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 dep_graph::DepNode;
  16 use hir::map as ast_map;
  17 use hir::{self, pat_util, PatKind};
  18 use hir::intravisit::{self, Visitor};
  19
  20 use middle::privacy;
  21 use ty::{self, TyCtxt};
  22 use hir::def::Def;
  23 use hir::def_id::{DefId};
  24 use lint;
  25
  26 use std::collections::HashSet;
  27 use syntax::{ast, codemap};
  28 use syntax::attr::{self, AttrMetaMethods};
  29
  30 // Any local node that may call something in its body block should be
  31 // explored. For example, if it's a live NodeItem that is a
  32 // function, then we should explore its block to check for codes that
  33 // may need to be marked as live.
  34 fn should_explore(tcx: &TyCtxt, node_id: ast::NodeId) -> bool {
  35     match tcx.map.find(node_id) {
  36         Some(ast_map::NodeItem(..)) |
  37         Some(ast_map::NodeImplItem(..)) |
  38         Some(ast_map::NodeForeignItem(..)) |
  39         Some(ast_map::NodeTraitItem(..)) =>
  40             true,
  41         _ =>
  42             false
  43     }
  44 }
  45
  46 struct MarkSymbolVisitor<'a, 'tcx: 'a> {
  47     worklist: Vec<ast::NodeId>,
  48     tcx: &'a TyCtxt<'tcx>,
  49     live_symbols: Box<HashSet<ast::NodeId>>,
  50     struct_has_extern_repr: bool,
  51     ignore_non_const_paths: bool,
  52     inherited_pub_visibility: bool,
  53     ignore_variant_stack: Vec<DefId>,
  54 }
  55
  56 impl<'a, 'tcx> MarkSymbolVisitor<'a, 'tcx> {
  57     fn new(tcx: &'a TyCtxt<'tcx>,
  58            worklist: Vec<ast::NodeId>) -> MarkSymbolVisitor<'a, 'tcx> {
  59         MarkSymbolVisitor {
  60             worklist: worklist,
  61             tcx: tcx,
  62             live_symbols: box HashSet::new(),
  63             struct_has_extern_repr: false,
  64             ignore_non_const_paths: false,
  65             inherited_pub_visibility: false,
  66             ignore_variant_stack: vec![],
  67         }
  68     }
  69
  70     fn check_def_id(&mut self, def_id: DefId) {
  71         if let Some(node_id) = self.tcx.map.as_local_node_id(def_id) {
  72             if should_explore(self.tcx, node_id) {
  73                 self.worklist.push(node_id);
  74             }
  75             self.live_symbols.insert(node_id);
  76         }
  77     }
  78
  79     fn insert_def_id(&mut self, def_id: DefId) {
  80         if let Some(node_id) = self.tcx.map.as_local_node_id(def_id) {
  81             debug_assert!(!should_explore(self.tcx, node_id));
  82             self.live_symbols.insert(node_id);
  83         }
  84     }
  85
  86     fn lookup_and_handle_definition(&mut self, id: &ast::NodeId) {
  87         use ty::TypeVariants::{TyEnum, TyStruct};
  88
  89         // If `bar` is a trait item, make sure to mark Foo as alive in `Foo::bar`
  90         self.tcx.tables.borrow().item_substs.get(id)
  91             .and_then(|substs| substs.substs.self_ty())
  92             .map(|ty| match ty.sty {
  93                 TyEnum(tyid, _) | TyStruct(tyid, _) => self.check_def_id(tyid.did),
  94                 _ => (),
  95             });
  96
  97         self.tcx.def_map.borrow().get(id).map(|def| {
  98             match def.full_def() {
  99                 Def::Const(_) | Def::AssociatedConst(..) => {
 100                     self.check_def_id(def.def_id());
 101                 }
 102                 _ if self.ignore_non_const_paths => (),
 103                 Def::PrimTy(_) => (),
 104                 Def::SelfTy(..) => (),
 105                 Def::Variant(enum_id, variant_id) => {
 106                     self.check_def_id(enum_id);
 107                     if !self.ignore_variant_stack.contains(&variant_id) {
