src/librustc_typeck/check/compare_method.rs

   1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use rustc::hir::{self, ImplItemKind, TraitItemKind};
  12 use rustc::infer::{self, InferOk};
  13 use rustc::middle::free_region::FreeRegionMap;
  14 use rustc::middle::region::RegionMaps;
  15 use rustc::ty::{self, TyCtxt};
  16 use rustc::traits::{self, ObligationCause, ObligationCauseCode, Reveal};
  17 use rustc::ty::error::{ExpectedFound, TypeError};
  18 use rustc::ty::subst::{Subst, Substs};
  19 use rustc::util::common::ErrorReported;
  20
  21 use syntax_pos::Span;
  22
  23 use super::{Inherited, FnCtxt};
  24 use astconv::ExplicitSelf;
  25
  26 /// Checks that a method from an impl conforms to the signature of
  27 /// the same method as declared in the trait.
  28 ///
  29 /// # Parameters
  30 ///
  31 /// - impl_m: type of the method we are checking
  32 /// - impl_m_span: span to use for reporting errors
  33 /// - trait_m: the method in the trait
  34 /// - impl_trait_ref: the TraitRef corresponding to the trait implementation
  35
  36 pub fn compare_impl_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  37                                      impl_m: &ty::AssociatedItem,
  38                                      impl_m_span: Span,
  39                                      trait_m: &ty::AssociatedItem,
  40                                      impl_trait_ref: ty::TraitRef<'tcx>,
  41                                      trait_item_span: Option<Span>,
  42                                      old_broken_mode: bool) {
  43     debug!("compare_impl_method(impl_trait_ref={:?})",
  44            impl_trait_ref);
  45
  46     if let Err(ErrorReported) = compare_self_type(tcx,
  47                                                   impl_m,
  48                                                   impl_m_span,
  49                                                   trait_m,
  50                                                   impl_trait_ref) {
  51         return;
  52     }
  53
  54     if let Err(ErrorReported) = compare_number_of_generics(tcx,
  55                                                            impl_m,
  56                                                            impl_m_span,
  57                                                            trait_m,
  58                                                            trait_item_span) {
  59         return;
  60     }
  61
  62     if let Err(ErrorReported) = compare_number_of_method_arguments(tcx,
  63                                                                    impl_m,
  64                                                                    impl_m_span,
  65                                                                    trait_m,
  66                                                                    trait_item_span) {
  67         return;
  68     }
  69
  70     if let Err(ErrorReported) = compare_predicate_entailment(tcx,
  71                                                              impl_m,
  72                                                              impl_m_span,
  73                                                              trait_m,
  74                                                              impl_trait_ref,
  75                                                              old_broken_mode) {
  76         return;
  77     }
  78 }
  79
  80 fn compare_predicate_entailment<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  81                                           impl_m: &ty::AssociatedItem,
  82                                           impl_m_span: Span,
  83                                           trait_m: &ty::AssociatedItem,
  84                                           impl_trait_ref: ty::TraitRef<'tcx>,
  85                                           old_broken_mode: bool)
  86                                           -> Result<(), ErrorReported> {
  87     let trait_to_impl_substs = impl_trait_ref.substs;
  88
  89     // This node-id should be used for the `body_id` field on each
  90     // `ObligationCause` (and the `FnCtxt`). This is what
  91     // `regionck_item` expects.
  92     let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
  93
  94     let cause = ObligationCause {
  95         span: impl_m_span,
  96         body_id: impl_m_node_id,
  97         code: ObligationCauseCode::CompareImplMethodObligation {
  98             item_name: impl_m.name,
  99             impl_item_def_id: impl_m.def_id,
 100             trait_item_def_id: trait_m.def_id,
 101             lint_id: if !old_broken_mode { Some(impl_m_node_id) } else { None },
 102         },
 103     };
 104
 105     // This code is best explained by example. Consider a trait:
 106     //
 107     //     trait Trait<'t,T> {
 108     //          fn method<'a,M>(t: &'t T, m: &'a M) -> Self;
 109     //     }
 110     //
 111     // And an impl:
 112     //
 113     //     impl<'i, 'j, U> Trait<'j, &'i U> for Foo {
 114     //          fn method<'b,N>(t: &'j &'i U, m: &'b N) -> Foo;
 115     //     }
 116     //
 117     // We wish to decide if those two method types are compatible.
