src/librustc_typeck/check/compare_method.rs

   1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 use rustc::hir::{self, ImplItemKind, TraitItemKind};
  12 use rustc::infer::{self, InferOk};
  13 use rustc::middle::free_region::FreeRegionMap;
  14 use rustc::middle::region::RegionMaps;
  15 use rustc::ty::{self, TyCtxt};
  16 use rustc::traits::{self, ObligationCause, ObligationCauseCode, Reveal};
  17 use rustc::ty::error::{ExpectedFound, TypeError};
  18 use rustc::ty::subst::{Subst, Substs};
  19 use rustc::util::common::ErrorReported;
  20
  21 use syntax_pos::Span;
  22
  23 use super::{Inherited, FnCtxt};
  24 use astconv::ExplicitSelf;
  25
  26 /// Checks that a method from an impl conforms to the signature of
  27 /// the same method as declared in the trait.
  28 ///
  29 /// # Parameters
  30 ///
  31 /// - impl_m: type of the method we are checking
  32 /// - impl_m_span: span to use for reporting errors
  33 /// - trait_m: the method in the trait
  34 /// - impl_trait_ref: the TraitRef corresponding to the trait implementation
  35
  36 pub fn compare_impl_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  37                                      impl_m: &ty::AssociatedItem,
  38                                      impl_m_span: Span,
  39                                      trait_m: &ty::AssociatedItem,
  40                                      impl_trait_ref: ty::TraitRef<'tcx>,
  41                                      trait_item_span: Option<Span>,
  42                                      old_broken_mode: bool) {
  43     debug!("compare_impl_method(impl_trait_ref={:?})",
  44            impl_trait_ref);
  45
  46     if let Err(ErrorReported) = compare_self_type(tcx,
  47                                                   impl_m,
  48                                                   impl_m_span,
  49                                                   trait_m,
  50                                                   impl_trait_ref) {
  51         return;
  52     }
  53
  54     if let Err(ErrorReported) = compare_number_of_generics(tcx,
  55                                                            impl_m,
  56                                                            impl_m_span,
  57                                                            trait_m,
  58                                                            trait_item_span) {
  59         return;
  60     }
  61
  62     if let Err(ErrorReported) = compare_number_of_method_arguments(tcx,
  63                                                                    impl_m,
  64                                                                    impl_m_span,
  65                                                                    trait_m,
  66                                                                    trait_item_span) {
  67         return;
  68     }
  69
  70     if let Err(ErrorReported) = compare_predicate_entailment(tcx,
  71                                                              impl_m,
  72                                                              impl_m_span,
  73                                                              trait_m,
  74                                                              impl_trait_ref,
  75                                                              old_broken_mode) {
  76         return;
  77     }
  78 }
  79
  80 fn compare_predicate_entailment<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  81                                           impl_m: &ty::AssociatedItem,
  82                                           impl_m_span: Span,
  83                                           trait_m: &ty::AssociatedItem,
  84                                           impl_trait_ref: ty::TraitRef<'tcx>,
  85                                           old_broken_mode: bool)
  86                                           -> Result<(), ErrorReported> {
  87     let trait_to_impl_substs = impl_trait_ref.substs;
  88
  89     // This node-id should be used for the `body_id` field on each
  90     // `ObligationCause` (and the `FnCtxt`). This is what
  91     // `regionck_item` expects.
  92     let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
  93
  94     let cause = ObligationCause {
  95         span: impl_m_span,
  96         body_id: impl_m_node_id,
  97         code: ObligationCauseCode::CompareImplMethodObligation {
  98             item_name: impl_m.name,
  99             impl_item_def_id: impl_m.def_id,
 100             trait_item_def_id: trait_m.def_id,
 101             lint_id: if !old_broken_mode { Some(impl_m_node_id) } else { None },
 102         },
 103     };
 104
 105     // This code is best explained by example. Consider a trait:
 106     //
 107     //     trait Trait<'t,T> {
 108     //          fn method<'a,M>(t: &'t T, m: &'a M) -> Self;
 109     //     }
 110     //
 111     // And an impl:
 112     //
 113     //     impl<'i, 'j, U> Trait<'j, &'i U> for Foo {
 114     //          fn method<'b,N>(t: &'j &'i U, m: &'b N) -> Foo;
 115     //     }
 116     //
 117     // We wish to decide if those two method types are compatible.
