See Reference types - Learning Rust // The exact lifetime of `'a` is determined at each call site. We'll explore // what this means in more depth later. // // The lifetime of `b` works the same, we just didn't give it a name. I would write the first part as // The exact lifetime of `a` is determined at each call site. or // The exact lifetime `'a` is determined at each call site. Note, that in the first version I have removed the ' so that we refer to the variable directly, and that I have removed in the second version the "of", so that we refer to the generic lifetime directly. (a lifetime of a generic lifetime would be a bit strange?) Perhaps we should actually write // The exact lifetime `'a` of parameter `a`is determined at each call site.