1 //! Checks for uses of const which the type is not `Freeze` (`Cell`-free).
3 //! This lint is **deny** by default.
7 use rustc_hir::def::{DefKind, Res};
8 use rustc_hir::{Expr, ExprKind, ImplItem, ImplItemKind, Item, ItemKind, Node, TraitItem, TraitItemKind, UnOp};
9 use rustc_lint::{LateContext, LateLintPass, Lint};
10 use rustc_middle::ty::adjustment::Adjust;
11 use rustc_middle::ty::{Ty, TypeFlags};
12 use rustc_session::{declare_lint_pass, declare_tool_lint};
13 use rustc_span::{InnerSpan, Span, DUMMY_SP};
14 use rustc_typeck::hir_ty_to_ty;
16 use crate::utils::{in_constant, is_copy, qpath_res, span_lint_and_then};
18 declare_clippy_lint! {
19 /// **What it does:** Checks for declaration of `const` items which is interior
20 /// mutable (e.g., contains a `Cell`, `Mutex`, `AtomicXxxx`, etc.).
22 /// **Why is this bad?** Consts are copied everywhere they are referenced, i.e.,
23 /// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
24 /// or `AtomicXxxx` will be created, which defeats the whole purpose of using
25 /// these types in the first place.
27 /// The `const` should better be replaced by a `static` item if a global
28 /// variable is wanted, or replaced by a `const fn` if a constructor is wanted.
30 /// **Known problems:** A "non-constant" const item is a legacy way to supply an
31 /// initialized value to downstream `static` items (e.g., the
32 /// `std::sync::ONCE_INIT` constant). In this case the use of `const` is legit,
33 /// and this lint should be suppressed.
37 /// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
40 /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
41 /// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
42 /// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
45 /// static STATIC_ATOM: AtomicUsize = AtomicUsize::new(15);
46 /// STATIC_ATOM.store(9, SeqCst);
47 /// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
49 pub DECLARE_INTERIOR_MUTABLE_CONST,
51 "declaring `const` with interior mutability"
54 declare_clippy_lint! {
55 /// **What it does:** Checks if `const` items which is interior mutable (e.g.,
56 /// contains a `Cell`, `Mutex`, `AtomicXxxx`, etc.) has been borrowed directly.
58 /// **Why is this bad?** Consts are copied everywhere they are referenced, i.e.,
59 /// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
60 /// or `AtomicXxxx` will be created, which defeats the whole purpose of using
61 /// these types in the first place.
63 /// The `const` value should be stored inside a `static` item.
65 /// **Known problems:** None
69 /// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
70 /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
73 /// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
74 /// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
77 /// static STATIC_ATOM: AtomicUsize = CONST_ATOM;
78 /// STATIC_ATOM.store(9, SeqCst);
79 /// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
81 pub BORROW_INTERIOR_MUTABLE_CONST,
83 "referencing `const` with interior mutability"
87 #[derive(Copy, Clone)]
90 Assoc { item: Span, ty: Span },
96 fn lint(&self) -> (&'static Lint, &'static str, Span) {
98 Self::Item { item } | Self::Assoc { item, .. } => (
99 DECLARE_INTERIOR_MUTABLE_CONST,
100 "a `const` item should never be interior mutable",
103 Self::Expr { expr } => (
104 BORROW_INTERIOR_MUTABLE_CONST,
105 "a `const` item with interior mutability should not be borrowed",
112 fn verify_ty_bound<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, ty: Ty<'tcx>, source: Source) {
113 if ty.is_freeze(cx.tcx, cx.param_env, DUMMY_SP) || is_copy(cx, ty) {
114 // An `UnsafeCell` is `!Copy`, and an `UnsafeCell` is also the only type which
115 // is `!Freeze`, thus if our type is `Copy` we can be sure it must be `Freeze`
120 let (lint, msg, span) = source.lint();
121 span_lint_and_then(cx, lint, span, msg, |diag| {
122 if span.from_expansion() {
123 return; // Don't give suggestions into macros.
