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.
5 use rustc::hir::map::Map;
6 use rustc::middle::codegen_fn_attrs::CodegenFnAttrFlags;
7 use rustc::middle::privacy;
8 use rustc::ty::{self, DefIdTree, TyCtxt};
9 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
11 use rustc_hir::def::{CtorOf, DefKind, Res};
12 use rustc_hir::def_id::{DefId, LOCAL_CRATE};
13 use rustc_hir::intravisit::{self, NestedVisitorMap, Visitor};
14 use rustc_hir::itemlikevisit::ItemLikeVisitor;
15 use rustc_hir::{Node, PatKind, TyKind};
16 use rustc_session::lint;
19 use rustc_span::symbol::sym;
20 use syntax::{ast, attr};
22 // Any local node that may call something in its body block should be
23 // explored. For example, if it's a live Node::Item that is a
24 // function, then we should explore its block to check for codes that
25 // may need to be marked as live.
26 fn should_explore(tcx: TyCtxt<'_>, hir_id: hir::HirId) -> bool {
27 match tcx.hir().find(hir_id) {
29 | Some(Node::ImplItem(..))
30 | Some(Node::ForeignItem(..))
31 | Some(Node::TraitItem(..))
32 | Some(Node::Variant(..))
33 | Some(Node::AnonConst(..))
34 | Some(Node::Pat(..)) => true,
39 struct MarkSymbolVisitor<'a, 'tcx> {
40 worklist: Vec<hir::HirId>,
42 tables: &'a ty::TypeckTables<'tcx>,
43 live_symbols: FxHashSet<hir::HirId>,
44 repr_has_repr_c: 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>,
52 impl<'a, 'tcx> MarkSymbolVisitor<'a, 'tcx> {
53 fn check_def_id(&mut self, def_id: DefId) {
54 if let Some(hir_id) = self.tcx.hir().as_local_hir_id(def_id) {
55 if should_explore(self.tcx, hir_id) || self.struct_constructors.contains_key(&hir_id) {
56 self.worklist.push(hir_id);
58 self.live_symbols.insert(hir_id);
62 fn insert_def_id(&mut self, def_id: DefId) {
63 if let Some(hir_id) = self.tcx.hir().as_local_hir_id(def_id) {
64 debug_assert!(!should_explore(self.tcx, hir_id));
65 self.live_symbols.insert(hir_id);
69 fn handle_res(&mut self, res: Res) {
71 Res::Def(DefKind::Const, _)
72 | Res::Def(DefKind::AssocConst, _)
73 | Res::Def(DefKind::TyAlias, _) => {
74 self.check_def_id(res.def_id());
76 _ if self.in_pat => {}
77 Res::PrimTy(..) | Res::SelfCtor(..) | Res::Local(..) => {}
78 Res::Def(DefKind::Ctor(CtorOf::Variant, ..), ctor_def_id) => {
79 let variant_id = self.tcx.parent(ctor_def_id).unwrap();
80 let enum_id = self.tcx.parent(variant_id).unwrap();
81 self.check_def_id(enum_id);
82 if !self.ignore_variant_stack.contains(&ctor_def_id) {
83 self.check_def_id(variant_id);
86 Res::Def(DefKind::Variant, variant_id) => {
87 let enum_id = self.tcx.parent(variant_id).unwrap();
88 self.check_def_id(enum_id);
89 if !self.ignore_variant_stack.contains(&variant_id) {
90 self.check_def_id(variant_id);
93 Res::SelfTy(t, i) => {
101 Res::ToolMod | Res::NonMacroAttr(..) | Res::Err => {}
103 self.check_def_id(res.def_id());
108 fn lookup_and_handle_method(&mut self, id: hir::HirId) {
109 if let Some(def_id) = self.tables.type_dependent_def_id(id) {
110 self.check_def_id(def_id);
112 bug!("no type-dependent def for method");
116 fn handle_field_access(&mut self, lhs: &hir::Expr<'_>, hir_id: hir::HirId) {
117 match self.tables.expr_ty_adjusted(lhs).kind {
119 let index = self.tcx.field_index(hir_id, self.tables);
120 self.insert_def_id(def.non_enum_variant().fields[index].did);
123 _ => span_bug!(lhs.span, "named field access on non-ADT"),
127 fn handle_field_pattern_match(
131 pats: &[hir::FieldPat<'_>],
133 let variant = match self.tables.node_type(lhs.hir_id).kind {
134 ty::Adt(adt, _) => adt.variant_of_res(res),
135 _ => span_bug!(lhs.span, "non-ADT in struct pattern"),
138 if let PatKind::Wild = pat.pat.kind {
141 let index = self.tcx.field_index(pat.hir_id, self.tables);