 118     //
 119     // We start out with trait_to_impl_substs, that maps the trait
 120     // type parameters to impl type parameters. This is taken from the
 121     // impl trait reference:
 122     //
 123     //     trait_to_impl_substs = {'t => 'j, T => &'i U, Self => Foo}
 124     //
 125     // We create a mapping `dummy_substs` that maps from the impl type
 126     // parameters to fresh types and regions. For type parameters,
 127     // this is the identity transform, but we could as well use any
 128     // skolemized types. For regions, we convert from bound to free
 129     // regions (Note: but only early-bound regions, i.e., those
 130     // declared on the impl or used in type parameter bounds).
 131     //
 132     //     impl_to_skol_substs = {'i => 'i0, U => U0, N => N0 }
 133     //
 134     // Now we can apply skol_substs to the type of the impl method
 135     // to yield a new function type in terms of our fresh, skolemized
 136     // types:
 137     //
 138     //     <'b> fn(t: &'i0 U0, m: &'b) -> Foo
 139     //
 140     // We now want to extract and substitute the type of the *trait*
 141     // method and compare it. To do so, we must create a compound
 142     // substitution by combining trait_to_impl_substs and
 143     // impl_to_skol_substs, and also adding a mapping for the method
 144     // type parameters. We extend the mapping to also include
 145     // the method parameters.
 146     //
 147     //     trait_to_skol_substs = { T => &'i0 U0, Self => Foo, M => N0 }
 148     //
 149     // Applying this to the trait method type yields:
 150     //
 151     //     <'a> fn(t: &'i0 U0, m: &'a) -> Foo
 152     //
 153     // This type is also the same but the name of the bound region ('a
 154     // vs 'b).  However, the normal subtyping rules on fn types handle
 155     // this kind of equivalency just fine.
 156     //
 157     // We now use these substitutions to ensure that all declared bounds are
 158     // satisfied by the implementation's method.
 159     //
 160     // We do this by creating a parameter environment which contains a
 161     // substitution corresponding to impl_to_skol_substs. We then build
 162     // trait_to_skol_substs and use it to convert the predicates contained
 163     // in the trait_m.generics to the skolemized form.
 164     //
 165     // Finally we register each of these predicates as an obligation in
 166     // a fresh FulfillmentCtxt, and invoke select_all_or_error.
 167
 168     // Create a parameter environment that represents the implementation's
 169     // method.
 170     let impl_param_env = 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_node_id);
 221     let trait_param_env = impl_param_env.with_caller_bounds(
 222         tcx.intern_predicates(&hybrid_preds.predicates));
 223     let trait_param_env = traits::normalize_param_env_or_error(tcx,
 224                                                                impl_m.def_id,
 225                                                                trait_param_env,
 226                                                                normalize_cause.clone());
 227
 228     tcx.infer_ctxt(trait_param_env, Reveal::UserFacing).enter(|infcx| {
 229         let inh = Inherited::new(infcx);
 230         let infcx = &inh.infcx;
 231
 232         debug!("compare_impl_method: caller_bounds={:?}",
 233                infcx.parameter_environment.caller_bounds);
 234
 235         let mut selcx = traits::SelectionContext::new(&infcx);
 236
 237         let impl_m_own_bounds = impl_m_predicates.instantiate_own(tcx, impl_to_skol_substs);
 238         let (impl_m_own_bounds, _) = infcx.replace_late_bound_regions_with_fresh_var(impl_m_span,
 239                                                        infer::HigherRankedType,
 240                                                        &ty::Binder(impl_m_own_bounds.predicates));
 241         for predicate in impl_m_own_bounds {
 242             let traits::Normalized { value: predicate, obligations } =
 243                 traits::normalize(&mut selcx, normalize_cause.clone(), &predicate);
 244
 245             inh.register_predicates(obligations);
 246             inh.register_predicate(traits::Obligation::new(cause.clone(), predicate));
 247         }
 248
 249         // We now need to check that the signature of the impl method is
 250         // compatible with that of the trait method. We do this by
 251         // checking that `impl_fty <: trait_fty`.
