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 middle::free_region::FreeRegionMap;
  12 use rustc::infer::{self, InferOk, TypeOrigin};
  13 use rustc::ty;
  14 use rustc::traits::{self, Reveal};
  15 use rustc::ty::error::{ExpectedFound, TypeError};
  16 use rustc::ty::subst::{Subst, Substs};
  17 use rustc::hir::{ImplItemKind, TraitItem_, Ty_};
  18
  19 use syntax::ast;
  20 use syntax_pos::Span;
  21
  22 use CrateCtxt;
  23 use super::assoc;
  24
  25 /// Checks that a method from an impl conforms to the signature of
  26 /// the same method as declared in the trait.
  27 ///
  28 /// # Parameters
  29 ///
  30 /// - impl_m: type of the method we are checking
  31 /// - impl_m_span: span to use for reporting errors
  32 /// - impl_m_body_id: id of the method body
  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>(ccx: &CrateCtxt<'a, 'tcx>,
  37                                      impl_m: &ty::Method<'tcx>,
  38                                      impl_m_span: Span,
  39                                      impl_m_body_id: ast::NodeId,
  40                                      trait_m: &ty::Method<'tcx>,
  41                                      impl_trait_ref: &ty::TraitRef<'tcx>) {
  42     debug!("compare_impl_method(impl_trait_ref={:?})",
  43            impl_trait_ref);
  44
  45     debug!("compare_impl_method: impl_trait_ref (liberated) = {:?}",
  46            impl_trait_ref);
  47
  48     let tcx = ccx.tcx;
  49
  50     let trait_to_impl_substs = &impl_trait_ref.substs;
  51
  52     // Try to give more informative error messages about self typing
  53     // mismatches.  Note that any mismatch will also be detected
  54     // below, where we construct a canonical function type that
  55     // includes the self parameter as a normal parameter.  It's just
  56     // that the error messages you get out of this code are a bit more
  57     // inscrutable, particularly for cases where one method has no
  58     // self.
  59     match (&trait_m.explicit_self, &impl_m.explicit_self) {
  60         (&ty::ExplicitSelfCategory::Static,
  61          &ty::ExplicitSelfCategory::Static) => {}
  62         (&ty::ExplicitSelfCategory::Static, _) => {
  63             let mut err = struct_span_err!(tcx.sess, impl_m_span, E0185,
  64                 "method `{}` has a `{}` declaration in the impl, \
  65                         but not in the trait",
  66                         trait_m.name,
  67                         impl_m.explicit_self);
  68             err.span_label(impl_m_span, &format!("`{}` used in impl",
  69                                                  impl_m.explicit_self));
  70             if let Some(span) = tcx.map.span_if_local(trait_m.def_id) {
  71                 err.span_label(span, &format!("trait declared without `{}`",
  72                                               impl_m.explicit_self));
  73             }
  74             err.emit();
  75             return;
  76         }
  77         (_, &ty::ExplicitSelfCategory::Static) => {
  78             let mut err = struct_span_err!(tcx.sess, impl_m_span, E0186,
  79                 "method `{}` has a `{}` declaration in the trait, \
  80                         but not in the impl",
  81                         trait_m.name,
  82                         trait_m.explicit_self);
  83             err.span_label(impl_m_span, &format!("expected `{}` in impl",
  84                                                   trait_m.explicit_self));
  85             if let Some(span) = tcx.map.span_if_local(trait_m.def_id) {
  86                 err.span_label(span, & format!("`{}` used in trait",
  87                                                trait_m.explicit_self));
  88             }
  89             err.emit();
  90             return;
  91         }
  92         _ => {
  93             // Let the type checker catch other errors below
  94         }
  95     }
  96
  97     let num_impl_m_type_params = impl_m.generics.types.len();
  98     let num_trait_m_type_params = trait_m.generics.types.len();
  99     if num_impl_m_type_params != num_trait_m_type_params {
 100         span_err!(tcx.sess, impl_m_span, E0049,
 101             "method `{}` has {} type parameter{} \
