src/librustc/middle/dead.rs

   1 // Copyright 2013 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 // This implements the dead-code warning pass. It follows middle::reachable
  12 // closely. The idea is that all reachable symbols are live, codes called
  13 // from live codes are live, and everything else is dead.
  14
  15 use hir::map as hir_map;
  16 use hir::{self, PatKind};
  17 use hir::intravisit::{self, Visitor, NestedVisitorMap};
  18 use hir::itemlikevisit::ItemLikeVisitor;
  19
  20 use middle::privacy;
  21 use ty::{self, TyCtxt};
  22 use hir::def::Def;
  23 use hir::def_id::{DefId, LOCAL_CRATE};
  24 use lint;
  25 use util::nodemap::FxHashSet;
  26
  27 use syntax::{ast, codemap};
  28 use syntax::attr;
  29 use syntax_pos;
  30
  31 // Any local node that may call something in its body block should be
  32 // explored. For example, if it's a live NodeItem that is a
  33 // function, then we should explore its block to check for codes that
  34 // may need to be marked as live.
  35 fn should_explore<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
  36                             node_id: ast::NodeId) -> bool {
  37     match tcx.hir.find(node_id) {
  38         Some(hir_map::NodeItem(..)) |
  39         Some(hir_map::NodeImplItem(..)) |
  40         Some(hir_map::NodeForeignItem(..)) |
  41         Some(hir_map::NodeTraitItem(..)) =>
  42             true,
  43         _ =>
  44             false
  45     }
  46 }
  47
  48 struct MarkSymbolVisitor<'a, 'tcx: 'a> {
  49     worklist: Vec<ast::NodeId>,
  50     tcx: TyCtxt<'a, 'tcx, 'tcx>,
  51     tables: &'a ty::TypeckTables<'tcx>,
  52     live_symbols: Box<FxHashSet<ast::NodeId>>,
  53     struct_has_extern_repr: bool,
  54     ignore_non_const_paths: bool,
  55     inherited_pub_visibility: bool,
  56     ignore_variant_stack: Vec<DefId>,
  57 }
  58
  59 impl<'a, 'tcx> MarkSymbolVisitor<'a, 'tcx> {
  60     fn check_def_id(&mut self, def_id: DefId) {
  61         if let Some(node_id) = self.tcx.hir.as_local_node_id(def_id) {
  62             if should_explore(self.tcx, node_id) {
  63                 self.worklist.push(node_id);
  64             }
  65             self.live_symbols.insert(node_id);
  66         }
  67     }
  68
  69     fn insert_def_id(&mut self, def_id: DefId) {
  70         if let Some(node_id) = self.tcx.hir.as_local_node_id(def_id) {
  71             debug_assert!(!should_explore(self.tcx, node_id));
  72             self.live_symbols.insert(node_id);
  73         }
  74     }
  75
  76     fn handle_definition(&mut self, def: Def) {
  77         match def {
  78             Def::Const(_) | Def::AssociatedConst(..) => {
  79                 self.check_def_id(def.def_id());
  80             }
  81             _ if self.ignore_non_const_paths => (),
  82             Def::PrimTy(..) | Def::SelfTy(..) => (),
  83             Def::Variant(variant_id) | Def::VariantCtor(variant_id, ..) => {
  84                 if let Some(enum_id) = self.tcx.parent_def_id(variant_id) {
  85                     self.check_def_id(enum_id);
  86                 }
  87                 if !self.ignore_variant_stack.contains(&variant_id) {
  88                     self.check_def_id(variant_id);
  89                 }
  90             }
  91             _ => {
  92                 self.check_def_id(def.def_id());
  93             }
  94         }
  95     }
  96
  97     fn lookup_and_handle_method(&mut self, id: ast::NodeId) {
  98         self.check_def_id(self.tables.type_dependent_defs[&id].def_id());
  99     }
 100
 101     fn handle_field_access(&mut self, lhs: &hir::Expr, name: ast::Name) {
 102         match self.tables.expr_ty_adjusted(lhs).sty {
 103             ty::TyAdt(def, _) => {
 104                 self.insert_def_id(def.struct_variant().field_named(name).did);
 105             }
 106             _ => span_bug!(lhs.span, "named field access on non-ADT"),
 107         }
 108     }
 109
 110     fn handle_tup_field_access(&mut self, lhs: &hir::Expr, idx: usize) {
 111         match self.tables.expr_ty_adjusted(lhs).sty {
 112             ty::TyAdt(def, _) => {
