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 ast_map;
  16 use middle::{def, pat_util, privacy, ty};
  17 use lint;
  18 use util::nodemap::NodeSet;
  19
  20 use std::collections::HashSet;
  21 use syntax::{ast, codemap};
  22 use syntax::ast_util::{local_def, is_local};
  23 use syntax::attr::{self, AttrMetaMethods};
  24 use syntax::visit::{self, Visitor};
  25
  26 // Any local node that may call something in its body block should be
  27 // explored. For example, if it's a live NodeItem that is a
  28 // function, then we should explore its block to check for codes that
  29 // may need to be marked as live.
  30 fn should_explore(tcx: &ty::ctxt, def_id: ast::DefId) -> bool {
  31     if !is_local(def_id) {
  32         return false;
  33     }
  34
  35     match tcx.map.find(def_id.node) {
  36         Some(ast_map::NodeItem(..))
  37         | Some(ast_map::NodeImplItem(..))
  38         | Some(ast_map::NodeForeignItem(..))
  39         | Some(ast_map::NodeTraitItem(..)) => true,
  40         _ => false
  41     }
  42 }
  43
  44 struct MarkSymbolVisitor<'a, 'tcx: 'a> {
  45     worklist: Vec<ast::NodeId>,
  46     tcx: &'a ty::ctxt<'tcx>,
  47     live_symbols: Box<HashSet<ast::NodeId>>,
  48     struct_has_extern_repr: bool,
  49     ignore_non_const_paths: bool,
  50     inherited_pub_visibility: bool,
  51     ignore_variant_stack: Vec<ast::NodeId>,
  52 }
  53
  54 impl<'a, 'tcx> MarkSymbolVisitor<'a, 'tcx> {
  55     fn new(tcx: &'a ty::ctxt<'tcx>,
  56            worklist: Vec<ast::NodeId>) -> MarkSymbolVisitor<'a, 'tcx> {
  57         MarkSymbolVisitor {
  58             worklist: worklist,
  59             tcx: tcx,
  60             live_symbols: box HashSet::new(),
  61             struct_has_extern_repr: false,
  62             ignore_non_const_paths: false,
  63             inherited_pub_visibility: false,
  64             ignore_variant_stack: vec![],
  65         }
  66     }
  67
  68     fn check_def_id(&mut self, def_id: ast::DefId) {
  69         if should_explore(self.tcx, def_id) {
  70             self.worklist.push(def_id.node);
  71         }
  72         self.live_symbols.insert(def_id.node);
  73     }
  74
  75     fn lookup_and_handle_definition(&mut self, id: &ast::NodeId) {
  76         self.tcx.def_map.borrow().get(id).map(|def| {
  77             match def.full_def() {
  78                 def::DefConst(_) | def::DefAssociatedConst(..) => {
  79                     self.check_def_id(def.def_id())
  80                 }
  81                 _ if self.ignore_non_const_paths => (),
  82                 def::DefPrimTy(_) => (),
  83                 def::DefVariant(enum_id, variant_id, _) => {
  84                     self.check_def_id(enum_id);
  85                     if !self.ignore_variant_stack.contains(&variant_id.node) {
  86                         self.check_def_id(variant_id);
  87                     }
  88                 }
  89                 _ => {
  90                     self.check_def_id(def.def_id());
  91                 }
  92             }
  93         });
  94     }
  95
  96     fn lookup_and_handle_method(&mut self, id: ast::NodeId,
  97                                 span: codemap::Span) {
  98         let method_call = ty::MethodCall::expr(id);
  99         match self.tcx.method_map.borrow().get(&method_call) {
 100             Some(method) => {
 101                 match method.origin {
 102                     ty::MethodStatic(def_id) => {
 103                         match ty::provided_source(self.tcx, def_id) {
 104                             Some(p_did) => self.check_def_id(p_did),
 105                             None => self.check_def_id(def_id)
 106                         }
 107                     }
 108                     ty::MethodStaticClosure(_) => {}
 109                     ty::MethodTypeParam(ty::MethodParam {
 110                         ref trait_ref,
 111                         method_num: index,
 112                         ..
 113                     }) |
 114                     ty::MethodTraitObject(ty::MethodObject {
 115                         ref trait_ref,
 116                         method_num: index,
 117                         ..
