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 clippy_utils::macros::macro_backtrace;
10 use if_chain::if_chain;
11 use rustc_hir::def::{DefKind, Res};
12 use rustc_hir::def_id::DefId;
14 BodyId, Expr, ExprKind, HirId, Impl, ImplItem, ImplItemKind, Item, ItemKind, Node, TraitItem, TraitItemKind, UnOp,
16 use rustc_hir_analysis::hir_ty_to_ty;
17 use rustc_lint::{LateContext, LateLintPass, Lint};
18 use rustc_middle::mir;
19 use rustc_middle::mir::interpret::{ConstValue, ErrorHandled};
20 use rustc_middle::ty::adjustment::Adjust;
21 use rustc_middle::ty::{self, Ty};
22 use rustc_session::{declare_lint_pass, declare_tool_lint};
23 use rustc_span::{sym, InnerSpan, Span};
25 // FIXME: this is a correctness problem but there's no suitable
26 // warn-by-default category.
27 declare_clippy_lint! {
29 /// Checks for declaration of `const` items which is interior
30 /// mutable (e.g., contains a `Cell`, `Mutex`, `AtomicXxxx`, etc.).
32 /// ### Why is this bad?
33 /// Consts are copied everywhere they are referenced, i.e.,
34 /// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
35 /// or `AtomicXxxx` will be created, which defeats the whole purpose of using
36 /// these types in the first place.
38 /// The `const` should better be replaced by a `static` item if a global
39 /// variable is wanted, or replaced by a `const fn` if a constructor is wanted.
41 /// ### Known problems
42 /// A "non-constant" const item is a legacy way to supply an
43 /// initialized value to downstream `static` items (e.g., the
44 /// `std::sync::ONCE_INIT` constant). In this case the use of `const` is legit,
45 /// and this lint should be suppressed.
47 /// Even though the lint avoids triggering on a constant whose type has enums that have variants
48 /// with interior mutability, and its value uses non interior mutable variants (see
49 /// [#3962](https://github.com/rust-lang/rust-clippy/issues/3962) and
50 /// [#3825](https://github.com/rust-lang/rust-clippy/issues/3825) for examples);
51 /// it complains about associated constants without default values only based on its types;
52 /// which might not be preferable.
53 /// There're other enums plus associated constants cases that the lint cannot handle.
55 /// Types that have underlying or potential interior mutability trigger the lint whether
56 /// the interior mutable field is used or not. See issues
57 /// [#5812](https://github.com/rust-lang/rust-clippy/issues/5812) and
61 /// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
63 /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
64 /// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
65 /// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
70 /// # use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
71 /// static STATIC_ATOM: AtomicUsize = AtomicUsize::new(15);
72 /// STATIC_ATOM.store(9, SeqCst);
73 /// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
75 #[clippy::version = "pre 1.29.0"]
76 pub DECLARE_INTERIOR_MUTABLE_CONST,
78 "declaring `const` with interior mutability"
81 // FIXME: this is a correctness problem but there's no suitable
82 // warn-by-default category.
83 declare_clippy_lint! {
85 /// Checks if `const` items which is interior mutable (e.g.,
86 /// contains a `Cell`, `Mutex`, `AtomicXxxx`, etc.) has been borrowed directly.
88 /// ### Why is this bad?
89 /// Consts are copied everywhere they are referenced, i.e.,
90 /// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
91 /// or `AtomicXxxx` will be created, which defeats the whole purpose of using
92 /// these types in the first place.
94 /// The `const` value should be stored inside a `static` item.
96 /// ### Known problems
97 /// When an enum has variants with interior mutability, use of its non
98 /// interior mutable variants can generate false positives. See issue
99 /// [#3962](https://github.com/rust-lang/rust-clippy/issues/3962)
101 /// Types that have underlying or potential interior mutability trigger the lint whether
102 /// the interior mutable field is used or not. See issues
103 /// [#5812](https://github.com/rust-lang/rust-clippy/issues/5812) and
104 /// [#3825](https://github.com/rust-lang/rust-clippy/issues/3825)
108 /// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
109 /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
111 /// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
112 /// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
117 /// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
118 /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
120 /// static STATIC_ATOM: AtomicUsize = CONST_ATOM;
121 /// STATIC_ATOM.store(9, SeqCst);
122 /// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
124 #[clippy::version = "pre 1.29.0"]
125 pub BORROW_INTERIOR_MUTABLE_CONST,
127 "referencing `const` with interior mutability"
130 fn is_unfrozen<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
131 // Ignore types whose layout is unknown since `is_freeze` reports every generic types as `!Freeze`,
132 // making it indistinguishable from `UnsafeCell`. i.e. it isn't a tool to prove a type is
133 // 'unfrozen'. However, this code causes a false negative in which
134 // a type contains a layout-unknown type, but also an unsafe cell like `const CELL: Cell<T>`.
