src/librustc_typeck/check/compare_method.rs

   1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use rustc::hir::{self, ImplItemKind, TraitItemKind};
  12 use rustc::infer::{self, InferOk};
  13 use rustc::middle::free_region::FreeRegionMap;
  14 use rustc::middle::region::RegionMaps;
  15 use rustc::ty::{self, TyCtxt};
  16 use rustc::traits::{self, ObligationCause, ObligationCauseCode, Reveal};
  17 use rustc::ty::error::{ExpectedFound, TypeError};
  18 use rustc::ty::subst::{Subst, Substs};
  19 use rustc::util::common::ErrorReported;
  20
  21 use syntax_pos::Span;
  22
  23 use super::{Inherited, FnCtxt};
  24 use astconv::ExplicitSelf;
  25
  26 /// Checks that a method from an impl conforms to the signature of
  27 /// the same method as declared in the trait.
  28 ///
  29 /// # Parameters
  30 ///
  31 /// - impl_m: type of the method we are checking
  32 /// - impl_m_span: span to use for reporting errors
  33 /// - trait_m: the method in the trait
  34 /// - impl_trait_ref: the TraitRef corresponding to the trait implementation
  35
  36 pub fn compare_impl_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  37                                      impl_m: &ty::AssociatedItem,
  38                                      impl_m_span: Span,
  39                                      trait_m: &ty::AssociatedItem,
  40                                      impl_trait_ref: ty::TraitRef<'tcx>,
  41                                      trait_item_span: Option<Span>,
  42                                      old_broken_mode: bool) {
  43     debug!("compare_impl_method(impl_trait_ref={:?})",
  44            impl_trait_ref);
  45
  46     if let Err(ErrorReported) = compare_self_type(tcx,
  47                                                   impl_m,
  48                                                   impl_m_span,
  49                                                   trait_m,
  50                                                   impl_trait_ref) {
  51         return;
  52     }
  53
  54     if let Err(ErrorReported) = compare_number_of_generics(tcx,
  55                                                            impl_m,
  56                                                            impl_m_span,
  57                                                            trait_m,
  58                                                            trait_item_span) {
  59         return;
  60     }
  61
  62     if let Err(ErrorReported) = compare_number_of_method_arguments(tcx,
  63                                                                    impl_m,
  64                                                                    impl_m_span,
  65                                                                    trait_m,
  66                                                                    trait_item_span) {
  67         return;
  68     }
  69
  70     if let Err(ErrorReported) = compare_predicate_entailment(tcx,
  71                                                              impl_m,
  72                                                              impl_m_span,
  73                                                              trait_m,
  74                                                              impl_trait_ref,
  75                                                              old_broken_mode) {
  76         return;
  77     }
  78 }
  79
  80 fn compare_predicate_entailment<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  81                                           impl_m: &ty::AssociatedItem,
  82                                           impl_m_span: Span,
  83                                           trait_m: &ty::AssociatedItem,
  84                                           impl_trait_ref: ty::TraitRef<'tcx>,
  85                                           old_broken_mode: bool)
  86                                           -> Result<(), ErrorReported> {
  87     let trait_to_impl_substs = impl_trait_ref.substs;
  88
  89     // This node-id should be used for the `body_id` field on each
  90     // `ObligationCause` (and the `FnCtxt`). This is what
  91     // `regionck_item` expects.
  92     let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
  93
  94     let cause = ObligationCause {
  95         span: impl_m_span,
  96         body_id: impl_m_node_id,
  97         code: ObligationCauseCode::CompareImplMethodObligation {
  98             item_name: impl_m.name,
  99             impl_item_def_id: impl_m.def_id,
 100             trait_item_def_id: trait_m.def_id,
 101             lint_id: if !old_broken_mode { Some(impl_m_node_id) } else { None },
 102         },
 103     };
 104
 105     // This code is best explained by example. Consider a trait:
 106     //
 107     //     trait Trait<'t,T> {
 108     //          fn method<'a,M>(t: &'t T, m: &'a M) -> Self;
 109     //     }
 110     //
 111     // And an impl:
 112     //
 113     //     impl<'i, 'j, U> Trait<'j, &'i U> for Foo {
 114     //          fn method<'b,N>(t: &'j &'i U, m: &'b N) -> Foo;
 115     //     }
 116     //
 117     // We wish to decide if those two method types are compatible.
