--- /dev/null
+//! Checks for uses of const which the type is not `Freeze` (`Cell`-free).
+//!
+//! This lint is **warn** by default.
+
+use std::ptr;
+
+use rustc_hir::def::{DefKind, Res};
+use rustc_hir::def_id::DefId;
+use rustc_hir::{
+ BodyId, Expr, ExprKind, HirId, Impl, ImplItem, ImplItemKind, Item, ItemKind, Node, TraitItem, TraitItemKind, UnOp,
+};
+use rustc_infer::traits::specialization_graph;
+use rustc_lint::{LateContext, LateLintPass, Lint};
+use rustc_middle::mir::interpret::{ConstValue, ErrorHandled};
+use rustc_middle::ty::adjustment::Adjust;
+use rustc_middle::ty::{self, AssocKind, Const, Ty};
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::{InnerSpan, Span, DUMMY_SP};
+use rustc_typeck::hir_ty_to_ty;
+
+use crate::utils::{in_constant, span_lint_and_then};
+use if_chain::if_chain;
+
+// FIXME: this is a correctness problem but there's no suitable
+// warn-by-default category.
+declare_clippy_lint! {
+ /// **What it does:** Checks for declaration of `const` items which is interior
+ /// mutable (e.g., contains a `Cell`, `Mutex`, `AtomicXxxx`, etc.).
+ ///
+ /// **Why is this bad?** Consts are copied everywhere they are referenced, i.e.,
+ /// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
+ /// or `AtomicXxxx` will be created, which defeats the whole purpose of using
+ /// these types in the first place.
+ ///
+ /// The `const` should better be replaced by a `static` item if a global
+ /// variable is wanted, or replaced by a `const fn` if a constructor is wanted.
+ ///
+ /// **Known problems:** A "non-constant" const item is a legacy way to supply an
+ /// initialized value to downstream `static` items (e.g., the
+ /// `std::sync::ONCE_INIT` constant). In this case the use of `const` is legit,
+ /// and this lint should be suppressed.
+ ///
+ /// Even though the lint avoids triggering on a constant whose type has enums that have variants
+ /// with interior mutability, and its value uses non interior mutable variants (see
+ /// [#3962](https://github.com/rust-lang/rust-clippy/issues/3962) and
+ /// [#3825](https://github.com/rust-lang/rust-clippy/issues/3825) for examples);
+ /// it complains about associated constants without default values only based on its types;
+ /// which might not be preferable.
+ /// There're other enums plus associated constants cases that the lint cannot handle.
+ ///
+ /// Types that have underlying or potential interior mutability trigger the lint whether
+ /// the interior mutable field is used or not. See issues
+ /// [#5812](https://github.com/rust-lang/rust-clippy/issues/5812) and
+ ///
+ /// **Example:**
+ /// ```rust
+ /// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
+ ///
+ /// // Bad.
+ /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
+ /// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
+ /// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
+ ///
+ /// // Good.
+ /// static STATIC_ATOM: AtomicUsize = AtomicUsize::new(15);
+ /// STATIC_ATOM.store(9, SeqCst);
+ /// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
+ /// ```
+ pub DECLARE_INTERIOR_MUTABLE_CONST,
+ style,
+ "declaring `const` with interior mutability"
+}
+
+// FIXME: this is a correctness problem but there's no suitable
+// warn-by-default category.
+declare_clippy_lint! {
+ /// **What it does:** Checks if `const` items which is interior mutable (e.g.,
+ /// contains a `Cell`, `Mutex`, `AtomicXxxx`, etc.) has been borrowed directly.
+ ///
+ /// **Why is this bad?** Consts are copied everywhere they are referenced, i.e.,
+ /// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
+ /// or `AtomicXxxx` will be created, which defeats the whole purpose of using
+ /// these types in the first place.
+ ///
+ /// The `const` value should be stored inside a `static` item.
