Up to now in the Git world, Git's Rust code has been opt-in at build-time and optional. With Git 2.55, the Rust code is still optional but is now being enabled by default. Git 2.55+ will now build with the Rust support enabled by default unless opting out with the "NO_RUST" option. But for Git 3.0 or some later point is when the Git Rust support may become mandatory.
Git 2.55 also further improves repacking with incremental multi-pack indexes, more easier fixing of earlier staged commits with the git history fixup to apply changes to an earlier staged commit, and a variety of other low-level improvements. The git history fixup allows instantly merging staged changes directly into an older commit without having to go through an interactive rebase. The git history sub-command at large though is still considered experimental in Git 2.55.
The full list of Git 2.55 changes can be found via today's release announcement . There is also an overview of the prominent Git 2.55 changes via the GitHub Blog .