 118                     }) => {
 119                         let trait_item = ty::trait_item(self.tcx,
 120                                                         trait_ref.def_id,
 121                                                         index);
 122                         self.check_def_id(trait_item.def_id());
 123                     }
 124                 }
 125             }
 126             None => {
 127                 self.tcx.sess.span_bug(span,
 128                                        "method call expression not \
 129                                         in method map?!")
 130             }
 131         }
 132     }
 133
 134     fn handle_field_access(&mut self, lhs: &ast::Expr, name: ast::Name) {
 135         match ty::expr_ty_adjusted(self.tcx, lhs).sty {
 136             ty::TyStruct(id, _) => {
 137                 let fields = ty::lookup_struct_fields(self.tcx, id);
 138                 let field_id = fields.iter()
 139                     .find(|field| field.name == name).unwrap().id;
 140                 self.live_symbols.insert(field_id.node);
 141             },
 142             _ => ()
 143         }
 144     }
 145
 146     fn handle_tup_field_access(&mut self, lhs: &ast::Expr, idx: usize) {
 147         match ty::expr_ty_adjusted(self.tcx, lhs).sty {
 148             ty::TyStruct(id, _) => {
 149                 let fields = ty::lookup_struct_fields(self.tcx, id);
 150                 let field_id = fields[idx].id;
 151                 self.live_symbols.insert(field_id.node);
 152             },
 153             _ => ()
 154         }
 155     }
 156
 157     fn handle_field_pattern_match(&mut self, lhs: &ast::Pat,
 158                                   pats: &[codemap::Spanned<ast::FieldPat>]) {
 159         let id = match self.tcx.def_map.borrow().get(&lhs.id).unwrap().full_def() {
 160             def::DefVariant(_, id, _) => id,
 161             _ => {
 162                 match ty::ty_to_def_id(ty::node_id_to_type(self.tcx,
 163                                                            lhs.id)) {
 164                     None => {
 165                         self.tcx.sess.span_bug(lhs.span,
 166                                                "struct pattern wasn't of a \
 167                                                 type with a def ID?!")
 168                     }
 169                     Some(def_id) => def_id,
 170                 }
 171             }
 172         };
 173         let fields = ty::lookup_struct_fields(self.tcx, id);
 174         for pat in pats {
 175             if let ast::PatWild(ast::PatWildSingle) = pat.node.pat.node {
 176                 continue;
 177             }
 178             let field_id = fields.iter()
 179                 .find(|field| field.name == pat.node.ident.name).unwrap().id;
 180             self.live_symbols.insert(field_id.node);
 181         }
 182     }
 183
 184     fn mark_live_symbols(&mut self) {
 185         let mut scanned = HashSet::new();
 186         while !self.worklist.is_empty() {
 187             let id = self.worklist.pop().unwrap();
 188             if scanned.contains(&id) {
 189                 continue
 190             }
 191             scanned.insert(id);
 192
 193             match self.tcx.map.find(id) {
 194                 Some(ref node) => {
 195                     self.live_symbols.insert(id);
 196                     self.visit_node(node);
 197                 }
 198                 None => (),
 199             }
 200         }
 201     }
 202
 203     fn visit_node(&mut self, node: &ast_map::Node) {
 204         let had_extern_repr = self.struct_has_extern_repr;
 205         self.struct_has_extern_repr = false;
 206         let had_inherited_pub_visibility = self.inherited_pub_visibility;
 207         self.inherited_pub_visibility = false;
 208         match *node {
 209             ast_map::NodeItem(item) => {
 210                 match item.node {
 211                     ast::ItemStruct(..) => {
 212                         self.struct_has_extern_repr = item.attrs.iter().any(|attr| {
 213                             attr::find_repr_attrs(self.tcx.sess.diagnostic(), attr)
 214                                 .contains(&attr::ReprExtern)
 215                         });
 216
 217                         visit::walk_item(self, &*item);
 218                     }
 219                     ast::ItemEnum(..) => {
 220                         self.inherited_pub_visibility = item.vis == ast::Public;
 221                         visit::walk_item(self, &*item);
 222                     }
 223                     ast::ItemFn(..)
 224                     | ast::ItemTy(..)
 225                     | ast::ItemStatic(..)
