A closure satisfies Send and Sync when it can, automatically. (I.e. with or without captures.) Satisfying Clone when the contents are Clone would be valuable fo

A closure satisfies Send and Sync when it can, automatically. (I.e. with or without captures.) Satisfying Clone when the contents are Clone would be valuable for all closures, whether or not they capture anything. The most valuable special case IMO for captureless closure would be allowing them to coerce to (extern) fn types, useful for writing e.g. C callbacks directly inline.