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::ty::{self, TyCtxt};
  15 use rustc::traits::{self, ObligationCause, ObligationCauseCode, Reveal};
  16 use rustc::ty::error::{ExpectedFound, TypeError};
  17 use rustc::ty::subst::{Subst, Substs};
  18 use rustc::util::common::ErrorReported;
  19
  20 use syntax::ast;
  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 /// - impl_m_body_id: id of the method body
  34 /// - trait_m: the method in the trait
  35 /// - impl_trait_ref: the TraitRef corresponding to the trait implementation
  36
  37 pub fn compare_impl_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  38                                      impl_m: &ty::AssociatedItem,
  39                                      impl_m_span: Span,
  40                                      impl_m_body_id: ast::NodeId,
  41                                      trait_m: &ty::AssociatedItem,
  42                                      impl_trait_ref: ty::TraitRef<'tcx>,
  43                                      trait_item_span: Option<Span>,
  44                                      old_broken_mode: bool) {
  45     debug!("compare_impl_method(impl_trait_ref={:?})",
  46            impl_trait_ref);
  47
  48     if let Err(ErrorReported) = compare_self_type(tcx,
  49                                                   impl_m,
  50                                                   impl_m_span,
  51                                                   trait_m,
  52                                                   impl_trait_ref) {
  53         return;
  54     }
  55
  56     if let Err(ErrorReported) = compare_number_of_generics(tcx,
  57                                                            impl_m,
  58                                                            impl_m_span,
  59                                                            trait_m,
  60                                                            trait_item_span) {
  61         return;
  62     }
  63
  64     if let Err(ErrorReported) = compare_number_of_method_arguments(tcx,
  65                                                                    impl_m,
  66                                                                    impl_m_span,
  67                                                                    trait_m,
  68                                                                    trait_item_span) {
  69         return;
  70     }
  71
  72     if let Err(ErrorReported) = compare_predicate_entailment(tcx,
  73                                                              impl_m,
  74                                                              impl_m_span,
  75                                                              impl_m_body_id,
  76                                                              trait_m,
  77                                                              impl_trait_ref,
  78                                                              old_broken_mode) {
  79         return;
  80     }
  81 }
  82
  83 fn compare_predicate_entailment<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  84                                           impl_m: &ty::AssociatedItem,
  85                                           impl_m_span: Span,
  86                                           impl_m_body_id: ast::NodeId,
  87                                           trait_m: &ty::AssociatedItem,
  88                                           impl_trait_ref: ty::TraitRef<'tcx>,
  89                                           old_broken_mode: bool)
  90                                           -> Result<(), ErrorReported> {
  91     let trait_to_impl_substs = impl_trait_ref.substs;
  92
  93     let cause = ObligationCause {
  94         span: impl_m_span,
  95         body_id: impl_m_body_id,
  96         code: ObligationCauseCode::CompareImplMethodObligation {
  97             item_name: impl_m.name,
  98             impl_item_def_id: impl_m.def_id,
  99             trait_item_def_id: trait_m.def_id,
 100             lint_id: if !old_broken_mode { Some(impl_m_body_id) } else { None },
 101         },
 102     };
 103
 104     // This code is best explained by example. Consider a trait:
 105     //
 106     //     trait Trait<'t,T> {
 107     //          fn method<'a,M>(t: &'t T, m: &'a M) -> Self;
 108     //     }
 109     //
 110     // And an impl:
 111     //
 112     //     impl<'i, 'j, U> Trait<'j, &'i U> for Foo {
 113     //          fn method<'b,N>(t: &'j &'i U, m: &'b N) -> Foo;
 114     //     }
 115     //
 116     // We wish to decide if those two method types are compatible.
 117     //
 118     // We start out with trait_to_impl_substs, that maps the trait
 119     // type parameters to impl type parameters. This is taken from the
 120     // impl trait reference:
 121     //
 122     //     trait_to_impl_substs = {'t => 'j, T => &'i U, Self => Foo}
 123     //
 124     // We create a mapping `dummy_substs` that maps from the impl type
 125     // parameters to fresh types and regions. For type parameters,
 126     // this is the identity transform, but we could as well use any
 127     // skolemized types. For regions, we convert from bound to free
 128     // regions (Note: but only early-bound regions, i.e., those
 129     // declared on the impl or used in type parameter bounds).