 118     //
 119     // We start out with trait_to_impl_substs, that maps the trait
 120     // type parameters to impl type parameters. This is taken from the
 121     // impl trait reference:
 122     //
 123     //     trait_to_impl_substs = {'t => 'j, T => &'i U, Self => Foo}
 124     //
 125     // We create a mapping `dummy_substs` that maps from the impl type
 126     // parameters to fresh types and regions. For type parameters,
 127     // this is the identity transform, but we could as well use any
 128     // skolemized types. For regions, we convert from bound to free
 129     // regions (Note: but only early-bound regions, i.e., those
 130     // declared on the impl or used in type parameter bounds).
 131     //
 132     //     impl_to_skol_substs = {'i => 'i0, U => U0, N => N0 }
 133     //
 134     // Now we can apply skol_substs to the type of the impl method
 135     // to yield a new function type in terms of our fresh, skolemized
 136     // types:
 137     //
 138     //     <'b> fn(t: &'i0 U0, m: &'b) -> Foo
 139     //
 140     // We now want to extract and substitute the type of the *trait*
 141     // method and compare it. To do so, we must create a compound
 142     // substitution by combining trait_to_impl_substs and
 143     // impl_to_skol_substs, and also adding a mapping for the method
 144     // type parameters. We extend the mapping to also include
 145     // the method parameters.
 146     //
 147     //     trait_to_skol_substs = { T => &'i0 U0, Self => Foo, M => N0 }
 148     //
 149     // Applying this to the trait method type yields:
 150     //
 151     //     <'a> fn(t: &'i0 U0, m: &'a) -> Foo
 152     //
 153     // This type is also the same but the name of the bound region ('a
 154     // vs 'b).  However, the normal subtyping rules on fn types handle
 155     // this kind of equivalency just fine.
 156     //
 157     // We now use these substitutions to ensure that all declared bounds are
 158     // satisfied by the implementation's method.
 159     //
 160     // We do this by creating a parameter environment which contains a
 161     // substitution corresponding to impl_to_skol_substs. We then build
 162     // trait_to_skol_substs and use it to convert the predicates contained
 163     // in the trait_m.generics to the skolemized form.
 164     //
 165     // Finally we register each of these predicates as an obligation in
 166     // a fresh FulfillmentCtxt, and invoke select_all_or_error.
 167
 168     // Create mapping from impl to skolemized.
 169     let impl_to_skol_substs = Substs::identity_for_item(tcx, impl_m.def_id);
 170
 171     // Create mapping from trait to skolemized.
 172     let trait_to_skol_substs = impl_to_skol_substs.rebase_onto(tcx,
 173                                                                impl_m.container.id(),
 174                                                                trait_to_impl_substs);
 175     debug!("compare_impl_method: trait_to_skol_substs={:?}",
 176            trait_to_skol_substs);
 177
 178     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 179     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 180     let impl_m_predicates = tcx.predicates_of(impl_m.def_id);
 181     let trait_m_predicates = tcx.predicates_of(trait_m.def_id);
 182
 183     // Check region bounds.
 184     check_region_bounds_on_impl_method(tcx,
 185                                        impl_m_span,
 186                                        impl_m,
 187                                        &trait_m_generics,
 188                                        &impl_m_generics,
 189                                        trait_to_skol_substs)?;
 190
 191     // Create obligations for each predicate declared by the impl
 192     // definition in the context of the trait's parameter
 193     // environment. We can't just use `impl_env.caller_bounds`,
 194     // however, because we want to replace all late-bound regions with
 195     // region variables.
 196     let impl_predicates = tcx.predicates_of(impl_m_predicates.parent.unwrap());
 197     let mut hybrid_preds = impl_predicates.instantiate_identity(tcx);
 198
 199     debug!("compare_impl_method: impl_bounds={:?}", hybrid_preds);
 200
 201     // This is the only tricky bit of the new way we check implementation methods
 202     // We need to build a set of predicates where only the method-level bounds
 203     // are from the trait and we assume all other bounds from the implementation
 204     // to be previously satisfied.
 205     //
 206     // We then register the obligations from the impl_m and check to see
 207     // if all constraints hold.
