Announcing Rust 1.46.0

There are several core language features you can now use in a const fn:

• if, if let, and match
• while, while let, and loop
• the && and || operators

You can also cast to a slice:

const fn foo() {
  let x = [1, 2, 3, 4, 5];

  // cast the array to a slice
  let y: &[_] = &x;
}

While these features may not feel new, given that you could use them all outside of const fn, they add a lot of compile-time computation power! As an example, the const-sha1 crate can let you compute SHA-1 hashes at compile time. This led to a 40x performance improvement in Microsoft’s WinRT bindings for Rust.

Back in March, the release of Rust 1.42 introduced better error messages when unwrap and related functions would panic. At the time, we mentioned that the way this was implemented was not yet stable. Rust 1.46 stabilizes this feature.

This attribute is called #[track_caller], which was originally proposed in RFC 2091 way back in July of 2017! If you’re writing a function like unwrap that may panic, you can put this annotation on your functions, and the default panic formatter will use its caller as the location in its error message. For example, here is unwrap previously:

pub fn unwrap(self) -> T {
    match self {
        Some(val) => val,
        None => panic!("called `Option::unwrap()` on a `None` value"),
    }
}

It now looks like this:

#[track_caller]
pub fn unwrap(self) -> T {
    match self {
        Some(val) => val,
        None => panic!("called `Option::unwrap()` on a `None` value"),
    }
}

That’s it!

If you are implementing a panic hook yourself, you can use the caller method on std::panic::Location to get access to this information.

Keeping with the theme of const fn improvements, std::mem::forget is now a const fn. Additionally, two new APIs were stabilized this release:

• Option::zip
• vec::Drain::as_slice

See the detailed release notes for more.