+ ///
+ /// **Known problems:** When an enum has variants with interior mutability, use of its non
+ /// interior mutable variants can generate false positives. See issue
+ /// [#3962](https://github.com/rust-lang/rust-clippy/issues/3962)
+ ///
+ /// Types that have underlying or potential interior mutability trigger the lint whether
+ /// the interior mutable field is used or not. See issues
+ /// [#5812](https://github.com/rust-lang/rust-clippy/issues/5812) and
+ /// [#3825](https://github.com/rust-lang/rust-clippy/issues/3825)
+ ///
+ /// **Example:**
+ /// ```rust
+ /// use std::sync::atomic::{AtomicUsize, Ordering::SeqCst};
+ /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
+ ///
+ /// // Bad.
+ /// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
+ /// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
+ ///
+ /// // Good.
+ /// static STATIC_ATOM: AtomicUsize = CONST_ATOM;
+ /// STATIC_ATOM.store(9, SeqCst);
+ /// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
+ /// ```
+ pub BORROW_INTERIOR_MUTABLE_CONST,
+ style,
+ "referencing `const` with interior mutability"
+}
+
+fn is_unfrozen<'tcx>(cx: &LateContext<'tcx>, ty: Ty<'tcx>) -> bool {
+ // Ignore types whose layout is unknown since `is_freeze` reports every generic types as `!Freeze`,
+ // making it indistinguishable from `UnsafeCell`. i.e. it isn't a tool to prove a type is
+ // 'unfrozen'. However, this code causes a false negative in which
+ // a type contains a layout-unknown type, but also a unsafe cell like `const CELL: Cell<T>`.
+ // Yet, it's better than `ty.has_type_flags(TypeFlags::HAS_TY_PARAM | TypeFlags::HAS_PROJECTION)`
+ // since it works when a pointer indirection involves (`Cell<*const T>`).
+ // Making up a `ParamEnv` where every generic params and assoc types are `Freeze`is another option;
+ // but I'm not sure whether it's a decent way, if possible.
+ cx.tcx.layout_of(cx.param_env.and(ty)).is_ok() && !ty.is_freeze(cx.tcx.at(DUMMY_SP), cx.param_env)
+}
+
+fn is_value_unfrozen_raw<'tcx>(
+ cx: &LateContext<'tcx>,
+ result: Result<ConstValue<'tcx>, ErrorHandled>,
+ ty: Ty<'tcx>,
+) -> bool {
+ fn inner<'tcx>(cx: &LateContext<'tcx>, val: &'tcx Const<'tcx>) -> bool {
+ match val.ty.kind() {
+ // the fact that we have to dig into every structs to search enums
+ // leads us to the point checking `UnsafeCell` directly is the only option.
+ ty::Adt(ty_def, ..) if Some(ty_def.did) == cx.tcx.lang_items().unsafe_cell_type() => true,
+ ty::Array(..) | ty::Adt(..) | ty::Tuple(..) => {
+ let val = cx.tcx.destructure_const(cx.param_env.and(val));
+ val.fields.iter().any(|field| inner(cx, field))
+ },
+ _ => false,
+ }
+ }
+
+ result.map_or_else(
+ |err| {
+ // Consider `TooGeneric` cases as being unfrozen.
+ // This causes a false positive where an assoc const whose type is unfrozen
+ // have a value that is a frozen variant with a generic param (an example is
+ // `declare_interior_mutable_const::enums::BothOfCellAndGeneric::GENERIC_VARIANT`).
+ // However, it prevents a number of false negatives that is, I think, important:
+ // 1. assoc consts in trait defs referring to consts of themselves
+ // (an example is `declare_interior_mutable_const::traits::ConcreteTypes::ANOTHER_ATOMIC`).
+ // 2. a path expr referring to assoc consts whose type is doesn't have
+ // any frozen variants in trait defs (i.e. without substitute for `Self`).
