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