1 //! Checks for uses of const which the type is not `Freeze` (`Cell`-free).
3 //! This lint is **warn** by default.
7 use clippy_utils::diagnostics::span_lint_and_then;
8 use clippy_utils::in_constant;
9 use if_chain::if_chain;
10 use rustc_hir::def::{DefKind, Res};
11 use rustc_hir::def_id::DefId;
13 BodyId, Expr, ExprKind, HirId, Impl, ImplItem, ImplItemKind, Item, ItemKind, Node, TraitItem, TraitItemKind, UnOp,
15 use rustc_lint::{LateContext, LateLintPass, Lint};
16 use rustc_middle::mir::interpret::{ConstValue, ErrorHandled};
17 use rustc_middle::ty::adjustment::Adjust;
18 use rustc_middle::ty::{self, Const, Ty};
19 use rustc_session::{declare_lint_pass, declare_tool_lint};
20 use rustc_span::{InnerSpan, Span, DUMMY_SP};
21 use rustc_typeck::hir_ty_to_ty;
23 // FIXME: this is a correctness problem but there's no suitable
24 // warn-by-default category.
25 declare_clippy_lint! {
27 /// Checks for declaration of `const` items which is interior
28 /// mutable (e.g., contains a `Cell`, `Mutex`, `AtomicXxxx`, etc.).
30 /// ### Why is this bad?
31 /// Consts are copied everywhere they are referenced, i.e.,
32 /// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
33 /// or `AtomicXxxx` will be created, which defeats the whole purpose of using
34 /// these types in the first place.
36 /// The `const` should better be replaced by a `static` item if a global
37 /// variable is wanted, or replaced by a `const fn` if a constructor is wanted.
39 /// ### Known problems
40 /// A "non-constant" const item is a legacy way to supply an
41 /// initialized value to downstream `static` items (e.g., the
42 /// `std::sync::ONCE_INIT` constant). In this case the use of `const` is legit,
43 /// and this lint should be suppressed.
45 /// Even though the lint avoids triggering on a constant whose type has enums that have variants
46 /// with interior mutability, and its value uses non interior mutable variants (see
47 /// [#3962](https://github.com/rust-lang/rust-clippy/issues/3962) and
48 /// [#3825](https://github.com/rust-lang/rust-clippy/issues/3825) for examples);
49 /// it complains about associated constants without default values only based on its types;
50 /// which might not be preferable.
51 /// There're other enums plus associated constants cases that the lint cannot handle.
53 /// Types that have underlying or potential interior mutability trigger the lint whether
54 /// the interior mutable field is used or not. See issues
55 /// [#5812](https://github.com/rust-lang/rust-clippy/issues/5812) and
59 /// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
61 /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
62 /// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
63 /// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
68 /// # use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
69 /// static STATIC_ATOM: AtomicUsize = AtomicUsize::new(15);
70 /// STATIC_ATOM.store(9, SeqCst);
71 /// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
73 #[clippy::version = "pre 1.29.0"]
74 pub DECLARE_INTERIOR_MUTABLE_CONST,
76 "declaring `const` with interior mutability"
79 // FIXME: this is a correctness problem but there's no suitable
80 // warn-by-default category.
81 declare_clippy_lint! {
83 /// Checks if `const` items which is interior mutable (e.g.,
84 /// contains a `Cell`, `Mutex`, `AtomicXxxx`, etc.) has been borrowed directly.
86 /// ### Why is this bad?
87 /// Consts are copied everywhere they are referenced, i.e.,
88 /// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
89 /// or `AtomicXxxx` will be created, which defeats the whole purpose of using
90 /// these types in the first place.
92 /// The `const` value should be stored inside a `static` item.