+ // (e.g. borrowing `borrow_interior_mutable_const::trait::ConcreteTypes::ATOMIC`)
+ // 3. similar to the false positive above;
+ // but the value is an unfrozen variant, or the type has no enums. (An example is
+ // `declare_interior_mutable_const::enums::BothOfCellAndGeneric::UNFROZEN_VARIANT`
+ // and `declare_interior_mutable_const::enums::BothOfCellAndGeneric::NO_ENUM`).
+ // One might be able to prevent these FNs correctly, and replace this with `false`;
+ // e.g. implementing `has_frozen_variant` described above, and not running this function
+ // when the type doesn't have any frozen variants would be the 'correct' way for the 2nd
+ // case (that actually removes another suboptimal behavior (I won't say 'false positive') where,
+ // similar to 2., but with the a frozen variant) (e.g. borrowing
+ // `borrow_interior_mutable_const::enums::AssocConsts::TO_BE_FROZEN_VARIANT`).
+ // I chose this way because unfrozen enums as assoc consts are rare (or, hopefully, none).
+ err == ErrorHandled::TooGeneric
+ },
+ |val| inner(cx, Const::from_value(cx.tcx, val, ty)),
+ )
+}
+
+fn is_value_unfrozen_poly<'tcx>(cx: &LateContext<'tcx>, body_id: BodyId, ty: Ty<'tcx>) -> bool {
+ let result = cx.tcx.const_eval_poly(body_id.hir_id.owner.to_def_id());
+ is_value_unfrozen_raw(cx, result, ty)
+}
+
+fn is_value_unfrozen_expr<'tcx>(cx: &LateContext<'tcx>, hir_id: HirId, def_id: DefId, ty: Ty<'tcx>) -> bool {
+ let substs = cx.typeck_results().node_substs(hir_id);
+
+ let result = cx
+ .tcx
+ .const_eval_resolve(cx.param_env, ty::WithOptConstParam::unknown(def_id), substs, None, None);
+ is_value_unfrozen_raw(cx, result, ty)
+}
+
+#[derive(Copy, Clone)]
+enum Source {
+ Item { item: Span },
+ Assoc { item: Span },
+ Expr { expr: Span },
+}
+
+impl Source {
+ #[must_use]
+ fn lint(&self) -> (&'static Lint, &'static str, Span) {
+ match self {
+ Self::Item { item } | Self::Assoc { item, .. } => (
+ DECLARE_INTERIOR_MUTABLE_CONST,
+ "a `const` item should never be interior mutable",
+ *item,
+ ),
+ Self::Expr { expr } => (
+ BORROW_INTERIOR_MUTABLE_CONST,
+ "a `const` item with interior mutability should not be borrowed",
+ *expr,
+ ),
+ }
+ }
+}
+
+fn lint(cx: &LateContext<'_>, source: Source) {
+ let (lint, msg, span) = source.lint();
+ span_lint_and_then(cx, lint, span, msg, |diag| {
+ if span.from_expansion() {
+ return; // Don't give suggestions into macros.
+ }
+ match source {
+ Source::Item { .. } => {
+ let const_kw_span = span.from_inner(InnerSpan::new(0, 5));
+ diag.span_label(const_kw_span, "make this a static item (maybe with lazy_static)");
+ },
+ Source::Assoc { .. } => (),
+ Source::Expr { .. } => {
+ diag.help("assign this const to a local or static variable, and use the variable here");
+ },
+ }
+ });
+}
+
+declare_lint_pass!(NonCopyConst => [DECLARE_INTERIOR_MUTABLE_CONST, BORROW_INTERIOR_MUTABLE_CONST]);
+
+impl<'tcx> LateLintPass<'tcx> for NonCopyConst {
+ fn check_item(&mut self, cx: &LateContext<'tcx>, it: &'tcx Item<'_>) {
+ if let ItemKind::Const(hir_ty, body_id) = it.kind {
+ let ty = hir_ty_to_ty(cx.tcx, hir_ty);
+
+ if is_unfrozen(cx, ty) && is_value_unfrozen_poly(cx, body_id, ty) {
+ lint(cx, Source::Item { item: it.span });
+ }
+ }
+ }
+
+ fn check_trait_item(&mut self, cx: &LateContext<'tcx>, trait_item: &'tcx TraitItem<'_>) {
+ if let TraitItemKind::Const(hir_ty, body_id_opt) = &trait_item.kind {
+ let ty = hir_ty_to_ty(cx.tcx, hir_ty);
+
+ // Normalize assoc types because ones originated from generic params
+ // bounded other traits could have their bound.
