src/librustc/middle/dead.rs

   1 // This implements the dead-code warning pass. It follows middle::reachable
   2 // closely. The idea is that all reachable symbols are live, codes called
   3 // from live codes are live, and everything else is dead.
   4
   5 use hir::Node;
   6 use hir::{self, PatKind};
   7 use hir::intravisit::{self, Visitor, NestedVisitorMap};
   8 use hir::itemlikevisit::ItemLikeVisitor;
   9
  10 use hir::def::Def;
  11 use hir::CodegenFnAttrFlags;
  12 use hir::def_id::{DefId, LOCAL_CRATE};
  13 use lint;
  14 use middle::privacy;
  15 use ty::{self, TyCtxt};
  16 use util::nodemap::FxHashSet;
  17
  18 use rustc_data_structures::fx::FxHashMap;
  19
  20 use syntax::{ast, source_map};
  21 use syntax::attr;
  22 use syntax_pos;
  23
  24 // Any local node that may call something in its body block should be
  25 // explored. For example, if it's a live Node::Item that is a
  26 // function, then we should explore its block to check for codes that
  27 // may need to be marked as live.
  28 fn should_explore<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  29                             node_id: ast::NodeId) -> bool {
  30     match tcx.hir().find(node_id) {
  31         Some(Node::Item(..)) |
  32         Some(Node::ImplItem(..)) |
  33         Some(Node::ForeignItem(..)) |
  34         Some(Node::TraitItem(..)) =>
  35             true,
  36         _ =>
  37             false
  38     }
  39 }
  40
  41 struct MarkSymbolVisitor<'a, 'tcx: 'a> {
  42     worklist: Vec<ast::NodeId>,
  43     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  44     tables: &'a ty::TypeckTables<'tcx>,
  45     live_symbols: FxHashSet<ast::NodeId>,
  46     repr_has_repr_c: bool,
  47     in_pat: bool,
  48     inherited_pub_visibility: bool,
  49     ignore_variant_stack: Vec<DefId>,
  50     // maps from tuple struct constructors to tuple struct items
  51     struct_constructors: FxHashMap<ast::NodeId, ast::NodeId>,
  52 }
  53
  54 impl<'a, 'tcx> MarkSymbolVisitor<'a, 'tcx> {
  55     fn check_def_id(&mut self, def_id: DefId) {
  56         if let Some(node_id) = self.tcx.hir().as_local_node_id(def_id) {
  57             if should_explore(self.tcx, node_id) ||
  58                self.struct_constructors.contains_key(&node_id) {
  59                 self.worklist.push(node_id);
  60             }
  61             self.live_symbols.insert(node_id);
  62         }
  63     }
  64
  65     fn insert_def_id(&mut self, def_id: DefId) {
  66         if let Some(node_id) = self.tcx.hir().as_local_node_id(def_id) {
  67             debug_assert!(!should_explore(self.tcx, node_id));
  68             self.live_symbols.insert(node_id);
  69         }
  70     }
  71
  72     fn handle_definition(&mut self, def: Def) {
  73         match def {
  74             Def::Const(_) | Def::AssociatedConst(..) | Def::TyAlias(_) => {
  75                 self.check_def_id(def.def_id());
  76             }
  77             _ if self.in_pat => (),
  78             Def::PrimTy(..) | Def::SelfTy(..) | Def::SelfCtor(..) |
  79             Def::Local(..) | Def::Upvar(..) => {}
  80             Def::Variant(variant_id) | Def::VariantCtor(variant_id, ..) => {
  81                 if let Some(enum_id) = self.tcx.parent_def_id(variant_id) {
  82                     self.check_def_id(enum_id);
  83                 }
  84                 if !self.ignore_variant_stack.contains(&variant_id) {
  85                     self.check_def_id(variant_id);
  86                 }
  87             }
