src/librustc_typeck/check/compare_method.rs

   1 use rustc::hir::{self, GenericParamKind, ImplItemKind, TraitItemKind};
   2 use rustc::hir::def::{Res, DefKind};
   3 use rustc::infer::{self, InferOk};
   4 use rustc::ty::{self, TyCtxt, GenericParamDefKind};
   5 use rustc::ty::util::ExplicitSelf;
   6 use rustc::traits::{self, ObligationCause, ObligationCauseCode, Reveal};
   7 use rustc::ty::error::{ExpectedFound, TypeError};
   8 use rustc::ty::subst::{Subst, InternalSubsts, SubstsRef};
   9 use rustc::util::common::ErrorReported;
  10 use errors::{Applicability, DiagnosticId};
  11
  12 use syntax_pos::Span;
  13
  14 use super::{Inherited, FnCtxt, potentially_plural_count};
  15
  16 /// Checks that a method from an impl conforms to the signature of
  17 /// the same method as declared in the trait.
  18 ///
  19 /// # Parameters
  20 ///
  21 /// - `impl_m`: type of the method we are checking
  22 /// - `impl_m_span`: span to use for reporting errors
  23 /// - `trait_m`: the method in the trait
  24 /// - `impl_trait_ref`: the TraitRef corresponding to the trait implementation
  25
  26 pub fn compare_impl_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  27                                      impl_m: &ty::AssociatedItem,
  28                                      impl_m_span: Span,
  29                                      trait_m: &ty::AssociatedItem,
  30                                      impl_trait_ref: ty::TraitRef<'tcx>,
  31                                      trait_item_span: Option<Span>) {
  32     debug!("compare_impl_method(impl_trait_ref={:?})",
  33            impl_trait_ref);
  34
  35     let impl_m_span = tcx.sess.source_map().def_span(impl_m_span);
  36
  37     if let Err(ErrorReported) = compare_self_type(tcx,
  38                                                   impl_m,
  39                                                   impl_m_span,
  40                                                   trait_m,
  41                                                   impl_trait_ref) {
  42         return;
  43     }
  44
  45     if let Err(ErrorReported) = compare_number_of_generics(tcx,
  46                                                            impl_m,
  47                                                            impl_m_span,
  48                                                            trait_m,
  49                                                            trait_item_span) {
  50         return;
  51     }
  52
  53     if let Err(ErrorReported) = compare_number_of_method_arguments(tcx,
  54                                                                    impl_m,
  55                                                                    impl_m_span,
  56                                                                    trait_m,
  57                                                                    trait_item_span) {
  58         return;
  59     }
  60
  61     if let Err(ErrorReported) = compare_synthetic_generics(tcx,
  62                                                            impl_m,
  63                                                            trait_m) {
  64         return;
  65     }
  66
  67     if let Err(ErrorReported) = compare_predicate_entailment(tcx,
  68                                                              impl_m,
  69                                                              impl_m_span,
  70                                                              trait_m,
  71                                                              impl_trait_ref) {
  72         return;
  73     }
  74 }
  75
  76 fn compare_predicate_entailment<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  77                                           impl_m: &ty::AssociatedItem,
  78                                           impl_m_span: Span,
  79                                           trait_m: &ty::AssociatedItem,
  80                                           impl_trait_ref: ty::TraitRef<'tcx>)
  81                                           -> Result<(), ErrorReported> {
  82     let trait_to_impl_substs = impl_trait_ref.substs;
  83
  84     // This node-id should be used for the `body_id` field on each
  85     // `ObligationCause` (and the `FnCtxt`). This is what
  86     // `regionck_item` expects.
  87     let impl_m_hir_id = tcx.hir().as_local_hir_id(impl_m.def_id).unwrap();
  88
  89     let cause = ObligationCause {
  90         span: impl_m_span,
  91         body_id: impl_m_hir_id,
  92         code: ObligationCauseCode::CompareImplMethodObligation {
  93             item_name: impl_m.ident.name,
  94             impl_item_def_id: impl_m.def_id,
  95             trait_item_def_id: trait_m.def_id,
  96         },
  97     };
  98
  99     // This code is best explained by example. Consider a trait:
 100     //
 101     //     trait Trait<'t,T> {
 102     //          fn method<'a,M>(t: &'t T, m: &'a M) -> Self;
 103     //     }
 104     //
 105     // And an impl:
 106     //
 107     //     impl<'i, 'j, U> Trait<'j, &'i U> for Foo {
 108     //          fn method<'b,N>(t: &'j &'i U, m: &'b N) -> Foo;
 109     //     }
 110     //
 111     // We wish to decide if those two method types are compatible.
 112     //
 113     // We start out with trait_to_impl_substs, that maps the trait
 114     // type parameters to impl type parameters. This is taken from the
 115     // impl trait reference:
 116     //
 117     //     trait_to_impl_substs = {'t => 'j, T => &'i U, Self => Foo}
 118     //
 119     // We create a mapping `dummy_substs` that maps from the impl type
 120     // parameters to fresh types and regions. For type parameters,
 121     // this is the identity transform, but we could as well use any
 122     // placeholder types. For regions, we convert from bound to free
 123     // regions (Note: but only early-bound regions, i.e., those
 124     // declared on the impl or used in type parameter bounds).
