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::intravisit::{self, NestedVisitorMap, Visitor};
6 use rustc::hir::itemlikevisit::ItemLikeVisitor;
8 use rustc::hir::{self, PatKind, TyKind};
10 use rustc::hir::def::{CtorOf, DefKind, Res};
11 use rustc::hir::def_id::{DefId, LOCAL_CRATE};
13 use rustc::middle::codegen_fn_attrs::CodegenFnAttrFlags;
14 use rustc::middle::privacy;
15 use rustc::ty::{self, DefIdTree, TyCtxt};
16 use rustc_data_structures::fx::{FxHashMap, FxHashSet};
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 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
215 NestedVisitorMap::None
218 fn visit_nested_body(&mut self, body: hir::BodyId) {
219 let old_tables = self.tables;
220 self.tables = self.tcx.body_tables(body);
221 let body = self.tcx.hir().body(body);
222 self.visit_body(body);
223 self.tables = old_tables;
226 fn visit_variant_data(
228 def: &'tcx hir::VariantData<'tcx>,
230 _: &hir::Generics<'_>,
234 let has_repr_c = self.repr_has_repr_c;
235 let inherited_pub_visibility = self.inherited_pub_visibility;
236 let live_fields = def
239 .filter(|f| has_repr_c || inherited_pub_visibility || f.vis.node.is_pub());
240 self.live_symbols.extend(live_fields.map(|f| f.hir_id));
242 intravisit::walk_struct_def(self, def);
245 fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) {
247 hir::ExprKind::Path(ref qpath @ hir::QPath::TypeRelative(..)) => {
248 let res = self.tables.qpath_res(qpath, expr.hir_id);
249 self.handle_res(res);
251 hir::ExprKind::MethodCall(..) => {
252 self.lookup_and_handle_method(expr.hir_id);
254 hir::ExprKind::Field(ref lhs, ..) => {
255 self.handle_field_access(&lhs, expr.hir_id);
257 hir::ExprKind::Struct(_, ref fields, _) => {
258 if let ty::Adt(ref adt, _) = self.tables.expr_ty(expr).kind {
259 self.mark_as_used_if_union(adt, fields);
265 intravisit::walk_expr(self, expr);
268 fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
269 // Inside the body, ignore constructions of variants
270 // necessary for the pattern to match. Those construction sites
271 // can't be reached unless the variant is constructed elsewhere.
272 let len = self.ignore_variant_stack.len();
273 self.ignore_variant_stack.extend(arm.pat.necessary_variants());
274 intravisit::walk_arm(self, arm);
275 self.ignore_variant_stack.truncate(len);
278 fn visit_pat(&mut self, pat: &'tcx hir::Pat<'tcx>) {
280 PatKind::Struct(ref path, ref fields, _) => {
281 let res = self.tables.qpath_res(path, pat.hir_id);
282 self.handle_field_pattern_match(pat, res, fields);
284 PatKind::Path(ref qpath) => {
285 let res = self.tables.qpath_res(qpath, pat.hir_id);
286 self.handle_res(res);
292 intravisit::walk_pat(self, pat);
296 fn visit_path(&mut self, path: &'tcx hir::Path<'tcx>, _: hir::HirId) {
297 self.handle_res(path.res);
298 intravisit::walk_path(self, path);
301 fn visit_ty(&mut self, ty: &'tcx hir::Ty<'tcx>) {
303 TyKind::Def(item_id, _) => {
304 let item = self.tcx.hir().expect_item(item_id.id);
305 intravisit::walk_item(self, item);
309 intravisit::walk_ty(self, ty);
312 fn visit_anon_const(&mut self, c: &'tcx hir::AnonConst) {
313 self.live_symbols.insert(c.hir_id);
314 intravisit::walk_anon_const(self, c);
318 fn has_allow_dead_code_or_lang_attr(
321 attrs: &[ast::Attribute],
323 if attr::contains_name(attrs, sym::lang) {
327 // Stable attribute for #[lang = "panic_impl"]
328 if attr::contains_name(attrs, sym::panic_handler) {
332 // (To be) stable attribute for #[lang = "oom"]
333 if attr::contains_name(attrs, sym::alloc_error_handler) {
337 let def_id = tcx.hir().local_def_id(id);
338 let cg_attrs = tcx.codegen_fn_attrs(def_id);
340 // #[used], #[no_mangle], #[export_name], etc also keeps the item alive
341 // forcefully, e.g., for placing it in a specific section.
