Comment by Panzerschrek

Extern "fil-C" can't be compatible with Rust code or something similar. It requires a metadata block attached to each allocation. So, if a memory block has been allocated in Rust and a pointer to it is passed to a fil-C function, it can't access it correctly. The only way to allow such cross-langauge-and-abi calls is to compile Rust code itself like fil-C, which requires doubled memory consumption, expensive runtime checks and GC overhead.