 252         //
 253         // FIXME. Unfortunately, this doesn't quite work right now because
 254         // associated type normalization is not integrated into subtype
 255         // checks. For the comparison to be valid, we need to
 256         // normalize the associated types in the impl/trait methods
 257         // first. However, because function types bind regions, just
 258         // calling `normalize_associated_types_in` would have no effect on
 259         // any associated types appearing in the fn arguments or return
 260         // type.
 261
 262         // Compute skolemized form of impl and trait method tys.
 263         let tcx = infcx.tcx;
 264
 265         let m_sig = |method: &ty::AssociatedItem| {
 266             match tcx.type_of(method.def_id).sty {
 267                 ty::TyFnDef(_, _, f) => f,
 268                 _ => bug!()
 269             }
 270         };
 271
 272         let (impl_sig, _) =
 273             infcx.replace_late_bound_regions_with_fresh_var(impl_m_span,
 274                                                             infer::HigherRankedType,
 275                                                             &m_sig(impl_m));
 276         let impl_sig =
 277             impl_sig.subst(tcx, impl_to_skol_substs);
 278         let impl_sig =
 279             inh.normalize_associated_types_in(impl_m_span,
 280                                               impl_m_node_id,
 281                                               &impl_sig);
 282         let impl_fty = tcx.mk_fn_ptr(ty::Binder(impl_sig));
 283         debug!("compare_impl_method: impl_fty={:?}", impl_fty);
 284
 285         let trait_sig = tcx.liberate_late_bound_regions(
 286             infcx.parameter_environment.free_id_outlive,
 287             &m_sig(trait_m));
 288         let trait_sig =
 289             trait_sig.subst(tcx, trait_to_skol_substs);
 290         let trait_sig =
 291             inh.normalize_associated_types_in(impl_m_span,
 292                                               impl_m_node_id,
 293                                               &trait_sig);
 294         let trait_fty = tcx.mk_fn_ptr(ty::Binder(trait_sig));
 295
 296         debug!("compare_impl_method: trait_fty={:?}", trait_fty);
 297
 298         let sub_result = infcx.sub_types(false, &cause, impl_fty, trait_fty)
 299                               .map(|InferOk { obligations, .. }| {
 300                                   inh.register_predicates(obligations);
 301                               });
 302
 303         if let Err(terr) = sub_result {
 304             debug!("sub_types failed: impl ty {:?}, trait ty {:?}",
 305                    impl_fty,
 306                    trait_fty);
 307
 308             let (impl_err_span, trait_err_span) = extract_spans_for_error_reporting(&infcx,
 309                                                                                     &terr,
 310                                                                                     &cause,
 311                                                                                     impl_m,
 312                                                                                     impl_sig,
 313                                                                                     trait_m,
 314                                                                                     trait_sig);
 315
 316             let cause = ObligationCause {
 317                 span: impl_err_span,
 318                 ..cause.clone()
 319             };
 320
 321             let mut diag = struct_span_err!(tcx.sess,
 322                                             cause.span,
 323                                             E0053,
 324                                             "method `{}` has an incompatible type for trait",
 325                                             trait_m.name);
 326
 327             infcx.note_type_err(&mut diag,
 328                                 &cause,
 329                                 trait_err_span.map(|sp| (sp, format!("type in trait"))),
 330                                 Some(infer::ValuePairs::Types(ExpectedFound {
 331                                     expected: trait_fty,
 332                                     found: impl_fty,
 333                                 })),
 334                                 &terr);
 335             diag.emit();
 336             return Err(ErrorReported);
 337         }
 338
 339         // Check that all obligations are satisfied by the implementation's
 340         // version.
 341         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
 342             infcx.report_fulfillment_errors(errors);
 343             return Err(ErrorReported);
 344         }
 345
 346         // Finally, resolve all regions. This catches wily misuses of
 347         // lifetime parameters.
