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 a parameter environment that represents the implementation's
 169     // method.
 170     let impl_param_env = tcx.parameter_environment(impl_m.def_id);
 171
 172     // Create mapping from impl to skolemized.
 173     let impl_to_skol_substs = Substs::identity_for_item(tcx, impl_m.def_id);
 174
 175     // Create mapping from trait to skolemized.
 176     let trait_to_skol_substs = impl_to_skol_substs.rebase_onto(tcx,
 177                                                                impl_m.container.id(),
 178                                                                trait_to_impl_substs);
 179     debug!("compare_impl_method: trait_to_skol_substs={:?}",
 180            trait_to_skol_substs);
 181
 182     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 183     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 184     let impl_m_predicates = tcx.predicates_of(impl_m.def_id);
 185     let trait_m_predicates = tcx.predicates_of(trait_m.def_id);
 186
 187     // Check region bounds.
 188     check_region_bounds_on_impl_method(tcx,
 189                                        impl_m_span,
 190                                        impl_m,
 191                                        &trait_m_generics,
 192                                        &impl_m_generics,
 193                                        trait_to_skol_substs)?;
 194
 195     // Create obligations for each predicate declared by the impl
 196     // definition in the context of the trait's parameter
 197     // environment. We can't just use `impl_env.caller_bounds`,
 198     // however, because we want to replace all late-bound regions with
 199     // region variables.
 200     let impl_predicates = tcx.predicates_of(impl_m_predicates.parent.unwrap());
 201     let mut hybrid_preds = impl_predicates.instantiate_identity(tcx);
 202
 203     debug!("compare_impl_method: impl_bounds={:?}", hybrid_preds);
 204
 205     // This is the only tricky bit of the new way we check implementation methods
 206     // We need to build a set of predicates where only the method-level bounds
 207     // are from the trait and we assume all other bounds from the implementation
 208     // to be previously satisfied.
 209     //
 210     // We then register the obligations from the impl_m and check to see
 211     // if all constraints hold.
 212     hybrid_preds.predicates
 213                 .extend(trait_m_predicates.instantiate_own(tcx, trait_to_skol_substs).predicates);
 214
 215     // Construct trait parameter environment and then shift it into the skolemized viewpoint.
 216     // The key step here is to update the caller_bounds's predicates to be
 217     // the new hybrid bounds we computed.
 218     let normalize_cause = traits::ObligationCause::misc(impl_m_span, impl_m_node_id);
 219     let trait_param_env = impl_param_env.with_caller_bounds(
 220         tcx.intern_predicates(&hybrid_preds.predicates));
 221     let trait_param_env = traits::normalize_param_env_or_error(tcx,
 222                                                                impl_m.def_id,
 223                                                                trait_param_env,
 224                                                                normalize_cause.clone());
 225
 226     tcx.infer_ctxt(trait_param_env, Reveal::UserFacing).enter(|infcx| {
 227         let inh = Inherited::new(infcx, impl_m.def_id);
 228         let infcx = &inh.infcx;
 229
 230         debug!("compare_impl_method: caller_bounds={:?}",
 231                infcx.parameter_environment.caller_bounds);
 232
 233         let mut selcx = traits::SelectionContext::new(&infcx);
 234
 235         let impl_m_own_bounds = impl_m_predicates.instantiate_own(tcx, impl_to_skol_substs);
 236         let (impl_m_own_bounds, _) = infcx.replace_late_bound_regions_with_fresh_var(impl_m_span,
 237                                                        infer::HigherRankedType,
 238                                                        &ty::Binder(impl_m_own_bounds.predicates));
 239         for predicate in impl_m_own_bounds {
 240             let traits::Normalized { value: predicate, obligations } =
 241                 traits::normalize(&mut selcx, normalize_cause.clone(), &predicate);
 242
 243             inh.register_predicates(obligations);
 244             inh.register_predicate(traits::Obligation::new(cause.clone(), predicate));
 245         }
 246
 247         // We now need to check that the signature of the impl method is
 248         // compatible with that of the trait method. We do this by
 249         // checking that `impl_fty <: trait_fty`.
 250         //
 251         // FIXME. Unfortunately, this doesn't quite work right now because
 252         // associated type normalization is not integrated into subtype
 253         // checks. For the comparison to be valid, we need to
 254         // normalize the associated types in the impl/trait methods
 255         // first. However, because function types bind regions, just
 256         // calling `normalize_associated_types_in` would have no effect on
 257         // any associated types appearing in the fn arguments or return
 258         // type.
