature(&tcx));
 549             }
 550             err.emit();
 551             return Err(ErrorReported);
 552         }
 553
 554         (true, false) => {
 555             let self_descr = self_string(trait_m);
 556             let mut err = struct_span_err!(tcx.sess,
 557                                            impl_m_span,
 558                                            E0186,
 559                                            "method `{}` has a `{}` declaration in the trait, but \
 560                                             not in the impl",
 561                                            trait_m.ident,
 562                                            self_descr);
 563             err.span_label(impl_m_span, format!("expected `{}` in impl", self_descr));
 564             if let Some(span) = tcx.hir().span_if_local(trait_m.def_id) {
 565                 err.span_label(span, format!("`{}` used in trait", self_descr));
 566             } else {
 567                 err.note_trait_signature(trait_m.ident.to_string(),
 568                                          trait_m.signature(&tcx));
 569             }
 570             err.emit();
 571             return Err(ErrorReported);
 572         }
 573     }
 574
 575     Ok(())
 576 }
 577
 578 fn compare_number_of_generics<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 579                                         impl_m: &ty::AssociatedItem,
 580                                         impl_m_span: Span,
 581                                         trait_m: &ty::AssociatedItem,
 582                                         trait_item_span: Option<Span>)
 583                                         -> Result<(), ErrorReported> {
 584     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 585     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 586     let num_impl_m_type_params = impl_m_generics.own_counts().types;
 587     let num_trait_m_type_params = trait_m_generics.own_counts().types;
 588
 589     if num_impl_m_type_params != num_trait_m_type_params {
 590         let impl_m_node_id = tcx.hir().as_local_node_id(impl_m.def_id).unwrap();
 591         let impl_m_item = tcx.hir().expect_impl_item(impl_m_node_id);
 592         let span = if impl_m_item.generics.params.is_empty()
 593             || impl_m_item.generics.span.is_dummy()  // impl Trait in argument position (#55374)
 594         {
 595             impl_m_span
 596         } else {
 597             impl_m_item.generics.span
 598         };
 599
 600         let mut err = struct_span_err!(tcx.sess, span, E0049,
 601             "method `{}` has {} but its trait declaration has {}",
 602             trait_m.ident,
 603             potentially_plural_count(num_impl_m_type_params, "type parameter"),
 604             potentially_plural_count(num_trait_m_type_params, "type parameter")
 605         );
 606
 607         let mut suffix = None;
 608
 609         if let Some(span) = trait_item_span {
 610             err.span_label(span, format!("expected {}",
 611                 potentially_plural_count(num_trait_m_type_params, "type parameter")));
 612         } else {
 613             suffix = Some(format!(", expected {}", num_trait_m_type_params));
 614         }
 615
 616         err.span_label(span,
 617                        format!("found {}{}",
 618                            potentially_plural_count(num_impl_m_type_params, "type parameter"),
 619                            suffix.as_ref().map(|s| &s[..]).unwrap_or("")));
 620
 621         err.emit();
 622
 623         return Err(ErrorReported);
 624     }
 625
 626     Ok(())
 627 }
 628
 629 fn compare_number_of_method_arguments<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 630                                                 impl_m: &ty::AssociatedItem,
 631                                                 impl_m_span: Span,
 632                                                 trait_m: &ty::AssociatedItem,
 633                                                 trait_item_span: Option<Span>)
 634                                                 -> Result<(), ErrorReported> {
 635     let impl_m_fty = tcx.fn_sig(impl_m.def_id);
 636     let trait_m_fty = tcx.fn_sig(trait_m.def_id);
 637     let trait_number_args = trait_m_fty.inputs().skip_binder().len();
 638     let impl_number_args = impl_m_fty.inputs().skip_binder().len();
 639     if trait_number_args != impl_number_args {
 640         let trait_m_node_id = tcx.hir().as_local_node_id(trait_m.def_id);
 641         let trait_span = if let Some(trait_id) = trait_m_node_id {
 642             match tcx.hir().expect_trait_item(trait_id).node {
 643                 TraitItemKind::Method(ref trait_m_sig, _) => {
 644                     let pos = if trait_number_args > 0 {
 645                         trait_number_args - 1
 646                     } else {
 647                         0
 648                     };
 649                     if let Some(arg) = trait_m_sig.decl.inputs.get(pos) {
 650                         Some(if pos == 0 {
 651                             arg.span
 652                         } else {
 653                             Span::new(trait_m_sig.decl.inputs[0].span.lo(),
 654                                       arg.span.hi(),
 655                                       arg.span.ctxt())
 656                         })
 657                     } else {
 658                         trait_item_span
 659                     }
 660                 }
 661                 _ => bug!("{:?} is not a method", impl_m),
 662             }
 663         } else {
 664             trait_item_span
 665         };
 666         let impl_m_node_id = tcx.hir().as_local_node_id(impl_m.def_id).unwrap();
 667         let impl_span = match tcx.hir().expect_impl_item(impl_m_node_id).node {
 668             ImplItemKind::Method(ref impl_m_sig, _) => {
