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_middle::ty;
   9 use rustc_middle::ty::error::{ExpectedFound, TypeError};
  10 use rustc_middle::ty::subst::{InternalSubsts, Subst};
  11 use rustc_middle::ty::util::ExplicitSelf;
  12 use rustc_middle::ty::{GenericParamDefKind, ToPredicate, TyCtxt};
  13 use rustc_span::Span;
  14 use rustc_trait_selection::traits::error_reporting::InferCtxtExt;
  15 use rustc_trait_selection::traits::{self, ObligationCause, ObligationCauseCode, Reveal};
  16
  17 use super::{potentially_plural_count, FnCtxt, Inherited};
  18
  19 /// Checks that a method from an impl conforms to the signature of
  20 /// the same method as declared in the trait.
  21 ///
  22 /// # Parameters
  23 ///
  24 /// - `impl_m`: type of the method we are checking
  25 /// - `impl_m_span`: span to use for reporting errors
  26 /// - `trait_m`: the method in the trait
  27 /// - `impl_trait_ref`: the TraitRef corresponding to the trait implementation
  28
  29 crate fn compare_impl_method<'tcx>(
  30     tcx: TyCtxt<'tcx>,
  31     impl_m: &ty::AssocItem,
  32     impl_m_span: Span,
  33     trait_m: &ty::AssocItem,
  34     impl_trait_ref: ty::TraitRef<'tcx>,
  35     trait_item_span: Option<Span>,
  36 ) {
  37     debug!("compare_impl_method(impl_trait_ref={:?})", impl_trait_ref);
  38
  39     let impl_m_span = tcx.sess.source_map().guess_head_span(impl_m_span);
  40
  41     if let Err(ErrorReported) = compare_self_type(tcx, impl_m, impl_m_span, trait_m, impl_trait_ref)
  42     {
  43         return;
  44     }
  45
  46     if let Err(ErrorReported) =
  47         compare_number_of_generics(tcx, impl_m, impl_m_span, trait_m, trait_item_span)
  48     {
  49         return;
  50     }
  51
  52     if let Err(ErrorReported) =
  53         compare_number_of_method_arguments(tcx, impl_m, impl_m_span, trait_m, trait_item_span)
  54     {
  55         return;
  56     }
  57
  58     if let Err(ErrorReported) = compare_synthetic_generics(tcx, impl_m, trait_m) {
  59         return;
  60     }
  61
  62     if let Err(ErrorReported) =
  63         compare_predicate_entailment(tcx, impl_m, impl_m_span, trait_m, impl_trait_ref)
  64     {
  65         return;
  66     }
  67 }
  68
  69 fn compare_predicate_entailment<'tcx>(
  70     tcx: TyCtxt<'tcx>,
  71     impl_m: &ty::AssocItem,
  72     impl_m_span: Span,
  73     trait_m: &ty::AssocItem,
  74     impl_trait_ref: ty::TraitRef<'tcx>,
  75 ) -> Result<(), ErrorReported> {
  76     let trait_to_impl_substs = impl_trait_ref.substs;
  77
  78     // This node-id should be used for the `body_id` field on each
  79     // `ObligationCause` (and the `FnCtxt`). This is what
  80     // `regionck_item` expects.
  81     let impl_m_hir_id = tcx.hir().local_def_id_to_hir_id(impl_m.def_id.expect_local());
  82
  83     // We sometimes modify the span further down.
  84     let mut cause = ObligationCause::new(
  85         impl_m_span,
  86         impl_m_hir_id,
  87         ObligationCauseCode::CompareImplMethodObligation {
  88             item_name: impl_m.ident.name,
  89             impl_item_def_id: impl_m.def_id,
  90             trait_item_def_id: trait_m.def_id,
  91         },
  92     );
  93
  94     // This code is best explained by example. Consider a trait:
  95     //
  96     //     trait Trait<'t, T> {
  97     //         fn method<'a, M>(t: &'t T, m: &'a M) -> Self;
  98     //     }
  99     //
 100     // And an impl:
 101     //
 102     //     impl<'i, 'j, U> Trait<'j, &'i U> for Foo {
 103     //          fn method<'b, N>(t: &'j &'i U, m: &'b N) -> Foo;
 104     //     }
 105     //
 106     // We wish to decide if those two method types are compatible.