 125     //
 126     //     impl_to_skol_substs = {'i => 'i0, U => U0, N => N0 }
 127     //
 128     // Now we can apply skol_substs to the type of the impl method
 129     // to yield a new function type in terms of our fresh, placeholder
 130     // types:
 131     //
 132     //     <'b> fn(t: &'i0 U0, m: &'b) -> Foo
 133     //
 134     // We now want to extract and substitute the type of the *trait*
 135     // method and compare it. To do so, we must create a compound
 136     // substitution by combining trait_to_impl_substs and
 137     // impl_to_skol_substs, and also adding a mapping for the method
 138     // type parameters. We extend the mapping to also include
 139     // the method parameters.
 140     //
 141     //     trait_to_skol_substs = { T => &'i0 U0, Self => Foo, M => N0 }
 142     //
 143     // Applying this to the trait method type yields:
 144     //
 145     //     <'a> fn(t: &'i0 U0, m: &'a) -> Foo
 146     //
 147     // This type is also the same but the name of the bound region ('a
 148     // vs 'b).  However, the normal subtyping rules on fn types handle
 149     // this kind of equivalency just fine.
 150     //
 151     // We now use these substitutions to ensure that all declared bounds are
 152     // satisfied by the implementation's method.
 153     //
 154     // We do this by creating a parameter environment which contains a
 155     // substitution corresponding to impl_to_skol_substs. We then build
 156     // trait_to_skol_substs and use it to convert the predicates contained
 157     // in the trait_m.generics to the placeholder form.
 158     //
 159     // Finally we register each of these predicates as an obligation in
 160     // a fresh FulfillmentCtxt, and invoke select_all_or_error.
 161
 162     // Create mapping from impl to placeholder.
 163     let impl_to_skol_substs = InternalSubsts::identity_for_item(tcx, impl_m.def_id);
 164
 165     // Create mapping from trait to placeholder.
 166     let trait_to_skol_substs = impl_to_skol_substs.rebase_onto(tcx,
 167                                                                impl_m.container.id(),
 168                                                                trait_to_impl_substs);
 169     debug!("compare_impl_method: trait_to_skol_substs={:?}",
 170            trait_to_skol_substs);
 171
 172     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 173     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 174     let impl_m_predicates = tcx.predicates_of(impl_m.def_id);
 175     let trait_m_predicates = tcx.predicates_of(trait_m.def_id);
 176
 177     // Check region bounds.
 178     check_region_bounds_on_impl_method(tcx,
 179                                        impl_m_span,
 180                                        impl_m,
 181                                        trait_m,
 182                                        &trait_m_generics,
 183                                        &impl_m_generics,
 184                                        trait_to_skol_substs)?;
 185
 186     // Create obligations for each predicate declared by the impl
 187     // definition in the context of the trait's parameter
 188     // environment. We can't just use `impl_env.caller_bounds`,
 189     // however, because we want to replace all late-bound regions with
 190     // region variables.
 191     let impl_predicates = tcx.predicates_of(impl_m_predicates.parent.unwrap());
 192     let mut hybrid_preds = impl_predicates.instantiate_identity(tcx);
 193
 194     debug!("compare_impl_method: impl_bounds={:?}", hybrid_preds);
 195
 196     // This is the only tricky bit of the new way we check implementation methods
 197     // We need to build a set of predicates where only the method-level bounds
 198     // are from the trait and we assume all other bounds from the implementation
 199     // to be previously satisfied.
 200     //
 201     // We then register the obligations from the impl_m and check to see
 202     // if all constraints hold.
 203     hybrid_preds.predicates.extend(
 204         trait_m_predicates.instantiate_own(tcx, trait_to_skol_substs).predicates);
 205
 206     // Construct trait parameter environment and then shift it into the placeholder viewpoint.
 207     // The key step here is to update the caller_bounds's predicates to be
 208     // the new hybrid bounds we computed.
 209     let normalize_cause = traits::ObligationCause::misc(impl_m_span, impl_m_hir_id);
 210     let param_env = ty::ParamEnv::new(
 211         tcx.intern_predicates(&hybrid_preds.predicates),
 212         Reveal::UserFacing,
 213         None
 214     );
 215     let param_env = traits::normalize_param_env_or_error(tcx,
 216                                                          impl_m.def_id,
 217                                                          param_env,
 218                                                          normalize_cause.clone());
 219
 220     tcx.infer_ctxt().enter(|infcx| {
 221         let inh = Inherited::new(infcx, impl_m.def_id);
 222         let infcx = &inh.infcx;
 223
 224         debug!("compare_impl_method: caller_bounds={:?}",
 225                param_env.caller_bounds);
 226
 227         let mut selcx = traits::SelectionContext::new(&infcx);
 228
 229         let impl_m_own_bounds = impl_m_predicates.instantiate_own(tcx, impl_to_skol_substs);
 230         let (impl_m_own_bounds, _) = infcx.replace_bound_vars_with_fresh_vars(
 231             impl_m_span,
 232             infer::HigherRankedType,
 233             &ty::Binder::bind(impl_m_own_bounds.predicates)
 234         );
 235         for predicate in impl_m_own_bounds {
 236             let traits::Normalized { value: predicate, obligations } =
 237                 traits::normalize(&mut selcx, param_env, normalize_cause.clone(), &predicate);
 238
 239             inh.register_predicates(obligations);
 240             inh.register_predicate(traits::Obligation::new(cause.clone(), param_env, predicate));
 241         }
 242
 243         // We now need to check that the signature of the impl method is
 244         // compatible with that of the trait method. We do this by
 245         // checking that `impl_fty <: trait_fty`.
