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 itertools::Itertools;
   6 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
   7 use rustc_errors::{pluralize, MultiSpan};
   8 use rustc_hir as hir;
   9 use rustc_hir::def::{CtorOf, DefKind, Res};
  10 use rustc_hir::def_id::{DefId, LocalDefId};
  11 use rustc_hir::intravisit::{self, Visitor};
  12 use rustc_hir::{Node, PatKind, TyKind};
  13 use rustc_middle::hir::nested_filter;
  14 use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
  15 use rustc_middle::middle::privacy;
  16 use rustc_middle::ty::query::Providers;
  17 use rustc_middle::ty::{self, DefIdTree, TyCtxt};
  18 use rustc_session::lint;
  19 use rustc_span::symbol::{sym, Symbol};
  20 use rustc_span::Span;
  21 use std::mem;
  22
  23 // Any local node that may call something in its body block should be
  24 // explored. For example, if it's a live Node::Item that is a
  25 // function, then we should explore its block to check for codes that
  26 // may need to be marked as live.
  27 fn should_explore(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool {
  28     matches!(
  29         tcx.hir().find_by_def_id(def_id),
  30         Some(
  31             Node::Item(..)
  32                 | Node::ImplItem(..)
  33                 | Node::ForeignItem(..)
  34                 | Node::TraitItem(..)
  35                 | Node::Variant(..)
  36                 | Node::AnonConst(..)
  37         )
  38     )
  39 }
  40
  41 struct MarkSymbolVisitor<'tcx> {
  42     worklist: Vec<LocalDefId>,
  43     tcx: TyCtxt<'tcx>,
  44     maybe_typeck_results: Option<&'tcx ty::TypeckResults<'tcx>>,
  45     live_symbols: FxHashSet<LocalDefId>,
  46     repr_has_repr_c: bool,
  47     in_pat: bool,
  48     ignore_variant_stack: Vec<DefId>,
  49     // maps from tuple struct constructors to tuple struct items
  50     struct_constructors: FxHashMap<LocalDefId, LocalDefId>,
  51     // maps from ADTs to ignored derived traits (e.g. Debug and Clone)
  52     // and the span of their respective impl (i.e., part of the derive
  53     // macro)
  54     ignored_derived_traits: FxHashMap<LocalDefId, Vec<(DefId, DefId)>>,
  55 }
  56
  57 impl<'tcx> MarkSymbolVisitor<'tcx> {
  58     /// Gets the type-checking results for the current body.
  59     /// As this will ICE if called outside bodies, only call when working with
  60     /// `Expr` or `Pat` nodes (they are guaranteed to be found only in bodies).
  61     #[track_caller]
  62     fn typeck_results(&self) -> &'tcx ty::TypeckResults<'tcx> {
  63         self.maybe_typeck_results
  64             .expect("`MarkSymbolVisitor::typeck_results` called outside of body")
  65     }
  66
  67     fn check_def_id(&mut self, def_id: DefId) {
  68         if let Some(def_id) = def_id.as_local() {
  69             if should_explore(self.tcx, def_id) || self.struct_constructors.contains_key(&def_id) {
  70                 self.worklist.push(def_id);
  71             }
  72             self.live_symbols.insert(def_id);
  73         }
  74     }
  75
  76     fn insert_def_id(&mut self, def_id: DefId) {
  77         if let Some(def_id) = def_id.as_local() {
  78             debug_assert!(!should_explore(self.tcx, def_id));
  79             self.live_symbols.insert(def_id);
  80         }
  81     }
  82
  83     fn handle_res(&mut self, res: Res) {
  84         match res {
  85             Res::Def(DefKind::Const | DefKind::AssocConst | DefKind::TyAlias, _) => {
  86                 self.check_def_id(res.def_id());
  87             }
  88             _ if self.in_pat => {}
  89             Res::PrimTy(..) | Res::SelfCtor(..) | Res::Local(..) => {}
  90             Res::Def(DefKind::Ctor(CtorOf::Variant, ..), ctor_def_id) => {
  91                 let variant_id = self.tcx.parent(ctor_def_id);
  92                 let enum_id = self.tcx.parent(variant_id);
  93                 self.check_def_id(enum_id);
  94                 if !self.ignore_variant_stack.contains(&ctor_def_id) {
  95                     self.check_def_id(variant_id);
  96                 }
  97             }
  98             Res::Def(DefKind::Variant, variant_id) => {
  99                 let enum_id = self.tcx.parent(variant_id);
 100                 self.check_def_id(enum_id);
 101                 if !self.ignore_variant_stack.contains(&variant_id) {
 102                     self.check_def_id(variant_id);
 103                 }
 104             }
 105             Res::SelfTy { trait_: t, alias_to: i } => {
 106                 if let Some(t) = t {
 107                     self.check_def_id(t);
 108                 }
 109                 if let Some((i, _)) = i {
 110                     self.check_def_id(i);
 111                 }
 112             }
 113             Res::ToolMod | Res::NonMacroAttr(..) | Res::Err => {}
 114             _ => {
 115                 self.check_def_id(res.def_id());
 116             }
 117         }
 118     }
 119
 120     fn lookup_and_handle_method(&mut self, id: hir::HirId) {
 121         if let Some(def_id) = self.typeck_results().type_dependent_def_id(id) {
 122             self.check_def_id(def_id);
 123         } else {
 124             bug!("no type-dependent def for method");
 125         }
 126     }
 127
 128     fn handle_field_access(&mut self, lhs: &hir::Expr<'_>, hir_id: hir::HirId) {
 129         match self.typeck_results().expr_ty_adjusted(lhs).kind() {
 130             ty::Adt(def, _) => {
 131                 let index = self.tcx.field_index(hir_id, self.typeck_results());
 132                 self.insert_def_id(def.non_enum_variant().fields[index].did);
 133             }
 134             ty::Tuple(..) => {}
 135             _ => span_bug!(lhs.span, "named field access on non-ADT"),
 136         }
 137     }
 138
 139     #[allow(dead_code)] // FIXME(81658): should be used + lint reinstated after #83171 relands.
