Rust already supports switching between borrow checker implementations.
It has migrated from a scope-based borrow checker to non-lexical borrow checker, and has next experimental Polonius implementation as an option. However, once the new implementation becomes production-ready, the old one gets discarded, because there's no reason to choose it. Borrow checking is fast, and the newer ones accept strictly more (correct) programs.
You also have Rc and RefCell types which give you greater flexibility at cost of some runtime checks.
It has migrated from a scope-based borrow checker to non-lexical borrow checker, and has next experimental Polonius implementation as an option. However, once the new implementation becomes production-ready, the old one gets discarded, because there's no reason to choose it. Borrow checking is fast, and the newer ones accept strictly more (correct) programs.
You also have Rc and RefCell types which give you greater flexibility at cost of some runtime checks.