Disallow references to static mut

Summary

• The static_mut_refs lint level is now deny by default. This checks for taking a shared or mutable reference to a static mut.

Details

The static_mut_refs lint detects taking a reference to a static mut. In the 2024 Edition, this lint is now deny by default to emphasize that you should avoid making these references.

#![allow(unused)]
fn main() {
static mut X: i32 = 23;
static mut Y: i32 = 24;

unsafe {
    let y = &X;             // ERROR: shared reference to mutable static
    let ref x = X;          // ERROR: shared reference to mutable static
    let (x, y) = (&X, &Y);  // ERROR: shared reference to mutable static
}
}

Merely taking such a reference in violation of Rust's mutability XOR aliasing requirement has always been instantaneous undefined behavior, even if the reference is never read from or written to. Furthermore, upholding mutability XOR aliasing for a static mut requires reasoning about your code globally, which can be particularly difficult in the face of reentrancy and/or multithreading.

Note that there are some cases where implicit references are automatically created without a visible & operator. For example, these situations will also trigger the lint:

#![allow(unused)]
fn main() {
static mut NUMS: &[u8; 3] = &[0, 1, 2];

unsafe {
    println!("{NUMS:?}");   // ERROR: shared reference to mutable static
    let n = NUMS.len();     // ERROR: shared reference to mutable static
}
}

Alternatives

Wherever possible, it is strongly recommended to use instead an immutable static of a type that provides interior mutability behind some locally-reasoned abstraction (which greatly reduces the complexity of ensuring that Rust's mutability XOR aliasing requirement is upheld).

In situations where no locally-reasoned abstraction is possible and you are therefore compelled still to reason globally about accesses to your static variable, you must now use raw pointers such as can be obtained via the &raw const or &raw mut operators. By first obtaining a raw pointer rather than directly taking a reference, (the safety requirements of) accesses through that pointer will be more familiar to unsafe developers and can be deferred until/limited to smaller regions of code.

Migration

There is no automatic migration to fix these references to static mut. To avoid undefined behavior you must rewrite your code to use a different approach as recommended in the Alternatives section.