 118     //
 119     // We start out with trait_to_impl_substs, that maps the trait
 120     // type parameters to impl type parameters. This is taken from the
 121     // impl trait reference:
 122     //
 123     //     trait_to_impl_substs = {'t => 'j, T => &'i U, Self => Foo}
 124     //
 125     // We create a mapping `dummy_substs` that maps from the impl type
 126     // parameters to fresh types and regions. For type parameters,
 127     // this is the identity transform, but we could as well use any
 128     // skolemized types. For regions, we convert from bound to free
 129     // regions (Note: but only early-bound regions, i.e., those
 130     // declared on the impl or used in type parameter bounds).
 131     //
 132     //     impl_to_skol_substs = {'i => 'i0, U => U0, N => N0 }
 133     //
 134     // Now we can apply skol_substs to the type of the impl method
 135     // to yield a new function type in terms of our fresh, skolemized
 136     // types:
 137     //
 138     //     <'b> fn(t: &'i0 U0, m: &'b) -> Foo
 139     //
 140     // We now want to extract and substitute the type of the *trait*
 141     // method and compare it. To do so, we must create a compound
 142     // substitution by combining trait_to_impl_substs and
 143     // impl_to_skol_substs, and also adding a mapping for the method
 144     // type parameters. We extend the mapping to also include
 145     // the method parameters.
 146     //
 147     //     trait_to_skol_substs = { T => &'i0 U0, Self => Foo, M => N0 }
 148     //
 149     // Applying this to the trait method type yields:
 150     //
 151     //     <'a> fn(t: &'i0 U0, m: &'a) -> Foo
 152     //
 153     // This type is also the same but the name of the bound region ('a
 154     // vs 'b).  However, the normal subtyping rules on fn types handle
 155     // this kind of equivalency just fine.
 156     //
 157     // We now use these substitutions to ensure that all declared bounds are
 158     // satisfied by the implementation's method.
 159     //
 160     // We do this by creating a parameter environment which contains a
 161     // substitution corresponding to impl_to_skol_substs. We then build
 162     // trait_to_skol_substs and use it to convert the predicates contained
 163     // in the trait_m.generics to the skolemized form.
 164     //
 165     // Finally we register each of these predicates as an obligation in
 166     // a fresh FulfillmentCtxt, and invoke select_all_or_error.
 167
 168     // Create mapping from impl to skolemized.
 169     let impl_to_skol_substs = Substs::identity_for_item(tcx, impl_m.def_id);
 170
 171     // Create mapping from trait to skolemized.
 172     let trait_to_skol_substs = impl_to_skol_substs.rebase_onto(tcx,
 173                                                                impl_m.container.id(),
 174                                                                trait_to_impl_substs);
 175     debug!("compare_impl_method: trait_to_skol_substs={:?}",
 176            trait_to_skol_substs);
 177
 178     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 179     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 180     let impl_m_predicates = tcx.predicates_of(impl_m.def_id);
 181     let trait_m_predicates = tcx.predicates_of(trait_m.def_id);
 182
 183     // Check region bounds.
 184     check_region_bounds_on_impl_method(tcx,
 185                                        impl_m_span,
 186                                        impl_m,
 187                                        &trait_m_generics,
 188                                        &impl_m_generics,
 189                                        trait_to_skol_substs)?;
 190
 191     // Create obligations for each predicate declared by the impl
 192     // definition in the context of the trait's parameter
 193     // environment. We can't just use `impl_env.caller_bounds`,
 194     // however, because we want to replace all late-bound regions with
 195     // region variables.
 196     let impl_predicates = tcx.predicates_of(impl_m_predicates.parent.unwrap());
 197     let mut hybrid_preds = impl_predicates.instantiate_identity(tcx);
 198
 199     debug!("compare_impl_method: impl_bounds={:?}", hybrid_preds);
 200
 201     // This is the only tricky bit of the new way we check implementation methods
 202     // We need to build a set of predicates where only the method-level bounds
 203     // are from the trait and we assume all other bounds from the implementation
 204     // to be previously satisfied.
 205     //
 206     // We then register the obligations from the impl_m and check to see
 207     // if all constraints hold.
 208     hybrid_preds.predicates
 209                 .extend(trait_m_predicates.instantiate_own(tcx, trait_to_skol_substs).predicates);
 210
 211     // Construct trait parameter environment and then shift it into the skolemized viewpoint.
 212     // The key step here is to update the caller_bounds's predicates to be
 213     // the new hybrid bounds we computed.
