The responsibility of proving safety can belong to the compiler or it could belong to the programmer. The compiler can do a very good job, but there are edge ca

The responsibility of proving safety can belong to the compiler or it could belong to the programmer. The compiler can do a very good job, but there are edge cases where your code can become convoluted and things aren't expressed easily. The programmer can do a good job of expressing things, but is usually not able to prove safety of large blocks. unsafe blocks are just a compromise where the programmer only has to prove safety of small blocks of code which aren't easily expressed in the type system.