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 front::map as ast_map;
  16 use rustc_front::hir;
  17 use rustc_front::intravisit::{self, Visitor};
  18
  19 use middle::{def, pat_util, privacy, ty};
  20 use middle::def_id::{DefId};
  21 use lint;
  22
  23 use std::collections::HashSet;
  24 use syntax::{ast, codemap};
  25 use syntax::attr::{self, AttrMetaMethods};
  26
  27 // Any local node that may call something in its body block should be
  28 // explored. For example, if it's a live NodeItem that is a
  29 // function, then we should explore its block to check for codes that
  30 // may need to be marked as live.
  31 fn should_explore(tcx: &ty::ctxt, node_id: ast::NodeId) -> bool {
  32     match tcx.map.find(node_id) {
  33         Some(ast_map::NodeItem(..)) |
  34         Some(ast_map::NodeImplItem(..)) |
  35         Some(ast_map::NodeForeignItem(..)) |
  36         Some(ast_map::NodeTraitItem(..)) =>
  37             true,
  38         _ =>
  39             false
  40     }
  41 }
  42
  43 struct MarkSymbolVisitor<'a, 'tcx: 'a> {
  44     worklist: Vec<ast::NodeId>,
  45     tcx: &'a ty::ctxt<'tcx>,
  46     live_symbols: Box<HashSet<ast::NodeId>>,
  47     struct_has_extern_repr: bool,
  48     ignore_non_const_paths: bool,
  49     inherited_pub_visibility: bool,
  50     ignore_variant_stack: Vec<DefId>,
  51 }
  52
  53 impl<'a, 'tcx> MarkSymbolVisitor<'a, 'tcx> {
  54     fn new(tcx: &'a ty::ctxt<'tcx>,
  55            worklist: Vec<ast::NodeId>) -> MarkSymbolVisitor<'a, 'tcx> {
  56         MarkSymbolVisitor {
  57             worklist: worklist,
  58             tcx: tcx,
  59             live_symbols: box HashSet::new(),
  60             struct_has_extern_repr: false,
  61             ignore_non_const_paths: false,
  62             inherited_pub_visibility: false,
  63             ignore_variant_stack: vec![],
  64         }
  65     }
  66
  67     fn check_def_id(&mut self, def_id: DefId) {
  68         if let Some(node_id) = self.tcx.map.as_local_node_id(def_id) {
  69             if should_explore(self.tcx, node_id) {
  70                 self.worklist.push(node_id);
  71             }
  72             self.live_symbols.insert(node_id);
  73         }
  74     }
  75
  76     fn insert_def_id(&mut self, def_id: DefId) {
  77         if let Some(node_id) = self.tcx.map.as_local_node_id(def_id) {
  78             debug_assert!(!should_explore(self.tcx, node_id));
  79             self.live_symbols.insert(node_id);
  80         }
  81     }
  82
  83     fn lookup_and_handle_definition(&mut self, id: &ast::NodeId) {
  84         use middle::ty::TypeVariants::{TyEnum, TyStruct};
  85
  86         // If `bar` is a trait item, make sure to mark Foo as alive in `Foo::bar`
  87         self.tcx.tables.borrow().item_substs.get(id)
  88             .and_then(|substs| substs.substs.self_ty())
  89             .map(|ty| match ty.sty {
  90                 TyEnum(tyid, _) | TyStruct(tyid, _) => self.check_def_id(tyid.did),
  91                 _ => (),
  92             });
  93
  94         self.tcx.def_map.borrow().get(id).map(|def| {
  95             match def.full_def() {
  96                 def::DefConst(_) | def::DefAssociatedConst(..) => {
  97                     self.check_def_id(def.def_id());
  98                 }
  99                 _ if self.ignore_non_const_paths => (),
 100                 def::DefPrimTy(_) => (),
 101                 def::DefSelfTy(..) => (),
 102                 def::DefVariant(enum_id, variant_id, _) => {
 103                     self.check_def_id(enum_id);
 104                     if !self.ignore_variant_stack.contains(&variant_id) {
 105                         self.check_def_id(variant_id);
 106                     }
 107                 }
 108                 _ => {
 109                     self.check_def_id(def.def_id());
 110                 }
 111             }
 112         });
 113     }
 114
 115     fn lookup_and_handle_method(&mut self, id: ast::NodeId) {
 116         let method_call = ty::MethodCall::expr(id);
 117         let method = self.tcx.tables.borrow().method_map[&method_call];
 118         self.check_def_id(method.def_id);
 119     }
 120
 121     fn handle_field_access(&mut self, lhs: &hir::Expr, name: ast::Name) {
 122         if let ty::TyStruct(def, _) = self.tcx.expr_ty_adjusted(lhs).sty {
 123             self.insert_def_id(def.struct_variant().field_named(name).did);
 124         } else {
 125             self.tcx.sess.span_bug(lhs.span, "named field access on non-struct")
 126         }
 127     }
 128
 129     fn handle_tup_field_access(&mut self, lhs: &hir::Expr, idx: usize) {
 130         if let ty::TyStruct(def, _) = self.tcx.expr_ty_adjusted(lhs).sty {
 131             self.insert_def_id(def.struct_variant().fields[idx].did);