 208     hybrid_preds.predicates
 209                 .extend(trait_m_predicates.instantiate_own(tcx, trait_to_skol_substs).predicates);
 210
 211     // Construct trait parameter environment and then shift it into the skolemized viewpoint.
 212     // The key step here is to update the caller_bounds's predicates to be
 213     // the new hybrid bounds we computed.
 214     let normalize_cause = traits::ObligationCause::misc(impl_m_span, impl_m_node_id);
 215     let param_env = ty::ParamEnv::new(tcx.intern_predicates(&hybrid_preds.predicates));
 216     let param_env = traits::normalize_param_env_or_error(tcx,
 217                                                          impl_m.def_id,
 218                                                          param_env,
 219                                                          normalize_cause.clone());
 220
 221     tcx.infer_ctxt(param_env, Reveal::UserFacing).enter(|infcx| {
 222         let inh = Inherited::new(infcx, impl_m.def_id);
 223         let infcx = &inh.infcx;
 224
 225         debug!("compare_impl_method: caller_bounds={:?}",
 226                infcx.param_env.caller_bounds);
 227
 228         let mut selcx = traits::SelectionContext::new(&infcx);
 229
 230         let impl_m_own_bounds = impl_m_predicates.instantiate_own(tcx, impl_to_skol_substs);
 231         let (impl_m_own_bounds, _) = infcx.replace_late_bound_regions_with_fresh_var(impl_m_span,
 232                                                        infer::HigherRankedType,
 233                                                        &ty::Binder(impl_m_own_bounds.predicates));
 234         for predicate in impl_m_own_bounds {
 235             let traits::Normalized { value: predicate, obligations } =
 236                 traits::normalize(&mut selcx, normalize_cause.clone(), &predicate);
 237
 238             inh.register_predicates(obligations);
 239             inh.register_predicate(traits::Obligation::new(cause.clone(), predicate));
 240         }
 241
 242         // We now need to check that the signature of the impl method is
 243         // compatible with that of the trait method. We do this by
 244         // checking that `impl_fty <: trait_fty`.
 245         //
 246         // FIXME. Unfortunately, this doesn't quite work right now because
 247         // associated type normalization is not integrated into subtype
 248         // checks. For the comparison to be valid, we need to
 249         // normalize the associated types in the impl/trait methods
 250         // first. However, because function types bind regions, just
 251         // calling `normalize_associated_types_in` would have no effect on
 252         // any associated types appearing in the fn arguments or return
 253         // type.
 254
 255         // Compute skolemized form of impl and trait method tys.
 256         let tcx = infcx.tcx;
 257
 258         let m_sig = |method: &ty::AssociatedItem| {
 259             match tcx.type_of(method.def_id).sty {
 260                 ty::TyFnDef(_, _, f) => f,
 261                 _ => bug!()
 262             }
 263         };
 264
 265         let (impl_sig, _) =
 266             infcx.replace_late_bound_regions_with_fresh_var(impl_m_span,
 267                                                             infer::HigherRankedType,
 268                                                             &m_sig(impl_m));
 269         let impl_sig =
 270             inh.normalize_associated_types_in(impl_m_span,
 271                                               impl_m_node_id,
 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 = inh.liberate_late_bound_regions(
 277             impl_m.def_id,
 278             &m_sig(trait_m));
 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                                               &trait_sig);
 285         let trait_fty = tcx.mk_fn_ptr(ty::Binder(trait_sig));
 286
 287         debug!("compare_impl_method: trait_fty={:?}", trait_fty);
 288
 289         let sub_result = infcx.sub_types(false, &cause, impl_fty, trait_fty)
 290                               .map(|InferOk { obligations, .. }| {
 291                                   inh.register_predicates(obligations);
 292                               });
 293
 294         if let Err(terr) = sub_result {
 295             debug!("sub_types failed: impl ty {:?}, trait ty {:?}",
 296                    impl_fty,
 297                    trait_fty);
 298
 299             let (impl_err_span, trait_err_span) = extract_spans_for_error_reporting(&infcx,
 300                                                                                     &terr,
 301                                                                                     &cause,
 302                                                                                     impl_m,
 303                                                                                     impl_sig,
 304                                                                                     trait_m,
 305                                                                                     trait_sig);
 306
 307             let cause = ObligationCause {
 308                 span: impl_err_span,
 309                 ..cause.clone()
 310             };
 311
 312             let mut diag = struct_span_err!(tcx.sess,
 313                                             cause.span,
 314                                             E0053,
 315                                             "method `{}` has an incompatible type for trait",
 316                                             trait_m.name);
 317
 318             infcx.note_type_err(&mut diag,
 319                                 &cause,
 320                                 trait_err_span.map(|sp| (sp, format!("type in trait"))),
 321                                 Some(infer::ValuePairs::Types(ExpectedFound {
 322                                     expected: trait_fty,
 323                                     found: impl_fty,
 324                                 })),
 325                                 &terr);
 326             diag.emit();
 327             return Err(ErrorReported);
 328         }
 329
 330         // Check that all obligations are satisfied by the implementation's
 331         // version.