  88             _ => {
  89                 self.check_def_id(def.def_id());
  90             }
  91         }
  92     }
  93
  94     fn lookup_and_handle_method(&mut self, id: hir::HirId) {
  95         if let Some(def) = self.tables.type_dependent_defs().get(id) {
  96             self.check_def_id(def.def_id());
  97         } else {
  98             bug!("no type-dependent def for method");
  99         }
 100     }
 101
 102     fn handle_field_access(&mut self, lhs: &hir::Expr, node_id: ast::NodeId) {
 103         match self.tables.expr_ty_adjusted(lhs).sty {
 104             ty::Adt(def, _) => {
 105                 let index = self.tcx.field_index(node_id, self.tables);
 106                 self.insert_def_id(def.non_enum_variant().fields[index].did);
 107             }
 108             ty::Tuple(..) => {}
 109             _ => span_bug!(lhs.span, "named field access on non-ADT"),
 110         }
 111     }
 112
 113     fn handle_field_pattern_match(&mut self, lhs: &hir::Pat, def: Def,
 114                                   pats: &[source_map::Spanned<hir::FieldPat>]) {
 115         let variant = match self.tables.node_id_to_type(lhs.hir_id).sty {
 116             ty::Adt(adt, _) => adt.variant_of_def(def),
 117             _ => span_bug!(lhs.span, "non-ADT in struct pattern")
 118         };
 119         for pat in pats {
 120             if let PatKind::Wild = pat.node.pat.node {
 121                 continue;
 122             }
 123             let index = self.tcx.field_index(pat.node.id, self.tables);
 124             self.insert_def_id(variant.fields[index].did);
 125         }
 126     }
 127
 128     fn mark_live_symbols(&mut self) {
 129         let mut scanned = FxHashSet::default();
 130         while let Some(id) = self.worklist.pop() {
 131             if !scanned.insert(id) {
 132                 continue
 133             }
 134
 135             // in the case of tuple struct constructors we want to check the item, not the generated
 136             // tuple struct constructor function
 137             let id = self.struct_constructors.get(&id).cloned().unwrap_or(id);
 138
 139             if let Some(node) = self.tcx.hir().find(id) {
 140                 self.live_symbols.insert(id);
 141                 self.visit_node(node);
 142             }
 143         }
 144     }
 145
 146     fn visit_node(&mut self, node: Node<'tcx>) {
 147         let had_repr_c = self.repr_has_repr_c;
 148         self.repr_has_repr_c = false;
 149         let had_inherited_pub_visibility = self.inherited_pub_visibility;
 150         self.inherited_pub_visibility = false;
 151         match node {
 152             Node::Item(item) => {
 153                 match item.node {
 154                     hir::ItemKind::Struct(..) | hir::ItemKind::Union(..) => {
 155                         let def_id = self.tcx.hir().local_def_id(item.id);
 156                         let def = self.tcx.adt_def(def_id);
 157                         self.repr_has_repr_c = def.repr.c();
 158
 159                         intravisit::walk_item(self, &item);
 160                     }
 161                     hir::ItemKind::Enum(..) => {
 162                         self.inherited_pub_visibility = item.vis.node.is_pub();
 163                         intravisit::walk_item(self, &item);
 164                     }
 165                     hir::ItemKind::Fn(..)
 166                     | hir::ItemKind::Ty(..)
 167                     | hir::ItemKind::Static(..)
 168                     | hir::ItemKind::Existential(..)