 246         //
 247         // FIXME. Unfortunately, this doesn't quite work right now because
 248         // associated type normalization is not integrated into subtype
 249         // checks. For the comparison to be valid, we need to
 250         // normalize the associated types in the impl/trait methods
 251         // first. However, because function types bind regions, just
 252         // calling `normalize_associated_types_in` would have no effect on
 253         // any associated types appearing in the fn arguments or return
 254         // type.
 255
 256         // Compute placeholder form of impl and trait method tys.
 257         let tcx = infcx.tcx;
 258
 259         let (impl_sig, _) = infcx.replace_bound_vars_with_fresh_vars(
 260             impl_m_span,
 261             infer::HigherRankedType,
 262             &tcx.fn_sig(impl_m.def_id)
 263         );
 264         let impl_sig =
 265             inh.normalize_associated_types_in(impl_m_span,
 266                                               impl_m_hir_id,
 267                                               param_env,
 268                                               &impl_sig);
 269         let impl_fty = tcx.mk_fn_ptr(ty::Binder::bind(impl_sig));
 270         debug!("compare_impl_method: impl_fty={:?}", impl_fty);
 271
 272         let trait_sig = tcx.liberate_late_bound_regions(
 273             impl_m.def_id,
 274             &tcx.fn_sig(trait_m.def_id));
 275         let trait_sig =
 276             trait_sig.subst(tcx, trait_to_skol_substs);
 277         let trait_sig =
 278             inh.normalize_associated_types_in(impl_m_span,
 279                                               impl_m_hir_id,
 280                                               param_env,
 281                                               &trait_sig);
 282         let trait_fty = tcx.mk_fn_ptr(ty::Binder::bind(trait_sig));
 283
 284         debug!("compare_impl_method: trait_fty={:?}", trait_fty);
 285
 286         let sub_result = infcx.at(&cause, param_env)
 287                               .sup(trait_fty, impl_fty)
 288                               .map(|InferOk { obligations, .. }| {
 289                                   inh.register_predicates(obligations);
 290                               });
 291
 292         if let Err(terr) = sub_result {
 293             debug!("sub_types failed: impl ty {:?}, trait ty {:?}",
 294                    impl_fty,
 295                    trait_fty);
 296
 297             let (impl_err_span, trait_err_span) = extract_spans_for_error_reporting(&infcx,
 298                                                                                     param_env,
 299                                                                                     &terr,
 300                                                                                     &cause,
 301                                                                                     impl_m,
 302                                                                                     impl_sig,
 303                                                                                     trait_m,
 304                                                                                     trait_sig);
 305
 306             let cause = ObligationCause {
 307                 span: impl_err_span,
 308                 ..cause.clone()
 309             };
 310
 311             let mut diag = struct_span_err!(tcx.sess,
 312                                             cause.span(tcx),
 313                                             E0053,
 314                                             "method `{}` has an incompatible type for trait",
 315                                             trait_m.ident);
 316             if let TypeError::Mutability = terr {
 317                 if let Some(trait_err_span) = trait_err_span {
 318                     if let Ok(trait_err_str) = tcx.sess.source_map()
 319                                                        .span_to_snippet(trait_err_span) {
 320                         diag.span_suggestion(
 321                             impl_err_span,
 322                             "consider change the type to match the mutability in trait",
 323                             trait_err_str,
 324                             Applicability::MachineApplicable,
 325                         );
 326                     }
 327                 }
 328             }
 329
 330             infcx.note_type_err(&mut diag,
 331                                 &cause,
 332                                 trait_err_span.map(|sp| (sp, "type in trait".to_owned())),
 333                                 Some(infer::ValuePairs::Types(ExpectedFound {
 334                                     expected: trait_fty,
 335                                     found: impl_fty,
 336                                 })),
 337                                 &terr);
 338             diag.emit();
 339             return Err(ErrorReported);
 340         }
 341
 342         // Check that all obligations are satisfied by the implementation's
 343         // version.
 344         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
 345             infcx.report_fulfillment_errors(errors, None, false);
 346             return Err(ErrorReported);
 347         }
 348
 349         // Finally, resolve all regions. This catches wily misuses of
 350         // lifetime parameters.