 140     fn handle_assign(&mut self, expr: &'tcx hir::Expr<'tcx>) {
 141         if self
 142             .typeck_results()
 143             .expr_adjustments(expr)
 144             .iter()
 145             .any(|adj| matches!(adj.kind, ty::adjustment::Adjust::Deref(_)))
 146         {
 147             self.visit_expr(expr);
 148         } else if let hir::ExprKind::Field(base, ..) = expr.kind {
 149             // Ignore write to field
 150             self.handle_assign(base);
 151         } else {
 152             self.visit_expr(expr);
 153         }
 154     }
 155
 156     #[allow(dead_code)] // FIXME(81658): should be used + lint reinstated after #83171 relands.
 157     fn check_for_self_assign(&mut self, assign: &'tcx hir::Expr<'tcx>) {
 158         fn check_for_self_assign_helper<'tcx>(
 159             typeck_results: &'tcx ty::TypeckResults<'tcx>,
 160             lhs: &'tcx hir::Expr<'tcx>,
 161             rhs: &'tcx hir::Expr<'tcx>,
 162         ) -> bool {
 163             match (&lhs.kind, &rhs.kind) {
 164                 (hir::ExprKind::Path(ref qpath_l), hir::ExprKind::Path(ref qpath_r)) => {
 165                     if let (Res::Local(id_l), Res::Local(id_r)) = (
 166                         typeck_results.qpath_res(qpath_l, lhs.hir_id),
 167                         typeck_results.qpath_res(qpath_r, rhs.hir_id),
 168                     ) {
 169                         if id_l == id_r {
 170                             return true;
 171                         }
 172                     }
 173                     return false;
 174                 }
 175                 (hir::ExprKind::Field(lhs_l, ident_l), hir::ExprKind::Field(lhs_r, ident_r)) => {
 176                     if ident_l == ident_r {
 177                         return check_for_self_assign_helper(typeck_results, lhs_l, lhs_r);
 178                     }
 179                     return false;
 180                 }
 181                 _ => {
 182                     return false;
 183                 }
 184             }
 185         }
 186
 187         if let hir::ExprKind::Assign(lhs, rhs, _) = assign.kind
 188             && check_for_self_assign_helper(self.typeck_results(), lhs, rhs)
 189                 && !assign.span.from_expansion()
 190         {
 191                 let is_field_assign = matches!(lhs.kind, hir::ExprKind::Field(..));
 192                 self.tcx.struct_span_lint_hir(
 193                     lint::builtin::DEAD_CODE,
 194                     assign.hir_id,
 195                     assign.span,
 196                     |lint| {
 197                         lint.build(&format!(
 198                             "useless assignment of {} of type `{}` to itself",
 199                             if is_field_assign { "field" } else { "variable" },
 200                             self.typeck_results().expr_ty(lhs),
 201                         ))
 202                         .emit();
 203                     },
 204                 )
 205         }
 206     }
 207
 208     fn handle_field_pattern_match(
 209         &mut self,
 210         lhs: &hir::Pat<'_>,
 211         res: Res,
 212         pats: &[hir::PatField<'_>],
 213     ) {
 214         let variant = match self.typeck_results().node_type(lhs.hir_id).kind() {
 215             ty::Adt(adt, _) => adt.variant_of_res(res),
 216             _ => span_bug!(lhs.span, "non-ADT in struct pattern"),
 217         };
 218         for pat in pats {
 219             if let PatKind::Wild = pat.pat.kind {
 220                 continue;
 221             }
 222             let index = self.tcx.field_index(pat.hir_id, self.typeck_results());
 223             self.insert_def_id(variant.fields[index].did);
 224         }
 225     }
 226
 227     fn mark_live_symbols(&mut self) {
 228         let mut scanned = FxHashSet::default();
 229         while let Some(id) = self.worklist.pop() {
 230             if !scanned.insert(id) {
 231                 continue;
 232             }
 233
 234             // in the case of tuple struct constructors we want to check the item, not the generated
 235             // tuple struct constructor function
 236             let id = self.struct_constructors.get(&id).copied().unwrap_or(id);
 237
 238             if let Some(node) = self.tcx.hir().find_by_def_id(id) {
 239                 self.live_symbols.insert(id);
 240                 self.visit_node(node);
 241             }
 242         }
 243     }
 244
 245     /// Automatically generated items marked with `rustc_trivial_field_reads`
 246     /// will be ignored for the purposes of dead code analysis (see PR #85200
 247     /// for discussion).