 132         }
 133     }
 134
 135     fn handle_field_pattern_match(&mut self, lhs: &hir::Pat,
 136                                   pats: &[codemap::Spanned<hir::FieldPat>]) {
 137         let def = self.tcx.def_map.borrow().get(&lhs.id).unwrap().full_def();
 138         let pat_ty = self.tcx.node_id_to_type(lhs.id);
 139         let variant = match pat_ty.sty {
 140             ty::TyStruct(adt, _) | ty::TyEnum(adt, _) => adt.variant_of_def(def),
 141             _ => self.tcx.sess.span_bug(lhs.span, "non-ADT in struct pattern")
 142         };
 143         for pat in pats {
 144             if let hir::PatWild = pat.node.pat.node {
 145                 continue;
 146             }
 147             self.insert_def_id(variant.field_named(pat.node.name).did);
 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                     hir::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                         intravisit::walk_item(self, &*item);
 185                     }
 186                     hir::ItemEnum(..) => {
 187                         self.inherited_pub_visibility = item.vis == hir::Public;
 188                         intravisit::walk_item(self, &*item);
 189                     }
 190                     hir::ItemFn(..)
 191                     | hir::ItemTy(..)
 192                     | hir::ItemStatic(..)
 193                     | hir::ItemConst(..) => {
 194                         intravisit::walk_item(self, &*item);
 195                     }
 196                     _ => ()
 197                 }
 198             }
 199             ast_map::NodeTraitItem(trait_item) => {
 200                 intravisit::walk_trait_item(self, trait_item);
 201             }
 202             ast_map::NodeImplItem(impl_item) => {
 203                 intravisit::walk_impl_item(self, impl_item);
 204             }
 205             ast_map::NodeForeignItem(foreign_item) => {
 206                 intravisit::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_variant_data(&mut self, def: &hir::VariantData, _: ast::Name,
 218                         _: &hir::Generics, _: ast::NodeId, _: codemap::Span) {
 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                 hir::NamedField(_, hir::Public) => true,
 224                 _ => false
 225             }
 226         });
 227         self.live_symbols.extend(live_fields.map(|f| f.node.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             hir::PatStruct(_, 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                     match impl_item.node {
 355                         hir::ImplItemKind::Const(..) |
 356                         hir::ImplItemKind::Method(..) => {
 357                             if opt_trait.is_some() ||
 358                                     has_allow_dead_code_or_lang_attr(&impl_item.attrs) {
 359                                 self.worklist.push(impl_item.id);
 360                             }
 361                         }
 362                         hir::ImplItemKind::Type(_) => {}
 363                     }
 364                 }
 365             }
 366             _ => ()
 367         }
 368     }
 369 }
 370
 371 fn create_and_seed_worklist(tcx: &ty::ctxt,
 372                             access_levels: &privacy::AccessLevels,
 373                             krate: &hir::Crate) -> 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     match *tcx.sess.entry_fn.borrow() {
 381         Some((id, _)) => worklist.push(id),
 382         None => ()
 383     }
 384
 385     // Seed implemented trait items
 386     let mut life_seeder = LifeSeeder {
 387         worklist: worklist
 388     };
 389     krate.visit_all_items(&mut life_seeder);
 390
 391     return life_seeder.worklist;
 392 }
 393
 394 fn find_live(tcx: &ty::ctxt,
 395              access_levels: &privacy::AccessLevels,
 396              krate: &hir::Crate)
 397              -> Box<HashSet<ast::NodeId>> {
 398     let worklist = create_and_seed_worklist(tcx, access_levels, krate);
 399     let mut symbol_visitor = MarkSymbolVisitor::new(tcx, worklist);
 400     symbol_visitor.mark_live_symbols();
 401     symbol_visitor.live_symbols
 402 }
 403
 404 fn get_struct_ctor_id(item: &hir::Item) -> Option<ast::NodeId> {
 405     match item.node {
 406         hir::ItemStruct(ref struct_def, _) if !struct_def.is_struct() => {
 407             Some(struct_def.id())
 408         }
 409         _ => None
 410     }
 411 }
 412
 413 struct DeadVisitor<'a, 'tcx: 'a> {
 414     tcx: &'a ty::ctxt<'tcx>,
 415     live_symbols: Box<HashSet<ast::NodeId>>,
 416 }
 417
 418 impl<'a, 'tcx> DeadVisitor<'a, 'tcx> {
 419     fn should_warn_about_item(&mut self, item: &hir::Item) -> bool {
 420         let should_warn = match item.node {
 421             hir::ItemStatic(..)