+ let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
+ if is_unfrozen(cx, normalized)
+ // When there's no default value, lint it only according to its type;
+ // in other words, lint consts whose value *could* be unfrozen, not definitely is.
+ // This feels inconsistent with how the lint treats generic types,
+ // which avoids linting types which potentially become unfrozen.
+ // One could check whether a unfrozen type have a *frozen variant*
+ // (like `body_id_opt.map_or_else(|| !has_frozen_variant(...), ...)`),
+ // and do the same as the case of generic types at impl items.
+ // Note that it isn't sufficient to check if it has an enum
+ // since all of that enum's variants can be unfrozen:
+ // i.e. having an enum doesn't necessary mean a type has a frozen variant.
+ // And, implementing it isn't a trivial task; it'll probably end up
+ // re-implementing the trait predicate evaluation specific to `Freeze`.
+ && body_id_opt.map_or(true, |body_id| is_value_unfrozen_poly(cx, body_id, normalized))
+ {
+ lint(cx, Source::Assoc { item: trait_item.span });
+ }
+ }
+ }
+
+ fn check_impl_item(&mut self, cx: &LateContext<'tcx>, impl_item: &'tcx ImplItem<'_>) {
+ if let ImplItemKind::Const(hir_ty, body_id) = &impl_item.kind {
+ let item_hir_id = cx.tcx.hir().get_parent_node(impl_item.hir_id());
+ let item = cx.tcx.hir().expect_item(item_hir_id);
+
+ match &item.kind {
+ ItemKind::Impl(Impl {
+ of_trait: Some(of_trait_ref),
+ ..
+ }) => {
+ if_chain! {
+ // Lint a trait impl item only when the definition is a generic type,
+ // assuming a assoc const is not meant to be a interior mutable type.
+ if let Some(of_trait_def_id) = of_trait_ref.trait_def_id();
+ if let Some(of_assoc_item) = specialization_graph::Node::Trait(of_trait_def_id)
+ .item(cx.tcx, impl_item.ident, AssocKind::Const, of_trait_def_id);
+ if cx
+ .tcx
+ .layout_of(cx.tcx.param_env(of_trait_def_id).and(
+ // Normalize assoc types because ones originated from generic params
+ // bounded other traits could have their bound at the trait defs;
+ // and, in that case, the definition is *not* generic.
+ cx.tcx.normalize_erasing_regions(
+ cx.tcx.param_env(of_trait_def_id),
+ cx.tcx.type_of(of_assoc_item.def_id),
+ ),
+ ))
+ .is_err();
+ // If there were a function like `has_frozen_variant` described above,
+ // we should use here as a frozen variant is a potential to be frozen
+ // similar to unknown layouts.
+ // e.g. `layout_of(...).is_err() || has_frozen_variant(...);`
+ then {
+ let ty = hir_ty_to_ty(cx.tcx, hir_ty);
+ let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
+ if is_unfrozen(cx, normalized)
+ && is_value_unfrozen_poly(cx, *body_id, normalized)
+ {
+ lint(
+ cx,
+ Source::Assoc {
+ item: impl_item.span,
+ },
+ );
+ }
+ }
+ }
+ },
+ ItemKind::Impl(Impl { of_trait: None, .. }) => {
+ let ty = hir_ty_to_ty(cx.tcx, hir_ty);
+ // Normalize assoc types originated from generic params.
