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 **deny** by default.
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use std::ptr;
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use rustc::declare_lint_pass;
use rustc::lint::{LateContext, LateLintPass, Lint, LintArray, LintPass};
use rustc::ty::adjustment::Adjust;
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use rustc::ty::{Ty, TypeFlags};
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use rustc_hir::def::{DefKind, Res};
use rustc_hir::*;
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use rustc_session::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, is_copy, qpath_res, span_lint_and_then};
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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.
///
/// **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,
correctness,
"declaring const with interior mutability"
}
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:** None
///
/// **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,
correctness,
"referencing const with interior mutability"
}
#[allow(dead_code)]
#[derive(Copy, Clone)]
enum Source {
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Item { item: Span },
Assoc { item: Span, ty: 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,
),
}
}
}
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fn verify_ty_bound<'a, 'tcx>(cx: &LateContext<'a, 'tcx>, ty: Ty<'tcx>, source: Source) {
if ty.is_freeze(cx.tcx, cx.param_env, DUMMY_SP) || is_copy(cx, ty) {
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// An `UnsafeCell` is `!Copy`, and an `UnsafeCell` is also the only type which
// is `!Freeze`, thus if our type is `Copy` we can be sure it must be `Freeze`
// as well.
return;
}
let (lint, msg, span) = source.lint();
span_lint_and_then(cx, lint, span, msg, |db| {
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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));
db.span_label(const_kw_span, "make this a static item (maybe with lazy_static)");
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},
Source::Assoc { ty: ty_span, .. } => {
if ty.flags.contains(TypeFlags::HAS_FREE_LOCAL_NAMES) {
db.span_label(ty_span, &format!("consider requiring `{}` to be `Copy`", ty));
}
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},
Source::Expr { .. } => {
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db.help("assign this const to a local or static variable, and use the variable here");
},
}
});
}
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declare_lint_pass!(NonCopyConst => [DECLARE_INTERIOR_MUTABLE_CONST, BORROW_INTERIOR_MUTABLE_CONST]);
impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NonCopyConst {
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fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, it: &'tcx Item<'_>) {
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if let ItemKind::Const(hir_ty, ..) = &it.kind {
let ty = hir_ty_to_ty(cx.tcx, hir_ty);
verify_ty_bound(cx, ty, Source::Item { item: it.span });
}
}
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fn check_trait_item(&mut self, cx: &LateContext<'a, 'tcx>, trait_item: &'tcx TraitItem<'_>) {
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if let TraitItemKind::Const(hir_ty, ..) = &trait_item.kind {
let ty = hir_ty_to_ty(cx.tcx, hir_ty);
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verify_ty_bound(
cx,
ty,
Source::Assoc {
ty: hir_ty.span,
item: trait_item.span,
},
);
}
}
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fn check_impl_item(&mut self, cx: &LateContext<'a, 'tcx>, impl_item: &'tcx ImplItem<'_>) {
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if let ImplItemKind::Const(hir_ty, ..) = &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);
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// Ensure the impl is an inherent impl.
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if let ItemKind::Impl(_, _, _, _, None, _, _) = item.kind {
let ty = hir_ty_to_ty(cx.tcx, hir_ty);
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verify_ty_bound(
cx,
ty,
Source::Assoc {
ty: hir_ty.span,
item: impl_item.span,
},
);
}
}
}
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fn check_expr(&mut self, cx: &LateContext<'a, '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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match qpath_res(cx, qpath, expr.hir_id) {
Res::Def(DefKind::Const, _) | Res::Def(DefKind::AssocConst, _) => {},
_ => 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(..) => {
dereferenced_expr = parent_expr;
needs_check_adjustment = true;
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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 {
let adjustments = cx.tables.expr_adjustments(dereferenced_expr);
if let Some(i) = adjustments.iter().position(|adj| match adj.kind {
Adjust::Borrow(_) | Adjust::Deref(_) => true,
_ => false,
}) {
if i == 0 {
cx.tables.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 {
cx.tables.expr_ty(dereferenced_expr)
};
verify_ty_bound(cx, ty, Source::Expr { expr: expr.span });
}
}
}