 130     //
 131     //     impl_to_skol_substs = {'i => 'i0, U => U0, N => N0 }
 132     //
 133     // Now we can apply skol_substs to the type of the impl method
 134     // to yield a new function type in terms of our fresh, skolemized
 135     // types:
 136     //
 137     //     <'b> fn(t: &'i0 U0, m: &'b) -> Foo
 138     //
 139     // We now want to extract and substitute the type of the *trait*
 140     // method and compare it. To do so, we must create a compound
 141     // substitution by combining trait_to_impl_substs and
 142     // impl_to_skol_substs, and also adding a mapping for the method
 143     // type parameters. We extend the mapping to also include
 144     // the method parameters.
 145     //
 146     //     trait_to_skol_substs = { T => &'i0 U0, Self => Foo, M => N0 }
 147     //
 148     // Applying this to the trait method type yields:
 149     //
 150     //     <'a> fn(t: &'i0 U0, m: &'a) -> Foo
 151     //
 152     // This type is also the same but the name of the bound region ('a
 153     // vs 'b).  However, the normal subtyping rules on fn types handle
 154     // this kind of equivalency just fine.
 155     //
 156     // We now use these substitutions to ensure that all declared bounds are
 157     // satisfied by the implementation's method.
 158     //
 159     // We do this by creating a parameter environment which contains a
 160     // substitution corresponding to impl_to_skol_substs. We then build
 161     // trait_to_skol_substs and use it to convert the predicates contained
 162     // in the trait_m.generics to the skolemized form.
 163     //
 164     // Finally we register each of these predicates as an obligation in
 165     // a fresh FulfillmentCtxt, and invoke select_all_or_error.
 166
 167     // Create a parameter environment that represents the implementation's
 168     // method.
 169     let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
 170     let impl_param_env = ty::ParameterEnvironment::for_item(tcx, impl_m_node_id);
 171
 172     // Create mapping from impl to skolemized.
 173     let impl_to_skol_substs = &impl_param_env.free_substs;
 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.subst(tcx,
 179                                                                           impl_to_skol_substs));
 180     debug!("compare_impl_method: trait_to_skol_substs={:?}",
 181            trait_to_skol_substs);
 182
 183     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 184     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 185     let impl_m_predicates = tcx.predicates_of(impl_m.def_id);
 186     let trait_m_predicates = tcx.predicates_of(trait_m.def_id);
 187
 188     // Check region bounds.
 189     check_region_bounds_on_impl_method(tcx,
 190                                        impl_m_span,
 191                                        impl_m,
 192                                        &trait_m_generics,
 193                                        &impl_m_generics,
 194                                        trait_to_skol_substs,
 195                                        impl_to_skol_substs)?;
 196
 197     // Create obligations for each predicate declared by the impl
 198     // definition in the context of the trait's parameter
 199     // environment. We can't just use `impl_env.caller_bounds`,
 200     // however, because we want to replace all late-bound regions with
 201     // region variables.
 202     let impl_predicates = tcx.predicates_of(impl_m_predicates.parent.unwrap());
 203     let mut hybrid_preds = impl_predicates.instantiate(tcx, impl_to_skol_substs);
 204
 205     debug!("compare_impl_method: impl_bounds={:?}", hybrid_preds);
 206
 207     // This is the only tricky bit of the new way we check implementation methods
 208     // We need to build a set of predicates where only the method-level bounds
 209     // are from the trait and we assume all other bounds from the implementation
 210     // to be previously satisfied.
 211     //
 212     // We then register the obligations from the impl_m and check to see
 213     // if all constraints hold.
 214     hybrid_preds.predicates
 215                 .extend(trait_m_predicates.instantiate_own(tcx, trait_to_skol_substs).predicates);
 216
 217     // Construct trait parameter environment and then shift it into the skolemized viewpoint.