 248     fn should_ignore_item(&mut self, def_id: DefId) -> bool {
 249         if let Some(impl_of) = self.tcx.impl_of_method(def_id) {
 250             if !self.tcx.has_attr(impl_of, sym::automatically_derived) {
 251                 return false;
 252             }
 253
 254             if let Some(trait_of) = self.tcx.trait_id_of_impl(impl_of)
 255                 && self.tcx.has_attr(trait_of, sym::rustc_trivial_field_reads)
 256             {
 257                 let trait_ref = self.tcx.impl_trait_ref(impl_of).unwrap();
 258                 if let ty::Adt(adt_def, _) = trait_ref.self_ty().kind()
 259                     && let Some(adt_def_id) = adt_def.did().as_local()
 260                 {
 261                     self.ignored_derived_traits
 262                         .entry(adt_def_id)
 263                         .or_default()
 264                         .push((trait_of, impl_of));
 265                 }
 266                 return true;
 267             }
 268         }
 269
 270         return false;
 271     }
 272
 273     fn visit_node(&mut self, node: Node<'tcx>) {
 274         if let Node::ImplItem(hir::ImplItem { def_id, .. }) = node
 275             && self.should_ignore_item(def_id.to_def_id())
 276         {
 277             return;
 278         }
 279
 280         let had_repr_c = self.repr_has_repr_c;
 281         self.repr_has_repr_c = false;
 282         match node {
 283             Node::Item(item) => match item.kind {
 284                 hir::ItemKind::Struct(..) | hir::ItemKind::Union(..) => {
 285                     let def = self.tcx.adt_def(item.def_id);
 286                     self.repr_has_repr_c = def.repr().c();
 287
 288                     intravisit::walk_item(self, &item);
 289                 }
 290                 hir::ItemKind::Enum(..) => {
 291                     intravisit::walk_item(self, &item);
 292                 }
 293                 hir::ItemKind::ForeignMod { .. } => {}
 294                 _ => {
 295                     intravisit::walk_item(self, &item);
 296                 }
 297             },
 298             Node::TraitItem(trait_item) => {
 299                 intravisit::walk_trait_item(self, trait_item);
 300             }
 301             Node::ImplItem(impl_item) => {
 302                 intravisit::walk_impl_item(self, impl_item);
 303             }
 304             Node::ForeignItem(foreign_item) => {
 305                 intravisit::walk_foreign_item(self, &foreign_item);
 306             }
 307             _ => {}
 308         }
 309         self.repr_has_repr_c = had_repr_c;
 310     }
 311
 312     fn mark_as_used_if_union(&mut self, adt: ty::AdtDef<'tcx>, fields: &[hir::ExprField<'_>]) {
 313         if adt.is_union() && adt.non_enum_variant().fields.len() > 1 && adt.did().is_local() {
 314             for field in fields {
 315                 let index = self.tcx.field_index(field.hir_id, self.typeck_results());
 316                 self.insert_def_id(adt.non_enum_variant().fields[index].did);
 317             }
 318         }
 319     }
 320 }
 321
 322 impl<'tcx> Visitor<'tcx> for MarkSymbolVisitor<'tcx> {
 323     fn visit_nested_body(&mut self, body: hir::BodyId) {
 324         let old_maybe_typeck_results =
 325             self.maybe_typeck_results.replace(self.tcx.typeck_body(body));
 326         let body = self.tcx.hir().body(body);
 327         self.visit_body(body);
 328         self.maybe_typeck_results = old_maybe_typeck_results;
 329     }
 330
 331     fn visit_variant_data(
 332         &mut self,
 333         def: &'tcx hir::VariantData<'tcx>,
 334         _: Symbol,
 335         _: &hir::Generics<'_>,
 336         _: hir::HirId,
 337         _: rustc_span::Span,
 338     ) {
 339         let tcx = self.tcx;
 340         let has_repr_c = self.repr_has_repr_c;
 341         let live_fields = def.fields().iter().filter_map(|f| {
 342             let def_id = tcx.hir().local_def_id(f.hir_id);
 343             if has_repr_c {
 344                 return Some(def_id);
 345             }
 346             if !tcx.visibility(f.hir_id.owner).is_public() {
 347                 return None;
 348             }
 349             if tcx.visibility(def_id).is_public() { Some(def_id) } else { None }
 350         });
 351         self.live_symbols.extend(live_fields);
 352
 353         intravisit::walk_struct_def(self, def);
 354     }
 355
 356     fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) {
 357         match expr.kind {
 358             hir::ExprKind::Path(ref qpath @ hir::QPath::TypeRelative(..)) => {
 359                 let res = self.typeck_results().qpath_res(qpath, expr.hir_id);
 360                 self.handle_res(res);
 361             }
 362             hir::ExprKind::MethodCall(..) => {
 363                 self.lookup_and_handle_method(expr.hir_id);
 364             }
 365             hir::ExprKind::Field(ref lhs, ..) => {
 366                 self.handle_field_access(&lhs, expr.hir_id);
 367             }
 368             hir::ExprKind::Struct(ref qpath, ref fields, _) => {
 369                 let res = self.typeck_results().qpath_res(qpath, expr.hir_id);
 370                 self.handle_res(res);
 371                 if let ty::Adt(adt, _) = self.typeck_results().expr_ty(expr).kind() {
 372                     self.mark_as_used_if_union(*adt, fields);
 373                 }
 374             }
 375             _ => (),
 376         }
 377
 378         intravisit::walk_expr(self, expr);
 379     }
 380
 381     fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
 382         // Inside the body, ignore constructions of variants
 383         // necessary for the pattern to match. Those construction sites
 384         // can't be reached unless the variant is constructed elsewhere.
