If you have a Vec<u8> of utf8 data there's three ways to get a String:

• from_utf8(vec: Vec<u8>) -> Result<String, FromUtf8Error>
• from_utf8_lossy(v: &'a [u8]) -> Cow<'a, str>
• unsafe fn from_utf8_unchecked(bytes: Vec<u8>) -> String

There doesn't appear to be any way to hand over the existing Vec<u8> and have any necessary character changes into � be done in place. The std::string::FromUtf8Error type doesn't, for example, have a way to just do a lossy in-place conversion.