 113                 self.insert_def_id(def.struct_variant().fields[idx].did);
 114             }
 115             ty::TyTuple(..) => {}
 116             _ => span_bug!(lhs.span, "numeric field access on non-ADT"),
 117         }
 118     }
 119
 120     fn handle_field_pattern_match(&mut self, lhs: &hir::Pat, def: Def,
 121                                   pats: &[codemap::Spanned<hir::FieldPat>]) {
 122         let variant = match self.tables.node_id_to_type(lhs.id).sty {
 123             ty::TyAdt(adt, _) => adt.variant_of_def(def),
 124             _ => span_bug!(lhs.span, "non-ADT in struct pattern")
 125         };
 126         for pat in pats {
 127             if let PatKind::Wild = pat.node.pat.node {
 128                 continue;
 129             }
 130             self.insert_def_id(variant.field_named(pat.node.name).did);
 131         }
 132     }
 133
 134     fn mark_live_symbols(&mut self) {
 135         let mut scanned = FxHashSet();
 136         while !self.worklist.is_empty() {
 137             let id = self.worklist.pop().unwrap();
 138             if scanned.contains(&id) {
 139                 continue
 140             }
 141             scanned.insert(id);
 142
 143             if let Some(ref node) = self.tcx.hir.find(id) {
 144                 self.live_symbols.insert(id);
 145                 self.visit_node(node);
 146             }
 147         }
 148     }
 149
 150     fn visit_node(&mut self, node: &hir_map::Node<'tcx>) {
 151         let had_extern_repr = self.struct_has_extern_repr;
 152         self.struct_has_extern_repr = false;
 153         let had_inherited_pub_visibility = self.inherited_pub_visibility;
 154         self.inherited_pub_visibility = false;
 155         match *node {
 156             hir_map::NodeItem(item) => {
 157                 match item.node {
 158                     hir::ItemStruct(..) | hir::ItemUnion(..) => {
 159                         let def_id = self.tcx.hir.local_def_id(item.id);
 160                         let def = self.tcx.adt_def(def_id);
 161                         self.struct_has_extern_repr = def.repr.c();
 162
 163                         intravisit::walk_item(self, &item);
 164                     }
 165                     hir::ItemEnum(..) => {
 166                         self.inherited_pub_visibility = item.vis == hir::Public;
 167                         intravisit::walk_item(self, &item);
 168                     }
 169                     hir::ItemFn(..)
 170                     | hir::ItemTy(..)
 171                     | hir::ItemStatic(..)
 172                     | hir::ItemConst(..) => {
 173                         intravisit::walk_item(self, &item);
 174                     }
 175                     _ => ()
 176                 }
 177             }
 178             hir_map::NodeTraitItem(trait_item) => {
 179                 intravisit::walk_trait_item(self, trait_item);
 180             }
 181             hir_map::NodeImplItem(impl_item) => {
 182                 intravisit::walk_impl_item(self, impl_item);
 183             }
 184             hir_map::NodeForeignItem(foreign_item) => {
 185                 intravisit::walk_foreign_item(self, &foreign_item);
 186             }
 187             _ => ()
 188         }
 189         self.struct_has_extern_repr = had_extern_repr;
 190         self.inherited_pub_visibility = had_inherited_pub_visibility;
 191     }
 192 }
 193
 194 impl<'a, 'tcx> Visitor<'tcx> for MarkSymbolVisitor<'a, 'tcx> {
 195     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
 196         NestedVisitorMap::None
 197     }
 198
 199     fn visit_nested_body(&mut self, body: hir::BodyId) {
 200         let old_tables = self.tables;
 201         self.tables = self.tcx.body_tables(body);
 202         let body = self.tcx.hir.body(body);
 203         self.visit_body(body);
 204         self.tables = old_tables;
 205     }
 206
 207     fn visit_variant_data(&mut self, def: &'tcx hir::VariantData, _: ast::Name,
 208                         _: &hir::Generics, _: ast::NodeId, _: syntax_pos::Span) {
 209         let has_extern_repr = self.struct_has_extern_repr;