 669                 let pos = if impl_number_args > 0 {
 670                     impl_number_args - 1
 671                 } else {
 672                     0
 673                 };
 674                 if let Some(arg) = impl_m_sig.decl.inputs.get(pos) {
 675                     if pos == 0 {
 676                         arg.span
 677                     } else {
 678                         Span::new(impl_m_sig.decl.inputs[0].span.lo(),
 679                                   arg.span.hi(),
 680                                   arg.span.ctxt())
 681                     }
 682                 } else {
 683                     impl_m_span
 684                 }
 685             }
 686             _ => bug!("{:?} is not a method", impl_m),
 687         };
 688         let mut err = struct_span_err!(tcx.sess,
 689                                        impl_span,
 690                                        E0050,
 691                                        "method `{}` has {} but the declaration in \
 692                                         trait `{}` has {}",
 693                                        trait_m.ident,
 694                                        potentially_plural_count(impl_number_args, "parameter"),
 695                                        tcx.item_path_str(trait_m.def_id),
 696                                        trait_number_args);
 697         if let Some(trait_span) = trait_span {
 698             err.span_label(trait_span, format!("trait requires {}",
 699                 potentially_plural_count(trait_number_args, "parameter")));
 700         } else {
 701             err.note_trait_signature(trait_m.ident.to_string(),
 702                                      trait_m.signature(&tcx));
 703         }
 704         err.span_label(impl_span, format!("expected {}, found {}",
 705             potentially_plural_count(trait_number_args, "parameter"), impl_number_args));
 706         err.emit();
 707         return Err(ErrorReported);
 708     }
 709
 710     Ok(())
 711 }
 712
 713 fn compare_synthetic_generics<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 714                                         impl_m: &ty::AssociatedItem,
 715                                         trait_m: &ty::AssociatedItem)
 716                                         -> Result<(), ErrorReported> {
 717     // FIXME(chrisvittal) Clean up this function, list of FIXME items:
 718     //     1. Better messages for the span labels
 719     //     2. Explanation as to what is going on
 720     // If we get here, we already have the same number of generics, so the zip will
 721     // be okay.
 722     let mut error_found = false;
 723     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 724     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 725     let impl_m_type_params = impl_m_generics.params.iter().filter_map(|param| match param.kind {
 726         GenericParamDefKind::Type { synthetic, .. } => Some((param.def_id, synthetic)),
 727         GenericParamDefKind::Lifetime => None,
 728     });
 729     let trait_m_type_params = trait_m_generics.params.iter().filter_map(|param| {
 730         match param.kind {
 731             GenericParamDefKind::Type { synthetic, .. } => Some((param.def_id, synthetic)),
 732             GenericParamDefKind::Lifetime => None,
 733         }
 734     });
 735     for ((impl_def_id, impl_synthetic), (trait_def_id, trait_synthetic))
 736         in impl_m_type_params.zip(trait_m_type_params)
 737     {
 738         if impl_synthetic != trait_synthetic {
 739             let impl_hir_id = tcx.hir().as_local_hir_id(impl_def_id).unwrap();
 740             let impl_span = tcx.hir().span_by_hir_id(impl_hir_id);
 741             let trait_span = tcx.def_span(trait_def_id);
 742             let mut err = struct_span_err!(tcx.sess,
 743                                            impl_span,
 744                                            E0643,
 745                                            "method `{}` has incompatible signature for trait",
 746                                            trait_m.ident);
 747             err.span_label(trait_span, "declaration in trait here");
 748             match (impl_synthetic, trait_synthetic) {
 749                 // The case where the impl method uses `impl Trait` but the trait method uses
 750                 // explicit generics
 751                 (Some(hir::SyntheticTyParamKind::ImplTrait), None) => {
 752                     err.span_label(impl_span, "expected generic parameter, found `impl Trait`");
 753                     (|| {
 754                         // try taking the name from the trait impl
 755                         // FIXME: this is obviously suboptimal since the name can already be used
 756                         // as another generic argument
 757                         let new_name = tcx
 758                             .sess
 759                             .source_map()
 760                             .span_to_snippet(trait_span)
 761                             .ok()?;
 762                         let trait_m = tcx.hir().as_local_node_id(trait_m.def_id)?;
 763                         let trait_m = tcx.hir().trait_item(hir::TraitItemId { node_id: trait_m });
 764
 765                         let impl_m = tcx.hir().as_local_node_id(impl_m.def_id)?;
 766                         let impl_m = tcx.hir().impl_item(hir::ImplItemId { node_id: impl_m });
 767
 768                         // in case there are no generics, take the spot between the function name
 769                         // and the opening paren of the argument list
 770                         let new_generics_span = tcx
 771                             .sess
 772                             .source_map()
 773                             .generate_fn_name_span(impl_span)?
