Understanding Rust Error E0712

Introduction to Rust Error E0712

Rust Error E0712 occurs when a thread-local variable is borrowed inside a function, and the borrow outlasts the function's lifetime. This is a common mistake when working with thread-local variables and multi-threading in Rust.

Official Example of Erroneous Code

#![feature(thread_local)]

#[thread_local]
static FOO: u8 = 3;

fn main() {
    let a = &FOO; // error: thread-local variable borrowed past end of function

    std::thread::spawn(move || {
        println!("{}", a);
    });
}

Explanation of the Erroneous Code

In this example, a thread-local variable FOO is defined as a static value. The main() function borrows a reference to FOO and attempts to use the borrowed reference inside a newly spawned thread. This causes the error, as the borrowed reference is used outside of the function's lifetime.

How to Fix Rust Error E0712

To fix this error, you should avoid borrowing the thread-local variable and using the reference in a different scope. Instead, pass a copy of the value to the new thread or use a synchronization primitive like Mutex or Arc for shared access to the value.

Fixed Code Example

#![feature(thread_local)]

#[thread_local]
static FOO: u8 = 3;

fn main() {
    let a = FOO; // copy the value of FOO, not a borrowed reference

    std::thread::spawn(move || {
        println!("{}", a);
    });
}

Conclusion

Rust Error E0712 occurs when a thread-local variable is borrowed within a function and the borrow outlives the function's lifetime. This error can be resolved by avoiding borrowing thread-local variables and instead copying their values or using synchronization primitives such as Mutex or Arc for shared access.