 210         let inherited_pub_visibility = self.inherited_pub_visibility;
 211         let live_fields = def.fields().iter().filter(|f| {
 212             has_extern_repr || inherited_pub_visibility || f.vis == hir::Public
 213         });
 214         self.live_symbols.extend(live_fields.map(|f| f.id));
 215
 216         intravisit::walk_struct_def(self, def);
 217     }
 218
 219     fn visit_expr(&mut self, expr: &'tcx hir::Expr) {
 220         match expr.node {
 221             hir::ExprPath(ref qpath @ hir::QPath::TypeRelative(..)) => {
 222                 let def = self.tables.qpath_def(qpath, expr.id);
 223                 self.handle_definition(def);
 224             }
 225             hir::ExprMethodCall(..) => {
 226                 self.lookup_and_handle_method(expr.id);
 227             }
 228             hir::ExprField(ref lhs, ref name) => {
 229                 self.handle_field_access(&lhs, name.node);
 230             }
 231             hir::ExprTupField(ref lhs, idx) => {
 232                 self.handle_tup_field_access(&lhs, idx.node);
 233             }
 234             _ => ()
 235         }
 236
 237         intravisit::walk_expr(self, expr);
 238     }
 239
 240     fn visit_arm(&mut self, arm: &'tcx hir::Arm) {
 241         if arm.pats.len() == 1 {
 242             let variants = arm.pats[0].necessary_variants();
 243
 244             // Inside the body, ignore constructions of variants
 245             // necessary for the pattern to match. Those construction sites
 246             // can't be reached unless the variant is constructed elsewhere.
 247             let len = self.ignore_variant_stack.len();
 248             self.ignore_variant_stack.extend_from_slice(&variants);
 249             intravisit::walk_arm(self, arm);
 250             self.ignore_variant_stack.truncate(len);
 251         } else {
 252             intravisit::walk_arm(self, arm);
 253         }
 254     }
 255
 256     fn visit_pat(&mut self, pat: &'tcx hir::Pat) {
 257         match pat.node {
 258             PatKind::Struct(hir::QPath::Resolved(_, ref path), ref fields, _) => {
 259                 self.handle_field_pattern_match(pat, path.def, fields);
 260             }
 261             PatKind::Path(ref qpath @ hir::QPath::TypeRelative(..)) => {
 262                 let def = self.tables.qpath_def(qpath, pat.id);
 263                 self.handle_definition(def);
 264             }
 265             _ => ()
 266         }
 267
 268         self.ignore_non_const_paths = true;
 269         intravisit::walk_pat(self, pat);
 270         self.ignore_non_const_paths = false;
 271     }
 272
 273     fn visit_path(&mut self, path: &'tcx hir::Path, _: ast::NodeId) {
 274         self.handle_definition(path.def);
 275         intravisit::walk_path(self, path);
 276     }
 277 }
 278
 279 fn has_allow_dead_code_or_lang_attr(attrs: &[ast::Attribute]) -> bool {
 280     if attr::contains_name(attrs, "lang") {
 281         return true;
 282     }
 283
 284     // #[used] also keeps the item alive forcefully,
 285     // e.g. for placing it in a specific section.
 286     if attr::contains_name(attrs, "used") {
 287         return true;
 288     }
 289
 290     // Don't lint about global allocators
 291     if attr::contains_name(attrs, "global_allocator") {
 292         return true;
 293     }
 294
 295     let dead_code = lint::builtin::DEAD_CODE.name_lower();
 296     for attr in lint::gather_attrs(attrs) {
 297         match attr {
 298             Ok((name, lint::Allow, _)) if name == &*dead_code => return true,
 299             _ => (),
 300         }
 301     }
 302     false
 303 }
 304
 305 // This visitor seeds items that
 306 //   1) We want to explicitly consider as live:
 307 //     * Item annotated with #[allow(dead_code)]
 308 //         - This is done so that if we want to suppress warnings for a
 309 //           group of dead functions, we only have to annotate the "root".
 310 //           For example, if both `f` and `g` are dead and `f` calls `g`,
 311 //           then annotating `f` with `#[allow(dead_code)]` will suppress
 312 //           warning for both `f` and `g`.
