compiler/rustc_passes/src/dead.rs

   1 // This implements the dead-code warning pass. It follows middle::reachable
   2 // closely. The idea is that all reachable symbols are live, codes called
   3 // from live codes are live, and everything else is dead.
   4
   5 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
   6 use rustc_hir as hir;
   7 use rustc_hir::def::{CtorOf, DefKind, Res};
   8 use rustc_hir::def_id::DefId;
   9 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
  10 use rustc_hir::itemlikevisit::ItemLikeVisitor;
  11 use rustc_hir::{Node, PatKind, TyKind};
  12 use rustc_middle::hir::map::Map;
  13 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
  14 use rustc_middle::middle::privacy;
  15 use rustc_middle::ty::{self, DefIdTree, TyCtxt};
  16 use rustc_session::lint;
  17
  18 use rustc_span::symbol::{sym, Symbol};
  19
  20 // Any local node that may call something in its body block should be
  21 // explored. For example, if it's a live Node::Item that is a
  22 // function, then we should explore its block to check for codes that
  23 // may need to be marked as live.
  24 fn should_explore(tcx: TyCtxt<'_>, hir_id: hir::HirId) -> bool {
  25     matches!(
  26         tcx.hir().find(hir_id),
  27         Some(
  28             Node::Item(..)
  29                 | Node::ImplItem(..)
  30                 | Node::ForeignItem(..)
  31                 | Node::TraitItem(..)
  32                 | Node::Variant(..)
  33                 | Node::AnonConst(..)
  34                 | Node::Pat(..),
  35         )
  36     )
  37 }
  38
  39 struct MarkSymbolVisitor<'tcx> {
  40     worklist: Vec<hir::HirId>,
  41     tcx: TyCtxt<'tcx>,
  42     maybe_typeck_results: Option<&'tcx ty::TypeckResults<'tcx>>,
  43     live_symbols: FxHashSet<hir::HirId>,
  44     repr_has_repr_c: bool,
  45     in_pat: bool,
  46     inherited_pub_visibility: bool,
  47     pub_visibility: bool,
  48     ignore_variant_stack: Vec<DefId>,
  49     // maps from tuple struct constructors to tuple struct items
  50     struct_constructors: FxHashMap<hir::HirId, hir::HirId>,
  51 }
  52
  53 impl<'tcx> MarkSymbolVisitor<'tcx> {
  54     /// Gets the type-checking results for the current body.
  55     /// As this will ICE if called outside bodies, only call when working with
  56     /// `Expr` or `Pat` nodes (they are guaranteed to be found only in bodies).
  57     #[track_caller]
  58     fn typeck_results(&self) -> &'tcx ty::TypeckResults<'tcx> {
  59         self.maybe_typeck_results
  60             .expect("`MarkSymbolVisitor::typeck_results` called outside of body")
  61     }
  62
  63     fn check_def_id(&mut self, def_id: DefId) {
  64         if let Some(def_id) = def_id.as_local() {
  65             let hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
  66             if should_explore(self.tcx, hir_id) || self.struct_constructors.contains_key(&hir_id) {
  67                 self.worklist.push(hir_id);
  68             }
  69             self.live_symbols.insert(hir_id);
  70         }
  71     }
  72
  73     fn insert_def_id(&mut self, def_id: DefId) {
  74         if let Some(def_id) = def_id.as_local() {
  75             let hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
  76             debug_assert!(!should_explore(self.tcx, hir_id));
  77             self.live_symbols.insert(hir_id);
  78         }
  79     }
  80
  81     fn handle_res(&mut self, res: Res) {
  82         match res {
  83             Res::Def(DefKind::Const | DefKind::AssocConst | DefKind::TyAlias, _) => {
  84                 self.check_def_id(res.def_id());
  85             }
  86             _ if self.in_pat => {}
  87             Res::PrimTy(..) | Res::SelfCtor(..) | Res::Local(..) => {}
  88             Res::Def(DefKind::Ctor(CtorOf::Variant, ..), ctor_def_id) => {
  89                 let variant_id = self.tcx.parent(ctor_def_id).unwrap();
