src/librustc_typeck/check/wfcheck.rs

   1 use crate::check::{Inherited, FnCtxt};
   2 use crate::constrained_generic_params::{identify_constrained_generic_params, Parameter};
   3
   4 use crate::hir::def_id::DefId;
   5 use rustc::traits::{self, ObligationCauseCode};
   6 use rustc::ty::{self, Lift, Ty, TyCtxt, GenericParamDefKind, TypeFoldable, ToPredicate};
   7 use rustc::ty::subst::{Subst, InternalSubsts};
   8 use rustc::util::nodemap::{FxHashSet, FxHashMap};
   9 use rustc::mir::interpret::ConstValue;
  10 use rustc::middle::lang_items;
  11 use rustc::infer::opaque_types::may_define_existential_type;
  12
  13 use syntax::ast;
  14 use syntax::feature_gate::{self, GateIssue};
  15 use syntax_pos::Span;
  16 use syntax::symbol::sym;
  17 use errors::{DiagnosticBuilder, DiagnosticId};
  18
  19 use rustc::hir::itemlikevisit::ParItemLikeVisitor;
  20 use rustc::hir;
  21
  22 /// Helper type of a temporary returned by `.for_item(...)`.
  23 /// This is necessary because we can't write the following bound:
  24 ///
  25 /// ```rust
  26 /// F: for<'b, 'tcx> where 'gcx: 'tcx FnOnce(FnCtxt<'b, 'gcx, 'tcx>)
  27 /// ```
  28 struct CheckWfFcxBuilder<'a, 'gcx: 'a+'tcx, 'tcx: 'a> {
  29     inherited: super::InheritedBuilder<'a, 'gcx, 'tcx>,
  30     id: hir::HirId,
  31     span: Span,
  32     param_env: ty::ParamEnv<'tcx>,
  33 }
  34
  35 impl<'a, 'gcx, 'tcx> CheckWfFcxBuilder<'a, 'gcx, 'tcx> {
  36     fn with_fcx<F>(&'tcx mut self, f: F) where
  37         F: for<'b> FnOnce(&FnCtxt<'b, 'gcx, 'tcx>,
  38                          TyCtxt<'b, 'gcx, 'gcx>) -> Vec<Ty<'tcx>>
  39     {
  40         let id = self.id;
  41         let span = self.span;
  42         let param_env = self.param_env;
  43         self.inherited.enter(|inh| {
  44             let fcx = FnCtxt::new(&inh, param_env, id);
  45             if !inh.tcx.features().trivial_bounds {
  46                 // As predicates are cached rather than obligations, this
  47                 // needsto be called first so that they are checked with an
  48                 // empty `param_env`.
  49                 check_false_global_bounds(&fcx, span, id);
  50             }
  51             let wf_tys = f(&fcx, fcx.tcx.global_tcx());
  52             fcx.select_all_obligations_or_error();
  53             fcx.regionck_item(id, span, &wf_tys);
  54         });
  55     }
  56 }
  57
  58 /// Checks that the field types (in a struct def'n) or argument types (in an enum def'n) are
  59 /// well-formed, meaning that they do not require any constraints not declared in the struct
  60 /// definition itself. For example, this definition would be illegal:
  61 ///
  62 /// ```rust
  63 /// struct Ref<'a, T> { x: &'a T }
  64 /// ```
  65 ///
  66 /// because the type did not declare that `T:'a`.
  67 ///
  68 /// We do this check as a pre-pass before checking fn bodies because if these constraints are
  69 /// not included it frequently leads to confusing errors in fn bodies. So it's better to check
  70 /// the types first.
  71 pub fn check_item_well_formed<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) {
  72     let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
  73     let item = tcx.hir().expect_item_by_hir_id(hir_id);
  74
  75     debug!("check_item_well_formed(it.hir_id={:?}, it.name={})",
  76            item.hir_id,
  77            tcx.def_path_str(def_id));
  78
  79     match item.node {
  80         // Right now we check that every default trait implementation
  81         // has an implementation of itself. Basically, a case like:
  82         //
  83         //     impl Trait for T {}
  84         //
  85         // has a requirement of `T: Trait` which was required for default
  86         // method implementations. Although this could be improved now that
  87         // there's a better infrastructure in place for this, it's being left
  88         // for a follow-up work.
  89         //
  90         // Since there's such a requirement, we need to check *just* positive
  91         // implementations, otherwise things like:
  92         //
  93         //     impl !Send for T {}
  94         //
  95         // won't be allowed unless there's an *explicit* implementation of `Send`
  96         // for `T`
  97         hir::ItemKind::Impl(_, polarity, defaultness, _, ref trait_ref, ref self_ty, _) => {
  98             let is_auto = tcx.impl_trait_ref(tcx.hir().local_def_id_from_hir_id(item.hir_id))
  99                                 .map_or(false, |trait_ref| tcx.trait_is_auto(trait_ref.def_id));
 100             if let (hir::Defaultness::Default { .. }, true) = (defaultness, is_auto) {
 101                 tcx.sess.span_err(item.span, "impls of auto traits cannot be default");
 102             }
 103             if polarity == hir::ImplPolarity::Positive {
 104                 check_impl(tcx, item, self_ty, trait_ref);
 105             } else {
 106                 // FIXME(#27579): what amount of WF checking do we need for neg impls?