 313 //     * Item annotated with #[lang=".."]
 314 //         - This is because lang items are always callable from elsewhere.
 315 //   or
 316 //   2) We are not sure to be live or not
 317 //     * Implementation of a trait method
 318 struct LifeSeeder<'k> {
 319     worklist: Vec<ast::NodeId>,
 320     krate: &'k hir::Crate,
 321 }
 322
 323 impl<'v, 'k> ItemLikeVisitor<'v> for LifeSeeder<'k> {
 324     fn visit_item(&mut self, item: &hir::Item) {
 325         let allow_dead_code = has_allow_dead_code_or_lang_attr(&item.attrs);
 326         if allow_dead_code {
 327             self.worklist.push(item.id);
 328         }
 329         match item.node {
 330             hir::ItemEnum(ref enum_def, _) if allow_dead_code => {
 331                 self.worklist.extend(enum_def.variants.iter()
 332                                                       .map(|variant| variant.node.data.id()));
 333             }
 334             hir::ItemTrait(.., ref trait_item_refs) => {
 335                 for trait_item_ref in trait_item_refs {
 336                     let trait_item = self.krate.trait_item(trait_item_ref.id);
 337                     match trait_item.node {
 338                         hir::TraitItemKind::Const(_, Some(_)) |
 339                         hir::TraitItemKind::Method(_, hir::TraitMethod::Provided(_)) => {
 340                             if has_allow_dead_code_or_lang_attr(&trait_item.attrs) {
 341                                 self.worklist.push(trait_item.id);
 342                             }
 343                         }
 344                         _ => {}
 345                     }
 346                 }
 347             }
 348             hir::ItemImpl(.., ref opt_trait, _, ref impl_item_refs) => {
 349                 for impl_item_ref in impl_item_refs {
 350                     let impl_item = self.krate.impl_item(impl_item_ref.id);
 351                     if opt_trait.is_some() ||
 352                             has_allow_dead_code_or_lang_attr(&impl_item.attrs) {
 353                         self.worklist.push(impl_item_ref.id.node_id);
 354                     }
 355                 }
 356             }
 357             _ => ()
 358         }
 359     }
 360
 361     fn visit_trait_item(&mut self, _item: &hir::TraitItem) {
 362         // ignore: we are handling this in `visit_item` above
 363     }
 364
 365     fn visit_impl_item(&mut self, _item: &hir::ImplItem) {
 366         // ignore: we are handling this in `visit_item` above
 367     }
 368 }
 369
 370 fn create_and_seed_worklist<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 371                                       access_levels: &privacy::AccessLevels,
 372                                       krate: &hir::Crate)
 373                                       -> Vec<ast::NodeId> {
 374     let mut worklist = Vec::new();
 375     for (id, _) in &access_levels.map {
 376         worklist.push(*id);
 377     }
 378
 379     // Seed entry point
 380     if let Some((id, _)) = *tcx.sess.entry_fn.borrow() {
 381         worklist.push(id);
 382     }
 383
 384     // Seed implemented trait items
 385     let mut life_seeder = LifeSeeder {
 386         worklist,
 387         krate,
 388     };
 389     krate.visit_all_item_likes(&mut life_seeder);
 390
 391     return life_seeder.worklist;
 392 }
 393
 394 fn find_live<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>,
 395                        access_levels: &privacy::AccessLevels,
 396                        krate: &hir::Crate)
 397                        -> Box<FxHashSet<ast::NodeId>> {
 398     let worklist = create_and_seed_worklist(tcx, access_levels, krate);
 399     let mut symbol_visitor = MarkSymbolVisitor {
 400         worklist,
 401         tcx,
 402         tables: &ty::TypeckTables::empty(),
 403         live_symbols: box FxHashSet(),
 404         struct_has_extern_repr: false,
 405         ignore_non_const_paths: false,
 406         inherited_pub_visibility: false,
 407         ignore_variant_stack: vec![],
 408     };
 409     symbol_visitor.mark_live_symbols();
 410     symbol_visitor.live_symbols
 411 }
 412
 413 fn get_struct_ctor_id(item: &hir::Item) -> Option<ast::NodeId> {
 414     match item.node {
 415         hir::ItemStruct(ref struct_def, _) if !struct_def.is_struct() => {
 416             Some(struct_def.id())
 417         }
 418         _ => None
 419     }
 420 }
 421
 422 struct DeadVisitor<'a, 'tcx: 'a> {
 423     tcx: TyCtxt<'a, 'tcx, 'tcx>,
 424     live_symbols: Box<FxHashSet<ast::NodeId>>,
 425 }
 426
 427 impl<'a, 'tcx> DeadVisitor<'a, 'tcx> {
 428     fn should_warn_about_item(&mut self, item: &hir::Item) -> bool {
 429         let should_warn = match item.node {
 430             hir::ItemStatic(..)