342 if cg_attrs.contains_extern_indicator() || cg_attrs.flags.contains(CodegenFnAttrFlags::USED) {
346 tcx.lint_level_at_node(lint::builtin::DEAD_CODE, id).0 == lint::Allow
349 // This visitor seeds items that
350 // 1) We want to explicitly consider as live:
351 // * Item annotated with #[allow(dead_code)]
352 // - This is done so that if we want to suppress warnings for a
353 // group of dead functions, we only have to annotate the "root".
354 // For example, if both `f` and `g` are dead and `f` calls `g`,
355 // then annotating `f` with `#[allow(dead_code)]` will suppress
356 // warning for both `f` and `g`.
357 // * Item annotated with #[lang=".."]
358 // - This is because lang items are always callable from elsewhere.
360 // 2) We are not sure to be live or not
361 // * Implementation of a trait method
362 struct LifeSeeder<'k, 'tcx> {
363 worklist: Vec<hir::HirId>,
364 krate: &'k hir::Crate<'k>,
366 // see `MarkSymbolVisitor::struct_constructors`
367 struct_constructors: FxHashMap<hir::HirId, hir::HirId>,
370 impl<'v, 'k, 'tcx> ItemLikeVisitor<'v> for LifeSeeder<'k, 'tcx> {
371 fn visit_item(&mut self, item: &hir::Item<'_>) {
372 let allow_dead_code = has_allow_dead_code_or_lang_attr(self.tcx, item.hir_id, &item.attrs);
374 self.worklist.push(item.hir_id);
377 hir::ItemKind::Enum(ref enum_def, _) => {
379 self.worklist.extend(enum_def.variants.iter().map(|variant| variant.id));
382 for variant in enum_def.variants {
383 if let Some(ctor_hir_id) = variant.data.ctor_hir_id() {
384 self.struct_constructors.insert(ctor_hir_id, variant.id);
388 hir::ItemKind::Trait(.., trait_item_refs) => {
389 for trait_item_ref in trait_item_refs {
390 let trait_item = self.krate.trait_item(trait_item_ref.id);
391 match trait_item.kind {
392 hir::TraitItemKind::Const(_, Some(_))
393 | hir::TraitItemKind::Method(_, hir::TraitMethod::Provided(_)) => {
394 if has_allow_dead_code_or_lang_attr(
399 self.worklist.push(trait_item.hir_id);
406 hir::ItemKind::Impl(.., ref opt_trait, _, impl_item_refs) => {
407 for impl_item_ref in impl_item_refs {
408 let impl_item = self.krate.impl_item(impl_item_ref.id);
409 if opt_trait.is_some()
410 || has_allow_dead_code_or_lang_attr(
416 self.worklist.push(impl_item_ref.id.hir_id);
420 hir::ItemKind::Struct(ref variant_data, _) => {
421 if let Some(ctor_hir_id) = variant_data.ctor_hir_id() {
422 self.struct_constructors.insert(ctor_hir_id, item.hir_id);
429 fn visit_trait_item(&mut self, _item: &hir::TraitItem<'_>) {
430 // ignore: we are handling this in `visit_item` above
433 fn visit_impl_item(&mut self, _item: &hir::ImplItem<'_>) {
434 // ignore: we are handling this in `visit_item` above
438 fn create_and_seed_worklist<'tcx>(
440 access_levels: &privacy::AccessLevels,
441 krate: &hir::Crate<'_>,
442 ) -> (Vec<hir::HirId>, FxHashMap<hir::HirId, hir::HirId>) {
443 let worklist = access_levels
448 if level >= &privacy::AccessLevel::Reachable { Some(id) } else { None }
453 tcx.entry_fn(LOCAL_CRATE).map(|(def_id, _)| tcx.hir().as_local_hir_id(def_id).unwrap()),
455 .collect::<Vec<_>>();
457 // Seed implemented trait items
458 let mut life_seeder =