  90                 let enum_id = self.tcx.parent(variant_id).unwrap();
  91                 self.check_def_id(enum_id);
  92                 if !self.ignore_variant_stack.contains(&ctor_def_id) {
  93                     self.check_def_id(variant_id);
  94                 }
  95             }
  96             Res::Def(DefKind::Variant, variant_id) => {
  97                 let enum_id = self.tcx.parent(variant_id).unwrap();
  98                 self.check_def_id(enum_id);
  99                 if !self.ignore_variant_stack.contains(&variant_id) {
 100                     self.check_def_id(variant_id);
 101                 }
 102             }
 103             Res::SelfTy(t, i) => {
 104                 if let Some(t) = t {
 105                     self.check_def_id(t);
 106                 }
 107                 if let Some((i, _)) = i {
 108                     self.check_def_id(i);
 109                 }
 110             }
 111             Res::ToolMod | Res::NonMacroAttr(..) | Res::Err => {}
 112             _ => {
 113                 self.check_def_id(res.def_id());
 114             }
 115         }
 116     }
 117
 118     fn lookup_and_handle_method(&mut self, id: hir::HirId) {
 119         if let Some(def_id) = self.typeck_results().type_dependent_def_id(id) {
 120             self.check_def_id(def_id);
 121         } else {
 122             bug!("no type-dependent def for method");
 123         }
 124     }
 125
 126     fn handle_field_access(&mut self, lhs: &hir::Expr<'_>, hir_id: hir::HirId) {
 127         match self.typeck_results().expr_ty_adjusted(lhs).kind() {
 128             ty::Adt(def, _) => {
 129                 let index = self.tcx.field_index(hir_id, self.typeck_results());
 130                 self.insert_def_id(def.non_enum_variant().fields[index].did);
 131             }
 132             ty::Tuple(..) => {}
 133             _ => span_bug!(lhs.span, "named field access on non-ADT"),
 134         }
 135     }
 136
 137     #[allow(dead_code)] // FIXME(81658): should be used + lint reinstated after #83171 relands.
 138     fn handle_assign(&mut self, expr: &'tcx hir::Expr<'tcx>) {
 139         if self
 140             .typeck_results()
 141             .expr_adjustments(expr)
 142             .iter()
 143             .any(|adj| matches!(adj.kind, ty::adjustment::Adjust::Deref(_)))
 144         {
 145             self.visit_expr(expr);
 146         } else if let hir::ExprKind::Field(base, ..) = expr.kind {
 147             // Ignore write to field
 148             self.handle_assign(base);
 149         } else {
 150             self.visit_expr(expr);
 151         }
 152     }
 153
 154     fn handle_field_pattern_match(
 155         &mut self,
 156         lhs: &hir::Pat<'_>,
 157         res: Res,
 158         pats: &[hir::PatField<'_>],
 159     ) {
 160         let variant = match self.typeck_results().node_type(lhs.hir_id).kind() {
 161             ty::Adt(adt, _) => adt.variant_of_res(res),
 162             _ => span_bug!(lhs.span, "non-ADT in struct pattern"),
 163         };
 164         for pat in pats {
 165             if let PatKind::Wild = pat.pat.kind {
 166                 continue;
 167             }
 168             let index = self.tcx.field_index(pat.hir_id, self.typeck_results());
 169             self.insert_def_id(variant.fields[index].did);
 170         }
 171     }
 172
 173     fn mark_live_symbols(&mut self) {
 174         let mut scanned = FxHashSet::default();
 175         while let Some(id) = self.worklist.pop() {
 176             if !scanned.insert(id) {
 177                 continue;
 178             }
 179
 180             // in the case of tuple struct constructors we want to check the item, not the generated
 181             // tuple struct constructor function
 182             let id = self.struct_constructors.get(&id).cloned().unwrap_or(id);
 183
