// This implements the dead-code warning pass. // All reachable symbols are live, code called from live code is live, code with certain lint // expectations such as `#[expect(unused)]` and `#[expect(dead_code)]` is live, and everything else // is dead. use std::mem; use hir::ItemKind; use hir::def_id::{LocalDefIdMap, LocalDefIdSet}; use rustc_abi::FieldIdx; use rustc_data_structures::unord::UnordSet; use rustc_errors::MultiSpan; use rustc_hir::def::{CtorOf, DefKind, Res}; use rustc_hir::def_id::{DefId, LocalDefId, LocalModDefId}; use rustc_hir::intravisit::{self, Visitor}; use rustc_hir::{self as hir, Node, PatKind, TyKind}; use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags; use rustc_middle::middle::privacy::Level; use rustc_middle::query::Providers; use rustc_middle::ty::{self, TyCtxt}; use rustc_middle::{bug, span_bug}; use rustc_session::lint; use rustc_session::lint::builtin::DEAD_CODE; use rustc_span::{Symbol, sym}; use crate::errors::{ ChangeFields, IgnoredDerivedImpls, MultipleDeadCodes, ParentInfo, UselessAssignment, }; // Any local node that may call something in its body block should be // explored. For example, if it's a live Node::Item that is a // function, then we should explore its block to check for codes that // may need to be marked as live. fn should_explore(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool { matches!( tcx.hir_node_by_def_id(def_id), Node::Item(..) | Node::ImplItem(..) | Node::ForeignItem(..) | Node::TraitItem(..) | Node::Variant(..) | Node::AnonConst(..) | Node::OpaqueTy(..) ) } fn ty_ref_to_pub_struct(tcx: TyCtxt<'_>, ty: &hir::Ty<'_>) -> bool { if let TyKind::Path(hir::QPath::Resolved(_, path)) = ty.kind && let Res::Def(def_kind, def_id) = path.res && def_id.is_local() && matches!(def_kind, DefKind::Struct | DefKind::Enum | DefKind::Union) { tcx.visibility(def_id).is_public() } else { true } } /// Determine if a work from the worklist is coming from a `#[allow]` /// or a `#[expect]` of `dead_code` #[derive(Debug, Copy, Clone, Eq, PartialEq, Hash)] enum ComesFromAllowExpect { Yes, No, } struct MarkSymbolVisitor<'tcx> { worklist: Vec<(LocalDefId, ComesFromAllowExpect)>, tcx: TyCtxt<'tcx>, maybe_typeck_results: Option<&'tcx ty::TypeckResults<'tcx>>, live_symbols: LocalDefIdSet, repr_unconditionally_treats_fields_as_live: bool, repr_has_repr_simd: bool, in_pat: bool, ignore_variant_stack: Vec, // maps from tuple struct constructors to tuple struct items struct_constructors: LocalDefIdMap, // maps from ADTs to ignored derived traits (e.g. Debug and Clone) // and the span of their respective impl (i.e., part of the derive // macro) ignored_derived_traits: LocalDefIdMap>, } impl<'tcx> MarkSymbolVisitor<'tcx> { /// Gets the type-checking results for the current body. /// As this will ICE if called outside bodies, only call when working with /// `Expr` or `Pat` nodes (they are guaranteed to be found only in bodies). #[track_caller] fn typeck_results(&self) -> &'tcx ty::TypeckResults<'tcx> { self.maybe_typeck_results .expect("`MarkSymbolVisitor::typeck_results` called outside of body") } fn check_def_id(&mut self, def_id: DefId) { if let Some(def_id) = def_id.as_local() { if should_explore(self.tcx, def_id) || self.struct_constructors.contains_key(&def_id) { self.worklist.push((def_id, ComesFromAllowExpect::No)); } self.live_symbols.insert(def_id); } } fn insert_def_id(&mut self, def_id: DefId) { if let Some(def_id) = def_id.as_local() { debug_assert!(!should_explore(self.tcx, def_id)); self.live_symbols.insert(def_id); } } fn handle_res(&mut self, res: Res) { match res { Res::Def(DefKind::Const | DefKind::AssocConst | DefKind::TyAlias, def_id) => { self.check_def_id(def_id); } _ if self.in_pat => {} Res::PrimTy(..) | Res::SelfCtor(..) | Res::Local(..) => {} Res::Def(DefKind::Ctor(CtorOf::Variant, ..), ctor_def_id) => { let variant_id = self.tcx.parent(ctor_def_id); let enum_id = self.tcx.parent(variant_id); self.check_def_id(enum_id); if !self.ignore_variant_stack.contains(&ctor_def_id) { self.check_def_id(variant_id); } } Res::Def(DefKind::Variant, variant_id) => { let enum_id = self.tcx.parent(variant_id); self.check_def_id(enum_id); if !self.ignore_variant_stack.contains(&variant_id) { self.check_def_id(variant_id); } } Res::Def(_, def_id) => self.check_def_id(def_id), Res::SelfTyParam { trait_: t } => self.check_def_id(t), Res::SelfTyAlias { alias_to: i, .. } => self.check_def_id(i), Res::ToolMod | Res::NonMacroAttr(..) | Res::Err => {} } } fn lookup_and_handle_method(&mut self, id: hir::HirId) { if let Some(def_id) = self.typeck_results().type_dependent_def_id(id) { self.check_def_id(def_id); } else { assert!