142 self.insert_def_id(variant.fields[index].did);
146 fn mark_live_symbols(&mut self) {
147 let mut scanned = FxHashSet::default();
148 while let Some(id) = self.worklist.pop() {
149 if !scanned.insert(id) {
153 // in the case of tuple struct constructors we want to check the item, not the generated
154 // tuple struct constructor function
155 let id = self.struct_constructors.get(&id).cloned().unwrap_or(id);
157 if let Some(node) = self.tcx.hir().find(id) {
158 self.live_symbols.insert(id);
159 self.visit_node(node);
164 fn visit_node(&mut self, node: Node<'tcx>) {
165 let had_repr_c = self.repr_has_repr_c;
166 self.repr_has_repr_c = false;
167 let had_inherited_pub_visibility = self.inherited_pub_visibility;
168 self.inherited_pub_visibility = false;
170 Node::Item(item) => match item.kind {
171 hir::ItemKind::Struct(..) | hir::ItemKind::Union(..) => {
172 let def_id = self.tcx.hir().local_def_id(item.hir_id);
173 let def = self.tcx.adt_def(def_id);
174 self.repr_has_repr_c = def.repr.c();
176 intravisit::walk_item(self, &item);
178 hir::ItemKind::Enum(..) => {
179 self.inherited_pub_visibility = item.vis.node.is_pub();
181 intravisit::walk_item(self, &item);
183 hir::ItemKind::ForeignMod(..) => {}
185 intravisit::walk_item(self, &item);
188 Node::TraitItem(trait_item) => {
189 intravisit::walk_trait_item(self, trait_item);
191 Node::ImplItem(impl_item) => {
192 intravisit::walk_impl_item(self, impl_item);
194 Node::ForeignItem(foreign_item) => {
195 intravisit::walk_foreign_item(self, &foreign_item);
199 self.repr_has_repr_c = had_repr_c;
200 self.inherited_pub_visibility = had_inherited_pub_visibility;
203 fn mark_as_used_if_union(&mut self, adt: &ty::AdtDef, fields: &[hir::Field<'_>]) {
204 if adt.is_union() && adt.non_enum_variant().fields.len() > 1 && adt.did.is_local() {
205 for field in fields {
206 let index = self.tcx.field_index(field.hir_id, self.tables);
207 self.insert_def_id(adt.non_enum_variant().fields[index].did);
213 impl<'a, 'tcx> Visitor<'tcx> for MarkSymbolVisitor<'a, 'tcx> {
214 type Map = Map<'tcx>;
216 fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<'_, Self::Map> {
217 NestedVisitorMap::None
220 fn visit_nested_body(&mut self, body: hir::BodyId) {
221 let old_tables = self.tables;
222 self.tables = self.tcx.body_tables(body);
223 let body = self.tcx.hir().body(body);
224 self.visit_body(body);
225 self.tables = old_tables;
228 fn visit_variant_data(
230 def: &'tcx hir::VariantData<'tcx>,
232 _: &hir::Generics<'_>,
236 let has_repr_c = self.repr_has_repr_c;
237 let inherited_pub_visibility = self.inherited_pub_visibility;
238 let live_fields = def
241 .filter(|f| has_repr_c || inherited_pub_visibility || f.vis.node.is_pub());
242 self.live_symbols.extend(live_fields.map(|f| f.hir_id));
244 intravisit::walk_struct_def(self, def);
247 fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) {
249 hir::ExprKind::Path(ref qpath @ hir::QPath::TypeRelative(..)) => {
250 let res = self.tables.qpath_res(qpath, expr.hir_id);
251 self.handle_res(res);
253 hir::ExprKind::MethodCall(..) => {
254 self.lookup_and_handle_method(expr.hir_id);
256 hir::ExprKind::Field(ref lhs, ..) => {
257 self.handle_field_access(&lhs, expr.hir_id);
259 hir::ExprKind::Struct(_, ref fields, _) => {
260 if let ty::Adt(ref adt, _) = self.tables.expr_ty(expr).kind {
261 self.mark_as_used_if_union(adt, fields);
267 intravisit::walk_expr(self, expr);
270 fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
271 // Inside the body, ignore constructions of variants
272 // necessary for the pattern to match. Those construction sites
273 // can't be reached unless the variant is constructed elsewhere.