 107     //
 108     // We start out with trait_to_impl_substs, that maps the trait
 109     // type parameters to impl type parameters. This is taken from the
 110     // impl trait reference:
 111     //
 112     //     trait_to_impl_substs = {'t => 'j, T => &'i U, Self => Foo}
 113     //
 114     // We create a mapping `dummy_substs` that maps from the impl type
 115     // parameters to fresh types and regions. For type parameters,
 116     // this is the identity transform, but we could as well use any
 117     // placeholder types. For regions, we convert from bound to free
 118     // regions (Note: but only early-bound regions, i.e., those
 119     // declared on the impl or used in type parameter bounds).
 120     //
 121     //     impl_to_placeholder_substs = {'i => 'i0, U => U0, N => N0 }
 122     //
 123     // Now we can apply placeholder_substs to the type of the impl method
 124     // to yield a new function type in terms of our fresh, placeholder
 125     // types:
 126     //
 127     //     <'b> fn(t: &'i0 U0, m: &'b) -> Foo
 128     //
 129     // We now want to extract and substitute the type of the *trait*
 130     // method and compare it. To do so, we must create a compound
 131     // substitution by combining trait_to_impl_substs and
 132     // impl_to_placeholder_substs, and also adding a mapping for the method
 133     // type parameters. We extend the mapping to also include
 134     // the method parameters.
 135     //
 136     //     trait_to_placeholder_substs = { T => &'i0 U0, Self => Foo, M => N0 }
 137     //
 138     // Applying this to the trait method type yields:
 139     //
 140     //     <'a> fn(t: &'i0 U0, m: &'a) -> Foo
 141     //
 142     // This type is also the same but the name of the bound region ('a
 143     // vs 'b).  However, the normal subtyping rules on fn types handle
 144     // this kind of equivalency just fine.
 145     //
 146     // We now use these substitutions to ensure that all declared bounds are
 147     // satisfied by the implementation's method.
 148     //
 149     // We do this by creating a parameter environment which contains a
 150     // substitution corresponding to impl_to_placeholder_substs. We then build
 151     // trait_to_placeholder_substs and use it to convert the predicates contained
 152     // in the trait_m.generics to the placeholder form.
 153     //
 154     // Finally we register each of these predicates as an obligation in
 155     // a fresh FulfillmentCtxt, and invoke select_all_or_error.
 156
 157     // Create mapping from impl to placeholder.
 158     let impl_to_placeholder_substs = InternalSubsts::identity_for_item(tcx, impl_m.def_id);
 159
 160     // Create mapping from trait to placeholder.
 161     let trait_to_placeholder_substs =
 162         impl_to_placeholder_substs.rebase_onto(tcx, impl_m.container.id(), trait_to_impl_substs);
 163     debug!("compare_impl_method: trait_to_placeholder_substs={:?}", trait_to_placeholder_substs);
 164
 165     let impl_m_generics = tcx.generics_of(impl_m.def_id);
 166     let trait_m_generics = tcx.generics_of(trait_m.def_id);
 167     let impl_m_predicates = tcx.predicates_of(impl_m.def_id);
 168     let trait_m_predicates = tcx.predicates_of(trait_m.def_id);
 169
 170     // Check region bounds.
 171     check_region_bounds_on_impl_item(
 172         tcx,
 173         impl_m_span,
 174         impl_m,
 175         trait_m,
 176         &trait_m_generics,
 177         &impl_m_generics,
 178     )?;
 179
 180     // Create obligations for each predicate declared by the impl
 181     // definition in the context of the trait's parameter
 182     // environment. We can't just use `impl_env.caller_bounds`,
 183     // however, because we want to replace all late-bound regions with
 184     // region variables.