( self.typeck_results().tainted_by_errors.is_some(), "no type-dependent def for method" ); } } fn handle_field_access(&mut self, lhs: &hir::Expr<'_>, hir_id: hir::HirId) { match self.typeck_results().expr_ty_adjusted(lhs).kind() { ty::Adt(def, _) => { let index = self.typeck_results().field_index(hir_id); self.insert_def_id(def.non_enum_variant().fields[index].did); } ty::Tuple(..) => {} ty::Error(_) => {} kind => span_bug!(lhs.span, "named field access on non-ADT: {kind:?}"), } } #[allow(dead_code)] // FIXME(81658): should be used + lint reinstated after #83171 relands. fn handle_assign(&mut self, expr: &'tcx hir::Expr<'tcx>) { if self .typeck_results() .expr_adjustments(expr) .iter() .any(|adj| matches!(adj.kind, ty::adjustment::Adjust::Deref(_))) { self.visit_expr(expr); } else if let hir::ExprKind::Field(base, ..) = expr.kind { // Ignore write to field self.handle_assign(base); } else { self.visit_expr(expr); } } #[allow(dead_code)] // FIXME(81658): should be used + lint reinstated after #83171 relands. fn check_for_self_assign(&mut self, assign: &'tcx hir::Expr<'tcx>) { fn check_for_self_assign_helper<'tcx>( typeck_results: &'tcx ty::TypeckResults<'tcx>, lhs: &'tcx hir::Expr<'tcx>, rhs: &'tcx hir::Expr<'tcx>, ) -> bool { match (&lhs.kind, &rhs.kind) { (hir::ExprKind::Path(qpath_l), hir::ExprKind::Path(qpath_r)) => { if let (Res::Local(id_l), Res::Local(id_r)) = ( typeck_results.qpath_res(qpath_l, lhs.hir_id), typeck_results.qpath_res(qpath_r, rhs.hir_id), ) { if id_l == id_r { return true; } } return false; } (hir::ExprKind::Field(lhs_l, ident_l), hir::ExprKind::Field(lhs_r, ident_r)) => { if ident_l == ident_r { return check_for_self_assign_helper(typeck_results, lhs_l, lhs_r); } return false; } _ => { return false; } } } if let hir::ExprKind::Assign(lhs, rhs, _) = assign.kind && check_for_self_assign_helper(self.typeck_results(), lhs, rhs) && !assign.span.from_expansion() { let is_field_assign = matches!(lhs.kind, hir::ExprKind::Field(..)); self.tcx.emit_node_span_lint( lint::builtin::DEAD_CODE, assign.hir_id, assign.span, UselessAssignment { is_field_assign, ty: self.typeck_results().expr_ty(lhs) }, ) } } fn handle_field_pattern_match( &mut self, lhs: &hir::Pat<'_>, res: Res, pats: &[hir::PatField<'_>], ) { let variant = match self.typeck_results().node_type(lhs.hir_id).kind() { ty::Adt(adt, _) => adt.variant_of_res(res), _ => span_bug!(lhs.span, "non-ADT in struct pattern"), }; for pat in pats { if let PatKind::Wild = pat.pat.kind { continue; } let index = self.typeck_results().field_index(pat.hir_id); self.insert_def_id(variant.fields[index].did); } } fn handle_tuple_field_pattern_match( &mut self, lhs: &hir::Pat<'_>, res: Res, pats: &[hir::Pat<'_>], dotdot: hir::DotDotPos, ) { let variant = match self.typeck_results().node_type(lhs.hir_id).kind() { ty::Adt(adt, _) => adt.variant_of_res(res), _ => { self.tcx.dcx().span_delayed_bug(lhs.span, "non-ADT in tuple struct pattern"); return; } }; let dotdot = dotdot.as_opt_usize().unwrap_or(pats.len()); let first_n = pats.iter().enumerate().take(dotdot); let missing = variant.fields.len() - pats.len(); let last_n = pats.iter().enumerate().skip(dotdot).map(|(idx, pat)| (idx + missing, pat)); for (idx, pat) in first_n.chain(last_n) { if let PatKind::Wild = pat.kind { continue; } self.insert_def_id(variant.fields[FieldIdx::from_usize(idx)].did); } } fn handle_offset_of(&mut self, expr: &'tcx hir::Expr<'tcx>) { let data = self.typeck_results().offset_of_data(); let &(container, ref indices) = data.get(expr.hir_id).expect("no offset_of_data for offset_of"); let body_did = self.typeck_results().hir_owner.to_def_id(); let typing_env = ty::TypingEnv::non_body_analysis(self.tcx, body_did); let mut current_ty = container; for &(variant, field) in indices { match current_ty.kind() { ty::Adt(def, args) => { let field = &def.variant(variant).fields[field]; self.insert_def_id(field.did); let field_ty = field.ty(self.tcx, args); current_ty = self.tcx.normalize_erasing_regions(typing_env, field_ty); } // we don't need to mark tuple fields as live, // but we may need to mark subfields ty::Tuple(tys) => { current_ty = self.tcx.normalize_erasing_regions(typing_env, tys[field.as_usize()]); } _ => span_bug!