In today’s Rust, there are a number of builtin traits (sometimes called “kinds”): Send, Freeze, Share, and Pod (in the future, perhaps Sized). These are expressed as traits, but they are quite unlike other traits in certain ways. One way is that they do not have any methods; instead, implementing a trait like Freeze indicates that the type has certain properties (defined below). The biggest difference, though, is that these traits are not implemented manually by users. Instead, the compiler decides automatically whether or not a type implements them based on the contents of the type.