 185     let impl_predicates = tcx.predicates_of(impl_m_predicates.parent.unwrap());
 186     let mut hybrid_preds = impl_predicates.instantiate_identity(tcx);
 187
 188     debug!("compare_impl_method: impl_bounds={:?}", hybrid_preds);
 189
 190     // This is the only tricky bit of the new way we check implementation methods
 191     // We need to build a set of predicates where only the method-level bounds
 192     // are from the trait and we assume all other bounds from the implementation
 193     // to be previously satisfied.
 194     //
 195     // We then register the obligations from the impl_m and check to see
 196     // if all constraints hold.
 197     hybrid_preds
 198         .predicates
 199         .extend(trait_m_predicates.instantiate_own(tcx, trait_to_placeholder_substs).predicates);
 200
 201     // Construct trait parameter environment and then shift it into the placeholder viewpoint.
 202     // The key step here is to update the caller_bounds's predicates to be
 203     // the new hybrid bounds we computed.
 204     let normalize_cause = traits::ObligationCause::misc(impl_m_span, impl_m_hir_id);
 205     let param_env =
 206         ty::ParamEnv::new(tcx.intern_predicates(&hybrid_preds.predicates), Reveal::UserFacing);
 207     let param_env = traits::normalize_param_env_or_error(
 208         tcx,
 209         impl_m.def_id,
 210         param_env,
 211         normalize_cause.clone(),
 212     );
 213
 214     tcx.infer_ctxt().enter(|infcx| {
 215         let inh = Inherited::new(infcx, impl_m.def_id.expect_local());
 216         let infcx = &inh.infcx;
 217
 218         debug!("compare_impl_method: caller_bounds={:?}", param_env.caller_bounds());
 219
 220         let mut selcx = traits::SelectionContext::new(&infcx);
 221
 222         let impl_m_own_bounds = impl_m_predicates.instantiate_own(tcx, impl_to_placeholder_substs);
 223         let (impl_m_own_bounds, _) = infcx.replace_bound_vars_with_fresh_vars(
 224             impl_m_span,
 225             infer::HigherRankedType,
 226             &ty::Binder::bind(impl_m_own_bounds.predicates),
 227         );
 228         for predicate in impl_m_own_bounds {
 229             let traits::Normalized { value: predicate, obligations } =
 230                 traits::normalize(&mut selcx, param_env, normalize_cause.clone(), &predicate);
 231
 232             inh.register_predicates(obligations);
 233             inh.register_predicate(traits::Obligation::new(cause.clone(), param_env, predicate));
 234         }
 235
 236         // We now need to check that the signature of the impl method is
 237         // compatible with that of the trait method. We do this by
 238         // checking that `impl_fty <: trait_fty`.
 239         //
 240         // FIXME. Unfortunately, this doesn't quite work right now because
 241         // associated type normalization is not integrated into subtype
 242         // checks. For the comparison to be valid, we need to
 243         // normalize the associated types in the impl/trait methods
 244         // first. However, because function types bind regions, just
 245         // calling `normalize_associated_types_in` would have no effect on
 246         // any associated types appearing in the fn arguments or return
 247         // type.
 248
 249         // Compute placeholder form of impl and trait method tys.
