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