compiler/rustc_typeck/src/check/compare_method.rs

   1 use crate::errors::LifetimesOrBoundsMismatchOnTrait;
   2 use rustc_errors::{pluralize, struct_span_err, Applicability, DiagnosticId, ErrorReported};
   3 use rustc_hir as hir;
   4 use rustc_hir::def::{DefKind, Res};
   5 use rustc_hir::intravisit;
   6 use rustc_hir::{GenericParamKind, ImplItemKind, TraitItemKind};
   7 use rustc_infer::infer::{self, InferOk, TyCtxtInferExt};
   8 use rustc_infer::traits::util;
   9 use rustc_middle::ty;
  10 use rustc_middle::ty::error::{ExpectedFound, TypeError};
  11 use rustc_middle::ty::subst::{InternalSubsts, Subst};
  12 use rustc_middle::ty::util::ExplicitSelf;
  13 use rustc_middle::ty::{GenericParamDefKind, ToPredicate, TyCtxt};
  14 use rustc_span::Span;
  15 use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
  16 use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode, Reveal};
  17
  18 use super::{potentially_plural_count, FnCtxt, Inherited};
  19
  20 /// Checks that a method from an impl conforms to the signature of
  21 /// the same method as declared in the trait.
  22 ///
  23 /// # Parameters
  24 ///
  25 /// - `impl_m`: type of the method we are checking
  26 /// - `impl_m_span`: span to use for reporting errors
  27 /// - `trait_m`: the method in the trait
  28 /// - `impl_trait_ref`: the TraitRef corresponding to the trait implementation
  29
  30 crate fn compare_impl_method<'tcx>(
  31     tcx: TyCtxt<'tcx>,
  32     impl_m: &ty::AssocItem,
  33     impl_m_span: Span,
  34     trait_m: &ty::AssocItem,
  35     impl_trait_ref: ty::TraitRef<'tcx>,
  36     trait_item_span: Option<Span>,
  37 ) {
  38     debug!("compare_impl_method(impl_trait_ref={:?})", impl_trait_ref);
  39
  40     let impl_m_span = tcx.sess.source_map().guess_head_span(impl_m_span);
  41
  42     if let Err(ErrorReported) = compare_self_type(tcx, impl_m, impl_m_span, trait_m, impl_trait_ref)
  43     {
  44         return;
  45     }
  46
  47     if let Err(ErrorReported) =
  48         compare_number_of_generics(tcx, impl_m, impl_m_span, trait_m, trait_item_span)
  49     {
  50         return;
  51     }
  52
  53     if let Err(ErrorReported) =
  54         compare_number_of_method_arguments(tcx, impl_m, impl_m_span, trait_m, trait_item_span)
  55     {
  56         return;
  57     }
  58
  59     if let Err(ErrorReported) = compare_synthetic_generics(tcx, impl_m, trait_m) {
  60         return;
  61     }
  62
  63     if let Err(ErrorReported) =
  64         compare_predicate_entailment(tcx, impl_m, impl_m_span, trait_m, impl_trait_ref)
  65     {
  66         return;
  67     }
  68 }
  69
  70 fn compare_predicate_entailment<'tcx>(
  71     tcx: TyCtxt<'tcx>,
  72     impl_m: &ty::AssocItem,
  73     impl_m_span: Span,
  74     trait_m: &ty::AssocItem,
  75     impl_trait_ref: ty::TraitRef<'tcx>,
  76 ) -> Result<(), ErrorReported> {
  77     let trait_to_impl_substs = impl_trait_ref.substs;
  78
  79     // This node-id should be used for the `body_id` field on each
  80     // `ObligationCause` (and the `FnCtxt`). This is what
  81     // `regionck_item` expects.
  82     let impl_m_hir_id = tcx.hir().local_def_id_to_hir_id(impl_m.def_id.expect_local());
  83
  84     // We sometimes modify the span further down.
  85     let mut cause = ObligationCause::new(
  86         impl_m_span,
  87         impl_m_hir_id,
  88         ObligationCauseCode::CompareImplMethodObligation {
  89             item_name: impl_m.ident.name,
  90             impl_item_def_id: impl_m.def_id,
  91             trait_item_def_id: trait_m.def_id,
  92         },
  93     );
  94
  95     // This code is best explained by example. Consider a trait:
  96     //
  97     //     trait Trait<'t, T> {
  98     //         fn method<'a, M>(t: &'t T, m: &'a M) -> Self;
  99     //     }
 100     //
 101     // And an impl:
 102     //
 103     //     impl<'i, 'j, U> Trait<'j, &'i U> for Foo {
 104     //          fn method<'b, N>(t: &'j &'i U, m: &'b N) -> Foo;
 105     //     }
 106     //
 107     // We wish to decide if those two method types are compatible.
 108     //
 109     // We start out with trait_to_impl_substs, that maps the trait
 110     // type parameters to impl type parameters. This is taken from the
 111     // impl trait reference:
 112     //
 113     //     trait_to_impl_substs = {'t => 'j, T => &'i U, Self => Foo}
 114     //
 115     // We create a mapping `dummy_substs` that maps from the impl type
 116     // parameters to fresh types and regions. For type parameters,
 117     // this is the identity transform, but we could as well use any
 118     // placeholder types. For regions, we convert from bound to free
 119     // regions (Note: but only early-bound regions, i.e., those
 120     // declared on the impl or used in type parameter bounds).
 121     //
 122     //     impl_to_placeholder_substs = {'i => 'i0, U => U0, N => N0 }
 123     //
 124     // Now we can apply placeholder_substs to the type of the impl method
 125     // to yield a new function type in terms of our fresh, placeholder
 126     // types:
 127     //
 128     //     <'b> fn(t: &'i0 U0, m: &'b) -> Foo
 129     //
 130     // We now want to extract and substitute the type of the *trait*
 131     // method and compare it. To do so, we must create a compound
 132     // substitution by combining trait_to_impl_substs and
 133     // impl_to_placeholder_substs, and also adding a mapping for the method
 134     // type parameters. We extend the mapping to also include
 135     // the method parameters.
 136     //
 137     //     trait_to_placeholder_substs = { T => &'i0 U0, Self => Foo, M => N0 }
 138     //
 139     // Applying this to the trait method type yields:
 140     //
 141     //     <'a> fn(t: &'i0 U0, m: &'a) -> Foo
 142     //
 143     // This type is also the same but the name of the bound region ('a
 144     // vs 'b).  However, the normal subtyping rules on fn types handle
 145     // this kind of equivalency just fine.
 146     //
 147     // We now use these substitutions to ensure that all declared bounds are
 148     // satisfied by the implementation's method.
 149     //
 150     // We do this by creating a parameter environment which contains a
 151     // substitution corresponding to impl_to_placeholder_substs. We then build
 152     // trait_to_placeholder_substs and use it to convert the predicates contained
 153     // in the trait_m.generics to the placeholder form.
 154     //
 155     // Finally we register each of these predicates as an obligation in
 156     // a fresh FulfillmentCtxt, and invoke select_all_or_error.
 157
 158     // Create mapping from impl to placeholder.
