The Rust team has announced that it is enabling the Polonius Alpha borrow checker on nightly releases for testing. The team expects to fully stabilize Polonius Alpha later in the year.

The announcement was made in a blog post on August 4. In the blog post, Rust team member Jack Huey said there are no known remaining issues with Polonius Alpha and performance is generally acceptable for stabilization.

Enabling the Polonius Alpha borrow checker on nightly for testing is being done to help find serious performance regressions, unsoundness in the formulation, and diagnostic issues that need to be improved, Huey said. Issues can be reported in GitHub or Zulip.

Developers can disable Polonius Alpha and use the stable NLL borrow checker by passing -Zpolonius=off to rustc , using RUSTFLAGS=-Zpolonius=off, or including the following in a project’s .cargo/config.toml configuration file:

[target.x86_64-unknown-linux-gnu]
rustflags = ["-Zpolonius=off"]

The Polonius borrow checker has been in the works since 2018. In 2023, a new formulation of a the Polonius borrow checker was proposed that required a minimal re-architecture of the existing NLL (non-lexical lifetime) implementation and could be extended to allow more sound code to compile, according to Huey. Stabilization of that implementation has been delayed until now.

The borrow checker is a component of the Rust compiler that enforces strict rules on references, or borrowing. These include ensuring that all variables are initialized before they are used, that the same value isn’t moved twice, that a value isn’t moved while it is borrowed, that a place isn’t accessed while it is mutably borrowed (except through the reference), and that a place isn’t mutated while it is immutably borrowed, according to the Rust documentation.

Java Development Kit (JDK) 28, a non-LTS (Long-Term Support) or “feature release” of standard Java due in March 2027, has started to take shape. Features listed for JDK 28 now include a preview of value objects, switching the default mode of the Shenandoah garbage collector to generational mode, and a preview of strict field initialization in the Java Virtual Machine (JVM).

As a non-LTS release, JDK 28 will be backed by six months of support by Oracle.

The three features currently targeted to JDK 28 include the following:

• Introduce value objects, which are immutable and lack object identity. Value objects are distinguished by the values of their fields, and can be represented by the JVM in ways that improve performance. A goal of the feature is to give developers a programming model for immutable data in which the == operator, and all other operations, distinguish objects by the values of their fields rather than their identities. Value objects is a preview language and VM feature.
• Switch the default mode of the Shenandoah Garbage Collector (GC) to the generational mode and deprecate the non-generational mode, with the intent to remove it in a future release. Goals include signaling the intent that future development will focus on generational mode, and reducing the maintenance cost of supporting two different modes. However, it is not a goal to remove non-generational mode at this time.
• Introduce strictly-initialized fields in the JVM. Such fields must be initialized before they are read, thus default values such as 0 or null are never observed. For strictly-initialized fields that are final, the same value is always observed. A goal is offering designers of JVM-based programming languages a model for field initialization, which has stronger integrity guarantees than the present model. This is a preview VM feature.

The predecessor to JDK 28, JDK 27, is due September 15. JDK 27 also is a non-LTS release that will be backed by six months of support by Oracle.