 102              but its trait declaration has {} type parameter{}",
 103             trait_m.name,
 104             num_impl_m_type_params,
 105             if num_impl_m_type_params == 1 {""} else {"s"},
 106             num_trait_m_type_params,
 107             if num_trait_m_type_params == 1 {""} else {"s"});
 108         return;
 109     }
 110
 111     if impl_m.fty.sig.0.inputs.len() != trait_m.fty.sig.0.inputs.len() {
 112         span_err!(tcx.sess, impl_m_span, E0050,
 113             "method `{}` has {} parameter{} \
 114              but the declaration in trait `{}` has {}",
 115             trait_m.name,
 116             impl_m.fty.sig.0.inputs.len(),
 117             if impl_m.fty.sig.0.inputs.len() == 1 {""} else {"s"},
 118             tcx.item_path_str(trait_m.def_id),
 119             trait_m.fty.sig.0.inputs.len());
 120         return;
 121     }
 122
 123     // This code is best explained by example. Consider a trait:
 124     //
 125     //     trait Trait<'t,T> {
 126     //          fn method<'a,M>(t: &'t T, m: &'a M) -> Self;
 127     //     }
 128     //
 129     // And an impl:
 130     //
 131     //     impl<'i, 'j, U> Trait<'j, &'i U> for Foo {
 132     //          fn method<'b,N>(t: &'j &'i U, m: &'b N) -> Foo;
 133     //     }
 134     //
 135     // We wish to decide if those two method types are compatible.
 136     //
 137     // We start out with trait_to_impl_substs, that maps the trait
 138     // type parameters to impl type parameters. This is taken from the
 139     // impl trait reference:
 140     //
 141     //     trait_to_impl_substs = {'t => 'j, T => &'i U, Self => Foo}
 142     //
 143     // We create a mapping `dummy_substs` that maps from the impl type
 144     // parameters to fresh types and regions. For type parameters,
 145     // this is the identity transform, but we could as well use any
 146     // skolemized types. For regions, we convert from bound to free
 147     // regions (Note: but only early-bound regions, i.e., those
 148     // declared on the impl or used in type parameter bounds).
 149     //
 150     //     impl_to_skol_substs = {'i => 'i0, U => U0, N => N0 }
 151     //
 152     // Now we can apply skol_substs to the type of the impl method
 153     // to yield a new function type in terms of our fresh, skolemized
 154     // types:
 155     //
 156     //     <'b> fn(t: &'i0 U0, m: &'b) -> Foo
 157     //
 158     // We now want to extract and substitute the type of the *trait*
 159     // method and compare it. To do so, we must create a compound
 160     // substitution by combining trait_to_impl_substs and
 161     // impl_to_skol_substs, and also adding a mapping for the method
 162     // type parameters. We extend the mapping to also include
 163     // the method parameters.
 164     //
 165     //     trait_to_skol_substs = { T => &'i0 U0, Self => Foo, M => N0 }
 166     //
 167     // Applying this to the trait method type yields:
 168     //
 169     //     <'a> fn(t: &'i0 U0, m: &'a) -> Foo
 170     //
 171     // This type is also the same but the name of the bound region ('a
 172     // vs 'b).  However, the normal subtyping rules on fn types handle
 173     // this kind of equivalency just fine.
 174     //
 175     // We now use these substitutions to ensure that all declared bounds are
 176     // satisfied by the implementation's method.
 177     //
 178     // We do this by creating a parameter environment which contains a
 179     // substitution corresponding to impl_to_skol_substs. We then build
 180     // trait_to_skol_substs and use it to convert the predicates contained
 181     // in the trait_m.generics to the skolemized form.
 182     //
 183     // Finally we register each of these predicates as an obligation in
 184     // a fresh FulfillmentCtxt, and invoke select_all_or_error.
 185
 186     // Create a parameter environment that represents the implementation's
 187     // method.
 188     let impl_m_node_id = tcx.map.as_local_node_id(impl_m.def_id).unwrap();
 189     let impl_param_env = ty::ParameterEnvironment::for_item(tcx, impl_m_node_id);
 190
 191     // Create mapping from impl to skolemized.