 259
 260         // Compute skolemized form of impl and trait method tys.
 261         let tcx = infcx.tcx;
 262
 263         let m_sig = |method: &ty::AssociatedItem| {
 264             match tcx.type_of(method.def_id).sty {
 265                 ty::TyFnDef(_, _, f) => f,
 266                 _ => bug!()
 267             }
 268         };
 269
 270         let (impl_sig, _) =
 271             infcx.replace_late_bound_regions_with_fresh_var(impl_m_span,
 272                                                             infer::HigherRankedType,
 273                                                             &m_sig(impl_m));
 274         let impl_sig =
 275             inh.normalize_associated_types_in(impl_m_span,
 276                                               impl_m_node_id,
 277                                               &impl_sig);
 278         let impl_fty = tcx.mk_fn_ptr(ty::Binder(impl_sig));
 279         debug!("compare_impl_method: impl_fty={:?}", impl_fty);
 280
 281         let trait_sig = inh.liberate_late_bound_regions(
 282             impl_m.def_id,
 283             &m_sig(trait_m));
 284         let trait_sig =
 285             trait_sig.subst(tcx, trait_to_skol_substs);
 286         let trait_sig =
 287             inh.normalize_associated_types_in(impl_m_span,
 288                                               impl_m_node_id,
 289                                               &trait_sig);
 290         let trait_fty = tcx.mk_fn_ptr(ty::Binder(trait_sig));
 291
 292         debug!("compare_impl_method: trait_fty={:?}", trait_fty);
 293
 294         let sub_result = infcx.sub_types(false, &cause, impl_fty, trait_fty)
 295                               .map(|InferOk { obligations, .. }| {
 296                                   inh.register_predicates(obligations);
 297                               });
 298
 299         if let Err(terr) = sub_result {
 300             debug!("sub_types failed: impl ty {:?}, trait ty {:?}",
 301                    impl_fty,
 302                    trait_fty);
 303
 304             let (impl_err_span, trait_err_span) = extract_spans_for_error_reporting(&infcx,
 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(
 353                 &infcx.parameter_environment.caller_bounds);
 354             infcx.resolve_regions_and_report_errors(impl_m.def_id, &region_maps, &free_regions);
 355         } else {
 356             let fcx = FnCtxt::new(&inh, impl_m_node_id);
 357             fcx.regionck_item(impl_m_node_id, impl_m_span, &[]);
 358         }
 359
 360         Ok(())
 361     })
 362 }
 363
 364 fn check_region_bounds_on_impl_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 365                                                 span: Span,
 366                                                 impl_m: &ty::AssociatedItem,
 367                                                 trait_generics: &ty::Generics,
 368                                                 impl_generics: &ty::Generics,
 369                                                 trait_to_skol_substs: &Substs<'tcx>)
 370                                                 -> Result<(), ErrorReported> {
 371     let trait_params = &trait_generics.regions[..];
 372     let impl_params = &impl_generics.regions[..];
 373
 374     debug!("check_region_bounds_on_impl_method: \
 375             trait_generics={:?} \
 376             impl_generics={:?} \
 377             trait_to_skol_substs={:?}",
 378            trait_generics,
 379            impl_generics,
 380            trait_to_skol_substs);
 381
 382     // Must have same number of early-bound lifetime parameters.
 383     // Unfortunately, if the user screws up the bounds, then this
 384     // will change classification between early and late.  E.g.,
 385     // if in trait we have `<'a,'b:'a>`, and in impl we just have
 386     // `<'a,'b>`, then we have 2 early-bound lifetime parameters
 387     // in trait but 0 in the impl. But if we report "expected 2
 388     // but found 0" it's confusing, because it looks like there
 389     // are zero. Since I don't quite know how to phrase things at
 390     // the moment, give a kind of vague error message.