 332         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
 333             infcx.report_fulfillment_errors(errors);
 334             return Err(ErrorReported);
 335         }
 336
 337         // Finally, resolve all regions. This catches wily misuses of
 338         // lifetime parameters.
 339         if old_broken_mode {
 340             // FIXME(#18937) -- this is how the code used to
 341             // work. This is buggy because the fulfillment cx creates
 342             // region obligations that get overlooked.  The right
 343             // thing to do is the code below. But we keep this old
 344             // pass around temporarily.
 345             let region_maps = RegionMaps::new();
 346             let mut free_regions = FreeRegionMap::new();
 347             free_regions.relate_free_regions_from_predicates(
 348                 &infcx.param_env.caller_bounds);
 349             infcx.resolve_regions_and_report_errors(impl_m.def_id, &region_maps, &free_regions);
 350         } else {
 351             let fcx = FnCtxt::new(&inh, impl_m_node_id);
 352             fcx.regionck_item(impl_m_node_id, impl_m_span, &[]);
 353         }
 354
 355         Ok(())
 356     })
 357 }
 358
 359 fn check_region_bounds_on_impl_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 360                                                 span: Span,
 361                                                 impl_m: &ty::AssociatedItem,
 362                                                 trait_generics: &ty::Generics,
 363                                                 impl_generics: &ty::Generics,
 364                                                 trait_to_skol_substs: &Substs<'tcx>)
 365                                                 -> Result<(), ErrorReported> {
 366     let trait_params = &trait_generics.regions[..];
 367     let impl_params = &impl_generics.regions[..];
 368
 369     debug!("check_region_bounds_on_impl_method: \
 370             trait_generics={:?} \
 371             impl_generics={:?} \
 372             trait_to_skol_substs={:?}",
 373            trait_generics,
 374            impl_generics,
 375            trait_to_skol_substs);
 376
 377     // Must have same number of early-bound lifetime parameters.
 378     // Unfortunately, if the user screws up the bounds, then this
 379     // will change classification between early and late.  E.g.,
 380     // if in trait we have `<'a,'b:'a>`, and in impl we just have
 381     // `<'a,'b>`, then we have 2 early-bound lifetime parameters
 382     // in trait but 0 in the impl. But if we report "expected 2
 383     // but found 0" it's confusing, because it looks like there
 384     // are zero. Since I don't quite know how to phrase things at
 385     // the moment, give a kind of vague error message.