 159     let impl_to_placeholder_substs = InternalSubsts::identity_for_item(tcx, impl_m.def_id);
 160
 161     // Create mapping from trait to placeholder.
 162     let trait_to_placeholder_substs =
 163         impl_to_placeholder_substs.rebase_onto(tcx, impl_m.container.id(), trait_to_impl_substs);
 164     debug!("compare_impl_method: trait_to_placeholder_substs={:?}", trait_to_placeholder_substs);
 165
 166     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 167     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 168     let impl_m_predicates = tcx.predicates_of(impl_m.def_id);
 169     let trait_m_predicates = tcx.predicates_of(trait_m.def_id);
 170
 171     // Check region bounds.
 172     check_region_bounds_on_impl_item(
 173         tcx,
 174         impl_m_span,
 175         impl_m,
 176         trait_m,
 177         &trait_m_generics,
 178         &impl_m_generics,
 179     )?;
 180
 181     // Create obligations for each predicate declared by the impl
 182     // definition in the context of the trait's parameter
 183     // environment. We can't just use `impl_env.caller_bounds`,
 184     // however, because we want to replace all late-bound regions with
 185     // region variables.
 186     let impl_predicates = tcx.predicates_of(impl_m_predicates.parent.unwrap());
 187     let mut hybrid_preds = impl_predicates.instantiate_identity(tcx);
 188
 189     debug!("compare_impl_method: impl_bounds={:?}", hybrid_preds);
 190
 191     // This is the only tricky bit of the new way we check implementation methods
 192     // We need to build a set of predicates where only the method-level bounds
 193     // are from the trait and we assume all other bounds from the implementation
 194     // to be previously satisfied.
 195     //
 196     // We then register the obligations from the impl_m and check to see
 197     // if all constraints hold.
 198     hybrid_preds
 199         .predicates
 200         .extend(trait_m_predicates.instantiate_own(tcx, trait_to_placeholder_substs).predicates);
 201
 202     // Construct trait parameter environment and then shift it into the placeholder viewpoint.
 203     // The key step here is to update the caller_bounds's predicates to be
 204     // the new hybrid bounds we computed.
 205     let normalize_cause = traits::ObligationCause::misc(impl_m_span, impl_m_hir_id);
 206     let param_env =
 207         ty::ParamEnv::new(tcx.intern_predicates(&hybrid_preds.predicates), Reveal::UserFacing);
 208     let param_env = traits::normalize_param_env_or_error(
 209         tcx,
 210         impl_m.def_id,
 211         param_env,
 212         normalize_cause.clone(),
 213     );
 214
 215     tcx.infer_ctxt().enter(|infcx| {
 216         let inh = Inherited::new(infcx, impl_m.def_id.expect_local());
 217         let infcx = &inh.infcx;
 218
 219         debug!("compare_impl_method: caller_bounds={:?}", param_env.caller_bounds());
 220
 221         let mut selcx = traits::SelectionContext::new(&infcx);
 222
 223         let impl_m_own_bounds = impl_m_predicates.instantiate_own(tcx, impl_to_placeholder_substs);
 224         let (impl_m_own_bounds, _) = infcx.replace_bound_vars_with_fresh_vars(
 225             impl_m_span,
 226             infer::HigherRankedType,
 227             ty::Binder::bind(impl_m_own_bounds.predicates),
 228         );
 229         for predicate in impl_m_own_bounds {
 230             let traits::Normalized { value: predicate, obligations } =
 231                 traits::normalize(&mut selcx, param_env, normalize_cause.clone(), predicate);
 232
 233             inh.register_predicates(obligations);
 234             inh.register_predicate(traits::Obligation::new(cause.clone(), param_env, predicate));
 235         }
 236
 237         // We now need to check that the signature of the impl method is
 238         // compatible with that of the trait method. We do this by
 239         // checking that `impl_fty <: trait_fty`.
 240         //
 241         // FIXME. Unfortunately, this doesn't quite work right now because
 242         // associated type normalization is not integrated into subtype
 243         // checks. For the comparison to be valid, we need to
 244         // normalize the associated types in the impl/trait methods
 245         // first. However, because function types bind regions, just
 246         // calling `normalize_associated_types_in` would have no effect on
 247         // any associated types appearing in the fn arguments or return
 248         // type.
 249
 250         // Compute placeholder form of impl and trait method tys.
 251         let tcx = infcx.tcx;
 252
 253         let (impl_sig, _) = infcx.replace_bound_vars_with_fresh_vars(
 254             impl_m_span,
 255             infer::HigherRankedType,
 256             tcx.fn_sig(impl_m.def_id),
 257         );
 258         let impl_sig =
 259             inh.normalize_associated_types_in(impl_m_span, impl_m_hir_id, param_env, impl_sig);
 260         let impl_fty = tcx.mk_fn_ptr(ty::Binder::bind(impl_sig));
 261         debug!("compare_impl_method: impl_fty={:?}", impl_fty);
 262
 263         let trait_sig = tcx.liberate_late_bound_regions(impl_m.def_id, tcx.fn_sig(trait_m.def_id));
 264         let trait_sig = trait_sig.subst(tcx, trait_to_placeholder_substs);
 265         let trait_sig =
 266             inh.normalize_associated_types_in(impl_m_span, impl_m_hir_id, param_env, trait_sig);
 267         let trait_fty = tcx.mk_fn_ptr(ty::Binder::bind(trait_sig));
 268
 269         debug!("compare_impl_method: trait_fty={:?}", trait_fty);
 270
 271         let sub_result = infcx.at(&cause, param_env).sup(trait_fty, impl_fty).map(
 272             |InferOk { obligations, .. }| {
 273                 inh.register_predicates(obligations);
 274             },
 275         );
 276
 277         if let Err(terr) = sub_result {
 278             debug!("sub_types failed: impl ty {:?}, trait ty {:?}", impl_fty, trait_fty);
 279
 280             let (impl_err_span, trait_err_span) = extract_spans_for_error_reporting(
 281                 &infcx, param_env, &terr, &cause, impl_m, impl_sig, trait_m, trait_sig,
 282             );
 283
 284             cause.make_mut().span = impl_err_span;
 285
 286             let mut diag = struct_span_err!(
 287                 tcx.sess,
 288                 cause.span(tcx),
 289                 E0053,
 290                 "method `{}` has an incompatible type for trait",
 291                 trait_m.ident
 292             );
 293             if let TypeError::Mutability = terr {
 294                 if let Some(trait_err_span) = trait_err_span {
 295                     if let Ok(trait_err_str) = tcx.sess.source_map().span_to_snippet(trait_err_span)
 296                     {
 297                         diag.span_suggestion(
 298                             impl_err_span,
 299                             "consider changing the mutability to match the trait",
 300                             trait_err_str,
 301                             Applicability::MachineApplicable,
 302                         );
 303                     }
 304                 }
 305             }
 306
 307             infcx.note_type_err(
 308                 &mut diag,
 309                 &cause,
 310                 trait_err_span.map(|sp| (sp, "type in trait".to_owned())),
 311                 Some(infer::ValuePairs::Types(ExpectedFound {
 312                     expected: trait_fty,
 313                     found: impl_fty,
 314                 })),
 315                 &terr,
 316             );
 317             diag.emit();
 318             return Err(ErrorReported);
 319         }
 320
 321         // Check that all obligations are satisfied by the implementation's
 322         // version.