274 let len = self.ignore_variant_stack.len();
275 self.ignore_variant_stack.extend(arm.pat.necessary_variants());
276 intravisit::walk_arm(self, arm);
277 self.ignore_variant_stack.truncate(len);
280 fn visit_pat(&mut self, pat: &'tcx hir::Pat<'tcx>) {
282 PatKind::Struct(ref path, ref fields, _) => {
283 let res = self.tables.qpath_res(path, pat.hir_id);
284 self.handle_field_pattern_match(pat, res, fields);
286 PatKind::Path(ref qpath) => {
287 let res = self.tables.qpath_res(qpath, pat.hir_id);
288 self.handle_res(res);
294 intravisit::walk_pat(self, pat);
298 fn visit_path(&mut self, path: &'tcx hir::Path<'tcx>, _: hir::HirId) {
299 self.handle_res(path.res);
300 intravisit::walk_path(self, path);
303 fn visit_ty(&mut self, ty: &'tcx hir::Ty<'tcx>) {
305 TyKind::Def(item_id, _) => {
306 let item = self.tcx.hir().expect_item(item_id.id);
307 intravisit::walk_item(self, item);
311 intravisit::walk_ty(self, ty);
314 fn visit_anon_const(&mut self, c: &'tcx hir::AnonConst) {
315 self.live_symbols.insert(c.hir_id);
316 intravisit::walk_anon_const(self, c);
320 fn has_allow_dead_code_or_lang_attr(
323 attrs: &[ast::Attribute],
325 if attr::contains_name(attrs, sym::lang) {
329 // Stable attribute for #[lang = "panic_impl"]
330 if attr::contains_name(attrs, sym::panic_handler) {
334 // (To be) stable attribute for #[lang = "oom"]
335 if attr::contains_name(attrs, sym::alloc_error_handler) {
339 let def_id = tcx.hir().local_def_id(id);
340 let cg_attrs = tcx.codegen_fn_attrs(def_id);
342 // #[used], #[no_mangle], #[export_name], etc also keeps the item alive
343 // forcefully, e.g., for placing it in a specific section.
344 if cg_attrs.contains_extern_indicator() || cg_attrs.flags.contains(CodegenFnAttrFlags::USED) {
348 tcx.lint_level_at_node(lint::builtin::DEAD_CODE, id).0 == lint::Allow
351 // This visitor seeds items that
352 // 1) We want to explicitly consider as live:
353 // * Item annotated with #[allow(dead_code)]
354 // - This is done so that if we want to suppress warnings for a
355 // group of dead functions, we only have to annotate the "root".
356 // For example, if both `f` and `g` are dead and `f` calls `g`,
357 // then annotating `f` with `#[allow(dead_code)]` will suppress
358 // warning for both `f` and `g`.
359 // * Item annotated with #[lang=".."]
360 // - This is because lang items are always callable from elsewhere.
362 // 2) We are not sure to be live or not
363 // * Implementation of a trait method
364 struct LifeSeeder<'k, 'tcx> {
365 worklist: Vec<hir::HirId>,
366 krate: &'k hir::Crate<'k>,
368 // see `MarkSymbolVisitor::struct_constructors`
369 struct_constructors: FxHashMap<hir::HirId, hir::HirId>,
372 impl<'v, 'k, 'tcx> ItemLikeVisitor<'v> for LifeSeeder<'k, 'tcx> {
373 fn visit_item(&mut self, item: &hir::Item<'_>) {
374 let allow_dead_code = has_allow_dead_code_or_lang_attr(self.tcx, item.hir_id, &item.attrs);
376 self.worklist.push(item.hir_id);
379 hir::ItemKind::Enum(ref enum_def, _) => {
381 self.worklist.extend(enum_def.variants.iter().map(|variant| variant.id));
384 for variant in enum_def.variants {
385 if let Some(ctor_hir_id) = variant.data.ctor_hir_id() {
386 self.struct_constructors.insert(ctor_hir_id, variant.id);
390 hir::ItemKind::Trait(.., trait_item_refs) => {
391 for trait_item_ref in trait_item_refs {
392 let trait_item = self.krate.trait_item(trait_item_ref.id);
393 match trait_item.kind {
394 hir::TraitItemKind::Const(_, Some(_))