 107                 if trait_ref.is_some() && !is_auto {
 108                     span_err!(tcx.sess, item.span, E0192,
 109                               "negative impls are only allowed for \
 110                                auto traits (e.g., `Send` and `Sync`)")
 111                 }
 112             }
 113         }
 114         hir::ItemKind::Fn(..) => {
 115             check_item_fn(tcx, item);
 116         }
 117         hir::ItemKind::Static(ref ty, ..) => {
 118             check_item_type(tcx, item.hir_id, ty.span, false);
 119         }
 120         hir::ItemKind::Const(ref ty, ..) => {
 121             check_item_type(tcx, item.hir_id, ty.span, false);
 122         }
 123         hir::ItemKind::ForeignMod(ref module) => for it in module.items.iter() {
 124             if let hir::ForeignItemKind::Static(ref ty, ..) = it.node {
 125                 check_item_type(tcx, it.hir_id, ty.span, true);
 126             }
 127         },
 128         hir::ItemKind::Struct(ref struct_def, ref ast_generics) => {
 129             check_type_defn(tcx, item, false, |fcx| {
 130                 vec![fcx.non_enum_variant(struct_def)]
 131             });
 132
 133             check_variances_for_type_defn(tcx, item, ast_generics);
 134         }
 135         hir::ItemKind::Union(ref struct_def, ref ast_generics) => {
 136             check_type_defn(tcx, item, true, |fcx| {
 137                 vec![fcx.non_enum_variant(struct_def)]
 138             });
 139
 140             check_variances_for_type_defn(tcx, item, ast_generics);
 141         }
 142         hir::ItemKind::Enum(ref enum_def, ref ast_generics) => {
 143             check_type_defn(tcx, item, true, |fcx| {
 144                 fcx.enum_variants(enum_def)
 145             });
 146
 147             check_variances_for_type_defn(tcx, item, ast_generics);
 148         }
 149         hir::ItemKind::Trait(..) => {
 150             check_trait(tcx, item);
 151         }
 152         hir::ItemKind::TraitAlias(..) => {
 153             check_trait(tcx, item);
 154         }
 155         _ => {}
 156     }
 157 }
 158
 159 pub fn check_trait_item<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) {
 160     let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
 161     let trait_item = tcx.hir().expect_trait_item(hir_id);
 162
 163     let method_sig = match trait_item.node {
 164         hir::TraitItemKind::Method(ref sig, _) => Some(sig),
 165         _ => None
 166     };
 167     check_associated_item(tcx, trait_item.hir_id, trait_item.span, method_sig);
 168 }
 169
 170 pub fn check_impl_item<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, def_id: DefId) {
 171     let hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
 172     let impl_item = tcx.hir().expect_impl_item(hir_id);
 173
 174     let method_sig = match impl_item.node {
 175         hir::ImplItemKind::Method(ref sig, _) => Some(sig),
 176         _ => None
 177     };
 178     check_associated_item(tcx, impl_item.hir_id, impl_item.span, method_sig);
 179 }
 180
 181 fn check_associated_item<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 182                                    item_id: hir::HirId,
 183                                    span: Span,
 184                                    sig_if_method: Option<&hir::MethodSig>) {
 185     debug!("check_associated_item: {:?}", item_id);
 186
 187     let code = ObligationCauseCode::MiscObligation;
 188     for_id(tcx, item_id, span).with_fcx(|fcx, tcx| {
 189         let item = fcx.tcx.associated_item(fcx.tcx.hir().local_def_id_from_hir_id(item_id));
 190
 191         let (mut implied_bounds, self_ty) = match item.container {
 192             ty::TraitContainer(_) => (vec![], fcx.tcx.mk_self_type()),
 193             ty::ImplContainer(def_id) => (fcx.impl_implied_bounds(def_id, span),
 194                                           fcx.tcx.type_of(def_id))
 195         };
 196
 197         match item.kind {
 198             ty::AssocKind::Const => {
 199                 let ty = fcx.tcx.type_of(item.def_id);
 200                 let ty = fcx.normalize_associated_types_in(span, &ty);
 201                 fcx.register_wf_obligation(ty, span, code.clone());
 202             }
 203             ty::AssocKind::Method => {
 204                 reject_shadowing_parameters(fcx.tcx, item.def_id);
 205                 let sig = fcx.tcx.fn_sig(item.def_id);
 206                 let sig = fcx.normalize_associated_types_in(span, &sig);
 207                 check_fn_or_method(tcx, fcx, span, sig,
 208                                    item.def_id, &mut implied_bounds);
 209                 let sig_if_method = sig_if_method.expect("bad signature for method");
 210                 check_method_receiver(fcx, sig_if_method, &item, self_ty);
 211             }
 212             ty::AssocKind::Type => {
 213                 if item.defaultness.has_value() {
 214                     let ty = fcx.tcx.type_of(item.def_id);
 215                     let ty = fcx.normalize_associated_types_in(span, &ty);
 216                     fcx.register_wf_obligation(ty, span, code.clone());
 217                 }
 218             }
 219             ty::AssocKind::Existential => {
 220                 // do nothing, existential types check themselves
 221             }
 222         }
 223
 224         implied_bounds
 225     })
 226 }
 227
 228 fn for_item<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'gcx>, item: &hir::Item)
 229                             -> CheckWfFcxBuilder<'a, 'gcx, 'tcx> {
 230     for_id(tcx, item.hir_id, item.span)
 231 }
 232
 233 fn for_id<'a, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'gcx>, id: hir::HirId, span: Span)
 234                           -> CheckWfFcxBuilder<'a, 'gcx, 'tcx> {
 235     let def_id = tcx.hir().local_def_id_from_hir_id(id);
 236     CheckWfFcxBuilder {
 237         inherited: Inherited::build(tcx, def_id),
 238         id,
 239         span,
 240         param_env: tcx.param_env(def_id),
 241     }
 242 }
 243
 244 /// In a type definition, we check that to ensure that the types of the fields are well-formed.
 245 fn check_type_defn<'a, 'tcx, F>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 246                                 item: &hir::Item, all_sized: bool, mut lookup_fields: F)
 247     where F: for<'fcx, 'gcx, 'tcx2> FnMut(&FnCtxt<'fcx, 'gcx, 'tcx2>) -> Vec<AdtVariant<'tcx2>>
 248 {
 249     for_item(tcx, item).with_fcx(|fcx, fcx_tcx| {
 250         let variants = lookup_fields(fcx);
 251         let def_id = fcx.tcx.hir().local_def_id_from_hir_id(item.hir_id);
 252         let packed = fcx.tcx.adt_def(def_id).repr.packed();
 253
 254         for variant in &variants {
 255             // For DST, or when drop needs to copy things around, all
 256             // intermediate types must be sized.
 257             let needs_drop_copy = || {
 258                 packed && {
 259                     let ty = variant.fields.last().unwrap().ty;
 260                     fcx.tcx.erase_regions(&ty).lift_to_tcx(fcx_tcx)
 261                         .map(|ty| ty.needs_drop(fcx_tcx, fcx_tcx.param_env(def_id)))
 262                         .unwrap_or_else(|| {
 263                             fcx_tcx.sess.delay_span_bug(
 264                                 item.span, &format!("inference variables in {:?}", ty));
 265                             // Just treat unresolved type expression as if it needs drop.