 108                         self.check_def_id(variant_id);
 109                     }
 110                 }
 111                 _ => {
 112                     self.check_def_id(def.def_id());
 113                 }
 114             }
 115         });
 116     }
 117
 118     fn lookup_and_handle_method(&mut self, id: ast::NodeId) {
 119         let method_call = ty::MethodCall::expr(id);
 120         let method = self.tcx.tables.borrow().method_map[&method_call];
 121         self.check_def_id(method.def_id);
 122     }
 123
 124     fn handle_field_access(&mut self, lhs: &hir::Expr, name: ast::Name) {
 125         if let ty::TyStruct(def, _) = self.tcx.expr_ty_adjusted(lhs).sty {
 126             self.insert_def_id(def.struct_variant().field_named(name).did);
 127         } else {
 128             span_bug!(lhs.span, "named field access on non-struct")
 129         }
 130     }
 131
 132     fn handle_tup_field_access(&mut self, lhs: &hir::Expr, idx: usize) {
 133         if let ty::TyStruct(def, _) = self.tcx.expr_ty_adjusted(lhs).sty {
 134             self.insert_def_id(def.struct_variant().fields[idx].did);
 135         }
 136     }
 137
 138     fn handle_field_pattern_match(&mut self, lhs: &hir::Pat,
 139                                   pats: &[codemap::Spanned<hir::FieldPat>]) {
 140         let def = self.tcx.def_map.borrow().get(&lhs.id).unwrap().full_def();
 141         let pat_ty = self.tcx.node_id_to_type(lhs.id);
 142         let variant = match pat_ty.sty {
 143             ty::TyStruct(adt, _) | ty::TyEnum(adt, _) => adt.variant_of_def(def),
 144             _ => span_bug!(lhs.span, "non-ADT in struct pattern")
 145         };
 146         for pat in pats {
 147             if let PatKind::Wild = pat.node.pat.node {
 148                 continue;
 149             }
 150             self.insert_def_id(variant.field_named(pat.node.name).did);
 151         }
 152     }
 153
 154     fn mark_live_symbols(&mut self) {
 155         let mut scanned = HashSet::new();
 156         while !self.worklist.is_empty() {
 157             let id = self.worklist.pop().unwrap();
 158             if scanned.contains(&id) {
 159                 continue
 160             }
 161             scanned.insert(id);
 162
 163             match self.tcx.map.find(id) {
 164                 Some(ref node) => {
 165                     self.live_symbols.insert(id);
 166                     self.visit_node(node);
 167                 }
 168                 None => (),
 169             }
 170         }
 171     }
 172
 173     fn visit_node(&mut self, node: &ast_map::Node) {
 174         let had_extern_repr = self.struct_has_extern_repr;
 175         self.struct_has_extern_repr = false;
 176         let had_inherited_pub_visibility = self.inherited_pub_visibility;
 177         self.inherited_pub_visibility = false;
 178         match *node {
 179             ast_map::NodeItem(item) => {
 180                 match item.node {
 181                     hir::ItemStruct(..) => {
 182                         self.struct_has_extern_repr = item.attrs.iter().any(|attr| {
 183                             attr::find_repr_attrs(self.tcx.sess.diagnostic(), attr)
 184                                 .contains(&attr::ReprExtern)
 185                         });
 186
 187                         intravisit::walk_item(self, &item);
 188                     }
 189                     hir::ItemEnum(..) => {
 190                         self.inherited_pub_visibility = item.vis == hir::Public;
 191                         intravisit::walk_item(self, &item);
 192                     }
 193                     hir::ItemFn(..)
 194                     | hir::ItemTy(..)
 195                     | hir::ItemStatic(..)
