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_data_structures::fx::{FxHashMap, FxHashSet};
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;
18 use rustc_ast::{ast, attr};
19 use rustc_span::symbol::sym;
21 // Any local node that may call something in its body block should be
22 // explored. For example, if it's a live Node::Item that is a
23 // function, then we should explore its block to check for codes that
24 // may need to be marked as live.
25 fn should_explore(tcx: TyCtxt<'_>, hir_id: hir::HirId) -> bool {
26 match tcx.hir().find(hir_id) {
30 | Node::ForeignItem(..)
40 struct MarkSymbolVisitor<'a, 'tcx> {
41 worklist: Vec<hir::HirId>,
43 tables: &'a ty::TypeckTables<'tcx>,
44 live_symbols: FxHashSet<hir::HirId>,
45 repr_has_repr_c: bool,
47 inherited_pub_visibility: bool,
48 ignore_variant_stack: Vec<DefId>,
49 // maps from tuple struct constructors to tuple struct items
50 struct_constructors: FxHashMap<hir::HirId, hir::HirId>,
53 impl<'a, 'tcx> MarkSymbolVisitor<'a, 'tcx> {
54 fn check_def_id(&mut self, def_id: DefId) {
55 if let Some(def_id) = def_id.as_local() {
56 let hir_id = self.tcx.hir().as_local_hir_id(def_id);
57 if should_explore(self.tcx, hir_id) || self.struct_constructors.contains_key(&hir_id) {
58 self.worklist.push(hir_id);
60 self.live_symbols.insert(hir_id);
64 fn insert_def_id(&mut self, def_id: DefId) {
65 if let Some(def_id) = def_id.as_local() {
66 let hir_id = self.tcx.hir().as_local_hir_id(def_id);
67 debug_assert!(!should_explore(self.tcx, hir_id));
68 self.live_symbols.insert(hir_id);
72 fn handle_res(&mut self, res: Res) {
74 Res::Def(DefKind::Const | DefKind::AssocConst | DefKind::TyAlias, _) => {
75 self.check_def_id(res.def_id());
77 _ if self.in_pat => {}
78 Res::PrimTy(..) | Res::SelfCtor(..) | Res::Local(..) => {}
79 Res::Def(DefKind::Ctor(CtorOf::Variant, ..), ctor_def_id) => {
80 let variant_id = self.tcx.parent(ctor_def_id).unwrap();
81 let enum_id = self.tcx.parent(variant_id).unwrap();
82 self.check_def_id(enum_id);
83 if !self.ignore_variant_stack.contains(&ctor_def_id) {
84 self.check_def_id(variant_id);
87 Res::Def(DefKind::Variant, variant_id) => {
88 let enum_id = self.tcx.parent(variant_id).unwrap();
89 self.check_def_id(enum_id);
90 if !self.ignore_variant_stack.contains(&variant_id) {
91 self.check_def_id(variant_id);
94 Res::SelfTy(t, i) => {
102 Res::ToolMod | Res::NonMacroAttr(..) | Res::Err => {}
104 self.check_def_id(res.def_id());
109 fn lookup_and_handle_method(&mut self, id: hir::HirId) {
110 if let Some(def_id) = self.tables.type_dependent_def_id(id) {
111 self.check_def_id(def_id);
113 bug!("no type-dependent def for method");
117 fn handle_field_access(&mut self, lhs: &hir::Expr<'_>, hir_id: hir::HirId) {
118 match self.tables.expr_ty_adjusted(lhs).kind {
120 let index = self.tcx.field_index(hir_id, self.tables);
121 self.insert_def_id(def.non_enum_variant().fields[index].did);
124 _ => span_bug!(lhs.span, "named field access on non-ADT"),
128 fn handle_field_pattern_match(
132 pats: &[hir::FieldPat<'_>],
134 let variant = match self.tables.node_type(lhs.hir_id).kind {
135 ty::Adt(adt, _) => adt.variant_of_res(res),
136 _ => span_bug!(lhs.span, "non-ADT in struct pattern"),
139 if let PatKind::Wild = pat.pat.kind {
142 let index = self.tcx.field_index(pat.hir_id, self.tables);
143 self.insert_def_id(variant.fields[index].did);
147 fn mark_live_symbols(&mut self) {
148 let mut scanned = FxHashSet::default();
149 while let Some(id) = self.worklist.pop() {
150 if !scanned.insert(id) {
154 // in the case of tuple struct constructors we want to check the item, not the generated
155 // tuple struct constructor function
156 let id = self.struct_constructors.get(&id).cloned().unwrap_or(id);
158 if let Some(node) = self.tcx.hir().find(id) {
159 self.live_symbols.insert(id);
160 self.visit_node(node);
165 fn visit_node(&mut self, node: Node<'tcx>) {
166 let had_repr_c = self.repr_has_repr_c;
167 self.repr_has_repr_c = false;
168 let had_inherited_pub_visibility = self.inherited_pub_visibility;
169 self.inherited_pub_visibility = false;
171 Node::Item(item) => match item.kind {
172 hir::ItemKind::Struct(..) | hir::ItemKind::Union(..) => {
173 let def_id = self.tcx.hir().local_def_id(item.hir_id);
174 let def = self.tcx.adt_def(def_id);
175 self.repr_has_repr_c = def.repr.c();
177 intravisit::walk_item(self, &item);
179 hir::ItemKind::Enum(..) => {
180 self.inherited_pub_visibility = item.vis.node.is_pub();
182 intravisit::walk_item(self, &item);
184 hir::ItemKind::ForeignMod(..) => {}
186 intravisit::walk_item(self, &item);
189 Node::TraitItem(trait_item) => {
