Rust 1.85: What's New, Release History & Support Lifecycle

What Is New in Rust 1.85

What new language features does Rust 1.85 introduce?

Rust 1.85 introduces the Rust 2024 edition, async closures, and several safety-related language changes.

• Rust 2024 edition - the largest edition to date, adding RPIT lifetime capture rules, new temporary scopes, unsafe extern blocks, and more.
• Async closures - write async || { ... } blocks that capture their environment and return a Future.
• Diagnostic hint - the #[diagnostic::do_not_recommend] attribute hides blanket impl suggestions from error messages.
• Tuple iterator extensions - FromIterator and Extend are now implemented for tuples up to twelve elements.
• New unsafe functions - std::env::set_var, std::env::remove_var, and std::os::unix::process::CommandExt::before_exec are now marked unsafe.

How can I write asynchronous closures in Rust 1.85?

Async closures are now stable and let you write async || { ... } blocks that capture their environment and return a Future.

let mut log = Vec::new();

let fetch = async || {
    // The closure can borrow `log` mutably.
    log.push("started".to_string());
    async { 42 }.await
};

let result = futures::executor::block_on(fetch());
println!("Result: {result}, log: {log:?}");

They also enable higher-ranked signatures via the new AsyncFn, AsyncFnMut, and AsyncFnOnce traits in the prelude.

What breaking changes should I be aware of when upgrading to Rust 1.85?

Several safety checks have been hardened, making previously accepted code now a compile-time error.

• Unsafe extern blocks - extern blocks must now be prefixed with unsafe.
• Unsafe attributes - export_name, link_section, and no_mangle require the unsafe keyword.
• unsafe_op_in_unsafe_fn - the lint now warns by default; you must wrap unsafe operations in an explicit unsafe { ... } block inside unsafe functions.
• Static mut references - any reference to a static mut item is denied unless wrapped in an unsafe block.
• Macro fragment specifiers - missing fragment specifiers are now a hard error, forcing explicit $(...:expr) style.
• Never type fallback - the fallback coercion for ! is now denied, tightening type inference.

Watch out for these changes in existing codebases; most can be fixed by adding the required unsafe blocks or updating macro patterns.

How have Cargo, Rustdoc, and Rustfmt been improved in this release?

Cargo now respects the rust-version field during dependency resolution, Rustdoc runs doctests in a single binary, and Rustfmt adds style editions and sorting fixes.

• Cargo - the resolver checks the rust-version field, preventing accidental use of crates that require a newer compiler.
• Rustdoc - doctests are combined into one executable, dramatically speeding up documentation testing.
• Rustfmt - introduces "style editions" so formatting can be decoupled from language edition, plus fixes for raw identifier sorting and version-aware identifier ordering.