459 LifeSeeder { worklist, krate, tcx, struct_constructors: Default::default() };
460 krate.visit_all_item_likes(&mut life_seeder);
462 (life_seeder.worklist, life_seeder.struct_constructors)
467 access_levels: &privacy::AccessLevels,
468 krate: &hir::Crate<'_>,
469 ) -> FxHashSet<hir::HirId> {
470 let (worklist, struct_constructors) = create_and_seed_worklist(tcx, access_levels, krate);
471 let mut symbol_visitor = MarkSymbolVisitor {
474 tables: &ty::TypeckTables::empty(None),
475 live_symbols: Default::default(),
476 repr_has_repr_c: false,
478 inherited_pub_visibility: false,
479 ignore_variant_stack: vec![],
482 symbol_visitor.mark_live_symbols();
483 symbol_visitor.live_symbols
486 struct DeadVisitor<'tcx> {
488 live_symbols: FxHashSet<hir::HirId>,
491 impl DeadVisitor<'tcx> {
492 fn should_warn_about_item(&mut self, item: &hir::Item<'_>) -> bool {
493 let should_warn = match item.kind {
494 hir::ItemKind::Static(..)
495 | hir::ItemKind::Const(..)
496 | hir::ItemKind::Fn(..)
497 | hir::ItemKind::TyAlias(..)
498 | hir::ItemKind::Enum(..)
499 | hir::ItemKind::Struct(..)
500 | hir::ItemKind::Union(..) => true,
503 should_warn && !self.symbol_is_live(item.hir_id)
506 fn should_warn_about_field(&mut self, field: &hir::StructField<'_>) -> bool {
507 let field_type = self.tcx.type_of(self.tcx.hir().local_def_id(field.hir_id));
508 !field.is_positional()
509 && !self.symbol_is_live(field.hir_id)
510 && !field_type.is_phantom_data()
511 && !has_allow_dead_code_or_lang_attr(self.tcx, field.hir_id, &field.attrs)
514 fn should_warn_about_variant(&mut self, variant: &hir::Variant<'_>) -> bool {
515 !self.symbol_is_live(variant.id)
516 && !has_allow_dead_code_or_lang_attr(self.tcx, variant.id, &variant.attrs)
519 fn should_warn_about_foreign_item(&mut self, fi: &hir::ForeignItem<'_>) -> bool {
520 !self.symbol_is_live(fi.hir_id)
521 && !has_allow_dead_code_or_lang_attr(self.tcx, fi.hir_id, &fi.attrs)
524 // id := HIR id of an item's definition.
525 fn symbol_is_live(&mut self, id: hir::HirId) -> bool {
526 if self.live_symbols.contains(&id) {
529 // If it's a type whose items are live, then it's live, too.
530 // This is done to handle the case where, for example, the static
531 // method of a private type is used, but the type itself is never
533 let def_id = self.tcx.hir().local_def_id(id);
534 let inherent_impls = self.tcx.inherent_impls(def_id);
535 for &impl_did in inherent_impls.iter() {
536 for &item_did in &self.tcx.associated_item_def_ids(impl_did)[..] {
537 if let Some(item_hir_id) = self.tcx.hir().as_local_hir_id(item_did) {
538 if self.live_symbols.contains(&item_hir_id) {
550 span: rustc_span::Span,
555 if !name.as_str().starts_with("_") {
557 lint::builtin::DEAD_CODE,
560 &format!("{} is never {}: `{}`", node_type, participle, name),
566 impl Visitor<'tcx> for DeadVisitor<'tcx> {
567 /// Walk nested items in place so that we don't report dead-code
568 /// on inner functions when the outer function is already getting
569 /// an error. We could do this also by checking the parents, but
570 /// this is how the code is setup and it seems harmless enough.