 184             if let Some(node) = self.tcx.hir().find(id) {
 185                 self.live_symbols.insert(id);
 186                 self.visit_node(node);
 187             }
 188         }
 189     }
 190
 191     fn visit_node(&mut self, node: Node<'tcx>) {
 192         let had_repr_c = self.repr_has_repr_c;
 193         let had_inherited_pub_visibility = self.inherited_pub_visibility;
 194         let had_pub_visibility = self.pub_visibility;
 195         self.repr_has_repr_c = false;
 196         self.inherited_pub_visibility = false;
 197         self.pub_visibility = false;
 198         match node {
 199             Node::Item(item) => {
 200                 self.pub_visibility = item.vis.node.is_pub();
 201
 202                 match item.kind {
 203                     hir::ItemKind::Struct(..) | hir::ItemKind::Union(..) => {
 204                         let def = self.tcx.adt_def(item.def_id);
 205                         self.repr_has_repr_c = def.repr.c();
 206
 207                         intravisit::walk_item(self, &item);
 208                     }
 209                     hir::ItemKind::Enum(..) => {
 210                         self.inherited_pub_visibility = self.pub_visibility;
 211
 212                         intravisit::walk_item(self, &item);
 213                     }
 214                     hir::ItemKind::ForeignMod { .. } => {}
 215                     _ => {
 216                         intravisit::walk_item(self, &item);
 217                     }
 218                 }
 219             }
 220             Node::TraitItem(trait_item) => {
 221                 intravisit::walk_trait_item(self, trait_item);
 222             }
 223             Node::ImplItem(impl_item) => {
 224                 intravisit::walk_impl_item(self, impl_item);
 225             }
 226             Node::ForeignItem(foreign_item) => {
 227                 intravisit::walk_foreign_item(self, &foreign_item);
 228             }
 229             _ => {}
 230         }
 231         self.pub_visibility = had_pub_visibility;
 232         self.inherited_pub_visibility = had_inherited_pub_visibility;
 233         self.repr_has_repr_c = had_repr_c;
 234     }
 235
 236     fn mark_as_used_if_union(&mut self, adt: &ty::AdtDef, fields: &[hir::ExprField<'_>]) {
 237         if adt.is_union() && adt.non_enum_variant().fields.len() > 1 && adt.did.is_local() {
 238             for field in fields {
 239                 let index = self.tcx.field_index(field.hir_id, self.typeck_results());
 240                 self.insert_def_id(adt.non_enum_variant().fields[index].did);
 241             }
 242         }
 243     }
 244 }
 245
 246 impl<'tcx> Visitor<'tcx> for MarkSymbolVisitor<'tcx> {
 247     type Map = intravisit::ErasedMap<'tcx>;
 248
 249     fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<Self::Map> {
 250         NestedVisitorMap::None
 251     }
 252
 253     fn visit_nested_body(&mut self, body: hir::BodyId) {
 254         let old_maybe_typeck_results =
 255             self.maybe_typeck_results.replace(self.tcx.typeck_body(body));
 256         let body = self.tcx.hir().body(body);
 257         self.visit_body(body);
 258         self.maybe_typeck_results = old_maybe_typeck_results;
 259     }
 260
 261     fn visit_variant_data(
 262         &mut self,
 263         def: &'tcx hir::VariantData<'tcx>,
 264         _: Symbol,
 265         _: &hir::Generics<'_>,
 266         _: hir::HirId,
 267         _: rustc_span::Span,
 268     ) {
 269         let has_repr_c = self.repr_has_repr_c;
 270         let inherited_pub_visibility = self.inherited_pub_visibility;
 271         let pub_visibility = self.pub_visibility;
 272         let live_fields = def.fields().iter().filter(|f| {
 273             has_repr_c || (pub_visibility && (inherited_pub_visibility || f.vis.node.is_pub()))
 274         });
 275         self.live_symbols.extend(live_fields.map(|f| f.hir_id));