(expr.span, "named field access on non-ADT"), } } } fn mark_live_symbols(&mut self) { let mut scanned = UnordSet::default(); while let Some(work) = self.worklist.pop() { if !scanned.insert(work) { continue; } let (id, comes_from_allow_expect) = work; // Avoid accessing the HIR for the synthesized associated type generated for RPITITs. if self.tcx.is_impl_trait_in_trait(id.to_def_id()) { self.live_symbols.insert(id); continue; } // in the case of tuple struct constructors we want to check the item, not the generated // tuple struct constructor function let id = self.struct_constructors.get(&id).copied().unwrap_or(id); // When using `#[allow]` or `#[expect]` of `dead_code`, we do a QOL improvement // by declaring fn calls, statics, ... within said items as live, as well as // the item itself, although technically this is not the case. // // This means that the lint for said items will never be fired. // // This doesn't make any difference for the item declared with `#[allow]`, as // the lint firing will be a nop, as it will be silenced by the `#[allow]` of // the item. // // However, for `#[expect]`, the presence or absence of the lint is relevant, // so we don't add it to the list of live symbols when it comes from a // `#[expect]`. This means that we will correctly report an item as live or not // for the `#[expect]` case. // // Note that an item can and will be duplicated on the worklist with different // `ComesFromAllowExpect`, particularly if it was added from the // `effective_visibilities` query or from the `#[allow]`/`#[expect]` checks, // this "duplication" is essential as otherwise a function with `#[expect]` // called from a `pub fn` may be falsely reported as not live, falsely // triggering the `unfulfilled_lint_expectations` lint. if comes_from_allow_expect != ComesFromAllowExpect::Yes { self.live_symbols.insert(id); } self.visit_node(self.tcx.hir_node_by_def_id(id)); } } /// Automatically generated items marked with `rustc_trivial_field_reads` /// will be ignored for the purposes of dead code analysis (see PR #85200 /// for discussion). fn should_ignore_item(&mut self, def_id: DefId) -> bool { if let Some(impl_of) = self.tcx.impl_of_method(def_id) { if !self.tcx.is_automatically_derived(impl_of) { return false; } // don't ignore impls for Enums and pub Structs whose methods don't have self receiver, // cause external crate may call such methods to construct values of these types if let Some(local_impl_of) = impl_of.as_local() && let Some(local_def_id) = def_id.as_local() && let Some(fn_sig) = self.tcx.hir_fn_sig_by_hir_id(self.tcx.local_def_id_to_hir_id(local_def_id)) && matches!(fn_sig.decl.implicit_self, hir::ImplicitSelfKind::None) && let TyKind::Path(hir::QPath::Resolved(_, path)) = self.tcx.hir().expect_item(local_impl_of).expect_impl().self_ty.kind && let Res::Def(def_kind, did) = path.res { match def_kind { // for example, #[derive(Default)] pub struct T(i32); // external crate can call T::default() to construct T, // so that don't ignore impl Default for pub Enum and Structs DefKind::Struct | DefKind::Union if self.tcx.visibility(did).is_public() => { return false; } // don't ignore impl Default for Enums, // cause we don't know which variant is constructed DefKind::Enum => return false, _ => (), }; } if let Some(trait_of) = self.tcx.trait_id_of_impl(impl_of) && self.tcx.has_attr(trait_of, sym::rustc_trivial_field_reads) { let trait_ref = self.tcx.impl_trait_ref(impl_of).unwrap().instantiate_identity(); if let ty::Adt(adt_def, _) = trait_ref.self_ty().kind() && let Some(adt_def_id) = adt_def.did().as_local() { self.ignored_derived_traits .entry(adt_def_id) .or_default() .push((trait_of, impl_of)); } return true; } } false } fn visit_node(&mut self, node: Node<'tcx>) { if let Node::ImplItem(hir::ImplItem { owner_id, .. }) = node && self.should_ignore_item(owner_id.to_def_id()) { return; } let unconditionally_treated_fields_as_live = self.repr_unconditionally_treats_fields_as_live; let had_repr_simd = self.repr_has_repr_simd; self.repr_unconditionally_treats_fields_as_live = false; self.repr_has_repr_simd = false; match node { Node::Item(item) => match item.kind { hir::ItemKind::Struct(..) | hir::ItemKind::Union(..) => { let def = self.tcx.adt_def(item.owner_id); self.repr_unconditionally_treats_fields_as_live = def.repr().c() || def.repr().transparent(); self.repr_has_repr_simd = def.repr().simd(); intravisit::walk_item(self, item) } hir::ItemKind::ForeignMod { .. } => {} hir::ItemKind::Trait(..) => { for impl_def_id in self.tcx.all_impls(item.owner_id.to_def_id()) { if let Some(local_def_id) = impl_def_id.as_local() && let ItemKind::Impl(impl_ref) = self.tcx.hir().expect_item(local_def_id).kind { // skip items // mark dependent traits live intravisit::walk_generics(self, impl_ref.generics); // mark dependent parameters live intravisit::walk_path(self, impl_ref.of_trait.unwrap().path); } } intravisit::walk_item(self, item) } _ => intravisit::walk_item(self, item), }, Node::TraitItem(trait_item) => { // mark corresponding ImplTerm live let trait_item_id = trait_item.owner_id.to_def_id(); if let Some(trait_id) = self.tcx.trait_of_item(trait_item_id) { // mark the trait live self.check_def_id(trait_id); for impl_id in self.tcx.all_impls(trait_id) { if let Some(local_impl_id) = impl_id.as_local() && let ItemKind::Impl(impl_ref) = self.tcx.hir().expect_item(local_impl_id).kind { if !matches!