Understanding Rust Error E0764: Mutable References in Constants

Rust Error E0764: Mutable References in Constants

This error occurs when a mutable reference is used in a constant. Using mutable references in constants or statics is disallowed as it could lead to mutable constants, which is against the immutability guarantees Rust provides.

Erroneous code example

#![feature(const_mut_refs)]

fn main() {
    const OH_NO: &'static mut usize = &mut 1; // error!
}

To prevent the creation of constants that have a mutable reference in their final value, Rust does not allow mutable references to be used in constant functions, statics, or constants. To approach this issue, you can use constant functions that don't return new mutable references.

Using const fn to avoid Rust Error E0764

The following example demonstrates how to use a constant function to perform operations on mutable references without causing Rust Error E0764.

Correct code example

#![feature(const_mut_refs)]

const fn foo(x: usize) -> usize {
    let mut y = 1;
    let z = &mut y;
    *z += x;
    y
}

fn main() {
    const FOO: usize = foo(10); // ok!
}

In this example, the const fn foo() takes a usize parameter 'x', then creates a mutable variable 'y' and a mutable reference 'z'. The function modifies 'y' using the mutable reference and returns the final value of 'y'. The constant 'FOO' is then initialized with the result of foo(10), without causing any errors.

Conclusion