 385         let len = self.ignore_variant_stack.len();
 386         self.ignore_variant_stack.extend(arm.pat.necessary_variants());
 387         intravisit::walk_arm(self, arm);
 388         self.ignore_variant_stack.truncate(len);
 389     }
 390
 391     fn visit_pat(&mut self, pat: &'tcx hir::Pat<'tcx>) {
 392         self.in_pat = true;
 393         match pat.kind {
 394             PatKind::Struct(ref path, ref fields, _) => {
 395                 let res = self.typeck_results().qpath_res(path, pat.hir_id);
 396                 self.handle_field_pattern_match(pat, res, fields);
 397             }
 398             PatKind::Path(ref qpath) => {
 399                 let res = self.typeck_results().qpath_res(qpath, pat.hir_id);
 400                 self.handle_res(res);
 401             }
 402             _ => (),
 403         }
 404
 405         intravisit::walk_pat(self, pat);
 406         self.in_pat = false;
 407     }
 408
 409     fn visit_path(&mut self, path: &'tcx hir::Path<'tcx>, _: hir::HirId) {
 410         self.handle_res(path.res);
 411         intravisit::walk_path(self, path);
 412     }
 413
 414     fn visit_ty(&mut self, ty: &'tcx hir::Ty<'tcx>) {
 415         if let TyKind::OpaqueDef(item_id, _) = ty.kind {
 416             let item = self.tcx.hir().item(item_id);
 417             intravisit::walk_item(self, item);
 418         }
 419         intravisit::walk_ty(self, ty);
 420     }
 421
 422     fn visit_anon_const(&mut self, c: &'tcx hir::AnonConst) {
 423         // When inline const blocks are used in pattern position, paths
 424         // referenced by it should be considered as used.
 425         let in_pat = mem::replace(&mut self.in_pat, false);
 426
 427         self.live_symbols.insert(self.tcx.hir().local_def_id(c.hir_id));
 428         intravisit::walk_anon_const(self, c);
 429
 430         self.in_pat = in_pat;
 431     }
 432 }
 433
 434 fn has_allow_dead_code_or_lang_attr(tcx: TyCtxt<'_>, id: hir::HirId) -> bool {
 435     let attrs = tcx.hir().attrs(id);
 436     if tcx.sess.contains_name(attrs, sym::lang) {
 437         return true;
 438     }
 439
 440     // Stable attribute for #[lang = "panic_impl"]
 441     if tcx.sess.contains_name(attrs, sym::panic_handler) {
 442         return true;
 443     }
 444
 445     // (To be) stable attribute for #[lang = "oom"]
 446     if tcx.sess.contains_name(attrs, sym::alloc_error_handler) {
 447         return true;
 448     }
 449
 450     let def_id = tcx.hir().local_def_id(id);
 451     if tcx.def_kind(def_id).has_codegen_attrs() {
 452         let cg_attrs = tcx.codegen_fn_attrs(def_id);
 453
 454         // #[used], #[no_mangle], #[export_name], etc also keeps the item alive
 455         // forcefully, e.g., for placing it in a specific section.
 456         if cg_attrs.contains_extern_indicator()
 457             || cg_attrs.flags.contains(CodegenFnAttrFlags::USED)
 458             || cg_attrs.flags.contains(CodegenFnAttrFlags::USED_LINKER)
 459         {
 460             return true;
 461         }
 462     }
 463
 464     tcx.lint_level_at_node(lint::builtin::DEAD_CODE, id).0 == lint::Allow
 465 }
 466
 467 // These check_* functions seeds items that
 468 //   1) We want to explicitly consider as live:
 469 //     * Item annotated with #[allow(dead_code)]
 470 //         - This is done so that if we want to suppress warnings for a
 471 //           group of dead functions, we only have to annotate the "root".
 472 //           For example, if both `f` and `g` are dead and `f` calls `g`,
 473 //           then annotating `f` with `#[allow(dead_code)]` will suppress
 474 //           warning for both `f` and `g`.
 475 //     * Item annotated with #[lang=".."]
 476 //         - This is because lang items are always callable from elsewhere.
