1 //! Checks for uses of const which the type is not Freeze (Cell-free).
3 //! This lint is **deny** by default.
5 use rustc::lint::{LateContext, LateLintPass, Lint, LintArray, LintPass};
7 use rustc::hir::def::Def;
8 use rustc::ty::{self, TypeFlags};
9 use rustc::ty::adjustment::Adjust;
10 use rustc_errors::Applicability;
11 use rustc_typeck::hir_ty_to_ty;
12 use syntax_pos::{DUMMY_SP, Span};
14 use crate::utils::{in_constant, in_macro, is_copy, span_lint_and_then};
16 /// **What it does:** Checks for declaration of `const` items which is interior
17 /// mutable (e.g. contains a `Cell`, `Mutex`, `AtomicXxxx` etc).
19 /// **Why is this bad?** Consts are copied everywhere they are referenced, i.e.
20 /// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
21 /// or `AtomicXxxx` will be created, which defeats the whole purpose of using
22 /// these types in the first place.
24 /// The `const` should better be replaced by a `static` item if a global
25 /// variable is wanted, or replaced by a `const fn` if a constructor is wanted.
27 /// **Known problems:** A "non-constant" const item is a legacy way to supply an
28 /// initialized value to downstream `static` items (e.g. the
29 /// `std::sync::ONCE_INIT` constant). In this case the use of `const` is legit,
30 /// and this lint should be suppressed.
34 /// use std::sync::atomic::{Ordering::SeqCst, AtomicUsize};
37 /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
38 /// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
39 /// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
42 /// static STATIC_ATOM: AtomicUsize = AtomicUsize::new(15);
43 /// STATIC_ATOM.store(9, SeqCst);
44 /// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
46 declare_clippy_lint! {
47 pub DECLARE_INTERIOR_MUTABLE_CONST,
49 "declaring const with interior mutability"
52 /// **What it does:** Checks if `const` items which is interior mutable (e.g.
53 /// contains a `Cell`, `Mutex`, `AtomicXxxx` etc) has been borrowed directly.
55 /// **Why is this bad?** Consts are copied everywhere they are referenced, i.e.
56 /// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
57 /// or `AtomicXxxx` will be created, which defeats the whole purpose of using
58 /// these types in the first place.
60 /// The `const` value should be stored inside a `static` item.
62 /// **Known problems:** None
66 /// use std::sync::atomic::{Ordering::SeqCst, AtomicUsize};
67 /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
70 /// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
71 /// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
74 /// static STATIC_ATOM: AtomicUsize = CONST_ATOM;
75 /// STATIC_ATOM.store(9, SeqCst);
76 /// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
78 declare_clippy_lint! {
79 pub BORROW_INTERIOR_MUTABLE_CONST,
81 "referencing const with interior mutability"
84 #[derive(Copy, Clone)]
99 fn lint(&self) -> (&'static Lint, &'static str, Span) {
101 Source::Item { item } | Source::Assoc { item, .. } => (
102 DECLARE_INTERIOR_MUTABLE_CONST,
103 "a const item should never be interior mutable",
106 Source::Expr { expr } => (
107 BORROW_INTERIOR_MUTABLE_CONST,
108 "a const item with interior mutability should not be borrowed",
115 fn verify_ty_bound<'a, 'tcx>(
116 cx: &LateContext<'a, 'tcx>,
120 if ty.is_freeze(cx.tcx, cx.param_env, DUMMY_SP) || is_copy(cx, ty) {
121 // an UnsafeCell is !Copy, and an UnsafeCell is also the only type which
122 // is !Freeze, thus if our type is Copy we can be sure it must be Freeze
127 let (lint, msg, span) = source.lint();
128 span_lint_and_then(cx, lint, span, msg, |db| {
130 return; // Don't give suggestions into macros.