 196                     | hir::ItemConst(..) => {
 197                         intravisit::walk_item(self, &item);
 198                     }
 199                     _ => ()
 200                 }
 201             }
 202             ast_map::NodeTraitItem(trait_item) => {
 203                 intravisit::walk_trait_item(self, trait_item);
 204             }
 205             ast_map::NodeImplItem(impl_item) => {
 206                 intravisit::walk_impl_item(self, impl_item);
 207             }
 208             ast_map::NodeForeignItem(foreign_item) => {
 209                 intravisit::walk_foreign_item(self, &foreign_item);
 210             }
 211             _ => ()
 212         }
 213         self.struct_has_extern_repr = had_extern_repr;
 214         self.inherited_pub_visibility = had_inherited_pub_visibility;
 215     }
 216 }
 217
 218 impl<'a, 'tcx, 'v> Visitor<'v> for MarkSymbolVisitor<'a, 'tcx> {
 219
 220     fn visit_variant_data(&mut self, def: &hir::VariantData, _: ast::Name,
 221                         _: &hir::Generics, _: ast::NodeId, _: codemap::Span) {
 222         let has_extern_repr = self.struct_has_extern_repr;
 223         let inherited_pub_visibility = self.inherited_pub_visibility;
 224         let live_fields = def.fields().iter().filter(|f| {
 225             has_extern_repr || inherited_pub_visibility || f.vis == hir::Public
 226         });
 227         self.live_symbols.extend(live_fields.map(|f| f.id));
 228
 229         intravisit::walk_struct_def(self, def);
 230     }
 231
 232     fn visit_expr(&mut self, expr: &hir::Expr) {
 233         match expr.node {
 234             hir::ExprMethodCall(..) => {
 235                 self.lookup_and_handle_method(expr.id);
 236             }
 237             hir::ExprField(ref lhs, ref name) => {
 238                 self.handle_field_access(&lhs, name.node);
 239             }
 240             hir::ExprTupField(ref lhs, idx) => {
 241                 self.handle_tup_field_access(&lhs, idx.node);
 242             }
 243             _ => ()
 244         }
 245
 246         intravisit::walk_expr(self, expr);
 247     }
 248
 249     fn visit_arm(&mut self, arm: &hir::Arm) {
 250         if arm.pats.len() == 1 {
 251             let pat = &*arm.pats[0];
 252             let variants = pat_util::necessary_variants(&self.tcx.def_map.borrow(), 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.extend_from_slice(&variants);
 259             intravisit::walk_arm(self, arm);
 260             self.ignore_variant_stack.truncate(len);
 261         } else {
 262             intravisit::walk_arm(self, arm);
 263         }
 264     }
 265
 266     fn visit_pat(&mut self, pat: &hir::Pat) {
 267         let def_map = &self.tcx.def_map;
 268         match pat.node {
 269             PatKind::Struct(_, ref fields, _) => {
 270                 self.handle_field_pattern_match(pat, fields);
 271             }
 272             _ if pat_util::pat_is_const(&def_map.borrow(), 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         intravisit::walk_pat(self, pat);
 281         self.ignore_non_const_paths = false;
 282     }
 283
 284     fn visit_path(&mut self, path: &hir::Path, id: ast::NodeId) {
 285         self.lookup_and_handle_definition(&id);
 286         intravisit::walk_path(self, path);
 287     }
 288
 289     fn visit_path_list_item(&mut self, path: &hir::Path, item: &hir::PathListItem) {
 290         self.lookup_and_handle_definition(&item.node.id());
 291         intravisit::walk_path_list_item(self, path, item);
 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: &hir::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             hir::ItemEnum(ref enum_def, _) if allow_dead_code => {
 336                 self.worklist.extend(enum_def.variants.iter()
 337                                                       .map(|variant| variant.node.data.id()));
 338             }
 339             hir::ItemTrait(_, _, _, ref trait_items) => {
 340                 for trait_item in trait_items {
 341                     match trait_item.node {
 342                         hir::ConstTraitItem(_, Some(_)) |
 343                         hir::MethodTraitItem(_, Some(_)) => {
 344                             if has_allow_dead_code_or_lang_attr(&trait_item.attrs) {
 345                                 self.worklist.push(trait_item.id);
 346                             }
 347                         }
 348                         _ => {}
 349                     }
 350                 }
 351             }
