Rust 1.85: Async Closures Stabilized and New Borrow Checker

The Rust team has released version 1.85, bringing several highly anticipated features to stable Rust.

Async Closures

The async || { ... } syntax is now stable, allowing closures that return futures. This has been one of the most requested features in the async ecosystem:

let fetch = async |url: &str| {
    let response = reqwest::get(url).await?;
    response.text().await
};

Previously, creating closures that captured their environment and returned futures required complex workarounds with Box<dyn Future>.

Polonius Borrow Checker

The new Polonius borrow checker is now enabled by default. It is more permissive than the previous NLL (Non-Lexical Lifetimes) checker, accepting more valid programs while maintaining Rust's safety guarantees. Code that previously required awkward restructuring to satisfy the borrow checker may now compile directly.

Trait System Improvements

• Return position impl Trait in traits — Methods can now use -> impl Trait in trait definitions
• Associated type bounds — Simplified syntax for bounding associated types
• Diagnostic attributes — Custom error messages for trait implementation failures

The Rust ecosystem continues to grow rapidly, with the language now ranked in the top 10 of the TIOBE index.