 218     // The key step here is to update the caller_bounds's predicates to be
 219     // the new hybrid bounds we computed.
 220     let normalize_cause = traits::ObligationCause::misc(impl_m_span, impl_m_body_id);
 221     let trait_param_env = impl_param_env.with_caller_bounds(hybrid_preds.predicates);
 222     let trait_param_env = traits::normalize_param_env_or_error(tcx,
 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);
 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             impl_sig.subst(tcx, impl_to_skol_substs);
 276         let impl_sig =
 277             inh.normalize_associated_types_in(impl_m_span,
 278                                               impl_m_body_id,
 279                                               &impl_sig);
 280         let impl_fty = tcx.mk_fn_ptr(ty::Binder(impl_sig));
 281         debug!("compare_impl_method: impl_fty={:?}", impl_fty);
 282
 283         let trait_sig = tcx.liberate_late_bound_regions(
 284             infcx.parameter_environment.free_id_outlive,
 285             &m_sig(trait_m));
 286         let trait_sig =
 287             trait_sig.subst(tcx, trait_to_skol_substs);
 288         let trait_sig =
 289             inh.normalize_associated_types_in(impl_m_span,
 290                                               impl_m_body_id,
 291                                               &trait_sig);
 292         let trait_fty = tcx.mk_fn_ptr(ty::Binder(trait_sig));
 293
 294         debug!("compare_impl_method: trait_fty={:?}", trait_fty);
 295
 296         let sub_result = infcx.sub_types(false, &cause, impl_fty, trait_fty)
 297                               .map(|InferOk { obligations, .. }| {
 298                                   inh.register_predicates(obligations);
 299                               });
 300
 301         if let Err(terr) = sub_result {
 302             debug!("sub_types failed: impl ty {:?}, trait ty {:?}",
 303                    impl_fty,
 304                    trait_fty);
 305
 306             let (impl_err_span, trait_err_span) = extract_spans_for_error_reporting(&infcx,
 307                                                                                     &terr,
 308                                                                                     &cause,
 309                                                                                     impl_m,
 310                                                                                     impl_sig,
 311                                                                                     trait_m,
 312                                                                                     trait_sig);
 313
 314             let cause = ObligationCause {
 315                 span: impl_err_span,
 316                 ..cause.clone()
 317             };
 318
 319             let mut diag = struct_span_err!(tcx.sess,
 320                                             cause.span,
 321                                             E0053,
 322                                             "method `{}` has an incompatible type for trait",
 323                                             trait_m.name);
 324
 325             infcx.note_type_err(&mut diag,
 326                                 &cause,
 327                                 trait_err_span.map(|sp| (sp, format!("type in trait"))),
 328                                 Some(infer::ValuePairs::Types(ExpectedFound {
 329                                     expected: trait_fty,
 330                                     found: impl_fty,
 331                                 })),
 332                                 &terr);
 333             diag.emit();
 334             return Err(ErrorReported);
 335         }
 336
 337         // Check that all obligations are satisfied by the implementation's
 338         // version.
 339         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
 340             infcx.report_fulfillment_errors(errors);
 341             return Err(ErrorReported);
 342         }
 343
 344         // Finally, resolve all regions. This catches wily misuses of
 345         // lifetime parameters.
 346         if old_broken_mode {
 347             // FIXME(#18937) -- this is how the code used to
 348             // work. This is buggy because the fulfillment cx creates
 349             // region obligations that get overlooked.  The right
 350             // thing to do is the code below. But we keep this old
 351             // pass around temporarily.