 422             | hir::ItemConst(..)
 423             | hir::ItemFn(..)
 424             | hir::ItemEnum(..)
 425             | hir::ItemStruct(..) => true,
 426             _ => false
 427         };
 428         let ctor_id = get_struct_ctor_id(item);
 429         should_warn && !self.symbol_is_live(item.id, ctor_id)
 430     }
 431
 432     fn should_warn_about_field(&mut self, node: &hir::StructField_) -> bool {
 433         let is_named = node.name().is_some();
 434         let field_type = self.tcx.node_id_to_type(node.id);
 435         let is_marker_field = match field_type.ty_to_def_id() {
 436             Some(def_id) => self.tcx.lang_items.items().any(|(_, item)| *item == Some(def_id)),
 437             _ => false
 438         };
 439         is_named
 440             && !self.symbol_is_live(node.id, None)
 441             && !is_marker_field
 442             && !has_allow_dead_code_or_lang_attr(&node.attrs)
 443     }
 444
 445     fn should_warn_about_variant(&mut self, variant: &hir::Variant_) -> bool {
 446         !self.symbol_is_live(variant.data.id(), None)
 447             && !has_allow_dead_code_or_lang_attr(&variant.attrs)
 448     }
 449
 450     // id := node id of an item's definition.
 451     // ctor_id := `Some` if the item is a struct_ctor (tuple struct),
 452     //            `None` otherwise.
 453     // If the item is a struct_ctor, then either its `id` or
 454     // `ctor_id` (unwrapped) is in the live_symbols set. More specifically,
 455     // DefMap maps the ExprPath of a struct_ctor to the node referred by
 456     // `ctor_id`. On the other hand, in a statement like
 457     // `type <ident> <generics> = <ty>;` where <ty> refers to a struct_ctor,
 458     // DefMap maps <ty> to `id` instead.
 459     fn symbol_is_live(&mut self,
 460                       id: ast::NodeId,
 461                       ctor_id: Option<ast::NodeId>)
 462                       -> bool {
 463         if self.live_symbols.contains(&id)
 464            || ctor_id.map_or(false,
 465                              |ctor| self.live_symbols.contains(&ctor)) {
 466             return true;
 467         }
 468         // If it's a type whose items are live, then it's live, too.
 469         // This is done to handle the case where, for example, the static
 470         // method of a private type is used, but the type itself is never
 471         // called directly.