(trait_item.kind, hir::TraitItemKind::Type(..)) && !ty_ref_to_pub_struct(self.tcx, impl_ref.self_ty) { // skip methods of private ty, // they would be solved in `solve_rest_impl_items` continue; } // mark self_ty live intravisit::walk_unambig_ty(self, impl_ref.self_ty); if let Some(&impl_item_id) = self.tcx.impl_item_implementor_ids(impl_id).get(&trait_item_id) { self.check_def_id(impl_item_id); } } } } intravisit::walk_trait_item(self, trait_item); } Node::ImplItem(impl_item) => { let item = self.tcx.local_parent(impl_item.owner_id.def_id); if self.tcx.impl_trait_ref(item).is_none() { //// If it's a type whose items are live, then it's live, too. //// This is done to handle the case where, for example, the static //// method of a private type is used, but the type itself is never //// called directly. let self_ty = self.tcx.type_of(item).instantiate_identity(); match *self_ty.kind() { ty::Adt(def, _) => self.check_def_id(def.did()), ty::Foreign(did) => self.check_def_id(did), ty::Dynamic(data, ..) => { if let Some(def_id) = data.principal_def_id() { self.check_def_id(def_id) } } _ => {} } } intravisit::walk_impl_item(self, impl_item); } Node::ForeignItem(foreign_item) => { intravisit::walk_foreign_item(self, foreign_item); } Node::OpaqueTy(opaq) => intravisit::walk_opaque_ty(self, opaq), _ => {} } self.repr_has_repr_simd = had_repr_simd; self.repr_unconditionally_treats_fields_as_live = unconditionally_treated_fields_as_live; } fn mark_as_used_if_union(&mut self, adt: ty::AdtDef<'tcx>, fields: &[hir::ExprField<'_>]) { if adt.is_union() && adt.non_enum_variant().fields.len() > 1 && adt.did().is_local() { for field in fields { let index = self.typeck_results().field_index(field.hir_id); self.insert_def_id(adt.non_enum_variant().fields[index].did); } } } fn solve_rest_impl_items(&mut self, mut unsolved_impl_items: Vec<(hir::ItemId, LocalDefId)>) { let mut ready; (ready, unsolved_impl_items) = unsolved_impl_items.into_iter().partition(|&(impl_id, impl_item_id)| { self.impl_item_with_used_self(impl_id, impl_item_id) }); while !ready.is_empty() { self.worklist = ready.into_iter().map(|(_, id)| (id, ComesFromAllowExpect::No)).collect(); self.mark_live_symbols(); (ready, unsolved_impl_items) = unsolved_impl_items.into_iter().partition(|&(impl_id, impl_item_id)| { self.impl_item_with_used_self(impl_id, impl_item_id) }); } } fn impl_item_with_used_self(&mut self, impl_id: hir::ItemId, impl_item_id: LocalDefId) -> bool { if let TyKind::Path(hir::QPath::Resolved(_, path)) = self.tcx.hir_item(impl_id).expect_impl().self_ty.kind && let Res::Def(def_kind, def_id) = path.res && let Some(local_def_id) = def_id.as_local() && matches!(def_kind, DefKind::Struct | DefKind::Enum | DefKind::Union) { if self.tcx.visibility(impl_item_id).is_public() { // for the public method, we don't know the trait item is used or not, // so we mark the method live if the self is used return self.live_symbols.contains(&local_def_id); } if let Some(trait_item_id) = self.tcx.associated_item(impl_item_id).trait_item_def_id && let Some(local_id) = trait_item_id.as_local() { // for the private method, we can know the trait item is used or not, // so we mark the method live if the self is used and the trait item is used return self.live_symbols.contains(&local_id) && self.live_symbols.contains(&local_def_id); } } false } } impl<'tcx> Visitor<'tcx> for MarkSymbolVisitor<'tcx> { fn visit_nested_body(&mut self, body: hir::BodyId) { let old_maybe_typeck_results = self.maybe_typeck_results.replace(self.tcx.typeck_body(body)); let body = self.tcx.hir_body(body); self.visit_body(body); self.maybe_typeck_results = old_maybe_typeck_results; } fn visit_variant_data(&mut self, def: &'tcx hir::VariantData<'tcx>) { let tcx = self.tcx; let unconditionally_treat_fields_as_live = self.repr_unconditionally_treats_fields_as_live; let has_repr_simd = self.repr_has_repr_simd; let effective_visibilities = &tcx.effective_visibilities(()); let live_fields = def.fields().iter().filter_map(|f| { let def_id = f.def_id; if unconditionally_treat_fields_as_live || (f.is_positional() && has_repr_simd) { return Some(def_id); } if !effective_visibilities.is_reachable(f.hir_id.owner.def_id) { return None; } if effective_visibilities.is_reachable(def_id) { Some(def_id) } else { None } }); self.live_symbols.extend(live_fields); intravisit::walk_struct_def(self, def); } fn visit_expr(&mut self, expr: &'tcx hir::Expr<'tcx>) { match expr.kind { hir::ExprKind::Path(ref qpath @ hir::QPath::TypeRelative(..)) => { let res = self.typeck_results().qpath_res(qpath, expr.hir_id); self.handle_res(res); } hir::ExprKind::MethodCall(..) => { self.lookup_and_handle_method(expr.hir_id); } hir::ExprKind::Field(ref lhs, ..) => { if