 323         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
 324             infcx.report_fulfillment_errors(errors, None, false);
 325             return Err(ErrorReported);
 326         }
 327
 328         // Finally, resolve all regions. This catches wily misuses of
 329         // lifetime parameters.
 330         let fcx = FnCtxt::new(&inh, param_env, impl_m_hir_id);
 331         fcx.regionck_item(impl_m_hir_id, impl_m_span, trait_sig.inputs_and_output);
 332
 333         Ok(())
 334     })
 335 }
 336
 337 fn check_region_bounds_on_impl_item<'tcx>(
 338     tcx: TyCtxt<'tcx>,
 339     span: Span,
 340     impl_m: &ty::AssocItem,
 341     trait_m: &ty::AssocItem,
 342     trait_generics: &ty::Generics,
 343     impl_generics: &ty::Generics,
 344 ) -> Result<(), ErrorReported> {
 345     let trait_params = trait_generics.own_counts().lifetimes;
 346     let impl_params = impl_generics.own_counts().lifetimes;
 347
 348     debug!(
 349         "check_region_bounds_on_impl_item: \
 350             trait_generics={:?} \
 351             impl_generics={:?}",
 352         trait_generics, impl_generics
 353     );
 354
 355     // Must have same number of early-bound lifetime parameters.
 356     // Unfortunately, if the user screws up the bounds, then this
 357     // will change classification between early and late.  E.g.,
 358     // if in trait we have `<'a,'b:'a>`, and in impl we just have
 359     // `<'a,'b>`, then we have 2 early-bound lifetime parameters
 360     // in trait but 0 in the impl. But if we report "expected 2
 361     // but found 0" it's confusing, because it looks like there
 362     // are zero. Since I don't quite know how to phrase things at
 363     // the moment, give a kind of vague error message.
 364     if trait_params != impl_params {
 365         let item_kind = assoc_item_kind_str(impl_m);
 366         let def_span = tcx.sess.source_map().guess_head_span(span);
 367         let span = tcx.hir().get_generics(impl_m.def_id).map(|g| g.span).unwrap_or(def_span);
 368         let generics_span = if let Some(sp) = tcx.hir().span_if_local(trait_m.def_id) {
 369             let def_sp = tcx.sess.source_map().guess_head_span(sp);
 370             Some(tcx.hir().get_generics(trait_m.def_id).map(|g| g.span).unwrap_or(def_sp))
 371         } else {
 372             None
 373         };
 374         tcx.sess.emit_err(LifetimesOrBoundsMismatchOnTrait {
 375             span,
 376             item_kind,
 377             ident: impl_m.ident,
 378             generics_span,
 379         });
 380         return Err(ErrorReported);
 381     }
 382
 383     Ok(())
 384 }
 385
 386 fn extract_spans_for_error_reporting<'a, 'tcx>(
 387     infcx: &infer::InferCtxt<'a, 'tcx>,
 388     param_env: ty::ParamEnv<'tcx>,
 389     terr: &TypeError<'_>,
 390     cause: &ObligationCause<'tcx>,
 391     impl_m: &ty::AssocItem,
 392     impl_sig: ty::FnSig<'tcx>,
 393     trait_m: &ty::AssocItem,
 394     trait_sig: ty::FnSig<'tcx>,
 395 ) -> (Span, Option<Span>) {
 396     let tcx = infcx.tcx;
 397     let impl_m_hir_id = tcx.hir().local_def_id_to_hir_id(impl_m.def_id.expect_local());
 398     let (impl_m_output, impl_m_iter) = match tcx.hir().expect_impl_item(impl_m_hir_id).kind {
 399         ImplItemKind::Fn(ref impl_m_sig, _) => {
 400             (&impl_m_sig.decl.output, impl_m_sig.decl.inputs.iter())
 401         }
 402         _ => bug!("{:?} is not a method", impl_m),
 403     };
 404
 405     match *terr {
 406         TypeError::Mutability => {
 407             if let Some(def_id) = trait_m.def_id.as_local() {
 408                 let trait_m_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 409                 let trait_m_iter = match tcx.hir().expect_trait_item(trait_m_hir_id).kind {
 410                     TraitItemKind::Fn(ref trait_m_sig, _) => trait_m_sig.decl.inputs.iter(),
 411                     _ => bug!("{:?} is not a TraitItemKind::Fn", trait_m),
 412                 };
 413
 414                 impl_m_iter
 415                     .zip(trait_m_iter)
 416                     .find(|&(ref impl_arg, ref trait_arg)| {
 417                         match (&impl_arg.kind, &trait_arg.kind) {
 418                             (
 419                                 &hir::TyKind::Rptr(_, ref impl_mt),
 420                                 &hir::TyKind::Rptr(_, ref trait_mt),
 421                             )
 422                             | (&hir::TyKind::Ptr(ref impl_mt), &hir::TyKind::Ptr(ref trait_mt)) => {
 423                                 impl_mt.mutbl != trait_mt.mutbl
 424                             }
 425                             _ => false,
 426                         }
 427                     })
 428                     .map(|(ref impl_arg, ref trait_arg)| (impl_arg.span, Some(trait_arg.span)))
 429                     .unwrap_or_else(|| (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id)))
 430             } else {
 431                 (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id))
 432             }
 433         }
 434         TypeError::Sorts(ExpectedFound { .. }) => {
 435             if let Some(def_id) = trait_m.def_id.as_local() {
 436                 let trait_m_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 437                 let (trait_m_output, trait_m_iter) =
 438                     match tcx.hir().expect_trait_item(trait_m_hir_id).kind {
 439                         TraitItemKind::Fn(ref trait_m_sig, _) => {
 440                             (&trait_m_sig.decl.output, trait_m_sig.decl.inputs.iter())
 441                         }
 442                         _ => bug!("{:?} is not a TraitItemKind::Fn", trait_m),
 443                     };
 444
 445                 let impl_iter = impl_sig.inputs().iter();
 446                 let trait_iter = trait_sig.inputs().iter();
 447                 impl_iter
 448                     .zip(trait_iter)
 449                     .zip(impl_m_iter)
 450                     .zip(trait_m_iter)
 451                     .find_map(|(((&impl_arg_ty, &trait_arg_ty), impl_arg), trait_arg)| match infcx
 452                         .at(&cause, param_env)
 453                         .sub(trait_arg_ty, impl_arg_ty)
 454                     {
 455                         Ok(_) => None,
 456                         Err(_) => Some((impl_arg.span, Some(trait_arg.span))),
 457                     })
 458                     .unwrap_or_else(|| {
 459                         if infcx
 460                             .at(&cause, param_env)
 461                             .sup(trait_sig.output(), impl_sig.output())
 462                             .is_err()
 463                         {
 464                             (impl_m_output.span(), Some(trait_m_output.span()))
 465                         } else {
 466                             (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id))
 467                         }
 468                     })
 469             } else {
 470                 (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id))
 471             }
 472         }
 473         _ => (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id)),
 474     }
 475 }
 476
 477 fn compare_self_type<'tcx>(
 478     tcx: TyCtxt<'tcx>,
 479     impl_m: &ty::AssocItem,
 480     impl_m_span: Span,
 481     trait_m: &ty::AssocItem,
 482     impl_trait_ref: ty::TraitRef<'tcx>,
 483 ) -> Result<(), ErrorReported> {
 484     // Try to give more informative error messages about self typing
 485     // mismatches.  Note that any mismatch will also be detected
 486     // below, where we construct a canonical function type that
 487     // includes the self parameter as a normal parameter.  It's just
 488     // that the error messages you get out of this code are a bit more
 489     // inscrutable, particularly for cases where one method has no
 490     // self.