133 Source::Item { .. } => {
134 let const_kw_span = span.from_inner_byte_pos(0, 5);
135 db.span_suggestion_with_applicability(
137 "make this a static item",
138 "static".to_string(),
139 Applicability::MachineApplicable,
142 Source::Assoc { ty: ty_span, .. } => {
143 if ty.flags.contains(TypeFlags::HAS_FREE_LOCAL_NAMES) {
144 db.span_help(ty_span, &format!("consider requiring `{}` to be `Copy`", ty));
147 Source::Expr { .. } => {
149 "assign this const to a local or static variable, and use the variable here",
157 pub struct NonCopyConst;
159 impl LintPass for NonCopyConst {
160 fn get_lints(&self) -> LintArray {
161 lint_array!(DECLARE_INTERIOR_MUTABLE_CONST, BORROW_INTERIOR_MUTABLE_CONST)
165 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NonCopyConst {
166 fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, it: &'tcx Item) {
167 if let ItemConst(hir_ty, ..) = &it.node {
168 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
169 verify_ty_bound(cx, ty, Source::Item { item: it.span });
173 fn check_trait_item(&mut self, cx: &LateContext<'a, 'tcx>, trait_item: &'tcx TraitItem) {
174 if let TraitItemKind::Const(hir_ty, ..) = &trait_item.node {
175 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
176 verify_ty_bound(cx, ty, Source::Assoc { ty: hir_ty.span, item: trait_item.span });
180 fn check_impl_item(&mut self, cx: &LateContext<'a, 'tcx>, impl_item: &'tcx ImplItem) {
181 if let ImplItemKind::Const(hir_ty, ..) = &impl_item.node {
182 let item_node_id = cx.tcx.hir.get_parent_node(impl_item.id);
183 let item = cx.tcx.hir.expect_item(item_node_id);
184 // ensure the impl is an inherent impl.
185 if let ItemImpl(_, _, _, _, None, _, _) = item.node {
186 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
187 verify_ty_bound(cx, ty, Source::Assoc { ty: hir_ty.span, item: impl_item.span });
192 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
193 if let ExprKind::Path(qpath) = &expr.node {
194 // Only lint if we use the const item inside a function.
195 if in_constant(cx, expr.id) {
199 // make sure it is a const item.
200 match cx.tables.qpath_def(qpath, expr.hir_id) {
201 Def::Const(_) | Def::AssociatedConst(_) => {},
205 // climb up to resolve any field access and explicit referencing.
206 let mut cur_expr = expr;
207 let mut dereferenced_expr = expr;
208 let mut needs_check_adjustment = true;
210 let parent_id = cx.tcx.hir.get_parent_node(cur_expr.id);
211 if parent_id == cur_expr.id {
214 if let Some(map::NodeExpr(parent_expr)) = cx.tcx.hir.find(parent_id) {
215 match &parent_expr.node {
216 ExprKind::AddrOf(..) => {
217 // `&e` => `e` must be referenced
218 needs_check_adjustment = false;
220 ExprKind::Field(..) => {
221 dereferenced_expr = parent_expr;
222 needs_check_adjustment = true;
224 ExprKind::Index(e, _) if ptr::eq(&**e, cur_expr) => {
225 // `e[i]` => desugared to `*Index::index(&e, i)`,
226 // meaning `e` must be referenced.
227 // no need to go further up since a method call is involved now.
228 needs_check_adjustment = false;
231 ExprKind::Unary(UnDeref, _) => {
232 // `*e` => desugared to `*Deref::deref(&e)`,
233 // meaning `e` must be referenced.
234 // no need to go further up since a method call is involved now.
235 needs_check_adjustment = false;
240 cur_expr = parent_expr;
246 let ty = if !needs_check_adjustment {
247 cx.tables.expr_ty(dereferenced_expr)
249 let adjustments = cx.tables.expr_adjustments(dereferenced_expr);
250 if let Some(i) = adjustments.iter().position(|adj| match adj.kind {
251 Adjust::Borrow(_) | Adjust::Deref(_) => true,
255 cx.tables.expr_ty(dereferenced_expr)
257 adjustments[i - 1].target
260 // No borrow adjustments = the entire const is moved.
265 verify_ty_bound(cx, ty, Source::Expr { expr: expr.span });