 431             | hir::ItemConst(..)
 432             | hir::ItemFn(..)
 433             | hir::ItemTy(..)
 434             | hir::ItemEnum(..)
 435             | hir::ItemStruct(..)
 436             | hir::ItemUnion(..) => true,
 437             _ => false
 438         };
 439         let ctor_id = get_struct_ctor_id(item);
 440         should_warn && !self.symbol_is_live(item.id, ctor_id)
 441     }
 442
 443     fn should_warn_about_field(&mut self, field: &hir::StructField) -> bool {
 444         let field_type = self.tcx.type_of(self.tcx.hir.local_def_id(field.id));
 445         let is_marker_field = match field_type.ty_to_def_id() {
 446             Some(def_id) => self.tcx.lang_items.items().iter().any(|item| *item == Some(def_id)),
 447             _ => false
 448         };
 449         !field.is_positional()
 450             && !self.symbol_is_live(field.id, None)
 451             && !is_marker_field
 452             && !has_allow_dead_code_or_lang_attr(&field.attrs)
 453     }
 454
 455     fn should_warn_about_variant(&mut self, variant: &hir::Variant_) -> bool {
 456         !self.symbol_is_live(variant.data.id(), None)
 457             && !has_allow_dead_code_or_lang_attr(&variant.attrs)
 458     }
 459
 460     fn should_warn_about_foreign_item(&mut self, fi: &hir::ForeignItem) -> bool {
 461         !self.symbol_is_live(fi.id, None)
 462             && !has_allow_dead_code_or_lang_attr(&fi.attrs)
 463     }
 464
 465     // id := node id of an item's definition.
 466     // ctor_id := `Some` if the item is a struct_ctor (tuple struct),
 467     //            `None` otherwise.
 468     // If the item is a struct_ctor, then either its `id` or
 469     // `ctor_id` (unwrapped) is in the live_symbols set. More specifically,
 470     // DefMap maps the ExprPath of a struct_ctor to the node referred by
 471     // `ctor_id`. On the other hand, in a statement like
 472     // `type <ident> <generics> = <ty>;` where <ty> refers to a struct_ctor,
 473     // DefMap maps <ty> to `id` instead.
 474     fn symbol_is_live(&mut self,
 475                       id: ast::NodeId,
 476                       ctor_id: Option<ast::NodeId>)
 477                       -> bool {
 478         if self.live_symbols.contains(&id)
 479            || ctor_id.map_or(false,
 480                              |ctor| self.live_symbols.contains(&ctor)) {
 481             return true;
 482         }
 483         // If it's a type whose items are live, then it's live, too.
 484         // This is done to handle the case where, for example, the static
 485         // method of a private type is used, but the type itself is never
 486         // called directly.
 487         let def_id = self.tcx.hir.local_def_id(id);
 488         let inherent_impls = self.tcx.inherent_impls(def_id);
 489         for &impl_did in inherent_impls.iter() {
 490             for &item_did in &self.tcx.associated_item_def_ids(impl_did)[..] {
 491                 if let Some(item_node_id) = self.tcx.hir.as_local_node_id(item_did) {
 492                     if self.live_symbols.contains(&item_node_id) {
 493                         return true;
 494                     }
 495                 }
 496             }
 497         }
 498         false
 499     }
 500
 501     fn warn_dead_code(&mut self,
 502                       id: ast::NodeId,
 503                       span: syntax_pos::Span,
 504                       name: ast::Name,
 505                       node_type: &str) {
 506         if !name.as_str().starts_with("_") {
 507             self.tcx
 508                 .sess
 509                 .add_lint(lint::builtin::DEAD_CODE,
 510                           id,
 511                           span,
 512                           format!("{} is never used: `{}`", node_type, name));
 513         }
 514     }
 515 }
 516
 517 impl<'a, 'tcx> Visitor<'tcx> for DeadVisitor<'a, 'tcx> {
 518     /// Walk nested items in place so that we don't report dead-code
 519     /// on inner functions when the outer function is already getting
 520     /// an error. We could do this also by checking the parents, but
 521     /// this is how the code is setup and it seems harmless enough.