+ let normalized = cx.tcx.normalize_erasing_regions(cx.param_env, ty);
+
+ if is_unfrozen(cx, ty) && is_value_unfrozen_poly(cx, *body_id, normalized) {
+ lint(cx, Source::Assoc { item: impl_item.span });
+ }
+ },
+ _ => (),
+ }
+ }
+ }
+
+ fn check_expr(&mut self, cx: &LateContext<'tcx>, expr: &'tcx Expr<'_>) {
+ if let ExprKind::Path(qpath) = &expr.kind {
+ // Only lint if we use the const item inside a function.
+ if in_constant(cx, expr.hir_id) {
+ return;
+ }
+
+ // Make sure it is a const item.
+ let item_def_id = match cx.qpath_res(qpath, expr.hir_id) {
+ Res::Def(DefKind::Const | DefKind::AssocConst, did) => did,
+ _ => return,
+ };
+
+ // Climb up to resolve any field access and explicit referencing.
+ let mut cur_expr = expr;
+ let mut dereferenced_expr = expr;
+ let mut needs_check_adjustment = true;
+ loop {
+ let parent_id = cx.tcx.hir().get_parent_node(cur_expr.hir_id);
+ if parent_id == cur_expr.hir_id {
+ break;
+ }
+ if let Some(Node::Expr(parent_expr)) = cx.tcx.hir().find(parent_id) {
+ match &parent_expr.kind {
+ ExprKind::AddrOf(..) => {
+ // `&e` => `e` must be referenced.
+ needs_check_adjustment = false;
+ },
+ ExprKind::Field(..) => {
+ needs_check_adjustment = true;
+
+ // Check whether implicit dereferences happened;
+ // if so, no need to go further up
+ // because of the same reason as the `ExprKind::Unary` case.
+ if cx
+ .typeck_results()
+ .expr_adjustments(dereferenced_expr)
+ .iter()
+ .any(|adj| matches!(adj.kind, Adjust::Deref(_)))
+ {
+ break;
+ }
+
+ dereferenced_expr = parent_expr;
+ },
+ ExprKind::Index(e, _) if ptr::eq(&**e, cur_expr) => {
+ // `e[i]` => desugared to `*Index::index(&e, i)`,
+ // meaning `e` must be referenced.
+ // no need to go further up since a method call is involved now.
+ needs_check_adjustment = false;
+ break;
+ },
+ ExprKind::Unary(UnOp::Deref, _) => {
+ // `*e` => desugared to `*Deref::deref(&e)`,
+ // meaning `e` must be referenced.
+ // no need to go further up since a method call is involved now.
+ needs_check_adjustment = false;
+ break;
+ },
+ _ => break,
+ }
+ cur_expr = parent_expr;
+ } else {
+ break;
+ }
+ }
+
+ let ty = if needs_check_adjustment {
+ let adjustments = cx.typeck_results().expr_adjustments(dereferenced_expr);
+ if let Some(i) = adjustments
+ .iter()
+ .position(|adj| matches!(adj.kind, Adjust::Borrow(_) | Adjust::Deref(_)))
+ {
+ if i == 0 {
+ cx.typeck_results().expr_ty(dereferenced_expr)
+ } else {
+ adjustments[i - 1].target
+ }
+ } else {
+ // No borrow adjustments means the entire const is moved.
+ return;
+ }
+ } else {
+ cx.typeck_results().expr_ty(dereferenced_expr)
+ };
+
+ if is_unfrozen(cx, ty) && is_value_unfrozen_expr(cx, expr.hir_id, item_def_id, ty) {
+ lint(cx, Source::Expr { expr: expr.span });
+ }
+ }
+ }
+}
projects / rustc.git / blobdiff