 491
 492     let self_string = |method: &ty::AssocItem| {
 493         let untransformed_self_ty = match method.container {
 494             ty::ImplContainer(_) => impl_trait_ref.self_ty(),
 495             ty::TraitContainer(_) => tcx.types.self_param,
 496         };
 497         let self_arg_ty = tcx.fn_sig(method.def_id).input(0);
 498         let param_env = ty::ParamEnv::reveal_all();
 499
 500         tcx.infer_ctxt().enter(|infcx| {
 501             let self_arg_ty = tcx.liberate_late_bound_regions(method.def_id, self_arg_ty);
 502             let can_eq_self = |ty| infcx.can_eq(param_env, untransformed_self_ty, ty).is_ok();
 503             match ExplicitSelf::determine(self_arg_ty, can_eq_self) {
 504                 ExplicitSelf::ByValue => "self".to_owned(),
 505                 ExplicitSelf::ByReference(_, hir::Mutability::Not) => "&self".to_owned(),
 506                 ExplicitSelf::ByReference(_, hir::Mutability::Mut) => "&mut self".to_owned(),
 507                 _ => format!("self: {}", self_arg_ty),
 508             }
 509         })
 510     };
 511
 512     match (trait_m.fn_has_self_parameter, impl_m.fn_has_self_parameter) {
 513         (false, false) | (true, true) => {}
 514
 515         (false, true) => {
 516             let self_descr = self_string(impl_m);
 517             let mut err = struct_span_err!(
 518                 tcx.sess,
 519                 impl_m_span,
 520                 E0185,
 521                 "method `{}` has a `{}` declaration in the impl, but \
 522                                             not in the trait",
 523                 trait_m.ident,
 524                 self_descr
 525             );
 526             err.span_label(impl_m_span, format!("`{}` used in impl", self_descr));
 527             if let Some(span) = tcx.hir().span_if_local(trait_m.def_id) {
 528                 err.span_label(span, format!("trait method declared without `{}`", self_descr));
 529             } else {
 530                 err.note_trait_signature(trait_m.ident.to_string(), trait_m.signature(tcx));
 531             }
 532             err.emit();
 533             return Err(ErrorReported);
 534         }
 535
 536         (true, false) => {
 537             let self_descr = self_string(trait_m);
 538             let mut err = struct_span_err!(
 539                 tcx.sess,
 540                 impl_m_span,
 541                 E0186,
 542                 "method `{}` has a `{}` declaration in the trait, but \
 543                                             not in the impl",
 544                 trait_m.ident,
 545                 self_descr
 546             );
 547             err.span_label(impl_m_span, format!("expected `{}` in impl", self_descr));
 548             if let Some(span) = tcx.hir().span_if_local(trait_m.def_id) {
 549                 err.span_label(span, format!("`{}` used in trait", self_descr));
 550             } else {
 551                 err.note_trait_signature(trait_m.ident.to_string(), trait_m.signature(tcx));
 552             }
 553             err.emit();
 554             return Err(ErrorReported);
 555         }
 556     }
 557
 558     Ok(())
 559 }
 560
 561 fn compare_number_of_generics<'tcx>(
 562     tcx: TyCtxt<'tcx>,
 563     impl_: &ty::AssocItem,
 564     _impl_span: Span,
 565     trait_: &ty::AssocItem,
 566     trait_span: Option<Span>,
 567 ) -> Result<(), ErrorReported> {
 568     let trait_own_counts = tcx.generics_of(trait_.def_id).own_counts();
 569     let impl_own_counts = tcx.generics_of(impl_.def_id).own_counts();
 570
 571     let matchings = [
 572         ("type", trait_own_counts.types, impl_own_counts.types),
 573         ("const", trait_own_counts.consts, impl_own_counts.consts),
 574     ];
 575
 576     let item_kind = assoc_item_kind_str(impl_);
 577
 578     let mut err_occurred = false;
 579     for &(kind, trait_count, impl_count) in &matchings {
 580         if impl_count != trait_count {
 581             err_occurred = true;
 582
 583             let (trait_spans, impl_trait_spans) = if let Some(def_id) = trait_.def_id.as_local() {
 584                 let trait_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 585                 let trait_item = tcx.hir().expect_trait_item(trait_hir_id);
 586                 if trait_item.generics.params.is_empty() {
 587                     (Some(vec![trait_item.generics.span]), vec![])
 588                 } else {
 589                     let arg_spans: Vec<Span> =
 590                         trait_item.generics.params.iter().map(|p| p.span).collect();
 591                     let impl_trait_spans: Vec<Span> = trait_item
 592                         .generics
 593                         .params
 594                         .iter()
 595                         .filter_map(|p| match p.kind {
 596                             GenericParamKind::Type {
 597                                 synthetic: Some(hir::SyntheticTyParamKind::ImplTrait),
 598                                 ..
 599                             } => Some(p.span),
 600                             _ => None,
 601                         })
 602                         .collect();
 603                     (Some(arg_spans), impl_trait_spans)
 604                 }
 605             } else {
 606                 (trait_span.map(|s| vec![s]), vec![])
 607             };
 608
 609             let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_.def_id.expect_local());
 610             let impl_item = tcx.hir().expect_impl_item(impl_hir_id);
 611             let impl_item_impl_trait_spans: Vec<Span> = impl_item
 612                 .generics
 613                 .params
 614                 .iter()
 615                 .filter_map(|p| match p.kind {
 616                     GenericParamKind::Type {
 617                         synthetic: Some(hir::SyntheticTyParamKind::ImplTrait),
 618                         ..