 169                     | hir::ItemKind::Const(..) => {
 170                         intravisit::walk_item(self, &item);
 171                     }
 172                     _ => ()
 173                 }
 174             }
 175             Node::TraitItem(trait_item) => {
 176                 intravisit::walk_trait_item(self, trait_item);
 177             }
 178             Node::ImplItem(impl_item) => {
 179                 intravisit::walk_impl_item(self, impl_item);
 180             }
 181             Node::ForeignItem(foreign_item) => {
 182                 intravisit::walk_foreign_item(self, &foreign_item);
 183             }
 184             _ => ()
 185         }
 186         self.repr_has_repr_c = had_repr_c;
 187         self.inherited_pub_visibility = had_inherited_pub_visibility;
 188     }
 189
 190     fn mark_as_used_if_union(&mut self, adt: &ty::AdtDef, fields: &hir::HirVec<hir::Field>) {
 191         if adt.is_union() && adt.non_enum_variant().fields.len() > 1 && adt.did.is_local() {
 192             for field in fields {
 193                 let index = self.tcx.field_index(field.id, self.tables);
 194                 self.insert_def_id(adt.non_enum_variant().fields[index].did);
 195             }
 196         }
 197     }
 198 }
 199
 200 impl<'a, 'tcx> Visitor<'tcx> for MarkSymbolVisitor<'a, 'tcx> {
 201     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
 202         NestedVisitorMap::None
 203     }
 204
 205     fn visit_nested_body(&mut self, body: hir::BodyId) {
 206         let old_tables = self.tables;
 207         self.tables = self.tcx.body_tables(body);
 208         let body = self.tcx.hir().body(body);
 209         self.visit_body(body);
 210         self.tables = old_tables;
 211     }
 212
 213     fn visit_variant_data(&mut self, def: &'tcx hir::VariantData, _: ast::Name,
 214                           _: &hir::Generics, _: ast::NodeId, _: syntax_pos::Span) {
 215         let has_repr_c = self.repr_has_repr_c;
 216         let inherited_pub_visibility = self.inherited_pub_visibility;
 217         let live_fields = def.fields().iter().filter(|f| {
 218             has_repr_c || inherited_pub_visibility || f.vis.node.is_pub()
 219         });
 220         self.live_symbols.extend(live_fields.map(|f| f.id));
 221
 222         intravisit::walk_struct_def(self, def);
 223     }
 224
 225     fn visit_expr(&mut self, expr: &'tcx hir::Expr) {
 226         match expr.node {
 227             hir::ExprKind::Path(ref qpath @ hir::QPath::TypeRelative(..)) => {
 228                 let def = self.tables.qpath_def(qpath, expr.hir_id);
 229                 self.handle_definition(def);
 230             }
 231             hir::ExprKind::MethodCall(..) => {
 232                 self.lookup_and_handle_method(expr.hir_id);
 233             }
 234             hir::ExprKind::Field(ref lhs, ..) => {
 235                 self.handle_field_access(&lhs, expr.id);
 236             }
 237             hir::ExprKind::Struct(_, ref fields, _) => {
 238                 if let ty::Adt(ref adt, _) = self.tables.expr_ty(expr).sty {
 239                     self.mark_as_used_if_union(adt, fields);
 240                 }
 241             }
 242             _ => ()
 243         }
 244
 245         intravisit::walk_expr(self, expr);
 246     }
 247
 248     fn visit_arm(&mut self, arm: &'tcx hir::Arm) {
 249         if arm.pats.len() == 1 {
 250             let variants = arm.pats[0].necessary_variants();
 251
 252             // Inside the body, ignore constructions of variants
 253             // necessary for the pattern to match. Those construction sites
 254             // can't be reached unless the variant is constructed elsewhere.