 214     let normalize_cause = traits::ObligationCause::misc(impl_m_span, impl_m_node_id);
 215     let param_env = ty::ParamEnv::new(tcx.intern_predicates(&hybrid_preds.predicates),
 216                                       Reveal::UserFacing);
 217     let param_env = traits::normalize_param_env_or_error(tcx,
 218                                                          impl_m.def_id,
 219                                                          param_env,
 220                                                          normalize_cause.clone());
 221
 222     tcx.infer_ctxt(()).enter(|infcx| {
 223         let inh = Inherited::new(infcx, impl_m.def_id);
 224         let infcx = &inh.infcx;
 225
 226         debug!("compare_impl_method: caller_bounds={:?}",
 227                param_env.caller_bounds);
 228
 229         let mut selcx = traits::SelectionContext::new(&infcx);
 230
 231         let impl_m_own_bounds = impl_m_predicates.instantiate_own(tcx, impl_to_skol_substs);
 232         let (impl_m_own_bounds, _) = infcx.replace_late_bound_regions_with_fresh_var(impl_m_span,
 233                                                        infer::HigherRankedType,
 234                                                        &ty::Binder(impl_m_own_bounds.predicates));
 235         for predicate in impl_m_own_bounds {
 236             let traits::Normalized { value: predicate, obligations } =
 237                 traits::normalize(&mut selcx, param_env, normalize_cause.clone(), &predicate);
 238
 239             inh.register_predicates(obligations);
 240             inh.register_predicate(traits::Obligation::new(cause.clone(), param_env, predicate));
 241         }
 242
 243         // We now need to check that the signature of the impl method is
 244         // compatible with that of the trait method. We do this by
 245         // checking that `impl_fty <: trait_fty`.
 246         //
 247         // FIXME. Unfortunately, this doesn't quite work right now because
 248         // associated type normalization is not integrated into subtype
 249         // checks. For the comparison to be valid, we need to
 250         // normalize the associated types in the impl/trait methods
 251         // first. However, because function types bind regions, just
 252         // calling `normalize_associated_types_in` would have no effect on
 253         // any associated types appearing in the fn arguments or return
 254         // type.
 255
 256         // Compute skolemized form of impl and trait method tys.
 257         let tcx = infcx.tcx;
 258
 259         let m_sig = |method: &ty::AssociatedItem| {
 260             match tcx.type_of(method.def_id).sty {
 261                 ty::TyFnDef(_, _, f) => f,
 262                 _ => bug!()
 263             }
 264         };
 265
 266         let (impl_sig, _) =
 267             infcx.replace_late_bound_regions_with_fresh_var(impl_m_span,
 268                                                             infer::HigherRankedType,
 269                                                             &m_sig(impl_m));
 270         let impl_sig =
 271             inh.normalize_associated_types_in(impl_m_span,
 272                                               impl_m_node_id,
 273                                               param_env,
 274                                               &impl_sig);
 275         let impl_fty = tcx.mk_fn_ptr(ty::Binder(impl_sig));
 276         debug!("compare_impl_method: impl_fty={:?}", impl_fty);
 277
 278         let trait_sig = inh.liberate_late_bound_regions(
 279             impl_m.def_id,
 280             &m_sig(trait_m));
 281         let trait_sig =
 282             trait_sig.subst(tcx, trait_to_skol_substs);
 283         let trait_sig =
 284             inh.normalize_associated_types_in(impl_m_span,
 285                                               impl_m_node_id,
 286                                               param_env,
 287                                               &trait_sig);
 288         let trait_fty = tcx.mk_fn_ptr(ty::Binder(trait_sig));
 289
 290         debug!("compare_impl_method: trait_fty={:?}", trait_fty);
 291
 292         let sub_result = infcx.at(&cause, param_env)
 293                               .sup(trait_fty, impl_fty)
 294                               .map(|InferOk { obligations, .. }| {
 295                                   inh.register_predicates(obligations);
 296                               });
 297
 298         if let Err(terr) = sub_result {
 299             debug!("sub_types failed: impl ty {:?}, trait ty {:?}",
 300                    impl_fty,
 301                    trait_fty);
 302
 303             let (impl_err_span, trait_err_span) = extract_spans_for_error_reporting(&infcx,
 304                                                                                     param_env,
 305                                                                                     &terr,
 306                                                                                     &cause,
 307                                                                                     impl_m,
 308                                                                                     impl_sig,
 309                                                                                     trait_m,
 310                                                                                     trait_sig);
 311
 312             let cause = ObligationCause {
 313                 span: impl_err_span,
 314                 ..cause.clone()
 315             };
 316
 317             let mut diag = struct_span_err!(tcx.sess,
 318                                             cause.span,
 319                                             E0053,
 320                                             "method `{}` has an incompatible type for trait",
 321                                             trait_m.name);
 322
 323             infcx.note_type_err(&mut diag,
 324                                 &cause,
 325                                 trait_err_span.map(|sp| (sp, format!("type in trait"))),
 326                                 Some(infer::ValuePairs::Types(ExpectedFound {
 327                                     expected: trait_fty,
 328                                     found: impl_fty,
 329                                 })),
 330                                 &terr);
 331             diag.emit();
 332             return Err(ErrorReported);
 333         }
 334
 335         // Check that all obligations are satisfied by the implementation's
 336         // version.