 774                             .shrink_to_hi();
 775                         // in case there are generics, just replace them
 776                         let generics_span = impl_m
 777                             .generics
 778                             .span
 779                             .substitute_dummy(new_generics_span);
 780                         // replace with the generics from the trait
 781                         let new_generics = tcx
 782                             .sess
 783                             .source_map()
 784                             .span_to_snippet(trait_m.generics.span)
 785                             .ok()?;
 786
 787                         err.multipart_suggestion(
 788                             "try changing the `impl Trait` argument to a generic parameter",
 789                             vec![
 790                                 // replace `impl Trait` with `T`
 791                                 (impl_span, new_name),
 792                                 // replace impl method generics with trait method generics
 793                                 // This isn't quite right, as users might have changed the names
 794                                 // of the generics, but it works for the common case
 795                                 (generics_span, new_generics),
 796                             ],
 797                             Applicability::MaybeIncorrect,
 798                         );
 799                         Some(())
 800                     })();
 801                 },
 802                 // The case where the trait method uses `impl Trait`, but the impl method uses
 803                 // explicit generics.
 804                 (None, Some(hir::SyntheticTyParamKind::ImplTrait)) => {
 805                     err.span_label(impl_span, "expected `impl Trait`, found generic parameter");
 806                     (|| {
 807                         let impl_m = tcx.hir().as_local_node_id(impl_m.def_id)?;
 808                         let impl_m = tcx.hir().impl_item(hir::ImplItemId { node_id: impl_m });
 809                         let input_tys = match impl_m.node {
 810                             hir::ImplItemKind::Method(ref sig, _) => &sig.decl.inputs,
 811                             _ => unreachable!(),
 812                         };
 813                         struct Visitor(Option<Span>, hir::def_id::DefId);
 814                         impl<'v> hir::intravisit::Visitor<'v> for Visitor {
 815                             fn visit_ty(&mut self, ty: &'v hir::Ty) {
 816                                 hir::intravisit::walk_ty(self, ty);
 817                                 if let hir::TyKind::Path(
 818                                     hir::QPath::Resolved(None, ref path)) = ty.node
 819                                 {
 820                                     if let hir::def::Def::TyParam(def_id) = path.def {
 821                                         if def_id == self.1 {
 822                                             self.0 = Some(ty.span);
 823                                         }
 824                                     }
 825                                 }
 826                             }
 827                             fn nested_visit_map<'this>(
 828                                 &'this mut self
 829                             ) -> hir::intravisit::NestedVisitorMap<'this, 'v> {
 830                                 hir::intravisit::NestedVisitorMap::None
 831                             }
 832                         }
 833                         let mut visitor = Visitor(None, impl_def_id);
 834                         for ty in input_tys {
 835                             hir::intravisit::Visitor::visit_ty(&mut visitor, ty);
 836                         }
 837                         let span = visitor.0?;
 838
 839                         let bounds = impl_m.generics.params.iter().find_map(|param| {
 840                             match param.kind {
 841                                 GenericParamKind::Lifetime { .. } => None,
 842                                 GenericParamKind::Type { .. } => {
 843                                     if param.hir_id == impl_hir_id {
 844                                         Some(&param.bounds)
 845                                     } else {
 846                                         None
 847                                     }
 848                                 }
 849                             }
 850                         })?;
 851                         let bounds = bounds.first()?.span().to(bounds.last()?.span());
 852                         let bounds = tcx
 853                             .sess
 854                             .source_map()
 855                             .span_to_snippet(bounds)
 856                             .ok()?;
 857
 858                         err.multipart_suggestion(
 859                             "try removing the generic parameter and using `impl Trait` instead",
 860                             vec![
 861                                 // delete generic parameters
 862                                 (impl_m.generics.span, String::new()),
 863                                 // replace param usage with `impl Trait`
 864                                 (span, format!("impl {}", bounds)),
 865                             ],
 866                             Applicability::MaybeIncorrect,
 867                         );
 868                         Some(())
 869                     })();
 870                 },
 871                 _ => unreachable!(),
 872             }
 873             err.emit();
 874             error_found = true;
 875         }
 876     }
 877     if error_found {
 878         Err(ErrorReported)
 879     } else {
 880         Ok(())
 881     }
 882 }
 883
 884 pub fn compare_const_impl<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 885                                     impl_c: &ty::AssociatedItem,
 886                                     impl_c_span: Span,
 887                                     trait_c: &ty::AssociatedItem,
 888                                     impl_trait_ref: ty::TraitRef<'tcx>) {
 889     debug!("compare_const_impl(impl_trait_ref={:?})", impl_trait_ref);
 890
 891     tcx.infer_ctxt().enter(|infcx| {
 892         let param_env = ty::ParamEnv::empty();
 893         let inh = Inherited::new(infcx, impl_c.def_id);
 894         let infcx = &inh.infcx;
 895
 896         // The below is for the most part highly similar to the procedure
 897         // for methods above. It is simpler in many respects, especially
 898         // because we shouldn't really have to deal with lifetimes or
 899         // predicates. In fact some of this should probably be put into
 900         // shared functions because of DRY violations...