 352             hir::ItemImpl(_, _, _, ref opt_trait, _, ref impl_items) => {
 353                 for impl_item in impl_items {
 354                     if opt_trait.is_some() ||
 355                             has_allow_dead_code_or_lang_attr(&impl_item.attrs) {
 356                         self.worklist.push(impl_item.id);
 357                     }
 358                 }
 359             }
 360             _ => ()
 361         }
 362     }
 363 }
 364
 365 fn create_and_seed_worklist(tcx: &TyCtxt,
 366                             access_levels: &privacy::AccessLevels,
 367                             krate: &hir::Crate) -> Vec<ast::NodeId> {
 368     let mut worklist = Vec::new();
 369     for (id, _) in &access_levels.map {
 370         worklist.push(*id);
 371     }
 372
 373     // Seed entry point
 374     match *tcx.sess.entry_fn.borrow() {
 375         Some((id, _)) => worklist.push(id),
 376         None => ()
 377     }
 378
 379     // Seed implemented trait items
 380     let mut life_seeder = LifeSeeder {
 381         worklist: worklist
 382     };
 383     krate.visit_all_items(&mut life_seeder);
 384
 385     return life_seeder.worklist;
 386 }
 387
 388 fn find_live(tcx: &TyCtxt,
 389              access_levels: &privacy::AccessLevels,
 390              krate: &hir::Crate)
 391              -> Box<HashSet<ast::NodeId>> {
 392     let worklist = create_and_seed_worklist(tcx, access_levels, krate);
 393     let mut symbol_visitor = MarkSymbolVisitor::new(tcx, worklist);
 394     symbol_visitor.mark_live_symbols();
 395     symbol_visitor.live_symbols
 396 }
 397
 398 fn get_struct_ctor_id(item: &hir::Item) -> Option<ast::NodeId> {
 399     match item.node {
 400         hir::ItemStruct(ref struct_def, _) if !struct_def.is_struct() => {
 401             Some(struct_def.id())
 402         }
 403         _ => None
 404     }
 405 }
 406
 407 struct DeadVisitor<'a, 'tcx: 'a> {
 408     tcx: &'a TyCtxt<'tcx>,
 409     live_symbols: Box<HashSet<ast::NodeId>>,
 410 }
 411
 412 impl<'a, 'tcx> DeadVisitor<'a, 'tcx> {
 413     fn should_warn_about_item(&mut self, item: &hir::Item) -> bool {
 414         let should_warn = match item.node {
 415             hir::ItemStatic(..)
 416             | hir::ItemConst(..)
 417             | hir::ItemFn(..)
 418             | hir::ItemEnum(..)
 419             | hir::ItemStruct(..) => true,
 420             _ => false
 421         };
 422         let ctor_id = get_struct_ctor_id(item);
 423         should_warn && !self.symbol_is_live(item.id, ctor_id)
 424     }
 425
 426     fn should_warn_about_field(&mut self, field: &hir::StructField) -> bool {
 427         let field_type = self.tcx.node_id_to_type(field.id);
 428         let is_marker_field = match field_type.ty_to_def_id() {
 429             Some(def_id) => self.tcx.lang_items.items().iter().any(|item| *item == Some(def_id)),
 430             _ => false
 431         };
 432         !field.is_positional()
 433             && !self.symbol_is_live(field.id, None)
 434             && !is_marker_field
 435             && !has_allow_dead_code_or_lang_attr(&field.attrs)
 436     }
 437
 438     fn should_warn_about_variant(&mut self, variant: &hir::Variant_) -> bool {
 439         !self.symbol_is_live(variant.data.id(), None)
 440             && !has_allow_dead_code_or_lang_attr(&variant.attrs)
 441     }
 442
 443     // id := node id of an item's definition.
 444     // ctor_id := `Some` if the item is a struct_ctor (tuple struct),
 445     //            `None` otherwise.
 446     // If the item is a struct_ctor, then either its `id` or
 447     // `ctor_id` (unwrapped) is in the live_symbols set. More specifically,
 448     // DefMap maps the ExprPath of a struct_ctor to the node referred by
 449     // `ctor_id`. On the other hand, in a statement like
 450     // `type <ident> <generics> = <ty>;` where <ty> refers to a struct_ctor,
 451     // DefMap maps <ty> to `id` instead.
 452     fn symbol_is_live(&mut self,
 453                       id: ast::NodeId,
 454                       ctor_id: Option<ast::NodeId>)
 455                       -> bool {
 456         if self.live_symbols.contains(&id)
 457            || ctor_id.map_or(false,
 458                              |ctor| self.live_symbols.contains(&ctor)) {
 459             return true;
 460         }
 461         // If it's a type whose items are live, then it's live, too.
 462         // This is done to handle the case where, for example, the static
 463         // method of a private type is used, but the type itself is never
 464         // called directly.
