1 //! Checks for uses of const which the type is not `Freeze` (`Cell`-free).
3 //! This lint is **warn** by default.
7 use rustc_hir::def::{DefKind, Res};
8 use rustc_hir::def_id::DefId;
10 BodyId, Expr, ExprKind, HirId, ImplItem, ImplItemKind, Item, ItemKind, Node, TraitItem, TraitItemKind, UnOp,
12 use rustc_infer::traits::specialization_graph;
13 use rustc_lint::{LateContext, LateLintPass, Lint};
14 use rustc_middle::mir::interpret::{ConstValue, ErrorHandled};
15 use rustc_middle::ty::adjustment::Adjust;
16 use rustc_middle::ty::{self, AssocKind, Const, Ty};
17 use rustc_session::{declare_lint_pass, declare_tool_lint};
18 use rustc_span::{InnerSpan, Span, DUMMY_SP};
19 use rustc_typeck::hir_ty_to_ty;
21 use crate::utils::{in_constant, qpath_res, span_lint_and_then};
22 use if_chain::if_chain;
24 // FIXME: this is a correctness problem but there's no suitable
25 // warn-by-default category.
26 declare_clippy_lint! {
27 /// **What it does:** Checks for declaration of `const` items which is interior
28 /// mutable (e.g., contains a `Cell`, `Mutex`, `AtomicXxxx`, etc.).
30 /// **Why is this bad?** Consts are copied everywhere they are referenced, i.e.,
31 /// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
32 /// or `AtomicXxxx` will be created, which defeats the whole purpose of using
33 /// these types in the first place.
35 /// The `const` should better be replaced by a `static` item if a global
36 /// variable is wanted, or replaced by a `const fn` if a constructor is wanted.
38 /// **Known problems:** A "non-constant" const item is a legacy way to supply an
39 /// initialized value to downstream `static` items (e.g., the
40 /// `std::sync::ONCE_INIT` constant). In this case the use of `const` is legit,
41 /// and this lint should be suppressed.
43 /// Even though the lint avoids triggering on a constant whose type has enums that have variants
44 /// with interior mutability, and its value uses non interior mutable variants (see
45 /// [#3962](https://github.com/rust-lang/rust-clippy/issues/3962) and
46 /// [#3825](https://github.com/rust-lang/rust-clippy/issues/3825) for examples);
47 /// it complains about associated constants without default values only based on its types;
48 /// which might not be preferable.
49 /// There're other enums plus associated constants cases that the lint cannot handle.
51 /// Types that have underlying or potential interior mutability trigger the lint whether
52 /// the interior mutable field is used or not. See issues
53 /// [#5812](https://github.com/rust-lang/rust-clippy/issues/5812) and
57 /// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
60 /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
61 /// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
62 /// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
65 /// static STATIC_ATOM: AtomicUsize = AtomicUsize::new(15);
66 /// STATIC_ATOM.store(9, SeqCst);
67 /// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
69 pub DECLARE_INTERIOR_MUTABLE_CONST,
71 "declaring `const` with interior mutability"
74 // FIXME: this is a correctness problem but there's no suitable
75 // warn-by-default category.
76 declare_clippy_lint! {
77 /// **What it does:** Checks if `const` items which is interior mutable (e.g.,
78 /// contains a `Cell`, `Mutex`, `AtomicXxxx`, etc.) has been borrowed directly.
80 /// **Why is this bad?** Consts are copied everywhere they are referenced, i.e.,
81 /// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
82 /// or `AtomicXxxx` will be created, which defeats the whole purpose of using
83 /// these types in the first place.
85 /// The `const` value should be stored inside a `static` item.
