The Rust language specifically gives more information and thus more optimization opportunities to the compiler. Plus, when comparing Rust code compiled with rus

The Rust language specifically gives more information and thus more optimization opportunities to the compiler. Plus, when comparing Rust code compiled with rustc and C code compiled with Clang, both are using LLVM as the compiler backend, so what primarily comes into play is how expressive each language is, and how idiomatic it is to write code that will be optimized by the compiler. Both C and Rust are capable of just inlining optimal assembly, so comparing "pure speed potential" is pointless.