 337         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
 338             infcx.report_fulfillment_errors(errors);
 339             return Err(ErrorReported);
 340         }
 341
 342         // Finally, resolve all regions. This catches wily misuses of
 343         // lifetime parameters.
 344         if old_broken_mode {
 345             // FIXME(#18937) -- this is how the code used to
 346             // work. This is buggy because the fulfillment cx creates
 347             // region obligations that get overlooked.  The right
 348             // thing to do is the code below. But we keep this old
 349             // pass around temporarily.
 350             let region_maps = RegionMaps::new();
 351             let mut free_regions = FreeRegionMap::new();
 352             free_regions.relate_free_regions_from_predicates(&param_env.caller_bounds);
 353             infcx.resolve_regions_and_report_errors(impl_m.def_id, &region_maps, &free_regions);
 354         } else {
 355             let fcx = FnCtxt::new(&inh, param_env, impl_m_node_id);
 356             fcx.regionck_item(impl_m_node_id, impl_m_span, &[]);
 357         }
 358
 359         Ok(())
 360     })
 361 }
 362
 363 fn check_region_bounds_on_impl_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 364                                                 span: Span,
 365                                                 impl_m: &ty::AssociatedItem,
 366                                                 trait_generics: &ty::Generics,
 367                                                 impl_generics: &ty::Generics,
 368                                                 trait_to_skol_substs: &Substs<'tcx>)
 369                                                 -> Result<(), ErrorReported> {
 370     let trait_params = &trait_generics.regions[..];
 371     let impl_params = &impl_generics.regions[..];
 372
 373     debug!("check_region_bounds_on_impl_method: \
 374             trait_generics={:?} \
 375             impl_generics={:?} \
 376             trait_to_skol_substs={:?}",
 377            trait_generics,
 378            impl_generics,
 379            trait_to_skol_substs);
 380
 381     // Must have same number of early-bound lifetime parameters.
 382     // Unfortunately, if the user screws up the bounds, then this
 383     // will change classification between early and late.  E.g.,
 384     // if in trait we have `<'a,'b:'a>`, and in impl we just have
 385     // `<'a,'b>`, then we have 2 early-bound lifetime parameters
 386     // in trait but 0 in the impl. But if we report "expected 2
 387     // but found 0" it's confusing, because it looks like there
 388     // are zero. Since I don't quite know how to phrase things at
 389     // the moment, give a kind of vague error message.
