Why isn't fil-C ABI compatible with C?

https://fil-c.org/runtime

Apparently this was done intentionally. The rationale given is that we don't want to allow non-safe C to be used in fil-C programs. Fair enough.

But why should we prevent fil-C programs to be used from Rust (or C, for that matter)?

I don't understand much about compilers, but I guess allowing one would also allow the other? i.e. it's not possible to make fil-C ABI compatible with C (so that it can be more easily called), while also not letting you use call C from it?

You're looking too much at the complete memory safety goal; the important part here is the interop non-goal. Designing a foreign function interface between Fil-C and regular C, in a way that upholds both languages' expectations about the environment the code's running in, is a very difficult problem and nobody seems to have much of an idea of how such a thing could work.

I worry that this same problem will doom the extern "Fil-C" idea that this post advocates. Zig is not really an encouraging precedent because Zig's proposed memory-safe mode adds runtime checks to everything just like Fil-C does, whereas the post author wants to avoid those checks for Rust code that's already statically known to be memory-safe. That said, it's possible I'm missing something.

It's also because Fil-C calls need to carry along a bunch of extra information, to support the safety checks