 391     if trait_params.len() != impl_params.len() {
 392         struct_span_err!(tcx.sess,
 393                          span,
 394                          E0195,
 395                          "lifetime parameters or bounds on method `{}` do not match the \
 396                           trait declaration",
 397                          impl_m.name)
 398             .span_label(span, "lifetimes do not match trait")
 399             .emit();
 400         return Err(ErrorReported);
 401     }
 402
 403     return Ok(());
 404 }
 405
 406 fn extract_spans_for_error_reporting<'a, 'gcx, 'tcx>(infcx: &infer::InferCtxt<'a, 'gcx, '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.sub_types(true, &cause, 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 infcx.sub_types(false, &cause, impl_sig.output(),
 473                                                 trait_sig.output())
 474                                      .is_err() {
 475                                          (impl_m_output.span(), Some(trait_m_output.span()))
 476                                      } else {
 477                                          (cause.span, tcx.hir.span_if_local(trait_m.def_id))
 478                                      }
 479                          })
 480             } else {
 481                 (cause.span, tcx.hir.span_if_local(trait_m.def_id))
 482             }
 483         }
 484         _ => (cause.span, tcx.hir.span_if_local(trait_m.def_id)),
 485     }
 486 }
 487
 488 fn compare_self_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 489                                impl_m: &ty::AssociatedItem,
 490                                impl_m_span: Span,
 491                                trait_m: &ty::AssociatedItem,
 492                                impl_trait_ref: ty::TraitRef<'tcx>)
 493                                -> Result<(), ErrorReported>
 494 {
 495     // Try to give more informative error messages about self typing
 496     // mismatches.  Note that any mismatch will also be detected
 497     // below, where we construct a canonical function type that
 498     // includes the self parameter as a normal parameter.  It's just
 499     // that the error messages you get out of this code are a bit more
 500     // inscrutable, particularly for cases where one method has no
 501     // self.
 502
 503     let self_string = |method: &ty::AssociatedItem| {
 504         let untransformed_self_ty = match method.container {
 505             ty::ImplContainer(_) => impl_trait_ref.self_ty(),
 506             ty::TraitContainer(_) => tcx.mk_self_type()
 507         };
 508         let method_ty = tcx.type_of(method.def_id);
 509         let self_arg_ty = *method_ty.fn_sig().input(0).skip_binder();
 510         match ExplicitSelf::determine(untransformed_self_ty, self_arg_ty) {
 511             ExplicitSelf::ByValue => "self".to_string(),
 512             ExplicitSelf::ByReference(_, hir::MutImmutable) => "&self".to_string(),
 513             ExplicitSelf::ByReference(_, hir::MutMutable) => "&mut self".to_string(),
 514             _ => format!("self: {}", self_arg_ty)
 515         }
 516     };
 517
 518     match (trait_m.method_has_self_argument, impl_m.method_has_self_argument) {
 519         (false, false) | (true, true) => {}
 520
 521         (false, true) => {
 522             let self_descr = self_string(impl_m);
 523             let mut err = struct_span_err!(tcx.sess,
 524                                            impl_m_span,
 525                                            E0185,
 526                                            "method `{}` has a `{}` declaration in the impl, but \
 527                                             not in the trait",
 528                                            trait_m.name,
 529                                            self_descr);
 530             err.span_label(impl_m_span, format!("`{}` used in impl", self_descr));
 531             if let Some(span) = tcx.hir.span_if_local(trait_m.def_id) {
 532                 err.span_label(span, format!("trait declared without `{}`", self_descr));
 533             }
 534             err.emit();
 535             return Err(ErrorReported);
 536         }
 537
 538         (true, false) => {
 539             let self_descr = self_string(trait_m);
 540             let mut err = struct_span_err!(tcx.sess,
 541                                            impl_m_span,
 542                                            E0186,
 543                                            "method `{}` has a `{}` declaration in the trait, but \
 544                                             not in the impl",
 545                                            trait_m.name,
 546                                            self_descr);
 547             err.span_label(impl_m_span,
 548                            format!("expected `{}` in impl", self_descr));
 549             if let Some(span) = tcx.hir.span_if_local(trait_m.def_id) {
 550                 err.span_label(span, format!("`{}` used in trait", self_descr));
 551             }
 552             err.emit();
 553             return Err(ErrorReported);
 554         }
 555     }
 556
 557     Ok(())
 558 }
 559
 560 fn compare_number_of_generics<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 561                                         impl_m: &ty::AssociatedItem,
 562                                         impl_m_span: Span,
 563                                         trait_m: &ty::AssociatedItem,
 564                                         trait_item_span: Option<Span>)