 390     if trait_params.len() != impl_params.len() {
 391         struct_span_err!(tcx.sess,
 392                          span,
 393                          E0195,
 394                          "lifetime parameters or bounds on method `{}` do not match the \
 395                           trait declaration",
 396                          impl_m.name)
 397             .span_label(span, "lifetimes do not match trait")
 398             .emit();
 399         return Err(ErrorReported);
 400     }
 401
 402     return Ok(());
 403 }
 404
 405 fn extract_spans_for_error_reporting<'a, 'gcx, 'tcx>(infcx: &infer::InferCtxt<'a, 'gcx, 'tcx>,
 406                                                      param_env: ty::ParamEnv<'tcx>,
 407                                                      terr: &TypeError,
 408                                                      cause: &ObligationCause<'tcx>,
 409                                                      impl_m: &ty::AssociatedItem,
 410                                                      impl_sig: ty::FnSig<'tcx>,
 411                                                      trait_m: &ty::AssociatedItem,
 412                                                      trait_sig: ty::FnSig<'tcx>)
 413                                                      -> (Span, Option<Span>) {
 414     let tcx = infcx.tcx;
 415     let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
 416     let (impl_m_output, impl_m_iter) = match tcx.hir.expect_impl_item(impl_m_node_id).node {
 417         ImplItemKind::Method(ref impl_m_sig, _) => {
 418             (&impl_m_sig.decl.output, impl_m_sig.decl.inputs.iter())
 419         }
 420         _ => bug!("{:?} is not a method", impl_m),
 421     };
 422
 423     match *terr {
 424         TypeError::Mutability => {
 425             if let Some(trait_m_node_id) = tcx.hir.as_local_node_id(trait_m.def_id) {
 426                 let trait_m_iter = match tcx.hir.expect_trait_item(trait_m_node_id).node {
 427                     TraitItemKind::Method(ref trait_m_sig, _) => {
 428                         trait_m_sig.decl.inputs.iter()
 429                     }
 430                     _ => bug!("{:?} is not a TraitItemKind::Method", trait_m),
 431                 };
 432
 433                 impl_m_iter.zip(trait_m_iter).find(|&(ref impl_arg, ref trait_arg)| {
 434                     match (&impl_arg.node, &trait_arg.node) {
 435                         (&hir::TyRptr(_, ref impl_mt), &hir::TyRptr(_, ref trait_mt)) |
 436                         (&hir::TyPtr(ref impl_mt), &hir::TyPtr(ref trait_mt)) => {
 437                             impl_mt.mutbl != trait_mt.mutbl
 438                         }
 439                         _ => false,
 440                     }
 441                 }).map(|(ref impl_arg, ref trait_arg)| {
 442                     (impl_arg.span, Some(trait_arg.span))
 443                 })
 444                 .unwrap_or_else(|| (cause.span, tcx.hir.span_if_local(trait_m.def_id)))
 445             } else {
 446                 (cause.span, tcx.hir.span_if_local(trait_m.def_id))
 447             }
 448         }
 449         TypeError::Sorts(ExpectedFound { .. }) => {
 450             if let Some(trait_m_node_id) = tcx.hir.as_local_node_id(trait_m.def_id) {
 451                 let (trait_m_output, trait_m_iter) =
 452                     match tcx.hir.expect_trait_item(trait_m_node_id).node {
 453                         TraitItemKind::Method(ref trait_m_sig, _) => {
 454                             (&trait_m_sig.decl.output, trait_m_sig.decl.inputs.iter())
 455                         }
 456                         _ => bug!("{:?} is not a TraitItemKind::Method", trait_m),
 457                     };
 458
 459                 let impl_iter = impl_sig.inputs().iter();
 460                 let trait_iter = trait_sig.inputs().iter();
 461                 impl_iter.zip(trait_iter)
 462                          .zip(impl_m_iter)
 463                          .zip(trait_m_iter)
 464                          .filter_map(|(((&impl_arg_ty, &trait_arg_ty), impl_arg), trait_arg)| {
 465                              match infcx.at(&cause, param_env).sub(trait_arg_ty, impl_arg_ty) {
 466                                  Ok(_) => None,
 467                                  Err(_) => Some((impl_arg.span, Some(trait_arg.span))),
 468                              }
 469                          })
 470                          .next()
 471                          .unwrap_or_else(|| {
 472                              if
 473                                  infcx.at(&cause, param_env)
 474                                       .sup(trait_sig.output(), impl_sig.output())
 475                                       .is_err()
 476                              {
 477                                  (impl_m_output.span(), Some(trait_m_output.span()))
 478                              } else {
 479                                  (cause.span, tcx.hir.span_if_local(trait_m.def_id))
 480                              }
 481                          })
 482             } else {
 483                 (cause.span, tcx.hir.span_if_local(trait_m.def_id))
 484             }
 485         }
 486         _ => (cause.span, tcx.hir.span_if_local(trait_m.def_id)),
 487     }
 488 }
 489
 490 fn compare_self_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 491                                impl_m: &ty::AssociatedItem,
 492                                impl_m_span: Span,
 493                                trait_m: &ty::AssociatedItem,
 494                                impl_trait_ref: ty::TraitRef<'tcx>)
 495                                -> Result<(), ErrorReported>
 496 {
 497     // Try to give more informative error messages about self typing
 498     // mismatches.  Note that any mismatch will also be detected
 499     // below, where we construct a canonical function type that
 500     // includes the self parameter as a normal parameter.  It's just
 501     // that the error messages you get out of this code are a bit more
 502     // inscrutable, particularly for cases where one method has no
 503     // self.