 465         let impl_items = self.tcx.impl_items.borrow();
 466         match self.tcx.inherent_impls.borrow().get(&self.tcx.map.local_def_id(id)) {
 467             None => (),
 468             Some(impl_list) => {
 469                 for impl_did in impl_list.iter() {
 470                     for item_did in impl_items.get(impl_did).unwrap().iter() {
 471                         if let Some(item_node_id) =
 472                                 self.tcx.map.as_local_node_id(item_did.def_id()) {
 473                             if self.live_symbols.contains(&item_node_id) {
 474                                 return true;
 475                             }
 476                         }
 477                     }
 478                 }
 479             }
 480         }
 481         false
 482     }
 483
 484     fn warn_dead_code(&mut self,
 485                       id: ast::NodeId,
 486                       span: codemap::Span,
 487                       name: ast::Name,
 488                       node_type: &str) {
 489         let name = name.as_str();
 490         if !name.starts_with("_") {
 491             self.tcx
 492                 .sess
 493                 .add_lint(lint::builtin::DEAD_CODE,
 494                           id,
 495                           span,
 496                           format!("{} is never used: `{}`", node_type, name));
 497         }
 498     }
 499 }
 500
 501 impl<'a, 'tcx, 'v> Visitor<'v> for DeadVisitor<'a, 'tcx> {
 502     /// Walk nested items in place so that we don't report dead-code
 503     /// on inner functions when the outer function is already getting
 504     /// an error. We could do this also by checking the parents, but
 505     /// this is how the code is setup and it seems harmless enough.
 506     fn visit_nested_item(&mut self, item: hir::ItemId) {
 507         self.visit_item(self.tcx.map.expect_item(item.id))
 508     }
 509
 510     fn visit_item(&mut self, item: &hir::Item) {
 511         if self.should_warn_about_item(item) {
 512             self.warn_dead_code(
 513                 item.id,
 514                 item.span,
 515                 item.name,
 516                 item.node.descriptive_variant()
 517             );
 518         } else {
 519             // Only continue if we didn't warn
 520             intravisit::walk_item(self, item);
 521         }
 522     }
 523
 524     fn visit_variant(&mut self, variant: &hir::Variant, g: &hir::Generics, id: ast::NodeId) {
 525         if self.should_warn_about_variant(&variant.node) {
 526             self.warn_dead_code(variant.node.data.id(), variant.span,
 527                                 variant.node.name, "variant");
 528         } else {
 529             intravisit::walk_variant(self, variant, g, id);
 530         }
 531     }
 532
 533     fn visit_foreign_item(&mut self, fi: &hir::ForeignItem) {
 534         if !self.symbol_is_live(fi.id, None) {
 535             self.warn_dead_code(fi.id, fi.span, fi.name, fi.node.descriptive_variant());
 536         }
 537         intravisit::walk_foreign_item(self, fi);
 538     }
 539
 540     fn visit_struct_field(&mut self, field: &hir::StructField) {
 541         if self.should_warn_about_field(&field) {
 542             self.warn_dead_code(field.id, field.span,
 543                                 field.name, "struct field");
 544         }
 545
 546         intravisit::walk_struct_field(self, field);
 547     }
 548
 549     fn visit_impl_item(&mut self, impl_item: &hir::ImplItem) {
 550         match impl_item.node {
 551             hir::ImplItemKind::Const(_, ref expr) => {
 552                 if !self.symbol_is_live(impl_item.id, None) {
 553                     self.warn_dead_code(impl_item.id, impl_item.span,
 554                                         impl_item.name, "associated const");
 555                 }
 556                 intravisit::walk_expr(self, expr)
 557             }
 558             hir::ImplItemKind::Method(_, ref body) => {
 559                 if !self.symbol_is_live(impl_item.id, None) {
 560                     self.warn_dead_code(impl_item.id, impl_item.span,
 561                                         impl_item.name, "method");
 562                 }
 563                 intravisit::walk_block(self, body)
 564             }
 565             hir::ImplItemKind::Type(..) => {}
 566         }
 567     }
 568
 569     // Overwrite so that we don't warn the trait item itself.
 570     fn visit_trait_item(&mut self, trait_item: &hir::TraitItem) {
 571         match trait_item.node {
 572             hir::ConstTraitItem(_, Some(ref expr)) => {
 573                 intravisit::walk_expr(self, expr)
 574             }
 575             hir::MethodTraitItem(_, Some(ref body)) => {
 576                 intravisit::walk_block(self, body)
 577             }
 578             hir::ConstTraitItem(_, None) |
 579             hir::MethodTraitItem(_, None) |
 580             hir::TypeTraitItem(..) => {}
 581         }
 582     }
 583 }
 584
 585 pub fn check_crate(tcx: &TyCtxt, access_levels: &privacy::AccessLevels) {
 586     let _task = tcx.dep_graph.in_task(DepNode::DeadCheck);
 587     let krate = tcx.map.krate();
 588     let live_symbols = find_live(tcx, access_levels, krate);
 589     let mut visitor = DeadVisitor { tcx: tcx, live_symbols: live_symbols };
 590     intravisit::walk_crate(&mut visitor, krate);
 591 }