 619                     } => Some(p.span),
 620                     _ => None,
 621                 })
 622                 .collect();
 623             let spans = impl_item.generics.spans();
 624             let span = spans.primary_span();
 625
 626             let mut err = tcx.sess.struct_span_err_with_code(
 627                 spans,
 628                 &format!(
 629                     "{} `{}` has {} {kind} parameter{} but its trait \
 630                      declaration has {} {kind} parameter{}",
 631                     item_kind,
 632                     trait_.ident,
 633                     impl_count,
 634                     pluralize!(impl_count),
 635                     trait_count,
 636                     pluralize!(trait_count),
 637                     kind = kind,
 638                 ),
 639                 DiagnosticId::Error("E0049".into()),
 640             );
 641
 642             let mut suffix = None;
 643
 644             if let Some(spans) = trait_spans {
 645                 let mut spans = spans.iter();
 646                 if let Some(span) = spans.next() {
 647                     err.span_label(
 648                         *span,
 649                         format!(
 650                             "expected {} {} parameter{}",
 651                             trait_count,
 652                             kind,
 653                             pluralize!(trait_count),
 654                         ),
 655                     );
 656                 }
 657                 for span in spans {
 658                     err.span_label(*span, "");
 659                 }
 660             } else {
 661                 suffix = Some(format!(", expected {}", trait_count));
 662             }
 663
 664             if let Some(span) = span {
 665                 err.span_label(
 666                     span,
 667                     format!(
 668                         "found {} {} parameter{}{}",
 669                         impl_count,
 670                         kind,
 671                         pluralize!(impl_count),
 672                         suffix.unwrap_or_else(String::new),
 673                     ),
 674                 );
 675             }
 676
 677             for span in impl_trait_spans.iter().chain(impl_item_impl_trait_spans.iter()) {
 678                 err.span_label(*span, "`impl Trait` introduces an implicit type parameter");
 679             }
 680
 681             err.emit();
 682         }
 683     }
 684
 685     if err_occurred { Err(ErrorReported) } else { Ok(()) }
 686 }
 687
 688 fn compare_number_of_method_arguments<'tcx>(
 689     tcx: TyCtxt<'tcx>,
 690     impl_m: &ty::AssocItem,
 691     impl_m_span: Span,
 692     trait_m: &ty::AssocItem,
 693     trait_item_span: Option<Span>,
 694 ) -> Result<(), ErrorReported> {
 695     let impl_m_fty = tcx.fn_sig(impl_m.def_id);
 696     let trait_m_fty = tcx.fn_sig(trait_m.def_id);
 697     let trait_number_args = trait_m_fty.inputs().skip_binder().len();
 698     let impl_number_args = impl_m_fty.inputs().skip_binder().len();
 699     if trait_number_args != impl_number_args {
 700         let trait_span = if let Some(def_id) = trait_m.def_id.as_local() {
 701             let trait_id = tcx.hir().local_def_id_to_hir_id(def_id);
 702             match tcx.hir().expect_trait_item(trait_id).kind {
 703                 TraitItemKind::Fn(ref trait_m_sig, _) => {
 704                     let pos = if trait_number_args > 0 { trait_number_args - 1 } else { 0 };
 705                     if let Some(arg) = trait_m_sig.decl.inputs.get(pos) {
 706                         Some(if pos == 0 {
 707                             arg.span
 708                         } else {
 709                             Span::new(
 710                                 trait_m_sig.decl.inputs[0].span.lo(),
 711                                 arg.span.hi(),
 712                                 arg.span.ctxt(),
 713                             )
 714                         })
 715                     } else {
 716                         trait_item_span
 717                     }
 718                 }
 719                 _ => bug!("{:?} is not a method", impl_m),
 720             }
 721         } else {
 722             trait_item_span
 723         };
 724         let impl_m_hir_id = tcx.hir().local_def_id_to_hir_id(impl_m.def_id.expect_local());
 725         let impl_span = match tcx.hir().expect_impl_item(impl_m_hir_id).kind {
 726             ImplItemKind::Fn(ref impl_m_sig, _) => {
 727                 let pos = if impl_number_args > 0 { impl_number_args - 1 } else { 0 };
 728                 if let Some(arg) = impl_m_sig.decl.inputs.get(pos) {
 729                     if pos == 0 {
 730                         arg.span
 731                     } else {
 732                         Span::new(
 733                             impl_m_sig.decl.inputs[0].span.lo(),
 734                             arg.span.hi(),
 735                             arg.span.ctxt(),
 736                         )
 737                     }
 738                 } else {
 739                     impl_m_span
 740                 }
 741             }
 742             _ => bug!("{:?} is not a method", impl_m),
 743         };
 744         let mut err = struct_span_err!(
 745             tcx.sess,
 746             impl_span,
 747             E0050,
 748             "method `{}` has {} but the declaration in \
 749                                         trait `{}` has {}",
 750             trait_m.ident,
 751             potentially_plural_count(impl_number_args, "parameter"),
 752             tcx.def_path_str(trait_m.def_id),
 753             trait_number_args
 754         );
 755         if let Some(trait_span) = trait_span {
 756             err.span_label(
 757                 trait_span,
 758                 format!(
 759                     "trait requires {}",
 760                     potentially_plural_count(trait_number_args, "parameter")
 761                 ),
 762             );
 763         } else {
 764             err.note_trait_signature(trait_m.ident.to_string(), trait_m.signature(tcx));
 765         }
 766         err.span_label(
 767             impl_span,
 768             format!(
 769                 "expected {}, found {}",
 770                 potentially_plural_count(trait_number_args, "parameter"),
 771                 impl_number_args
 772             ),
 773         );
 774         err.emit();
 775         return Err(ErrorReported);
 776     }
 777
 778     Ok(())
 779 }
 780
 781 fn compare_synthetic_generics<'tcx>(
 782     tcx: TyCtxt<'tcx>,
 783     impl_m: &ty::AssocItem,
 784     trait_m: &ty::AssocItem,
 785 ) -> Result<(), ErrorReported> {
 786     // FIXME(chrisvittal) Clean up this function, list of FIXME items:
 787     //     1. Better messages for the span labels
 788     //     2. Explanation as to what is going on
 789     // If we get here, we already have the same number of generics, so the zip will
 790     // be okay.