 351         let fcx = FnCtxt::new(&inh, param_env, impl_m_hir_id);
 352         fcx.regionck_item(impl_m_hir_id, impl_m_span, &[]);
 353
 354         Ok(())
 355     })
 356 }
 357
 358 fn check_region_bounds_on_impl_method<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 359                                                 span: Span,
 360                                                 impl_m: &ty::AssociatedItem,
 361                                                 trait_m: &ty::AssociatedItem,
 362                                                 trait_generics: &ty::Generics,
 363                                                 impl_generics: &ty::Generics,
 364                                                 trait_to_skol_substs: SubstsRef<'tcx>)
 365                                                 -> Result<(), ErrorReported> {
 366     let trait_params = trait_generics.own_counts().lifetimes;
 367     let impl_params = impl_generics.own_counts().lifetimes;
 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 != impl_params {
 387         let def_span = tcx.sess.source_map().def_span(span);
 388         let span = tcx.hir().get_generics_span(impl_m.def_id).unwrap_or(def_span);
 389         let mut err = struct_span_err!(
 390             tcx.sess,
 391             span,
 392             E0195,
 393             "lifetime parameters or bounds on method `{}` do not match the trait declaration",
 394             impl_m.ident,
 395         );
 396         err.span_label(span, "lifetimes do not match method in trait");
 397         if let Some(sp) = tcx.hir().span_if_local(trait_m.def_id) {
 398             let def_sp = tcx.sess.source_map().def_span(sp);
 399             let sp = tcx.hir().get_generics_span(trait_m.def_id).unwrap_or(def_sp);
 400             err.span_label(sp, "lifetimes in impl do not match this method in trait");
 401         }
 402         err.emit();
 403         return Err(ErrorReported);
 404     }
 405
 406     Ok(())
 407 }
 408
 409 fn extract_spans_for_error_reporting<'a, 'gcx, 'tcx>(infcx: &infer::InferCtxt<'a, 'gcx, 'tcx>,
 410                                                      param_env: ty::ParamEnv<'tcx>,
 411                                                      terr: &TypeError<'_>,
 412                                                      cause: &ObligationCause<'tcx>,
 413                                                      impl_m: &ty::AssociatedItem,
 414                                                      impl_sig: ty::FnSig<'tcx>,
 415                                                      trait_m: &ty::AssociatedItem,
 416                                                      trait_sig: ty::FnSig<'tcx>)
 417                                                      -> (Span, Option<Span>) {
 418     let tcx = infcx.tcx;
 419     let impl_m_hir_id = tcx.hir().as_local_hir_id(impl_m.def_id).unwrap();
 420     let (impl_m_output, impl_m_iter) = match tcx.hir()
 421                                                 .expect_impl_item(impl_m_hir_id)
 422                                                 .node {
 423         ImplItemKind::Method(ref impl_m_sig, _) => {
 424             (&impl_m_sig.decl.output, impl_m_sig.decl.inputs.iter())
 425         }
 426         _ => bug!("{:?} is not a method", impl_m),
 427     };
 428
 429     match *terr {
 430         TypeError::Mutability => {
 431             if let Some(trait_m_hir_id) = tcx.hir().as_local_hir_id(trait_m.def_id) {
 432                 let trait_m_iter = match tcx.hir()
 433                                             .expect_trait_item(trait_m_hir_id)
 434                                             .node {
 435                     TraitItemKind::Method(ref trait_m_sig, _) => {
 436                         trait_m_sig.decl.inputs.iter()
 437                     }
 438                     _ => bug!("{:?} is not a TraitItemKind::Method", trait_m),
 439                 };
 440
 441                 impl_m_iter.zip(trait_m_iter).find(|&(ref impl_arg, ref trait_arg)| {
 442                     match (&impl_arg.node, &trait_arg.node) {
 443                         (&hir::TyKind::Rptr(_, ref impl_mt), &hir::TyKind::Rptr(_, ref trait_mt)) |
 444                         (&hir::TyKind::Ptr(ref impl_mt), &hir::TyKind::Ptr(ref trait_mt)) => {
 445                             impl_mt.mutbl != trait_mt.mutbl
 446                         }
 447                         _ => false,
 448                     }
 449                 }).map(|(ref impl_arg, ref trait_arg)| {
 450                     (impl_arg.span, Some(trait_arg.span))
 451                 })
 452                 .unwrap_or_else(|| (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id)))
 453             } else {
 454                 (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id))
 455             }
 456         }
 457         TypeError::Sorts(ExpectedFound { .. }) => {
 458             if let Some(trait_m_hir_id) = tcx.hir().as_local_hir_id(trait_m.def_id) {
 459                 let (trait_m_output, trait_m_iter) =
 460                     match tcx.hir().expect_trait_item(trait_m_hir_id).node {
 461                         TraitItemKind::Method(ref trait_m_sig, _) => {
 462                             (&trait_m_sig.decl.output, trait_m_sig.decl.inputs.iter())
 463                         }
 464                         _ => bug!("{:?} is not a TraitItemKind::Method", trait_m),
 465                     };
 466
 467                 let impl_iter = impl_sig.inputs().iter();
 468                 let trait_iter = trait_sig.inputs().iter();
 469                 impl_iter.zip(trait_iter)
 470                          .zip(impl_m_iter)
 471                          .zip(trait_m_iter)
 472                          .filter_map(|(((&impl_arg_ty, &trait_arg_ty), impl_arg), trait_arg)|
 473                              match infcx.at(&cause, param_env).sub(trait_arg_ty, impl_arg_ty) {
 474                                  Ok(_) => None,
 475                                  Err(_) => Some((impl_arg.span, Some(trait_arg.span))),
 476                              }
 477                          )
 478                          .next()
 479                          .unwrap_or_else(||
 480                              if
 481                                  infcx.at(&cause, param_env)
 482                                       .sup(trait_sig.output(), impl_sig.output())
 483                                       .is_err()
 484                              {
 485                                  (impl_m_output.span(), Some(trait_m_output.span()))
 486                              } else {
 487                                  (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id))
 488                              }
 489                          )
 490             } else {
 491                 (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id))
 492             }
 493         }
 494         _ => (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id)),
 495     }
 496 }
 497
 498 fn compare_self_type<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 499                                impl_m: &ty::AssociatedItem,
 500                                impl_m_span: Span,
 501                                trait_m: &ty::AssociatedItem,
 502                                impl_trait_ref: ty::TraitRef<'tcx>)
 503                                -> Result<(), ErrorReported>
 504 {
 505     // Try to give more informative error messages about self typing
 506     // mismatches.  Note that any mismatch will also be detected
 507     // below, where we construct a canonical function type that
 508     // includes the self parameter as a normal parameter.  It's just
 509     // that the error messages you get out of this code are a bit more
 510     // inscrutable, particularly for cases where one method has no
 511     // self.
