Additions to the prelude

🚧 The 2024 Edition has not yet been released and hence this section is still "under construction". More information may be found in the tracking issue at https://github.com/rust-lang/rust/issues/121042.

Summary

• The Future and IntoFuture traits are now part of the prelude.
• This might make calls to trait methods ambiguous which could make some code fail to compile.

Details

The prelude of the standard library is the module containing everything that is automatically imported in every module. It contains commonly used items such as Option, Vec, drop, and Clone.

The Rust compiler prioritizes any manually imported items over those from the prelude, to make sure additions to the prelude will not break any existing code. For example, if you have a crate or module called example containing a pub struct Option;, then use example::*; will make Option unambiguously refer to the one from example; not the one from the standard library.

However, adding a trait to the prelude can break existing code in a subtle way. For example, a call to x.poll() which comes from a MyPoller trait might fail to compile if std's Future is also imported, because the call to poll is now ambiguous and could come from either trait.

As a solution, Rust 2024 will use a new prelude. It's identical to the current one, except for two new additions:

• std::future::Future
• std::future::IntoFuture

Migration

🚧 The automatic migration for this has not yet been implemented.

Migration needed

Conflicting trait methods

When two traits that are in scope have the same method name, it is ambiguous which trait method should be used. For example:

trait MyPoller {
    // This name is the same as the `poll` method on the `Future` trait from `std`.
    fn poll(&self) {
        println!("polling");
    }
}

impl<T> MyPoller for T {}

fn main() {
    // Pin<&mut async {}> implements both `std::future::Future` and `MyPoller`.
    // If both traits are in scope (as would be the case in Rust 2024),
    // then it becomes ambiguous which `poll` method to call
    core::pin::pin!(async {}).poll();
}

We can fix this by using fully qualified syntax:

fn main() {
    // Now it is clear which trait method we're referring to
    <_ as MyPoller>::poll(&core::pin::pin!(async {}));
}