 791     let mut error_found = false;
 792     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 793     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 794     let impl_m_type_params = impl_m_generics.params.iter().filter_map(|param| match param.kind {
 795         GenericParamDefKind::Type { synthetic, .. } => Some((param.def_id, synthetic)),
 796         GenericParamDefKind::Lifetime | GenericParamDefKind::Const => None,
 797     });
 798     let trait_m_type_params = trait_m_generics.params.iter().filter_map(|param| match param.kind {
 799         GenericParamDefKind::Type { synthetic, .. } => Some((param.def_id, synthetic)),
 800         GenericParamDefKind::Lifetime | GenericParamDefKind::Const => None,
 801     });
 802     for ((impl_def_id, impl_synthetic), (trait_def_id, trait_synthetic)) in
 803         impl_m_type_params.zip(trait_m_type_params)
 804     {
 805         if impl_synthetic != trait_synthetic {
 806             let impl_hir_id = tcx.hir().local_def_id_to_hir_id(impl_def_id.expect_local());
 807             let impl_span = tcx.hir().span(impl_hir_id);
 808             let trait_span = tcx.def_span(trait_def_id);
 809             let mut err = struct_span_err!(
 810                 tcx.sess,
 811                 impl_span,
 812                 E0643,
 813                 "method `{}` has incompatible signature for trait",
 814                 trait_m.ident
 815             );
 816             err.span_label(trait_span, "declaration in trait here");
 817             match (impl_synthetic, trait_synthetic) {
 818                 // The case where the impl method uses `impl Trait` but the trait method uses
 819                 // explicit generics
 820                 (Some(hir::SyntheticTyParamKind::ImplTrait), None) => {
 821                     err.span_label(impl_span, "expected generic parameter, found `impl Trait`");
 822                     (|| {
 823                         // try taking the name from the trait impl
 824                         // FIXME: this is obviously suboptimal since the name can already be used
 825                         // as another generic argument
 826                         let new_name = tcx.sess.source_map().span_to_snippet(trait_span).ok()?;
 827                         let trait_m = tcx.hir().local_def_id_to_hir_id(trait_m.def_id.as_local()?);
 828                         let trait_m = tcx.hir().trait_item(hir::TraitItemId { hir_id: trait_m });
 829
 830                         let impl_m = tcx.hir().local_def_id_to_hir_id(impl_m.def_id.as_local()?);
 831                         let impl_m = tcx.hir().impl_item(hir::ImplItemId { hir_id: impl_m });
 832
 833                         // in case there are no generics, take the spot between the function name
 834                         // and the opening paren of the argument list
 835                         let new_generics_span =
 836                             tcx.sess.source_map().generate_fn_name_span(impl_span)?.shrink_to_hi();
 837                         // in case there are generics, just replace them
 838                         let generics_span =
 839                             impl_m.generics.span.substitute_dummy(new_generics_span);
 840                         // replace with the generics from the trait
 841                         let new_generics =
 842                             tcx.sess.source_map().span_to_snippet(trait_m.generics.span).ok()?;
 843
 844                         err.multipart_suggestion(
 845                             "try changing the `impl Trait` argument to a generic parameter",
 846                             vec![
 847                                 // replace `impl Trait` with `T`
 848                                 (impl_span, new_name),
 849                                 // replace impl method generics with trait method generics
 850                                 // This isn't quite right, as users might have changed the names
 851                                 // of the generics, but it works for the common case
 852                                 (generics_span, new_generics),
 853                             ],
 854                             Applicability::MaybeIncorrect,
 855                         );
 856                         Some(())
 857                     })();
 858                 }
 859                 // The case where the trait method uses `impl Trait`, but the impl method uses
 860                 // explicit generics.
 861                 (None, Some(hir::SyntheticTyParamKind::ImplTrait)) => {
 862                     err.span_label(impl_span, "expected `impl Trait`, found generic parameter");
 863                     (|| {
 864                         let impl_m = tcx.hir().local_def_id_to_hir_id(impl_m.def_id.as_local()?);
 865                         let impl_m = tcx.hir().impl_item(hir::ImplItemId { hir_id: impl_m });
 866                         let input_tys = match impl_m.kind {
 867                             hir::ImplItemKind::Fn(ref sig, _) => sig.decl.inputs,
 868                             _ => unreachable!(),
 869                         };
 870                         struct Visitor(Option<Span>, hir::def_id::DefId);
 871                         impl<'v> intravisit::Visitor<'v> for Visitor {
 872                             fn visit_ty(&mut self, ty: &'v hir::Ty<'v>) {
 873                                 intravisit::walk_ty(self, ty);
 874                                 if let hir::TyKind::Path(hir::QPath::Resolved(None, ref path)) =
 875                                     ty.kind
 876                                 {
 877                                     if let Res::Def(DefKind::TyParam, def_id) = path.res {
 878                                         if def_id == self.1 {
 879                                             self.0 = Some(ty.span);
 880                                         }
 881                                     }
 882                                 }
 883                             }
 884                             type Map = intravisit::ErasedMap<'v>;
 885                             fn nested_visit_map(
 886                                 &mut self,
 887                             ) -> intravisit::NestedVisitorMap<Self::Map>
 888                             {
 889                                 intravisit::NestedVisitorMap::None
 890                             }
 891                         }
 892                         let mut visitor = Visitor(None, impl_def_id);
 893                         for ty in input_tys {
 894                             intravisit::Visitor::visit_ty(&mut visitor, ty);
 895                         }
 896                         let span = visitor.0?;
 897
 898                         let bounds =
 899                             impl_m.generics.params.iter().find_map(|param| match param.kind {
 900                                 GenericParamKind::Lifetime { .. } => None,
 901                                 GenericParamKind::Type { .. } | GenericParamKind::Const { .. } => {
 902                                     if param.hir_id == impl_hir_id {
 903                                         Some(&param.bounds)
 904                                     } else {
 905                                         None
 906                                     }
 907                                 }
 908                             })?;
 909                         let bounds = bounds.first()?.span().to(bounds.last()?.span());
 910                         let bounds = tcx.sess.source_map().span_to_snippet(bounds).ok()?;
 911
 912                         err.multipart_suggestion(
 913                             "try removing the generic parameter and using `impl Trait` instead",
 914                             vec![
 915                                 // delete generic parameters
 916                                 (impl_m.generics.span, String::new()),
 917                                 // replace param usage with `impl Trait`
 918                                 (span, format!("impl {}", bounds)),
 919                             ],
 920                             Applicability::MaybeIncorrect,
 921                         );
 922                         Some(())
 923                     })();
 924                 }
 925                 _ => unreachable!(),
 926             }
 927             err.emit();
 928             error_found = true;
 929         }
 930     }
 931     if error_found { Err(ErrorReported) } else { Ok(()) }
 932 }
 933
 934 crate fn compare_const_impl<'tcx>(
 935     tcx: TyCtxt<'tcx>,
 936     impl_c: &ty::AssocItem,
 937     impl_c_span: Span,
 938     trait_c: &ty::AssocItem,
 939     impl_trait_ref: ty::TraitRef<'tcx>,
 940 ) {
 941     debug!("compare_const_impl(impl_trait_ref={:?})", impl_trait_ref);
 942
 943     tcx.infer_ctxt().enter(|infcx| {
 944         let param_env = tcx.param_env(impl_c.def_id);
 945         let inh = Inherited::new(infcx, impl_c.def_id.expect_local());
 946         let infcx = &inh.infcx;
 947
 948         // The below is for the most part highly similar to the procedure
 949         // for methods above. It is simpler in many respects, especially
 950         // because we shouldn't really have to deal with lifetimes or
 951         // predicates. In fact some of this should probably be put into
 952         // shared functions because of DRY violations...
 953         let trait_to_impl_substs = impl_trait_ref.substs;
 954
 955         // Create a parameter environment that represents the implementation's
 956         // method.
 957         let impl_c_hir_id = tcx.hir().local_def_id_to_hir_id(impl_c.def_id.expect_local());
 958
 959         // Compute placeholder form of impl and trait const tys.
 960         let impl_ty = tcx.type_of(impl_c.def_id);
 961         let trait_ty = tcx.type_of(trait_c.def_id).subst(tcx, trait_to_impl_substs);
 962         let mut cause = ObligationCause::new(
 963             impl_c_span,
 964             impl_c_hir_id,
 965             ObligationCauseCode::CompareImplConstObligation,
 966         );
 967
 968         // There is no "body" here, so just pass dummy id.