 512
 513     let self_string = |method: &ty::AssociatedItem| {
 514         let untransformed_self_ty = match method.container {
 515             ty::ImplContainer(_) => impl_trait_ref.self_ty(),
 516             ty::TraitContainer(_) => tcx.mk_self_type()
 517         };
 518         let self_arg_ty = *tcx.fn_sig(method.def_id).input(0).skip_binder();
 519         let param_env = ty::ParamEnv::reveal_all();
 520
 521         tcx.infer_ctxt().enter(|infcx| {
 522             let self_arg_ty = tcx.liberate_late_bound_regions(
 523                 method.def_id,
 524                 &ty::Binder::bind(self_arg_ty)
 525             );
 526             let can_eq_self = |ty| infcx.can_eq(param_env, untransformed_self_ty, ty).is_ok();
 527             match ExplicitSelf::determine(self_arg_ty, can_eq_self) {
 528                 ExplicitSelf::ByValue => "self".to_owned(),
 529                 ExplicitSelf::ByReference(_, hir::MutImmutable) => "&self".to_owned(),
 530                 ExplicitSelf::ByReference(_, hir::MutMutable) => "&mut self".to_owned(),
 531                 _ => format!("self: {}", self_arg_ty)
 532             }
 533         })
 534     };
 535
 536     match (trait_m.method_has_self_argument, impl_m.method_has_self_argument) {
 537         (false, false) | (true, true) => {}
 538
 539         (false, true) => {
 540             let self_descr = self_string(impl_m);
 541             let mut err = struct_span_err!(tcx.sess,
 542                                            impl_m_span,
 543                                            E0185,
 544                                            "method `{}` has a `{}` declaration in the impl, but \
 545                                             not in the trait",
 546                                            trait_m.ident,
 547                                            self_descr);
 548             err.span_label(impl_m_span, format!("`{}` used in impl", self_descr));
 549             if let Some(span) = tcx.hir().span_if_local(trait_m.def_id) {
 550                 err.span_label(span, format!("trait method declared without `{}`", self_descr));
 551             } else {
 552                 err.note_trait_signature(trait_m.ident.to_string(),
 553                                          trait_m.signature(tcx));
 554             }
 555             err.emit();
 556             return Err(ErrorReported);
 557         }
 558
 559         (true, false) => {
 560             let self_descr = self_string(trait_m);
 561             let mut err = struct_span_err!(tcx.sess,
 562                                            impl_m_span,
 563                                            E0186,
 564                                            "method `{}` has a `{}` declaration in the trait, but \
 565                                             not in the impl",
 566                                            trait_m.ident,
 567                                            self_descr);
 568             err.span_label(impl_m_span, format!("expected `{}` in impl", self_descr));
 569             if let Some(span) = tcx.hir().span_if_local(trait_m.def_id) {
 570                 err.span_label(span, format!("`{}` used in trait", self_descr));
 571             } else {
 572                 err.note_trait_signature(trait_m.ident.to_string(),
 573                                          trait_m.signature(tcx));
 574             }
 575             err.emit();
 576             return Err(ErrorReported);
 577         }
 578     }
 579
 580     Ok(())
 581 }
 582
 583 fn compare_number_of_generics<'a, 'tcx>(
 584     tcx: TyCtxt<'a, 'tcx, 'tcx>,
 585     impl_: &ty::AssociatedItem,
 586     impl_span: Span,
 587     trait_: &ty::AssociatedItem,
 588     trait_span: Option<Span>,
 589 ) -> Result<(), ErrorReported> {
 590     let trait_own_counts = tcx.generics_of(trait_.def_id).own_counts();
 591     let impl_own_counts = tcx.generics_of(impl_.def_id).own_counts();
 592
 593     let matchings = [
 594         ("type", trait_own_counts.types, impl_own_counts.types),
 595         ("const", trait_own_counts.consts, impl_own_counts.consts),
 596     ];
 597
 598     let mut err_occurred = false;
 599     for &(kind, trait_count, impl_count) in &matchings {
 600         if impl_count != trait_count {
 601             err_occurred = true;
 602
 603             let impl_hir_id = tcx.hir().as_local_hir_id(impl_.def_id).unwrap();
 604             let impl_item = tcx.hir().expect_impl_item(impl_hir_id);
 605             let span = if impl_item.generics.params.is_empty()
 606                 || impl_item.generics.span.is_dummy() { // argument position impl Trait (#55374)
 607                 impl_span
 608             } else {
 609                 impl_item.generics.span
 610             };
 611
 612             let mut err = tcx.sess.struct_span_err_with_code(
 613                 span,
 614                 &format!(
 615                     "method `{}` has {} {kind} parameter{} but its trait \
 616                      declaration has {} {kind} parameter{}",
 617                     trait_.ident,
 618                     impl_count,
 619                     if impl_count != 1 { "s" } else { "" },
 620                     trait_count,
 621                     if trait_count != 1 { "s" } else { "" },
 622                     kind = kind,
 623                 ),
 624                 DiagnosticId::Error("E0049".into()),
 625             );
 626
 627             let mut suffix = None;
 628
 629             if let Some(span) = trait_span {
 630                 err.span_label(
 631                     span,
