> Double-frees are deterministic panics in Fil-C

What does "deterministic panic" mean here? It sounds like that's a runtime panic, but the whole point of the paragraph you were quoting is compile time safety.

WUFFS is able to guarantee safety of the codec at compile time.

Rust is able to offer only some of its guarantees at compile time, and the rest are delivered at runtime.

Fil-C delivers almost all of its safety guarantees at runtime.