 276
 277         intravisit::walk_struct_def(self, def);
 278     }
 279
 280     fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) {
 281         match expr.kind {
 282             hir::ExprKind::Path(ref qpath @ hir::QPath::TypeRelative(..)) => {
 283                 let res = self.typeck_results().qpath_res(qpath, expr.hir_id);
 284                 self.handle_res(res);
 285             }
 286             hir::ExprKind::MethodCall(..) => {
 287                 self.lookup_and_handle_method(expr.hir_id);
 288             }
 289             hir::ExprKind::Field(ref lhs, ..) => {
 290                 self.handle_field_access(&lhs, expr.hir_id);
 291             }
 292             hir::ExprKind::Struct(ref qpath, ref fields, _) => {
 293                 let res = self.typeck_results().qpath_res(qpath, expr.hir_id);
 294                 self.handle_res(res);
 295                 if let ty::Adt(ref adt, _) = self.typeck_results().expr_ty(expr).kind() {
 296                     self.mark_as_used_if_union(adt, fields);
 297                 }
 298             }
 299             _ => (),
 300         }
 301
 302         intravisit::walk_expr(self, expr);
 303     }
 304
 305     fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
 306         // Inside the body, ignore constructions of variants
 307         // necessary for the pattern to match. Those construction sites
 308         // can't be reached unless the variant is constructed elsewhere.
 309         let len = self.ignore_variant_stack.len();
 310         self.ignore_variant_stack.extend(arm.pat.necessary_variants());
 311         intravisit::walk_arm(self, arm);
 312         self.ignore_variant_stack.truncate(len);
 313     }
 314
 315     fn visit_pat(&mut self, pat: &'tcx hir::Pat<'tcx>) {
 316         self.in_pat = true;
 317         match pat.kind {
 318             PatKind::Struct(ref path, ref fields, _) => {
 319                 let res = self.typeck_results().qpath_res(path, pat.hir_id);
 320                 self.handle_field_pattern_match(pat, res, fields);
 321             }
 322             PatKind::Path(ref qpath) => {
 323                 let res = self.typeck_results().qpath_res(qpath, pat.hir_id);
 324                 self.handle_res(res);
 325             }
 326             _ => (),
 327         }
 328
 329         intravisit::walk_pat(self, pat);
 330         self.in_pat = false;
 331     }
 332
 333     fn visit_path(&mut self, path: &'tcx hir::Path<'tcx>, _: hir::HirId) {
 334         self.handle_res(path.res);
 335         intravisit::walk_path(self, path);
 336     }
 337
 338     fn visit_ty(&mut self, ty: &'tcx hir::Ty<'tcx>) {
 339         if let TyKind::OpaqueDef(item_id, _) = ty.kind {
 340             let item = self.tcx.hir().item(item_id);
 341             intravisit::walk_item(self, item);
 342         }
 343         intravisit::walk_ty(self, ty);
 344     }
 345
 346     fn visit_anon_const(&mut self, c: &'tcx hir::AnonConst) {
 347         self.live_symbols.insert(c.hir_id);
 348         intravisit::walk_anon_const(self, c);
 349     }
 350 }
 351
 352 fn has_allow_dead_code_or_lang_attr(tcx: TyCtxt<'_>, id: hir::HirId) -> bool {
 353     let attrs = tcx.hir().attrs(id);
 354     if tcx.sess.contains_name(attrs, sym::lang) {
 355         return true;
 356     }
 357
 358     // Stable attribute for #[lang = "panic_impl"]
 359     if tcx.sess.contains_name(attrs, sym::panic_handler) {
 360         return true;
 361     }
 362
 363     // (To be) stable attribute for #[lang = "oom"]
 364     if tcx.sess.contains_name(attrs, sym::alloc_error_handler) {
 365         return true;
 366     }
 367
 368     let def_id = tcx.hir().local_def_id(id);
 369     let cg_attrs = tcx.codegen_fn_attrs(def_id);
 370
 371     // #[used], #[no_mangle], #[export_name], etc also keeps the item alive
 372     // forcefully, e.g., for placing it in a specific section.