 632                     format!("expected {} {} parameter{}", trait_count, kind,
 633                         if trait_count != 1 { "s" } else { "" })
 634                 );
 635             } else {
 636                 suffix = Some(format!(", expected {}", trait_count));
 637             }
 638
 639             err.span_label(
 640                 span,
 641                 format!("found {} {} parameter{}{}", impl_count, kind,
 642                     if impl_count != 1 { "s" } else { "" },
 643                     suffix.unwrap_or_else(|| String::new())),
 644             );
 645
 646             err.emit();
 647         }
 648     }
 649
 650     if err_occurred {
 651         Err(ErrorReported)
 652     } else {
 653         Ok(())
 654     }
 655 }
 656
 657 fn compare_number_of_method_arguments<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 658                                                 impl_m: &ty::AssociatedItem,
 659                                                 impl_m_span: Span,
 660                                                 trait_m: &ty::AssociatedItem,
 661                                                 trait_item_span: Option<Span>)
 662                                                 -> Result<(), ErrorReported> {
 663     let impl_m_fty = tcx.fn_sig(impl_m.def_id);
 664     let trait_m_fty = tcx.fn_sig(trait_m.def_id);
 665     let trait_number_args = trait_m_fty.inputs().skip_binder().len();
 666     let impl_number_args = impl_m_fty.inputs().skip_binder().len();
 667     if trait_number_args != impl_number_args {
 668         let trait_m_hir_id = tcx.hir().as_local_hir_id(trait_m.def_id);
 669         let trait_span = if let Some(trait_id) = trait_m_hir_id {
 670             match tcx.hir().expect_trait_item(trait_id).node {
 671                 TraitItemKind::Method(ref trait_m_sig, _) => {
 672                     let pos = if trait_number_args > 0 {
 673                         trait_number_args - 1
 674                     } else {
 675                         0
 676                     };
 677                     if let Some(arg) = trait_m_sig.decl.inputs.get(pos) {
 678                         Some(if pos == 0 {
 679                             arg.span
 680                         } else {
 681                             Span::new(trait_m_sig.decl.inputs[0].span.lo(),
 682                                       arg.span.hi(),
 683                                       arg.span.ctxt())
 684                         })
 685                     } else {
 686                         trait_item_span
 687                     }
 688                 }
 689                 _ => bug!("{:?} is not a method", impl_m),
 690             }
 691         } else {
 692             trait_item_span
 693         };
 694         let impl_m_hir_id = tcx.hir().as_local_hir_id(impl_m.def_id).unwrap();
 695         let impl_span = match tcx.hir().expect_impl_item(impl_m_hir_id).node {
 696             ImplItemKind::Method(ref impl_m_sig, _) => {
 697                 let pos = if impl_number_args > 0 {
 698                     impl_number_args - 1
 699                 } else {
 700                     0
 701                 };
 702                 if let Some(arg) = impl_m_sig.decl.inputs.get(pos) {
 703                     if pos == 0 {
 704                         arg.span
 705                     } else {
 706                         Span::new(impl_m_sig.decl.inputs[0].span.lo(),
 707                                   arg.span.hi(),
 708                                   arg.span.ctxt())
 709                     }
 710                 } else {
 711                     impl_m_span
 712                 }
 713             }
 714             _ => bug!("{:?} is not a method", impl_m),
 715         };
 716         let mut err = struct_span_err!(tcx.sess,
 717                                        impl_span,
 718                                        E0050,
 719                                        "method `{}` has {} but the declaration in \
 720                                         trait `{}` has {}",
 721                                        trait_m.ident,
 722                                        potentially_plural_count(impl_number_args, "parameter"),
 723                                        tcx.def_path_str(trait_m.def_id),
 724                                        trait_number_args);
 725         if let Some(trait_span) = trait_span {
 726             err.span_label(trait_span, format!("trait requires {}",
 727                 potentially_plural_count(trait_number_args, "parameter")));
 728         } else {
 729             err.note_trait_signature(trait_m.ident.to_string(),
 730                                      trait_m.signature(tcx));
 731         }
 732         err.span_label(impl_span, format!("expected {}, found {}",
 733             potentially_plural_count(trait_number_args, "parameter"), impl_number_args));
 734         err.emit();
 735         return Err(ErrorReported);
 736     }
 737
 738     Ok(())
 739 }
 740
 741 fn compare_synthetic_generics<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 742                                         impl_m: &ty::AssociatedItem,
 743                                         trait_m: &ty::AssociatedItem)
 744                                         -> Result<(), ErrorReported> {
 745     // FIXME(chrisvittal) Clean up this function, list of FIXME items:
 746     //     1. Better messages for the span labels
 747     //     2. Explanation as to what is going on
 748     // If we get here, we already have the same number of generics, so the zip will
 749     // be okay.