135 // Yet, it's better than `ty.has_type_flags(TypeFlags::HAS_TY_PARAM | TypeFlags::HAS_PROJECTION)`
136 // since it works when a pointer indirection involves (`Cell<*const T>`).
137 // Making up a `ParamEnv` where every generic params and assoc types are `Freeze`is another option;
138 // but I'm not sure whether it's a decent way, if possible.
139 cx.tcx.layout_of(cx.param_env.and(ty)).is_ok() && !ty.is_freeze(cx.tcx, cx.param_env)
142 fn is_value_unfrozen_raw<'tcx>(
143 cx: &LateContext<'tcx>,
144 result: Result<ConstValue<'tcx>, ErrorHandled>,
147 fn inner<'tcx>(cx: &LateContext<'tcx>, val: mir::ConstantKind<'tcx>) -> bool {
148 match val.ty().kind() {
149 // the fact that we have to dig into every structs to search enums
150 // leads us to the point checking `UnsafeCell` directly is the only option.
151 ty::Adt(ty_def, ..) if ty_def.is_unsafe_cell() => true,
152 // As of 2022-09-08 miri doesn't track which union field is active so there's no safe way to check the
154 ty::Adt(def, ..) if def.is_union() => false,
155 ty::Array(..) | ty::Adt(..) | ty::Tuple(..) => {
156 let val = cx.tcx.destructure_mir_constant(cx.param_env, val);
157 val.fields.iter().any(|field| inner(cx, *field))
164 // Consider `TooGeneric` cases as being unfrozen.
165 // This causes a false positive where an assoc const whose type is unfrozen
166 // have a value that is a frozen variant with a generic param (an example is
167 // `declare_interior_mutable_const::enums::BothOfCellAndGeneric::GENERIC_VARIANT`).
168 // However, it prevents a number of false negatives that is, I think, important:
169 // 1. assoc consts in trait defs referring to consts of themselves
170 // (an example is `declare_interior_mutable_const::traits::ConcreteTypes::ANOTHER_ATOMIC`).
171 // 2. a path expr referring to assoc consts whose type is doesn't have
172 // any frozen variants in trait defs (i.e. without substitute for `Self`).
173 // (e.g. borrowing `borrow_interior_mutable_const::trait::ConcreteTypes::ATOMIC`)
174 // 3. similar to the false positive above;
175 // but the value is an unfrozen variant, or the type has no enums. (An example is
176 // `declare_interior_mutable_const::enums::BothOfCellAndGeneric::UNFROZEN_VARIANT`
177 // and `declare_interior_mutable_const::enums::BothOfCellAndGeneric::NO_ENUM`).
178 // One might be able to prevent these FNs correctly, and replace this with `false`;
179 // e.g. implementing `has_frozen_variant` described above, and not running this function
180 // when the type doesn't have any frozen variants would be the 'correct' way for the 2nd
181 // case (that actually removes another suboptimal behavior (I won't say 'false positive') where,
182 // similar to 2., but with the a frozen variant) (e.g. borrowing
183 // `borrow_interior_mutable_const::enums::AssocConsts::TO_BE_FROZEN_VARIANT`).
184 // I chose this way because unfrozen enums as assoc consts are rare (or, hopefully, none).