 969         let impl_ty =
 970             inh.normalize_associated_types_in(impl_c_span, impl_c_hir_id, param_env, impl_ty);
 971
 972         debug!("compare_const_impl: impl_ty={:?}", impl_ty);
 973
 974         let trait_ty =
 975             inh.normalize_associated_types_in(impl_c_span, impl_c_hir_id, param_env, trait_ty);
 976
 977         debug!("compare_const_impl: trait_ty={:?}", trait_ty);
 978
 979         let err = infcx
 980             .at(&cause, param_env)
 981             .sup(trait_ty, impl_ty)
 982             .map(|ok| inh.register_infer_ok_obligations(ok));
 983
 984         if let Err(terr) = err {
 985             debug!(
 986                 "checking associated const for compatibility: impl ty {:?}, trait ty {:?}",
 987                 impl_ty, trait_ty
 988             );
 989
 990             // Locate the Span containing just the type of the offending impl
 991             match tcx.hir().expect_impl_item(impl_c_hir_id).kind {
 992                 ImplItemKind::Const(ref ty, _) => cause.make_mut().span = ty.span,
 993                 _ => bug!("{:?} is not a impl const", impl_c),
 994             }
 995
 996             let mut diag = struct_span_err!(
 997                 tcx.sess,
 998                 cause.span,
 999                 E0326,
1000                 "implemented const `{}` has an incompatible type for \
1001                                              trait",
1002                 trait_c.ident
1003             );
1004
1005             let trait_c_hir_id =
1006                 trait_c.def_id.as_local().map(|def_id| tcx.hir().local_def_id_to_hir_id(def_id));
1007             let trait_c_span = trait_c_hir_id.map(|trait_c_hir_id| {
1008                 // Add a label to the Span containing just the type of the const
1009                 match tcx.hir().expect_trait_item(trait_c_hir_id).kind {
1010                     TraitItemKind::Const(ref ty, _) => ty.span,
1011                     _ => bug!("{:?} is not a trait const", trait_c),
1012                 }
1013             });
1014
1015             infcx.note_type_err(
1016                 &mut diag,
1017                 &cause,
1018                 trait_c_span.map(|span| (span, "type in trait".to_owned())),
1019                 Some(infer::ValuePairs::Types(ExpectedFound {
1020                     expected: trait_ty,
1021                     found: impl_ty,
1022                 })),
1023                 &terr,
1024             );
1025             diag.emit();
1026         }
1027
1028         // Check that all obligations are satisfied by the implementation's
1029         // version.
1030         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
1031             infcx.report_fulfillment_errors(errors, None, false);
1032             return;
1033         }
1034
1035         let fcx = FnCtxt::new(&inh, param_env, impl_c_hir_id);
1036         fcx.regionck_item(impl_c_hir_id, impl_c_span, &[]);
1037     });
1038 }
1039
1040 crate fn compare_ty_impl<'tcx>(
1041     tcx: TyCtxt<'tcx>,
1042     impl_ty: &ty::AssocItem,
1043     impl_ty_span: Span,
1044     trait_ty: &ty::AssocItem,
1045     impl_trait_ref: ty::TraitRef<'tcx>,
1046     trait_item_span: Option<Span>,
1047 ) {
1048     debug!("compare_impl_type(impl_trait_ref={:?})", impl_trait_ref);
1049
1050     let _: Result<(), ErrorReported> = (|| {
1051         compare_number_of_generics(tcx, impl_ty, impl_ty_span, trait_ty, trait_item_span)?;
1052
1053         compare_type_predicate_entailment(tcx, impl_ty, impl_ty_span, trait_ty, impl_trait_ref)?;
1054
1055         check_type_bounds(tcx, trait_ty, impl_ty, impl_ty_span, impl_trait_ref)
1056     })();
1057 }
1058
1059 /// The equivalent of [compare_predicate_entailment], but for associated types
1060 /// instead of associated functions.
1061 fn compare_type_predicate_entailment<'tcx>(
1062     tcx: TyCtxt<'tcx>,
1063     impl_ty: &ty::AssocItem,
1064     impl_ty_span: Span,
1065     trait_ty: &ty::AssocItem,
1066     impl_trait_ref: ty::TraitRef<'tcx>,
1067 ) -> Result<(), ErrorReported> {
1068     let impl_substs = InternalSubsts::identity_for_item(tcx, impl_ty.def_id);
1069     let trait_to_impl_substs =
1070         impl_substs.rebase_onto(tcx, impl_ty.container.id(), impl_trait_ref.substs);
1071
1072     let impl_ty_generics = tcx.generics_of(impl_ty.def_id);
1073     let trait_ty_generics = tcx.generics_of(trait_ty.def_id);
1074     let impl_ty_predicates = tcx.predicates_of(impl_ty.def_id);
1075     let trait_ty_predicates = tcx.predicates_of(trait_ty.def_id);
1076
1077     check_region_bounds_on_impl_item(
1078         tcx,
1079         impl_ty_span,
1080         impl_ty,
1081         trait_ty,
1082         &trait_ty_generics,
1083         &impl_ty_generics,
1084     )?;
1085
1086     let impl_ty_own_bounds = impl_ty_predicates.instantiate_own(tcx, impl_substs);
1087
1088     if impl_ty_own_bounds.is_empty() {
1089         // Nothing to check.
1090         return Ok(());
1091     }
1092
1093     // This `HirId` should be used for the `body_id` field on each
1094     // `ObligationCause` (and the `FnCtxt`). This is what
1095     // `regionck_item` expects.
1096     let impl_ty_hir_id = tcx.hir().local_def_id_to_hir_id(impl_ty.def_id.expect_local());
1097     let cause = ObligationCause::new(
1098         impl_ty_span,
1099         impl_ty_hir_id,
1100         ObligationCauseCode::CompareImplTypeObligation {
1101             item_name: impl_ty.ident.name,
1102             impl_item_def_id: impl_ty.def_id,
1103             trait_item_def_id: trait_ty.def_id,
1104         },
1105     );
1106
1107     debug!("compare_type_predicate_entailment: trait_to_impl_substs={:?}", trait_to_impl_substs);
1108
1109     // The predicates declared by the impl definition, the trait and the
1110     // associated type in the trait are assumed.
1111     let impl_predicates = tcx.predicates_of(impl_ty_predicates.parent.unwrap());
1112     let mut hybrid_preds = impl_predicates.instantiate_identity(tcx);
1113     hybrid_preds
1114         .predicates
1115         .extend(trait_ty_predicates.instantiate_own(tcx, trait_to_impl_substs).predicates);
1116
1117     debug!("compare_type_predicate_entailment: bounds={:?}", hybrid_preds);
1118
1119     let normalize_cause = traits::ObligationCause::misc(impl_ty_span, impl_ty_hir_id);
1120     let param_env =
1121         ty::ParamEnv::new(tcx.intern_predicates(&hybrid_preds.predicates), Reveal::UserFacing);
1122     let param_env = traits::normalize_param_env_or_error(
1123         tcx,
1124         impl_ty.def_id,
1125         param_env,
1126         normalize_cause.clone(),
1127     );
1128     tcx.infer_ctxt().enter(|infcx| {
1129         let inh = Inherited::new(infcx, impl_ty.def_id.expect_local());
1130         let infcx = &inh.infcx;
1131
1132         debug!("compare_type_predicate_entailment: caller_bounds={:?}", param_env.caller_bounds());
1133
1134         let mut selcx = traits::SelectionContext::new(&infcx);
1135
1136         for predicate in impl_ty_own_bounds.predicates {
1137             let traits::Normalized { value: predicate, obligations } =
1138                 traits::normalize(&mut selcx, param_env, normalize_cause.clone(), predicate);
1139
1140             inh.register_predicates(obligations);
1141             inh.register_predicate(traits::Obligation::new(cause.clone(), param_env, predicate));
1142         }
1143
1144         // Check that all obligations are satisfied by the implementation's
1145         // version.