94 /// ### Known problems
95 /// When an enum has variants with interior mutability, use of its non
96 /// interior mutable variants can generate false positives. See issue
97 /// [#3962](https://github.com/rust-lang/rust-clippy/issues/3962)
99 /// Types that have underlying or potential interior mutability trigger the lint whether
100 /// the interior mutable field is used or not. See issues
101 /// [#5812](https://github.com/rust-lang/rust-clippy/issues/5812) and
102 /// [#3825](https://github.com/rust-lang/rust-clippy/issues/3825)
106 /// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
107 /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
109 /// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
110 /// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
115 /// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
116 /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
118 /// static STATIC_ATOM: AtomicUsize = CONST_ATOM;
119 /// STATIC_ATOM.store(9, SeqCst);
120 /// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
122 #[clippy::version = "pre 1.29.0"]
123 pub BORROW_INTERIOR_MUTABLE_CONST,
125 "referencing `const` with interior mutability"
128 fn is_unfrozen<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
129 // Ignore types whose layout is unknown since `is_freeze` reports every generic types as `!Freeze`,
130 // making it indistinguishable from `UnsafeCell`. i.e. it isn't a tool to prove a type is
131 // 'unfrozen'. However, this code causes a false negative in which
132 // a type contains a layout-unknown type, but also an unsafe cell like `const CELL: Cell<T>`.
133 // Yet, it's better than `ty.has_type_flags(TypeFlags::HAS_TY_PARAM | TypeFlags::HAS_PROJECTION)`
134 // since it works when a pointer indirection involves (`Cell<*const T>`).
135 // Making up a `ParamEnv` where every generic params and assoc types are `Freeze`is another option;
136 // but I'm not sure whether it's a decent way, if possible.
137 cx.tcx.layout_of(cx.param_env.and(ty)).is_ok() && !ty.is_freeze(cx.tcx.at(DUMMY_SP), cx.param_env)
140 fn is_value_unfrozen_raw<'tcx>(
141 cx: &LateContext<'tcx>,
142 result: Result<ConstValue<'tcx>, ErrorHandled>,
145 fn inner<'tcx>(cx: &LateContext<'tcx>, val: Const<'tcx>) -> bool {
146 match val.ty().kind() {
147 // the fact that we have to dig into every structs to search enums
148 // leads us to the point checking `UnsafeCell` directly is the only option.
149 ty::Adt(ty_def, ..) if Some(ty_def.did()) == cx.tcx.lang_items().unsafe_cell_type() => true,
150 ty::Array(..) | ty::Adt(..) | ty::Tuple(..) => {
151 let val = cx.tcx.destructure_const(cx.param_env.and(val));
152 val.fields.iter().any(|field| inner(cx, *field))
160 // Consider `TooGeneric` cases as being unfrozen.
161 // This causes a false positive where an assoc const whose type is unfrozen
162 // have a value that is a frozen variant with a generic param (an example is
163 // `declare_interior_mutable_const::enums::BothOfCellAndGeneric::GENERIC_VARIANT`).
164 // However, it prevents a number of false negatives that is, I think, important:
165 // 1. assoc consts in trait defs referring to consts of themselves
166 // (an example is `declare_interior_mutable_const::traits::ConcreteTypes::ANOTHER_ATOMIC`).
167 // 2. a path expr referring to assoc consts whose type is doesn't have
168 // any frozen variants in trait defs (i.e. without substitute for `Self`).
169 // (e.g. borrowing `borrow_interior_mutable_const::trait::ConcreteTypes::ATOMIC`)
170 // 3. similar to the false positive above;
171 // but the value is an unfrozen variant, or the type has no enums. (An example is
172 // `declare_interior_mutable_const::enums::BothOfCellAndGeneric::UNFROZEN_VARIANT`
173 // and `declare_interior_mutable_const::enums::BothOfCellAndGeneric::NO_ENUM`).
174 // One might be able to prevent these FNs correctly, and replace this with `false`;
175 // e.g. implementing `has_frozen_variant` described above, and not running this function
176 // when the type doesn't have any frozen variants would be the 'correct' way for the 2nd
177 // case (that actually removes another suboptimal behavior (I won't say 'false positive') where,
178 // similar to 2., but with the a frozen variant) (e.g. borrowing
179 // `borrow_interior_mutable_const::enums::AssocConsts::TO_BE_FROZEN_VARIANT`).
180 // I chose this way because unfrozen enums as assoc consts are rare (or, hopefully, none).