87 /// **Known problems:** When an enum has variants with interior mutability, use of its non
88 /// interior mutable variants can generate false positives. See issue
89 /// [#3962](https://github.com/rust-lang/rust-clippy/issues/3962)
91 /// Types that have underlying or potential interior mutability trigger the lint whether
92 /// the interior mutable field is used or not. See issues
93 /// [#5812](https://github.com/rust-lang/rust-clippy/issues/5812) and
94 /// [#3825](https://github.com/rust-lang/rust-clippy/issues/3825)
98 /// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
99 /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
102 /// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
103 /// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
106 /// static STATIC_ATOM: AtomicUsize = CONST_ATOM;
107 /// STATIC_ATOM.store(9, SeqCst);
108 /// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
110 pub BORROW_INTERIOR_MUTABLE_CONST,
112 "referencing `const` with interior mutability"
115 fn is_unfrozen<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
116 // Ignore types whose layout is unknown since `is_freeze` reports every generic types as `!Freeze`,
117 // making it indistinguishable from `UnsafeCell`. i.e. it isn't a tool to prove a type is
118 // 'unfrozen'. However, this code causes a false negative in which
119 // a type contains a layout-unknown type, but also a unsafe cell like `const CELL: Cell<T>`.
120 // Yet, it's better than `ty.has_type_flags(TypeFlags::HAS_TY_PARAM | TypeFlags::HAS_PROJECTION)`
121 // since it works when a pointer indirection involves (`Cell<*const T>`).
122 // Making up a `ParamEnv` where every generic params and assoc types are `Freeze`is another option;
123 // but I'm not sure whether it's a decent way, if possible.
124 cx.tcx.layout_of(cx.param_env.and(ty)).is_ok() && !ty.is_freeze(cx.tcx.at(DUMMY_SP), cx.param_env)
127 fn is_value_unfrozen_raw<'tcx>(
128 cx: &LateContext<'tcx>,
129 result: Result<ConstValue<'tcx>, ErrorHandled>,
132 fn inner<'tcx>(cx: &LateContext<'tcx>, val: &'tcx Const<'tcx>) -> bool {
133 match val.ty.kind() {
134 // the fact that we have to dig into every structs to search enums
135 // leads us to the point checking `UnsafeCell` directly is the only option.
136 ty::Adt(ty_def, ..) if Some(ty_def.did) == cx.tcx.lang_items().unsafe_cell_type() => true,
137 ty::Array(..) | ty::Adt(..) | ty::Tuple(..) => {
138 let val = cx.tcx.destructure_const(cx.param_env.and(val));
139 val.fields.iter().any(|field| inner(cx, field))
147 // Consider `TooGeneric` cases as being unfrozen.
148 // This causes a false positive where an assoc const whose type is unfrozen
149 // have a value that is a frozen variant with a generic param (an example is
150 // `declare_interior_mutable_const::enums::BothOfCellAndGeneric::GENERIC_VARIANT`).
151 // However, it prevents a number of false negatives that is, I think, important:
152 // 1. assoc consts in trait defs referring to consts of themselves
153 // (an example is `declare_interior_mutable_const::traits::ConcreteTypes::ANOTHER_ATOMIC`).
154 // 2. a path expr referring to assoc consts whose type is doesn't have
155 // any frozen variants in trait defs (i.e. without substitute for `Self`).
156 // (e.g. borrowing `borrow_interior_mutable_const::trait::ConcreteTypes::ATOMIC`)
157 // 3. similar to the false positive above;
158 // but the value is an unfrozen variant, or the type has no enums. (An example is
159 // `declare_interior_mutable_const::enums::BothOfCellAndGeneric::UNFROZEN_VARIANT`
160 // and `declare_interior_mutable_const::enums::BothOfCellAndGeneric::NO_ENUM`).
161 // One might be able to prevent these FNs correctly, and replace this with `false`;
162 // e.g. implementing `has_frozen_variant` described above, and not running this function
163 // when the type doesn't have any frozen variants would be the 'correct' way for the 2nd
164 // case (that actually removes another suboptimal behavior (I won't say 'false positive') where,
165 // similar to 2., but with the a frozen variant) (e.g. borrowing
166 // `borrow_interior_mutable_const::enums::AssocConsts::TO_BE_FROZEN_VARIANT`).
167 // I chose this way because unfrozen enums as assoc consts are rare (or, hopefully, none).