 226                     | ast::ItemConst(..) => {
 227                         visit::walk_item(self, &*item);
 228                     }
 229                     _ => ()
 230                 }
 231             }
 232             ast_map::NodeTraitItem(trait_item) => {
 233                 visit::walk_trait_item(self, trait_item);
 234             }
 235             ast_map::NodeImplItem(impl_item) => {
 236                 visit::walk_impl_item(self, impl_item);
 237             }
 238             ast_map::NodeForeignItem(foreign_item) => {
 239                 visit::walk_foreign_item(self, &*foreign_item);
 240             }
 241             _ => ()
 242         }
 243         self.struct_has_extern_repr = had_extern_repr;
 244         self.inherited_pub_visibility = had_inherited_pub_visibility;
 245     }
 246 }
 247
 248 impl<'a, 'tcx, 'v> Visitor<'v> for MarkSymbolVisitor<'a, 'tcx> {
 249
 250     fn visit_struct_def(&mut self, def: &ast::StructDef, _: ast::Ident,
 251                         _: &ast::Generics, _: ast::NodeId) {
 252         let has_extern_repr = self.struct_has_extern_repr;
 253         let inherited_pub_visibility = self.inherited_pub_visibility;
 254         let live_fields = def.fields.iter().filter(|f| {
 255             has_extern_repr || inherited_pub_visibility || match f.node.kind {
 256                 ast::NamedField(_, ast::Public) => true,
 257                 _ => false
 258             }
 259         });
 260         self.live_symbols.extend(live_fields.map(|f| f.node.id));
 261
 262         visit::walk_struct_def(self, def);
 263     }
 264
 265     fn visit_expr(&mut self, expr: &ast::Expr) {
 266         match expr.node {
 267             ast::ExprMethodCall(..) => {
 268                 self.lookup_and_handle_method(expr.id, expr.span);
 269             }
 270             ast::ExprField(ref lhs, ref ident) => {
 271                 self.handle_field_access(&**lhs, ident.node.name);
 272             }
 273             ast::ExprTupField(ref lhs, idx) => {
 274                 self.handle_tup_field_access(&**lhs, idx.node);
 275             }
 276             _ => ()
 277         }
 278
 279         visit::walk_expr(self, expr);
 280     }
 281
 282     fn visit_arm(&mut self, arm: &ast::Arm) {
 283         if arm.pats.len() == 1 {
 284             let pat = &*arm.pats[0];
 285             let variants = pat_util::necessary_variants(&self.tcx.def_map, pat);
 286
 287             // Inside the body, ignore constructions of variants
 288             // necessary for the pattern to match. Those construction sites
 289             // can't be reached unless the variant is constructed elsewhere.
 290             let len = self.ignore_variant_stack.len();
 291             self.ignore_variant_stack.push_all(&*variants);
 292             visit::walk_arm(self, arm);
 293             self.ignore_variant_stack.truncate(len);
 294         } else {
 295             visit::walk_arm(self, arm);
 296         }
 297     }
 298
 299     fn visit_pat(&mut self, pat: &ast::Pat) {
 300         let def_map = &self.tcx.def_map;
 301         match pat.node {
 302             ast::PatStruct(_, ref fields, _) => {
 303                 self.handle_field_pattern_match(pat, fields);
 304             }
 305             _ if pat_util::pat_is_const(def_map, pat) => {
 306                 // it might be the only use of a const
 307                 self.lookup_and_handle_definition(&pat.id)
 308             }
 309             _ => ()
 310         }
 311
 312         self.ignore_non_const_paths = true;
 313         visit::walk_pat(self, pat);
 314         self.ignore_non_const_paths = false;
 315     }
 316
 317     fn visit_path(&mut self, path: &ast::Path, id: ast::NodeId) {
 318         self.lookup_and_handle_definition(&id);
 319         visit::walk_path(self, path);
 320     }
 321
 322     fn visit_item(&mut self, _: &ast::Item) {
 323         // Do not recurse into items. These items will be added to the
 324         // worklist and recursed into manually if necessary.