 901         let trait_to_impl_substs = impl_trait_ref.substs;
 902
 903         // Create a parameter environment that represents the implementation's
 904         // method.
 905         let impl_c_node_id = tcx.hir().as_local_node_id(impl_c.def_id).unwrap();
 906
 907         // Compute placeholder form of impl and trait const tys.
 908         let impl_ty = tcx.type_of(impl_c.def_id);
 909         let trait_ty = tcx.type_of(trait_c.def_id).subst(tcx, trait_to_impl_substs);
 910         let mut cause = ObligationCause::misc(impl_c_span, impl_c_node_id);
 911
 912         // There is no "body" here, so just pass dummy id.
 913         let impl_ty = inh.normalize_associated_types_in(impl_c_span,
 914                                                         impl_c_node_id,
 915                                                         param_env,
 916                                                         &impl_ty);
 917
 918         debug!("compare_const_impl: impl_ty={:?}", impl_ty);
 919
 920         let trait_ty = inh.normalize_associated_types_in(impl_c_span,
 921                                                          impl_c_node_id,
 922                                                          param_env,
 923                                                          &trait_ty);
 924
 925         debug!("compare_const_impl: trait_ty={:?}", trait_ty);
 926
 927         let err = infcx.at(&cause, param_env)
 928                        .sup(trait_ty, impl_ty)
 929                        .map(|ok| inh.register_infer_ok_obligations(ok));
 930
 931         if let Err(terr) = err {
 932             debug!("checking associated const for compatibility: impl ty {:?}, trait ty {:?}",
 933                    impl_ty,
 934                    trait_ty);
 935
 936             // Locate the Span containing just the type of the offending impl
 937             match tcx.hir().expect_impl_item(impl_c_node_id).node {
 938                 ImplItemKind::Const(ref ty, _) => cause.span = ty.span,
 939                 _ => bug!("{:?} is not a impl const", impl_c),
 940             }
 941
 942             let mut diag = struct_span_err!(tcx.sess,
 943                                             cause.span,
 944                                             E0326,
 945                                             "implemented const `{}` has an incompatible type for \
 946                                              trait",
 947                                             trait_c.ident);
 948
 949             let trait_c_node_id = tcx.hir().as_local_node_id(trait_c.def_id);
 950             let trait_c_span = trait_c_node_id.map(|trait_c_node_id| {
 951                 // Add a label to the Span containing just the type of the const
 952                 match tcx.hir().expect_trait_item(trait_c_node_id).node {
 953                     TraitItemKind::Const(ref ty, _) => ty.span,
 954                     _ => bug!("{:?} is not a trait const", trait_c),
 955                 }
 956             });
 957
 958             infcx.note_type_err(&mut diag,
 959                                 &cause,
 960                                 trait_c_span.map(|span| (span, "type in trait".to_owned())),
 961                                 Some(infer::ValuePairs::Types(ExpectedFound {
 962                                     expected: trait_ty,
 963                                     found: impl_ty,
 964                                 })),
 965                                 &terr);
 966             diag.emit();
 967         }
 968
 969         // Check that all obligations are satisfied by the implementation's
 970         // version.
 971         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
 972             infcx.report_fulfillment_errors(errors, None, false);
 973             return;
 974         }
 975
 976         let fcx = FnCtxt::new(&inh, param_env, impl_c_node_id);
 977         fcx.regionck_item(impl_c_node_id, impl_c_span, &[]);
 978     });
 979 }