Understanding and Resolving Rust Error E0729

Introduction to Rust Error E0729

Rust Error E0729 is related to Non-Lexical Lifetimes (NLL) issues and the new borrow checker implemented in the Rust compiler since version 1.31. This error, which was previously emitted by the compiler, is mostly replaced by warnings to address unsound code and possible memory safety issues resulting from borrow checker bugs. Although this compile-time warning may not cause immediate issues, it is essential to understand and resolve it, as the compiler may stop accepting the affected code in future releases.

What Triggers Rust Error E0729?

Error E0729 is triggered by the presence of unsound code that got through the old borrow checker, thus potentially causing memory safety problems. Code that does not involve borrowing and still generates warnings related to E0729 is likely affected by known bugs. Ultimately, you should change your code to fix the error before it potentially turns into a hard error.

How to Resolve Rust Error E0729

To resolve E0729, look for instances of unsound code that cause the warning and correct them. Here's an example of a problematic code:

fn main() {
    let mut x = 5;
    let y = &x;
    *x += 1;
    println!("{}", y);
}

The problem with this code is due to the aliasing of the mutable reference x and the immutable reference y. To resolve this error, you should update the mutable reference after the immutable reference is no longer in use. Here's the corrected code:

fn main() {
    let mut x = 5;
    let y = &x;
    println!("{}", y);
    *x += 1;
}

The fix in this example avoids unsound code and ensures the error warning no longer triggers.

Why Not Fix the Old Borrow Checker?

Fixing the old borrow checker is not a viable solution, as it has multiple inherent soundness issues. The NLL-based new borrow checker was introduced to address these problems and provide a more accurate and reliable compiler. Transitioning to the NLL-based checker is the recommended solution for handling Rust Error E0729.

Conclusion