It is in general an escape hatch for being able to write code that isn't possible in safe Rust. Limitations of the borrow checker (e.g. for backpointers in circ

It is in general an escape hatch for being able to write code that isn't possible in safe Rust. Limitations of the borrow checker (e.g. for backpointers in circular data structures) definitely fall into that category. Even without FFI Rust would not be practical without unsafe.