 348         if old_broken_mode {
 349             // FIXME(#18937) -- this is how the code used to
 350             // work. This is buggy because the fulfillment cx creates
 351             // region obligations that get overlooked.  The right
 352             // thing to do is the code below. But we keep this old
 353             // pass around temporarily.
 354             let region_maps = RegionMaps::new();
 355             let mut free_regions = FreeRegionMap::new();
 356             free_regions.relate_free_regions_from_predicates(
 357                 &infcx.parameter_environment.caller_bounds);
 358             infcx.resolve_regions_and_report_errors(impl_m.def_id, &region_maps, &free_regions);
 359         } else {
 360             let fcx = FnCtxt::new(&inh, impl_m_node_id);
 361             fcx.regionck_item(impl_m_node_id, impl_m_span, &[]);
 362         }
 363
 364         Ok(())
 365     })
 366 }
 367
 368 fn check_region_bounds_on_impl_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 369                                                 span: Span,
 370                                                 impl_m: &ty::AssociatedItem,
 371                                                 trait_generics: &ty::Generics,
 372                                                 impl_generics: &ty::Generics,
 373                                                 trait_to_skol_substs: &Substs<'tcx>,
 374                                                 impl_to_skol_substs: &Substs<'tcx>)
 375                                                 -> Result<(), ErrorReported> {
 376     let trait_params = &trait_generics.regions[..];
 377     let impl_params = &impl_generics.regions[..];
 378
 379     debug!("check_region_bounds_on_impl_method: \
 380             trait_generics={:?} \
 381             impl_generics={:?} \
 382             trait_to_skol_substs={:?} \
 383             impl_to_skol_substs={:?}",
 384            trait_generics,
 385            impl_generics,
 386            trait_to_skol_substs,
 387            impl_to_skol_substs);
 388
 389     // Must have same number of early-bound lifetime parameters.
 390     // Unfortunately, if the user screws up the bounds, then this
 391     // will change classification between early and late.  E.g.,
 392     // if in trait we have `<'a,'b:'a>`, and in impl we just have
 393     // `<'a,'b>`, then we have 2 early-bound lifetime parameters
 394     // in trait but 0 in the impl. But if we report "expected 2
 395     // but found 0" it's confusing, because it looks like there
 396     // are zero. Since I don't quite know how to phrase things at
 397     // the moment, give a kind of vague error message.
 398     if trait_params.len() != impl_params.len() {
 399         struct_span_err!(tcx.sess,
 400                          span,
 401                          E0195,
 402                          "lifetime parameters or bounds on method `{}` do not match the \
 403                           trait declaration",
 404                          impl_m.name)
 405             .span_label(span, &format!("lifetimes do not match trait"))
 406             .emit();
 407         return Err(ErrorReported);
 408     }
 409
 410     return Ok(());
 411 }
 412
 413 fn extract_spans_for_error_reporting<'a, 'gcx, 'tcx>(infcx: &infer::InferCtxt<'a, 'gcx, 'tcx>,
 414                                                      terr: &TypeError,
 415                                                      cause: &ObligationCause<'tcx>,
 416                                                      impl_m: &ty::AssociatedItem,
 417                                                      impl_sig: ty::FnSig<'tcx>,
 418                                                      trait_m: &ty::AssociatedItem,
 419                                                      trait_sig: ty::FnSig<'tcx>)
 420                                                      -> (Span, Option<Span>) {
 421     let tcx = infcx.tcx;
 422     let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
 423     let (impl_m_output, impl_m_iter) = match tcx.hir.expect_impl_item(impl_m_node_id).node {
 424         ImplItemKind::Method(ref impl_m_sig, _) => {
 425             (&impl_m_sig.decl.output, impl_m_sig.decl.inputs.iter())
 426         }
 427         _ => bug!("{:?} is not a method", impl_m),
 428     };
 429
 430     match *terr {
