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;
19 use rustc_span::symbol::{sym, Symbol};
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<'tcx> {
41 worklist: Vec<hir::HirId>,
43 maybe_typeck_results: Option<&'tcx ty::TypeckResults<'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<'tcx> MarkSymbolVisitor<'tcx> {
54 /// Gets the type-checking results for the current body.
55 /// As this will ICE if called outside bodies, only call when working with
56 /// `Expr` or `Pat` nodes (they are guaranteed to be found only in bodies).
58 fn typeck_results(&self) -> &'tcx ty::TypeckResults<'tcx> {
59 self.maybe_typeck_results
60 .expect("`MarkSymbolVisitor::typeck_results` called outside of body")
63 fn check_def_id(&mut self, def_id: DefId) {
64 if let Some(def_id) = def_id.as_local() {
65 let hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
66 if should_explore(self.tcx, hir_id) || self.struct_constructors.contains_key(&hir_id) {
67 self.worklist.push(hir_id);
69 self.live_symbols.insert(hir_id);
73 fn insert_def_id(&mut self, def_id: DefId) {
74 if let Some(def_id) = def_id.as_local() {
75 let hir_id = self.tcx.hir().local_def_id_to_hir_id(def_id);
76 debug_assert!(!should_explore(self.tcx, hir_id));
77 self.live_symbols.insert(hir_id);
81 fn handle_res(&mut self, res: Res) {
83 Res::Def(DefKind::Const | DefKind::AssocConst | DefKind::TyAlias, _) => {
84 self.check_def_id(res.def_id());
86 _ if self.in_pat => {}
87 Res::PrimTy(..) | Res::SelfCtor(..) | Res::Local(..) => {}
88 Res::Def(DefKind::Ctor(CtorOf::Variant, ..), ctor_def_id) => {
89 let variant_id = self.tcx.parent(ctor_def_id).unwrap();
90 let enum_id = self.tcx.parent(variant_id).unwrap();
91 self.check_def_id(enum_id);
92 if !self.ignore_variant_stack.contains(&ctor_def_id) {
93 self.check_def_id(variant_id);
96 Res::Def(DefKind::Variant, variant_id) => {
97 let enum_id = self.tcx.parent(variant_id).unwrap();
98 self.check_def_id(enum_id);
99 if !self.ignore_variant_stack.contains(&variant_id) {
100 self.check_def_id(variant_id);
103 Res::SelfTy(t, i) => {
105 self.check_def_id(t);
107 if let Some((i, _)) = i {
108 self.check_def_id(i);
111 Res::ToolMod | Res::NonMacroAttr(..) | Res::Err => {}
113 self.check_def_id(res.def_id());
118 fn lookup_and_handle_method(&mut self, id: hir::HirId) {
119 if let Some(def_id) = self.typeck_results().type_dependent_def_id(id) {
120 self.check_def_id(def_id);
122 bug!("no type-dependent def for method");
126 fn handle_field_access(&mut self, lhs: &hir::Expr<'_>, hir_id: hir::HirId) {
127 match self.typeck_results().expr_ty_adjusted(lhs).kind() {
129 let index = self.tcx.field_index(hir_id, self.typeck_results());
130 self.insert_def_id(def.non_enum_variant().fields[index].did);
133 _ => span_bug!(lhs.span, "named field access on non-ADT"),
137 fn handle_field_pattern_match(
141 pats: &[hir::FieldPat<'_>],
143 let variant = match self.typeck_results().node_type(lhs.hir_id).kind() {
144 ty::Adt(adt, _) => adt.variant_of_res(res),
145 _ => span_bug!(lhs.span, "non-ADT in struct pattern"),
148 if let PatKind::Wild = pat.pat.kind {
151 let index = self.tcx.field_index(pat.hir_id, self.typeck_results());
152 self.insert_def_id(variant.fields[index].did);
156 fn mark_live_symbols(&mut self) {
157 let mut scanned = FxHashSet::default();
158 while let Some(id) = self.worklist.pop() {
159 if !scanned.insert(id) {
163 // in the case of tuple struct constructors we want to check the item, not the generated
164 // tuple struct constructor function
