Passing Go/Rust callbacks to a C library

WHAT IT TESTS: whether you can bridge a C function-pointer callback from Go or Rust safely. ANSWER OUTLINE: C wants a bare function pointer with C calling convention. In Go, you define an //export-ed function and pass it via cgo, often threading state through a registered handle since you cannot pass Go closures directly. In Rust, you pass an extern "C" fn; capturing closures need a separate void* user-data pointer cast back inside. Key risks: unwinding across FFI is UB, and state lifetime must outlive the C call.