 266                             true
 267                         })
 268                 }
 269             };
 270             let all_sized =
 271                 all_sized ||
 272                 variant.fields.is_empty() ||
 273                 needs_drop_copy();
 274             let unsized_len = if all_sized {
 275                 0
 276             } else {
 277                 1
 278             };
 279             for (idx, field) in variant.fields[..variant.fields.len() - unsized_len]
 280                 .iter()
 281                 .enumerate()
 282             {
 283                 let last = idx == variant.fields.len() - 1;
 284                 fcx.register_bound(
 285                     field.ty,
 286                     fcx.tcx.require_lang_item(lang_items::SizedTraitLangItem),
 287                     traits::ObligationCause::new(
 288                         field.span,
 289                         fcx.body_id,
 290                         traits::FieldSized {
 291                             adt_kind: match item.node.adt_kind() {
 292                                 Some(i) => i,
 293                                 None => bug!(),
 294                             },
 295                             last
 296                         }
 297                     )
 298                 );
 299             }
 300
 301             // All field types must be well-formed.
 302             for field in &variant.fields {
 303                 fcx.register_wf_obligation(field.ty, field.span,
 304                     ObligationCauseCode::MiscObligation)
 305             }
 306         }
 307
 308         check_where_clauses(tcx, fcx, item.span, def_id, None);
 309
 310         // No implied bounds in a struct definition.
 311         vec![]
 312     });
 313 }
 314
 315 fn check_trait<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, item: &hir::Item) {
 316     debug!("check_trait: {:?}", item.hir_id);
 317
 318     let trait_def_id = tcx.hir().local_def_id_from_hir_id(item.hir_id);
 319
 320     let trait_def = tcx.trait_def(trait_def_id);
 321     if trait_def.is_marker {
 322         for associated_def_id in &*tcx.associated_item_def_ids(trait_def_id) {
 323             struct_span_err!(
 324                 tcx.sess,
 325                 tcx.def_span(*associated_def_id),
 326                 E0714,
 327                 "marker traits cannot have associated items",
 328             ).emit();
 329         }
 330     }
 331
 332     for_item(tcx, item).with_fcx(|fcx, _| {
 333         check_where_clauses(tcx, fcx, item.span, trait_def_id, None);
 334         vec![]
 335     });
 336 }
 337
 338 fn check_item_fn<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, item: &hir::Item) {
 339     for_item(tcx, item).with_fcx(|fcx, tcx| {
 340         let def_id = fcx.tcx.hir().local_def_id_from_hir_id(item.hir_id);
 341         let sig = fcx.tcx.fn_sig(def_id);
 342         let sig = fcx.normalize_associated_types_in(item.span, &sig);
 343         let mut implied_bounds = vec![];
 344         check_fn_or_method(tcx, fcx, item.span, sig,
 345                            def_id, &mut implied_bounds);
 346         implied_bounds
 347     })
 348 }
 349
 350 fn check_item_type<'a, 'tcx>(
 351     tcx: TyCtxt<'a, 'tcx, 'tcx>,
 352     item_id: hir::HirId,
 353     ty_span: Span,
 354     allow_foreign_ty: bool,
 355 ) {
 356     debug!("check_item_type: {:?}", item_id);
 357
 358     for_id(tcx, item_id, ty_span).with_fcx(|fcx, gcx| {
 359         let ty = gcx.type_of(gcx.hir().local_def_id_from_hir_id(item_id));
 360         let item_ty = fcx.normalize_associated_types_in(ty_span, &ty);
 361
 362         let mut forbid_unsized = true;
 363         if allow_foreign_ty {
 364             if let ty::Foreign(_) = fcx.tcx.struct_tail(item_ty).sty {
 365                 forbid_unsized = false;
 366             }
 367         }
 368
 369         fcx.register_wf_obligation(item_ty, ty_span, ObligationCauseCode::MiscObligation);
 370         if forbid_unsized {
 371             fcx.register_bound(
 372                 item_ty,
 373                 fcx.tcx.require_lang_item(lang_items::SizedTraitLangItem),
 374                 traits::ObligationCause::new(ty_span, fcx.body_id, traits::MiscObligation),
 375             );
 376         }
 377
 378         // No implied bounds in a const, etc.
 379         vec![]
 380     });
 381 }
 382
 383 fn check_impl<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 384                         item: &hir::Item,
 385                         ast_self_ty: &hir::Ty,
 386                         ast_trait_ref: &Option<hir::TraitRef>)
 387 {
 388     debug!("check_impl: {:?}", item);
 389
 390     for_item(tcx, item).with_fcx(|fcx, tcx| {
 391         let item_def_id = fcx.tcx.hir().local_def_id_from_hir_id(item.hir_id);
 392
 393         match *ast_trait_ref {
 394             Some(ref ast_trait_ref) => {
 395                 let trait_ref = fcx.tcx.impl_trait_ref(item_def_id).unwrap();
 396                 let trait_ref =
 397                     fcx.normalize_associated_types_in(
 398                         ast_trait_ref.path.span, &trait_ref);
 399                 let obligations =
 400                     ty::wf::trait_obligations(fcx,
 401                                                 fcx.param_env,
 402                                                 fcx.body_id,
 403                                                 &trait_ref,
 404                                                 ast_trait_ref.path.span);
 405                 for obligation in obligations {
 406                     fcx.register_predicate(obligation);
 407                 }
 408             }
 409             None => {
 410                 let self_ty = fcx.tcx.type_of(item_def_id);
 411                 let self_ty = fcx.normalize_associated_types_in(item.span, &self_ty);
 412                 fcx.register_wf_obligation(self_ty, ast_self_ty.span,
 413                     ObligationCauseCode::MiscObligation);
 414             }
 415         }
 416
 417         check_where_clauses(tcx, fcx, item.span, item_def_id, None);
 418
 419         fcx.impl_implied_bounds(item_def_id, item.span)
 420     });
 421 }
 422
 423 /// Checks where-clauses and inline bounds that are declared on `def_id`.