 522     fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
 523         NestedVisitorMap::All(&self.tcx.hir)
 524     }
 525
 526     fn visit_item(&mut self, item: &'tcx hir::Item) {
 527         if self.should_warn_about_item(item) {
 528             self.warn_dead_code(
 529                 item.id,
 530                 item.span,
 531                 item.name,
 532                 item.node.descriptive_variant()
 533             );
 534         } else {
 535             // Only continue if we didn't warn
 536             intravisit::walk_item(self, item);
 537         }
 538     }
 539
 540     fn visit_variant(&mut self,
 541                      variant: &'tcx hir::Variant,
 542                      g: &'tcx hir::Generics,
 543                      id: ast::NodeId) {
 544         if self.should_warn_about_variant(&variant.node) {
 545             self.warn_dead_code(variant.node.data.id(), variant.span,
 546                                 variant.node.name, "variant");
 547         } else {
 548             intravisit::walk_variant(self, variant, g, id);
 549         }
 550     }
 551
 552     fn visit_foreign_item(&mut self, fi: &'tcx hir::ForeignItem) {
 553         if self.should_warn_about_foreign_item(fi) {
 554             self.warn_dead_code(fi.id, fi.span, fi.name, fi.node.descriptive_variant());
 555         }
 556         intravisit::walk_foreign_item(self, fi);
 557     }
 558
 559     fn visit_struct_field(&mut self, field: &'tcx hir::StructField) {
 560         if self.should_warn_about_field(&field) {
 561             self.warn_dead_code(field.id, field.span,
 562                                 field.name, "field");
 563         }
 564
 565         intravisit::walk_struct_field(self, field);
 566     }
 567
 568     fn visit_impl_item(&mut self, impl_item: &'tcx hir::ImplItem) {
 569         match impl_item.node {
 570             hir::ImplItemKind::Const(_, body_id) => {
 571                 if !self.symbol_is_live(impl_item.id, None) {
 572                     self.warn_dead_code(impl_item.id, impl_item.span,
 573                                         impl_item.name, "associated const");
 574                 }
 575                 self.visit_nested_body(body_id)
 576             }
 577             hir::ImplItemKind::Method(_, body_id) => {
 578                 if !self.symbol_is_live(impl_item.id, None) {
 579                     self.warn_dead_code(impl_item.id, impl_item.span,
 580                                         impl_item.name, "method");
 581                 }
 582                 self.visit_nested_body(body_id)
 583             }
 584             hir::ImplItemKind::Type(..) => {}
 585         }
 586     }
 587
 588     // Overwrite so that we don't warn the trait item itself.
 589     fn visit_trait_item(&mut self, trait_item: &'tcx hir::TraitItem) {
 590         match trait_item.node {
 591             hir::TraitItemKind::Const(_, Some(body_id)) |
 592             hir::TraitItemKind::Method(_, hir::TraitMethod::Provided(body_id)) => {
 593                 self.visit_nested_body(body_id)
 594             }
 595             hir::TraitItemKind::Const(_, None) |
 596             hir::TraitItemKind::Method(_, hir::TraitMethod::Required(_)) |
 597             hir::TraitItemKind::Type(..) => {}
 598         }
 599     }
 600 }
 601
 602 pub fn check_crate<'a, 'tcx>(tcx: TyCtxt<'a, 'tcx, 'tcx>) {
 603     let access_levels = &tcx.privacy_access_levels(LOCAL_CRATE);
 604     let krate = tcx.hir.krate();
 605     let live_symbols = find_live(tcx, access_levels, krate);
 606     let mut visitor = DeadVisitor { tcx: tcx, live_symbols: live_symbols };
 607     intravisit::walk_crate(&mut visitor, krate);
 608 }