 352             let mut free_regions = FreeRegionMap::new();
 353             free_regions.relate_free_regions_from_predicates(
 354                 &infcx.parameter_environment.caller_bounds);
 355             infcx.resolve_regions_and_report_errors(&free_regions, impl_m_body_id);
 356         } else {
 357             let fcx = FnCtxt::new(&inh, impl_m_body_id);
 358             fcx.regionck_item(impl_m_body_id, impl_m_span, &[]);
 359         }
 360
 361         Ok(())
 362     })
 363 }
 364
 365 fn check_region_bounds_on_impl_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 366                                                 span: Span,
 367                                                 impl_m: &ty::AssociatedItem,
 368                                                 trait_generics: &ty::Generics,
 369                                                 impl_generics: &ty::Generics,
 370                                                 trait_to_skol_substs: &Substs<'tcx>,
 371                                                 impl_to_skol_substs: &Substs<'tcx>)
 372                                                 -> Result<(), ErrorReported> {
 373     let trait_params = &trait_generics.regions[..];
 374     let impl_params = &impl_generics.regions[..];
 375
 376     debug!("check_region_bounds_on_impl_method: \
 377             trait_generics={:?} \
 378             impl_generics={:?} \
 379             trait_to_skol_substs={:?} \
 380             impl_to_skol_substs={:?}",
 381            trait_generics,
 382            impl_generics,
 383            trait_to_skol_substs,
 384            impl_to_skol_substs);
 385
 386     // Must have same number of early-bound lifetime parameters.
 387     // Unfortunately, if the user screws up the bounds, then this
 388     // will change classification between early and late.  E.g.,
 389     // if in trait we have `<'a,'b:'a>`, and in impl we just have
 390     // `<'a,'b>`, then we have 2 early-bound lifetime parameters
 391     // in trait but 0 in the impl. But if we report "expected 2
 392     // but found 0" it's confusing, because it looks like there
 393     // are zero. Since I don't quite know how to phrase things at
 394     // the moment, give a kind of vague error message.
 395     if trait_params.len() != impl_params.len() {
 396         struct_span_err!(tcx.sess,
 397                          span,
 398                          E0195,
 399                          "lifetime parameters or bounds on method `{}` do not match the \
 400                           trait declaration",
 401                          impl_m.name)
 402             .span_label(span, &format!("lifetimes do not match trait"))
 403             .emit();
 404         return Err(ErrorReported);
 405     }
 406
 407     return Ok(());
 408 }
 409
 410 fn extract_spans_for_error_reporting<'a, 'gcx, 'tcx>(infcx: &infer::InferCtxt<'a, 'gcx, 'tcx>,
 411                                                      terr: &TypeError,
 412                                                      cause: &ObligationCause<'tcx>,
 413                                                      impl_m: &ty::AssociatedItem,
 414                                                      impl_sig: ty::FnSig<'tcx>,
 415                                                      trait_m: &ty::AssociatedItem,
 416                                                      trait_sig: ty::FnSig<'tcx>)
 417                                                      -> (Span, Option<Span>) {
 418     let tcx = infcx.tcx;
 419     let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
 420     let (impl_m_output, impl_m_iter) = match tcx.hir.expect_impl_item(impl_m_node_id).node {
 421         ImplItemKind::Method(ref impl_m_sig, _) => {
 422             (&impl_m_sig.decl.output, impl_m_sig.decl.inputs.iter())
 423         }
 424         _ => bug!("{:?} is not a method", impl_m),
 425     };
 426
 427     match *terr {
 428         TypeError::Mutability => {
 429             if let Some(trait_m_node_id) = tcx.hir.as_local_node_id(trait_m.def_id) {
 430                 let trait_m_iter = match tcx.hir.expect_trait_item(trait_m_node_id).node {
