// 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::def_id::{LocalDefIdMap, LocalDefIdSet}; use rustc_abi::FieldIdx; use rustc_data_structures::fx::FxIndexSet; 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, QPath}; use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags; use rustc_middle::middle::privacy::Level; use rustc_middle::query::Providers; use rustc_middle::ty::{self, AssocTag, TyCtxt}; use rustc_middle::{bug, span_bug}; use rustc_session::lint::builtin::DEAD_CODE; use rustc_session::lint::{self, LintExpectationId}; use rustc_span::{Symbol, kw, sym}; use crate::errors::{ ChangeFields, IgnoredDerivedImpls, MultipleDeadCodes, ParentInfo, UselessAssignment, }; /// Any local definition that may call something in its body block should be explored. For example, /// if it's a live 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 { match tcx.def_kind(def_id) { DefKind::Mod | DefKind::Struct | DefKind::Union | DefKind::Enum | DefKind::Variant | DefKind::Trait | DefKind::TyAlias | DefKind::ForeignTy | DefKind::TraitAlias | DefKind::AssocTy | DefKind::Fn | DefKind::Const | DefKind::Static { .. } | DefKind::AssocFn | DefKind::AssocConst | DefKind::Macro(_) | DefKind::GlobalAsm | DefKind::Impl { .. } | DefKind::OpaqueTy | DefKind::AnonConst | DefKind::InlineConst | DefKind::ExternCrate | DefKind::Use | DefKind::Ctor(..) | DefKind::ForeignMod => true, DefKind::TyParam | DefKind::ConstParam | DefKind::Field | DefKind::LifetimeParam | DefKind::Closure | DefKind::SyntheticCoroutineBody => false, } } /// 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>>, scanned: LocalDefIdSet, live_symbols: LocalDefIdSet, repr_unconditionally_treats_fields_as_live: bool, repr_has_repr_simd: bool, in_pat: bool, ignore_variant_stack: Vec, // 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.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::AssocTy | 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:?}"), } } 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); } } 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, _) => { // Marks the ADT live if its variant appears as the pattern, // considering cases when we have `let T(x) = foo()` and `fn foo() -> T;`, // we will lose the liveness info of `T` cause we cannot mark it live when visiting `foo`. // Related issue: https://github.com/rust-lang/rust/issues/120770 self.check_def_id(adt.did()); 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, _) => { // Marks the ADT live if its variant appears as the pattern self.check_def_id(adt.did()); 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) { while let Some(work) = self.worklist.pop() { let (mut id, comes_from_allow_expect) = work; // in the case of tuple struct constructors we want to check the item, // not the generated tuple struct constructor function if let DefKind::Ctor(..) = self.tcx.def_kind(id) { id = self.tcx.local_parent(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. match comes_from_allow_expect { ComesFromAllowExpect::Yes => {} ComesFromAllowExpect::No => { self.live_symbols.insert(id); } } if !self.scanned.insert(id) { continue; } // 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; } 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_impl_item(&mut self, impl_item: &hir::ImplItem<'_>) -> bool { if let hir::ImplItemImplKind::Trait { .. } = impl_item.impl_kind && let impl_of = self.tcx.parent(impl_item.owner_id.to_def_id()) && self.tcx.is_automatically_derived(impl_of) && let trait_ref = self.tcx.impl_trait_ref(impl_of).unwrap().instantiate_identity() && self.tcx.has_attr(trait_ref.def_id, sym::rustc_trivial_field_reads) { 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().insert(trait_ref.def_id); } return true; } false } fn visit_node(&mut self, node: Node<'tcx>) { if let Node::ImplItem(impl_item) = node && self.should_ignore_impl_item(impl_item) { 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(.., trait_item_refs) => { // mark assoc ty live if the trait is live for trait_item in trait_item_refs { if matches!(self.tcx.def_kind(trait_item.owner_id), DefKind::AssocTy) { self.check_def_id(trait_item.owner_id.to_def_id()); } } intravisit::walk_item(self, item) } _ => intravisit::walk_item(self, item), }, Node::TraitItem(trait_item) => { // mark the trait live let trait_item_id = trait_item.owner_id.to_def_id(); if let Some(trait_id) = self.tcx.trait_of_assoc(trait_item_id) { self.check_def_id(trait_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 let hir::ImplItemImplKind::Inherent { .. } = impl_item.impl_kind { //// 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); } } } /// Returns whether `local_def_id` is potentially alive or not. /// `local_def_id` points to an impl or an impl item, /// both impl and impl item that may be passed to this function are of a trait, /// and added