 424 fn check_where_clauses<'a, 'gcx, 'fcx, 'tcx>(
 425     tcx: TyCtxt<'a, 'gcx, 'gcx>,
 426     fcx: &FnCtxt<'fcx, 'gcx, 'tcx>,
 427     span: Span,
 428     def_id: DefId,
 429     return_ty: Option<Ty<'tcx>>,
 430 ) {
 431     debug!("check_where_clauses(def_id={:?}, return_ty={:?})", def_id, return_ty);
 432
 433     let predicates = fcx.tcx.predicates_of(def_id);
 434     let generics = tcx.generics_of(def_id);
 435
 436     let is_our_default = |def: &ty::GenericParamDef| {
 437         match def.kind {
 438             GenericParamDefKind::Type { has_default, .. } => {
 439                 has_default && def.index >= generics.parent_count as u32
 440             }
 441             _ => unreachable!()
 442         }
 443     };
 444
 445     // Check that concrete defaults are well-formed. See test `type-check-defaults.rs`.
 446     // For example, this forbids the declaration:
 447     //
 448     //     struct Foo<T = Vec<[u32]>> { .. }
 449     //
 450     // Here, the default `Vec<[u32]>` is not WF because `[u32]: Sized` does not hold.
 451     for param in &generics.params {
 452         if let GenericParamDefKind::Type { .. } = param.kind {
 453             if is_our_default(&param) {
 454                 let ty = fcx.tcx.type_of(param.def_id);
 455                 // Ignore dependent defaults -- that is, where the default of one type
 456                 // parameter includes another (e.g., `<T, U = T>`). In those cases, we can't
 457                 // be sure if it will error or not as user might always specify the other.
 458                 if !ty.needs_subst() {
 459                     fcx.register_wf_obligation(ty, fcx.tcx.def_span(param.def_id),
 460                         ObligationCauseCode::MiscObligation);
 461                 }
 462             }
 463         }
 464     }
 465
 466     // Check that trait predicates are WF when params are substituted by their defaults.
 467     // We don't want to overly constrain the predicates that may be written but we want to
 468     // catch cases where a default my never be applied such as `struct Foo<T: Copy = String>`.
 469     // Therefore we check if a predicate which contains a single type param
 470     // with a concrete default is WF with that default substituted.
 471     // For more examples see tests `defaults-well-formedness.rs` and `type-check-defaults.rs`.
 472     //
 473     // First we build the defaulted substitution.
 474     let substs = InternalSubsts::for_item(fcx.tcx, def_id, |param, _| {
 475         match param.kind {
 476             GenericParamDefKind::Lifetime => {
 477                 // All regions are identity.
 478                 fcx.tcx.mk_param_from_def(param)
 479             }
 480
 481             GenericParamDefKind::Type { .. } => {
 482                 // If the param has a default, ...
 483                 if is_our_default(param) {
 484                     let default_ty = fcx.tcx.type_of(param.def_id);
 485                     // ... and it's not a dependent default, ...
 486                     if !default_ty.needs_subst() {
 487                         // ... then substitute it with the default.
 488                         return default_ty.into();
 489                     }
 490                 }
 491                 // Mark unwanted params as error.
 492                 fcx.tcx.types.err.into()
 493             }
 494
 495             GenericParamDefKind::Const => {
 496                 // FIXME(const_generics:defaults)
 497                 fcx.tcx.consts.err.into()
 498             }
 499         }
 500     });
 501
 502     // Now we build the substituted predicates.
 503     let default_obligations = predicates.predicates.iter().flat_map(|&(pred, _)| {
 504         #[derive(Default)]
 505         struct CountParams { params: FxHashSet<u32> }
 506         impl<'tcx> ty::fold::TypeVisitor<'tcx> for CountParams {
 507             fn visit_ty(&mut self, t: Ty<'tcx>) -> bool {
 508                 if let ty::Param(param) = t.sty {
 509                     self.params.insert(param.index);
 510                 }
 511                 t.super_visit_with(self)
 512             }
 513
 514             fn visit_region(&mut self, _: ty::Region<'tcx>) -> bool {
 515                 true
 516             }
 517
 518             fn visit_const(&mut self, c: &'tcx ty::Const<'tcx>) -> bool {
 519                 if let ConstValue::Param(param) = c.val {
 520                     self.params.insert(param.index);
 521                 }
 522                 c.super_visit_with(self)
 523             }
 524         }
 525         let mut param_count = CountParams::default();
 526         let has_region = pred.visit_with(&mut param_count);
 527         let substituted_pred = pred.subst(fcx.tcx, substs);
 528         // Don't check non-defaulted params, dependent defaults (including lifetimes)
 529         // or preds with multiple params.
 530         if substituted_pred.references_error() || param_count.params.len() > 1 || has_region {
 531             None
 532         } else if predicates.predicates.iter().any(|&(p, _)| p == substituted_pred) {
 533             // Avoid duplication of predicates that contain no parameters, for example.
 534             None
 535         } else {
 536             Some(substituted_pred)
 537         }
 538     }).map(|pred| {
 539         // Convert each of those into an obligation. So if you have
 540         // something like `struct Foo<T: Copy = String>`, we would
 541         // take that predicate `T: Copy`, substitute to `String: Copy`
 542         // (actually that happens in the previous `flat_map` call),
 543         // and then try to prove it (in this case, we'll fail).
 544         //
 545         // Note the subtle difference from how we handle `predicates`
 546         // below: there, we are not trying to prove those predicates
 547         // to be *true* but merely *well-formed*.
