I feel like writing Rust is a whole lot faster and less error-prone than writing something in C. But even assuming that it's slower, it's definitely much faster than writing C and doing formal verification. Naturally formal verification can help with other properties besides memory safety, and as far as I am aware Rust is still lacking a formal semantics so when you go all-in on formal methods C can reach a provable level of quality that Rust currently cannot. But very few people develop software that way.