 373     if cg_attrs.contains_extern_indicator() || cg_attrs.flags.contains(CodegenFnAttrFlags::USED) {
 374         return true;
 375     }
 376
 377     tcx.lint_level_at_node(lint::builtin::DEAD_CODE, id).0 == lint::Allow
 378 }
 379
 380 // This visitor seeds items that
 381 //   1) We want to explicitly consider as live:
 382 //     * Item annotated with #[allow(dead_code)]
 383 //         - This is done so that if we want to suppress warnings for a
 384 //           group of dead functions, we only have to annotate the "root".
 385 //           For example, if both `f` and `g` are dead and `f` calls `g`,
 386 //           then annotating `f` with `#[allow(dead_code)]` will suppress
 387 //           warning for both `f` and `g`.
 388 //     * Item annotated with #[lang=".."]
 389 //         - This is because lang items are always callable from elsewhere.
 390 //   or
 391 //   2) We are not sure to be live or not
 392 //     * Implementations of traits and trait methods
 393 struct LifeSeeder<'k, 'tcx> {
 394     worklist: Vec<hir::HirId>,
 395     krate: &'k hir::Crate<'k>,
 396     tcx: TyCtxt<'tcx>,
 397     // see `MarkSymbolVisitor::struct_constructors`
 398     struct_constructors: FxHashMap<hir::HirId, hir::HirId>,
 399 }
 400
 401 impl<'v, 'k, 'tcx> ItemLikeVisitor<'v> for LifeSeeder<'k, 'tcx> {
 402     fn visit_item(&mut self, item: &hir::Item<'_>) {
 403         let allow_dead_code = has_allow_dead_code_or_lang_attr(self.tcx, item.hir_id());
 404         if allow_dead_code {
 405             self.worklist.push(item.hir_id());
 406         }
 407         match item.kind {
 408             hir::ItemKind::Enum(ref enum_def, _) => {
 409                 if allow_dead_code {
 410                     self.worklist.extend(enum_def.variants.iter().map(|variant| variant.id));
 411                 }
 412
 413                 for variant in enum_def.variants {
 414                     if let Some(ctor_hir_id) = variant.data.ctor_hir_id() {
 415                         self.struct_constructors.insert(ctor_hir_id, variant.id);
 416                     }
 417                 }
 418             }
 419             hir::ItemKind::Impl(hir::Impl { ref of_trait, items, .. }) => {
 420                 if of_trait.is_some() {
 421                     self.worklist.push(item.hir_id());
 422                 }
 423                 for impl_item_ref in items {
 424                     let impl_item = self.krate.impl_item(impl_item_ref.id);
 425                     if of_trait.is_some()
 426                         || has_allow_dead_code_or_lang_attr(self.tcx, impl_item.hir_id())
 427                     {
 428                         self.worklist.push(impl_item_ref.id.hir_id());
 429                     }
 430                 }
 431             }
 432             hir::ItemKind::Struct(ref variant_data, _) => {
 433                 if let Some(ctor_hir_id) = variant_data.ctor_hir_id() {
 434                     self.struct_constructors.insert(ctor_hir_id, item.hir_id());
 435                 }
 436             }
 437             _ => (),
 438         }
 439     }
 440
 441     fn visit_trait_item(&mut self, trait_item: &hir::TraitItem<'_>) {
 442         use hir::TraitItemKind::{Const, Fn};
 443         if matches!(trait_item.kind, Const(_, Some(_)) | Fn(_, hir::TraitFn::Provided(_)))
 444             && has_allow_dead_code_or_lang_attr(self.tcx, trait_item.hir_id())
 445         {
 446             self.worklist.push(trait_item.hir_id());
 447         }
 448     }
 449
 450     fn visit_impl_item(&mut self, _item: &hir::ImplItem<'_>) {
 451         // ignore: we are handling this in `visit_item` above
 452     }
 453
 454     fn visit_foreign_item(&mut self, foreign_item: &hir::ForeignItem<'_>) {
 455         use hir::ForeignItemKind::{Fn, Static};
 456         if matches!(foreign_item.kind, Static(..) | Fn(..))