181 err == ErrorHandled::TooGeneric
183 |val| inner(cx, Const::from_value(cx.tcx, val, ty)),
187 fn is_value_unfrozen_poly<'tcx>(cx: &LateContext<'tcx>, body_id: BodyId, ty: Ty<'tcx>) -> bool {
188 let result = cx.tcx.const_eval_poly(body_id.hir_id.owner.to_def_id());
189 is_value_unfrozen_raw(cx, result, ty)
192 fn is_value_unfrozen_expr<'tcx>(cx: &LateContext<'tcx>, hir_id: HirId, def_id: DefId, ty: Ty<'tcx>) -> bool {
193 let substs = cx.typeck_results().node_substs(hir_id);
195 let result = cx.tcx.const_eval_resolve(
197 ty::Unevaluated::new(ty::WithOptConstParam::unknown(def_id), substs),
200 is_value_unfrozen_raw(cx, result, ty)
203 #[derive(Copy, Clone)]
206 Assoc { item: Span },
212 fn lint(&self) -> (&'static Lint, &'static str, Span) {
214 Self::Item { item } | Self::Assoc { item, .. } => (
215 DECLARE_INTERIOR_MUTABLE_CONST,
216 "a `const` item should never be interior mutable",
219 Self::Expr { expr } => (
220 BORROW_INTERIOR_MUTABLE_CONST,
221 "a `const` item with interior mutability should not be borrowed",
228 fn lint(cx: &LateContext<'_>, source: Source) {
229 let (lint, msg, span) = source.lint();
230 span_lint_and_then(cx, lint, span, msg, |diag| {
231 if span.from_expansion() {
232 return; // Don't give suggestions into macros.
235 Source::Item { .. } => {
236 let const_kw_span = span.from_inner(InnerSpan::new(0, 5));
237 diag.span_label(const_kw_span, "make this a static item (maybe with lazy_static)");
239 Source::Assoc { .. } => (),
240 Source::Expr { .. } => {
241 diag.help("assign this const to a local or static variable, and use the variable here");
247 declare_lint_pass!(NonCopyConst => [DECLARE_INTERIOR_MUTABLE_CONST, BORROW_INTERIOR_MUTABLE_CONST]);
249 impl<'tcx> LateLintPass<'tcx> for NonCopyConst {
250 fn check_item(&mut self, cx: &LateContext<'tcx>, it: &'tcx Item<'_>) {
251 if let ItemKind::Const(hir_ty, body_id) = it.kind {
252 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
254 if is_unfrozen(cx, ty) && is_value_unfrozen_poly(cx, body_id, ty) {
255 lint(cx, Source::Item { item: it.span });
260 fn check_trait_item(&mut self, cx: &LateContext<'tcx>, trait_item: &'tcx TraitItem<'_>) {
261 if let TraitItemKind::Const(hir_ty, body_id_opt) = &trait_item.kind {
262 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
264 // Normalize assoc types because ones originated from generic params
265 // bounded other traits could have their bound.
266 let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
267 if is_unfrozen(cx, normalized)
268 // When there's no default value, lint it only according to its type;
269 // in other words, lint consts whose value *could* be unfrozen, not definitely is.
270 // This feels inconsistent with how the lint treats generic types,
271 // which avoids linting types which potentially become unfrozen.
272 // One could check whether an unfrozen type have a *frozen variant*
273 // (like `body_id_opt.map_or_else(|| !has_frozen_variant(...), ...)`),
274 // and do the same as the case of generic types at impl items.
275 // Note that it isn't sufficient to check if it has an enum
276 // since all of that enum's variants can be unfrozen:
277 // i.e. having an enum doesn't necessary mean a type has a frozen variant.
278 // And, implementing it isn't a trivial task; it'll probably end up
279 // re-implementing the trait predicate evaluation specific to `Freeze`.
280 && body_id_opt.map_or(true, |body_id| is_value_unfrozen_poly(cx, body_id, normalized))
282 lint(cx, Source::Assoc { item: trait_item.span });
287 fn check_impl_item(&mut self, cx: &LateContext<'tcx>, impl_item: &'tcx ImplItem<'_>) {
288 if let ImplItemKind::Const(hir_ty, body_id) = &impl_item.kind {
289 let item_def_id = cx.tcx.hir().get_parent_item(impl_item.hir_id());
290 let item = cx.tcx.hir().expect_item(item_def_id);
293 ItemKind::Impl(Impl {
294 of_trait: Some(of_trait_ref),
298 // Lint a trait impl item only when the definition is a generic type,
299 // assuming an assoc const is not meant to be an interior mutable type.