 431         TypeError::Mutability => {
 432             if let Some(trait_m_node_id) = tcx.hir.as_local_node_id(trait_m.def_id) {
 433                 let trait_m_iter = match tcx.hir.expect_trait_item(trait_m_node_id).node {
 434                     TraitItemKind::Method(ref trait_m_sig, _) => {
 435                         trait_m_sig.decl.inputs.iter()
 436                     }
 437                     _ => bug!("{:?} is not a TraitItemKind::Method", trait_m),
 438                 };
 439
 440                 impl_m_iter.zip(trait_m_iter).find(|&(ref impl_arg, ref trait_arg)| {
 441                     match (&impl_arg.node, &trait_arg.node) {
 442                         (&hir::TyRptr(_, ref impl_mt), &hir::TyRptr(_, ref trait_mt)) |
 443                         (&hir::TyPtr(ref impl_mt), &hir::TyPtr(ref trait_mt)) => {
 444                             impl_mt.mutbl != trait_mt.mutbl
 445                         }
 446                         _ => false,
 447                     }
 448                 }).map(|(ref impl_arg, ref trait_arg)| {
 449                     (impl_arg.span, Some(trait_arg.span))
 450                 })
 451                 .unwrap_or_else(|| (cause.span, tcx.hir.span_if_local(trait_m.def_id)))
 452             } else {
 453                 (cause.span, tcx.hir.span_if_local(trait_m.def_id))
 454             }
 455         }
 456         TypeError::Sorts(ExpectedFound { .. }) => {
 457             if let Some(trait_m_node_id) = tcx.hir.as_local_node_id(trait_m.def_id) {
 458                 let (trait_m_output, trait_m_iter) =
 459                     match tcx.hir.expect_trait_item(trait_m_node_id).node {
 460                         TraitItemKind::Method(ref trait_m_sig, _) => {
 461                             (&trait_m_sig.decl.output, trait_m_sig.decl.inputs.iter())
 462                         }
 463                         _ => bug!("{:?} is not a TraitItemKind::Method", trait_m),
 464                     };
 465
 466                 let impl_iter = impl_sig.inputs().iter();
 467                 let trait_iter = trait_sig.inputs().iter();
 468                 impl_iter.zip(trait_iter)
 469                          .zip(impl_m_iter)
 470                          .zip(trait_m_iter)
 471                          .filter_map(|(((impl_arg_ty, trait_arg_ty), impl_arg), trait_arg)| {
 472                              match infcx.sub_types(true, &cause, trait_arg_ty, impl_arg_ty) {
 473                                  Ok(_) => None,
 474                                  Err(_) => Some((impl_arg.span, Some(trait_arg.span))),
 475                              }
 476                          })
 477                          .next()
 478                          .unwrap_or_else(|| {
 479                              if infcx.sub_types(false, &cause, impl_sig.output(),
 480                                                 trait_sig.output())
 481                                      .is_err() {
 482                                          (impl_m_output.span(), Some(trait_m_output.span()))
 483                                      } else {
 484                                          (cause.span, tcx.hir.span_if_local(trait_m.def_id))
 485                                      }
 486                          })
 487             } else {
 488                 (cause.span, tcx.hir.span_if_local(trait_m.def_id))
 489             }
 490         }
 491         _ => (cause.span, tcx.hir.span_if_local(trait_m.def_id)),
 492     }
 493 }
 494
 495 fn compare_self_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 496                                impl_m: &ty::AssociatedItem,
 497                                impl_m_span: Span,
 498                                trait_m: &ty::AssociatedItem,
 499                                impl_trait_ref: ty::TraitRef<'tcx>)
 500                                -> Result<(), ErrorReported>
 501 {
 502     // Try to give more informative error messages about self typing
 503     // mismatches.  Note that any mismatch will also be detected
 504     // below, where we construct a canonical function type that
 505     // includes the self parameter as a normal parameter.  It's just
 506     // that the error messages you get out of this code are a bit more
 507     // inscrutable, particularly for cases where one method has no
 508     // self.