395 | hir::TraitItemKind::Method(_, hir::TraitMethod::Provided(_)) => {
396 if has_allow_dead_code_or_lang_attr(
401 self.worklist.push(trait_item.hir_id);
408 hir::ItemKind::Impl(.., ref opt_trait, _, impl_item_refs) => {
409 for impl_item_ref in impl_item_refs {
410 let impl_item = self.krate.impl_item(impl_item_ref.id);
411 if opt_trait.is_some()
412 || has_allow_dead_code_or_lang_attr(
418 self.worklist.push(impl_item_ref.id.hir_id);
422 hir::ItemKind::Struct(ref variant_data, _) => {
423 if let Some(ctor_hir_id) = variant_data.ctor_hir_id() {
424 self.struct_constructors.insert(ctor_hir_id, item.hir_id);
431 fn visit_trait_item(&mut self, _item: &hir::TraitItem<'_>) {
432 // ignore: we are handling this in `visit_item` above
435 fn visit_impl_item(&mut self, _item: &hir::ImplItem<'_>) {
436 // ignore: we are handling this in `visit_item` above
440 fn create_and_seed_worklist<'tcx>(
442 access_levels: &privacy::AccessLevels,
443 krate: &hir::Crate<'_>,
444 ) -> (Vec<hir::HirId>, FxHashMap<hir::HirId, hir::HirId>) {
445 let worklist = access_levels
450 if level >= &privacy::AccessLevel::Reachable { Some(id) } else { None }
455 tcx.entry_fn(LOCAL_CRATE).map(|(def_id, _)| tcx.hir().as_local_hir_id(def_id).unwrap()),
457 .collect::<Vec<_>>();
459 // Seed implemented trait items
460 let mut life_seeder =
461 LifeSeeder { worklist, krate, tcx, struct_constructors: Default::default() };
462 krate.visit_all_item_likes(&mut life_seeder);
464 (life_seeder.worklist, life_seeder.struct_constructors)
469 access_levels: &privacy::AccessLevels,
470 krate: &hir::Crate<'_>,
471 ) -> FxHashSet<hir::HirId> {
472 let (worklist, struct_constructors) = create_and_seed_worklist(tcx, access_levels, krate);
473 let mut symbol_visitor = MarkSymbolVisitor {
476 tables: &ty::TypeckTables::empty(None),
477 live_symbols: Default::default(),
478 repr_has_repr_c: false,
480 inherited_pub_visibility: false,
481 ignore_variant_stack: vec![],
484 symbol_visitor.mark_live_symbols();
485 symbol_visitor.live_symbols
488 struct DeadVisitor<'tcx> {
490 live_symbols: FxHashSet<hir::HirId>,
493 impl DeadVisitor<'tcx> {
494 fn should_warn_about_item(&mut self, item: &hir::Item<'_>) -> bool {
495 let should_warn = match item.kind {
496 hir::ItemKind::Static(..)
497 | hir::ItemKind::Const(..)
498 | hir::ItemKind::Fn(..)
499 | hir::ItemKind::TyAlias(..)
500 | hir::ItemKind::Enum(..)
501 | hir::ItemKind::Struct(..)
502 | hir::ItemKind::Union(..) => true,
505 should_warn && !self.symbol_is_live(item.hir_id)
508 fn should_warn_about_field(&mut self, field: &hir::StructField<'_>) -> bool {
509 let field_type = self.tcx.type_of(self.tcx.hir().local_def_id(field.hir_id));
510 !field.is_positional()
511 && !self.symbol_is_live(field.hir_id)
512 && !field_type.is_phantom_data()
513 && !has_allow_dead_code_or_lang_attr(self.tcx, field.hir_id, &field.attrs)
516 fn should_warn_about_variant(&mut self, variant: &hir::Variant<'_>) -> bool {
517 !self.symbol_is_live(variant.id)
518 && !has_allow_dead_code_or_lang_attr(self.tcx, variant.id, &variant.attrs)
521 fn should_warn_about_foreign_item(&mut self, fi: &hir::ForeignItem<'_>) -> bool {
522 !self.symbol_is_live(fi.hir_id)
523 && !has_allow_dead_code_or_lang_attr(self.tcx, fi.hir_id, &fi.attrs)
526 // id := HIR id of an item's definition.
527 fn symbol_is_live(&mut self, id: hir::HirId) -> bool {
528 if self.live_symbols.contains(&id) {
531 // If it's a type whose items are live, then it's live, too.