1146         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
1147             infcx.report_fulfillment_errors(errors, None, false);
1148             return Err(ErrorReported);
1149         }
1150
1151         // Finally, resolve all regions. This catches wily misuses of
1152         // lifetime parameters.
1153         let fcx = FnCtxt::new(&inh, param_env, impl_ty_hir_id);
1154         fcx.regionck_item(impl_ty_hir_id, impl_ty_span, &[]);
1155
1156         Ok(())
1157     })
1158 }
1159
1160 /// Validate that `ProjectionCandidate`s created for this associated type will
1161 /// be valid.
1162 ///
1163 /// Usually given
1164 ///
1165 /// trait X { type Y: Copy } impl X for T { type Y = S; }
1166 ///
1167 /// We are able to normalize `<T as X>::U` to `S`, and so when we check the
1168 /// impl is well-formed we have to prove `S: Copy`.
1169 ///
1170 /// For default associated types the normalization is not possible (the value
1171 /// from the impl could be overridden). We also can't normalize generic
1172 /// associated types (yet) because they contain bound parameters.
1173 pub fn check_type_bounds<'tcx>(
1174     tcx: TyCtxt<'tcx>,
1175     trait_ty: &ty::AssocItem,
1176     impl_ty: &ty::AssocItem,
1177     impl_ty_span: Span,
1178     impl_trait_ref: ty::TraitRef<'tcx>,
1179 ) -> Result<(), ErrorReported> {
1180     // Given
1181     //
1182     // impl<A, B> Foo<u32> for (A, B) {
1183     //     type Bar<C> =...
1184     // }
1185     //
1186     // - `impl_substs` would be `[A, B, C]`
1187     // - `rebased_substs` would be `[(A, B), u32, C]`, combining the substs from
1188     //    the *trait* with the generic associated type parameters.
1189     let impl_ty_substs = InternalSubsts::identity_for_item(tcx, impl_ty.def_id);
1190     let rebased_substs =
1191         impl_ty_substs.rebase_onto(tcx, impl_ty.container.id(), impl_trait_ref.substs);
1192     let impl_ty_value = tcx.type_of(impl_ty.def_id);
1193
1194     let param_env = tcx.param_env(impl_ty.def_id);
1195
1196     // When checking something like
1197     //
1198     // trait X { type Y: PartialEq<<Self as X>::Y> }
1199     // impl X for T { default type Y = S; }
1200     //
1201     // We will have to prove the bound S: PartialEq<<T as X>::Y>. In this case
1202     // we want <T as X>::Y to normalize to S. This is valid because we are
1203     // checking the default value specifically here. Add this equality to the
1204     // ParamEnv for normalization specifically.
1205     let normalize_param_env = {
1206         let mut predicates = param_env.caller_bounds().iter().collect::<Vec<_>>();
1207         match impl_ty_value.kind() {
1208             ty::Projection(proj)
1209                 if proj.item_def_id == trait_ty.def_id && proj.substs == rebased_substs =>
1210             {
1211                 // Don't include this predicate if the projected type is
1212                 // exactly the same as the projection. This can occur in
1213                 // (somewhat dubious) code like this:
1214                 //
1215                 // impl<T> X for T where T: X { type Y = <T as X>::Y; }
1216             }
1217             _ => predicates.push(
1218                 ty::Binder::dummy(ty::ProjectionPredicate {
1219                     projection_ty: ty::ProjectionTy {
1220                         item_def_id: trait_ty.def_id,
1221                         substs: rebased_substs,
1222                     },
1223                     ty: impl_ty_value,
1224                 })
1225                 .to_predicate(tcx),
1226             ),
1227         };
1228         ty::ParamEnv::new(tcx.intern_predicates(&predicates), Reveal::UserFacing)
1229     };
1230
1231     tcx.infer_ctxt().enter(move |infcx| {
1232         let inh = Inherited::new(infcx, impl_ty.def_id.expect_local());
1233         let infcx = &inh.infcx;
1234         let mut selcx = traits::SelectionContext::new(&infcx);
1235
1236         let impl_ty_hir_id = tcx.hir().local_def_id_to_hir_id(impl_ty.def_id.expect_local());
1237         let normalize_cause = traits::ObligationCause::misc(impl_ty_span, impl_ty_hir_id);
1238         let mk_cause = |span| {
1239             ObligationCause::new(
1240                 impl_ty_span,
1241                 impl_ty_hir_id,
1242                 ObligationCauseCode::BindingObligation(trait_ty.def_id, span),
1243             )
1244         };
1245
1246         let obligations = tcx
1247             .explicit_item_bounds(trait_ty.def_id)
1248             .iter()
1249             .map(|&(bound, span)| {
1250                 let concrete_ty_bound = bound.subst(tcx, rebased_substs);
1251                 debug!("check_type_bounds: concrete_ty_bound = {:?}", concrete_ty_bound);
1252
1253                 traits::Obligation::new(mk_cause(span), param_env, concrete_ty_bound)
1254             })
1255             .collect();
1256         debug!("check_type_bounds: item_bounds={:?}", obligations);
1257
1258         for mut obligation in util::elaborate_obligations(tcx, obligations) {
1259             let traits::Normalized { value: normalized_predicate, obligations } = traits::normalize(
1260                 &mut selcx,
1261                 normalize_param_env,
1262                 normalize_cause.clone(),
1263                 obligation.predicate,
1264             );
1265             debug!("compare_projection_bounds: normalized predicate = {:?}", normalized_predicate);
1266             obligation.predicate = normalized_predicate;
1267
1268             inh.register_predicates(obligations);
1269             inh.register_predicate(obligation);
1270         }
1271
1272         // Check that all obligations are satisfied by the implementation's
1273         // version.
1274         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
1275             infcx.report_fulfillment_errors(errors, None, false);
1276             return Err(ErrorReported);
1277         }
1278
1279         // Finally, resolve all regions. This catches wily misuses of
1280         // lifetime parameters.
1281         let fcx = FnCtxt::new(&inh, param_env, impl_ty_hir_id);
1282         let implied_bounds = match impl_ty.container {
1283             ty::TraitContainer(_) => vec![],
1284             ty::ImplContainer(def_id) => fcx.impl_implied_bounds(def_id, impl_ty_span),
1285         };
1286         fcx.regionck_item(impl_ty_hir_id, impl_ty_span, &implied_bounds);
1287
1288         Ok(())
1289     })
1290 }
1291
1292 fn assoc_item_kind_str(impl_item: &ty::AssocItem) -> &'static str {
1293     match impl_item.kind {
1294         ty::AssocKind::Const => "const",
1295         ty::AssocKind::Fn => "method",
1296         ty::AssocKind::Type => "type",
1297     }
1298 }