string-to-unibyte

string-to-unibyte is a built-in function in `C source code'.

(string-to-unibyte STRING)

Return a unibyte string with the same individual chars as STRING.
If STRING is unibyte, the result is STRING itself.
Otherwise it is a newly created string, with no text properties,
where each `eight-bit' character is converted to the corresponding byte.
If STRING contains a non-ASCII, non-`eight-bit' character,
an error is signaled.