What are non-lexical lifetimes?

ghz 7months ago ⋅ 67 views

Rust has an RFC related to non-lexical lifetimes which has been approved to be implemented in the language for a long time. Recently, Rust's support of this feature has improved a lot and is considered complete.

My question is: what exactly is a non-lexical lifetime?

Answers

Non-lexical lifetimes (NLL) are a feature in Rust that enhance the borrow checker by making its analysis more precise. This allows Rust to accept more valid programs by reducing the conservatism associated with traditional, lexical lifetime scopes.

Lexical Lifetimes

Before NLL, Rust's borrow checker used lexical lifetimes. Lexical lifetimes are determined strictly by the syntactic structure of the code—essentially, the lifetimes of variables and borrows are tied directly to the block of code in which they are defined. This means that a borrow would last until the end of the scope in which it was created, even if it was no longer used.

For example:

fn main() {
    let mut x = 5;
    let y = &x; // Borrow of x starts here

    println!("{}", y); // Use y
    // Borrow of x ends here

    x = 6; // This would be an error with lexical lifetimes because the borrow of x is considered to be valid until the end of the scope
}

With lexical lifetimes, the borrow of x by y would be considered active until the end of the block, leading to an error when x is reassigned.

Non-Lexical Lifetimes

Non-lexical lifetimes improve upon this by allowing the compiler to determine the actual points in the code where references are used and where they can be safely considered no longer in use. This allows borrows to end as soon as the last use of the borrowed value, rather than at the end of the scope.

The previous example with non-lexical lifetimes:

fn main() {
    let mut x = 5;
    let y = &x; // Borrow of x starts here

    println!("{}", y); // Last use of y

    // Borrow of x ends here

    x = 6; // This is now allowed with non-lexical lifetimes
}

Here, the borrow of x by y ends immediately after y is last used in the println! statement, allowing x to be reassigned without any issue.

Advantages of Non-Lexical Lifetimes

  1. More Flexible Borrow Checking:

    • NLL allows the borrow checker to be more precise and flexible, accepting more programs that are logically correct but would have been rejected under the older, more conservative rules.

  2. Better Error Messages:

    • NLL enables the compiler to provide more accurate and helpful error messages because it has a better understanding of the actual lifetimes and usage patterns of references.

  3. Easier Refactoring:

    • Code refactoring becomes easier because lifetimes are more closely aligned with actual usage, reducing the need to restructure code simply to satisfy the borrow checker.

How Non-Lexical Lifetimes Work

Non-lexical lifetimes are implemented by analyzing the control flow of a program to determine where references are actually used. This involves:

  1. Lifetime Analysis:

    • The compiler performs a more detailed analysis of the control flow graph of the program to track the points at which borrows are created, used, and no longer needed.

  2. Region Inference:

    • Using this analysis, the compiler infers more precise regions (or lifetimes) for borrows and references, allowing them to be valid for only as long as they are actually needed.

Conclusion

Non-lexical lifetimes are a significant enhancement to Rust's borrow checker, making it more powerful and user-friendly. They allow for more precise lifetime tracking based on actual usage rather than lexical scope, which in turn allows for more valid programs and easier refactoring. This feature underscores Rust's commitment to providing strong guarantees of memory safety while also aiming to improve developer experience and productivity.

#rust