190 intravisit::walk_trait_item(self, trait_item);
192 Node::ImplItem(impl_item) => {
193 intravisit::walk_impl_item(self, impl_item);
195 Node::ForeignItem(foreign_item) => {
196 intravisit::walk_foreign_item(self, &foreign_item);
200 self.repr_has_repr_c = had_repr_c;
201 self.inherited_pub_visibility = had_inherited_pub_visibility;
204 fn mark_as_used_if_union(&mut self, adt: &ty::AdtDef, fields: &[hir::Field<'_>]) {
205 if adt.is_union() && adt.non_enum_variant().fields.len() > 1 && adt.did.is_local() {
206 for field in fields {
207 let index = self.tcx.field_index(field.hir_id, self.tables);
208 self.insert_def_id(adt.non_enum_variant().fields[index].did);
214 impl<'a, 'tcx> Visitor<'tcx> for MarkSymbolVisitor<'a, 'tcx> {
215 type Map = intravisit::ErasedMap<'tcx>;
217 fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<Self::Map> {
218 NestedVisitorMap::None
221 fn visit_nested_body(&mut self, body: hir::BodyId) {
222 let old_tables = self.tables;
223 self.tables = self.tcx.body_tables(body);
224 let body = self.tcx.hir().body(body);
225 self.visit_body(body);
226 self.tables = old_tables;
229 fn visit_variant_data(
231 def: &'tcx hir::VariantData<'tcx>,
233 _: &hir::Generics<'_>,
237 let has_repr_c = self.repr_has_repr_c;
238 let inherited_pub_visibility = self.inherited_pub_visibility;
239 let live_fields = def
242 .filter(|f| has_repr_c || inherited_pub_visibility || f.vis.node.is_pub());
243 self.live_symbols.extend(live_fields.map(|f| f.hir_id));
245 intravisit::walk_struct_def(self, def);
248 fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) {
250 hir::ExprKind::Path(ref qpath @ hir::QPath::TypeRelative(..)) => {
251 let res = self.tables.qpath_res(qpath, expr.hir_id);
252 self.handle_res(res);
254 hir::ExprKind::MethodCall(..) => {
255 self.lookup_and_handle_method(expr.hir_id);
257 hir::ExprKind::Field(ref lhs, ..) => {
258 self.handle_field_access(&lhs, expr.hir_id);
260 hir::ExprKind::Struct(ref qpath, ref fields, _) => {
261 let res = self.tables.qpath_res(qpath, expr.hir_id);
262 self.handle_res(res);
263 if let ty::Adt(ref adt, _) = self.tables.expr_ty(expr).kind {
264 self.mark_as_used_if_union(adt, fields);
270 intravisit::walk_expr(self, expr);
273 fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
274 // Inside the body, ignore constructions of variants
275 // necessary for the pattern to match. Those construction sites
276 // can't be reached unless the variant is constructed elsewhere.
277 let len = self.ignore_variant_stack.len();
278 self.ignore_variant_stack.extend(arm.pat.necessary_variants());
279 intravisit::walk_arm(self, arm);
280 self.ignore_variant_stack.truncate(len);
283 fn visit_pat(&mut self, pat: &'tcx hir::Pat<'tcx>) {
285 PatKind::Struct(ref path, ref fields, _) => {
286 let res = self.tables.qpath_res(path, pat.hir_id);
287 self.handle_field_pattern_match(pat, res, fields);
289 PatKind::Path(ref qpath) => {
290 let res = self.tables.qpath_res(qpath, pat.hir_id);
291 self.handle_res(res);
297 intravisit::walk_pat(self, pat);
301 fn visit_path(&mut self, path: &'tcx hir::Path<'tcx>, _: hir::HirId) {
302 self.handle_res(path.res);
303 intravisit::walk_path(self, path);
306 fn visit_ty(&mut self, ty: &'tcx hir::Ty<'tcx>) {
307 if let TyKind::Def(item_id, _) = ty.kind {
308 let item = self.tcx.hir().expect_item(item_id.id);
309 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::Fn(_, hir::TraitFn::Provided(_)) => {
396 if has_allow_dead_code_or_lang_attr(
401 self.worklist.push(trait_item.hir_id);
408 hir::ItemKind::Impl { ref of_trait, items, .. } => {
409 for impl_item_ref in items {
410 let impl_item = self.krate.impl_item(impl_item_ref.id);
411 if of_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)),
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(did) = item_did.as_local() {
540 let item_hir_id = self.tcx.hir().as_local_hir_id(did);
541 if self.live_symbols.contains(&item_hir_id) {
553 span: rustc_span::Span,
557 if !name.as_str().starts_with('_') {
558 self.tcx.struct_span_lint_hir(lint::builtin::DEAD_CODE, id, span, |lint| {
559 let def_id = self.tcx.hir().local_def_id(id);
560 let descr = self.tcx.def_kind(def_id).descr(def_id.to_def_id());
561 lint.build(&format!("{} is never {}: `{}`", descr, participle, name)).emit()
567 impl Visitor<'tcx> for DeadVisitor<'tcx> {
568 type Map = Map<'tcx>;
570 /// Walk nested items in place so that we don't report dead-code
571 /// on inner functions when the outer function is already getting
572 /// an error. We could do this also by checking the parents, but
573 /// this is how the code is setup and it seems harmless enough.
