Rust 1.98.0 becomes stable on August 20, 2026, six weeks after Rust 1.97.0 shipped on July 9, 2026.
Its one compatibility risk is a new fast path for derive(PartialOrd) when Ord is derived, which can change behaviour for types whose PartialOrd and Ord implementations disagree.
C variadic function definitions are not in Rust 1.98: the stabilization pull request carries the 1.99.0 milestone, expected around October 1, 2026.