into the unsolved_items during `create_and_seed_worklist` fn check_impl_or_impl_item_live(&mut self, local_def_id: LocalDefId) -> bool { let (impl_block_id, trait_def_id) = match self.tcx.def_kind(local_def_id) { // assoc impl items of traits are live if the corresponding trait items are live DefKind::AssocConst | DefKind::AssocTy | DefKind::AssocFn => { let trait_item_id = self.tcx.trait_item_of(local_def_id).and_then(|def_id| def_id.as_local()); (self.tcx.local_parent(local_def_id), trait_item_id) } // impl items are live if the corresponding traits are live DefKind::Impl { of_trait: true } => ( local_def_id, self.tcx .impl_trait_ref(local_def_id) .and_then(|trait_ref| trait_ref.skip_binder().def_id.as_local()), ), _ => bug!(), }; if let Some(trait_def_id) = trait_def_id && !self.live_symbols.contains(&trait_def_id) { return false; } // The impl or impl item is used if the corresponding trait or trait item is used and the ty is used. if let ty::Adt(adt, _) = self.tcx.type_of(impl_block_id).instantiate_identity().kind() && let Some(adt_def_id) = adt.did().as_local() && !self.live_symbols.contains(&adt_def_id) { return false; } true } } 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 @ 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); } hir::ExprKind::Assign(ref lhs, ..) => { self.handle_assign(lhs); self.check_for_self_assign(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) => { // mark the type of variant live when meeting E::V in expr if let ty::Adt(adt, _) = self.typeck_results().node_type(expr.hir_id).kind() { self.check_def_id(adt.did()); } 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 visit_trait_ref(&mut self, t: &'tcx hir::TraitRef<'tcx>) { if let Some(trait_def_id) = t.path.res.opt_def_id() && let Some(segment) = t.path.segments.last() && let Some(args) = segment.args { for constraint in args.constraints { if let Some(local_def_id) = self .tcx .associated_items(trait_def_id) .find_by_ident_and_kind( self.tcx, constraint.ident, AssocTag::Const, trait_def_id, ) .and_then(|item| item.def_id.as_local()) { self.worklist.push((local_def_id, ComesFromAllowExpect::No)); } } } intravisit::walk_trait_ref(self, t); } } 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).level; 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_COMPILER) || 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 } } /// Examine the given definition and record it in the worklist if it should be considered live. /// /// 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=".."] /// Lang items are always callable from elsewhere. /// /// For trait methods and implementations of traits, we are not certain that the definitions are /// live at this stage. We record them in `unsolved_items` for later examination. fn maybe_record_as_seed<'tcx>( tcx: TyCtxt<'tcx>, owner_id: hir::OwnerId, worklist: &mut Vec<(LocalDefId, ComesFromAllowExpect)>, unsolved_items: &mut Vec, ) { let allow_dead_code = has_allow_dead_code_or_lang_attr(tcx, owner_id.def_id); if let Some(comes_from_allow) = allow_dead_code { worklist.push((owner_id.def_id, comes_from_allow)); } match tcx.def_kind(owner_id) { DefKind::Enum => { if let Some(comes_from_allow) = allow_dead_code { let adt = tcx.adt_def(owner_id); worklist.extend( adt.variants() .iter() .map(|variant| (variant.def_id.expect_local(), comes_from_allow)), ); } } DefKind::AssocFn | DefKind::AssocConst | DefKind::AssocTy => { if allow_dead_code.is_none() { let parent = tcx.local_parent(owner_id.def_id); match tcx.def_kind(parent) { DefKind::Impl { of_trait: false } | DefKind::Trait => {} DefKind::Impl { of_trait: true } => { // We only care about associated items of traits, // because they cannot be visited directly, // so we later mark them as live if their corresponding traits // or trait items and self types are both live, // but inherent associated items can be visited and marked directly. unsolved_items.push(owner_id.def_id); } _ => bug!(), } } } DefKind::Impl { of_trait: true } => { if allow_dead_code.is_none() { unsolved_items.push(owner_id.def_id); } } DefKind::GlobalAsm => { // global_asm! is always live. worklist.push((owner_id.def_id, ComesFromAllowExpect::No)); } DefKind::Const => { if tcx.item_name(owner_id.def_id) == kw::Underscore { // `const _` is always live, as that syntax only exists for the side effects // of type checking and evaluating the constant expression, and marking them // as dead code would defeat that purpose. worklist.push((owner_id.def_id, ComesFromAllowExpect::No)); } } _ => {} } } fn create_and_seed_worklist( tcx: TyCtxt<'_>, ) -> (Vec<(LocalDefId, ComesFromAllowExpect)>, Vec) { let effective_visibilities = &tcx.effective_visibilities(()); let mut unsolved_impl_item = Vec::new(); 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.owners() { maybe_record_as_seed(tcx, id, &mut worklist, &mut