126 Source::Item { .. } => {
127 let const_kw_span = span.from_inner(InnerSpan::new(0, 5));
128 diag.span_label(const_kw_span, "make this a static item (maybe with lazy_static)");
130 Source::Assoc { ty: ty_span, .. } => {
131 if ty.flags.intersects(TypeFlags::HAS_FREE_LOCAL_NAMES) {
132 diag.span_label(ty_span, &format!("consider requiring `{}` to be `Copy`", ty));
135 Source::Expr { .. } => {
136 diag.help("assign this const to a local or static variable, and use the variable here");
142 declare_lint_pass!(NonCopyConst => [DECLARE_INTERIOR_MUTABLE_CONST, BORROW_INTERIOR_MUTABLE_CONST]);
144 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NonCopyConst {
145 fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, it: &'tcx Item<'_>) {
146 if let ItemKind::Const(hir_ty, ..) = &it.kind {
147 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
148 verify_ty_bound(cx, ty, Source::Item { item: it.span });
152 fn check_trait_item(&mut self, cx: &LateContext<'a, 'tcx>, trait_item: &'tcx TraitItem<'_>) {
153 if let TraitItemKind::Const(hir_ty, ..) = &trait_item.kind {
154 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
160 item: trait_item.span,
166 fn check_impl_item(&mut self, cx: &LateContext<'a, 'tcx>, impl_item: &'tcx ImplItem<'_>) {
167 if let ImplItemKind::Const(hir_ty, ..) = &impl_item.kind {
168 let item_hir_id = cx.tcx.hir().get_parent_node(impl_item.hir_id);
169 let item = cx.tcx.hir().expect_item(item_hir_id);
170 // Ensure the impl is an inherent impl.
171 if let ItemKind::Impl { of_trait: None, .. } = item.kind {
172 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
178 item: impl_item.span,
185 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr<'_>) {
186 if let ExprKind::Path(qpath) = &expr.kind {
187 // Only lint if we use the const item inside a function.
188 if in_constant(cx, expr.hir_id) {
192 // Make sure it is a const item.
193 match qpath_res(cx, qpath, expr.hir_id) {
194 Res::Def(DefKind::Const | DefKind::AssocConst, _) => {},
198 // Climb up to resolve any field access and explicit referencing.
199 let mut cur_expr = expr;
200 let mut dereferenced_expr = expr;
201 let mut needs_check_adjustment = true;
203 let parent_id = cx.tcx.hir().get_parent_node(cur_expr.hir_id);
204 if parent_id == cur_expr.hir_id {
207 if let Some(Node::Expr(parent_expr)) = cx.tcx.hir().find(parent_id) {
208 match &parent_expr.kind {
209 ExprKind::AddrOf(..) => {
210 // `&e` => `e` must be referenced.
211 needs_check_adjustment = false;
213 ExprKind::Field(..) => {
214 dereferenced_expr = parent_expr;
215 needs_check_adjustment = true;
217 ExprKind::Index(e, _) if ptr::eq(&**e, cur_expr) => {
218 // `e[i]` => desugared to `*Index::index(&e, i)`,
219 // meaning `e` must be referenced.
220 // no need to go further up since a method call is involved now.
221 needs_check_adjustment = false;
224 ExprKind::Unary(UnOp::UnDeref, _) => {
225 // `*e` => desugared to `*Deref::deref(&e)`,
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;
233 cur_expr = parent_expr;
239 let ty = if needs_check_adjustment {
240 let adjustments = cx.tables.expr_adjustments(dereferenced_expr);
241 if let Some(i) = adjustments.iter().position(|adj| match adj.kind {
242 Adjust::Borrow(_) | Adjust::Deref(_) => true,
246 cx.tables.expr_ty(dereferenced_expr)
248 adjustments[i - 1].target
251 // No borrow adjustments means the entire const is moved.
255 cx.tables.expr_ty(dereferenced_expr)
258 verify_ty_bound(cx, ty, Source::Expr { expr: expr.span });