185 err == ErrorHandled::TooGeneric
187 |val| inner(cx, mir::ConstantKind::from_value(val, ty)),
191 fn is_value_unfrozen_poly<'tcx>(cx: &LateContext<'tcx>, body_id: BodyId, ty: Ty<'tcx>) -> bool {
192 let result = cx.tcx.const_eval_poly(body_id.hir_id.owner.to_def_id());
193 is_value_unfrozen_raw(cx, result, ty)
196 fn is_value_unfrozen_expr<'tcx>(cx: &LateContext<'tcx>, hir_id: HirId, def_id: DefId, ty: Ty<'tcx>) -> bool {
197 let substs = cx.typeck_results().node_substs(hir_id);
199 let result = cx.tcx.const_eval_resolve(
201 mir::UnevaluatedConst::new(ty::WithOptConstParam::unknown(def_id), substs),
204 is_value_unfrozen_raw(cx, result, ty)
207 #[derive(Copy, Clone)]
210 Assoc { item: Span },
216 fn lint(&self) -> (&'static Lint, &'static str, Span) {
218 Self::Item { item } | Self::Assoc { item, .. } => (
219 DECLARE_INTERIOR_MUTABLE_CONST,
220 "a `const` item should never be interior mutable",
223 Self::Expr { expr } => (
224 BORROW_INTERIOR_MUTABLE_CONST,
225 "a `const` item with interior mutability should not be borrowed",
232 fn lint(cx: &LateContext<'_>, source: Source) {
233 let (lint, msg, span) = source.lint();
234 span_lint_and_then(cx, lint, span, msg, |diag| {
235 if span.from_expansion() {
236 return; // Don't give suggestions into macros.
239 Source::Item { .. } => {
240 let const_kw_span = span.from_inner(InnerSpan::new(0, 5));
241 diag.span_label(const_kw_span, "make this a static item (maybe with lazy_static)");
243 Source::Assoc { .. } => (),
244 Source::Expr { .. } => {
245 diag.help("assign this const to a local or static variable, and use the variable here");
251 declare_lint_pass!(NonCopyConst => [DECLARE_INTERIOR_MUTABLE_CONST, BORROW_INTERIOR_MUTABLE_CONST]);
253 impl<'tcx> LateLintPass<'tcx> for NonCopyConst {
254 fn check_item(&mut self, cx: &LateContext<'tcx>, it: &'tcx Item<'_>) {
255 if let ItemKind::Const(hir_ty, body_id) = it.kind {
256 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
257 if !ignored_macro(cx, it) && is_unfrozen(cx, ty) && is_value_unfrozen_poly(cx, body_id, ty) {
258 lint(cx, Source::Item { item: it.span });
263 fn check_trait_item(&mut self, cx: &LateContext<'tcx>, trait_item: &'tcx TraitItem<'_>) {
264 if let TraitItemKind::Const(hir_ty, body_id_opt) = &trait_item.kind {
265 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
267 // Normalize assoc types because ones originated from generic params
268 // bounded other traits could have their bound.
269 let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
270 if is_unfrozen(cx, normalized)
271 // When there's no default value, lint it only according to its type;
272 // in other words, lint consts whose value *could* be unfrozen, not definitely is.
273 // This feels inconsistent with how the lint treats generic types,
274 // which avoids linting types which potentially become unfrozen.
275 // One could check whether an unfrozen type have a *frozen variant*
276 // (like `body_id_opt.map_or_else(|| !has_frozen_variant(...), ...)`),
277 // and do the same as the case of generic types at impl items.
278 // Note that it isn't sufficient to check if it has an enum
279 // since all of that enum's variants can be unfrozen:
280 // i.e. having an enum doesn't necessary mean a type has a frozen variant.
281 // And, implementing it isn't a trivial task; it'll probably end up
282 // re-implementing the trait predicate evaluation specific to `Freeze`.
283 && body_id_opt.map_or(true, |body_id| is_value_unfrozen_poly(cx, body_id, normalized))
285 lint(cx, Source::Assoc { item: trait_item.span });
290 fn check_impl_item(&mut self, cx: &LateContext<'tcx>, impl_item: &'tcx ImplItem<'_>) {
291 if let ImplItemKind::Const(hir_ty, body_id) = &impl_item.kind {
292 let item_def_id = cx.tcx.hir().get_parent_item(impl_item.hir_id()).def_id;
293 let item = cx.tcx.hir().expect_item(item_def_id);
296 ItemKind::Impl(Impl {
297 of_trait: Some(of_trait_ref),
301 // Lint a trait impl item only when the definition is a generic type,
302 // assuming an assoc const is not meant to be an interior mutable type.