300 if let Some(of_trait_def_id) = of_trait_ref.trait_def_id();
301 if let Some(of_assoc_item) = cx
303 .associated_item(impl_item.def_id)
307 .layout_of(cx.tcx.param_env(of_trait_def_id).and(
308 // Normalize assoc types because ones originated from generic params
309 // bounded other traits could have their bound at the trait defs;
310 // and, in that case, the definition is *not* generic.
311 cx.tcx.normalize_erasing_regions(
312 cx.tcx.param_env(of_trait_def_id),
313 cx.tcx.type_of(of_assoc_item),
317 // If there were a function like `has_frozen_variant` described above,
318 // we should use here as a frozen variant is a potential to be frozen
319 // similar to unknown layouts.
320 // e.g. `layout_of(...).is_err() || has_frozen_variant(...);`
321 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
322 let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
323 if is_unfrozen(cx, normalized);
324 if is_value_unfrozen_poly(cx, *body_id, normalized);
329 item: impl_item.span,
335 ItemKind::Impl(Impl { of_trait: None, .. }) => {
336 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
337 // Normalize assoc types originated from generic params.
338 let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
340 if is_unfrozen(cx, ty) && is_value_unfrozen_poly(cx, *body_id, normalized) {
341 lint(cx, Source::Assoc { item: impl_item.span });
349 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
350 if let ExprKind::Path(qpath) = &expr.kind {
351 // Only lint if we use the const item inside a function.
352 if in_constant(cx, expr.hir_id) {
356 // Make sure it is a const item.
357 let item_def_id = match cx.qpath_res(qpath, expr.hir_id) {
358 Res::Def(DefKind::Const | DefKind::AssocConst, did) => did,
362 // Climb up to resolve any field access and explicit referencing.
363 let mut cur_expr = expr;
364 let mut dereferenced_expr = expr;
365 let mut needs_check_adjustment = true;
367 let parent_id = cx.tcx.hir().get_parent_node(cur_expr.hir_id);
368 if parent_id == cur_expr.hir_id {
371 if let Some(Node::Expr(parent_expr)) = cx.tcx.hir().find(parent_id) {
372 match &parent_expr.kind {
373 ExprKind::AddrOf(..) => {
374 // `&e` => `e` must be referenced.
375 needs_check_adjustment = false;
377 ExprKind::Field(..) => {
378 needs_check_adjustment = true;
380 // Check whether implicit dereferences happened;
381 // if so, no need to go further up
382 // because of the same reason as the `ExprKind::Unary` case.
385 .expr_adjustments(dereferenced_expr)
387 .any(|adj| matches!(adj.kind, Adjust::Deref(_)))
392 dereferenced_expr = parent_expr;
394 ExprKind::Index(e, _) if ptr::eq(&**e, cur_expr) => {
395 // `e[i]` => desugared to `*Index::index(&e, i)`,
396 // meaning `e` must be referenced.
397 // no need to go further up since a method call is involved now.
398 needs_check_adjustment = false;
401 ExprKind::Unary(UnOp::Deref, _) => {
402 // `*e` => desugared to `*Deref::deref(&e)`,
403 // meaning `e` must be referenced.
404 // no need to go further up since a method call is involved now.
405 needs_check_adjustment = false;
410 cur_expr = parent_expr;
416 let ty = if needs_check_adjustment {
417 let adjustments = cx.typeck_results().expr_adjustments(dereferenced_expr);
418 if let Some(i) = adjustments
420 .position(|adj| matches!(adj.kind, Adjust::Borrow(_) | Adjust::Deref(_)))
423 cx.typeck_results().expr_ty(dereferenced_expr)
425 adjustments[i - 1].target
428 // No borrow adjustments means the entire const is moved.
432 cx.typeck_results().expr_ty(dereferenced_expr)
435 if is_unfrozen(cx, ty) && is_value_unfrozen_expr(cx, expr.hir_id, item_def_id, ty) {
436 lint(cx, Source::Expr { expr: expr.span });