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