 548         let pred = fcx.normalize_associated_types_in(span, &pred);
 549         let cause = traits::ObligationCause::new(span, fcx.body_id, traits::ItemObligation(def_id));
 550         traits::Obligation::new(cause, fcx.param_env, pred)
 551     });
 552
 553     let mut predicates = predicates.instantiate_identity(fcx.tcx);
 554
 555     if let Some(return_ty) = return_ty {
 556         predicates.predicates.extend(check_existential_types(tcx, fcx, def_id, span, return_ty));
 557     }
 558
 559     let predicates = fcx.normalize_associated_types_in(span, &predicates);
 560
 561     debug!("check_where_clauses: predicates={:?}", predicates.predicates);
 562     let wf_obligations =
 563         predicates.predicates
 564                     .iter()
 565                     .flat_map(|p| ty::wf::predicate_obligations(fcx,
 566                                                                 fcx.param_env,
 567                                                                 fcx.body_id,
 568                                                                 p,
 569                                                                 span));
 570
 571     for obligation in wf_obligations.chain(default_obligations) {
 572         debug!("next obligation cause: {:?}", obligation.cause);
 573         fcx.register_predicate(obligation);
 574     }
 575 }
 576
 577 fn check_fn_or_method<'a, 'fcx, 'gcx, 'tcx>(tcx: TyCtxt<'a, 'gcx, 'gcx>,
 578                                             fcx: &FnCtxt<'fcx, 'gcx, 'tcx>,
 579                                             span: Span,
 580                                             sig: ty::PolyFnSig<'tcx>,
 581                                             def_id: DefId,
 582                                             implied_bounds: &mut Vec<Ty<'tcx>>)
 583 {
 584     let sig = fcx.normalize_associated_types_in(span, &sig);
 585     let sig = fcx.tcx.liberate_late_bound_regions(def_id, &sig);
 586
 587     for input_ty in sig.inputs() {
 588         fcx.register_wf_obligation(&input_ty, span, ObligationCauseCode::MiscObligation);
 589     }
 590     implied_bounds.extend(sig.inputs());
 591
 592     fcx.register_wf_obligation(sig.output(), span, ObligationCauseCode::MiscObligation);
 593
 594     // FIXME(#25759) return types should not be implied bounds
 595     implied_bounds.push(sig.output());
 596
 597     check_where_clauses(tcx, fcx, span, def_id, Some(sig.output()));
 598 }
 599
 600 /// Checks "defining uses" of existential types to ensure that they meet the restrictions laid for
 601 /// "higher-order pattern unification".
 602 /// This ensures that inference is tractable.
 603 /// In particular, definitions of existential types can only use other generics as arguments,
 604 /// and they cannot repeat an argument. Example:
 605 ///
 606 /// ```rust
 607 /// existential type Foo<A, B>;
 608 ///
 609 /// // Okay -- `Foo` is applied to two distinct, generic types.
 610 /// fn a<T, U>() -> Foo<T, U> { .. }
 611 ///
 612 /// // Not okay -- `Foo` is applied to `T` twice.
 613 /// fn b<T>() -> Foo<T, T> { .. }
 614 ///
 615 /// // Not okay -- `Foo` is applied to a non-generic type.
 616 /// fn b<T>() -> Foo<T, u32> { .. }
 617 /// ```
 618 ///
 619 fn check_existential_types<'a, 'fcx, 'gcx, 'tcx>(
 620     tcx: TyCtxt<'a, 'gcx, 'gcx>,
 621     fcx: &FnCtxt<'fcx, 'gcx, 'tcx>,
 622     fn_def_id: DefId,
 623     span: Span,
 624     ty: Ty<'tcx>,
 625 ) -> Vec<ty::Predicate<'tcx>> {
 626     trace!("check_existential_types(ty={:?})", ty);
 627     let mut substituted_predicates = Vec::new();
 628     ty.fold_with(&mut ty::fold::BottomUpFolder {
 629         tcx: fcx.tcx,
 630         ty_op: |ty| {
 631             if let ty::Opaque(def_id, substs) = ty.sty {
 632                 trace!("check_existential_types: opaque_ty, {:?}, {:?}", def_id, substs);
 633                 let generics = tcx.generics_of(def_id);
 634                 // Only check named existential types defined in this crate.
 635                 if generics.parent.is_none() && def_id.is_local() {
 636                     let opaque_hir_id = tcx.hir().as_local_hir_id(def_id).unwrap();
 637                     if may_define_existential_type(tcx, fn_def_id, opaque_hir_id) {
 638                         trace!("check_existential_types: may define, generics={:#?}", generics);
 639                         let mut seen: FxHashMap<_, Vec<_>> = FxHashMap::default();
 640                         for (subst, param) in substs.iter().zip(&generics.params) {
 641                             match subst.unpack() {
 642                                 ty::subst::UnpackedKind::Type(ty) => match ty.sty {
 643                                     ty::Param(..) => {}
 644                                     // Prevent `fn foo() -> Foo<u32>` from being defining.
 645                                     _ => {
 646                                         tcx.sess
 647                                             .struct_span_err(
 648                                                 span,
 649                                                 "non-defining existential type use \
 650                                                  in defining scope",
 651                                             )
 652                                             .span_note(
 653                                                 tcx.def_span(param.def_id),
 654                                                 &format!(
 655                                                     "used non-generic type {} for \
 656                                                      generic parameter",
 657                                                     ty,
 658                                                 ),
 659                                             )
 660                                             .emit();
 661                                     }
 662                                 }
 663
 664                                 ty::subst::UnpackedKind::Lifetime(region) => {
 665                                     let param_span = tcx.def_span(param.def_id);
 666                                     if let ty::ReStatic = region {
 667                                         tcx
 668                                             .sess
 669                                             .struct_span_err(
 670                                                 span,
 671                                                 "non-defining existential type use \
 672                                                     in defining scope",
 673                                             )
 674                                             .span_label(
 675                                                 param_span,
 676                                                 "cannot use static lifetime, use a bound lifetime \
 677                                                 instead or remove the lifetime parameter from the \
 678                                                 existential type",
 679                                             )
 680                                             .emit();
 681                                     } else {
 682                                         seen.entry(region).or_default().push(param_span);
 683                                     }
 684                                 }
 685
 686                                 ty::subst::UnpackedKind::Const(ct) => match ct.val {
 687                                     ConstValue::Param(_) => {}
 688                                     _ => {
 689                                         tcx.sess
 690                                             .struct_span_err(
 691                                                 span,
 692                                                 "non-defining existential type use \
 693                                                 in defining scope",
 694                                             )
 695                                             .span_note(
 696                                                 tcx.def_span(param.def_id),
 697                                                 &format!(
 698                                                     "used non-generic const {} for \
 699                                                     generic parameter",
 700                                                     ty,
 701                                                 ),
 702                                             )
 703                                             .emit();
 704                                     }
 705                                 }
 706                             } // match subst
 707                         } // for (subst, param)
 708                         for (_, spans) in seen {
 709                             if spans.len() > 1 {
 710                                 tcx
 711                                     .sess
 712                                     .struct_span_err(
 713                                         span,
 714                                         "non-defining existential type use \
 715                                             in defining scope",
 716                                     ).
