Your `fn rust_qsort` takes ownership of the vector, so it frees its memory after sorting and it can't be used after sorting in the caller function. And generic

Your `fn rust_qsort` takes ownership of the vector, so it frees its memory after sorting and it can't be used after sorting in the caller function. And generic `impl FnMut` won't work in `extern "C"`, it only accepts function pointers.