 457             && has_allow_dead_code_or_lang_attr(self.tcx, foreign_item.hir_id())
 458         {
 459             self.worklist.push(foreign_item.hir_id());
 460         }
 461     }
 462 }
 463
 464 fn create_and_seed_worklist<'tcx>(
 465     tcx: TyCtxt<'tcx>,
 466     access_levels: &privacy::AccessLevels,
 467     krate: &hir::Crate<'_>,
 468 ) -> (Vec<hir::HirId>, FxHashMap<hir::HirId, hir::HirId>) {
 469     let worklist = access_levels
 470         .map
 471         .iter()
 472         .filter_map(
 473             |(&id, &level)| {
 474                 if level >= privacy::AccessLevel::Reachable { Some(id) } else { None }
 475             },
 476         )
 477         .chain(
 478             // Seed entry point
 479             tcx.entry_fn(()).and_then(|(def_id, _)| {
 480                 def_id.as_local().map(|def_id| tcx.hir().local_def_id_to_hir_id(def_id))
 481             }),
 482         )
 483         .collect::<Vec<_>>();
 484
 485     // Seed implemented trait items
 486     let mut life_seeder =
 487         LifeSeeder { worklist, krate, tcx, struct_constructors: Default::default() };
 488     krate.visit_all_item_likes(&mut life_seeder);
 489
 490     (life_seeder.worklist, life_seeder.struct_constructors)
 491 }
 492
 493 fn find_live<'tcx>(
 494     tcx: TyCtxt<'tcx>,
 495     access_levels: &privacy::AccessLevels,
 496     krate: &hir::Crate<'_>,
 497 ) -> FxHashSet<hir::HirId> {
 498     let (worklist, struct_constructors) = create_and_seed_worklist(tcx, access_levels, krate);
 499     let mut symbol_visitor = MarkSymbolVisitor {
 500         worklist,
 501         tcx,
 502         maybe_typeck_results: None,
 503         live_symbols: Default::default(),
 504         repr_has_repr_c: false,
 505         in_pat: false,
 506         inherited_pub_visibility: false,
 507         pub_visibility: false,
 508         ignore_variant_stack: vec![],
 509         struct_constructors,
 510     };
 511     symbol_visitor.mark_live_symbols();
 512     symbol_visitor.live_symbols
 513 }
 514
 515 struct DeadVisitor<'tcx> {
 516     tcx: TyCtxt<'tcx>,
 517     live_symbols: FxHashSet<hir::HirId>,
 518 }
 519
 520 impl DeadVisitor<'tcx> {
 521     fn should_warn_about_item(&mut self, item: &hir::Item<'_>) -> bool {
 522         let should_warn = matches!(
 523             item.kind,
 524             hir::ItemKind::Static(..)
 525                 | hir::ItemKind::Const(..)
 526                 | hir::ItemKind::Fn(..)
 527                 | hir::ItemKind::TyAlias(..)
 528                 | hir::ItemKind::Enum(..)
 529                 | hir::ItemKind::Struct(..)
 530                 | hir::ItemKind::Union(..)