 255             let len = self.ignore_variant_stack.len();
 256             self.ignore_variant_stack.extend_from_slice(&variants);
 257             intravisit::walk_arm(self, arm);
 258             self.ignore_variant_stack.truncate(len);
 259         } else {
 260             intravisit::walk_arm(self, arm);
 261         }
 262     }
 263
 264     fn visit_pat(&mut self, pat: &'tcx hir::Pat) {
 265         match pat.node {
 266             PatKind::Struct(hir::QPath::Resolved(_, ref path), ref fields, _) => {
 267                 self.handle_field_pattern_match(pat, path.def, fields);
 268             }
 269             PatKind::Path(ref qpath @ hir::QPath::TypeRelative(..)) => {
 270                 let def = self.tables.qpath_def(qpath, pat.hir_id);
 271                 self.handle_definition(def);
 272             }
 273             _ => ()
 274         }
 275
 276         self.in_pat = true;
 277         intravisit::walk_pat(self, pat);
 278         self.in_pat = false;
 279     }
 280
 281     fn visit_path(&mut self, path: &'tcx hir::Path, _: hir::HirId) {
 282         self.handle_definition(path.def);
 283         intravisit::walk_path(self, path);
 284     }
 285 }
 286
 287 fn has_allow_dead_code_or_lang_attr(tcx: TyCtxt<'_, '_, '_>,
 288                                     id: ast::NodeId,
 289                                     attrs: &[ast::Attribute]) -> bool {
 290     if attr::contains_name(attrs, "lang") {
 291         return true;
 292     }
 293
 294     // Stable attribute for #[lang = "panic_impl"]
 295     if attr::contains_name(attrs, "panic_handler") {
 296         return true;
 297     }
 298
 299     // (To be) stable attribute for #[lang = "oom"]
 300     if attr::contains_name(attrs, "alloc_error_handler") {
 301         return true;
 302     }
 303
 304     // Don't lint about global allocators
 305     if attr::contains_name(attrs, "global_allocator") {
 306         return true;
 307     }
 308
 309     let def_id = tcx.hir().local_def_id(id);
 310     let cg_attrs = tcx.codegen_fn_attrs(def_id);
 311
 312     // #[used], #[no_mangle], #[export_name], etc also keeps the item alive
 313     // forcefully, e.g., for placing it in a specific section.
 314     if cg_attrs.contains_extern_indicator() ||
 315         cg_attrs.flags.contains(CodegenFnAttrFlags::USED) {
 316         return true;
 317     }
 318
 319     tcx.lint_level_at_node(lint::builtin::DEAD_CODE, id).0 == lint::Allow
 320 }
 321
 322 // This visitor seeds items that
 323 //   1) We want to explicitly consider as live:
 324 //     * Item annotated with #[allow(dead_code)]
 325 //         - This is done so that if we want to suppress warnings for a
 326 //           group of dead functions, we only have to annotate the "root".
 327 //           For example, if both `f` and `g` are dead and `f` calls `g`,
 328 //           then annotating `f` with `#[allow(dead_code)]` will suppress
 329 //           warning for both `f` and `g`.
 330 //     * Item annotated with #[lang=".."]
 331 //         - This is because lang items are always callable from elsewhere.
 332 //   or
 333 //   2) We are not sure to be live or not
 334 //     * Implementation of a trait method
 335 struct LifeSeeder<'k, 'tcx: 'k> {
 336     worklist: Vec<ast::NodeId>,
 337     krate: &'k hir::Crate,
 338     tcx: TyCtxt<'k, 'tcx, 'tcx>,
 339     // see `MarkSymbolVisitor::struct_constructors`
 340     struct_constructors: FxHashMap<ast::NodeId, ast::NodeId>,
 341 }
 342
 343 impl<'v, 'k, 'tcx> ItemLikeVisitor<'v> for LifeSeeder<'k, 'tcx> {
 344     fn visit_item(&mut self, item: &hir::Item) {
 345         let allow_dead_code = has_allow_dead_code_or_lang_attr(self.tcx,
 346                                                                item.id,
 347                                                                &item.attrs);
 348         if allow_dead_code {
 349             self.worklist.push(item.id);
 350         }
 351         match item.node {
 352             hir::ItemKind::Enum(ref enum_def, _) if allow_dead_code => {
 353                 self.worklist.extend(enum_def.variants.iter()
 354                                                       .map(|variant| variant.node.data.id()));
 355             }
