If you're trying to remember what the language is where there's no immediately obvious straightforward way to get the length of an array, it's not Rust or C++; you must have been thinking of C.
In Rust the arrays aren't stunted left over primitive types which weren't gifted modern features, array.len() works because all Rust's types get features like this, not just the latest and greatest stuff.
(C++ arrays are different from arrays in many other programming languages, though not necessarily Rust, in that their type specifies their length, so in a way this is something you already "have" but certainly there are cases where it is convenient to "get" this information from an object.)
