rust/clippy_lints/src/non_copy_const.rs

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//! Checks for uses of const which the type is not `Freeze` (`Cell`-free).
//!
//! This lint is **warn** by default.
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use std::ptr;
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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;
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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};
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use rustc_session::{declare_lint_pass, declare_tool_lint};
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use rustc_span::{InnerSpan, Span, DUMMY_SP};
use rustc_typeck::hir_ty_to_ty;
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use crate::utils::{in_constant, span_lint_and_then};
use if_chain::if_chain;
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// 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
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/// mutable (e.g., contains a `Cell`, `Mutex`, `AtomicXxxx`, etc.).
///
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/// **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
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/// 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,
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"declaring `const` with interior mutability"
}
// FIXME: this is a correctness problem but there's no suitable
// warn-by-default category.
declare_clippy_lint! {
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/// **What it does:** Checks if `const` items which is interior mutable (e.g.,
/// contains a `Cell`, `Mutex`, `AtomicXxxx`, etc.) has been borrowed directly.
///
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/// **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,
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"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 {
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Item { item: Span },
Assoc { item: Span },
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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,
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"a `const` item should never be interior mutable",
*item,
),
Self::Expr { expr } => (
BORROW_INTERIOR_MUTABLE_CONST,
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"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| {
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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)");
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},
Source::Assoc { .. } => (),
Source::Expr { .. } => {
diag.help("assign this const to a local or static variable, and use the variable here");
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},
}
});
}
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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<'_>) {
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if let ExprKind::Path(qpath) = &expr.kind {
// Only lint if we use the const item inside a function.
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if in_constant(cx, expr.hir_id) {
return;
}
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// Make sure it is a const item.
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let item_def_id = match cx.qpath_res(qpath, expr.hir_id) {
Res::Def(DefKind::Const | DefKind::AssocConst, did) => did,
_ => return,
};
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// 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);
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if parent_id == cur_expr.hir_id {
break;
}
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if let Some(Node::Expr(parent_expr)) = cx.tcx.hir().find(parent_id) {
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match &parent_expr.kind {
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ExprKind::AddrOf(..) => {
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// `&e` => `e` must be referenced.
needs_check_adjustment = false;
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},
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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;
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},
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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;
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},
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ExprKind::Unary(UnOp::UnDeref, _) => {
// `*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;
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},
_ => break,
}
cur_expr = parent_expr;
} else {
break;
}
}
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let ty = if needs_check_adjustment {
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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 {
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cx.typeck_results().expr_ty(dereferenced_expr)
} else {
adjustments[i - 1].target
}
} else {
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// No borrow adjustments means the entire const is moved.
return;
}
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} else {
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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 });
}
}
}
}