 509
 510     let self_string = |method: &ty::AssociatedItem| {
 511         let untransformed_self_ty = match method.container {
 512             ty::ImplContainer(_) => impl_trait_ref.self_ty(),
 513             ty::TraitContainer(_) => tcx.mk_self_type()
 514         };
 515         let method_ty = tcx.type_of(method.def_id);
 516         let self_arg_ty = *method_ty.fn_sig().input(0).skip_binder();
 517         match ExplicitSelf::determine(untransformed_self_ty, self_arg_ty) {
 518             ExplicitSelf::ByValue => "self".to_string(),
 519             ExplicitSelf::ByReference(_, hir::MutImmutable) => "&self".to_string(),
 520             ExplicitSelf::ByReference(_, hir::MutMutable) => "&mut self".to_string(),
 521             _ => format!("self: {}", self_arg_ty)
 522         }
 523     };
 524
 525     match (trait_m.method_has_self_argument, impl_m.method_has_self_argument) {
 526         (false, false) | (true, true) => {}
 527
 528         (false, true) => {
 529             let self_descr = self_string(impl_m);
 530             let mut err = struct_span_err!(tcx.sess,
 531                                            impl_m_span,
 532                                            E0185,
 533                                            "method `{}` has a `{}` declaration in the impl, but \
 534                                             not in the trait",
 535                                            trait_m.name,
 536                                            self_descr);
 537             err.span_label(impl_m_span, &format!("`{}` used in impl", self_descr));
 538             if let Some(span) = tcx.hir.span_if_local(trait_m.def_id) {
 539                 err.span_label(span, &format!("trait declared without `{}`", self_descr));
 540             }
 541             err.emit();
 542             return Err(ErrorReported);
 543         }
 544
 545         (true, false) => {
 546             let self_descr = self_string(trait_m);
 547             let mut err = struct_span_err!(tcx.sess,
 548                                            impl_m_span,
 549                                            E0186,
 550                                            "method `{}` has a `{}` declaration in the trait, but \
 551                                             not in the impl",
 552                                            trait_m.name,
 553                                            self_descr);
 554             err.span_label(impl_m_span,
 555                            &format!("expected `{}` in impl", self_descr));
 556             if let Some(span) = tcx.hir.span_if_local(trait_m.def_id) {
 557                 err.span_label(span, &format!("`{}` used in trait", self_descr));
 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 span = match tcx.hir.expect_impl_item(impl_m_node_id).node {
 580             ImplItemKind::Method(ref impl_m_sig, _) => {
 581                 if impl_m_sig.generics.is_parameterized() {
 582                     impl_m_sig.generics.span
 583                 } else {
 584                     impl_m_span
 585                 }
 586             }
 587             _ => bug!("{:?} is not a method", impl_m),
 588         };
 589
 590         let mut err = struct_span_err!(tcx.sess,
 591                                        span,
 592                                        E0049,
 593                                        "method `{}` has {} type parameter{} but its trait \
 594                                         declaration has {} type parameter{}",
 595                                        trait_m.name,
 596                                        num_impl_m_type_params,
 597                                        if num_impl_m_type_params == 1 { "" } else { "s" },
 598                                        num_trait_m_type_params,
 599                                        if num_trait_m_type_params == 1 {
 600                                            ""
 601                                        } else {
 602                                            "s"
 603                                        });
 604
 605         let mut suffix = None;
 606
 607         if let Some(span) = trait_item_span {
 608             err.span_label(span,
 609                            &format!("expected {}",
 610                                     &if num_trait_m_type_params != 1 {
 611                                         format!("{} type parameters", num_trait_m_type_params)
 612                                     } else {
 613                                         format!("{} type parameter", num_trait_m_type_params)
 614                                     }));
 615         } else {
 616             suffix = Some(format!(", expected {}", num_trait_m_type_params));
 617         }
 618
 619         err.span_label(span,
 620                        &format!("found {}{}",
 621                                 &if num_impl_m_type_params != 1 {
 622                                     format!("{} type parameters", num_impl_m_type_params)
 623                                 } else {
 624                                     format!("1 type parameter")
 625                                 },
 626                                 suffix.as_ref().map(|s| &s[..]).unwrap_or("")));
 627
 628         err.emit();
 629