303 if let Some(of_trait_def_id) = of_trait_ref.trait_def_id();
304 if let Some(of_assoc_item) = cx
306 .associated_item(impl_item.owner_id)
310 .layout_of(cx.tcx.param_env(of_trait_def_id).and(
311 // Normalize assoc types because ones originated from generic params
312 // bounded other traits could have their bound at the trait defs;
313 // and, in that case, the definition is *not* generic.
314 cx.tcx.normalize_erasing_regions(
315 cx.tcx.param_env(of_trait_def_id),
316 cx.tcx.type_of(of_assoc_item),
320 // If there were a function like `has_frozen_variant` described above,
321 // we should use here as a frozen variant is a potential to be frozen
322 // similar to unknown layouts.
323 // e.g. `layout_of(...).is_err() || has_frozen_variant(...);`
324 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
325 let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
326 if is_unfrozen(cx, normalized);
327 if is_value_unfrozen_poly(cx, *body_id, normalized);
332 item: impl_item.span,
338 ItemKind::Impl(Impl { of_trait: None, .. }) => {
339 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
340 // Normalize assoc types originated from generic params.
341 let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
343 if is_unfrozen(cx, ty) && is_value_unfrozen_poly(cx, *body_id, normalized) {
344 lint(cx, Source::Assoc { item: impl_item.span });
352 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
353 if let ExprKind::Path(qpath) = &expr.kind {
354 // Only lint if we use the const item inside a function.
355 if in_constant(cx, expr.hir_id) {
359 // Make sure it is a const item.
360 let Res::Def(DefKind::Const | DefKind::AssocConst, item_def_id) = cx.qpath_res(qpath, expr.hir_id) else {
364 // Climb up to resolve any field access and explicit referencing.
365 let mut cur_expr = expr;
366 let mut dereferenced_expr = expr;
367 let mut needs_check_adjustment = true;
369 let parent_id = cx.tcx.hir().get_parent_node(cur_expr.hir_id);
370 if parent_id == cur_expr.hir_id {
373 if let Some(Node::Expr(parent_expr)) = cx.tcx.hir().find(parent_id) {
374 match &parent_expr.kind {
375 ExprKind::AddrOf(..) => {
376 // `&e` => `e` must be referenced.
377 needs_check_adjustment = false;
379 ExprKind::Field(..) => {
380 needs_check_adjustment = true;
382 // Check whether implicit dereferences happened;
383 // if so, no need to go further up
384 // because of the same reason as the `ExprKind::Unary` case.
387 .expr_adjustments(dereferenced_expr)
389 .any(|adj| matches!(adj.kind, Adjust::Deref(_)))
394 dereferenced_expr = parent_expr;
396 ExprKind::Index(e, _) if ptr::eq(&**e, cur_expr) => {
397 // `e[i]` => desugared to `*Index::index(&e, i)`,
398 // meaning `e` must be referenced.
399 // no need to go further up since a method call is involved now.
400 needs_check_adjustment = false;
403 ExprKind::Unary(UnOp::Deref, _) => {
404 // `*e` => desugared to `*Deref::deref(&e)`,
405 // meaning `e` must be referenced.
406 // no need to go further up since a method call is involved now.
407 needs_check_adjustment = false;
412 cur_expr = parent_expr;
418 let ty = if needs_check_adjustment {
419 let adjustments = cx.typeck_results().expr_adjustments(dereferenced_expr);
420 if let Some(i) = adjustments
422 .position(|adj| matches!(adj.kind, Adjust::Borrow(_) | Adjust::Deref(_)))
425 cx.typeck_results().expr_ty(dereferenced_expr)
427 adjustments[i - 1].target
430 // No borrow adjustments means the entire const is moved.
434 cx.typeck_results().expr_ty(dereferenced_expr)
437 if is_unfrozen(cx, ty) && is_value_unfrozen_expr(cx, expr.hir_id, item_def_id, ty) {
438 lint(cx, Source::Expr { expr: expr.span });
444 fn ignored_macro(cx: &LateContext<'_>, it: &rustc_hir::Item<'_>) -> bool {
445 macro_backtrace(it.span).any(|macro_call| {
447 cx.tcx.get_diagnostic_name(macro_call.def_id),
448 Some(sym::thread_local_macro)