 250         let tcx = infcx.tcx;
 251
 252         let (impl_sig, _) = infcx.replace_bound_vars_with_fresh_vars(
 253             impl_m_span,
 254             infer::HigherRankedType,
 255             &tcx.fn_sig(impl_m.def_id),
 256         );
 257         let impl_sig =
 258             inh.normalize_associated_types_in(impl_m_span, impl_m_hir_id, param_env, &impl_sig);
 259         let impl_fty = tcx.mk_fn_ptr(ty::Binder::bind(impl_sig));
 260         debug!("compare_impl_method: impl_fty={:?}", impl_fty);
 261
 262         let trait_sig = tcx.liberate_late_bound_regions(impl_m.def_id, &tcx.fn_sig(trait_m.def_id));
 263         let trait_sig = trait_sig.subst(tcx, trait_to_placeholder_substs);
 264         let trait_sig =
 265             inh.normalize_associated_types_in(impl_m_span, impl_m_hir_id, param_env, &trait_sig);
 266         let trait_fty = tcx.mk_fn_ptr(ty::Binder::bind(trait_sig));
 267
 268         debug!("compare_impl_method: trait_fty={:?}", trait_fty);
 269
 270         let sub_result = infcx.at(&cause, param_env).sup(trait_fty, impl_fty).map(
 271             |InferOk { obligations, .. }| {
 272                 inh.register_predicates(obligations);
 273             },
 274         );
 275
 276         if let Err(terr) = sub_result {
 277             debug!("sub_types failed: impl ty {:?}, trait ty {:?}", impl_fty, trait_fty);
 278
 279             let (impl_err_span, trait_err_span) = extract_spans_for_error_reporting(
 280                 &infcx, param_env, &terr, &cause, impl_m, impl_sig, trait_m, trait_sig,
 281             );
 282
 283             cause.make_mut().span = impl_err_span;
 284
 285             let mut diag = struct_span_err!(
 286                 tcx.sess,
 287                 cause.span(tcx),
 288                 E0053,
 289                 "method `{}` has an incompatible type for trait",
 290                 trait_m.ident
 291             );
 292             if let TypeError::Mutability = terr {
 293                 if let Some(trait_err_span) = trait_err_span {
 294                     if let Ok(trait_err_str) = tcx.sess.source_map().span_to_snippet(trait_err_span)
 295                     {
 296                         diag.span_suggestion(
 297                             impl_err_span,
 298                             "consider change the type to match the mutability in trait",
 299                             trait_err_str,
 300                             Applicability::MachineApplicable,
 301                         );
 302                     }
 303                 }
 304             }
 305
 306             infcx.note_type_err(
 307                 &mut diag,
 308                 &cause,
 309                 trait_err_span.map(|sp| (sp, "type in trait".to_owned())),
 310                 Some(infer::ValuePairs::Types(ExpectedFound {
 311                     expected: trait_fty,
 312                     found: impl_fty,
 313                 })),
 314                 &terr,
 315             );
 316             diag.emit();
 317             return Err(ErrorReported);
 318         }
 319
 320         // Check that all obligations are satisfied by the implementation's
 321         // version.
 322         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
 323             infcx.report_fulfillment_errors(errors, None, false);
 324             return Err(ErrorReported);
 325         }
 326
 327         // Finally, resolve all regions. This catches wily misuses of
 328         // lifetime parameters.
 329         let fcx = FnCtxt::new(&inh, param_env, impl_m_hir_id);
 330         fcx.regionck_item(impl_m_hir_id, impl_m_span, &[]);
 331
 332         Ok(())
 333     })
 334 }
 335
 336 fn check_region_bounds_on_impl_item<'tcx>(
 337     tcx: TyCtxt<'tcx>,
 338     span: Span,
 339     impl_m: &ty::AssocItem,
 340     trait_m: &ty::AssocItem,
 341     trait_generics: &ty::Generics,
 342     impl_generics: &ty::Generics,
 343 ) -> Result<(), ErrorReported> {
 344     let trait_params = trait_generics.own_counts().lifetimes;
 345     let impl_params = impl_generics.own_counts().lifetimes;
 346
 347     debug!(
 348         "check_region_bounds_on_impl_item: \
 349             trait_generics={:?} \
 350             impl_generics={:?}",
 351         trait_generics, impl_generics
 352     );
 353
 354     // Must have same number of early-bound lifetime parameters.