 531         );
 532         should_warn && !self.symbol_is_live(item.hir_id())
 533     }
 534
 535     fn should_warn_about_field(&mut self, field: &hir::FieldDef<'_>) -> bool {
 536         let field_type = self.tcx.type_of(self.tcx.hir().local_def_id(field.hir_id));
 537         !field.is_positional()
 538             && !self.symbol_is_live(field.hir_id)
 539             && !field_type.is_phantom_data()
 540             && !has_allow_dead_code_or_lang_attr(self.tcx, field.hir_id)
 541     }
 542
 543     fn should_warn_about_variant(&mut self, variant: &hir::Variant<'_>) -> bool {
 544         !self.symbol_is_live(variant.id) && !has_allow_dead_code_or_lang_attr(self.tcx, variant.id)
 545     }
 546
 547     fn should_warn_about_foreign_item(&mut self, fi: &hir::ForeignItem<'_>) -> bool {
 548         !self.symbol_is_live(fi.hir_id())
 549             && !has_allow_dead_code_or_lang_attr(self.tcx, fi.hir_id())
 550     }
 551
 552     // id := HIR id of an item's definition.
 553     fn symbol_is_live(&mut self, id: hir::HirId) -> bool {
 554         if self.live_symbols.contains(&id) {
 555             return true;
 556         }
 557         // If it's a type whose items are live, then it's live, too.
 558         // This is done to handle the case where, for example, the static
 559         // method of a private type is used, but the type itself is never
 560         // called directly.
 561         let def_id = self.tcx.hir().local_def_id(id);
 562         let inherent_impls = self.tcx.inherent_impls(def_id);
 563         for &impl_did in inherent_impls.iter() {
 564             for item_did in self.tcx.associated_item_def_ids(impl_did) {
 565                 if let Some(did) = item_did.as_local() {
 566                     let item_hir_id = self.tcx.hir().local_def_id_to_hir_id(did);
 567                     if self.live_symbols.contains(&item_hir_id) {
 568                         return true;
 569                     }
 570                 }
 571             }
 572         }
 573         false
 574     }
 575
 576     fn warn_dead_code(
 577         &mut self,
 578         id: hir::HirId,
 579         span: rustc_span::Span,
 580         name: Symbol,
 581         participle: &str,
 582     ) {
 583         if !name.as_str().starts_with('_') {
 584             self.tcx.struct_span_lint_hir(lint::builtin::DEAD_CODE, id, span, |lint| {
 585                 let def_id = self.tcx.hir().local_def_id(id);
 586                 let descr = self.tcx.def_kind(def_id).descr(def_id.to_def_id());
 587                 lint.build(&format!("{} is never {}: `{}`", descr, participle, name)).emit()
 588             });
 589         }
 590     }
 591 }
 592
 593 impl Visitor<'tcx> for DeadVisitor<'tcx> {
 594     type Map = Map<'tcx>;
 595
 596     /// Walk nested items in place so that we don't report dead-code
 597     /// on inner functions when the outer function is already getting
 598     /// an error. We could do this also by checking the parents, but
 599     /// this is how the code is setup and it seems harmless enough.
 600     fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
 601         NestedVisitorMap::All(self.tcx.hir())
 602     }
 603
 604     fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {
 605         if self.should_warn_about_item(item) {
 606             // For most items, we want to highlight its identifier
 607             let span = match item.kind {
 608                 hir::ItemKind::Fn(..)
 609                 | hir::ItemKind::Mod(..)
 610                 | hir::ItemKind::Enum(..)
 611                 | hir::ItemKind::Struct(..)
 612                 | hir::ItemKind::Union(..)
 613                 | hir::ItemKind::Trait(..)
 614                 | hir::ItemKind::Impl { .. } => {
 615                     // FIXME(66095): Because item.span is annotated with things
 616                     // like expansion data, and ident.span isn't, we use the
 617                     // def_span method if it's part of a macro invocation
 618                     // (and thus has a source_callee set).
 619                     // We should probably annotate ident.span with the macro
 620                     // context, but that's a larger change.