 504
 505     let self_string = |method: &ty::AssociatedItem| {
 506         let untransformed_self_ty = match method.container {
 507             ty::ImplContainer(_) => impl_trait_ref.self_ty(),
 508             ty::TraitContainer(_) => tcx.mk_self_type()
 509         };
 510         let method_ty = tcx.type_of(method.def_id);
 511         let self_arg_ty = *method_ty.fn_sig().input(0).skip_binder();
 512         match ExplicitSelf::determine(untransformed_self_ty, self_arg_ty) {
 513             ExplicitSelf::ByValue => "self".to_string(),
 514             ExplicitSelf::ByReference(_, hir::MutImmutable) => "&self".to_string(),
 515             ExplicitSelf::ByReference(_, hir::MutMutable) => "&mut self".to_string(),
 516             _ => format!("self: {}", self_arg_ty)
 517         }
 518     };
 519
 520     match (trait_m.method_has_self_argument, impl_m.method_has_self_argument) {
 521         (false, false) | (true, true) => {}
 522
 523         (false, true) => {
 524             let self_descr = self_string(impl_m);
 525             let mut err = struct_span_err!(tcx.sess,
 526                                            impl_m_span,
 527                                            E0185,
 528                                            "method `{}` has a `{}` declaration in the impl, but \
 529                                             not in the trait",
 530                                            trait_m.name,
 531                                            self_descr);
 532             err.span_label(impl_m_span, format!("`{}` used in impl", self_descr));
 533             if let Some(span) = tcx.hir.span_if_local(trait_m.def_id) {
 534                 err.span_label(span, format!("trait declared without `{}`", self_descr));
 535             }
 536             err.emit();
 537             return Err(ErrorReported);
 538         }
 539
 540         (true, false) => {
 541             let self_descr = self_string(trait_m);
 542             let mut err = struct_span_err!(tcx.sess,
 543                                            impl_m_span,
 544                                            E0186,
 545                                            "method `{}` has a `{}` declaration in the trait, but \
 546                                             not in the impl",
 547                                            trait_m.name,
 548                                            self_descr);
 549             err.span_label(impl_m_span,
 550                            format!("expected `{}` in impl", self_descr));
 551             if let Some(span) = tcx.hir.span_if_local(trait_m.def_id) {
 552                 err.span_label(span, format!("`{}` used in trait", self_descr));
 553             } else {
 554                 err.note_trait_signature(trait_m.name.to_string(),
 555                                          trait_m.signature(&tcx));
 556             }
 557             err.emit();
 558             return Err(ErrorReported);
 559         }
 560     }
 561
 562     Ok(())
 563 }
 564
 565 fn compare_number_of_generics<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 566                                         impl_m: &ty::AssociatedItem,
 567                                         impl_m_span: Span,
 568                                         trait_m: &ty::AssociatedItem,
 569                                         trait_item_span: Option<Span>)
 570                                         -> Result<(), ErrorReported> {
 571     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 572     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 573     let num_impl_m_type_params = impl_m_generics.types.len();
 574     let num_trait_m_type_params = trait_m_generics.types.len();
 575     if num_impl_m_type_params != num_trait_m_type_params {
 576         let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
 577         let span = match tcx.hir.expect_impl_item(impl_m_node_id).node {
 578             ImplItemKind::Method(ref impl_m_sig, _) => {
 579                 if impl_m_sig.generics.is_parameterized() {
 580                     impl_m_sig.generics.span
 581                 } else {
 582                     impl_m_span
 583                 }
 584             }
 585             _ => bug!("{:?} is not a method", impl_m),
 586         };
 587
 588         let mut err = struct_span_err!(tcx.sess,
 589                                        span,
 590                                        E0049,
 591                                        "method `{}` has {} type parameter{} but its trait \
 592                                         declaration has {} type parameter{}",
 593                                        trait_m.name,
 594                                        num_impl_m_type_params,
 595                                        if num_impl_m_type_params == 1 { "" } else { "s" },
 596                                        num_trait_m_type_params,
 597                                        if num_trait_m_type_params == 1 {
 598                                            ""
 599                                        } else {
 600                                            "s"
 601                                        });
 602
 603         let mut suffix = None;
 604
 605         if let Some(span) = trait_item_span {
 606             err.span_label(span,
 607                            format!("expected {}",
 608                                     &if num_trait_m_type_params != 1 {
 609                                         format!("{} type parameters", num_trait_m_type_params)
 610                                     } else {
 611                                         format!("{} type parameter", num_trait_m_type_params)
 612                                     }));
 613         } else {
 614             suffix = Some(format!(", expected {}", num_trait_m_type_params));
 615         }
 616
 617         err.span_label(span,
 618                        format!("found {}{}",