 477 //   or
 478 //   2) We are not sure to be live or not
 479 //     * Implementations of traits and trait methods
 480 fn check_item<'tcx>(
 481     tcx: TyCtxt<'tcx>,
 482     worklist: &mut Vec<LocalDefId>,
 483     struct_constructors: &mut FxHashMap<LocalDefId, LocalDefId>,
 484     id: hir::ItemId,
 485 ) {
 486     let allow_dead_code = has_allow_dead_code_or_lang_attr(tcx, id.hir_id());
 487     if allow_dead_code {
 488         worklist.push(id.def_id);
 489     }
 490
 491     match tcx.def_kind(id.def_id) {
 492         DefKind::Enum => {
 493             let item = tcx.hir().item(id);
 494             if let hir::ItemKind::Enum(ref enum_def, _) = item.kind {
 495                 let hir = tcx.hir();
 496                 if allow_dead_code {
 497                     worklist.extend(
 498                         enum_def.variants.iter().map(|variant| hir.local_def_id(variant.id)),
 499                     );
 500                 }
 501
 502                 for variant in enum_def.variants {
 503                     if let Some(ctor_hir_id) = variant.data.ctor_hir_id() {
 504                         struct_constructors
 505                             .insert(hir.local_def_id(ctor_hir_id), hir.local_def_id(variant.id));
 506                     }
 507                 }
 508             }
 509         }
 510         DefKind::Impl => {
 511             let of_trait = tcx.impl_trait_ref(id.def_id);
 512
 513             if of_trait.is_some() {
 514                 worklist.push(id.def_id);
 515             }
 516
 517             // get DefIds from another query
 518             let local_def_ids = tcx
 519                 .associated_item_def_ids(id.def_id)
 520                 .iter()
 521                 .filter_map(|def_id| def_id.as_local());
 522
 523             // And we access the Map here to get HirId from LocalDefId
 524             for id in local_def_ids {
 525                 if of_trait.is_some()
 526                     || has_allow_dead_code_or_lang_attr(tcx, tcx.hir().local_def_id_to_hir_id(id))
 527                 {
 528                     worklist.push(id);
 529                 }
 530             }
 531         }
 532         DefKind::Struct => {
 533             let item = tcx.hir().item(id);
 534             if let hir::ItemKind::Struct(ref variant_data, _) = item.kind
 535                 && let Some(ctor_hir_id) = variant_data.ctor_hir_id()
 536             {
 537                 struct_constructors.insert(tcx.hir().local_def_id(ctor_hir_id), item.def_id);
 538             }
 539         }
 540         DefKind::GlobalAsm => {
 541             // global_asm! is always live.
 542             worklist.push(id.def_id);
 543         }
 544         _ => {}
 545     }
 546 }
 547
 548 fn check_trait_item<'tcx>(tcx: TyCtxt<'tcx>, worklist: &mut Vec<LocalDefId>, id: hir::TraitItemId) {
 549     use hir::TraitItemKind::{Const, Fn};
 550     if matches!(tcx.def_kind(id.def_id), DefKind::AssocConst | DefKind::AssocFn) {
 551         let trait_item = tcx.hir().trait_item(id);
 552         if matches!(trait_item.kind, Const(_, Some(_)) | Fn(_, hir::TraitFn::Provided(_)))
 553             && has_allow_dead_code_or_lang_attr(tcx, trait_item.hir_id())
 554         {
 555             worklist.push(trait_item.def_id);
 556         }
 557     }
 558 }
 559
 560 fn check_foreign_item<'tcx>(
 561     tcx: TyCtxt<'tcx>,
 562     worklist: &mut Vec<LocalDefId>,
 563     id: hir::ForeignItemId,
 564 ) {
 565     if matches!(tcx.def_kind(id.def_id), DefKind::Static(_) | DefKind::Fn)
 566         && has_allow_dead_code_or_lang_attr(tcx, id.hir_id())
 567     {
 568         worklist.push(id.def_id);
 569     }
 570 }
 571
 572 fn create_and_seed_worklist<'tcx>(
 573     tcx: TyCtxt<'tcx>,
 574 ) -> (Vec<LocalDefId>, FxHashMap<LocalDefId, LocalDefId>) {
 575     let access_levels = &tcx.privacy_access_levels(());
 576     // see `MarkSymbolVisitor::struct_constructors`
 577     let mut struct_constructors = Default::default();
 578     let mut worklist = access_levels
 579         .map
 580         .iter()
 581         .filter_map(
 582             |(&id, &level)| {
 583                 if level >= privacy::AccessLevel::Reachable { Some(id) } else { None }
 584             },
 585         )
 586         // Seed entry point
 587         .chain(tcx.entry_fn(()).and_then(|(def_id, _)| def_id.as_local()))
 588         .collect::<Vec<_>>();
 589
 590     let crate_items = tcx.hir_crate_items(());
 591     for id in crate_items.items() {
 592         check_item(tcx, &mut worklist, &mut struct_constructors, id);
 593     }
 594
 595     for id in crate_items.trait_items() {
 596         check_trait_item(tcx, &mut worklist, id);
 597     }
 598
 599     for id in crate_items.foreign_items() {
 600         check_foreign_item(tcx, &mut worklist, id);
 601     }
 602
 603     (worklist, struct_constructors)
 604 }
 605
 606 fn live_symbols_and_ignored_derived_traits<'tcx>(
 607     tcx: TyCtxt<'tcx>,
 608     (): (),
 609 ) -> (FxHashSet<LocalDefId>, FxHashMap<LocalDefId, Vec<(DefId, DefId)>>) {
 610     let (worklist, struct_constructors) = create_and_seed_worklist(tcx);
 611     let mut symbol_visitor = MarkSymbolVisitor {
 612         worklist,
 613         tcx,
 614         maybe_typeck_results: None,
 615         live_symbols: Default::default(),
 616         repr_has_repr_c: false,
 617         in_pat: false,
 618         ignore_variant_stack: vec![],
 619         struct_constructors,
 620         ignored_derived_traits: FxHashMap::default(),
 621     };
 622     symbol_visitor.mark_live_symbols();
 623     (symbol_visitor.live_symbols, symbol_visitor.ignored_derived_traits)
 624 }
 625
 626 struct DeadVariant {
 627     hir_id: hir::HirId,
 628     span: Span,
 629     name: Symbol,
 630     level: lint::Level,
 631 }
 632
 633 struct DeadVisitor<'tcx> {
 634     tcx: TyCtxt<'tcx>,
 635     live_symbols: &'tcx FxHashSet<LocalDefId>,
 636     ignored_derived_traits: &'tcx FxHashMap<LocalDefId, Vec<(DefId, DefId)>>,
 637 }
 638
 639 impl<'tcx> DeadVisitor<'tcx> {
 640     fn should_warn_about_item(&mut self, item: &hir::Item<'_>) -> bool {
 641         let should_warn = matches!(
 642             item.kind,
 643             hir::ItemKind::Static(..)
