string-as-unibyte

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

(string-as-unibyte STRING)

Return a unibyte string with the same individual bytes as STRING.
If STRING is unibyte, the result is STRING itself.
Otherwise it is a newly created string, with no text properties.
If STRING is multibyte and contains a character of charset
`eight-bit', it is converted to the corresponding single byte.