Understanding Rust Error E0625: Const Variable Referring to Thread-Local Static

Overview of Rust Error E0625

Rust Error E0625 occurs when a compile-time constant variable refers to a thread-local static variable. Although static and constant variables can reference other constant variables, a constant variable cannot reference a thread-local static variable.

Erroneous code example:

#![allow(unused)]
#![feature(thread_local)]

fn main() {
#[thread_local]
static X: usize = 12;

const Y: usize = 2 * X;
}

In the example above, E0625 occurs because the constant variable Y attempts to reference the thread-local static variable X.

Fixing the Error

To fix Rust Error E0625, the value of the thread-local static variable should be extracted as a constant, which can then be used in the constant expression.

Modified example without error:

#![allow(unused)]
#![feature(thread_local)]

fn main() {
const C: usize = 12;

#[thread_local]
static X: usize = C;

const Y: usize = 2 * C;
}

In this modified example, const C is used to store the value 12, which is used for both thread-local static variable X and constant variable Y. This eliminates the direct reference from Y to the thread-local static variable X, thus fixing Rust Error E0625.

Conclusion

Rust Error E0625 occurs when a constant variable refers to a thread-local static variable. This error can be easily fixed by extracting the value of the thread-local static variable into a separate const variable and then using it in the necessary expressions. Understanding this error and its solution ensures better code writing and the elimination of unnecessary thread-local static references.