We are in agreement that Julia is not the origin of ! for this, I was just pointing out that if you want a historical take (what influenced what), Julia most likely got it from Scheme (whether it originated in Scheme or not is immaterial). Rust doesn't use ! to signify "This is a mutating function", it uses it to signify "This is a macro". Whereas Julia and Scheme use it as an indicator for the same thing: functions that modify their arguments. And Julia is known to have been influenced by Scheme, which existed when Julia's development started while Rust (publicly) did not.