Understanding Rust Error E0434

Introduction to Rust Error E0434

Rust Error E0434 occurs when a variable used inside an inner function comes from a dynamic environment. This error is due to the inner function's lack of access to the containing environment.

Erroneous Code Example:

#![allow(unused)]
fn main() {
    fn foo() {
        let y = 5;
        fn bar() -> u32 {
            y // error: can't capture dynamic environment in a fn item; use the
            //        || { ... } closure form instead.
        }
    }
}

There are two ways to fix this error:

1. Replace the function with a closure:

Corrected Code Example:

#![allow(unused)]
fn main() {
    fn foo() {
        let y = 5;
        let bar = || {
            y
        };
    }
}

2. Replace the captured variable with a constant or a static item:

Corrected Code Example:

#![allow(unused)]
fn main() {
    fn foo() {
        static mut X: u32 = 4;
        const Y: u32 = 5;
        fn bar() -> u32 {
            unsafe {
                X = 3;
            }
            Y
        }
    }
}

Conclusion

Rust Error E0434 is an error regarding the use of dynamic environment variables in an inner function. To fix this error, either replace the function with a closure or replace the captured variable with a constant or static item. By implementing one of these solutions, your Rust code will compile successfully and eliminate the E0434 error.