 621                     if item.span.source_callee().is_some() {
 622                         self.tcx.sess.source_map().guess_head_span(item.span)
 623                     } else {
 624                         item.ident.span
 625                     }
 626                 }
 627                 _ => item.span,
 628             };
 629             let participle = match item.kind {
 630                 hir::ItemKind::Struct(..) => "constructed", // Issue #52325
 631                 _ => "used",
 632             };
 633             self.warn_dead_code(item.hir_id(), span, item.ident.name, participle);
 634         } else {
 635             // Only continue if we didn't warn
 636             intravisit::walk_item(self, item);
 637         }
 638     }
 639
 640     fn visit_variant(
 641         &mut self,
 642         variant: &'tcx hir::Variant<'tcx>,
 643         g: &'tcx hir::Generics<'tcx>,
 644         id: hir::HirId,
 645     ) {
 646         if self.should_warn_about_variant(&variant) {
 647             self.warn_dead_code(variant.id, variant.span, variant.ident.name, "constructed");
 648         } else {
 649             intravisit::walk_variant(self, variant, g, id);
 650         }
 651     }
 652
 653     fn visit_foreign_item(&mut self, fi: &'tcx hir::ForeignItem<'tcx>) {
 654         if self.should_warn_about_foreign_item(fi) {
 655             self.warn_dead_code(fi.hir_id(), fi.span, fi.ident.name, "used");
 656         }
 657         intravisit::walk_foreign_item(self, fi);
 658     }
 659
 660     fn visit_field_def(&mut self, field: &'tcx hir::FieldDef<'tcx>) {
 661         if self.should_warn_about_field(&field) {
 662             self.warn_dead_code(field.hir_id, field.span, field.ident.name, "read");
 663         }
 664         intravisit::walk_field_def(self, field);
 665     }
 666
 667     fn visit_impl_item(&mut self, impl_item: &'tcx hir::ImplItem<'tcx>) {
 668         match impl_item.kind {
 669             hir::ImplItemKind::Const(_, body_id) => {
 670                 if !self.symbol_is_live(impl_item.hir_id()) {
 671                     self.warn_dead_code(
 672                         impl_item.hir_id(),
 673                         impl_item.span,
 674                         impl_item.ident.name,
 675                         "used",
 676                     );
 677                 }
 678                 self.visit_nested_body(body_id)
 679             }
 680             hir::ImplItemKind::Fn(_, body_id) => {
 681                 if !self.symbol_is_live(impl_item.hir_id()) {
 682                     // FIXME(66095): Because impl_item.span is annotated with things
 683                     // like expansion data, and ident.span isn't, we use the
 684                     // def_span method if it's part of a macro invocation
 685                     // (and thus has a source_callee set).
 686                     // We should probably annotate ident.span with the macro
 687                     // context, but that's a larger change.
 688                     let span = if impl_item.span.source_callee().is_some() {
 689                         self.tcx.sess.source_map().guess_head_span(impl_item.span)
 690                     } else {
 691                         impl_item.ident.span
 692                     };
 693                     self.warn_dead_code(impl_item.hir_id(), span, impl_item.ident.name, "used");
 694                 }
 695                 self.visit_nested_body(body_id)
 696             }
 697             hir::ImplItemKind::TyAlias(..) => {}
 698         }
 699     }
 700
 701     // Overwrite so that we don't warn the trait item itself.
 702     fn visit_trait_item(&mut self, trait_item: &'tcx hir::TraitItem<'tcx>) {
 703         match trait_item.kind {
 704             hir::TraitItemKind::Const(_, Some(body_id))
 705             | hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(body_id)) => {
 706                 self.visit_nested_body(body_id)
 707             }
 708             hir::TraitItemKind::Const(_, None)
 709             | hir::TraitItemKind::Fn(_, hir::TraitFn::Required(_))
 710             | hir::TraitItemKind::Type(..) => {}
 711         }
 712     }
 713 }
 714
 715 pub fn check_crate(tcx: TyCtxt<'_>) {
 716     let access_levels = &tcx.privacy_access_levels(());
 717     let krate = tcx.hir().krate();
 718     let live_symbols = find_live(tcx, access_levels, krate);
 719     let mut visitor = DeadVisitor { tcx, live_symbols };
 720     intravisit::walk_crate(&mut visitor, krate);
 721 }