165 let id = self.struct_constructors.get(&id).cloned().unwrap_or(id);
167 if let Some(node) = self.tcx.hir().find(id) {
168 self.live_symbols.insert(id);
169 self.visit_node(node);
174 fn visit_node(&mut self, node: Node<'tcx>) {
175 let had_repr_c = self.repr_has_repr_c;
176 self.repr_has_repr_c = false;
177 let had_inherited_pub_visibility = self.inherited_pub_visibility;
178 self.inherited_pub_visibility = false;
180 Node::Item(item) => match item.kind {
181 hir::ItemKind::Struct(..) | hir::ItemKind::Union(..) => {
182 let def_id = self.tcx.hir().local_def_id(item.hir_id);
183 let def = self.tcx.adt_def(def_id);
184 self.repr_has_repr_c = def.repr.c();
186 intravisit::walk_item(self, &item);
188 hir::ItemKind::Enum(..) => {
189 self.inherited_pub_visibility = item.vis.node.is_pub();
191 intravisit::walk_item(self, &item);
193 hir::ItemKind::ForeignMod(..) => {}
195 intravisit::walk_item(self, &item);
198 Node::TraitItem(trait_item) => {
199 intravisit::walk_trait_item(self, trait_item);
201 Node::ImplItem(impl_item) => {
202 intravisit::walk_impl_item(self, impl_item);
204 Node::ForeignItem(foreign_item) => {
205 intravisit::walk_foreign_item(self, &foreign_item);
209 self.repr_has_repr_c = had_repr_c;
210 self.inherited_pub_visibility = had_inherited_pub_visibility;
213 fn mark_as_used_if_union(&mut self, adt: &ty::AdtDef, fields: &[hir::Field<'_>]) {
214 if adt.is_union() && adt.non_enum_variant().fields.len() > 1 && adt.did.is_local() {
215 for field in fields {
216 let index = self.tcx.field_index(field.hir_id, self.typeck_results());
217 self.insert_def_id(adt.non_enum_variant().fields[index].did);
223 impl<'tcx> Visitor<'tcx> for MarkSymbolVisitor<'tcx> {
224 type Map = intravisit::ErasedMap<'tcx>;
226 fn nested_visit_map(&mut self) -> intravisit::NestedVisitorMap<Self::Map> {
227 NestedVisitorMap::None
230 fn visit_nested_body(&mut self, body: hir::BodyId) {
231 let old_maybe_typeck_results =
232 self.maybe_typeck_results.replace(self.tcx.typeck_body(body));
233 let body = self.tcx.hir().body(body);
234 self.visit_body(body);
235 self.maybe_typeck_results = old_maybe_typeck_results;
238 fn visit_variant_data(
240 def: &'tcx hir::VariantData<'tcx>,
242 _: &hir::Generics<'_>,
246 let has_repr_c = self.repr_has_repr_c;
247 let inherited_pub_visibility = self.inherited_pub_visibility;
248 let live_fields = def
251 .filter(|f| has_repr_c || inherited_pub_visibility || f.vis.node.is_pub());
252 self.live_symbols.extend(live_fields.map(|f| f.hir_id));
254 intravisit::walk_struct_def(self, def);
257 fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) {
259 hir::ExprKind::Path(ref qpath @ hir::QPath::TypeRelative(..)) => {
260 let res = self.typeck_results().qpath_res(qpath, expr.hir_id);
261 self.handle_res(res);
263 hir::ExprKind::MethodCall(..) => {
264 self.lookup_and_handle_method(expr.hir_id);
266 hir::ExprKind::Field(ref lhs, ..) => {
267 self.handle_field_access(&lhs, expr.hir_id);
269 hir::ExprKind::Struct(ref qpath, ref fields, _) => {
270 let res = self.typeck_results().qpath_res(qpath, expr.hir_id);
271 self.handle_res(res);
272 if let ty::Adt(ref adt, _) = self.typeck_results().expr_ty(expr).kind() {
273 self.mark_as_used_if_union(adt, fields);
279 intravisit::walk_expr(self, expr);
282 fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) {
283 // Inside the body, ignore constructions of variants
284 // necessary for the pattern to match. Those construction sites
285 // can't be reached unless the variant is constructed elsewhere.
