Add static_mut lint

Fixes #12896

CHANGELOG.md

@@ -5820,6 +5820,7 @@ Released 2018-09-13
 [`skip_while_next`]: https://rust-lang.github.io/rust-clippy/master/index.html#skip_while_next
 [`slow_vector_initialization`]: https://rust-lang.github.io/rust-clippy/master/index.html#slow_vector_initialization
 [`stable_sort_primitive`]: https://rust-lang.github.io/rust-clippy/master/index.html#stable_sort_primitive
+[`static_mut`]: https://rust-lang.github.io/rust-clippy/master/index.html#static_mut
 [`std_instead_of_alloc`]: https://rust-lang.github.io/rust-clippy/master/index.html#std_instead_of_alloc
 [`std_instead_of_core`]: https://rust-lang.github.io/rust-clippy/master/index.html#std_instead_of_core
 [`str_split_at_newline`]: https://rust-lang.github.io/rust-clippy/master/index.html#str_split_at_newline

clippy_lints/src/declared_lints.rs

@@ -652,6 +652,7 @@ pub(crate) static LINTS: &[&crate::LintInfo] = &[
     crate::size_of_in_element_count::SIZE_OF_IN_ELEMENT_COUNT_INFO,
     crate::size_of_ref::SIZE_OF_REF_INFO,
     crate::slow_vector_initialization::SLOW_VECTOR_INITIALIZATION_INFO,
+    crate::static_mut::STATIC_MUT_INFO,
     crate::std_instead_of_core::ALLOC_INSTEAD_OF_CORE_INFO,
     crate::std_instead_of_core::STD_INSTEAD_OF_ALLOC_INFO,
     crate::std_instead_of_core::STD_INSTEAD_OF_CORE_INFO,

clippy_lints/src/lib.rs

@@ -325,6 +325,7 @@ mod single_range_in_vec_init;
 mod size_of_in_element_count;
 mod size_of_ref;
 mod slow_vector_initialization;
+mod static_mut;
 mod std_instead_of_core;
 mod string_patterns;
 mod strings;
@@ -1169,6 +1170,7 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
         })
     });
     store.register_late_pass(|_| Box::new(string_patterns::StringPatterns));
+    store.register_early_pass(|| Box::new(static_mut::StaticMut));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }
 

clippy_lints/src/static_mut.rs

@@ -0,0 +1,60 @@
+use clippy_utils::diagnostics::span_lint_and_help;
+use rustc_ast::ast::{Item, ItemKind, Mutability, StaticItem};
+use rustc_lint::{EarlyContext, EarlyLintPass, LintContext};
+use rustc_middle::lint::in_external_macro;
+use rustc_session::declare_lint_pass;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Produces warnings when a `static mut` is declared.
+    ///
+    /// ### Why is this bad?
+    /// `static mut` can [easily produce undefined behavior][1] and
+    /// [may be removed in the future][2].
+    ///
+    /// ### Example
+    /// ```no_run
+    /// static mut GLOBAL_INT: u8 = 0;
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// use std::sync::RwLock;
+    ///
+    /// static GLOBAL_INT: RwLock<u8> = RwLock::new(0);
+    /// ```
+    ///
+    /// [1]: https://doc.rust-lang.org/nightly/edition-guide/rust-2024/static-mut-reference.html
+    /// [2]: https://github.com/rust-lang/rfcs/pull/3560
+    #[clippy::version = "1.80.0"]
+    pub STATIC_MUT,
+    nursery,
+    "detect mutable static definitions"
+}
+
+declare_lint_pass!(StaticMut => [STATIC_MUT]);
+
+impl EarlyLintPass for StaticMut {
+    fn check_item(&mut self, cx: &EarlyContext<'_>, item: &Item) {
+        if in_external_macro(cx.sess(), item.span) {
+            return;
+        };
+        let ItemKind::Static(ref static_item_box) = item.kind else {
+            return;
+        };
+        let StaticItem {
+            mutability: Mutability::Mut,
+            ..
+        } = static_item_box.as_ref()
+        else {
+            return;
+        };
+        span_lint_and_help(
+            cx,
+            STATIC_MUT,
+            item.span,
+            "declaration of static mut",
+            None,
+            "remove the `mut` and use a type with interior mutibability that implements `Sync`, such as `std::sync::Mutex`",
+        );
+    }
+}

tests/ui/static_mut.rs

@@ -0,0 +1,3 @@
+#![warn(clippy::static_mut)]
+
+static mut NUMBER: u8 = 3;

tests/ui/static_mut.stderr

@@ -0,0 +1,12 @@
+error: declaration of static mut
+  --> tests/ui/static_mut.rs:3:1
+   |
+LL | static mut NUMBER: u8 = 3;
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: remove the `mut` and use a type with interior mutibability that implements `Sync`, such as `std::sync::Mutex`
+   = note: `-D clippy::static-mut` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::static_mut)]`
+
+error: aborting due to 1 previous error
+