 325     }
 326 }
 327
 328 fn has_allow_dead_code_or_lang_attr(attrs: &[ast::Attribute]) -> bool {
 329     if attr::contains_name(attrs, "lang") {
 330         return true;
 331     }
 332
 333     let dead_code = lint::builtin::DEAD_CODE.name_lower();
 334     for attr in lint::gather_attrs(attrs) {
 335         match attr {
 336             Ok((ref name, lint::Allow, _))
 337                 if &name[..] == dead_code => return true,
 338             _ => (),
 339         }
 340     }
 341     false
 342 }
 343
 344 // This visitor seeds items that
 345 //   1) We want to explicitly consider as live:
 346 //     * Item annotated with #[allow(dead_code)]
 347 //         - This is done so that if we want to suppress warnings for a
 348 //           group of dead functions, we only have to annotate the "root".
 349 //           For example, if both `f` and `g` are dead and `f` calls `g`,
 350 //           then annotating `f` with `#[allow(dead_code)]` will suppress
 351 //           warning for both `f` and `g`.
 352 //     * Item annotated with #[lang=".."]
 353 //         - This is because lang items are always callable from elsewhere.
 354 //   or
 355 //   2) We are not sure to be live or not
 356 //     * Implementation of a trait method
 357 struct LifeSeeder {
 358     worklist: Vec<ast::NodeId>
 359 }
 360
 361 impl<'v> Visitor<'v> for LifeSeeder {
 362     fn visit_item(&mut self, item: &ast::Item) {
 363         let allow_dead_code = has_allow_dead_code_or_lang_attr(&item.attrs);
 364         if allow_dead_code {
 365             self.worklist.push(item.id);
 366         }
 367         match item.node {
 368             ast::ItemEnum(ref enum_def, _) if allow_dead_code => {
 369                 self.worklist.extend(enum_def.variants.iter().map(|variant| variant.node.id));
 370             }
 371             ast::ItemTrait(_, _, _, ref trait_items) => {
 372                 for trait_item in trait_items {
 373                     match trait_item.node {
 374                         ast::ConstTraitItem(_, Some(_)) |
 375                         ast::MethodTraitItem(_, Some(_)) => {
 376                             if has_allow_dead_code_or_lang_attr(&trait_item.attrs) {
 377                                 self.worklist.push(trait_item.id);
 378                             }
 379                         }
 380                         _ => {}
 381                     }
 382                 }
 383             }
 384             ast::ItemImpl(_, _, _, ref opt_trait, _, ref impl_items) => {
 385                 for impl_item in impl_items {
 386                     match impl_item.node {
 387                         ast::ConstImplItem(..) |
 388                         ast::MethodImplItem(..) => {
 389                             if opt_trait.is_some() ||
 390                                     has_allow_dead_code_or_lang_attr(&impl_item.attrs) {
 391                                 self.worklist.push(impl_item.id);
 392                             }
 393                         }
 394                         ast::TypeImplItem(_) => {}
 395                         ast::MacImplItem(_) => panic!("unexpanded macro")
 396                     }
 397                 }
 398             }
 399             _ => ()
 400         }
 401         visit::walk_item(self, item);
 402     }
 403 }
 404
 405 fn create_and_seed_worklist(tcx: &ty::ctxt,
 406                             exported_items: &privacy::ExportedItems,
 407                             reachable_symbols: &NodeSet,
 408                             krate: &ast::Crate) -> Vec<ast::NodeId> {
 409     let mut worklist = Vec::new();
 410
 411     // Preferably, we would only need to seed the worklist with reachable
 412     // symbols. However, since the set of reachable symbols differs
 413     // depending on whether a crate is built as bin or lib, and we want
 414     // the warning to be consistent, we also seed the worklist with
 415     // exported symbols.
 416     for id in exported_items {
 417         worklist.push(*id);
 418     }
 419     for id in reachable_symbols {
 420         // Reachable variants can be dead, because we warn about
 421         // variants never constructed, not variants never used.