286 let len = self.ignore_variant_stack.len();
287 self.ignore_variant_stack.extend(arm.pat.necessary_variants());
288 intravisit::walk_arm(self, arm);
289 self.ignore_variant_stack.truncate(len);
292 fn visit_pat(&mut self, pat: &'tcx hir::Pat<'tcx>) {
294 PatKind::Struct(ref path, ref fields, _) => {
295 let res = self.typeck_results().qpath_res(path, pat.hir_id);
296 self.handle_field_pattern_match(pat, res, fields);
298 PatKind::Path(ref qpath) => {
299 let res = self.typeck_results().qpath_res(qpath, pat.hir_id);
300 self.handle_res(res);
306 intravisit::walk_pat(self, pat);
310 fn visit_path(&mut self, path: &'tcx hir::Path<'tcx>, _: hir::HirId) {
311 self.handle_res(path.res);
312 intravisit::walk_path(self, path);
315 fn visit_ty(&mut self, ty: &'tcx hir::Ty<'tcx>) {
316 if let TyKind::OpaqueDef(item_id, _) = ty.kind {
317 let item = self.tcx.hir().expect_item(item_id.id);
318 intravisit::walk_item(self, item);
320 intravisit::walk_ty(self, ty);
323 fn visit_anon_const(&mut self, c: &'tcx hir::AnonConst) {
324 self.live_symbols.insert(c.hir_id);
325 intravisit::walk_anon_const(self, c);
329 fn has_allow_dead_code_or_lang_attr(
332 attrs: &[ast::Attribute],
334 if tcx.sess.contains_name(attrs, sym::lang) {
338 // Stable attribute for #[lang = "panic_impl"]
339 if tcx.sess.contains_name(attrs, sym::panic_handler) {
343 // (To be) stable attribute for #[lang = "oom"]
344 if tcx.sess.contains_name(attrs, sym::alloc_error_handler) {
348 let def_id = tcx.hir().local_def_id(id);
349 let cg_attrs = tcx.codegen_fn_attrs(def_id);
351 // #[used], #[no_mangle], #[export_name], etc also keeps the item alive
352 // forcefully, e.g., for placing it in a specific section.
353 if cg_attrs.contains_extern_indicator() || cg_attrs.flags.contains(CodegenFnAttrFlags::USED) {
357 tcx.lint_level_at_node(lint::builtin::DEAD_CODE, id).0 == lint::Allow
360 // This visitor seeds items that
361 // 1) We want to explicitly consider as live:
362 // * Item annotated with #[allow(dead_code)]
363 // - This is done so that if we want to suppress warnings for a
364 // group of dead functions, we only have to annotate the "root".
365 // For example, if both `f` and `g` are dead and `f` calls `g`,
366 // then annotating `f` with `#[allow(dead_code)]` will suppress
367 // warning for both `f` and `g`.
368 // * Item annotated with #[lang=".."]
369 // - This is because lang items are always callable from elsewhere.
371 // 2) We are not sure to be live or not
372 // * Implementations of traits and trait methods
373 struct LifeSeeder<'k, 'tcx> {
374 worklist: Vec<hir::HirId>,
375 krate: &'k hir::Crate<'k>,
377 // see `MarkSymbolVisitor::struct_constructors`
378 struct_constructors: FxHashMap<hir::HirId, hir::HirId>,
381 impl<'v, 'k, 'tcx> ItemLikeVisitor<'v> for LifeSeeder<'k, 'tcx> {
382 fn visit_item(&mut self, item: &hir::Item<'_>) {
383 let allow_dead_code = has_allow_dead_code_or_lang_attr(self.tcx, item.hir_id, &item.attrs);
385 self.worklist.push(item.hir_id);
388 hir::ItemKind::Enum(ref enum_def, _) => {
390 self.worklist.extend(enum_def.variants.iter().map(|variant| variant.id));
393 for variant in enum_def.variants {
394 if let Some(ctor_hir_id) = variant.data.ctor_hir_id() {
395 self.struct_constructors.insert(ctor_hir_id, variant.id);
399 hir::ItemKind::Trait(.., trait_item_refs) => {
400 for trait_item_ref in trait_item_refs {
401 let trait_item = self.krate.trait_item(trait_item_ref.id);
402 match trait_item.kind {
403 hir::TraitItemKind::Const(_, Some(_))