574 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
575 NestedVisitorMap::All(self.tcx.hir())
578 fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {
579 if self.should_warn_about_item(item) {
580 // For most items, we want to highlight its identifier
581 let span = match item.kind {
582 hir::ItemKind::Fn(..)
583 | hir::ItemKind::Mod(..)
584 | hir::ItemKind::Enum(..)
585 | hir::ItemKind::Struct(..)
586 | hir::ItemKind::Union(..)
587 | hir::ItemKind::Trait(..)
588 | hir::ItemKind::Impl { .. } => {
589 // FIXME(66095): Because item.span is annotated with things
590 // like expansion data, and ident.span isn't, we use the
591 // def_span method if it's part of a macro invocation
592 // (and thus has asource_callee set).
593 // We should probably annotate ident.span with the macro
594 // context, but that's a larger change.
595 if item.span.source_callee().is_some() {
596 self.tcx.sess.source_map().guess_head_span(item.span)
603 let participle = match item.kind {
604 hir::ItemKind::Struct(..) => "constructed", // Issue #52325
607 self.warn_dead_code(item.hir_id, span, item.ident.name, participle);
609 // Only continue if we didn't warn
610 intravisit::walk_item(self, item);
616 variant: &'tcx hir::Variant<'tcx>,
617 g: &'tcx hir::Generics<'tcx>,
620 if self.should_warn_about_variant(&variant) {
621 self.warn_dead_code(variant.id, variant.span, variant.ident.name, "constructed");
623 intravisit::walk_variant(self, variant, g, id);
627 fn visit_foreign_item(&mut self, fi: &'tcx hir::ForeignItem<'tcx>) {
628 if self.should_warn_about_foreign_item(fi) {
629 self.warn_dead_code(fi.hir_id, fi.span, fi.ident.name, "used");
631 intravisit::walk_foreign_item(self, fi);
634 fn visit_struct_field(&mut self, field: &'tcx hir::StructField<'tcx>) {
635 if self.should_warn_about_field(&field) {
636 self.warn_dead_code(field.hir_id, field.span, field.ident.name, "read");
638 intravisit::walk_struct_field(self, field);
641 fn visit_impl_item(&mut self, impl_item: &'tcx hir::ImplItem<'tcx>) {
642 match impl_item.kind {
643 hir::ImplItemKind::Const(_, body_id) => {
644 if !self.symbol_is_live(impl_item.hir_id) {
648 impl_item.ident.name,
652 self.visit_nested_body(body_id)
654 hir::ImplItemKind::Fn(_, body_id) => {
655 if !self.symbol_is_live(impl_item.hir_id) {
656 let span = self.tcx.sess.source_map().guess_head_span(impl_item.span);
657 self.warn_dead_code(impl_item.hir_id, span, impl_item.ident.name, "used");
659 self.visit_nested_body(body_id)
661 hir::ImplItemKind::OpaqueTy(..) | hir::ImplItemKind::TyAlias(..) => {}
665 // Overwrite so that we don't warn the trait item itself.
666 fn visit_trait_item(&mut self, trait_item: &'tcx hir::TraitItem<'tcx>) {
667 match trait_item.kind {
668 hir::TraitItemKind::Const(_, Some(body_id))
669 | hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(body_id)) => {
670 self.visit_nested_body(body_id)
672 hir::TraitItemKind::Const(_, None)
673 | hir::TraitItemKind::Fn(_, hir::TraitFn::Required(_))
674 | hir::TraitItemKind::Type(..) => {}
679 pub fn check_crate(tcx: TyCtxt<'_>) {
680 let access_levels = &tcx.privacy_access_levels(LOCAL_CRATE);
681 let krate = tcx.hir().krate();
682 let live_symbols = find_live(tcx, access_levels, krate);
683 let mut visitor = DeadVisitor { tcx, live_symbols };
684 intravisit::walk_crate(&mut visitor, krate);