 355     // Unfortunately, if the user screws up the bounds, then this
 356     // will change classification between early and late.  E.g.,
 357     // if in trait we have `<'a,'b:'a>`, and in impl we just have
 358     // `<'a,'b>`, then we have 2 early-bound lifetime parameters
 359     // in trait but 0 in the impl. But if we report "expected 2
 360     // but found 0" it's confusing, because it looks like there
 361     // are zero. Since I don't quite know how to phrase things at
 362     // the moment, give a kind of vague error message.
 363     if trait_params != impl_params {
 364         let item_kind = assoc_item_kind_str(impl_m);
 365         let def_span = tcx.sess.source_map().guess_head_span(span);
 366         let span = tcx.hir().get_generics(impl_m.def_id).map(|g| g.span).unwrap_or(def_span);
 367         let generics_span = if let Some(sp) = tcx.hir().span_if_local(trait_m.def_id) {
 368             let def_sp = tcx.sess.source_map().guess_head_span(sp);
 369             Some(tcx.hir().get_generics(trait_m.def_id).map(|g| g.span).unwrap_or(def_sp))
 370         } else {
 371             None
 372         };
 373         tcx.sess.emit_err(LifetimesOrBoundsMismatchOnTrait {
 374             span,
 375             item_kind,
 376             ident: impl_m.ident,
 377             generics_span,
 378         });
 379         return Err(ErrorReported);
 380     }
 381
 382     Ok(())
 383 }
 384
 385 fn extract_spans_for_error_reporting<'a, 'tcx>(
 386     infcx: &infer::InferCtxt<'a, 'tcx>,
 387     param_env: ty::ParamEnv<'tcx>,
 388     terr: &TypeError<'_>,
 389     cause: &ObligationCause<'tcx>,
 390     impl_m: &ty::AssocItem,
 391     impl_sig: ty::FnSig<'tcx>,
 392     trait_m: &ty::AssocItem,
 393     trait_sig: ty::FnSig<'tcx>,
 394 ) -> (Span, Option<Span>) {
 395     let tcx = infcx.tcx;
 396     let impl_m_hir_id = tcx.hir().local_def_id_to_hir_id(impl_m.def_id.expect_local());
 397     let (impl_m_output, impl_m_iter) = match tcx.hir().expect_impl_item(impl_m_hir_id).kind {
 398         ImplItemKind::Fn(ref impl_m_sig, _) => {
 399             (&impl_m_sig.decl.output, impl_m_sig.decl.inputs.iter())
 400         }
 401         _ => bug!("{:?} is not a method", impl_m),
 402     };
 403
 404     match *terr {
 405         TypeError::Mutability => {
 406             if let Some(def_id) = trait_m.def_id.as_local() {
 407                 let trait_m_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 408                 let trait_m_iter = match tcx.hir().expect_trait_item(trait_m_hir_id).kind {
 409                     TraitItemKind::Fn(ref trait_m_sig, _) => trait_m_sig.decl.inputs.iter(),
 410                     _ => bug!("{:?} is not a TraitItemKind::Fn", trait_m),
 411                 };
 412
 413                 impl_m_iter
 414                     .zip(trait_m_iter)
 415                     .find(|&(ref impl_arg, ref trait_arg)| {
 416                         match (&impl_arg.kind, &trait_arg.kind) {
 417                             (
 418                                 &hir::TyKind::Rptr(_, ref impl_mt),
 419                                 &hir::TyKind::Rptr(_, ref trait_mt),
 420                             )
 421                             | (&hir::TyKind::Ptr(ref impl_mt), &hir::TyKind::Ptr(ref trait_mt)) => {
 422                                 impl_mt.mutbl != trait_mt.mutbl
 423                             }
 424                             _ => false,
 425                         }
 426                     })
 427                     .map(|(ref impl_arg, ref trait_arg)| (impl_arg.span, Some(trait_arg.span)))
 428                     .unwrap_or_else(|| (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id)))
 429             } else {
 430                 (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id))
 431             }
 432         }
 433         TypeError::Sorts(ExpectedFound { .. }) => {
 434             if let Some(def_id) = trait_m.def_id.as_local() {
 435                 let trait_m_hir_id = tcx.hir().local_def_id_to_hir_id(def_id);