 750     let mut error_found = false;
 751     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 752     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 753     let impl_m_type_params = impl_m_generics.params.iter().filter_map(|param| match param.kind {
 754         GenericParamDefKind::Type { synthetic, .. } => Some((param.def_id, synthetic)),
 755         GenericParamDefKind::Lifetime | GenericParamDefKind::Const => None,
 756     });
 757     let trait_m_type_params = trait_m_generics.params.iter().filter_map(|param| {
 758         match param.kind {
 759             GenericParamDefKind::Type { synthetic, .. } => Some((param.def_id, synthetic)),
 760             GenericParamDefKind::Lifetime | GenericParamDefKind::Const => None,
 761         }
 762     });
 763     for ((impl_def_id, impl_synthetic), (trait_def_id, trait_synthetic))
 764         in impl_m_type_params.zip(trait_m_type_params)
 765     {
 766         if impl_synthetic != trait_synthetic {
 767             let impl_hir_id = tcx.hir().as_local_hir_id(impl_def_id).unwrap();
 768             let impl_span = tcx.hir().span_by_hir_id(impl_hir_id);
 769             let trait_span = tcx.def_span(trait_def_id);
 770             let mut err = struct_span_err!(tcx.sess,
 771                                            impl_span,
 772                                            E0643,
 773                                            "method `{}` has incompatible signature for trait",
 774                                            trait_m.ident);
 775             err.span_label(trait_span, "declaration in trait here");
 776             match (impl_synthetic, trait_synthetic) {
 777                 // The case where the impl method uses `impl Trait` but the trait method uses
 778                 // explicit generics
 779                 (Some(hir::SyntheticTyParamKind::ImplTrait), None) => {
 780                     err.span_label(impl_span, "expected generic parameter, found `impl Trait`");
 781                     (|| {
 782                         // try taking the name from the trait impl
 783                         // FIXME: this is obviously suboptimal since the name can already be used
 784                         // as another generic argument
 785                         let new_name = tcx
 786                             .sess
 787                             .source_map()
 788                             .span_to_snippet(trait_span)
 789                             .ok()?;
 790                         let trait_m = tcx.hir().as_local_hir_id(trait_m.def_id)?;
 791                         let trait_m = tcx.hir().trait_item(hir::TraitItemId { hir_id: trait_m });
 792
 793                         let impl_m = tcx.hir().as_local_hir_id(impl_m.def_id)?;
 794                         let impl_m = tcx.hir().impl_item(hir::ImplItemId { hir_id: impl_m });
 795
 796                         // in case there are no generics, take the spot between the function name
 797                         // and the opening paren of the argument list
 798                         let new_generics_span = tcx
 799                             .sess
 800                             .source_map()
 801                             .generate_fn_name_span(impl_span)?
 802                             .shrink_to_hi();
 803                         // in case there are generics, just replace them
 804                         let generics_span = impl_m
 805                             .generics
 806                             .span
 807                             .substitute_dummy(new_generics_span);
 808                         // replace with the generics from the trait
 809                         let new_generics = tcx
 810                             .sess
 811                             .source_map()
 812                             .span_to_snippet(trait_m.generics.span)
 813                             .ok()?;
 814
 815                         err.multipart_suggestion(
 816                             "try changing the `impl Trait` argument to a generic parameter",
 817                             vec![
 818                                 // replace `impl Trait` with `T`
 819                                 (impl_span, new_name),
 820                                 // replace impl method generics with trait method generics
 821                                 // This isn't quite right, as users might have changed the names
 822                                 // of the generics, but it works for the common case
 823                                 (generics_span, new_generics),
 824                             ],
 825                             Applicability::MaybeIncorrect,
 826                         );
 827                         Some(())
 828                     })();
 829                 },
 830                 // The case where the trait method uses `impl Trait`, but the impl method uses
 831                 // explicit generics.
 832                 (None, Some(hir::SyntheticTyParamKind::ImplTrait)) => {
 833                     err.span_label(impl_span, "expected `impl Trait`, found generic parameter");
 834                     (|| {
 835                         let impl_m = tcx.hir().as_local_hir_id(impl_m.def_id)?;
 836                         let impl_m = tcx.hir().impl_item(hir::ImplItemId { hir_id: impl_m });
 837                         let input_tys = match impl_m.node {
 838                             hir::ImplItemKind::Method(ref sig, _) => &sig.decl.inputs,
 839                             _ => unreachable!(),
 840                         };
 841                         struct Visitor(Option<Span>, hir::def_id::DefId);
 842                         impl<'v> hir::intravisit::Visitor<'v> for Visitor {
 843                             fn visit_ty(&mut self, ty: &'v hir::Ty) {
 844                                 hir::intravisit::walk_ty(self, ty);
 845                                 if let hir::TyKind::Path(
 846                                     hir::QPath::Resolved(None, ref path)) = ty.node
 847                                 {
 848                                     if let Res::Def(DefKind::TyParam, def_id) = path.res {
 849                                         if def_id == self.1 {
 850                                             self.0 = Some(ty.span);
 851                                         }
 852                                     }
 853                                 }
 854                             }
 855                             fn nested_visit_map<'this>(
 856                                 &'this mut self
 857                             ) -> hir::intravisit::NestedVisitorMap<'this, 'v> {
 858                                 hir::intravisit::NestedVisitorMap::None
 859                             }
 860                         }
 861                         let mut visitor = Visitor(None, impl_def_id);
 862                         for ty in input_tys {
 863                             hir::intravisit::Visitor::visit_ty(&mut visitor, ty);
 864                         }
