SPARK is practically just a restricted version of Ada with a few added features for formal verification. You can write a program primarily in SPARK but disable said restrictions based on circumstance by setting the `SPARK_Mode` pragma/aspect to `off` on a package, procedure, function, etc. Mixing Ada and SPARK is trivial.

I guess it is similar to Rust code that uses `unsafe {}` as the other poster mentioned (maybe `unsafe fn` for a closer analogy). My knowledge of Ada/SPARK is much greater than what I know about Rust, so I might be guessing wrong.