168 err == ErrorHandled::TooGeneric
170 |val| inner(cx, Const::from_value(cx.tcx, val, ty)),
174 fn is_value_unfrozen_poly<'tcx>(cx: &LateContext<'tcx>, body_id: BodyId, ty: Ty<'tcx>) -> bool {
175 let result = cx.tcx.const_eval_poly(body_id.hir_id.owner.to_def_id());
176 is_value_unfrozen_raw(cx, result, ty)
179 fn is_value_unfrozen_expr<'tcx>(cx: &LateContext<'tcx>, hir_id: HirId, def_id: DefId, ty: Ty<'tcx>) -> bool {
180 let substs = cx.typeck_results().node_substs(hir_id);
184 .const_eval_resolve(cx.param_env, ty::WithOptConstParam::unknown(def_id), substs, None, None);
185 is_value_unfrozen_raw(cx, result, ty)
188 #[derive(Copy, Clone)]
191 Assoc { item: Span },
197 fn lint(&self) -> (&'static Lint, &'static str, Span) {
199 Self::Item { item } | Self::Assoc { item, .. } => (
200 DECLARE_INTERIOR_MUTABLE_CONST,
201 "a `const` item should never be interior mutable",
204 Self::Expr { expr } => (
205 BORROW_INTERIOR_MUTABLE_CONST,
206 "a `const` item with interior mutability should not be borrowed",
213 fn lint(cx: &LateContext<'_>, source: Source) {
214 let (lint, msg, span) = source.lint();
215 span_lint_and_then(cx, lint, span, msg, |diag| {
216 if span.from_expansion() {
217 return; // Don't give suggestions into macros.
220 Source::Item { .. } => {
221 let const_kw_span = span.from_inner(InnerSpan::new(0, 5));
222 diag.span_label(const_kw_span, "make this a static item (maybe with lazy_static)");
224 Source::Assoc { .. } => (),
225 Source::Expr { .. } => {
226 diag.help("assign this const to a local or static variable, and use the variable here");
232 declare_lint_pass!(NonCopyConst => [DECLARE_INTERIOR_MUTABLE_CONST, BORROW_INTERIOR_MUTABLE_CONST]);
234 impl<'tcx> LateLintPass<'tcx> for NonCopyConst {
235 fn check_item(&mut self, cx: &LateContext<'tcx>, it: &'tcx Item<'_>) {
236 if let ItemKind::Const(hir_ty, body_id) = it.kind {
237 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
239 if is_unfrozen(cx, ty) && is_value_unfrozen_poly(cx, body_id, ty) {
240 lint(cx, Source::Item { item: it.span });
245 fn check_trait_item(&mut self, cx: &LateContext<'tcx>, trait_item: &'tcx TraitItem<'_>) {
246 if let TraitItemKind::Const(hir_ty, body_id_opt) = &trait_item.kind {
247 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
249 // Normalize assoc types because ones originated from generic params
250 // bounded other traits could have their bound.
251 let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
252 if is_unfrozen(cx, normalized)
253 // When there's no default value, lint it only according to its type;
254 // in other words, lint consts whose value *could* be unfrozen, not definitely is.
255 // This feels inconsistent with how the lint treats generic types,
256 // which avoids linting types which potentially become unfrozen.
257 // One could check whether a unfrozen type have a *frozen variant*
258 // (like `body_id_opt.map_or_else(|| !has_frozen_variant(...), ...)`),
259 // and do the same as the case of generic types at impl items.
260 // Note that it isn't sufficient to check if it has an enum
261 // since all of that enum's variants can be unfrozen:
262 // i.e. having an enum doesn't necessary mean a type has a frozen variant.
263 // And, implementing it isn't a trivial task; it'll probably end up
264 // re-implementing the trait predicate evaluation specific to `Freeze`.
265 && body_id_opt.map_or(true, |body_id| is_value_unfrozen_poly(cx, body_id, normalized))
267 lint(cx, Source::Assoc { item: trait_item.span });
272 fn check_impl_item(&mut self, cx: &LateContext<'tcx>, impl_item: &'tcx ImplItem<'_>) {
273 if let ImplItemKind::Const(hir_ty, body_id) = &impl_item.kind {
274 let item_hir_id = cx.tcx.hir().get_parent_node(impl_item.hir_id);
275 let item = cx.tcx.hir().expect_item(item_hir_id);
279 of_trait: Some(of_trait_ref),
283 // Lint a trait impl item only when the definition is a generic type,
284 // assuming a assoc const is not meant to be a interior mutable type.