 356             hir::ItemKind::Trait(.., ref trait_item_refs) => {
 357                 for trait_item_ref in trait_item_refs {
 358                     let trait_item = self.krate.trait_item(trait_item_ref.id);
 359                     match trait_item.node {
 360                         hir::TraitItemKind::Const(_, Some(_)) |
 361                         hir::TraitItemKind::Method(_, hir::TraitMethod::Provided(_)) => {
 362                             if has_allow_dead_code_or_lang_attr(self.tcx,
 363                                                                 trait_item.id,
 364                                                                 &trait_item.attrs) {
 365                                 self.worklist.push(trait_item.id);
 366                             }
 367                         }
 368                         _ => {}
 369                     }
 370                 }
 371             }
 372             hir::ItemKind::Impl(.., ref opt_trait, _, ref impl_item_refs) => {
 373                 for impl_item_ref in impl_item_refs {
 374                     let impl_item = self.krate.impl_item(impl_item_ref.id);
 375                     if opt_trait.is_some() ||
 376                             has_allow_dead_code_or_lang_attr(self.tcx,
 377                                                              impl_item.id,
 378                                                              &impl_item.attrs) {
 379                         self.worklist.push(impl_item_ref.id.node_id);
 380                     }
 381                 }
 382             }
 383             hir::ItemKind::Struct(ref variant_data, _) => {
 384                 self.struct_constructors.insert(variant_data.id(), item.id);
 385             }
 386             _ => ()
 387         }
 388     }
 389
 390     fn visit_trait_item(&mut self, _item: &hir::TraitItem) {
 391         // ignore: we are handling this in `visit_item` above
 392     }
 393
 394     fn visit_impl_item(&mut self, _item: &hir::ImplItem) {
 395         // ignore: we are handling this in `visit_item` above
 396     }
 397 }
 398
 399 fn create_and_seed_worklist<'a, 'tcx>(
 400     tcx: TyCtxt<'a, 'tcx, 'tcx>,
 401     access_levels: &privacy::AccessLevels,
 402     krate: &hir::Crate,
 403 ) -> (Vec<ast::NodeId>, FxHashMap<ast::NodeId, ast::NodeId>) {
 404     let worklist = access_levels.map.iter().filter_map(|(&id, level)| {
 405         if level >= &privacy::AccessLevel::Reachable {
 406             Some(id)
 407         } else {
 408             None
 409         }
 410     }).chain(
 411         // Seed entry point
 412         tcx.sess.entry_fn.borrow().map(|(id, _, _)| id)
 413     ).collect::<Vec<_>>();
 414
 415     // Seed implemented trait items
 416     let mut life_seeder = LifeSeeder {
 417         worklist,
 418         krate,
 419         tcx,
 420         struct_constructors: Default::default(),
 421     };
 422     krate.visit_all_item_likes(&mut life_seeder);
 423
 424     (life_seeder.worklist, life_seeder.struct_constructors)
 425 }
 426
 427 fn find_live<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 428                        access_levels: &privacy::AccessLevels,
 429                        krate: &hir::Crate)
 430                        -> FxHashSet<ast::NodeId> {
 431     let (worklist, struct_constructors) = create_and_seed_worklist(tcx, access_levels, krate);
 432     let mut symbol_visitor = MarkSymbolVisitor {
 433         worklist,
 434         tcx,
 435         tables: &ty::TypeckTables::empty(None),
 436         live_symbols: Default::default(),
 437         repr_has_repr_c: false,
 438         in_pat: false,
 439         inherited_pub_visibility: false,
 440         ignore_variant_stack: vec![],
 441         struct_constructors,
 442     };
 443     symbol_visitor.mark_live_symbols();
 444     symbol_visitor.live_symbols
 445 }
 446
 447 struct DeadVisitor<'a, 'tcx: 'a> {
 448     tcx: TyCtxt<'a, 'tcx, 'tcx>,
 449     live_symbols: FxHashSet<ast::NodeId>,
 450 }
 451
 452 impl<'a, 'tcx> DeadVisitor<'a, 'tcx> {
 453     fn should_warn_about_item(&mut self, item: &hir::Item) -> bool {
 454         let should_warn = match item.node {
 455             hir::ItemKind::Static(..)
 456             | hir::ItemKind::Const(..)
