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.
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