 431                     TraitItemKind::Method(ref trait_m_sig, _) => {
 432                         trait_m_sig.decl.inputs.iter()
 433                     }
 434                     _ => bug!("{:?} is not a TraitItemKind::Method", trait_m),
 435                 };
 436
 437                 impl_m_iter.zip(trait_m_iter).find(|&(ref impl_arg, ref trait_arg)| {
 438                     match (&impl_arg.node, &trait_arg.node) {
 439                         (&hir::TyRptr(_, ref impl_mt), &hir::TyRptr(_, ref trait_mt)) |
 440                         (&hir::TyPtr(ref impl_mt), &hir::TyPtr(ref trait_mt)) => {
 441                             impl_mt.mutbl != trait_mt.mutbl
 442                         }
 443                         _ => false,
 444                     }
 445                 }).map(|(ref impl_arg, ref trait_arg)| {
 446                     (impl_arg.span, Some(trait_arg.span))
 447                 })
 448                 .unwrap_or_else(|| (cause.span, tcx.hir.span_if_local(trait_m.def_id)))
 449             } else {
 450                 (cause.span, tcx.hir.span_if_local(trait_m.def_id))
 451             }
 452         }
 453         TypeError::Sorts(ExpectedFound { .. }) => {
 454             if let Some(trait_m_node_id) = tcx.hir.as_local_node_id(trait_m.def_id) {
 455                 let (trait_m_output, trait_m_iter) =
 456                     match tcx.hir.expect_trait_item(trait_m_node_id).node {
 457                         TraitItemKind::Method(ref trait_m_sig, _) => {
 458                             (&trait_m_sig.decl.output, trait_m_sig.decl.inputs.iter())
 459                         }
 460                         _ => bug!("{:?} is not a TraitItemKind::Method", trait_m),
 461                     };
 462
 463                 let impl_iter = impl_sig.inputs().iter();
 464                 let trait_iter = trait_sig.inputs().iter();
 465                 impl_iter.zip(trait_iter)
 466                          .zip(impl_m_iter)
 467                          .zip(trait_m_iter)
 468                          .filter_map(|(((impl_arg_ty, trait_arg_ty), impl_arg), trait_arg)| {
 469                              match infcx.sub_types(true, &cause, trait_arg_ty, impl_arg_ty) {
 470                                  Ok(_) => None,
 471                                  Err(_) => Some((impl_arg.span, Some(trait_arg.span))),
 472                              }
 473                          })
 474                          .next()
 475                          .unwrap_or_else(|| {
 476                              if infcx.sub_types(false, &cause, impl_sig.output(),
 477                                                 trait_sig.output())
 478                                      .is_err() {
 479                                          (impl_m_output.span(), Some(trait_m_output.span()))
 480                                      } else {
 481                                          (cause.span, tcx.hir.span_if_local(trait_m.def_id))
 482                                      }
 483                          })
 484             } else {
 485                 (cause.span, tcx.hir.span_if_local(trait_m.def_id))
 486             }
 487         }
 488         _ => (cause.span, tcx.hir.span_if_local(trait_m.def_id)),
 489     }
 490 }
 491
 492 fn compare_self_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 493                                impl_m: &ty::AssociatedItem,
 494                                impl_m_span: Span,
 495                                trait_m: &ty::AssociatedItem,
 496                                impl_trait_ref: ty::TraitRef<'tcx>)
 497                                -> Result<(), ErrorReported>
 498 {
 499     // Try to give more informative error messages about self typing
 500     // mismatches.  Note that any mismatch will also be detected
 501     // below, where we construct a canonical function type that
 502     // includes the self parameter as a normal parameter.  It's just
 503     // that the error messages you get out of this code are a bit more
 504     // inscrutable, particularly for cases where one method has no
 505     // self.