 457             | hir::ItemKind::Fn(..)
 458             | hir::ItemKind::Ty(..)
 459             | hir::ItemKind::Enum(..)
 460             | hir::ItemKind::Struct(..)
 461             | hir::ItemKind::Union(..) => true,
 462             _ => false
 463         };
 464         should_warn && !self.symbol_is_live(item.id)
 465     }
 466
 467     fn should_warn_about_field(&mut self, field: &hir::StructField) -> bool {
 468         let field_type = self.tcx.type_of(self.tcx.hir().local_def_id(field.id));
 469         !field.is_positional()
 470             && !self.symbol_is_live(field.id)
 471             && !field_type.is_phantom_data()
 472             && !has_allow_dead_code_or_lang_attr(self.tcx, field.id, &field.attrs)
 473     }
 474
 475     fn should_warn_about_variant(&mut self, variant: &hir::VariantKind) -> bool {
 476         !self.symbol_is_live(variant.data.id())
 477             && !has_allow_dead_code_or_lang_attr(self.tcx,
 478                                                  variant.data.id(),
 479                                                  &variant.attrs)
 480     }
 481
 482     fn should_warn_about_foreign_item(&mut self, fi: &hir::ForeignItem) -> bool {
 483         !self.symbol_is_live(fi.id)
 484             && !has_allow_dead_code_or_lang_attr(self.tcx, fi.id, &fi.attrs)
 485     }
 486
 487     // id := node id of an item's definition.
 488     fn symbol_is_live(
 489         &mut self,
 490         id: ast::NodeId,
 491     ) -> bool {
 492         if self.live_symbols.contains(&id) {
 493             return true;
 494         }
 495         // If it's a type whose items are live, then it's live, too.
 496         // This is done to handle the case where, for example, the static
 497         // method of a private type is used, but the type itself is never
 498         // called directly.
 499         let def_id = self.tcx.hir().local_def_id(id);
 500         let inherent_impls = self.tcx.inherent_impls(def_id);
 501         for &impl_did in inherent_impls.iter() {
 502             for &item_did in &self.tcx.associated_item_def_ids(impl_did)[..] {
 503                 if let Some(item_node_id) = self.tcx.hir().as_local_node_id(item_did) {
 504                     if self.live_symbols.contains(&item_node_id) {
 505                         return true;
 506                     }
 507                 }
 508             }
 509         }
 510         false
 511     }
 512
 513     fn warn_dead_code(&mut self,
 514                       id: ast::NodeId,
 515                       span: syntax_pos::Span,
 516                       name: ast::Name,
 517                       node_type: &str,
 518                       participle: &str) {
 519         if !name.as_str().starts_with("_") {
 520             self.tcx
 521                 .lint_node(lint::builtin::DEAD_CODE,
 522                            id,
 523                            span,
 524                            &format!("{} is never {}: `{}`",
 525                                     node_type, participle, name));
 526         }
 527     }
 528 }
 529
 530 impl<'a, 'tcx> Visitor<'tcx> for DeadVisitor<'a, 'tcx> {
 531     /// Walk nested items in place so that we don't report dead-code
 532     /// on inner functions when the outer function is already getting
 533     /// an error. We could do this also by checking the parents, but
 534     /// this is how the code is setup and it seems harmless enough.
 535     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
 536         NestedVisitorMap::All(&self.tcx.hir())
 537     }
 538
 539     fn visit_item(&mut self, item: &'tcx hir::Item) {
 540         if self.should_warn_about_item(item) {
 541             // For items that have a definition with a signature followed by a
 542             // block, point only at the signature.