 717                                     span_note(
 718                                         spans,
 719                                         "lifetime used multiple times",
 720                                     )
 721                                     .emit();
 722                             }
 723                         }
 724                     } // if may_define_existential_type
 725
 726                     // Now register the bounds on the parameters of the existential type
 727                     // so the parameters given by the function need to fulfill them.
 728                     //
 729                     //     existential type Foo<T: Bar>: 'static;
 730                     //     fn foo<U>() -> Foo<U> { .. *}
 731                     //
 732                     // becomes
 733                     //
 734                     //     existential type Foo<T: Bar>: 'static;
 735                     //     fn foo<U: Bar>() -> Foo<U> { .. *}
 736                     let predicates = tcx.predicates_of(def_id);
 737                     trace!(
 738                         "check_existential_types: may define, predicates={:#?}",
 739                         predicates,
 740                     );
 741                     for &(pred, _) in predicates.predicates.iter() {
 742                         let substituted_pred = pred.subst(fcx.tcx, substs);
 743                         // Avoid duplication of predicates that contain no parameters, for example.
 744                         if !predicates.predicates.iter().any(|&(p, _)| p == substituted_pred) {
 745                             substituted_predicates.push(substituted_pred);
 746                         }
 747                     }
 748                 } // if is_named_existential_type
 749             } // if let Opaque
 750             ty
 751         },
 752         lt_op: |lt| lt,
 753         ct_op: |ct| ct,
 754     });
 755     substituted_predicates
 756 }
 757
 758 fn check_method_receiver<'fcx, 'gcx, 'tcx>(fcx: &FnCtxt<'fcx, 'gcx, 'tcx>,
 759                                            method_sig: &hir::MethodSig,
 760                                            method: &ty::AssocItem,
 761                                            self_ty: Ty<'tcx>)
 762 {
 763     // Check that the method has a valid receiver type, given the type `Self`.
 764     debug!("check_method_receiver({:?}, self_ty={:?})",
 765            method, self_ty);
 766
 767     if !method.method_has_self_argument {
 768         return;
 769     }
 770
 771     let span = method_sig.decl.inputs[0].span;
 772
 773     let sig = fcx.tcx.fn_sig(method.def_id);
 774     let sig = fcx.normalize_associated_types_in(span, &sig);
 775     let sig = fcx.tcx.liberate_late_bound_regions(method.def_id, &sig);
 776
 777     debug!("check_method_receiver: sig={:?}", sig);
 778
 779     let self_ty = fcx.normalize_associated_types_in(span, &self_ty);
 780     let self_ty = fcx.tcx.liberate_late_bound_regions(
 781         method.def_id,
 782         &ty::Binder::bind(self_ty)
 783     );
 784
 785     let receiver_ty = sig.inputs()[0];
 786
 787     let receiver_ty = fcx.normalize_associated_types_in(span, &receiver_ty);
 788     let receiver_ty = fcx.tcx.liberate_late_bound_regions(
 789         method.def_id,
 790         &ty::Binder::bind(receiver_ty)
 791     );
 792
 793     if fcx.tcx.features().arbitrary_self_types {
 794         if !receiver_is_valid(fcx, span, receiver_ty, self_ty, true) {
 795             // Report error; `arbitrary_self_types` was enabled.
 796             fcx.tcx.sess.diagnostic().mut_span_err(
 797                 span, &format!("invalid method receiver type: {:?}", receiver_ty)
 798             ).note("type of `self` must be `Self` or a type that dereferences to it")
 799             .help("consider changing to `self`, `&self`, `&mut self`, or `self: Box<Self>`")
 800             .code(DiagnosticId::Error("E0307".into()))
 801             .emit();
 802         }
 803     } else {
 804         if !receiver_is_valid(fcx, span, receiver_ty, self_ty, false) {
 805             if receiver_is_valid(fcx, span, receiver_ty, self_ty, true) {
 806                 // Report error; would have worked with `arbitrary_self_types`.
 807                 feature_gate::feature_err(
 808                     &fcx.tcx.sess.parse_sess,
 809                     sym::arbitrary_self_types,
 810                     span,
 811                     GateIssue::Language,
 812                     &format!(
 813                         "`{}` cannot be used as the type of `self` without \
 814                             the `arbitrary_self_types` feature",
 815                         receiver_ty,
 816                     ),
 817                 ).help("consider changing to `self`, `&self`, `&mut self`, or `self: Box<Self>`")
 818                 .emit();
 819             } else {
 820                 // Report error; would not have worked with `arbitrary_self_types`.
 821                 fcx.tcx.sess.diagnostic().mut_span_err(
 822                     span, &format!("invalid method receiver type: {:?}", receiver_ty)
 823                 ).note("type must be `Self` or a type that dereferences to it")
 824                 .help("consider changing to `self`, `&self`, `&mut self`, or `self: Box<Self>`")
 825                 .code(DiagnosticId::Error("E0307".into()))
 826                 .emit();
 827             }
 828         }
 829     }
 830 }
 831
 832 /// Returns whether `receiver_ty` would be considered a valid receiver type for `self_ty`. If
 833 /// `arbitrary_self_types` is enabled, `receiver_ty` must transitively deref to `self_ty`, possibly
 834 /// through a `*const/mut T` raw pointer. If the feature is not enabled, the requirements are more
 835 /// strict: `receiver_ty` must implement `Receiver` and directly implement
 836 /// `Deref<Target = self_ty>`.
 837 ///
 838 /// N.B., there are cases this function returns `true` but causes an error to be emitted,
 839 /// particularly when `receiver_ty` derefs to a type that is the same as `self_ty` but has the
 840 /// wrong lifetime. Be careful of this if you are calling this function speculatively.
 841 fn receiver_is_valid<'fcx, 'tcx, 'gcx>(
 842     fcx: &FnCtxt<'fcx, 'gcx, 'tcx>,
 843     span: Span,
 844     receiver_ty: Ty<'tcx>,
 845     self_ty: Ty<'tcx>,
 846     arbitrary_self_types_enabled: bool,
 847 ) -> bool {
 848     let cause = fcx.cause(span, traits::ObligationCauseCode::MethodReceiver);
 849
 850     let can_eq_self = |ty| fcx.infcx.can_eq(fcx.param_env, self_ty, ty).is_ok();
 851
 852     // `self: Self` is always valid.
