So, a few things, some of which others have touched on:

1. Fil-C is slower and bigger. Noticeably so. If you were OK with slower and bigger then the rewrite you should have considered wasn't to Rust in the last ten years but to Java or C# much earlier. That doesn't invalidate Fil'C's existence, but I want to point that out.

2. You're still writing C. If the program is finished or just occasionally doing a little bit of maintenance that's fine. I wrote C for most of my career, it's not a miserable language, and you are avoiding a rewrite. But if you're writing much new code Rust is just so much nicer. I stopped writing any C when I learned Rust.

3. This is runtime safety and you might need more. Rust gives you a bit more, often you can express at compile time things Fil-C would only have checked at runtime, but you might need everything and languages like WUFFS deliver that. WUFFS doesn't have runtime checks. It has proved to its satisfaction during compilation that your code is safe, so it can be executed at runtime in absolute safety. Your code might be wrong. Maybe your WUFFS GIF flipper actually makes frog GIFs purple instead of flipping them. But it can't crash, or execute x86 machine code hidden in the GIF, or whatever, that's the whole point.

[delayed]

Yes it's slower, but it works. It's being built by one single dad who focused on compatibility before speed.

I'm not convinced that tying the lifetimes into the type system is the correct way to do memory management. I've read too many articles of people being forced into refactoring the entire codebase to implement a feature.