 386     if trait_params.len() != impl_params.len() {
 387         struct_span_err!(tcx.sess,
 388                          span,
 389                          E0195,
 390                          "lifetime parameters or bounds on method `{}` do not match the \
 391                           trait declaration",
 392                          impl_m.name)
 393             .span_label(span, "lifetimes do not match trait")
 394             .emit();
 395         return Err(ErrorReported);
 396     }
 397
 398     return Ok(());
 399 }
 400
 401 fn extract_spans_for_error_reporting<'a, 'gcx, 'tcx>(infcx: &infer::InferCtxt<'a, 'gcx, 'tcx>,
 402                                                      terr: &TypeError,
 403                                                      cause: &ObligationCause<'tcx>,
 404                                                      impl_m: &ty::AssociatedItem,
 405                                                      impl_sig: ty::FnSig<'tcx>,
 406                                                      trait_m: &ty::AssociatedItem,
 407                                                      trait_sig: ty::FnSig<'tcx>)
 408                                                      -> (Span, Option<Span>) {
 409     let tcx = infcx.tcx;
 410     let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
 411     let (impl_m_output, impl_m_iter) = match tcx.hir.expect_impl_item(impl_m_node_id).node {
 412         ImplItemKind::Method(ref impl_m_sig, _) => {
 413             (&impl_m_sig.decl.output, impl_m_sig.decl.inputs.iter())
 414         }
 415         _ => bug!("{:?} is not a method", impl_m),
 416     };
 417
 418     match *terr {
 419         TypeError::Mutability => {
 420             if let Some(trait_m_node_id) = tcx.hir.as_local_node_id(trait_m.def_id) {
 421                 let trait_m_iter = match tcx.hir.expect_trait_item(trait_m_node_id).node {
 422                     TraitItemKind::Method(ref trait_m_sig, _) => {
 423                         trait_m_sig.decl.inputs.iter()
 424                     }
 425                     _ => bug!("{:?} is not a TraitItemKind::Method", trait_m),
 426                 };
 427
 428                 impl_m_iter.zip(trait_m_iter).find(|&(ref impl_arg, ref trait_arg)| {
 429                     match (&impl_arg.node, &trait_arg.node) {
 430                         (&hir::TyRptr(_, ref impl_mt), &hir::TyRptr(_, ref trait_mt)) |
 431                         (&hir::TyPtr(ref impl_mt), &hir::TyPtr(ref trait_mt)) => {
 432                             impl_mt.mutbl != trait_mt.mutbl
 433                         }
 434                         _ => false,
 435                     }
 436                 }).map(|(ref impl_arg, ref trait_arg)| {
 437                     (impl_arg.span, Some(trait_arg.span))
 438                 })
 439                 .unwrap_or_else(|| (cause.span, tcx.hir.span_if_local(trait_m.def_id)))
 440             } else {
 441                 (cause.span, tcx.hir.span_if_local(trait_m.def_id))
 442             }
 443         }
 444         TypeError::Sorts(ExpectedFound { .. }) => {
 445             if let Some(trait_m_node_id) = tcx.hir.as_local_node_id(trait_m.def_id) {
 446                 let (trait_m_output, trait_m_iter) =
 447                     match tcx.hir.expect_trait_item(trait_m_node_id).node {
 448                         TraitItemKind::Method(ref trait_m_sig, _) => {
 449                             (&trait_m_sig.decl.output, trait_m_sig.decl.inputs.iter())
 450                         }
 451                         _ => bug!("{:?} is not a TraitItemKind::Method", trait_m),
 452                     };
 453
 454                 let impl_iter = impl_sig.inputs().iter();
 455                 let trait_iter = trait_sig.inputs().iter();
 456                 impl_iter.zip(trait_iter)
 457                          .zip(impl_m_iter)
 458                          .zip(trait_m_iter)
 459                          .filter_map(|(((impl_arg_ty, trait_arg_ty), impl_arg), trait_arg)| {
 460                              match infcx.sub_types(true, &cause, trait_arg_ty, impl_arg_ty) {
 461                                  Ok(_) => None,
 462                                  Err(_) => Some((impl_arg.span, Some(trait_arg.span))),
 463                              }
 464                          })
 465                          .next()
 466                          .unwrap_or_else(|| {
 467                              if infcx.sub_types(false, &cause, impl_sig.output(),
 468                                                 trait_sig.output())
 469                                      .is_err() {
 470                                          (impl_m_output.span(), Some(trait_m_output.span()))
 471                                      } else {
 472                                          (cause.span, tcx.hir.span_if_local(trait_m.def_id))
 473                                      }
 474                          })
 475             } else {
 476                 (cause.span, tcx.hir.span_if_local(trait_m.def_id))
 477             }
 478         }
 479         _ => (cause.span, tcx.hir.span_if_local(trait_m.def_id)),
 480     }
 481 }
 482
 483 fn compare_self_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 484                                impl_m: &ty::AssociatedItem,
 485                                impl_m_span: Span,
 486                                trait_m: &ty::AssociatedItem,
 487                                impl_trait_ref: ty::TraitRef<'tcx>)
 488                                -> Result<(), ErrorReported>
 489 {
 490     // Try to give more informative error messages about self typing
 491     // mismatches.  Note that any mismatch will also be detected
 492     // below, where we construct a canonical function type that
 493     // includes the self parameter as a normal parameter.  It's just
 494     // that the error messages you get out of this code are a bit more
 495     // inscrutable, particularly for cases where one method has no
 496     // self.