 853     if can_eq_self(receiver_ty) {
 854         if let Some(mut err) = fcx.demand_eqtype_with_origin(&cause, self_ty, receiver_ty) {
 855             err.emit();
 856         }
 857         return true
 858     }
 859
 860     let mut autoderef = fcx.autoderef(span, receiver_ty);
 861
 862     // The `arbitrary_self_types` feature allows raw pointer receivers like `self: *const Self`.
 863     if arbitrary_self_types_enabled {
 864         autoderef = autoderef.include_raw_pointers();
 865     }
 866
 867     // The first type is `receiver_ty`, which we know its not equal to `self_ty`; skip it.
 868     autoderef.next();
 869
 870     // Keep dereferencing `receiver_ty` until we get to `self_ty`.
 871     loop {
 872         if let Some((potential_self_ty, _)) = autoderef.next() {
 873             debug!("receiver_is_valid: potential self type `{:?}` to match `{:?}`",
 874                 potential_self_ty, self_ty);
 875
 876             if can_eq_self(potential_self_ty) {
 877                 autoderef.finalize(fcx);
 878
 879                 if let Some(mut err) = fcx.demand_eqtype_with_origin(
 880                     &cause, self_ty, potential_self_ty
 881                 ) {
 882                     err.emit();
 883                 }
 884
 885                 break
 886             }
 887         } else {
 888             debug!("receiver_is_valid: type `{:?}` does not deref to `{:?}`",
 889                 receiver_ty, self_ty);
 890             // If he receiver already has errors reported due to it, consider it valid to avoid
 891             // unecessary errors (#58712).
 892             return receiver_ty.references_error();
 893         }
 894
 895         // Without the `arbitrary_self_types` feature, `receiver_ty` must directly deref to
 896         // `self_ty`. Enforce this by only doing one iteration of the loop.
 897         if !arbitrary_self_types_enabled {
 898             return false
 899         }
 900     }
 901
 902     // Without `feature(arbitrary_self_types)`, we require that `receiver_ty` implements `Receiver`.
 903     if !arbitrary_self_types_enabled {
 904         let trait_def_id = match fcx.tcx.lang_items().receiver_trait() {
 905             Some(did) => did,
 906             None => {
 907                 debug!("receiver_is_valid: missing Receiver trait");
 908                 return false
 909             }
 910         };
 911
 912         let trait_ref = ty::TraitRef{
 913             def_id: trait_def_id,
 914             substs: fcx.tcx.mk_substs_trait(receiver_ty, &[]),
 915         };
 916
 917         let obligation = traits::Obligation::new(
 918             cause.clone(),
 919             fcx.param_env,
 920             trait_ref.to_predicate()
 921         );
 922
 923         if !fcx.predicate_must_hold_modulo_regions(&obligation) {
 924             debug!("receiver_is_valid: type `{:?}` does not implement `Receiver` trait",
 925                 receiver_ty);
 926             return false
 927         }
 928     }
 929
 930     true
 931 }
 932
 933 fn check_variances_for_type_defn<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 934                                            item: &hir::Item,
 935                                            hir_generics: &hir::Generics)
 936 {
 937     let item_def_id = tcx.hir().local_def_id_from_hir_id(item.hir_id);
 938     let ty = tcx.type_of(item_def_id);
 939     if tcx.has_error_field(ty) {
 940         return;
 941     }
 942
 943     let ty_predicates = tcx.predicates_of(item_def_id);
 944     assert_eq!(ty_predicates.parent, None);
 945     let variances = tcx.variances_of(item_def_id);
 946
 947     let mut constrained_parameters: FxHashSet<_> =
 948         variances.iter().enumerate()
 949                         .filter(|&(_, &variance)| variance != ty::Bivariant)
 950                         .map(|(index, _)| Parameter(index as u32))
 951                         .collect();
 952
 953     identify_constrained_generic_params(
 954         tcx,
 955         &ty_predicates,
 956         None,
 957         &mut constrained_parameters,
 958     );
 959
 960     for (index, _) in variances.iter().enumerate() {
 961         if constrained_parameters.contains(&Parameter(index as u32)) {
 962             continue;
 963         }
 964
 965         let param = &hir_generics.params[index];
 966
 967         match param.name {
 968             hir::ParamName::Error => { }
 969             _ => report_bivariance(tcx, param.span, param.name.ident().name),
 970         }
 971     }
 972 }
 973
 974 fn report_bivariance<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 975                                span: Span,
 976                                param_name: ast::Name)
 977 {
 978     let mut err = error_392(tcx, span, param_name);
 979
 980     let suggested_marker_id = tcx.lang_items().phantom_data();
 981     // Help is available only in presence of lang items.
 982     if let Some(def_id) = suggested_marker_id {
 983         err.help(&format!("consider removing `{}` or using a marker such as `{}`",
 984                           param_name,
 985                           tcx.def_path_str(def_id)));
 986     }
 987     err.emit();
 988 }
 989
 990 fn reject_shadowing_parameters(tcx: TyCtxt<'_, '_, '_>, def_id: DefId) {
 991     let generics = tcx.generics_of(def_id);
 992     let parent = tcx.generics_of(generics.parent.unwrap());
 993     let impl_params: FxHashMap<_, _> = parent.params.iter().flat_map(|param| match param.kind {
 994         GenericParamDefKind::Lifetime => None,
 995         GenericParamDefKind::Type { .. } | GenericParamDefKind::Const => {
 996             Some((param.name, param.def_id))
 997         }
 998     }).collect();
 999
1000     for method_param in &generics.params {
1001         // Shadowing is checked in `resolve_lifetime`.
1002         if let GenericParamDefKind::Lifetime = method_param.kind {
1003             continue
1004         }
1005         if impl_params.contains_key(&method_param.name) {
1006             // Tighten up the span to focus on only the shadowing type.
1007             let type_span = tcx.def_span(method_param.def_id);
1008
1009             // The expectation here is that the original trait declaration is
1010             // local so it should be okay to just unwrap everything.