404 | hir::TraitItemKind::Fn(_, hir::TraitFn::Provided(_)) => {
405 if has_allow_dead_code_or_lang_attr(
410 self.worklist.push(trait_item.hir_id);
417 hir::ItemKind::Impl { ref of_trait, items, .. } => {
418 if of_trait.is_some() {
419 self.worklist.push(item.hir_id);
421 for impl_item_ref in items {
422 let impl_item = self.krate.impl_item(impl_item_ref.id);
423 if of_trait.is_some()
424 || has_allow_dead_code_or_lang_attr(
430 self.worklist.push(impl_item_ref.id.hir_id);
434 hir::ItemKind::Struct(ref variant_data, _) => {
435 if let Some(ctor_hir_id) = variant_data.ctor_hir_id() {
436 self.struct_constructors.insert(ctor_hir_id, item.hir_id);
443 fn visit_trait_item(&mut self, _item: &hir::TraitItem<'_>) {
444 // ignore: we are handling this in `visit_item` above
447 fn visit_impl_item(&mut self, _item: &hir::ImplItem<'_>) {
448 // ignore: we are handling this in `visit_item` above
452 fn create_and_seed_worklist<'tcx>(
454 access_levels: &privacy::AccessLevels,
455 krate: &hir::Crate<'_>,
456 ) -> (Vec<hir::HirId>, FxHashMap<hir::HirId, hir::HirId>) {
457 let worklist = access_levels
462 if level >= privacy::AccessLevel::Reachable { Some(id) } else { None }
467 tcx.entry_fn(LOCAL_CRATE).map(|(def_id, _)| tcx.hir().local_def_id_to_hir_id(def_id)),
469 .collect::<Vec<_>>();
471 // Seed implemented trait items
472 let mut life_seeder =
473 LifeSeeder { worklist, krate, tcx, struct_constructors: Default::default() };
474 krate.visit_all_item_likes(&mut life_seeder);
476 (life_seeder.worklist, life_seeder.struct_constructors)
481 access_levels: &privacy::AccessLevels,
482 krate: &hir::Crate<'_>,
483 ) -> FxHashSet<hir::HirId> {
484 let (worklist, struct_constructors) = create_and_seed_worklist(tcx, access_levels, krate);
485 let mut symbol_visitor = MarkSymbolVisitor {
488 maybe_typeck_results: None,
489 live_symbols: Default::default(),
490 repr_has_repr_c: false,
492 inherited_pub_visibility: false,
493 ignore_variant_stack: vec![],
496 symbol_visitor.mark_live_symbols();
497 symbol_visitor.live_symbols
500 struct DeadVisitor<'tcx> {
502 live_symbols: FxHashSet<hir::HirId>,
505 impl DeadVisitor<'tcx> {
506 fn should_warn_about_item(&mut self, item: &hir::Item<'_>) -> bool {
507 let should_warn = match item.kind {
508 hir::ItemKind::Static(..)
509 | hir::ItemKind::Const(..)
510 | hir::ItemKind::Fn(..)
511 | hir::ItemKind::TyAlias(..)
512 | hir::ItemKind::Enum(..)
513 | hir::ItemKind::Struct(..)
514 | hir::ItemKind::Union(..) => true,
517 should_warn && !self.symbol_is_live(item.hir_id)
520 fn should_warn_about_field(&mut self, field: &hir::StructField<'_>) -> bool {
521 let field_type = self.tcx.type_of(self.tcx.hir().local_def_id(field.hir_id));
522 !field.is_positional()
523 && !self.symbol_is_live(field.hir_id)
524 && !field_type.is_phantom_data()
525 && !has_allow_dead_code_or_lang_attr(self.tcx, field.hir_id, &field.attrs)
528 fn should_warn_about_variant(&mut self, variant: &hir::Variant<'_>) -> bool {
529 !self.symbol_is_live(variant.id)
530 && !has_allow_dead_code_or_lang_attr(self.tcx, variant.id, &variant.attrs)
533 fn should_warn_about_foreign_item(&mut self, fi: &hir::ForeignItem<'_>) -> bool {
534 !self.symbol_is_live(fi.hir_id)
535 && !has_allow_dead_code_or_lang_attr(self.tcx, fi.hir_id, &fi.attrs)
538 // id := HIR id of an item's definition.
539 fn symbol_is_live(&mut self, id: hir::HirId) -> bool {
540 if self.live_symbols.contains(&id) {
543 // If it's a type whose items are live, then it's live, too.