 192     let impl_to_skol_substs = &impl_param_env.free_substs;
 193
 194     // Create mapping from trait to skolemized.
 195     let trait_to_skol_substs =
 196         impl_to_skol_substs.rebase_onto(tcx, impl_m.container_id(),
 197                                         trait_to_impl_substs.subst(tcx, impl_to_skol_substs));
 198     debug!("compare_impl_method: trait_to_skol_substs={:?}",
 199            trait_to_skol_substs);
 200
 201     // Check region bounds. FIXME(@jroesch) refactor this away when removing
 202     // ParamBounds.
 203     if !check_region_bounds_on_impl_method(ccx,
 204                                            impl_m_span,
 205                                            impl_m,
 206                                            &trait_m.generics,
 207                                            &impl_m.generics,
 208                                            trait_to_skol_substs,
 209                                            impl_to_skol_substs) {
 210         return;
 211     }
 212
 213     tcx.infer_ctxt(None, None, Reveal::NotSpecializable).enter(|mut infcx| {
 214         let mut fulfillment_cx = traits::FulfillmentContext::new();
 215
 216         // Create obligations for each predicate declared by the impl
 217         // definition in the context of the trait's parameter
 218         // environment. We can't just use `impl_env.caller_bounds`,
 219         // however, because we want to replace all late-bound regions with
 220         // region variables.
 221         let impl_predicates = tcx.lookup_predicates(impl_m.predicates.parent.unwrap());
 222         let mut hybrid_preds = impl_predicates.instantiate(tcx, impl_to_skol_substs);
 223
 224         debug!("compare_impl_method: impl_bounds={:?}", hybrid_preds);
 225
 226         // This is the only tricky bit of the new way we check implementation methods
 227         // We need to build a set of predicates where only the method-level bounds
 228         // are from the trait and we assume all other bounds from the implementation
 229         // to be previously satisfied.
 230         //
 231         // We then register the obligations from the impl_m and check to see
 232         // if all constraints hold.
 233         hybrid_preds.predicates.extend(
 234             trait_m.predicates.instantiate_own(tcx, trait_to_skol_substs).predicates);
 235
 236         // Construct trait parameter environment and then shift it into the skolemized viewpoint.
 237         // The key step here is to update the caller_bounds's predicates to be
 238         // the new hybrid bounds we computed.
 239         let normalize_cause = traits::ObligationCause::misc(impl_m_span, impl_m_body_id);
 240         let trait_param_env = impl_param_env.with_caller_bounds(hybrid_preds.predicates);
 241         let trait_param_env = traits::normalize_param_env_or_error(tcx,
 242                                                                    trait_param_env,
 243                                                                    normalize_cause.clone());
 244         // FIXME(@jroesch) this seems ugly, but is a temporary change
 245         infcx.parameter_environment = trait_param_env;
 246
 247         debug!("compare_impl_method: caller_bounds={:?}",
 248             infcx.parameter_environment.caller_bounds);
 249
 250         let mut selcx = traits::SelectionContext::new(&infcx);
 251
 252         let impl_m_own_bounds = impl_m.predicates.instantiate_own(tcx, impl_to_skol_substs);
 253         let (impl_m_own_bounds, _) =
 254             infcx.replace_late_bound_regions_with_fresh_var(
 255                 impl_m_span,
 256                 infer::HigherRankedType,
 257                 &ty::Binder(impl_m_own_bounds.predicates));
 258         for predicate in impl_m_own_bounds {
 259             let traits::Normalized { value: predicate, .. } =
 260                 traits::normalize(&mut selcx, normalize_cause.clone(), &predicate);
 261
 262             let cause = traits::ObligationCause {
 263                 span: impl_m_span,
 264                 body_id: impl_m_body_id,
 265                 code: traits::ObligationCauseCode::CompareImplMethodObligation
 266             };
 267
 268             fulfillment_cx.register_predicate_obligation(
 269                 &infcx,
 270                 traits::Obligation::new(cause, predicate));
 271         }
 272
 273         // We now need to check that the signature of the impl method is
 274         // compatible with that of the trait method. We do this by
 275         // checking that `impl_fty <: trait_fty`.
