isAsciiString in Phobos?

[Andrej Mitrovic]

If I want to transfer some string to a C function that expects
ascii-only string. What can I use to verify there are no non-ascii
characters in a D string? I haven't seen anything in Phobos.

I was thinking of using:

bool isAscii = mystring.all!(a => a <= 0xFF);

Is this safe?

I'm thinking of whether a code point can consist of two code units
such as [C1][C2], where C2 may be in the range 0 - 0xFF. I don't know
if that's possible (not a unicode pro here..).