 619                                 &if num_impl_m_type_params != 1 {
 620                                     format!("{} type parameters", num_impl_m_type_params)
 621                                 } else {
 622                                     format!("1 type parameter")
 623                                 },
 624                                 suffix.as_ref().map(|s| &s[..]).unwrap_or("")));
 625
 626         err.emit();
 627
 628         return Err(ErrorReported);
 629     }
 630
 631     Ok(())
 632 }
 633
 634 fn compare_number_of_method_arguments<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 635                                                 impl_m: &ty::AssociatedItem,
 636                                                 impl_m_span: Span,
 637                                                 trait_m: &ty::AssociatedItem,
 638                                                 trait_item_span: Option<Span>)
 639                                                 -> Result<(), ErrorReported> {
 640     let m_fty = |method: &ty::AssociatedItem| {
 641         match tcx.type_of(method.def_id).sty {
 642             ty::TyFnDef(_, _, f) => f,
 643             _ => bug!()
 644         }
 645     };
 646     let impl_m_fty = m_fty(impl_m);
 647     let trait_m_fty = m_fty(trait_m);
 648     let trait_number_args = trait_m_fty.inputs().skip_binder().len();
 649     let impl_number_args = impl_m_fty.inputs().skip_binder().len();
 650     if trait_number_args != impl_number_args {
 651         let trait_m_node_id = tcx.hir.as_local_node_id(trait_m.def_id);
 652         let trait_span = if let Some(trait_id) = trait_m_node_id {
 653             match tcx.hir.expect_trait_item(trait_id).node {
 654                 TraitItemKind::Method(ref trait_m_sig, _) => {
 655                     if let Some(arg) = trait_m_sig.decl.inputs.get(if trait_number_args > 0 {
 656                         trait_number_args - 1
 657                     } else {
 658                         0
 659                     }) {
 660                         Some(arg.span)
 661                     } else {
 662                         trait_item_span
 663                     }
 664                 }
 665                 _ => bug!("{:?} is not a method", impl_m),
 666             }
 667         } else {
 668             trait_item_span
 669         };
 670         let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
 671         let impl_span = match tcx.hir.expect_impl_item(impl_m_node_id).node {
 672             ImplItemKind::Method(ref impl_m_sig, _) => {
 673                 if let Some(arg) = impl_m_sig.decl.inputs.get(if impl_number_args > 0 {
 674                     impl_number_args - 1
 675                 } else {
 676                     0
 677                 }) {
 678                     arg.span
 679                 } else {
 680                     impl_m_span
 681                 }
 682             }
 683             _ => bug!("{:?} is not a method", impl_m),
 684         };
 685         let mut err = struct_span_err!(tcx.sess,
 686                                        impl_span,
 687                                        E0050,
 688                                        "method `{}` has {} parameter{} but the declaration in \
 689                                         trait `{}` has {}",
 690                                        trait_m.name,
 691                                        impl_number_args,
 692                                        if impl_number_args == 1 { "" } else { "s" },
 693                                        tcx.item_path_str(trait_m.def_id),
 694                                        trait_number_args);
 695         if let Some(trait_span) = trait_span {
 696             err.span_label(trait_span,
 697                            format!("trait requires {}",
 698                                     &if trait_number_args != 1 {
 699                                         format!("{} parameters", trait_number_args)
 700                                     } else {
 701                                         format!("{} parameter", trait_number_args)
 702                                     }));
 703         } else {
 704             err.note_trait_signature(trait_m.name.to_string(),
 705                                      trait_m.signature(&tcx));
 706         }
 707         err.span_label(impl_span,
 708                        format!("expected {}, found {}",
 709                                 &if trait_number_args != 1 {
 710                                     format!("{} parameters", trait_number_args)
 711                                 } else {
 712                                     format!("{} parameter", trait_number_args)
 713                                 },
 714                                 impl_number_args));
 715         err.emit();
 716         return Err(ErrorReported);
 717     }
 718
 719     Ok(())
 720 }
 721
 722 pub fn compare_const_impl<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 723                                     impl_c: &ty::AssociatedItem,
 724                                     impl_c_span: Span,
 725                                     trait_c: &ty::AssociatedItem,
 726                                     impl_trait_ref: ty::TraitRef<'tcx>) {
 727     debug!("compare_const_impl(impl_trait_ref={:?})", impl_trait_ref);
 728
 729     tcx.infer_ctxt(()).enter(|infcx| {
 730         let param_env = ty::ParamEnv::empty(Reveal::UserFacing);
 731         let inh = Inherited::new(infcx, impl_c.def_id);
 732         let infcx = &inh.infcx;
 733
 734         // The below is for the most part highly similar to the procedure
 735         // for methods above. It is simpler in many respects, especially
 736         // because we shouldn't really have to deal with lifetimes or
 737         // predicates. In fact some of this should probably be put into
 738         // shared functions because of DRY violations...