285 if let Some(of_trait_def_id) = of_trait_ref.trait_def_id();
286 if let Some(of_assoc_item) = specialization_graph::Node::Trait(of_trait_def_id)
287 .item(cx.tcx, impl_item.ident, AssocKind::Const, of_trait_def_id);
290 .layout_of(cx.tcx.param_env(of_trait_def_id).and(
291 // Normalize assoc types because ones originated from generic params
292 // bounded other traits could have their bound at the trait defs;
293 // and, in that case, the definition is *not* generic.
294 cx.tcx.normalize_erasing_regions(
295 cx.tcx.param_env(of_trait_def_id),
296 cx.tcx.type_of(of_assoc_item.def_id),
300 // If there were a function like `has_frozen_variant` described above,
301 // we should use here as a frozen variant is a potential to be frozen
302 // similar to unknown layouts.
303 // e.g. `layout_of(...).is_err() || has_frozen_variant(...);`
305 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
306 let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
307 if is_unfrozen(cx, normalized)
308 && is_value_unfrozen_poly(cx, *body_id, normalized)
313 item: impl_item.span,
320 ItemKind::Impl { of_trait: None, .. } => {
321 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
322 // Normalize assoc types originated from generic params.
323 let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
325 if is_unfrozen(cx, ty) && is_value_unfrozen_poly(cx, *body_id, normalized) {
326 lint(cx, Source::Assoc { item: impl_item.span });
334 fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
335 if let ExprKind::Path(qpath) = &expr.kind {
336 // Only lint if we use the const item inside a function.
337 if in_constant(cx, expr.hir_id) {
341 // Make sure it is a const item.
342 let item_def_id = match qpath_res(cx, qpath, expr.hir_id) {
343 Res::Def(DefKind::Const | DefKind::AssocConst, did) => did,
347 // Climb up to resolve any field access and explicit referencing.
348 let mut cur_expr = expr;
349 let mut dereferenced_expr = expr;
350 let mut needs_check_adjustment = true;
352 let parent_id = cx.tcx.hir().get_parent_node(cur_expr.hir_id);
353 if parent_id == cur_expr.hir_id {
356 if let Some(Node::Expr(parent_expr)) = cx.tcx.hir().find(parent_id) {
357 match &parent_expr.kind {
358 ExprKind::AddrOf(..) => {
359 // `&e` => `e` must be referenced.
360 needs_check_adjustment = false;
362 ExprKind::Field(..) => {
363 needs_check_adjustment = true;
365 // Check whether implicit dereferences happened;
366 // if so, no need to go further up
367 // because of the same reason as the `ExprKind::Unary` case.
370 .expr_adjustments(dereferenced_expr)
372 .any(|adj| matches!(adj.kind, Adjust::Deref(_)))
377 dereferenced_expr = parent_expr;
379 ExprKind::Index(e, _) if ptr::eq(&**e, cur_expr) => {
380 // `e[i]` => desugared to `*Index::index(&e, i)`,
381 // meaning `e` must be referenced.
382 // no need to go further up since a method call is involved now.
383 needs_check_adjustment = false;
386 ExprKind::Unary(UnOp::UnDeref, _) => {
387 // `*e` => desugared to `*Deref::deref(&e)`,
388 // meaning `e` must be referenced.
389 // no need to go further up since a method call is involved now.
390 needs_check_adjustment = false;
395 cur_expr = parent_expr;
401 let ty = if needs_check_adjustment {
402 let adjustments = cx.typeck_results().expr_adjustments(dereferenced_expr);
403 if let Some(i) = adjustments
405 .position(|adj| matches!(adj.kind, Adjust::Borrow(_) | Adjust::Deref(_)))
408 cx.typeck_results().expr_ty(dereferenced_expr)
410 adjustments[i - 1].target
413 // No borrow adjustments means the entire const is moved.
417 cx.typeck_results().expr_ty(dereferenced_expr)
420 if is_unfrozen(cx, ty) && is_value_unfrozen_expr(cx, expr.hir_id, item_def_id, ty) {
421 lint(cx, Source::Expr { expr: expr.span });