 422         if let Some(ast_map::NodeVariant(..)) = tcx.map.find(*id) {
 423             continue;
 424         }
 425         worklist.push(*id);
 426     }
 427
 428     // Seed entry point
 429     match *tcx.sess.entry_fn.borrow() {
 430         Some((id, _)) => worklist.push(id),
 431         None => ()
 432     }
 433
 434     // Seed implemented trait items
 435     let mut life_seeder = LifeSeeder {
 436         worklist: worklist
 437     };
 438     visit::walk_crate(&mut life_seeder, krate);
 439
 440     return life_seeder.worklist;
 441 }
 442
 443 fn find_live(tcx: &ty::ctxt,
 444              exported_items: &privacy::ExportedItems,
 445              reachable_symbols: &NodeSet,
 446              krate: &ast::Crate)
 447              -> Box<HashSet<ast::NodeId>> {
 448     let worklist = create_and_seed_worklist(tcx, exported_items,
 449                                             reachable_symbols, krate);
 450     let mut symbol_visitor = MarkSymbolVisitor::new(tcx, worklist);
 451     symbol_visitor.mark_live_symbols();
 452     symbol_visitor.live_symbols
 453 }
 454
 455 fn get_struct_ctor_id(item: &ast::Item) -> Option<ast::NodeId> {
 456     match item.node {
 457         ast::ItemStruct(ref struct_def, _) => struct_def.ctor_id,
 458         _ => None
 459     }
 460 }
 461
 462 struct DeadVisitor<'a, 'tcx: 'a> {
 463     tcx: &'a ty::ctxt<'tcx>,
 464     live_symbols: Box<HashSet<ast::NodeId>>,
 465 }
 466
 467 impl<'a, 'tcx> DeadVisitor<'a, 'tcx> {
 468     fn should_warn_about_item(&mut self, item: &ast::Item) -> bool {
 469         let should_warn = match item.node {
 470             ast::ItemStatic(..)
 471             | ast::ItemConst(..)
 472             | ast::ItemFn(..)
 473             | ast::ItemEnum(..)
 474             | ast::ItemStruct(..) => true,
 475             _ => false
 476         };
 477         let ctor_id = get_struct_ctor_id(item);
 478         should_warn && !self.symbol_is_live(item.id, ctor_id)
 479     }
 480
 481     fn should_warn_about_field(&mut self, node: &ast::StructField_) -> bool {
 482         let is_named = node.ident().is_some();
 483         let field_type = ty::node_id_to_type(self.tcx, node.id);
 484         let is_marker_field = match ty::ty_to_def_id(field_type) {
 485             Some(def_id) => self.tcx.lang_items.items().any(|(_, item)| *item == Some(def_id)),
 486             _ => false
 487         };
 488         is_named
 489             && !self.symbol_is_live(node.id, None)
 490             && !is_marker_field
 491             && !has_allow_dead_code_or_lang_attr(&node.attrs)
 492     }
 493
 494     fn should_warn_about_variant(&mut self, variant: &ast::Variant_) -> bool {
 495         !self.symbol_is_live(variant.id, None)
 496             && !has_allow_dead_code_or_lang_attr(&variant.attrs)
 497     }
 498
 499     // id := node id of an item's definition.
 500     // ctor_id := `Some` if the item is a struct_ctor (tuple struct),
 501     //            `None` otherwise.
 502     // If the item is a struct_ctor, then either its `id` or
 503     // `ctor_id` (unwrapped) is in the live_symbols set. More specifically,
 504     // DefMap maps the ExprPath of a struct_ctor to the node referred by
 505     // `ctor_id`. On the other hand, in a statement like
 506     // `type <ident> <generics> = <ty>;` where <ty> refers to a struct_ctor,
 507     // DefMap maps <ty> to `id` instead.
 508     fn symbol_is_live(&mut self, id: ast::NodeId,
 509                       ctor_id: Option<ast::NodeId>) -> bool {
 510         if self.live_symbols.contains(&id)
 511            || ctor_id.map_or(false,
 512                              |ctor| self.live_symbols.contains(&ctor)) {
 513             return true;
 514         }
 515         // If it's a type whose items are live, then it's live, too.
 516         // This is done to handle the case where, for example, the static
 517         // method of a private type is used, but the type itself is never
 518         // called directly.
