string-make-multibyte

string-make-multibyte is a built-in function in `C source code'.

(string-make-multibyte STRING)

Return the multibyte equivalent of STRING.
If STRING is unibyte and contains non-ASCII characters, the function
`unibyte-char-to-multibyte' is used to convert each unibyte character
to a multibyte character. In this case, the returned string is a
newly created string with no text properties. If STRING is multibyte
or entirely ASCII, it is returned unchanged. In particular, when
STRING is unibyte and entirely ASCII, the returned string is unibyte.
(When the characters are all ASCII, Emacs primitives will treat the
string the same way whether it is unibyte or multibyte.)