Checking to see if the string is valid UTF-8 requires code to execute, which gets in the way of performance. Therefore, you can check with functions like:
I don't see much value in a distinct char type which has no real semantics beyond being an unsigned byte.
Calling validate on a whole string makes at least some sense, I don't love it, but it's not crazy. But it's obviously never worth "validating" a single char, so why not just call them bytes (or ubytes if that's the preferred D nomenclature)
There are some extra semantics that strings have in D, but you have a point. Why make it special?
It's because strings are such a basic thing in programming, that baking them into language has a way of standardizing them. Instead of everyone having their own string type, there is one string type.
One of the problems with strings not being a builtin type is what does one do with string tokens? (C++ has this problem with its library string type.) Being an array of bytes in a function signature gives no clue if it is intended to be a string, or an array of other data.
It's just a big advantage to building it in, and making it interact smoothly with the other core language features.
https://dlang.org/phobos/std_utf.html#validate
when it is convenient in your code to do so.