 543             let span = match item.node {
 544                 hir::ItemKind::Fn(..) |
 545                 hir::ItemKind::Mod(..) |
 546                 hir::ItemKind::Enum(..) |
 547                 hir::ItemKind::Struct(..) |
 548                 hir::ItemKind::Union(..) |
 549                 hir::ItemKind::Trait(..) |
 550                 hir::ItemKind::Impl(..) => self.tcx.sess.source_map().def_span(item.span),
 551                 _ => item.span,
 552             };
 553             let participle = match item.node {
 554                 hir::ItemKind::Struct(..) => "constructed", // Issue #52325
 555                 _ => "used"
 556             };
 557             self.warn_dead_code(
 558                 item.id,
 559                 span,
 560                 item.ident.name,
 561                 item.node.descriptive_variant(),
 562                 participle,
 563             );
 564         } else {
 565             // Only continue if we didn't warn
 566             intravisit::walk_item(self, item);
 567         }
 568     }
 569
 570     fn visit_variant(&mut self,
 571                      variant: &'tcx hir::Variant,
 572                      g: &'tcx hir::Generics,
 573                      id: ast::NodeId) {
 574         if self.should_warn_about_variant(&variant.node) {
 575             self.warn_dead_code(variant.node.data.id(), variant.span, variant.node.ident.name,
 576                                 "variant", "constructed");
 577         } else {
 578             intravisit::walk_variant(self, variant, g, id);
 579         }
 580     }
 581
 582     fn visit_foreign_item(&mut self, fi: &'tcx hir::ForeignItem) {
 583         if self.should_warn_about_foreign_item(fi) {
 584             self.warn_dead_code(fi.id, fi.span, fi.ident.name,
 585                                 fi.node.descriptive_variant(), "used");
 586         }
 587         intravisit::walk_foreign_item(self, fi);
 588     }
 589
 590     fn visit_struct_field(&mut self, field: &'tcx hir::StructField) {
 591         if self.should_warn_about_field(&field) {
 592             self.warn_dead_code(field.id, field.span, field.ident.name, "field", "used");
 593         }
 594         intravisit::walk_struct_field(self, field);
 595     }
 596
 597     fn visit_impl_item(&mut self, impl_item: &'tcx hir::ImplItem) {
 598         match impl_item.node {
 599             hir::ImplItemKind::Const(_, body_id) => {
 600                 if !self.symbol_is_live(impl_item.id) {
 601                     self.warn_dead_code(impl_item.id,
 602                                         impl_item.span,
 603                                         impl_item.ident.name,
 604                                         "associated const",
 605                                         "used");
 606                 }
 607                 self.visit_nested_body(body_id)
 608             }
 609             hir::ImplItemKind::Method(_, body_id) => {
 610                 if !self.symbol_is_live(impl_item.id) {
 611                     let span = self.tcx.sess.source_map().def_span(impl_item.span);
 612                     self.warn_dead_code(impl_item.id, span, impl_item.ident.name, "method", "used");
 613                 }
 614                 self.visit_nested_body(body_id)
 615             }
 616             hir::ImplItemKind::Existential(..) |
 617             hir::ImplItemKind::Type(..) => {}
 618         }
 619     }
 620
 621     // Overwrite so that we don't warn the trait item itself.
 622     fn visit_trait_item(&mut self, trait_item: &'tcx hir::TraitItem) {
 623         match trait_item.node {
 624             hir::TraitItemKind::Const(_, Some(body_id)) |
 625             hir::TraitItemKind::Method(_, hir::TraitMethod::Provided(body_id)) => {
 626                 self.visit_nested_body(body_id)
 627             }
 628             hir::TraitItemKind::Const(_, None) |
 629             hir::TraitItemKind::Method(_, hir::TraitMethod::Required(_)) |
 630             hir::TraitItemKind::Type(..) => {}
 631         }
 632     }
 633 }
 634
 635 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
 636     let access_levels = &tcx.privacy_access_levels(LOCAL_CRATE);
 637     let krate = tcx.hir().krate();
 638     let live_symbols = find_live(tcx, access_levels, krate);
 639     let mut visitor = DeadVisitor {
 640         tcx,
 641         live_symbols,
 642     };
 643     intravisit::walk_crate(&mut visitor, krate);
 644 }