1011             let trait_def_id = impl_params[&method_param.name];
1012             let trait_decl_span = tcx.def_span(trait_def_id);
1013             error_194(tcx, type_span, trait_decl_span, &method_param.name.as_str()[..]);
1014         }
1015     }
1016 }
1017
1018 /// Feature gates RFC 2056 -- trivial bounds, checking for global bounds that
1019 /// aren't true.
1020 fn check_false_global_bounds<'a, 'gcx, 'tcx>(
1021     fcx: &FnCtxt<'a, 'gcx, 'tcx>,
1022     span: Span,
1023     id: hir::HirId)
1024 {
1025     let empty_env = ty::ParamEnv::empty();
1026
1027     let def_id = fcx.tcx.hir().local_def_id_from_hir_id(id);
1028     let predicates = fcx.tcx.predicates_of(def_id).predicates
1029         .iter()
1030         .map(|(p, _)| *p)
1031         .collect();
1032     // Check elaborated bounds.
1033     let implied_obligations = traits::elaborate_predicates(fcx.tcx, predicates);
1034
1035     for pred in implied_obligations {
1036         // Match the existing behavior.
1037         if pred.is_global() && !pred.has_late_bound_regions() {
1038             let pred = fcx.normalize_associated_types_in(span, &pred);
1039             let obligation = traits::Obligation::new(
1040                 traits::ObligationCause::new(
1041                     span,
1042                     id,
1043                     traits::TrivialBound,
1044                 ),
1045                 empty_env,
1046                 pred,
1047             );
1048             fcx.register_predicate(obligation);
1049         }
1050     }
1051
1052     fcx.select_all_obligations_or_error();
1053 }
1054
1055 pub struct CheckTypeWellFormedVisitor<'a, 'tcx: 'a> {
1056     tcx: TyCtxt<'a, 'tcx, 'tcx>,
1057 }
1058
1059 impl<'a, 'gcx> CheckTypeWellFormedVisitor<'a, 'gcx> {
1060     pub fn new(tcx: TyCtxt<'a, 'gcx, 'gcx>)
1061                -> CheckTypeWellFormedVisitor<'a, 'gcx> {
1062         CheckTypeWellFormedVisitor {
1063             tcx,
1064         }
1065     }
1066 }
1067
1068 impl<'a, 'tcx> ParItemLikeVisitor<'tcx> for CheckTypeWellFormedVisitor<'a, 'tcx> {
1069     fn visit_item(&self, i: &'tcx hir::Item) {
1070         debug!("visit_item: {:?}", i);
1071         let def_id = self.tcx.hir().local_def_id_from_hir_id(i.hir_id);
1072         self.tcx.ensure().check_item_well_formed(def_id);
1073     }
1074
1075     fn visit_trait_item(&self, trait_item: &'tcx hir::TraitItem) {
1076         debug!("visit_trait_item: {:?}", trait_item);
1077         let def_id = self.tcx.hir().local_def_id_from_hir_id(trait_item.hir_id);
1078         self.tcx.ensure().check_trait_item_well_formed(def_id);
1079     }
1080
1081     fn visit_impl_item(&self, impl_item: &'tcx hir::ImplItem) {
1082         debug!("visit_impl_item: {:?}", impl_item);
1083         let def_id = self.tcx.hir().local_def_id_from_hir_id(impl_item.hir_id);
1084         self.tcx.ensure().check_impl_item_well_formed(def_id);
1085     }
1086 }
1087
1088 ///////////////////////////////////////////////////////////////////////////
1089 // ADT
1090
1091 struct AdtVariant<'tcx> {
1092     fields: Vec<AdtField<'tcx>>,
1093 }
1094
1095 struct AdtField<'tcx> {
1096     ty: Ty<'tcx>,
1097     span: Span,
1098 }
1099
1100 impl<'a, 'gcx, 'tcx> FnCtxt<'a, 'gcx, 'tcx> {
1101     fn non_enum_variant(&self, struct_def: &hir::VariantData) -> AdtVariant<'tcx> {
1102         let fields = struct_def.fields().iter().map(|field| {
1103             let field_ty = self.tcx.type_of(self.tcx.hir().local_def_id_from_hir_id(field.hir_id));
1104             let field_ty = self.normalize_associated_types_in(field.span,
1105                                                               &field_ty);
1106             AdtField { ty: field_ty, span: field.span }
1107         })
1108         .collect();
1109         AdtVariant { fields }
1110     }
1111
1112     fn enum_variants(&self, enum_def: &hir::EnumDef) -> Vec<AdtVariant<'tcx>> {
1113         enum_def.variants.iter()
1114             .map(|variant| self.non_enum_variant(&variant.node.data))
1115             .collect()
1116     }
1117
1118     fn impl_implied_bounds(&self, impl_def_id: DefId, span: Span) -> Vec<Ty<'tcx>> {
1119         match self.tcx.impl_trait_ref(impl_def_id) {
1120             Some(ref trait_ref) => {
1121                 // Trait impl: take implied bounds from all types that
1122                 // appear in the trait reference.
1123                 let trait_ref = self.normalize_associated_types_in(span, trait_ref);
1124                 trait_ref.substs.types().collect()
1125             }
1126
1127             None => {
1128                 // Inherent impl: take implied bounds from the `self` type.
1129                 let self_ty = self.tcx.type_of(impl_def_id);
1130                 let self_ty = self.normalize_associated_types_in(span, &self_ty);
1131                 vec![self_ty]
1132             }
1133         }
1134     }
1135 }
1136
1137 fn error_392<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>, span: Span, param_name: ast::Name)
1138                        -> DiagnosticBuilder<'tcx> {
1139     let mut err = struct_span_err!(tcx.sess, span, E0392,
1140                   "parameter `{}` is never used", param_name);
1141     err.span_label(span, "unused parameter");
1142     err
1143 }
1144
1145 fn error_194(tcx: TyCtxt<'_, '_, '_>, span: Span, trait_decl_span: Span, name: &str) {
1146     struct_span_err!(tcx.sess, span, E0194,
1147                      "type parameter `{}` shadows another type parameter of the same name",
1148                      name)
1149         .span_label(span, "shadows another type parameter")
1150         .span_label(trait_decl_span, format!("first `{}` declared here", name))
1151         .emit();
1152 }