Announcing Rust 1.72.0

Aug. 24, 2023 · The Rust Release Team

The Rust team is happy to announce a new version of Rust, 1.72.0. Rust is a programming language empowering everyone to build reliable and efficient software.

If you have a previous version of Rust installed via rustup, you can get 1.72.0 with:

$ rustup update stable

If you don't have it already, you can get rustup from the appropriate page on our website, and check out the detailed release notes for 1.72.0 on GitHub.

If you'd like to help us out by testing future releases, you might consider updating locally to use the beta channel (rustup default beta) or the nightly channel (rustup default nightly). Please report any bugs you might come across!

What's in 1.72.0 stable

Rust reports potentially useful cfg-disabled items in errors

You can conditionally enable Rust code using cfg, such as to provide certain functions only with certain crate features, or only on particular platforms. Previously, items disabled in this way would be effectively invisible to the compiler. Now, though, the compiler will remember the name and cfg conditions of those items, so it can report (for example) if a function you tried to call is unavailable because you need to enable a crate feature.

   Compiling my-project v0.1.0 (/tmp/my-project)
error[E0432]: unresolved import `rustix::io_uring`
   --> src/
1   | use rustix::io_uring;
    |     ^^^^^^^^^^^^^^^^ no `io_uring` in the root
note: found an item that was configured out
   --> /home/username/.cargo/registry/src/
213 | pub mod io_uring;
    |         ^^^^^^^^
    = note: the item is gated behind the `io_uring` feature

For more information about this error, try `rustc --explain E0432`.
error: could not compile `my-project` (bin "my-project") due to previous error

Const evaluation time is now unlimited

To prevent user-provided const evaluation from getting into a compile-time infinite loop or otherwise taking unbounded time at compile time, Rust previously limited the maximum number of statements run as part of any given constant evaluation. However, especially creative Rust code could hit these limits and produce a compiler error. Worse, whether code hit the limit could vary wildly based on libraries invoked by the user; if a library you invoked split a statement into two within one of its functions, your code could then fail to compile.

Now, you can do an unlimited amount of const evaluation at compile time. To avoid having long compilations without feedback, the compiler will always emit a message after your compile-time code has been running for a while, and repeat that message after a period that doubles each time. By default, the compiler will also emit a deny-by-default lint (const_eval_long_running) after a large number of steps to catch infinite loops, but you can allow(const_eval_long_running) to permit especially long const evaluation.

Uplifted lints from Clippy

Several lints from Clippy have been pulled into rustc:

  • clippy::undropped_manually_drops to undropped_manually_drops (deny)

    • ManuallyDrop does not drop its inner value, so calling std::mem::drop on it does nothing. Instead, the lint will suggest ManuallyDrop::into_inner first, or you may use the unsafe ManuallyDrop::drop to run the destructor in-place. This lint is denied by default.
  • clippy::invalid_utf8_in_unchecked to invalid_from_utf8_unchecked (deny) and invalid_from_utf8 (warn)

    • The first checks for calls to std::str::from_utf8_unchecked and std::str::from_utf8_unchecked_mut with an invalid UTF-8 literal, which violates their safety pre-conditions, resulting in undefined behavior. This lint is denied by default.
    • The second checks for calls to std::str::from_utf8 and std::str::from_utf8_mut with an invalid UTF-8 literal, which will always return an error. This lint is a warning by default.
  • clippy::cmp_nan to invalid_nan_comparisons (warn)

    • This checks for comparisons with f32::NAN or f64::NAN as one of the operands. NaN does not compare meaningfully to anything – not even itself – so those comparisons are always false. This lint is a warning by default, and will suggest calling the is_nan() method instead.
  • clippy::cast_ref_to_mut to invalid_reference_casting (allow)

    • This checks for casts of &T to &mut T without using interior mutability, which is immediate undefined behavior, even if the reference is unused. This lint is currently allowed by default due to potential false positives, but it is planned to be denied by default in 1.73 after implementation improvements.

Stabilized APIs

These APIs are now stable in const contexts:

Other changes

Check out everything that changed in Rust, Cargo, and Clippy.

Future Windows compatibility

In a future release we're planning to increase the minimum supported Windows version to 10. The accepted proposal in compiler MCP 651 is that Rust 1.75 will be the last to officially support Windows 7, 8, and 8.1. When Rust 1.76 is released in February 2024, only Windows 10 and later will be supported as tier-1 targets. This change will apply both as a host compiler and as a compilation target.

Update: The planned increase to Windows' minimum support level has been delayed until Rust 1.78, due to be released in May 2024.

Contributors to 1.72.0

Many people came together to create Rust 1.72.0. We couldn't have done it without all of you. Thanks!