532 // This is done to handle the case where, for example, the static
533 // method of a private type is used, but the type itself is never
535 let def_id = self.tcx.hir().local_def_id(id);
536 let inherent_impls = self.tcx.inherent_impls(def_id);
537 for &impl_did in inherent_impls.iter() {
538 for &item_did in &self.tcx.associated_item_def_ids(impl_did)[..] {
539 if let Some(item_hir_id) = self.tcx.hir().as_local_hir_id(item_did) {
540 if self.live_symbols.contains(&item_hir_id) {
552 span: rustc_span::Span,
557 if !name.as_str().starts_with("_") {
559 lint::builtin::DEAD_CODE,
562 &format!("{} is never {}: `{}`", node_type, participle, name),
568 impl Visitor<'tcx> for DeadVisitor<'tcx> {
569 type Map = Map<'tcx>;
571 /// Walk nested items in place so that we don't report dead-code
572 /// on inner functions when the outer function is already getting
573 /// an error. We could do this also by checking the parents, but
574 /// this is how the code is setup and it seems harmless enough.
575 fn nested_visit_map(&mut self) -> NestedVisitorMap<'_, Self::Map> {
576 NestedVisitorMap::All(&self.tcx.hir())
579 fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {
580 if self.should_warn_about_item(item) {
581 // For most items, we want to highlight its identifier
582 let span = match item.kind {
583 hir::ItemKind::Fn(..)
584 | hir::ItemKind::Mod(..)
585 | hir::ItemKind::Enum(..)
586 | hir::ItemKind::Struct(..)
587 | hir::ItemKind::Union(..)
588 | hir::ItemKind::Trait(..)
589 | hir::ItemKind::Impl(..) => {
590 // FIXME(66095): Because item.span is annotated with things
591 // like expansion data, and ident.span isn't, we use the
592 // def_span method if it's part of a macro invocation
593 // (and thus has asource_callee set).
594 // We should probably annotate ident.span with the macro
595 // context, but that's a larger change.
596 if item.span.source_callee().is_some() {
597 self.tcx.sess.source_map().def_span(item.span)
604 let participle = match item.kind {
605 hir::ItemKind::Struct(..) => "constructed", // Issue #52325
612 item.kind.descriptive_variant(),
616 // Only continue if we didn't warn
617 intravisit::walk_item(self, item);
623 variant: &'tcx hir::Variant<'tcx>,
624 g: &'tcx hir::Generics<'tcx>,
627 if self.should_warn_about_variant(&variant) {
636 intravisit::walk_variant(self, variant, g, id);
640 fn visit_foreign_item(&mut self, fi: &'tcx hir::ForeignItem<'tcx>) {
641 if self.should_warn_about_foreign_item(fi) {
646 fi.kind.descriptive_variant(),
650 intravisit::walk_foreign_item(self, fi);
653 fn visit_struct_field(&mut self, field: &'tcx hir::StructField<'tcx>) {
654 if self.should_warn_about_field(&field) {
655 self.warn_dead_code(field.hir_id, field.span, field.ident.name, "field", "read");
657 intravisit::walk_struct_field(self, field);
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) {
667 impl_item.ident.name,
672 self.visit_nested_body(body_id)
674 hir::ImplItemKind::Method(_, body_id) => {
675 if !self.symbol_is_live(impl_item.hir_id) {
676 let span = self.tcx.sess.source_map().def_span(impl_item.span);
680 impl_item.ident.name,
685 self.visit_nested_body(body_id)
687 hir::ImplItemKind::OpaqueTy(..) | hir::ImplItemKind::TyAlias(..) => {}
691 // Overwrite so that we don't warn the trait item itself.
692 fn visit_trait_item(&mut self, trait_item: &'tcx hir::TraitItem<'tcx>) {
693 match trait_item.kind {
694 hir::TraitItemKind::Const(_, Some(body_id))
695 | hir::TraitItemKind::Method(_, hir::TraitMethod::Provided(body_id)) => {
696 self.visit_nested_body(body_id)
698 hir::TraitItemKind::Const(_, None)
699 | hir::TraitItemKind::Method(_, hir::TraitMethod::Required(_))
700 | hir::TraitItemKind::Type(..) => {}
705 pub fn check_crate(tcx: TyCtxt<'_>) {
706 let access_levels = &tcx.privacy_access_levels(LOCAL_CRATE);
707 let krate = tcx.hir().krate();
708 let live_symbols = find_live(tcx, access_levels, krate);
709 let mut visitor = DeadVisitor { tcx, live_symbols };
710 intravisit::walk_crate(&mut visitor, krate);