string-as-multibyte

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

(string-as-multibyte STRING)

Return a multibyte string with the same individual bytes as STRING.
If STRING is multibyte, the result is STRING itself.
Otherwise it is a newly created string, with no text properties.

If STRING is unibyte and contains an individual 8-bit byte (i.e. not
part of a correct utf-8 sequence), it is converted to the corresponding
multibyte character of charset `eight-bit'.
See also `string-to-multibyte'.

Beware, this often doesn't really do what you think it does.
It is similar to (decode-coding-string STRING 'utf-8-emacs).
If you're not sure, whether to use `string-as-multibyte' or
`string-to-multibyte', use `string-to-multibyte'.