"trait Meower" just declares the trait. "impl Meower" tells the compiler to accept any type that implements the trait. It's the same as adding a generic paramet

"trait Meower" just declares the trait. "impl Meower" tells the compiler to accept any type that implements the trait. It's the same as adding a generic parameter "<M: Meower>" and using "M", as with the example above that one. (Except in the second example it's placed behind a shared reference; it still works either way with either syntax.)