 506
 507     let self_string = |method: &ty::AssociatedItem| {
 508         let untransformed_self_ty = match method.container {
 509             ty::ImplContainer(_) => impl_trait_ref.self_ty(),
 510             ty::TraitContainer(_) => tcx.mk_self_type()
 511         };
 512         let method_ty = tcx.type_of(method.def_id);
 513         let self_arg_ty = *method_ty.fn_sig().input(0).skip_binder();
 514         match ExplicitSelf::determine(untransformed_self_ty, self_arg_ty) {
 515             ExplicitSelf::ByValue => "self".to_string(),
 516             ExplicitSelf::ByReference(_, hir::MutImmutable) => "&self".to_string(),
 517             ExplicitSelf::ByReference(_, hir::MutMutable) => "&mut self".to_string(),
 518             _ => format!("self: {}", self_arg_ty)
 519         }
 520     };
 521
 522     match (trait_m.method_has_self_argument, impl_m.method_has_self_argument) {
 523         (false, false) | (true, true) => {}
 524
 525         (false, true) => {
 526             let self_descr = self_string(impl_m);
 527             let mut err = struct_span_err!(tcx.sess,
 528                                            impl_m_span,
 529                                            E0185,
 530                                            "method `{}` has a `{}` declaration in the impl, but \
 531                                             not in the trait",
 532                                            trait_m.name,
 533                                            self_descr);
 534             err.span_label(impl_m_span, &format!("`{}` used in impl", self_descr));
 535             if let Some(span) = tcx.hir.span_if_local(trait_m.def_id) {
 536                 err.span_label(span, &format!("trait declared without `{}`", self_descr));
 537             }
 538             err.emit();
 539             return Err(ErrorReported);
 540         }
 541
 542         (true, false) => {
 543             let self_descr = self_string(trait_m);
 544             let mut err = struct_span_err!(tcx.sess,
 545                                            impl_m_span,
 546                                            E0186,
 547                                            "method `{}` has a `{}` declaration in the trait, but \
 548                                             not in the impl",
 549                                            trait_m.name,
 550                                            self_descr);
 551             err.span_label(impl_m_span,
 552                            &format!("expected `{}` in impl", self_descr));
 553             if let Some(span) = tcx.hir.span_if_local(trait_m.def_id) {
 554                 err.span_label(span, &format!("`{}` used in trait", self_descr));
 555             }
 556             err.emit();
 557             return Err(ErrorReported);
 558         }
 559     }
 560
 561     Ok(())
 562 }
 563
 564 fn compare_number_of_generics<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 565                                         impl_m: &ty::AssociatedItem,
 566                                         impl_m_span: Span,
 567                                         trait_m: &ty::AssociatedItem,
 568                                         trait_item_span: Option<Span>)
 569                                         -> Result<(), ErrorReported> {
 570     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 571     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 572     let num_impl_m_type_params = impl_m_generics.types.len();
 573     let num_trait_m_type_params = trait_m_generics.types.len();
 574     if num_impl_m_type_params != num_trait_m_type_params {
 575         let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
 576         let span = match tcx.hir.expect_impl_item(impl_m_node_id).node {
 577             ImplItemKind::Method(ref impl_m_sig, _) => {
 578                 if impl_m_sig.generics.is_parameterized() {
 579                     impl_m_sig.generics.span
 580                 } else {
 581                     impl_m_span
 582                 }
 583             }
 584             _ => bug!("{:?} is not a method", impl_m),
 585         };
 586
 587         let mut err = struct_span_err!(tcx.sess,
 588                                        span,
 589                                        E0049,
 590                                        "method `{}` has {} type parameter{} but its trait \
 591                                         declaration has {} type parameter{}",
 592                                        trait_m.name,
 593                                        num_impl_m_type_params,
 594                                        if num_impl_m_type_params == 1 { "" } else { "s" },
 595                                        num_trait_m_type_params,
 596                                        if num_trait_m_type_params == 1 {
 597                                            ""
 598                                        } else {
 599                                            "s"
 600                                        });
 601
 602         let mut suffix = None;
 603
 604         if let Some(span) = trait_item_span {
 605             err.span_label(span,
 606                            &format!("expected {}",
 607                                     &if num_trait_m_type_params != 1 {
 608                                         format!("{} type parameters", num_trait_m_type_params)
 609                                     } else {
 610                                         format!("{} type parameter", num_trait_m_type_params)
 611                                     }));
 612         } else {
 613             suffix = Some(format!(", expected {}", num_trait_m_type_params));
 614         }
 615
 616         err.span_label(span,