self.typeck_results().opt_field_index(expr.hir_id).is_some() { self.handle_field_access(lhs, expr.hir_id); } else { self.tcx.dcx().span_delayed_bug(expr.span, "couldn't resolve index for field"); } } hir::ExprKind::Struct(qpath, fields, _) => { let res = self.typeck_results().qpath_res(qpath, expr.hir_id); self.handle_res(res); if let ty::Adt(adt, _) = self.typeck_results().expr_ty(expr).kind() { self.mark_as_used_if_union(*adt, fields); } } hir::ExprKind::Closure(cls) => { self.insert_def_id(cls.def_id.to_def_id()); } hir::ExprKind::OffsetOf(..) => { self.handle_offset_of(expr); } _ => (), } intravisit::walk_expr(self, expr); } fn visit_arm(&mut self, arm: &'tcx hir::Arm<'tcx>) { // Inside the body, ignore constructions of variants // necessary for the pattern to match. Those construction sites // can't be reached unless the variant is constructed elsewhere. let len = self.ignore_variant_stack.len(); self.ignore_variant_stack.extend(arm.pat.necessary_variants()); intravisit::walk_arm(self, arm); self.ignore_variant_stack.truncate(len); } fn visit_pat(&mut self, pat: &'tcx hir::Pat<'tcx>) { self.in_pat = true; match pat.kind { PatKind::Struct(ref path, fields, _) => { let res = self.typeck_results().qpath_res(path, pat.hir_id); self.handle_field_pattern_match(pat, res, fields); } PatKind::TupleStruct(ref qpath, fields, dotdot) => { let res = self.typeck_results().qpath_res(qpath, pat.hir_id); self.handle_tuple_field_pattern_match(pat, res, fields, dotdot); } _ => (), } intravisit::walk_pat(self, pat); self.in_pat = false; } fn visit_pat_expr(&mut self, expr: &'tcx rustc_hir::PatExpr<'tcx>) { match &expr.kind { rustc_hir::PatExprKind::Path(qpath) => { let res = self.typeck_results().qpath_res(qpath, expr.hir_id); self.handle_res(res); } _ => {} } intravisit::walk_pat_expr(self, expr); } fn visit_path(&mut self, path: &hir::Path<'tcx>, _: hir::HirId) { self.handle_res(path.res); intravisit::walk_path(self, path); } fn visit_anon_const(&mut self, c: &'tcx hir::AnonConst) { // When inline const blocks are used in pattern position, paths // referenced by it should be considered as used. let in_pat = mem::replace(&mut self.in_pat, false); self.live_symbols.insert(c.def_id); intravisit::walk_anon_const(self, c); self.in_pat = in_pat; } fn visit_inline_const(&mut self, c: &'tcx hir::ConstBlock) { // When inline const blocks are used in pattern position, paths // referenced by it should be considered as used. let in_pat = mem::replace(&mut self.in_pat, false); self.live_symbols.insert(c.def_id); intravisit::walk_inline_const(self, c); self.in_pat = in_pat; } } fn has_allow_dead_code_or_lang_attr( tcx: TyCtxt<'_>, def_id: LocalDefId, ) -> Option { fn has_lang_attr(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool { tcx.has_attr(def_id, sym::lang) // Stable attribute for #[lang = "panic_impl"] || tcx.has_attr(def_id, sym::panic_handler) } fn has_allow_expect_dead_code(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool { let hir_id = tcx.local_def_id_to_hir_id(def_id); let lint_level = tcx.lint_level_at_node(lint::builtin::DEAD_CODE, hir_id).0; matches!(lint_level, lint::Allow | lint::Expect(_)) } fn has_used_like_attr(tcx: TyCtxt<'_>, def_id: LocalDefId) -> bool { tcx.def_kind(def_id).has_codegen_attrs() && { let cg_attrs = tcx.codegen_fn_attrs(def_id); // #[used], #[no_mangle], #[export_name], etc also keeps the item alive // forcefully, e.g., for placing it in a specific section. cg_attrs.contains_extern_indicator() || cg_attrs.flags.contains(CodegenFnAttrFlags::USED) || cg_attrs.flags.contains(CodegenFnAttrFlags::USED_LINKER) } } if has_allow_expect_dead_code(tcx, def_id) { Some(ComesFromAllowExpect::Yes) } else if has_used_like_attr(tcx, def_id) || has_lang_attr(tcx, def_id) { Some(ComesFromAllowExpect::No) } else { None } } // These check_* functions seeds items that // 1) We want to explicitly consider as live: // * Item annotated with #[allow(dead_code)] // - This is done so that if we want to suppress warnings for a // group of dead functions, we only have to annotate the "root". // For example, if both `f` and `g` are dead and `f` calls `g`, // then annotating `f` with `#[allow(dead_code)]` will suppress // warning for both `f` and `g`. // * Item annotated with #[lang=".."] // - This is because lang items are always callable from elsewhere. // or // 2) We are not sure to be live or not // * Implementations of traits and trait methods fn check_item<'tcx>( tcx: TyCtxt<'tcx>, worklist: &mut Vec<(LocalDefId, ComesFromAllowExpect)>, struct_constructors: &mut LocalDefIdMap, unsolved_impl_items: &mut Vec<(hir::ItemId, LocalDefId)>, id: hir::ItemId, ) { let allow_dead_code = has_allow_dead_code_or_lang_attr(tcx, id.owner_id.def_id); if let Some(comes_from_allow) = allow_dead_code { worklist.push((id.owner_id.def_id, comes_from_allow)); } match tcx.def_kind(id.owner_id) { DefKind::Enum => { let item = tcx.hir_item(id); if let hir::ItemKind::Enum(ref enum_def, _) = item.kind { if let Some(comes_from_allow) = allow_dead_code { worklist.extend( enum_def.variants.iter().map(|variant| (variant.def_id, comes_from_allow)), ); } for variant in enum_def.variants { if let Some(ctor_def_id) = variant.data.ctor_def_id() { struct_constructors.insert(ctor_def_id, variant.def_id); } } } } DefKind::Impl { of_trait } => { // get DefIds from another query let local_def_ids = tcx .associated_item_def_ids(id.owner_id) .iter() .filter_map(|def_id| def_id.as_local()); let ty_is_pub = ty_ref_to_pub_struct(tcx, tcx.hir_item(id).expect_impl().self_ty); // And we access the Map here to get HirId from LocalDefId for local_def_id in local_def_ids { // check the function may construct Self let mut may_construct_self = false; if let Some(fn_sig) = tcx.hir_fn_sig_by_hir_id(tcx.local_def_id_to_hir_id(local_def_id)) { may_construct_self = matches!(fn_sig.decl.implicit_self, hir::ImplicitSelfKind::None); } // for trait impl blocks, // mark the method live if the self_ty is public, // or the method is public and may construct self if of_trait && (!matches!(tcx.def_kind(local_def_id), DefKind::AssocFn) || tcx.visibility(local_def_id).is_public() && (ty_is_pub || may_construct_self)) { worklist.push((local_def_id, ComesFromAllowExpect::No)); } else if let Some(comes_from_allow) = has_allow_dead_code_or_lang_attr(tcx, local_def_id) { worklist.push((local_def_id, comes_from_allow)); } else if of_trait { // private method || public method not constructs self unsolved_impl_items.push((id, local_def_id)); } } } DefKind::Struct => { let item = tcx.hir_item(id); if let hir::ItemKind::Struct(ref variant_data, _) = item.kind && let Some(ctor_def_id) = variant_data.ctor_def_id() { struct_constructors.insert(ctor_def_id, item.owner_id.def_id); } } DefKind::GlobalAsm => { // global_asm! is always live. worklist.push((id.owner_id.def_id, ComesFromAllowExpect::No)); } _ => {} } } fn check_trait_item( tcx: TyCtxt<'_>, worklist: &mut Vec<(LocalDefId, ComesFromAllowExpect)>, id: hir::TraitItemId, ) { use hir::TraitItemKind::{Const, Fn}; if matches!(tcx.def_kind(id.owner_id), DefKind::AssocConst | DefKind::AssocFn) { let trait_item = tcx.hir_trait_item(id); if matches!(trait_item.kind, Const(_, Some(_)) | Fn(..)) && let Some(comes_from_allow) = has_allow_dead_code_or_lang_attr(tcx, trait_item.owner_id.def_id) { worklist.push((trait_item.owner_id.def_id, comes_from_allow)); } } } fn check_foreign_item( tcx: TyCtxt<'_>, worklist: &mut Vec<(LocalDefId, ComesFromAllowExpect)>, id: hir::ForeignItemId, ) { if matches!(tcx.def_kind(id.owner_id), DefKind::Static { .. } | DefKind::Fn) && let Some(comes_from_allow) = has_allow_dead_code_or_lang_attr(tcx, id.owner_id.def_id) { worklist.push((id.owner_id.def_id, comes_from_allow)); } } fn create_and_seed_worklist( tcx: TyCtxt<'_>, ) -> ( Vec<(LocalDefId, ComesFromAllowExpect)>, LocalDefIdMap, Vec<(hir::ItemId, LocalDefId)>, ) { let effective_visibilities = &tcx.effective_visibilities(()); // see `MarkSymbolVisitor::struct_constructors` let mut unsolved_impl_item = Vec::new(); let mut struct_constructors = Default::default(); let mut worklist = effective_visibilities .iter() .filter_map(|(&id, effective_vis)| { effective_vis .is_public_at_level(Level::Reachable) .then_some(id) .map(|id| (id, ComesFromAllowExpect::No)) }) // Seed entry point .chain( tcx.entry_fn(()) .and_then(|(def_id, _)| def_id.as_local().map(|id| (id, ComesFromAllowExpect::No))), ) .collect::>(); let crate_items = tcx.hir_crate_items(()); for id in crate_items.free_items() { check_item(tcx, &mut worklist, &mut struct_constructors, &mut unsolved_impl_item, id); } for id in crate_items.trait_items() { check_trait_item(tcx, &mut worklist, id); } for id in crate_items.foreign_items() { check_foreign_item(tcx, &mut worklist, id); } (worklist, struct_constructors, unsolved_impl_item) } fn live_symbols_and_ignored_derived_traits( tcx: TyCtxt<'_>, (): (), ) -> (LocalDefIdSet, LocalDefIdMap>) { let (worklist, struct_constructors, unsolved_impl_items) = create_and_seed_worklist(tcx); let mut symbol_visitor = MarkSymbolVisitor { worklist, tcx, maybe_typeck_results: None, live_symbols: Default::default(), repr_unconditionally_treats_fields_as_live: false, repr_has_repr_simd: false, in_pat: false, ignore_variant_stack: vec![], struct_constructors, ignored_derived_traits: Default::default(), }; symbol_visitor.mark_live_symbols(); symbol_visitor.solve_rest_impl_items(unsolved_impl_items); (symbol_visitor.live_symbols, symbol_visitor.ignored_derived_traits) } struct DeadItem { def_id: LocalDefId, name: Symbol, level: lint::Level, } struct DeadVisitor<'tcx> { tcx: TyCtxt<'tcx>, live_symbols: &'tcx LocalDefIdSet, ignored_derived_traits: &'tcx LocalDefIdMap>, } enum ShouldWarnAboutField { Yes, No, } #[derive(Debug, Copy, Clone, PartialEq, Eq)] enum ReportOn { TupleField, NamedField, } impl<'tcx> DeadVisitor<'tcx> { fn should_warn_about_field(&mut self, field: &ty::FieldDef) -> ShouldWarnAboutField { if self.live_symbols.contains(&field.did.expect_local()) { return ShouldWarnAboutField::No; } let field_type = self.tcx.type_of(field.did).instantiate_identity(); if field_type.is_phantom_data() { return ShouldWarnAboutField::No; } let is_positional = field.name.as_str().starts_with(|c: char| c.is_ascii_digit()); if is_positional && self .tcx .layout_of( ty::TypingEnv::non_body_analysis(self.tcx, field.did) .as_query_input(field_type), ) .map_or(true, |layout| layout.is_zst()) { return ShouldWarnAboutField::No; } ShouldWarnAboutField::Yes } fn def_lint_level(&self, id: LocalDefId) -> lint::Level { let hir_id = self.tcx.local_def_id_to_hir_id(id); self.tcx.lint_level_at_node(DEAD_CODE, hir_id).0 } // # Panics // All `dead_codes` must have the same lint level, otherwise we will intentionally ICE. // This is because we emit a multi-spanned lint using the lint level of the `dead_codes`'s // first local def id. // Prefer calling `Self.warn_dead_code` or `Self.warn_dead_code_grouped_by_lint_level` // since those methods group by lint level before calling this method. fn lint_at_single_level( &self, dead_codes: &[&DeadItem], participle: &str, parent_item: Option, report_on: ReportOn, ) { fn get_parent_if_enum_variant<'tcx>( tcx: TyCtxt<'tcx>, may_variant: LocalDefId, ) -> LocalDefId { if let Node::Variant(_) = tcx.hir_node_by_def_id(may_variant) && let Some(enum_did) = tcx.opt_parent(may_variant.to_def_id()) && let Some(enum_local_id) = enum_did.as_local() && let Node::Item(item) = tcx.hir_node_by_def_id(enum_local_id) && let ItemKind::Enum(_, _) = item.kind { enum_local_id } else { may_variant } } let Some(&first_item) = dead_codes.first() else { return; }; let tcx = self.tcx; let first_lint_level = first_item.level; assert!(dead_codes.iter().skip(1).all(|item| item.level == first_lint_level)); let names: Vec<_> = dead_codes.iter().map(|item| item.name).collect(); let spans: Vec<_> = dead_codes .iter() .map(|item| match tcx.def_ident_span(item.def_id) { Some(s) => s.with_ctxt(tcx.def_span(item.def_id).ctxt()), None => tcx.def_span(item.def_id), }) .collect(); let descr = tcx.def_descr(first_item.def_id.to_def_id()); // `impl` blocks are "batched" and (unlike other batching) might // contain different kinds of associated items. let descr = if dead_codes.iter().any(|item| tcx.def_descr(item.def_id.to_def_id()) != descr) { "associated item" } else { descr }; let num = dead_codes.len(); let multiple = num > 6; let name_list = names.into(); let parent_info = if let Some(parent_item) = parent_item { let parent_descr = tcx.def_descr(parent_item.to_def_id()); let span = if let DefKind::Impl { .. } = tcx.def_kind(parent_item) { tcx.def_span(parent_item) } else { tcx.def_ident_span(parent_item).unwrap() }; Some(ParentInfo { num, descr, parent_descr, span }) } else { None }; let encl_def_id = parent_item.unwrap_or(first_item.def_id); // If parent of encl_def_id is an enum, use the parent ID instead. let encl_def_id = get_parent_if_enum_variant(tcx, encl_def_id); let ignored_derived_impls = if let Some(ign_traits) = self.ignored_derived_traits.get(&encl_def_id) { let trait_list = ign_traits .iter() .map(|(trait_id, _)| self.tcx.item_name(*trait_id)) .collect::>(); let trait_list_len = trait_list.len(); Some(IgnoredDerivedImpls { name: self.tcx.item_name(encl_def_id.to_def_id()), trait_list: trait_list.into(), trait_list_len, }) } else { None }; let diag = match report_on { ReportOn::TupleField => { let tuple_fields = if let Some(parent_id) = parent_item && let node = tcx.hir_node_by_def_id(parent_id) && let hir::Node::Item(hir::Item { kind: hir::ItemKind::Struct(hir::VariantData::Tuple(fields, _, _), _), .. }) = node { *fields } else { &[] }; let trailing_tuple_fields = if tuple_fields.len() >= dead_codes.len() { LocalDefIdSet::from_iter( tuple_fields .iter() .skip(tuple_fields.len() - dead_codes.len()) .map(|f| f.def_id), ) } else { LocalDefIdSet::default() }; let fields_suggestion = // Suggest removal if all tuple fields are at the end. // Otherwise suggest removal or changing to unit type if dead_codes.iter().all(|dc| trailing_tuple_fields.contains(&dc.def_id)) { ChangeFields::Remove { num } } else { ChangeFields::ChangeToUnitTypeOrRemove { num, spans: spans.clone() } }; MultipleDeadCodes::UnusedTupleStructFields { multiple, num, descr, participle, name_list, change_fields_suggestion: fields_suggestion, parent_info, ignored_derived_impls, } } ReportOn::NamedField => MultipleDeadCodes::DeadCodes { multiple, num, descr, participle, name_list, parent_info, ignored_derived_impls, }, }; let hir_id = tcx.local_def_id_to_hir_id(first_item.def_id); self.tcx.emit_node_span_lint(DEAD_CODE, hir_id, MultiSpan::from_spans(spans), diag); } fn warn_multiple( &self, def_id: LocalDefId, participle: &str, dead_codes: Vec, report_on: ReportOn, ) { let mut dead_codes = dead_codes .iter() .filter(|v| !v.name.as_str().starts_with('_')) .collect::>(); if dead_codes.is_empty() { return; } dead_codes.sort_by_key(|v| v.level); for group in dead_codes[..].chunk_by(|a, b| a.level == b.level) { self.lint_at_single_level(&group, participle, Some(def_id), report_on); } } fn warn_dead_code(&mut self, id: LocalDefId, participle: &str) { let item = DeadItem { def_id: id, name: self.tcx.item_name(id.to_def_id()), level: self.def_lint_level(id), }; self.lint_at_single_level(&[&item], participle, None, ReportOn::NamedField); } fn check_definition(&mut self, def_id: LocalDefId) { if self.is_live_code(def_id) { return; } match self.tcx.def_kind(def_id) { DefKind::AssocConst | DefKind::AssocFn | DefKind::Fn | DefKind::Static { .. } | DefKind::Const | DefKind::TyAlias | DefKind::Enum | DefKind::Union | DefKind::ForeignTy | DefKind::Trait => self.warn_dead_code(def_id, "used"), DefKind::Struct => self.warn_dead_code(def_id, "constructed"), DefKind::Variant | DefKind::Field => bug!("should be handled specially"), _ => {} } } fn is_live_code(&self, def_id: LocalDefId) -> bool { // if we cannot get a name for the item, then we just assume that it is // live. I mean, we can't really emit a lint. let Some(name) = self.tcx.opt_item_name(def_id.to_def_id()) else { return true; }; self.live_symbols.contains(&def_id) || name.as_str().starts_with('_') } } fn check_mod_deathness(tcx: TyCtxt<'_>, module: LocalModDefId) { let (live_symbols, ignored_derived_traits) = tcx.live_symbols_and_ignored_derived_traits(()); let mut visitor = DeadVisitor { tcx, live_symbols, ignored_derived_traits }; let module_items = tcx.hir_module_items(module); for item in module_items.free_items() { let def_kind = tcx.def_kind(item.owner_id); let mut dead_codes = Vec::new(); // if we have diagnosed the trait, do not diagnose unused methods if matches!(def_kind, DefKind::Impl { .. }) || (def_kind == DefKind::Trait && live_symbols.contains(&item.owner_id.def_id)) { for &def_id in tcx.associated_item_def_ids(item.owner_id.def_id) { // We have diagnosed unused methods in traits if matches!(def_kind, DefKind::Impl { of_trait: true }) && tcx.def_kind(def_id) == DefKind::AssocFn || def_kind == DefKind::Trait && tcx.def_kind(def_id) != DefKind::AssocFn { continue; } if let Some(local_def_id) = def_id.as_local() && !visitor.is_live_code(local_def_id) { let name = tcx.item_name(def_id); let level = visitor.def_lint_level(local_def_id); dead_codes.push(DeadItem { def_id: local_def_id, name, level }); } } } if !dead_codes.is_empty() { visitor.warn_multiple(item.owner_id.def_id, "used", dead_codes, ReportOn::NamedField); } if !live_symbols.contains(&item.owner_id.def_id) { let parent = tcx.local_parent(item.owner_id.def_id); if parent != module.to_local_def_id() && !live_symbols.contains(&parent) { // We already have diagnosed something. continue; } visitor.check_definition(item.owner_id.def_id); continue; } if let DefKind::Struct | DefKind::Union | DefKind::Enum = def_kind { let adt = tcx.adt_def(item.owner_id); let mut dead_variants = Vec::new(); for variant in adt.variants() { let def_id = variant.def_id.expect_local(); if !live_symbols.contains(&def_id) { // Record to group diagnostics. let level = visitor.def_lint_level(def_id); dead_variants.push(DeadItem { def_id, name: variant.name, level }); continue; } let is_positional = variant.fields.raw.first().is_some_and(|field| { field.name.as_str().starts_with(|c: char| c.is_ascii_digit()) }); let report_on = if is_positional { ReportOn::TupleField } else { ReportOn::NamedField }; let dead_fields = variant .fields .iter() .filter_map(|field| { let def_id = field.did.expect_local(); if let ShouldWarnAboutField::Yes = visitor.should_warn_about_field(field) { let level = visitor.def_lint_level(def_id); Some(DeadItem { def_id, name: field.name, level }) } else { None } }) .collect(); visitor.warn_multiple(def_id, "read", dead_fields, report_on); } visitor.warn_multiple( item.owner_id.def_id, "constructed", dead_variants, ReportOn::NamedField, ); } } for foreign_item in module_items.foreign_items() { visitor.check_definition(foreign_item.owner_id.def_id); } } pub(crate) fn provide(providers: &mut Providers) { *providers = Providers { live_symbols_and_ignored_derived_traits, check_mod_deathness, ..*providers }; }