 630         return Err(ErrorReported);
 631     }
 632
 633     Ok(())
 634 }
 635
 636 fn compare_number_of_method_arguments<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 637                                                 impl_m: &ty::AssociatedItem,
 638                                                 impl_m_span: Span,
 639                                                 trait_m: &ty::AssociatedItem,
 640                                                 trait_item_span: Option<Span>)
 641                                                 -> Result<(), ErrorReported> {
 642     let m_fty = |method: &ty::AssociatedItem| {
 643         match tcx.type_of(method.def_id).sty {
 644             ty::TyFnDef(_, _, f) => f,
 645             _ => bug!()
 646         }
 647     };
 648     let impl_m_fty = m_fty(impl_m);
 649     let trait_m_fty = m_fty(trait_m);
 650     let trait_number_args = trait_m_fty.inputs().skip_binder().len();
 651     let impl_number_args = impl_m_fty.inputs().skip_binder().len();
 652     if trait_number_args != impl_number_args {
 653         let trait_m_node_id = tcx.hir.as_local_node_id(trait_m.def_id);
 654         let trait_span = if let Some(trait_id) = trait_m_node_id {
 655             match tcx.hir.expect_trait_item(trait_id).node {
 656                 TraitItemKind::Method(ref trait_m_sig, _) => {
 657                     if let Some(arg) = trait_m_sig.decl.inputs.get(if trait_number_args > 0 {
 658                         trait_number_args - 1
 659                     } else {
 660                         0
 661                     }) {
 662                         Some(arg.span)
 663                     } else {
 664                         trait_item_span
 665                     }
 666                 }
 667                 _ => bug!("{:?} is not a method", impl_m),
 668             }
 669         } else {
 670             trait_item_span
 671         };
 672         let impl_m_node_id = tcx.hir.as_local_node_id(impl_m.def_id).unwrap();
 673         let impl_span = match tcx.hir.expect_impl_item(impl_m_node_id).node {
 674             ImplItemKind::Method(ref impl_m_sig, _) => {
 675                 if let Some(arg) = impl_m_sig.decl.inputs.get(if impl_number_args > 0 {
 676                     impl_number_args - 1
 677                 } else {
 678                     0
 679                 }) {
 680                     arg.span
 681                 } else {
 682                     impl_m_span
 683                 }
 684             }
 685             _ => bug!("{:?} is not a method", impl_m),
 686         };
 687         let mut err = struct_span_err!(tcx.sess,
 688                                        impl_span,
 689                                        E0050,
 690                                        "method `{}` has {} parameter{} but the declaration in \
 691                                         trait `{}` has {}",
 692                                        trait_m.name,
 693                                        impl_number_args,
 694                                        if impl_number_args == 1 { "" } else { "s" },
 695                                        tcx.item_path_str(trait_m.def_id),
 696                                        trait_number_args);
 697         if let Some(trait_span) = trait_span {
 698             err.span_label(trait_span,
 699                            &format!("trait requires {}",
 700                                     &if trait_number_args != 1 {
 701                                         format!("{} parameters", trait_number_args)
 702                                     } else {
 703                                         format!("{} parameter", trait_number_args)
 704                                     }));
 705         }
 706         err.span_label(impl_span,
 707                        &format!("expected {}, found {}",
 708                                 &if trait_number_args != 1 {
 709                                     format!("{} parameters", trait_number_args)
 710                                 } else {
 711                                     format!("{} parameter", trait_number_args)
 712                                 },
 713                                 impl_number_args));
 714         err.emit();
 715         return Err(ErrorReported);
 716     }
 717
 718     Ok(())
 719 }
 720
 721 pub fn compare_const_impl<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 722                                     impl_c: &ty::AssociatedItem,
 723                                     impl_c_span: Span,
 724                                     trait_c: &ty::AssociatedItem,
 725                                     impl_trait_ref: ty::TraitRef<'tcx>) {
 726     debug!("compare_const_impl(impl_trait_ref={:?})", impl_trait_ref);
 727
 728     tcx.infer_ctxt((), Reveal::UserFacing).enter(|infcx| {
 729         let inh = Inherited::new(infcx);
 730         let infcx = &inh.infcx;
 731
 732         // The below is for the most part highly similar to the procedure
 733         // for methods above. It is simpler in many respects, especially
 734         // because we shouldn't really have to deal with lifetimes or
 735         // predicates. In fact some of this should probably be put into
 736         // shared functions because of DRY violations...