 497
 498     let self_string = |method: &ty::AssociatedItem| {
 499         let untransformed_self_ty = match method.container {
 500             ty::ImplContainer(_) => impl_trait_ref.self_ty(),
 501             ty::TraitContainer(_) => tcx.mk_self_type()
 502         };
 503         let method_ty = tcx.type_of(method.def_id);
 504         let self_arg_ty = *method_ty.fn_sig().input(0).skip_binder();
 505         match ExplicitSelf::determine(untransformed_self_ty, self_arg_ty) {
 506             ExplicitSelf::ByValue => "self".to_string(),
 507             ExplicitSelf::ByReference(_, hir::MutImmutable) => "&self".to_string(),
 508             ExplicitSelf::ByReference(_, hir::MutMutable) => "&mut self".to_string(),
 509             _ => format!("self: {}", self_arg_ty)
 510         }
 511     };
 512
 513     match (trait_m.method_has_self_argument, impl_m.method_has_self_argument) {
 514         (false, false) | (true, true) => {}
 515
 516         (false, true) => {
 517             let self_descr = self_string(impl_m);
 518             let mut err = struct_span_err!(tcx.sess,
 519                                            impl_m_span,
 520                                            E0185,
 521                                            "method `{}` has a `{}` declaration in the impl, but \
 522                                             not in the trait",
 523                                            trait_m.name,
 524                                            self_descr);
 525             err.span_label(impl_m_span, format!("`{}` used in impl", self_descr));
 526             if let Some(span) = tcx.hir.span_if_local(trait_m.def_id) {
 527                 err.span_label(span, format!("trait declared without `{}`", self_descr));
 528             }
 529             err.emit();
 530             return Err(ErrorReported);
 531         }
 532
 533         (true, false) => {
 534             let self_descr = self_string(trait_m);
 535             let mut err = struct_span_err!(tcx.sess,
 536                                            impl_m_span,
 537                                            E0186,
 538                                            "method `{}` has a `{}` declaration in the trait, but \
 539                                             not in the impl",
 540                                            trait_m.name,
 541                                            self_descr);
 542             err.span_label(impl_m_span,
 543                            format!("expected `{}` in impl", self_descr));
 544             if let Some(span) = tcx.hir.span_if_local(trait_m.def_id) {
 545                 err.span_label(span, format!("`{}` used in trait", self_descr));
 546             }
 547             err.emit();
 548             return Err(ErrorReported);
 549         }
 550     }
 551
 552     Ok(())
 553 }
 554
 555 fn compare_number_of_generics<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 556                                         impl_m: &ty::AssociatedItem,
 557                                         impl_m_span: Span,
 558                                         trait_m: &ty::AssociatedItem,
 559                                         trait_item_span: Option<Span>)
 560                                         -> Result<(), ErrorReported> {
 561     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 562     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 563     let num_impl_m_type_params = impl_m_generics.types.len();
 564     let num_trait_m_type_params = trait_m_generics.types.len();
 565     if num_impl_m_type_params != num_trait_m_type_params {
 566         let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
 567         let span = match tcx.hir.expect_impl_item(impl_m_node_id).node {
 568             ImplItemKind::Method(ref impl_m_sig, _) => {
 569                 if impl_m_sig.generics.is_parameterized() {
 570                     impl_m_sig.generics.span
 571                 } else {
 572                     impl_m_span
 573                 }
 574             }
 575             _ => bug!("{:?} is not a method", impl_m),
 576         };
 577
 578         let mut err = struct_span_err!(tcx.sess,
 579                                        span,
 580                                        E0049,
 581                                        "method `{}` has {} type parameter{} but its trait \
 582                                         declaration has {} type parameter{}",
 583                                        trait_m.name,
 584                                        num_impl_m_type_params,
 585                                        if num_impl_m_type_params == 1 { "" } else { "s" },
 586                                        num_trait_m_type_params,
 587                                        if num_trait_m_type_params == 1 {
 588                                            ""
 589                                        } else {
 590                                            "s"
 591                                        });
 592
 593         let mut suffix = None;
 594
 595         if let Some(span) = trait_item_span {
 596             err.span_label(span,
 597                            format!("expected {}",
 598                                     &if num_trait_m_type_params != 1 {
 599                                         format!("{} type parameters", num_trait_m_type_params)
 600                                     } else {
 601                                         format!("{} type parameter", num_trait_m_type_params)
 602                                     }));
 603         } else {
 604             suffix = Some(format!(", expected {}", num_trait_m_type_params));
 605         }
 606
 607         err.span_label(span,