 519         let impl_items = self.tcx.impl_items.borrow();
 520         match self.tcx.inherent_impls.borrow().get(&local_def(id)) {
 521             None => (),
 522             Some(impl_list) => {
 523                 for impl_did in impl_list.iter() {
 524                     for item_did in impl_items.get(impl_did).unwrap().iter() {
 525                         if self.live_symbols.contains(&item_did.def_id()
 526                                                                .node) {
 527                             return true;
 528                         }
 529                     }
 530                 }
 531             }
 532         }
 533         false
 534     }
 535
 536     fn warn_dead_code(&mut self,
 537                       id: ast::NodeId,
 538                       span: codemap::Span,
 539                       name: ast::Name,
 540                       node_type: &str) {
 541         let name = name.as_str();
 542         if !name.starts_with("_") {
 543             self.tcx
 544                 .sess
 545                 .add_lint(lint::builtin::DEAD_CODE,
 546                           id,
 547                           span,
 548                           format!("{} is never used: `{}`", node_type, name));
 549         }
 550     }
 551 }
 552
 553 impl<'a, 'tcx, 'v> Visitor<'v> for DeadVisitor<'a, 'tcx> {
 554     fn visit_item(&mut self, item: &ast::Item) {
 555         if self.should_warn_about_item(item) {
 556             self.warn_dead_code(
 557                 item.id,
 558                 item.span,
 559                 item.ident.name,
 560                 item.node.descriptive_variant()
 561             );
 562         } else {
 563             match item.node {
 564                 ast::ItemEnum(ref enum_def, _) => {
 565                     for variant in &enum_def.variants {
 566                         if self.should_warn_about_variant(&variant.node) {
 567                             self.warn_dead_code(variant.node.id, variant.span,
 568                                                 variant.node.name.name, "variant");
 569                         }
 570                     }
 571                 },
 572                 _ => ()
 573             }
 574         }
 575         visit::walk_item(self, item);
 576     }
 577
 578     fn visit_foreign_item(&mut self, fi: &ast::ForeignItem) {
 579         if !self.symbol_is_live(fi.id, None) {
 580             self.warn_dead_code(fi.id, fi.span, fi.ident.name, fi.node.descriptive_variant());
 581         }
 582         visit::walk_foreign_item(self, fi);
 583     }
 584
 585     fn visit_struct_field(&mut self, field: &ast::StructField) {
 586         if self.should_warn_about_field(&field.node) {
 587             self.warn_dead_code(field.node.id, field.span,
 588                                 field.node.ident().unwrap().name, "struct field");
 589         }
 590
 591         visit::walk_struct_field(self, field);
 592     }
 593
 594     fn visit_impl_item(&mut self, impl_item: &ast::ImplItem) {
 595         match impl_item.node {
 596             ast::ConstImplItem(_, ref expr) => {
 597                 if !self.symbol_is_live(impl_item.id, None) {
 598                     self.warn_dead_code(impl_item.id, impl_item.span,
 599                                         impl_item.ident.name, "associated const");
 600                 }
 601                 visit::walk_expr(self, expr)
 602             }
 603             ast::MethodImplItem(_, ref body) => {
 604                 if !self.symbol_is_live(impl_item.id, None) {
 605                     self.warn_dead_code(impl_item.id, impl_item.span,
 606                                         impl_item.ident.name, "method");
 607                 }
 608                 visit::walk_block(self, body)
 609             }
 610             ast::TypeImplItem(..) |
 611             ast::MacImplItem(..) => {}
 612         }
 613     }
 614
 615     // Overwrite so that we don't warn the trait item itself.
 616     fn visit_trait_item(&mut self, trait_item: &ast::TraitItem) {
 617         match trait_item.node {
 618             ast::ConstTraitItem(_, Some(ref expr)) => {
 619                 visit::walk_expr(self, expr)
 620             }
 621             ast::MethodTraitItem(_, Some(ref body)) => {
 622                 visit::walk_block(self, body)
 623             }
 624             ast::ConstTraitItem(_, None) |
 625             ast::MethodTraitItem(_, None) |
 626             ast::TypeTraitItem(..) => {}
 627         }
 628     }
 629 }
 630
 631 pub fn check_crate(tcx: &ty::ctxt,
 632                    exported_items: &privacy::ExportedItems,
 633                    reachable_symbols: &NodeSet) {
 634     let krate = tcx.map.krate();
 635     let live_symbols = find_live(tcx, exported_items,
 636                                  reachable_symbols, krate);
 637     let mut visitor = DeadVisitor { tcx: tcx, live_symbols: live_symbols };
 638     visit::walk_crate(&mut visitor, krate);
 639 }