+foo is an old deprecated feature called a mode that is finally just about to be removed. Because the compiler is written in rust, it can take some time for dep

+foo is an old deprecated feature called a mode that is finally just about to be removed. Because the compiler is written in rust, it can take some time for deprecated things to be fully removed from the language. Modes were supposed to give hints to the compiler whether or not to implicitly copy or move a value into a function, but it was buggy and confusing. So we replaced it with always moving for unique types, and always copying for reference counted types.