 276         //
 277         // FIXME. Unfortunately, this doesn't quite work right now because
 278         // associated type normalization is not integrated into subtype
 279         // checks. For the comparison to be valid, we need to
 280         // normalize the associated types in the impl/trait methods
 281         // first. However, because function types bind regions, just
 282         // calling `normalize_associated_types_in` would have no effect on
 283         // any associated types appearing in the fn arguments or return
 284         // type.
 285
 286         // Compute skolemized form of impl and trait method tys.
 287         let tcx = infcx.tcx;
 288         let origin = TypeOrigin::MethodCompatCheck(impl_m_span);
 289
 290         let (impl_sig, _) =
 291             infcx.replace_late_bound_regions_with_fresh_var(impl_m_span,
 292                                                             infer::HigherRankedType,
 293                                                             &impl_m.fty.sig);
 294         let impl_sig =
 295             impl_sig.subst(tcx, impl_to_skol_substs);
 296         let impl_sig =
 297             assoc::normalize_associated_types_in(&infcx,
 298                                                  &mut fulfillment_cx,
 299                                                  impl_m_span,
 300                                                  impl_m_body_id,
 301                                                  &impl_sig);
 302         let impl_fty = tcx.mk_fn_ptr(tcx.mk_bare_fn(ty::BareFnTy {
 303             unsafety: impl_m.fty.unsafety,
 304             abi: impl_m.fty.abi,
 305             sig: ty::Binder(impl_sig.clone())
 306         }));
 307         debug!("compare_impl_method: impl_fty={:?}", impl_fty);
 308
 309         let trait_sig = tcx.liberate_late_bound_regions(
 310             infcx.parameter_environment.free_id_outlive,
 311             &trait_m.fty.sig);
 312         let trait_sig =
 313             trait_sig.subst(tcx, trait_to_skol_substs);
 314         let trait_sig =
 315             assoc::normalize_associated_types_in(&infcx,
 316                                                  &mut fulfillment_cx,
 317                                                  impl_m_span,
 318                                                  impl_m_body_id,
 319                                                  &trait_sig);
 320         let trait_fty = tcx.mk_fn_ptr(tcx.mk_bare_fn(ty::BareFnTy {
 321             unsafety: trait_m.fty.unsafety,
 322             abi: trait_m.fty.abi,
 323             sig: ty::Binder(trait_sig.clone())
 324         }));
 325
 326         debug!("compare_impl_method: trait_fty={:?}", trait_fty);
 327
 328         if let Err(terr) = infcx.sub_types(false, origin, impl_fty, trait_fty) {
 329             debug!("sub_types failed: impl ty {:?}, trait ty {:?}",
 330                    impl_fty,
 331                    trait_fty);
 332
 333             let (impl_err_span, trait_err_span) =
 334                 extract_spans_for_error_reporting(&infcx, &terr, origin, impl_m,
 335                     impl_sig, trait_m, trait_sig);
 336
 337             let origin = TypeOrigin::MethodCompatCheck(impl_err_span);
 338
 339             let mut diag = struct_span_err!(
 340                 tcx.sess, origin.span(), E0053,
 341                 "method `{}` has an incompatible type for trait", trait_m.name
 342             );
 343
 344             infcx.note_type_err(
 345                 &mut diag,
 346                 origin,
 347                 trait_err_span.map(|sp| (sp, format!("type in trait"))),
 348                 Some(infer::ValuePairs::Types(ExpectedFound {
 349                      expected: trait_fty,
 350                      found: impl_fty
 351                 })),
 352                 &terr
 353             );
 354             diag.emit();
 355             return
 356         }
 357
 358         // Check that all obligations are satisfied by the implementation's
 359         // version.