 472         let impl_items = self.tcx.impl_items.borrow();
 473         match self.tcx.inherent_impls.borrow().get(&self.tcx.map.local_def_id(id)) {
 474             None => (),
 475             Some(impl_list) => {
 476                 for impl_did in impl_list.iter() {
 477                     for item_did in impl_items.get(impl_did).unwrap().iter() {
 478                         if let Some(item_node_id) =
 479                                 self.tcx.map.as_local_node_id(item_did.def_id()) {
 480                             if self.live_symbols.contains(&item_node_id) {
 481                                 return true;
 482                             }
 483                         }
 484                     }
 485                 }
 486             }
 487         }
 488         false
 489     }
 490
 491     fn warn_dead_code(&mut self,
 492                       id: ast::NodeId,
 493                       span: codemap::Span,
 494                       name: ast::Name,
 495                       node_type: &str) {
 496         let name = name.as_str();
 497         if !name.starts_with("_") {
 498             self.tcx
 499                 .sess
 500                 .add_lint(lint::builtin::DEAD_CODE,
 501                           id,
 502                           span,
 503                           format!("{} is never used: `{}`", node_type, name));
 504         }
 505     }
 506 }
 507
 508 impl<'a, 'tcx, 'v> Visitor<'v> for DeadVisitor<'a, 'tcx> {
 509     /// Walk nested items in place so that we don't report dead-code
 510     /// on inner functions when the outer function is already getting
 511     /// an error. We could do this also by checking the parents, but
 512     /// this is how the code is setup and it seems harmless enough.
 513     fn visit_nested_item(&mut self, item: hir::ItemId) {
 514         self.visit_item(self.tcx.map.expect_item(item.id))
 515     }
 516
 517     fn visit_item(&mut self, item: &hir::Item) {
 518         if self.should_warn_about_item(item) {
 519             self.warn_dead_code(
 520                 item.id,
 521                 item.span,
 522                 item.name,
 523                 item.node.descriptive_variant()
 524             );
 525         } else {
 526             // Only continue if we didn't warn
 527             intravisit::walk_item(self, item);
 528         }
 529     }
 530
 531     fn visit_variant(&mut self, variant: &hir::Variant, g: &hir::Generics, id: ast::NodeId) {
 532         if self.should_warn_about_variant(&variant.node) {
 533             self.warn_dead_code(variant.node.data.id(), variant.span,
 534                                 variant.node.name, "variant");
 535         } else {
 536             intravisit::walk_variant(self, variant, g, id);
 537         }
 538     }
 539
 540     fn visit_foreign_item(&mut self, fi: &hir::ForeignItem) {
 541         if !self.symbol_is_live(fi.id, None) {
 542             self.warn_dead_code(fi.id, fi.span, fi.name, fi.node.descriptive_variant());
 543         }
 544         intravisit::walk_foreign_item(self, fi);
 545     }
 546
 547     fn visit_struct_field(&mut self, field: &hir::StructField) {
 548         if self.should_warn_about_field(&field.node) {
 549             self.warn_dead_code(field.node.id, field.span,
 550                                 field.node.name().unwrap(), "struct field");
 551         }
 552
 553         intravisit::walk_struct_field(self, field);
 554     }
 555
 556     fn visit_impl_item(&mut self, impl_item: &hir::ImplItem) {
 557         match impl_item.node {
 558             hir::ImplItemKind::Const(_, ref expr) => {
 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, "associated const");
 562                 }
 563                 intravisit::walk_expr(self, expr)
 564             }
 565             hir::ImplItemKind::Method(_, ref body) => {
 566                 if !self.symbol_is_live(impl_item.id, None) {
 567                     self.warn_dead_code(impl_item.id, impl_item.span,
 568                                         impl_item.name, "method");
 569                 }
 570                 intravisit::walk_block(self, body)
 571             }
 572             hir::ImplItemKind::Type(..) => {}
 573         }
 574     }
 575
 576     // Overwrite so that we don't warn the trait item itself.
 577     fn visit_trait_item(&mut self, trait_item: &hir::TraitItem) {
 578         match trait_item.node {
 579             hir::ConstTraitItem(_, Some(ref expr)) => {
 580                 intravisit::walk_expr(self, expr)
 581             }
 582             hir::MethodTraitItem(_, Some(ref body)) => {
 583                 intravisit::walk_block(self, body)
 584             }
 585             hir::ConstTraitItem(_, None) |
 586             hir::MethodTraitItem(_, None) |
 587             hir::TypeTraitItem(..) => {}
 588         }
 589     }
 590 }
 591
 592 pub fn check_crate(tcx: &ty::ctxt, access_levels: &privacy::AccessLevels) {
 593     let krate = tcx.map.krate();
 594     let live_symbols = find_live(tcx, access_levels, krate);
 595     let mut visitor = DeadVisitor { tcx: tcx, live_symbols: live_symbols };
 596     intravisit::walk_crate(&mut visitor, krate);
 597 }