 436                 let (trait_m_output, trait_m_iter) =
 437                     match tcx.hir().expect_trait_item(trait_m_hir_id).kind {
 438                         TraitItemKind::Fn(ref trait_m_sig, _) => {
 439                             (&trait_m_sig.decl.output, trait_m_sig.decl.inputs.iter())
 440                         }
 441                         _ => bug!("{:?} is not a TraitItemKind::Fn", trait_m),
 442                     };
 443
 444                 let impl_iter = impl_sig.inputs().iter();
 445                 let trait_iter = trait_sig.inputs().iter();
 446                 impl_iter
 447                     .zip(trait_iter)
 448                     .zip(impl_m_iter)
 449                     .zip(trait_m_iter)
 450                     .find_map(|(((&impl_arg_ty, &trait_arg_ty), impl_arg), trait_arg)| match infcx
 451                         .at(&cause, param_env)
 452                         .sub(trait_arg_ty, impl_arg_ty)
 453                     {
 454                         Ok(_) => None,
 455                         Err(_) => Some((impl_arg.span, Some(trait_arg.span))),
 456                     })
 457                     .unwrap_or_else(|| {
 458                         if infcx
 459                             .at(&cause, param_env)
 460                             .sup(trait_sig.output(), impl_sig.output())
 461                             .is_err()
 462                         {
 463                             (impl_m_output.span(), Some(trait_m_output.span()))
 464                         } else {
 465                             (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id))
 466                         }
 467                     })
 468             } else {
 469                 (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id))
 470             }
 471         }
 472         _ => (cause.span(tcx), tcx.hir().span_if_local(trait_m.def_id)),
 473     }
 474 }
 475
 476 fn compare_self_type<'tcx>(
 477     tcx: TyCtxt<'tcx>,
 478     impl_m: &ty::AssocItem,
 479     impl_m_span: Span,
 480     trait_m: &ty::AssocItem,
 481     impl_trait_ref: ty::TraitRef<'tcx>,
 482 ) -> Result<(), ErrorReported> {
 483     // Try to give more informative error messages about self typing
 484     // mismatches.  Note that any mismatch will also be detected
 485     // below, where we construct a canonical function type that
 486     // includes the self parameter as a normal parameter.  It's just
 487     // that the error messages you get out of this code are a bit more
 488     // inscrutable, particularly for cases where one method has no
 489     // self.
 490
 491     let self_string = |method: &ty::AssocItem| {
 492         let untransformed_self_ty = match method.container {
 493             ty::ImplContainer(_) => impl_trait_ref.self_ty(),
 494             ty::TraitContainer(_) => tcx.types.self_param,
 495         };
 496         let self_arg_ty = tcx.fn_sig(method.def_id).input(0).skip_binder();
 497         let param_env = ty::ParamEnv::reveal_all();
 498
 499         tcx.infer_ctxt().enter(|infcx| {
 500             let self_arg_ty =
 501                 tcx.liberate_late_bound_regions(method.def_id, &ty::Binder::bind(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         compare_projection_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 fn compare_projection_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     let have_gats = tcx.features().generic_associated_types;
1181     if impl_ty.defaultness.is_final() && !have_gats {
1182         // For "final", non-generic associate type implementations, we
1183         // don't need this as described above.
1184         return Ok(());
1185     }
1186
1187     // Given
1188     //
1189     // impl<A, B> Foo<u32> for (A, B) {
1190     //     type Bar<C> =...
1191     // }
1192     //
1193     // - `impl_substs` would be `[A, B, C]`
1194     // - `rebased_substs` would be `[(A, B), u32, C]`, combining the substs from
1195     //    the *trait* with the generic associated type parameters.
