Janet uses setjmp/longjmp for error handling because most alternatives in C don't compose well or are incredibly verbose/inefficient. The C API is not documented yet, but in general one could use setjmp at C++/Rust <-> C boundary points to prevent panics from jumping over C++/Rust stack frames. However, many functions in the C API will not panic (longjmp to the last save point). The overhead could be minimized by only calling setjmp for API calls that can panic.

Just spitballing here.