 565                                         -> Result<(), ErrorReported> {
 566     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 567     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 568     let num_impl_m_type_params = impl_m_generics.types.len();
 569     let num_trait_m_type_params = trait_m_generics.types.len();
 570     if num_impl_m_type_params != num_trait_m_type_params {
 571         let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
 572         let span = match tcx.hir.expect_impl_item(impl_m_node_id).node {
 573             ImplItemKind::Method(ref impl_m_sig, _) => {
 574                 if impl_m_sig.generics.is_parameterized() {
 575                     impl_m_sig.generics.span
 576                 } else {
 577                     impl_m_span
 578                 }
 579             }
 580             _ => bug!("{:?} is not a method", impl_m),
 581         };
 582
 583         let mut err = struct_span_err!(tcx.sess,
 584                                        span,
 585                                        E0049,
 586                                        "method `{}` has {} type parameter{} but its trait \
 587                                         declaration has {} type parameter{}",
 588                                        trait_m.name,
 589                                        num_impl_m_type_params,
 590                                        if num_impl_m_type_params == 1 { "" } else { "s" },
 591                                        num_trait_m_type_params,
 592                                        if num_trait_m_type_params == 1 {
 593                                            ""
 594                                        } else {
 595                                            "s"
 596                                        });
 597
 598         let mut suffix = None;
 599
 600         if let Some(span) = trait_item_span {
 601             err.span_label(span,
 602                            format!("expected {}",
 603                                     &if num_trait_m_type_params != 1 {
 604                                         format!("{} type parameters", num_trait_m_type_params)
 605                                     } else {
 606                                         format!("{} type parameter", num_trait_m_type_params)
 607                                     }));
 608         } else {
 609             suffix = Some(format!(", expected {}", num_trait_m_type_params));
 610         }
 611
 612         err.span_label(span,
 613                        format!("found {}{}",
 614                                 &if num_impl_m_type_params != 1 {
 615                                     format!("{} type parameters", num_impl_m_type_params)
 616                                 } else {
 617                                     format!("1 type parameter")
 618                                 },
 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 m_fty = |method: &ty::AssociatedItem| {
 636         match tcx.type_of(method.def_id).sty {
 637             ty::TyFnDef(_, _, f) => f,
 638             _ => bug!()
 639         }
 640     };
 641     let impl_m_fty = m_fty(impl_m);
 642     let trait_m_fty = m_fty(trait_m);
 643     let trait_number_args = trait_m_fty.inputs().skip_binder().len();
 644     let impl_number_args = impl_m_fty.inputs().skip_binder().len();
 645     if trait_number_args != impl_number_args {
 646         let trait_m_node_id = tcx.hir.as_local_node_id(trait_m.def_id);
 647         let trait_span = if let Some(trait_id) = trait_m_node_id {
 648             match tcx.hir.expect_trait_item(trait_id).node {
 649                 TraitItemKind::Method(ref trait_m_sig, _) => {
 650                     if let Some(arg) = trait_m_sig.decl.inputs.get(if trait_number_args > 0 {
 651                         trait_number_args - 1
 652                     } else {
 653                         0
 654                     }) {
 655                         Some(arg.span)
 656                     } else {
 657                         trait_item_span
 658                     }
 659                 }
 660                 _ => bug!("{:?} is not a method", impl_m),
 661             }
 662         } else {
 663             trait_item_span
 664         };
 665         let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
 666         let impl_span = match tcx.hir.expect_impl_item(impl_m_node_id).node {
 667             ImplItemKind::Method(ref impl_m_sig, _) => {
 668                 if let Some(arg) = impl_m_sig.decl.inputs.get(if impl_number_args > 0 {
 669                     impl_number_args - 1
 670                 } else {
 671                     0
 672                 }) {
 673                     arg.span
 674                 } else {
 675                     impl_m_span
 676                 }
 677             }
 678             _ => bug!("{:?} is not a method", impl_m),
 679         };
 680         let mut err = struct_span_err!(tcx.sess,
 681                                        impl_span,
 682                                        E0050,
 683                                        "method `{}` has {} parameter{} but the declaration in \
 684                                         trait `{}` has {}",
 685                                        trait_m.name,
 686                                        impl_number_args,
 687                                        if impl_number_args == 1 { "" } else { "s" },
 688                                        tcx.item_path_str(trait_m.def_id),
