I want extern "fil-C"

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.

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?

There is a solution that Mozilla uses in prod for legacy C codecs: https://rlbox.dev/ It's not a replacement for Fil-C's role as a precise ASAN/Valgrind, but it works great if you want to call a C library without letting it freely spray caller's memory.

Any reason why you don't use clang's `-fbounds-safety`? It offers ABI compatibility and incremental adoption. The author of Fil-C worked on it also :-) https://clang.llvm.org/docs/BoundsSafety.html#overview

This would also mean that you wouldn't need unsafe{} to call into the Fil-C ffi. The majority of unsafe{} in (non pure-rust) cargo dependencies is calling C ffi. Rust + Fil-C is a great match that fills a particular niche, I'd love to see it happen.