571 fn nested_visit_map<'this>(&'this mut self) -> NestedVisitorMap<'this, 'tcx> {
572 NestedVisitorMap::All(&self.tcx.hir())
575 fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {
576 if self.should_warn_about_item(item) {
577 // For most items, we want to highlight its identifier
578 let span = match item.kind {
579 hir::ItemKind::Fn(..)
580 | hir::ItemKind::Mod(..)
581 | hir::ItemKind::Enum(..)
582 | hir::ItemKind::Struct(..)
583 | hir::ItemKind::Union(..)
584 | hir::ItemKind::Trait(..)
585 | hir::ItemKind::Impl(..) => {
586 // FIXME(66095): Because item.span is annotated with things
587 // like expansion data, and ident.span isn't, we use the
588 // def_span method if it's part of a macro invocation
589 // (and thus has asource_callee set).
590 // We should probably annotate ident.span with the macro
591 // context, but that's a larger change.
592 if item.span.source_callee().is_some() {
593 self.tcx.sess.source_map().def_span(item.span)
600 let participle = match item.kind {
601 hir::ItemKind::Struct(..) => "constructed", // Issue #52325
608 item.kind.descriptive_variant(),
612 // Only continue if we didn't warn
613 intravisit::walk_item(self, item);
619 variant: &'tcx hir::Variant<'tcx>,
620 g: &'tcx hir::Generics<'tcx>,
623 if self.should_warn_about_variant(&variant) {
632 intravisit::walk_variant(self, variant, g, id);
636 fn visit_foreign_item(&mut self, fi: &'tcx hir::ForeignItem<'tcx>) {
637 if self.should_warn_about_foreign_item(fi) {
642 fi.kind.descriptive_variant(),
646 intravisit::walk_foreign_item(self, fi);
649 fn visit_struct_field(&mut self, field: &'tcx hir::StructField<'tcx>) {
650 if self.should_warn_about_field(&field) {
651 self.warn_dead_code(field.hir_id, field.span, field.ident.name, "field", "read");
653 intravisit::walk_struct_field(self, field);
656 fn visit_impl_item(&mut self, impl_item: &'tcx hir::ImplItem<'tcx>) {
657 match impl_item.kind {
658 hir::ImplItemKind::Const(_, body_id) => {
659 if !self.symbol_is_live(impl_item.hir_id) {
663 impl_item.ident.name,
668 self.visit_nested_body(body_id)
670 hir::ImplItemKind::Method(_, body_id) => {
671 if !self.symbol_is_live(impl_item.hir_id) {
672 let span = self.tcx.sess.source_map().def_span(impl_item.span);
676 impl_item.ident.name,
681 self.visit_nested_body(body_id)
683 hir::ImplItemKind::OpaqueTy(..) | hir::ImplItemKind::TyAlias(..) => {}
687 // Overwrite so that we don't warn the trait item itself.
688 fn visit_trait_item(&mut self, trait_item: &'tcx hir::TraitItem<'tcx>) {
689 match trait_item.kind {
690 hir::TraitItemKind::Const(_, Some(body_id))
691 | hir::TraitItemKind::Method(_, hir::TraitMethod::Provided(body_id)) => {
692 self.visit_nested_body(body_id)
694 hir::TraitItemKind::Const(_, None)
695 | hir::TraitItemKind::Method(_, hir::TraitMethod::Required(_))
696 | hir::TraitItemKind::Type(..) => {}
701 pub fn check_crate(tcx: TyCtxt<'_>) {
702 let access_levels = &tcx.privacy_access_levels(LOCAL_CRATE);
703 let krate = tcx.hir().krate();
704 let live_symbols = find_live(tcx, access_levels, krate);
705 let mut visitor = DeadVisitor { tcx, live_symbols };
706 intravisit::walk_crate(&mut visitor, krate);