1 // Copyright 2014-2018 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution.
4 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
5 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
6 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
7 // option. This file may not be copied, modified, or distributed
8 // except according to those terms.
11 //! Checks for uses of const which the type is not Freeze (Cell-free).
13 //! This lint is **deny** by default.
15 use crate::rustc::lint::{LateContext, LateLintPass, Lint, LintArray, LintPass};
16 use crate::rustc::{declare_tool_lint, lint_array};
17 use crate::rustc::hir::*;
18 use crate::rustc::hir::def::Def;
19 use crate::rustc::ty::{self, TypeFlags};
20 use crate::rustc::ty::adjustment::Adjust;
21 use crate::rustc_errors::Applicability;
22 use crate::rustc_typeck::hir_ty_to_ty;
23 use crate::syntax_pos::{DUMMY_SP, Span};
25 use crate::utils::{in_constant, in_macro, is_copy, span_lint_and_then};
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.
45 /// use std::sync::atomic::{Ordering::SeqCst, AtomicUsize};
48 /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
49 /// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
50 /// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
53 /// static STATIC_ATOM: AtomicUsize = AtomicUsize::new(15);
54 /// STATIC_ATOM.store(9, SeqCst);
55 /// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
57 declare_clippy_lint! {
58 pub DECLARE_INTERIOR_MUTABLE_CONST,
60 "declaring const with interior mutability"
63 /// **What it does:** Checks if `const` items which is interior mutable (e.g.
64 /// contains a `Cell`, `Mutex`, `AtomicXxxx` etc) has been borrowed directly.
66 /// **Why is this bad?** Consts are copied everywhere they are referenced, i.e.
67 /// every time you refer to the const a fresh instance of the `Cell` or `Mutex`
68 /// or `AtomicXxxx` will be created, which defeats the whole purpose of using
69 /// these types in the first place.
71 /// The `const` value should be stored inside a `static` item.
73 /// **Known problems:** None
77 /// use std::sync::atomic::{Ordering::SeqCst, AtomicUsize};
78 /// const CONST_ATOM: AtomicUsize = AtomicUsize::new(12);
81 /// CONST_ATOM.store(6, SeqCst); // the content of the atomic is unchanged
82 /// assert_eq!(CONST_ATOM.load(SeqCst), 12); // because the CONST_ATOM in these lines are distinct
85 /// static STATIC_ATOM: AtomicUsize = CONST_ATOM;
86 /// STATIC_ATOM.store(9, SeqCst);
87 /// assert_eq!(STATIC_ATOM.load(SeqCst), 9); // use a `static` item to refer to the same instance
89 declare_clippy_lint! {
90 pub BORROW_INTERIOR_MUTABLE_CONST,
92 "referencing const with interior mutability"
95 #[derive(Copy, Clone)]
110 fn lint(&self) -> (&'static Lint, &'static str, Span) {
112 Source::Item { item } | Source::Assoc { item, .. } => (
113 DECLARE_INTERIOR_MUTABLE_CONST,
114 "a const item should never be interior mutable",
117 Source::Expr { expr } => (
118 BORROW_INTERIOR_MUTABLE_CONST,
119 "a const item with interior mutability should not be borrowed",
126 fn verify_ty_bound<'a, 'tcx>(
127 cx: &LateContext<'a, 'tcx>,
131 if ty.is_freeze(cx.tcx, cx.param_env, DUMMY_SP) || is_copy(cx, ty) {
132 // an UnsafeCell is !Copy, and an UnsafeCell is also the only type which
133 // is !Freeze, thus if our type is Copy we can be sure it must be Freeze
138 let (lint, msg, span) = source.lint();
139 span_lint_and_then(cx, lint, span, msg, |db| {
141 return; // Don't give suggestions into macros.