 644                 | hir::ItemKind::Const(..)
 645                 | hir::ItemKind::Fn(..)
 646                 | hir::ItemKind::TyAlias(..)
 647                 | hir::ItemKind::Enum(..)
 648                 | hir::ItemKind::Struct(..)
 649                 | hir::ItemKind::Union(..)
 650         );
 651         should_warn && !self.symbol_is_live(item.def_id)
 652     }
 653
 654     fn should_warn_about_field(&mut self, field: &hir::FieldDef<'_>) -> bool {
 655         let def_id = self.tcx.hir().local_def_id(field.hir_id);
 656         let field_type = self.tcx.type_of(def_id);
 657         !field.is_positional()
 658             && !self.symbol_is_live(def_id)
 659             && !field_type.is_phantom_data()
 660             && !has_allow_dead_code_or_lang_attr(self.tcx, field.hir_id)
 661     }
 662
 663     fn should_warn_about_variant(&mut self, variant: &hir::Variant<'_>) -> bool {
 664         let def_id = self.tcx.hir().local_def_id(variant.id);
 665         !self.symbol_is_live(def_id) && !has_allow_dead_code_or_lang_attr(self.tcx, variant.id)
 666     }
 667
 668     fn should_warn_about_foreign_item(&mut self, fi: &hir::ForeignItem<'_>) -> bool {
 669         !self.symbol_is_live(fi.def_id) && !has_allow_dead_code_or_lang_attr(self.tcx, fi.hir_id())
 670     }
 671
 672     // id := HIR id of an item's definition.
 673     fn symbol_is_live(&mut self, def_id: LocalDefId) -> bool {
 674         if self.live_symbols.contains(&def_id) {
 675             return true;
 676         }
 677         // If it's a type whose items are live, then it's live, too.
 678         // This is done to handle the case where, for example, the static
 679         // method of a private type is used, but the type itself is never
 680         // called directly.
 681         let inherent_impls = self.tcx.inherent_impls(def_id);
 682         for &impl_did in inherent_impls.iter() {
 683             for item_did in self.tcx.associated_item_def_ids(impl_did) {
 684                 if let Some(def_id) = item_did.as_local()
 685                     && self.live_symbols.contains(&def_id)
 686                 {
 687                     return true;
 688                 }
 689             }
 690         }
 691         false
 692     }
 693
 694     fn warn_multiple_dead_codes(
 695         &self,
 696         dead_codes: &[(hir::HirId, Span, Symbol)],
 697         participle: &str,
 698         parent_hir_id: Option<hir::HirId>,
 699     ) {
 700         if let Some((id, _, name)) = dead_codes.first()
 701             && !name.as_str().starts_with('_')
 702         {
 703             self.tcx.struct_span_lint_hir(
 704                 lint::builtin::DEAD_CODE,
 705                 *id,
 706                 MultiSpan::from_spans(
 707                     dead_codes.iter().map(|(_, span, _)| *span).collect(),
 708                 ),
 709                 |lint| {
 710                     let def_id = self.tcx.hir().local_def_id(*id);
 711                     let descr = self.tcx.def_kind(def_id).descr(def_id.to_def_id());
 712                     let span_len = dead_codes.len();
 713                     let names = match &dead_codes.iter().map(|(_, _, n)| n.to_string()).collect::<Vec<_>>()[..]