 608                        format!("found {}{}",
 609                                 &if num_impl_m_type_params != 1 {
 610                                     format!("{} type parameters", num_impl_m_type_params)
 611                                 } else {
 612                                     format!("1 type parameter")
 613                                 },
 614                                 suffix.as_ref().map(|s| &s[..]).unwrap_or("")));
 615
 616         err.emit();
 617
 618         return Err(ErrorReported);
 619     }
 620
 621     Ok(())
 622 }
 623
 624 fn compare_number_of_method_arguments<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 625                                                 impl_m: &ty::AssociatedItem,
 626                                                 impl_m_span: Span,
 627                                                 trait_m: &ty::AssociatedItem,
 628                                                 trait_item_span: Option<Span>)
 629                                                 -> Result<(), ErrorReported> {
 630     let m_fty = |method: &ty::AssociatedItem| {
 631         match tcx.type_of(method.def_id).sty {
 632             ty::TyFnDef(_, _, f) => f,
 633             _ => bug!()
 634         }
 635     };
 636     let impl_m_fty = m_fty(impl_m);
 637     let trait_m_fty = m_fty(trait_m);
 638     let trait_number_args = trait_m_fty.inputs().skip_binder().len();
 639     let impl_number_args = impl_m_fty.inputs().skip_binder().len();
 640     if trait_number_args != impl_number_args {
 641         let trait_m_node_id = tcx.hir.as_local_node_id(trait_m.def_id);
 642         let trait_span = if let Some(trait_id) = trait_m_node_id {
 643             match tcx.hir.expect_trait_item(trait_id).node {
 644                 TraitItemKind::Method(ref trait_m_sig, _) => {
 645                     if let Some(arg) = trait_m_sig.decl.inputs.get(if trait_number_args > 0 {
 646                         trait_number_args - 1
 647                     } else {
 648                         0
 649                     }) {
 650                         Some(arg.span)
 651                     } else {
 652                         trait_item_span
 653                     }
 654                 }
 655                 _ => bug!("{:?} is not a method", impl_m),
 656             }
 657         } else {
 658             trait_item_span
 659         };
 660         let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
 661         let impl_span = match tcx.hir.expect_impl_item(impl_m_node_id).node {
 662             ImplItemKind::Method(ref impl_m_sig, _) => {
 663                 if let Some(arg) = impl_m_sig.decl.inputs.get(if impl_number_args > 0 {
 664                     impl_number_args - 1
 665                 } else {
 666                     0
 667                 }) {
 668                     arg.span
 669                 } else {
 670                     impl_m_span
 671                 }
 672             }
 673             _ => bug!("{:?} is not a method", impl_m),
 674         };
 675         let mut err = struct_span_err!(tcx.sess,
 676                                        impl_span,
 677                                        E0050,
 678                                        "method `{}` has {} parameter{} but the declaration in \
 679                                         trait `{}` has {}",
 680                                        trait_m.name,
 681                                        impl_number_args,
 682                                        if impl_number_args == 1 { "" } else { "s" },
 683                                        tcx.item_path_str(trait_m.def_id),
 684                                        trait_number_args);
 685         if let Some(trait_span) = trait_span {
 686             err.span_label(trait_span,
 687                            format!("trait requires {}",
 688                                     &if trait_number_args != 1 {
 689                                         format!("{} parameters", trait_number_args)
 690                                     } else {
 691                                         format!("{} parameter", trait_number_args)
 692                                     }));
 693         }
 694         err.span_label(impl_span,
 695                        format!("expected {}, found {}",
 696                                 &if trait_number_args != 1 {
 697                                     format!("{} parameters", trait_number_args)
 698                                 } else {
 699                                     format!("{} parameter", trait_number_args)
 700                                 },
 701                                 impl_number_args));
 702         err.emit();
 703         return Err(ErrorReported);
 704     }
 705
 706     Ok(())
 707 }
 708
 709 pub fn compare_const_impl<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 710                                     impl_c: &ty::AssociatedItem,
 711                                     impl_c_span: Span,
 712                                     trait_c: &ty::AssociatedItem,
 713                                     impl_trait_ref: ty::TraitRef<'tcx>) {
 714     debug!("compare_const_impl(impl_trait_ref={:?})", impl_trait_ref);
 715
 716     tcx.infer_ctxt((), Reveal::UserFacing).enter(|infcx| {
 717         let inh = Inherited::new(infcx, impl_c.def_id);
 718         let infcx = &inh.infcx;
 719
 720         // The below is for the most part highly similar to the procedure
 721         // for methods above. It is simpler in many respects, especially
 722         // because we shouldn't really have to deal with lifetimes or
 723         // predicates. In fact some of this should probably be put into
 724         // shared functions because of DRY violations...