1196     let impl_ty_substs = InternalSubsts::identity_for_item(tcx, impl_ty.def_id);
1197     let rebased_substs =
1198         impl_ty_substs.rebase_onto(tcx, impl_ty.container.id(), impl_trait_ref.substs);
1199     let impl_ty_value = tcx.type_of(impl_ty.def_id);
1200
1201     let param_env = tcx.param_env(impl_ty.def_id);
1202
1203     // When checking something like
1204     //
1205     // trait X { type Y: PartialEq<<Self as X>::Y> }
1206     // impl X for T { default type Y = S; }
1207     //
1208     // We will have to prove the bound S: PartialEq<<T as X>::Y>. In this case
1209     // we want <T as X>::Y to normalize to S. This is valid because we are
1210     // checking the default value specifically here. Add this equality to the
1211     // ParamEnv for normalization specifically.
1212     let normalize_param_env = {
1213         let mut predicates = param_env.caller_bounds().iter().collect::<Vec<_>>();
1214         predicates.push(
1215             ty::Binder::dummy(ty::ProjectionPredicate {
1216                 projection_ty: ty::ProjectionTy {
1217                     item_def_id: trait_ty.def_id,
1218                     substs: rebased_substs,
1219                 },
1220                 ty: impl_ty_value,
1221             })
1222             .to_predicate(tcx),
1223         );
1224         ty::ParamEnv::new(tcx.intern_predicates(&predicates), Reveal::UserFacing)
1225     };
1226
1227     tcx.infer_ctxt().enter(move |infcx| {
1228         let inh = Inherited::new(infcx, impl_ty.def_id.expect_local());
1229         let infcx = &inh.infcx;
1230         let mut selcx = traits::SelectionContext::new(&infcx);
1231
1232         let impl_ty_hir_id = tcx.hir().local_def_id_to_hir_id(impl_ty.def_id.expect_local());
1233         let normalize_cause = traits::ObligationCause::misc(impl_ty_span, impl_ty_hir_id);
1234         let cause = ObligationCause::new(
1235             impl_ty_span,
1236             impl_ty_hir_id,
1237             ObligationCauseCode::ItemObligation(trait_ty.def_id),
1238         );
1239
1240         let predicates = tcx.projection_predicates(trait_ty.def_id);
1241         debug!("compare_projection_bounds: projection_predicates={:?}", predicates);
1242
1243         for predicate in predicates {
1244             let concrete_ty_predicate = predicate.subst(tcx, rebased_substs);
1245             debug!("compare_projection_bounds: concrete predicate = {:?}", concrete_ty_predicate);
1246
1247             let traits::Normalized { value: normalized_predicate, obligations } = traits::normalize(
1248                 &mut selcx,
1249                 normalize_param_env,
1250                 normalize_cause.clone(),
1251                 &concrete_ty_predicate,
1252             );
1253             debug!("compare_projection_bounds: normalized predicate = {:?}", normalized_predicate);
1254
1255             inh.register_predicates(obligations);
1256             inh.register_predicate(traits::Obligation::new(
1257                 cause.clone(),
1258                 param_env,
1259                 normalized_predicate,
1260             ));
1261         }
1262
1263         // Check that all obligations are satisfied by the implementation's
1264         // version.
1265         if let Err(ref errors) = inh.fulfillment_cx.borrow_mut().select_all_or_error(&infcx) {
1266             infcx.report_fulfillment_errors(errors, None, false);
1267             return Err(ErrorReported);
1268         }
1269
1270         // Finally, resolve all regions. This catches wily misuses of
1271         // lifetime parameters.
1272         let fcx = FnCtxt::new(&inh, param_env, impl_ty_hir_id);
1273         fcx.regionck_item(impl_ty_hir_id, impl_ty_span, &[]);
1274
1275         Ok(())
1276     })
1277 }
1278
1279 fn assoc_item_kind_str(impl_item: &ty::AssocItem) -> &'static str {
1280     match impl_item.kind {
1281         ty::AssocKind::Const => "const",
1282         ty::AssocKind::Fn => "method",
1283         ty::AssocKind::Type => "type",
1284     }
1285 }