 689                                        trait_number_args);
 690         if let Some(trait_span) = trait_span {
 691             err.span_label(trait_span,
 692                            format!("trait requires {}",
 693                                     &if trait_number_args != 1 {
 694                                         format!("{} parameters", trait_number_args)
 695                                     } else {
 696                                         format!("{} parameter", trait_number_args)
 697                                     }));
 698         }
 699         err.span_label(impl_span,
 700                        format!("expected {}, found {}",
 701                                 &if trait_number_args != 1 {
 702                                     format!("{} parameters", trait_number_args)
 703                                 } else {
 704                                     format!("{} parameter", trait_number_args)
 705                                 },
 706                                 impl_number_args));
 707         err.emit();
 708         return Err(ErrorReported);
 709     }
 710
 711     Ok(())
 712 }
 713
 714 pub fn compare_const_impl<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 715                                     impl_c: &ty::AssociatedItem,
 716                                     impl_c_span: Span,
 717                                     trait_c: &ty::AssociatedItem,
 718                                     impl_trait_ref: ty::TraitRef<'tcx>) {
 719     debug!("compare_const_impl(impl_trait_ref={:?})", impl_trait_ref);
 720
 721     tcx.infer_ctxt((), Reveal::UserFacing).enter(|infcx| {
 722         let inh = Inherited::new(infcx, impl_c.def_id);
 723         let infcx = &inh.infcx;
 724
 725         // The below is for the most part highly similar to the procedure
 726         // for methods above. It is simpler in many respects, especially
 727         // because we shouldn't really have to deal with lifetimes or
 728         // predicates. In fact some of this should probably be put into
 729         // shared functions because of DRY violations...
 730         let trait_to_impl_substs = impl_trait_ref.substs;
 731
 732         // Create a parameter environment that represents the implementation's
 733         // method.
 734         let impl_c_node_id = tcx.hir.as_local_node_id(impl_c.def_id).unwrap();
 735
 736         // Compute skolemized form of impl and trait const tys.
 737         let impl_ty = tcx.type_of(impl_c.def_id);
 738         let trait_ty = tcx.type_of(trait_c.def_id).subst(tcx, trait_to_impl_substs);
 739         let mut cause = ObligationCause::misc(impl_c_span, impl_c_node_id);
 740
 741         // There is no "body" here, so just pass dummy id.
 742         let impl_ty = inh.normalize_associated_types_in(impl_c_span,
 743                                                         impl_c_node_id,
 744                                                         &impl_ty);
 745
 746         debug!("compare_const_impl: impl_ty={:?}", impl_ty);
 747
 748         let trait_ty = inh.normalize_associated_types_in(impl_c_span,
 749                                                          impl_c_node_id,
 750                                                          &trait_ty);
 751
 752         debug!("compare_const_impl: trait_ty={:?}", trait_ty);
 753
 754         let err = infcx.sub_types(false, &cause, impl_ty, trait_ty)
 755             .map(|ok| inh.register_infer_ok_obligations(ok));
 756
 757         if let Err(terr) = err {
 758             debug!("checking associated const for compatibility: impl ty {:?}, trait ty {:?}",
 759                    impl_ty,
 760                    trait_ty);
 761
 762             // Locate the Span containing just the type of the offending impl
 763             match tcx.hir.expect_impl_item(impl_c_node_id).node {
 764                 ImplItemKind::Const(ref ty, _) => cause.span = ty.span,
 765                 _ => bug!("{:?} is not a impl const", impl_c),
 766             }
 767
 768             let mut diag = struct_span_err!(tcx.sess,
 769                                             cause.span,
 770                                             E0326,
 771                                             "implemented const `{}` has an incompatible type for \
 772                                              trait",
 773                                             trait_c.name);
 774
 775             let trait_c_node_id = tcx.hir.as_local_node_id(trait_c.def_id);
 776             let trait_c_span = trait_c_node_id.map(|trait_c_node_id| {
 777                 // Add a label to the Span containing just the type of the const
 778                 match tcx.hir.expect_trait_item(trait_c_node_id).node {
 779                     TraitItemKind::Const(ref ty, _) => ty.span,
 780                     _ => bug!("{:?} is not a trait const", trait_c),
 781                 }
 782             });
 783
 784             infcx.note_type_err(&mut diag,
 785                                 &cause,
 786                                 trait_c_span.map(|span| (span, format!("type in trait"))),
 787                                 Some(infer::ValuePairs::Types(ExpectedFound {
 788                                     expected: trait_ty,
 789                                     found: impl_ty,
 790                                 })),
 791                                 &terr);
 792             diag.emit();
 793         }
 794
 795         // FIXME(#41323) Check the obligations in the fulfillment context.
 796     });
 797 }