unsolved_impl_item); } (worklist, unsolved_impl_item) } fn live_symbols_and_ignored_derived_traits( tcx: TyCtxt<'_>, (): (), ) -> (LocalDefIdSet, LocalDefIdMap>) { let (worklist, mut unsolved_items) = create_and_seed_worklist(tcx); let mut symbol_visitor = MarkSymbolVisitor { worklist, tcx, maybe_typeck_results: None, scanned: Default::default(), live_symbols: Default::default(), repr_unconditionally_treats_fields_as_live: false, repr_has_repr_simd: false, in_pat: false, ignore_variant_stack: vec![], ignored_derived_traits: Default::default(), }; symbol_visitor.mark_live_symbols(); // We have marked the primary seeds as live. We now need to process unsolved items from traits // and trait impls: add them to the work list if the trait or the implemented type is live. let mut items_to_check: Vec<_> = unsolved_items .extract_if(.., |&mut local_def_id| { symbol_visitor.check_impl_or_impl_item_live(local_def_id) }) .collect(); while !items_to_check.is_empty() { symbol_visitor .worklist .extend(items_to_check.drain(..).map(|id| (id, ComesFromAllowExpect::No))); symbol_visitor.mark_live_symbols(); items_to_check.extend(unsolved_items.extract_if(.., |&mut local_def_id| { symbol_visitor.check_impl_or_impl_item_live(local_def_id) })); } (symbol_visitor.live_symbols, symbol_visitor.ignored_derived_traits) } struct DeadItem { def_id: LocalDefId, name: Symbol, level: (lint::Level, Option), } 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 { /// Report on something that hasn't got a proper name to refer to TupleField, /// Report on something that has got a name, which could be a field but also a method 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, Option) { let hir_id = self.tcx.local_def_id_to_hir_id(id); let level = self.tcx.lint_level_at_node(DEAD_CODE, hir_id); (level.level, level.lint_id) } // # 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, ) { 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| { let span = tcx.def_span(item.def_id); let ident_span = tcx.def_ident_span(item.def_id); // FIXME(cjgillot) this SyntaxContext manipulation does not make any sense. ident_span.map(|s| s.with_ctxt(span.ctxt())).unwrap_or(span) }) .collect(); let mut 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. if dead_codes.iter().any(|item| tcx.def_descr(item.def_id.to_def_id()) != descr) { descr = "associated item" } let num = dead_codes.len(); let multiple = num > 6; let name_list = names.into(); let parent_info = parent_item.map(|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() }; ParentInfo { num, descr, parent_descr, span } }); let mut encl_def_id = parent_item.unwrap_or(first_item.def_id); // `ignored_derived_traits` is computed for the enum, not for the variants. if let DefKind::Variant = tcx.def_kind(encl_def_id) { encl_def_id = tcx.local_parent(encl_def_id); } let ignored_derived_impls = self.ignored_derived_traits.get(&encl_def_id).map(|ign_traits| { let trait_list = ign_traits .iter() .map(|trait_id| self.tcx.item_name(*trait_id)) .collect::>(); let trait_list_len = trait_list.len(); IgnoredDerivedImpls { name: self.tcx.item_name(encl_def_id.to_def_id()), trait_list: trait_list.into(), trait_list_len, } }); 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 => { let enum_variants_with_same_name = dead_codes .iter() .filter_map(|dead_item| { if let DefKind::AssocFn | DefKind::AssocConst = tcx.def_kind(dead_item.def_id) && let impl_did = tcx.local_parent(dead_item.def_id) && let DefKind::Impl { of_trait: false } = tcx.def_kind(impl_did) && let ty::Adt(maybe_enum, _) = tcx.type_of(impl_did).instantiate_identity().kind() && maybe_enum.is_enum() && let Some(variant) = maybe_enum.variants().iter().find(|i| i.name == dead_item.name) { Some(crate::errors::EnumVariantSameName { dead_descr: tcx.def_descr(dead_item.def_id.to_def_id()), dead_name: dead_item.name, variant_span: tcx.def_span(variant.def_id), }) } else { None } }) .collect(); MultipleDeadCodes::DeadCodes { multiple, num, descr, participle, name_list, parent_info, ignored_derived_impls, enum_variants_with_same_name, } } }; 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; } // FIXME: `dead_codes` should probably be morally equivalent to `IndexMap<(Level, LintExpectationId), (DefId, Symbol)>` dead_codes.sort_by_key(|v| v.level.0); 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::AssocTy | 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(); // Only diagnose unused assoc items in inherent impl and used trait, // for unused assoc items in impls of trait, // we have diagnosed them in the trait if they are unused, // for unused assoc items in unused trait, // we have diagnosed the unused trait. if matches!(def_kind, DefKind::Impl { of_trait: false }) || (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) { 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 }; }