 737         let trait_to_impl_substs = impl_trait_ref.substs;
 738
 739         // Create a parameter environment that represents the implementation's
 740         // method.
 741         let impl_c_node_id = tcx.hir.as_local_node_id(impl_c.def_id).unwrap();
 742         let impl_param_env = ty::ParameterEnvironment::for_item(tcx, impl_c_node_id);
 743
 744         // Create mapping from impl to skolemized.
 745         let impl_to_skol_substs = &impl_param_env.free_substs;
 746
 747         // Create mapping from trait to skolemized.
 748         let trait_to_skol_substs = impl_to_skol_substs.rebase_onto(tcx,
 749                                                                    impl_c.container.id(),
 750                                                                    trait_to_impl_substs.subst(tcx,
 751                                                                               impl_to_skol_substs));
 752         debug!("compare_const_impl: trait_to_skol_substs={:?}",
 753                trait_to_skol_substs);
 754
 755         // Compute skolemized form of impl and trait const tys.
 756         let impl_ty = tcx.type_of(impl_c.def_id).subst(tcx, impl_to_skol_substs);
 757         let trait_ty = tcx.type_of(trait_c.def_id).subst(tcx, trait_to_skol_substs);
 758         let mut cause = ObligationCause::misc(impl_c_span, impl_c_node_id);
 759
 760         // There is no "body" here, so just pass dummy id.
 761         let impl_ty = inh.normalize_associated_types_in(impl_c_span,
 762                                                         impl_c_node_id,
 763                                                         &impl_ty);
 764
 765         debug!("compare_const_impl: impl_ty={:?}", impl_ty);
 766
 767         let trait_ty = inh.normalize_associated_types_in(impl_c_span,
 768                                                          impl_c_node_id,
 769                                                          &trait_ty);
 770
 771         debug!("compare_const_impl: trait_ty={:?}", trait_ty);
 772
 773         let err = infcx.sub_types(false, &cause, impl_ty, trait_ty)
 774             .map(|ok| inh.register_infer_ok_obligations(ok));
 775
 776         if let Err(terr) = err {
 777             debug!("checking associated const for compatibility: impl ty {:?}, trait ty {:?}",
 778                    impl_ty,
 779                    trait_ty);
 780
 781             // Locate the Span containing just the type of the offending impl
 782             match tcx.hir.expect_impl_item(impl_c_node_id).node {
 783                 ImplItemKind::Const(ref ty, _) => cause.span = ty.span,
 784                 _ => bug!("{:?} is not a impl const", impl_c),
 785             }
 786
 787             let mut diag = struct_span_err!(tcx.sess,
 788                                             cause.span,
 789                                             E0326,
 790                                             "implemented const `{}` has an incompatible type for \
 791                                              trait",
 792                                             trait_c.name);
 793
 794             let trait_c_node_id = tcx.hir.as_local_node_id(trait_c.def_id);
 795             let trait_c_span = trait_c_node_id.map(|trait_c_node_id| {
 796                 // Add a label to the Span containing just the type of the const
 797                 match tcx.hir.expect_trait_item(trait_c_node_id).node {
 798                     TraitItemKind::Const(ref ty, _) => ty.span,
 799                     _ => bug!("{:?} is not a trait const", trait_c),
 800                 }
 801             });
 802
 803             infcx.note_type_err(&mut diag,
 804                                 &cause,
 805                                 trait_c_span.map(|span| (span, format!("type in trait"))),
 806                                 Some(infer::ValuePairs::Types(ExpectedFound {
 807                                     expected: trait_ty,
 808                                     found: impl_ty,
 809                                 })),
 810                                 &terr);
 811             diag.emit();
 812         }
 813
 814         // FIXME(#41323) Check the obligations in the fulfillment context.
 815     });
 816 }