 865                         let span = visitor.0?;
 866
 867                         let bounds = impl_m.generics.params.iter().find_map(|param| {
 868                             match param.kind {
 869                                 GenericParamKind::Lifetime { .. } => None,
 870                                 GenericParamKind::Type { .. } |
 871                                 GenericParamKind::Const { .. } => {
 872                                     if param.hir_id == impl_hir_id {
 873                                         Some(&param.bounds)
 874                                     } else {
 875                                         None
 876                                     }
 877                                 }
 878                             }
 879                         })?;
 880                         let bounds = bounds.first()?.span().to(bounds.last()?.span());
 881                         let bounds = tcx
 882                             .sess
 883                             .source_map()
 884                             .span_to_snippet(bounds)
 885                             .ok()?;
 886
 887                         err.multipart_suggestion(
 888                             "try removing the generic parameter and using `impl Trait` instead",
 889                             vec![
 890                                 // delete generic parameters
 891                                 (impl_m.generics.span, String::new()),
 892                                 // replace param usage with `impl Trait`
 893                                 (span, format!("impl {}", bounds)),
 894                             ],
 895                             Applicability::MaybeIncorrect,
 896                         );
 897                         Some(())
 898                     })();
 899                 },
 900                 _ => unreachable!(),
 901             }
 902             err.emit();
 903             error_found = true;
 904         }
 905     }
 906     if error_found {
 907         Err(ErrorReported)
 908     } else {
 909         Ok(())
 910     }
 911 }
 912
 913 pub fn compare_const_impl<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 914                                     impl_c: &ty::AssociatedItem,
 915                                     impl_c_span: Span,
 916                                     trait_c: &ty::AssociatedItem,
 917                                     impl_trait_ref: ty::TraitRef<'tcx>) {
 918     debug!("compare_const_impl(impl_trait_ref={:?})", impl_trait_ref);
 919
 920     tcx.infer_ctxt().enter(|infcx| {
 921         let param_env = tcx.param_env(impl_c.def_id);
 922         let inh = Inherited::new(infcx, impl_c.def_id);
 923         let infcx = &inh.infcx;
 924
 925         // The below is for the most part highly similar to the procedure
 926         // for methods above. It is simpler in many respects, especially
 927         // because we shouldn't really have to deal with lifetimes or
 928         // predicates. In fact some of this should probably be put into
 929         // shared functions because of DRY violations...
 930         let trait_to_impl_substs = impl_trait_ref.substs;
 931
 932         // Create a parameter environment that represents the implementation's
 933         // method.
 934         let impl_c_hir_id = tcx.hir().as_local_hir_id(impl_c.def_id).unwrap();
 935
 936         // Compute placeholder form of impl and trait const tys.
 937         let impl_ty = tcx.type_of(impl_c.def_id);
 938         let trait_ty = tcx.type_of(trait_c.def_id).subst(tcx, trait_to_impl_substs);
 939         let mut cause = ObligationCause::misc(impl_c_span, impl_c_hir_id);
 940
 941         // There is no "body" here, so just pass dummy id.
 942         let impl_ty = inh.normalize_associated_types_in(impl_c_span,
 943                                                         impl_c_hir_id,
 944                                                         param_env,
 945                                                         &impl_ty);
 946
 947         debug!("compare_const_impl: impl_ty={:?}", impl_ty);
 948
 949         let trait_ty = inh.normalize_associated_types_in(impl_c_span,
 950                                                          impl_c_hir_id,
 951                                                          param_env,
 952                                                          &trait_ty);
 953
 954         debug!("compare_const_impl: trait_ty={:?}", trait_ty);
 955
 956         let err = infcx.at(&cause, param_env)
 957                        .sup(trait_ty, impl_ty)
 958                        .map(|ok| inh.register_infer_ok_obligations(ok));
 959
 960         if let Err(terr) = err {
 961             debug!("checking associated const for compatibility: impl ty {:?}, trait ty {:?}",
 962                    impl_ty,
 963                    trait_ty);
 964
 965             // Locate the Span containing just the type of the offending impl
 966             match tcx.hir().expect_impl_item(impl_c_hir_id).node {
 967                 ImplItemKind::Const(ref ty, _) => cause.span = ty.span,
 968                 _ => bug!("{:?} is not a impl const", impl_c),
 969             }
 970
 971             let mut diag = struct_span_err!(tcx.sess,
 972                                             cause.span,
 973                                             E0326,
 974                                             "implemented const `{}` has an incompatible type for \
 975                                              trait",
 976                                             trait_c.ident);
 977
 978             let trait_c_hir_id = tcx.hir().as_local_hir_id(trait_c.def_id);
 979             let trait_c_span = trait_c_hir_id.map(|trait_c_hir_id| {
 980                 // Add a label to the Span containing just the type of the const
 981                 match tcx.hir().expect_trait_item(trait_c_hir_id).node {
 982                     TraitItemKind::Const(ref ty, _) => ty.span,
 983                     _ => bug!("{:?} is not a trait const", trait_c),
 984                 }
 985             });
 986
 987             infcx.note_type_err(&mut diag,
 988                                 &cause,
 989                                 trait_c_span.map(|span| (span, "type in trait".to_owned())),
 990                                 Some(infer::ValuePairs::Types(ExpectedFound {
 991                                     expected: trait_ty,
 992                                     found: impl_ty,
 993                                 })),
 994                                 &terr);
 995             diag.emit();
 996         }
 997
 998         // Check that all obligations are satisfied by the implementation's
 999         // version.
1000         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
1001             infcx.report_fulfillment_errors(errors, None, false);
1002             return;
1003         }
1004
1005         let fcx = FnCtxt::new(&inh, param_env, impl_c_hir_id);
1006         fcx.regionck_item(impl_c_hir_id, impl_c_span, &[]);
1007     });
1008 }