 360         if let Err(ref errors) = fulfillment_cx.select_all_or_error(&infcx) {
 361             infcx.report_fulfillment_errors(errors);
 362             return
 363         }
 364
 365         // Finally, resolve all regions. This catches wily misuses of
 366         // lifetime parameters. We have to build up a plausible lifetime
 367         // environment based on what we find in the trait. We could also
 368         // include the obligations derived from the method argument types,
 369         // but I don't think it's necessary -- after all, those are still
 370         // in effect when type-checking the body, and all the
 371         // where-clauses in the header etc should be implied by the trait
 372         // anyway, so it shouldn't be needed there either. Anyway, we can
 373         // always add more relations later (it's backwards compat).
 374         let mut free_regions = FreeRegionMap::new();
 375         free_regions.relate_free_regions_from_predicates(
 376             &infcx.parameter_environment.caller_bounds);
 377
 378         infcx.resolve_regions_and_report_errors(&free_regions, impl_m_body_id);
 379     });
 380
 381     fn check_region_bounds_on_impl_method<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
 382                                                     span: Span,
 383                                                     impl_m: &ty::Method<'tcx>,
 384                                                     trait_generics: &ty::Generics<'tcx>,
 385                                                     impl_generics: &ty::Generics<'tcx>,
 386                                                     trait_to_skol_substs: &Substs<'tcx>,
 387                                                     impl_to_skol_substs: &Substs<'tcx>)
 388                                                     -> bool
 389     {
 390
 391         let trait_params = &trait_generics.regions[..];
 392         let impl_params = &impl_generics.regions[..];
 393
 394         debug!("check_region_bounds_on_impl_method: \
 395                trait_generics={:?} \
 396                impl_generics={:?} \
 397                trait_to_skol_substs={:?} \
 398                impl_to_skol_substs={:?}",
 399                trait_generics,
 400                impl_generics,
 401                trait_to_skol_substs,
 402                impl_to_skol_substs);
 403
 404         // Must have same number of early-bound lifetime parameters.
 405         // Unfortunately, if the user screws up the bounds, then this
 406         // will change classification between early and late.  E.g.,
 407         // if in trait we have `<'a,'b:'a>`, and in impl we just have
 408         // `<'a,'b>`, then we have 2 early-bound lifetime parameters
 409         // in trait but 0 in the impl. But if we report "expected 2
 410         // but found 0" it's confusing, because it looks like there
 411         // are zero. Since I don't quite know how to phrase things at
 412         // the moment, give a kind of vague error message.
 413         if trait_params.len() != impl_params.len() {
 414             struct_span_err!(ccx.tcx.sess, span, E0195,
 415                 "lifetime parameters or bounds on method `{}` do \
 416                  not match the trait declaration",impl_m.name)
 417                 .span_label(span, &format!("lifetimes do not match trait"))
 418                 .emit();
 419             return false;
 420         }
 421
 422         return true;
 423     }
 424
 425     fn extract_spans_for_error_reporting<'a, 'gcx, 'tcx>(infcx: &infer::InferCtxt<'a, 'gcx, 'tcx>,
 426                                                          terr: &TypeError,
 427                                                          origin: TypeOrigin,
 428                                                          impl_m: &ty::Method,
 429                                                          impl_sig: ty::FnSig<'tcx>,
 430                                                          trait_m: &ty::Method,
 431                                                          trait_sig: ty::FnSig<'tcx>)
 432                                                         -> (Span, Option<Span>) {
 433         let tcx = infcx.tcx;
 434         let impl_m_node_id = tcx.map.as_local_node_id(impl_m.def_id).unwrap();
 435         let (impl_m_output, impl_m_iter) = match tcx.map.expect_impl_item(impl_m_node_id).node {
 436             ImplItemKind::Method(ref impl_m_sig, _) =>
 437                 (&impl_m_sig.decl.output, impl_m_sig.decl.inputs.iter()),
 438             _ => bug!("{:?} is not a method", impl_m)
 439         };
 440
 441         match *terr {
 442             TypeError::Mutability => {
 443                 if let Some(trait_m_node_id) = tcx.map.as_local_node_id(trait_m.def_id) {
