Rust changes for v6.3 More core additions, getting closer to a point where the first Rust modules can be upstreamed. The major ones being: - Sync: new types 'Arc', 'ArcBorrow' and 'UniqueArc'. - Types: new trait 'ForeignOwnable' and new type 'ScopeGuard'. There is also a substantial removal in terms of lines: - 'alloc' crate: remove the 'borrow' module (type 'Cow' and trait 'ToOwned').