 725         let trait_to_impl_substs = impl_trait_ref.substs;
 726
 727         // Create a parameter environment that represents the implementation's
 728         // method.
 729         let impl_c_node_id = tcx.hir.as_local_node_id(impl_c.def_id).unwrap();
 730
 731         // Compute skolemized form of impl and trait const tys.
 732         let impl_ty = tcx.type_of(impl_c.def_id);
 733         let trait_ty = tcx.type_of(trait_c.def_id).subst(tcx, trait_to_impl_substs);
 734         let mut cause = ObligationCause::misc(impl_c_span, impl_c_node_id);
 735
 736         // There is no "body" here, so just pass dummy id.
 737         let impl_ty = inh.normalize_associated_types_in(impl_c_span,
 738                                                         impl_c_node_id,
 739                                                         &impl_ty);
 740
 741         debug!("compare_const_impl: impl_ty={:?}", impl_ty);
 742
 743         let trait_ty = inh.normalize_associated_types_in(impl_c_span,
 744                                                          impl_c_node_id,
 745                                                          &trait_ty);
 746
 747         debug!("compare_const_impl: trait_ty={:?}", trait_ty);
 748
 749         let err = infcx.sub_types(false, &cause, impl_ty, trait_ty)
 750             .map(|ok| inh.register_infer_ok_obligations(ok));
 751
 752         if let Err(terr) = err {
 753             debug!("checking associated const for compatibility: impl ty {:?}, trait ty {:?}",
 754                    impl_ty,
 755                    trait_ty);
 756
 757             // Locate the Span containing just the type of the offending impl
 758             match tcx.hir.expect_impl_item(impl_c_node_id).node {
 759                 ImplItemKind::Const(ref ty, _) => cause.span = ty.span,
 760                 _ => bug!("{:?} is not a impl const", impl_c),
 761             }
 762
 763             let mut diag = struct_span_err!(tcx.sess,
 764                                             cause.span,
 765                                             E0326,
 766                                             "implemented const `{}` has an incompatible type for \
 767                                              trait",
 768                                             trait_c.name);
 769
 770             let trait_c_node_id = tcx.hir.as_local_node_id(trait_c.def_id);
 771             let trait_c_span = trait_c_node_id.map(|trait_c_node_id| {
 772                 // Add a label to the Span containing just the type of the const
 773                 match tcx.hir.expect_trait_item(trait_c_node_id).node {
 774                     TraitItemKind::Const(ref ty, _) => ty.span,
 775                     _ => bug!("{:?} is not a trait const", trait_c),
 776                 }
 777             });
 778
 779             infcx.note_type_err(&mut diag,
 780                                 &cause,
 781                                 trait_c_span.map(|span| (span, format!("type in trait"))),
 782                                 Some(infer::ValuePairs::Types(ExpectedFound {
 783                                     expected: trait_ty,
 784                                     found: impl_ty,
 785                                 })),
 786                                 &terr);
 787             diag.emit();
 788         }
 789
 790         // FIXME(#41323) Check the obligations in the fulfillment context.
 791     });
 792 }