 444                     let trait_m_iter = match tcx.map.expect_trait_item(trait_m_node_id).node {
 445                         TraitItem_::MethodTraitItem(ref trait_m_sig, _) =>
 446                             trait_m_sig.decl.inputs.iter(),
 447                         _ => bug!("{:?} is not a MethodTraitItem", trait_m)
 448                     };
 449
 450                     impl_m_iter.zip(trait_m_iter).find(|&(ref impl_arg, ref trait_arg)| {
 451                         match (&impl_arg.ty.node, &trait_arg.ty.node) {
 452                             (&Ty_::TyRptr(_, ref impl_mt), &Ty_::TyRptr(_, ref trait_mt)) |
 453                             (&Ty_::TyPtr(ref impl_mt), &Ty_::TyPtr(ref trait_mt)) =>
 454                                 impl_mt.mutbl != trait_mt.mutbl,
 455                             _ => false
 456                         }
 457                     }).map(|(ref impl_arg, ref trait_arg)| {
 458                         match (impl_arg.to_self(), trait_arg.to_self()) {
 459                             (Some(impl_self), Some(trait_self)) =>
 460                                 (impl_self.span, Some(trait_self.span)),
 461                             (None, None) => (impl_arg.ty.span, Some(trait_arg.ty.span)),
 462                             _ => bug!("impl and trait fns have different first args, \
 463                                        impl: {:?}, trait: {:?}", impl_arg, trait_arg)
 464                         }
 465                     }).unwrap_or((origin.span(), tcx.map.span_if_local(trait_m.def_id)))
 466                 } else {
 467                     (origin.span(), tcx.map.span_if_local(trait_m.def_id))
 468                 }
 469             }
 470             TypeError::Sorts(ExpectedFound { .. }) => {
 471                 if let Some(trait_m_node_id) = tcx.map.as_local_node_id(trait_m.def_id) {
 472                     let (trait_m_output, trait_m_iter) =
 473                     match tcx.map.expect_trait_item(trait_m_node_id).node {
 474                         TraitItem_::MethodTraitItem(ref trait_m_sig, _) =>
 475                             (&trait_m_sig.decl.output, trait_m_sig.decl.inputs.iter()),
 476                         _ => bug!("{:?} is not a MethodTraitItem", trait_m)
 477                     };
 478
 479                     let impl_iter = impl_sig.inputs.iter();
 480                     let trait_iter = trait_sig.inputs.iter();
 481                     impl_iter.zip(trait_iter).zip(impl_m_iter).zip(trait_m_iter)
 482                         .filter_map(|(((impl_arg_ty, trait_arg_ty), impl_arg), trait_arg)| {
 483                             match infcx.sub_types(true, origin, trait_arg_ty, impl_arg_ty) {
 484                                 Ok(_) => None,
 485                                 Err(_) => Some((impl_arg.ty.span, Some(trait_arg.ty.span)))
 486                             }
 487                         })
 488                         .next()
 489                         .unwrap_or_else(|| {
 490                             if infcx.sub_types(false, origin, impl_sig.output,
 491                                                trait_sig.output).is_err() {
 492                                 (impl_m_output.span(), Some(trait_m_output.span()))
 493                             } else {
 494                                 (origin.span(), tcx.map.span_if_local(trait_m.def_id))
 495                             }
 496                         })
 497                 } else {
 498                     (origin.span(), tcx.map.span_if_local(trait_m.def_id))
 499                 }
 500             }
 501             _ => (origin.span(), tcx.map.span_if_local(trait_m.def_id))
 502         }
 503     }
 504 }
 505
 506 pub fn compare_const_impl<'a, 'tcx>(ccx: &CrateCtxt<'a, 'tcx>,
 507                                     impl_c: &ty::AssociatedConst<'tcx>,
 508                                     impl_c_span: Span,
 509                                     trait_c: &ty::AssociatedConst<'tcx>,
 510                                     impl_trait_ref: &ty::TraitRef<'tcx>) {
 511     debug!("compare_const_impl(impl_trait_ref={:?})",
 512            impl_trait_ref);
 513
 514     let tcx = ccx.tcx;
 515     tcx.infer_ctxt(None, None, Reveal::NotSpecializable).enter(|infcx| {
 516         let mut fulfillment_cx = traits::FulfillmentContext::new();
 517
 518         // The below is for the most part highly similar to the procedure
 519         // for methods above. It is simpler in many respects, especially
 520         // because we shouldn't really have to deal with lifetimes or
 521         // predicates. In fact some of this should probably be put into
 522         // shared functions because of DRY violations...