 617                        &format!("found {}{}",
 618                                 &if num_impl_m_type_params != 1 {
 619                                     format!("{} type parameters", num_impl_m_type_params)
 620                                 } else {
 621                                     format!("1 type parameter")
 622                                 },
 623                                 suffix.as_ref().map(|s| &s[..]).unwrap_or("")));
 624
 625         err.emit();
 626
 627         return Err(ErrorReported);
 628     }
 629
 630     Ok(())
 631 }
 632
 633 fn compare_number_of_method_arguments<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 634                                                 impl_m: &ty::AssociatedItem,
 635                                                 impl_m_span: Span,
 636                                                 trait_m: &ty::AssociatedItem,
 637                                                 trait_item_span: Option<Span>)
 638                                                 -> Result<(), ErrorReported> {
 639     let m_fty = |method: &ty::AssociatedItem| {
 640         match tcx.type_of(method.def_id).sty {
 641             ty::TyFnDef(_, _, f) => f,
 642             _ => bug!()
 643         }
 644     };
 645     let impl_m_fty = m_fty(impl_m);
 646     let trait_m_fty = m_fty(trait_m);
 647     let trait_number_args = trait_m_fty.inputs().skip_binder().len();
 648     let impl_number_args = impl_m_fty.inputs().skip_binder().len();
 649     if trait_number_args != impl_number_args {
 650         let trait_m_node_id = tcx.hir.as_local_node_id(trait_m.def_id);
 651         let trait_span = if let Some(trait_id) = trait_m_node_id {
 652             match tcx.hir.expect_trait_item(trait_id).node {
 653                 TraitItemKind::Method(ref trait_m_sig, _) => {
 654                     if let Some(arg) = trait_m_sig.decl.inputs.get(if trait_number_args > 0 {
 655                         trait_number_args - 1
 656                     } else {
 657                         0
 658                     }) {
 659                         Some(arg.span)
 660                     } else {
 661                         trait_item_span
 662                     }
 663                 }
 664                 _ => bug!("{:?} is not a method", impl_m),
 665             }
 666         } else {
 667             trait_item_span
 668         };
 669         let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
 670         let impl_span = match tcx.hir.expect_impl_item(impl_m_node_id).node {
 671             ImplItemKind::Method(ref impl_m_sig, _) => {
 672                 if let Some(arg) = impl_m_sig.decl.inputs.get(if impl_number_args > 0 {
 673                     impl_number_args - 1
 674                 } else {
 675                     0
 676                 }) {
 677                     arg.span
 678                 } else {
 679                     impl_m_span
 680                 }
 681             }
 682             _ => bug!("{:?} is not a method", impl_m),
 683         };
 684         let mut err = struct_span_err!(tcx.sess,
 685                                        impl_span,
 686                                        E0050,
 687                                        "method `{}` has {} parameter{} but the declaration in \
 688                                         trait `{}` has {}",
 689                                        trait_m.name,
 690                                        impl_number_args,
 691                                        if impl_number_args == 1 { "" } else { "s" },
 692                                        tcx.item_path_str(trait_m.def_id),
 693                                        trait_number_args);
 694         if let Some(trait_span) = trait_span {
 695             err.span_label(trait_span,
 696                            &format!("trait requires {}",
 697                                     &if trait_number_args != 1 {
 698                                         format!("{} parameters", trait_number_args)
 699                                     } else {
 700                                         format!("{} parameter", trait_number_args)
 701                                     }));
 702         }
 703         err.span_label(impl_span,
 704                        &format!("expected {}, found {}",
 705                                 &if trait_number_args != 1 {
 706                                     format!("{} parameters", trait_number_args)
 707                                 } else {
 708                                     format!("{} parameter", trait_number_args)
 709                                 },
 710                                 impl_number_args));
 711         err.emit();
 712         return Err(ErrorReported);
 713     }
 714
 715     Ok(())
 716 }
 717
 718 pub fn compare_const_impl<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 719                                     impl_c: &ty::AssociatedItem,
 720                                     impl_c_span: Span,
 721                                     trait_c: &ty::AssociatedItem,
 722                                     impl_trait_ref: ty::TraitRef<'tcx>) {
 723     debug!("compare_const_impl(impl_trait_ref={:?})", impl_trait_ref);
 724
 725     tcx.infer_ctxt((), Reveal::UserFacing).enter(|infcx| {
 726         let inh = Inherited::new(infcx);
 727         let infcx = &inh.infcx;
 728
 729         // The below is for the most part highly similar to the procedure
 730         // for methods above. It is simpler in many respects, especially
 731         // because we shouldn't really have to deal with lifetimes or
 732         // predicates. In fact some of this should probably be put into
 733         // shared functions because of DRY violations...