544 // This is done to handle the case where, for example, the static
545 // method of a private type is used, but the type itself is never
547 let def_id = self.tcx.hir().local_def_id(id);
548 let inherent_impls = self.tcx.inherent_impls(def_id);
549 for &impl_did in inherent_impls.iter() {
550 for &item_did in &self.tcx.associated_item_def_ids(impl_did)[..] {
551 if let Some(did) = item_did.as_local() {
552 let item_hir_id = self.tcx.hir().local_def_id_to_hir_id(did);
553 if self.live_symbols.contains(&item_hir_id) {
565 span: rustc_span::Span,
569 if !name.as_str().starts_with('_') {
570 self.tcx.struct_span_lint_hir(lint::builtin::DEAD_CODE, id, span, |lint| {
571 let def_id = self.tcx.hir().local_def_id(id);
572 let descr = self.tcx.def_kind(def_id).descr(def_id.to_def_id());
573 lint.build(&format!("{} is never {}: `{}`", descr, participle, name)).emit()
579 impl Visitor<'tcx> for DeadVisitor<'tcx> {
580 type Map = Map<'tcx>;
582 /// Walk nested items in place so that we don't report dead-code
583 /// on inner functions when the outer function is already getting
584 /// an error. We could do this also by checking the parents, but
585 /// this is how the code is setup and it seems harmless enough.
586 fn nested_visit_map(&mut self) -> NestedVisitorMap<Self::Map> {
587 NestedVisitorMap::All(self.tcx.hir())
590 fn visit_item(&mut self, item: &'tcx hir::Item<'tcx>) {
591 if self.should_warn_about_item(item) {
592 // For most items, we want to highlight its identifier
593 let span = match item.kind {
594 hir::ItemKind::Fn(..)
595 | hir::ItemKind::Mod(..)
596 | hir::ItemKind::Enum(..)
597 | hir::ItemKind::Struct(..)
598 | hir::ItemKind::Union(..)
599 | hir::ItemKind::Trait(..)
600 | hir::ItemKind::Impl { .. } => {
601 // FIXME(66095): Because item.span is annotated with things
602 // like expansion data, and ident.span isn't, we use the
603 // def_span method if it's part of a macro invocation
604 // (and thus has a source_callee set).
605 // We should probably annotate ident.span with the macro
606 // context, but that's a larger change.
607 if item.span.source_callee().is_some() {
608 self.tcx.sess.source_map().guess_head_span(item.span)
615 let participle = match item.kind {
616 hir::ItemKind::Struct(..) => "constructed", // Issue #52325
619 self.warn_dead_code(item.hir_id, span, item.ident.name, participle);
621 // Only continue if we didn't warn
622 intravisit::walk_item(self, item);
628 variant: &'tcx hir::Variant<'tcx>,
629 g: &'tcx hir::Generics<'tcx>,
632 if self.should_warn_about_variant(&variant) {
633 self.warn_dead_code(variant.id, variant.span, variant.ident.name, "constructed");
635 intravisit::walk_variant(self, variant, g, id);
639 fn visit_foreign_item(&mut self, fi: &'tcx hir::ForeignItem<'tcx>) {
640 if self.should_warn_about_foreign_item(fi) {
641 self.warn_dead_code(fi.hir_id, fi.span, fi.ident.name, "used");
643 intravisit::walk_foreign_item(self, fi);
646 fn visit_struct_field(&mut self, field: &'tcx hir::StructField<'tcx>) {
647 if self.should_warn_about_field(&field) {
648 self.warn_dead_code(field.hir_id, field.span, field.ident.name, "read");
650 intravisit::walk_struct_field(self, field);
653 fn visit_impl_item(&mut self, impl_item: &'tcx hir::ImplItem<'tcx>) {
654 match impl_item.kind {
655 hir::ImplItemKind::Const(_, body_id) => {
656 if !self.symbol_is_live(impl_item.hir_id) {
660 impl_item.ident.name,
664 self.visit_nested_body(body_id)
666 hir::ImplItemKind::Fn(_, body_id) => {
667 if !self.symbol_is_live(impl_item.hir_id) {
668 // FIXME(66095): Because impl_item.span is annotated with things
669 // like expansion data, and ident.span isn't, we use the
670 // def_span method if it's part of a macro invocation
671 // (and thus has a source_callee set).
672 // We should probably annotate ident.span with the macro
673 // context, but that's a larger change.
674 let span = if impl_item.span.source_callee().is_some() {
675 self.tcx.sess.source_map().guess_head_span(impl_item.span)
679 self.warn_dead_code(impl_item.hir_id, span, impl_item.ident.name, "used");
681 self.visit_nested_body(body_id)
683 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::Fn(_, hir::TraitFn::Provided(body_id)) => {
692 self.visit_nested_body(body_id)
694 hir::TraitItemKind::Const(_, None)
695 | hir::TraitItemKind::Fn(_, hir::TraitFn::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);