 714                     {
 715                         _ if span_len > 6 => String::new(),
 716                         [name] => format!("`{name}` "),
 717                         [names @ .., last] => {
 718                             format!("{} and `{last}` ", names.iter().map(|name| format!("`{name}`")).join(", "))
 719                         }
 720                         [] => unreachable!(),
 721                     };
 722                     let mut err = lint.build(&format!(
 723                         "{these}{descr}{s} {names}{are} never {participle}",
 724                         these = if span_len > 6 { "multiple " } else { "" },
 725                         s = pluralize!(span_len),
 726                         are = pluralize!("is", span_len),
 727                     ));
 728                     let hir = self.tcx.hir();
 729                     if let Some(parent_hir_id) = parent_hir_id
 730                         && let Some(parent_node) = hir.find(parent_hir_id)
 731                         && let Node::Item(item) = parent_node
 732                     {
 733                         let def_id = self.tcx.hir().local_def_id(parent_hir_id);
 734                         let parent_descr = self.tcx.def_kind(def_id).descr(def_id.to_def_id());
 735                         err.span_label(
 736                             item.ident.span,
 737                             format!(
 738                                 "{descr}{s} in this {parent_descr}",
 739                                 s = pluralize!(span_len)
 740                             ),
 741                         );
 742                     }
 743                     if let Some(encl_scope) = hir.get_enclosing_scope(*id)
 744                         && let Some(encl_def_id) = hir.opt_local_def_id(encl_scope)
 745                         && let Some(ign_traits) = self.ignored_derived_traits.get(&encl_def_id)
 746                     {
 747                         let traits_str = ign_traits
 748                             .iter()
 749                             .map(|(trait_id, _)| format!("`{}`", self.tcx.item_name(*trait_id)))
 750                             .collect::<Vec<_>>()
 751                             .join(" and ");
 752                         let plural_s = pluralize!(ign_traits.len());
 753                         let article = if ign_traits.len() > 1 { "" } else { "a " };
 754                         let is_are = if ign_traits.len() > 1 { "these are" } else { "this is" };
 755                         let msg = format!(
 756                             "`{}` has {}derived impl{} for the trait{} {}, but {} \
 757                             intentionally ignored during dead code analysis",
 758                             self.tcx.item_name(encl_def_id.to_def_id()),
 759                             article,
 760                             plural_s,
 761                             plural_s,
 762                             traits_str,
 763                             is_are
 764                         );
 765                         err.note(&msg);
 766                     }
 767                         err.emit();
 768                     },
 769             );
 770         }
 771     }
 772
 773     fn warn_dead_fields_and_variants(
 774         &self,
 775         hir_id: hir::HirId,
 776         participle: &str,
 777         dead_codes: Vec<DeadVariant>,
 778     ) {
 779         let mut dead_codes = dead_codes
 780             .iter()
 781             .filter(|v| !v.name.as_str().starts_with('_'))
 782             .map(|v| v)
 783             .collect::<Vec<&DeadVariant>>();
 784         if dead_codes.is_empty() {
 785             return;
 786         }
 787         dead_codes.sort_by_key(|v| v.level);
 788         for (_, group) in &dead_codes.into_iter().group_by(|v| v.level) {
 789             self.warn_multiple_dead_codes(
 790                 &group
 791                     .map(|v| (v.hir_id, v.span, v.name))
 792                     .collect::<Vec<(hir::HirId, Span, Symbol)>>(),
 793                 participle,
 794                 Some(hir_id),
 795             );
 796         }
 797     }
 798
 799     fn warn_dead_code(
 800         &mut self,
 801         id: hir::HirId,
 802         span: rustc_span::Span,
 803         name: Symbol,
 804         participle: &str,
 805     ) {
 806         self.warn_multiple_dead_codes(&[(id, span, name)], participle, None);
 807     }
 808 }
 809
 810 impl<'tcx> Visitor<'tcx> for DeadVisitor<'tcx> {
 811     type NestedFilter = nested_filter::All;
 812
 813     /// Walk nested items in place so that we don't report dead-code
 814     /// on inner functions when the outer function is already getting
 815     /// an error. We could do this also by checking the parents, but
 816     /// this is how the code is setup and it seems harmless enough.
 817     fn nested_visit_map(&mut self) -> Self::Map {
 818         self.tcx.hir()
 819     }
 820
 821     fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {
 822         if self.should_warn_about_item(item) {
 823             // For most items, we want to highlight its identifier
 824             let span = match item.kind {
 825                 hir::ItemKind::Fn(..)
 826                 | hir::ItemKind::Mod(..)
 827                 | hir::ItemKind::Enum(..)
 828                 | hir::ItemKind::Struct(..)
 829                 | hir::ItemKind::Union(..)
 830                 | hir::ItemKind::Trait(..)
 831                 | hir::ItemKind::Impl { .. } => {
 832                     // FIXME(66095): Because item.span is annotated with things
 833                     // like expansion data, and ident.span isn't, we use the
 834                     // def_span method if it's part of a macro invocation
 835                     // (and thus has a source_callee set).
 836                     // We should probably annotate ident.span with the macro
 837                     // context, but that's a larger change.
 838                     if item.span.source_callee().is_some() {
 839                         self.tcx.sess.source_map().guess_head_span(item.span)
 840                     } else {
 841                         item.ident.span
 842                     }
 843                 }
 844                 _ => item.span,
 845             };
 846             let participle = match item.kind {
 847                 hir::ItemKind::Struct(..) => "constructed", // Issue #52325
 848                 _ => "used",
 849             };
 850             self.warn_dead_code(item.hir_id(), span, item.ident.name, participle);
 851         } else {
 852             // Only continue if we didn't warn
 853             intravisit::walk_item(self, item);
 854         }
 855     }
 856
 857     // This visitor should only visit a single module at a time.