 739         let trait_to_impl_substs = impl_trait_ref.substs;
 740
 741         // Create a parameter environment that represents the implementation's
 742         // method.
 743         let impl_c_node_id = tcx.hir.as_local_node_id(impl_c.def_id).unwrap();
 744
 745         // Compute skolemized form of impl and trait const tys.
 746         let impl_ty = tcx.type_of(impl_c.def_id);
 747         let trait_ty = tcx.type_of(trait_c.def_id).subst(tcx, trait_to_impl_substs);
 748         let mut cause = ObligationCause::misc(impl_c_span, impl_c_node_id);
 749
 750         // There is no "body" here, so just pass dummy id.
 751         let impl_ty = inh.normalize_associated_types_in(impl_c_span,
 752                                                         impl_c_node_id,
 753                                                         param_env,
 754                                                         &impl_ty);
 755
 756         debug!("compare_const_impl: impl_ty={:?}", impl_ty);
 757
 758         let trait_ty = inh.normalize_associated_types_in(impl_c_span,
 759                                                          impl_c_node_id,
 760                                                          param_env,
 761                                                          &trait_ty);
 762
 763         debug!("compare_const_impl: trait_ty={:?}", trait_ty);
 764
 765         let err = infcx.at(&cause, param_env)
 766                        .sup(trait_ty, impl_ty)
 767                        .map(|ok| inh.register_infer_ok_obligations(ok));
 768
 769         if let Err(terr) = err {
 770             debug!("checking associated const for compatibility: impl ty {:?}, trait ty {:?}",
 771                    impl_ty,
 772                    trait_ty);
 773
 774             // Locate the Span containing just the type of the offending impl
 775             match tcx.hir.expect_impl_item(impl_c_node_id).node {
 776                 ImplItemKind::Const(ref ty, _) => cause.span = ty.span,
 777                 _ => bug!("{:?} is not a impl const", impl_c),
 778             }
 779
 780             let mut diag = struct_span_err!(tcx.sess,
 781                                             cause.span,
 782                                             E0326,
 783                                             "implemented const `{}` has an incompatible type for \
 784                                              trait",
 785                                             trait_c.name);
 786
 787             let trait_c_node_id = tcx.hir.as_local_node_id(trait_c.def_id);
 788             let trait_c_span = trait_c_node_id.map(|trait_c_node_id| {
 789                 // Add a label to the Span containing just the type of the const
 790                 match tcx.hir.expect_trait_item(trait_c_node_id).node {
 791                     TraitItemKind::Const(ref ty, _) => ty.span,
 792                     _ => bug!("{:?} is not a trait const", trait_c),
 793                 }
 794             });
 795
 796             infcx.note_type_err(&mut diag,
 797                                 &cause,
 798                                 trait_c_span.map(|span| (span, format!("type in trait"))),
 799                                 Some(infer::ValuePairs::Types(ExpectedFound {
 800                                     expected: trait_ty,
 801                                     found: impl_ty,
 802                                 })),
 803                                 &terr);
 804             diag.emit();
 805         }
 806
 807         // Check that all obligations are satisfied by the implementation's
 808         // version.
 809         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
 810             infcx.report_fulfillment_errors(errors);
 811             return;
 812         }
 813
 814         let fcx = FnCtxt::new(&inh, param_env, impl_c_node_id);
 815         fcx.regionck_item(impl_c_node_id, impl_c_span, &[]);
 816     });
 817 }