 734         let trait_to_impl_substs = impl_trait_ref.substs;
 735
 736         // Create a parameter environment that represents the implementation's
 737         // method.
 738         let impl_c_node_id = tcx.hir.as_local_node_id(impl_c.def_id).unwrap();
 739         let impl_param_env = ty::ParameterEnvironment::for_item(tcx, impl_c_node_id);
 740
 741         // Create mapping from impl to skolemized.
 742         let impl_to_skol_substs = &impl_param_env.free_substs;
 743
 744         // Create mapping from trait to skolemized.
 745         let trait_to_skol_substs = impl_to_skol_substs.rebase_onto(tcx,
 746                                                                    impl_c.container.id(),
 747                                                                    trait_to_impl_substs.subst(tcx,
 748                                                                               impl_to_skol_substs));
 749         debug!("compare_const_impl: trait_to_skol_substs={:?}",
 750                trait_to_skol_substs);
 751
 752         // Compute skolemized form of impl and trait const tys.
 753         let impl_ty = tcx.type_of(impl_c.def_id).subst(tcx, impl_to_skol_substs);
 754         let trait_ty = tcx.type_of(trait_c.def_id).subst(tcx, trait_to_skol_substs);
 755         let mut cause = ObligationCause::misc(impl_c_span, impl_c_node_id);
 756
 757         // There is no "body" here, so just pass dummy id.
 758         let impl_ty = inh.normalize_associated_types_in(impl_c_span,
 759                                                         impl_c_node_id,
 760                                                         &impl_ty);
 761
 762         debug!("compare_const_impl: impl_ty={:?}", impl_ty);
 763
 764         let trait_ty = inh.normalize_associated_types_in(impl_c_span,
 765                                                          impl_c_node_id,
 766                                                          &trait_ty);
 767
 768         debug!("compare_const_impl: trait_ty={:?}", trait_ty);
 769
 770         let err = infcx.sub_types(false, &cause, impl_ty, trait_ty)
 771             .map(|ok| inh.register_infer_ok_obligations(ok));
 772
 773         if let Err(terr) = err {
 774             debug!("checking associated const for compatibility: impl ty {:?}, trait ty {:?}",
 775                    impl_ty,
 776                    trait_ty);
 777
 778             // Locate the Span containing just the type of the offending impl
 779             match tcx.hir.expect_impl_item(impl_c_node_id).node {
 780                 ImplItemKind::Const(ref ty, _) => cause.span = ty.span,
 781                 _ => bug!("{:?} is not a impl const", impl_c),
 782             }
 783
 784             let mut diag = struct_span_err!(tcx.sess,
 785                                             cause.span,
 786                                             E0326,
 787                                             "implemented const `{}` has an incompatible type for \
 788                                              trait",
 789                                             trait_c.name);
 790
 791             // Add a label to the Span containing just the type of the item
 792             let trait_c_node_id = tcx.hir.as_local_node_id(trait_c.def_id).unwrap();
 793             let trait_c_span = match tcx.hir.expect_trait_item(trait_c_node_id).node {
 794                 TraitItemKind::Const(ref ty, _) => ty.span,
 795                 _ => bug!("{:?} is not a trait const", trait_c),
 796             };
 797
 798             infcx.note_type_err(&mut diag,
 799                                 &cause,
 800                                 Some((trait_c_span, format!("type in trait"))),
 801                                 Some(infer::ValuePairs::Types(ExpectedFound {
 802                                     expected: trait_ty,
 803                                     found: impl_ty,
 804                                 })),
 805                                 &terr);
 806             diag.emit();
 807         }
 808
 809         // FIXME(#41323) Check the obligations in the fulfillment context.
 810     });
 811 }