I think they were referring to an actual difference between Rust and OCaml, Haskell, etc. Rust allows you to mutate data via mutable references that are guarant

I think they were referring to an actual difference between Rust and OCaml, Haskell, etc. Rust allows you to mutate data via mutable references that are guaranteed to be unique. Mainstream functional programming languages do not allow you to do this (though linear typing may land in GHC soon).