Right, but that's a subtly different type of infallibility: it allows the compiler to optimize instantiations of traits that are generic over error types by est

Right, but that's a subtly different type of infallibility: it allows the compiler to optimize instantiations of traits that are generic over error types by establishing that the error is impossible in a concrete implementation. The other kind of infallibility is "the absence of an error encoding for a function means the function cannot fail". The Infallible type can't provide that, since a function have an infallible `Err` case but still arbitrarily decide to panic at runtime. Edit: in other words, Rust's type system isn't strong enough to encode "full" infallibility. We'd need some kind of effect typing for that.