I think it would be useful if the distinction did exist in Rust, so that for example, "closures that have an empty environment" were indeed referred to as lambdas, and lambdas had the guarantee of being Sync, Send, Copy, Clone, etc., since they're more or less just like regular `fn` items as far as I know. As it is, a closure that captures nothing doesn't satisfy those bounds AFAIK, so their use is more restrictive than it needs to be. Instead, you're forced to write an actual `fn` definition, depriving you of the practicality and conciseness of closure syntax.