 858     fn visit_mod(&mut self, _: &'tcx hir::Mod<'tcx>, _: Span, _: hir::HirId) {}
 859
 860     fn visit_enum_def(
 861         &mut self,
 862         enum_definition: &'tcx hir::EnumDef<'tcx>,
 863         generics: &'tcx hir::Generics<'tcx>,
 864         item_id: hir::HirId,
 865         _: Span,
 866     ) {
 867         intravisit::walk_enum_def(self, enum_definition, generics, item_id);
 868         let dead_variants = enum_definition
 869             .variants
 870             .iter()
 871             .filter_map(|variant| {
 872                 if self.should_warn_about_variant(&variant) {
 873                     Some(DeadVariant {
 874                         hir_id: variant.id,
 875                         span: variant.span,
 876                         name: variant.ident.name,
 877                         level: self.tcx.lint_level_at_node(lint::builtin::DEAD_CODE, variant.id).0,
 878                     })
 879                 } else {
 880                     None
 881                 }
 882             })
 883             .collect();
 884         self.warn_dead_fields_and_variants(item_id, "constructed", dead_variants)
 885     }
 886
 887     fn visit_variant(
 888         &mut self,
 889         variant: &'tcx hir::Variant<'tcx>,
 890         g: &'tcx hir::Generics<'tcx>,
 891         id: hir::HirId,
 892     ) {
 893         if !self.should_warn_about_variant(&variant) {
 894             intravisit::walk_variant(self, variant, g, id);
 895         }
 896     }
 897
 898     fn visit_foreign_item(&mut self, fi: &'tcx hir::ForeignItem<'tcx>) {
 899         if self.should_warn_about_foreign_item(fi) {
 900             self.warn_dead_code(fi.hir_id(), fi.span, fi.ident.name, "used");
 901         }
 902         intravisit::walk_foreign_item(self, fi);
 903     }
 904
 905     fn visit_variant_data(
 906         &mut self,
 907         def: &'tcx hir::VariantData<'tcx>,
 908         _: Symbol,
 909         _: &hir::Generics<'_>,
 910         id: hir::HirId,
 911         _: rustc_span::Span,
 912     ) {
 913         intravisit::walk_struct_def(self, def);
 914         let dead_fields = def
 915             .fields()
 916             .iter()
 917             .filter_map(|field| {
 918                 if self.should_warn_about_field(&field) {
 919                     Some(DeadVariant {
 920                         hir_id: field.hir_id,
 921                         span: field.span,
 922                         name: field.ident.name,
 923                         level: self
 924                             .tcx
 925                             .lint_level_at_node(lint::builtin::DEAD_CODE, field.hir_id)
 926                             .0,
 927                     })
 928                 } else {
 929                     None
 930                 }
 931             })
 932             .collect();
 933         self.warn_dead_fields_and_variants(id, "read", dead_fields)
 934     }
 935
 936     fn visit_impl_item(&mut self, impl_item: &'tcx hir::ImplItem<'tcx>) {
 937         match impl_item.kind {
 938             hir::ImplItemKind::Const(_, body_id) => {
 939                 if !self.symbol_is_live(impl_item.def_id) {
 940                     self.warn_dead_code(
 941                         impl_item.hir_id(),
 942                         impl_item.span,
 943                         impl_item.ident.name,
 944                         "used",
 945                     );
 946                 }
 947                 self.visit_nested_body(body_id)
 948             }
 949             hir::ImplItemKind::Fn(_, body_id) => {
 950                 if !self.symbol_is_live(impl_item.def_id) {
 951                     // FIXME(66095): Because impl_item.span is annotated with things
 952                     // like expansion data, and ident.span isn't, we use the
 953                     // def_span method if it's part of a macro invocation
 954                     // (and thus has a source_callee set).
 955                     // We should probably annotate ident.span with the macro
 956                     // context, but that's a larger change.
 957                     let span = if impl_item.span.source_callee().is_some() {
 958                         self.tcx.sess.source_map().guess_head_span(impl_item.span)
 959                     } else {
 960                         impl_item.ident.span
 961                     };
 962                     self.warn_dead_code(impl_item.hir_id(), span, impl_item.ident.name, "used");
 963                 }
 964                 self.visit_nested_body(body_id)
 965             }
 966             hir::ImplItemKind::TyAlias(..) => {}
 967         }
 968     }
 969
 970     // Overwrite so that we don't warn the trait item itself.
 971     fn visit_trait_item(&mut self, trait_item: &'tcx hir::TraitItem<'tcx>) {
 972         match trait_item.kind {
 973             hir::TraitItemKind::Const(_, Some(body_id))
 974             | hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(body_id)) => {
 975                 self.visit_nested_body(body_id)
 976             }
 977             hir::TraitItemKind::Const(_, None)
 978             | hir::TraitItemKind::Fn(_, hir::TraitFn::Required(_))
 979             | hir::TraitItemKind::Type(..) => {}
 980         }
 981     }
 982 }
 983
 984 fn check_mod_deathness(tcx: TyCtxt<'_>, module: LocalDefId) {
 985     let (live_symbols, ignored_derived_traits) = tcx.live_symbols_and_ignored_derived_traits(());
 986     let mut visitor = DeadVisitor { tcx, live_symbols, ignored_derived_traits };
 987     let (module, _, module_id) = tcx.hir().get_module(module);
 988     // Do not use an ItemLikeVisitor since we may want to skip visiting some items
 989     // when a surrounding one is warned against or `_`.
 990     intravisit::walk_mod(&mut visitor, module, module_id);
 991 }
 992
 993 pub(crate) fn provide(providers: &mut Providers) {
 994     *providers =
 995         Providers { live_symbols_and_ignored_derived_traits, check_mod_deathness, ..*providers };
 996 }