144 Source::Item { .. } => {
145 let const_kw_span = span.from_inner_byte_pos(0, 5);
146 db.span_suggestion_with_applicability(
148 "make this a static item",
149 "static".to_string(),
150 Applicability::MachineApplicable,
153 Source::Assoc { ty: ty_span, .. } => {
154 if ty.flags.contains(TypeFlags::HAS_FREE_LOCAL_NAMES) {
155 db.span_help(ty_span, &format!("consider requiring `{}` to be `Copy`", ty));
158 Source::Expr { .. } => {
160 "assign this const to a local or static variable, and use the variable here",
168 pub struct NonCopyConst;
170 impl LintPass for NonCopyConst {
171 fn get_lints(&self) -> LintArray {
172 lint_array!(DECLARE_INTERIOR_MUTABLE_CONST, BORROW_INTERIOR_MUTABLE_CONST)
176 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for NonCopyConst {
177 fn check_item(&mut self, cx: &LateContext<'a, 'tcx>, it: &'tcx Item) {
178 if let ItemKind::Const(hir_ty, ..) = &it.node {
179 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
180 verify_ty_bound(cx, ty, Source::Item { item: it.span });
184 fn check_trait_item(&mut self, cx: &LateContext<'a, 'tcx>, trait_item: &'tcx TraitItem) {
185 if let TraitItemKind::Const(hir_ty, ..) = &trait_item.node {
186 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
187 verify_ty_bound(cx, ty, Source::Assoc { ty: hir_ty.span, item: trait_item.span });
191 fn check_impl_item(&mut self, cx: &LateContext<'a, 'tcx>, impl_item: &'tcx ImplItem) {
192 if let ImplItemKind::Const(hir_ty, ..) = &impl_item.node {
193 let item_node_id = cx.tcx.hir.get_parent_node(impl_item.id);
194 let item = cx.tcx.hir.expect_item(item_node_id);
195 // ensure the impl is an inherent impl.
196 if let ItemKind::Impl(_, _, _, _, None, _, _) = item.node {
197 let ty = hir_ty_to_ty(cx.tcx, hir_ty);
198 verify_ty_bound(cx, ty, Source::Assoc { ty: hir_ty.span, item: impl_item.span });
203 fn check_expr(&mut self, cx: &LateContext<'a, 'tcx>, expr: &'tcx Expr) {
204 if let ExprKind::Path(qpath) = &expr.node {
205 // Only lint if we use the const item inside a function.
206 if in_constant(cx, expr.id) {
210 // make sure it is a const item.
211 match cx.tables.qpath_def(qpath, expr.hir_id) {
212 Def::Const(_) | Def::AssociatedConst(_) => {},
216 // climb up to resolve any field access and explicit referencing.
217 let mut cur_expr = expr;
218 let mut dereferenced_expr = expr;
219 let mut needs_check_adjustment = true;
221 let parent_id = cx.tcx.hir.get_parent_node(cur_expr.id);
222 if parent_id == cur_expr.id {
225 if let Some(Node::Expr(parent_expr)) = cx.tcx.hir.find(parent_id) {
226 match &parent_expr.node {
227 ExprKind::AddrOf(..) => {
228 // `&e` => `e` must be referenced
229 needs_check_adjustment = false;
231 ExprKind::Field(..) => {
232 dereferenced_expr = parent_expr;
233 needs_check_adjustment = true;
235 ExprKind::Index(e, _) if ptr::eq(&**e, cur_expr) => {
236 // `e[i]` => desugared to `*Index::index(&e, i)`,
237 // meaning `e` must be referenced.
238 // no need to go further up since a method call is involved now.
239 needs_check_adjustment = false;
242 ExprKind::Unary(UnDeref, _) => {
243 // `*e` => desugared to `*Deref::deref(&e)`,
244 // meaning `e` must be referenced.
245 // no need to go further up since a method call is involved now.
246 needs_check_adjustment = false;
251 cur_expr = parent_expr;
257 let ty = if needs_check_adjustment {
258 let adjustments = cx.tables.expr_adjustments(dereferenced_expr);
259 if let Some(i) = adjustments.iter().position(|adj| match adj.kind {
260 Adjust::Borrow(_) | Adjust::Deref(_) => true,
264 cx.tables.expr_ty(dereferenced_expr)
266 adjustments[i - 1].target
269 // No borrow adjustments = the entire const is moved.
273 cx.tables.expr_ty(dereferenced_expr)
276 verify_ty_bound(cx, ty, Source::Expr { expr: expr.span });