 523         let trait_to_impl_substs = &impl_trait_ref.substs;
 524
 525         // Create a parameter environment that represents the implementation's
 526         // method.
 527         let impl_c_node_id = tcx.map.as_local_node_id(impl_c.def_id).unwrap();
 528         let impl_param_env = ty::ParameterEnvironment::for_item(tcx, impl_c_node_id);
 529
 530         // Create mapping from impl to skolemized.
 531         let impl_to_skol_substs = &impl_param_env.free_substs;
 532
 533         // Create mapping from trait to skolemized.
 534         let trait_to_skol_substs =
 535             impl_to_skol_substs.rebase_onto(tcx, impl_c.container.id(),
 536                                             trait_to_impl_substs.subst(tcx, impl_to_skol_substs));
 537         debug!("compare_const_impl: trait_to_skol_substs={:?}",
 538             trait_to_skol_substs);
 539
 540         // Compute skolemized form of impl and trait const tys.
 541         let impl_ty = impl_c.ty.subst(tcx, impl_to_skol_substs);
 542         let trait_ty = trait_c.ty.subst(tcx, trait_to_skol_substs);
 543         let mut origin = TypeOrigin::Misc(impl_c_span);
 544
 545         let err = infcx.commit_if_ok(|_| {
 546             // There is no "body" here, so just pass dummy id.
 547             let impl_ty =
 548                 assoc::normalize_associated_types_in(&infcx,
 549                                                      &mut fulfillment_cx,
 550                                                      impl_c_span,
 551                                                      0,
 552                                                      &impl_ty);
 553
 554             debug!("compare_const_impl: impl_ty={:?}",
 555                 impl_ty);
 556
 557             let trait_ty =
 558                 assoc::normalize_associated_types_in(&infcx,
 559                                                      &mut fulfillment_cx,
 560                                                      impl_c_span,
 561                                                      0,
 562                                                      &trait_ty);
 563
 564             debug!("compare_const_impl: trait_ty={:?}",
 565                 trait_ty);
 566
 567             infcx.sub_types(false, origin, impl_ty, trait_ty)
 568                  .map(|InferOk { obligations, .. }| {
 569                 // FIXME(#32730) propagate obligations
 570                 assert!(obligations.is_empty())
 571             })
 572         });
 573
 574         if let Err(terr) = err {
 575             debug!("checking associated const for compatibility: impl ty {:?}, trait ty {:?}",
 576                    impl_ty,
 577                    trait_ty);
 578
 579             // Locate the Span containing just the type of the offending impl
 580             match tcx.map.expect_impl_item(impl_c_node_id).node {
 581                 ImplItemKind::Const(ref ty, _) => origin = TypeOrigin::Misc(ty.span),
 582                 _ => bug!("{:?} is not a impl const", impl_c)
 583             }
 584
 585             let mut diag = struct_span_err!(
 586                 tcx.sess, origin.span(), E0326,
 587                 "implemented const `{}` has an incompatible type for trait",
 588                 trait_c.name
 589             );
 590
 591             // Add a label to the Span containing just the type of the item
 592             let trait_c_node_id = tcx.map.as_local_node_id(trait_c.def_id).unwrap();
 593             let trait_c_span = match tcx.map.expect_trait_item(trait_c_node_id).node {
 594                 TraitItem_::ConstTraitItem(ref ty, _) => ty.span,
 595                 _ => bug!("{:?} is not a trait const", trait_c)
 596             };
 597
 598             infcx.note_type_err(
 599                 &mut diag,
 600                 origin,
 601                 Some((trait_c_span, format!("type in trait"))),
 602                 Some(infer::ValuePairs::Types(ExpectedFound {
 603                     expected: trait_ty,
 604                     found: impl_ty
 605                 })), &terr
 606             );
 607             diag.emit();
 608         }
 609     });
 610 }