Function quoted_string::quote_if_needed

pub fn quote_if_needed<'a, FN>(
    input: &'a str, 
    valid_without_quoting: FN, 
    quoted_string_type: QuotedStringType
) -> Result<(QuotedStringType, Cow<'a, str>)> where
    FN: ValidWithoutQuotationCheck, 

quotes the input string if needed(RFC 5322/6532/822/2045)

The valid_without_quoting parameter accepts a function, which should only return true if the char is valid without quoting. So this function should never return true for e.g. \0. Use this function if some characters are only valid in a quoted-string context.

If the allowed_mail_type parameter is set to Ascii the algorithm will return a error if it stumbles over a non-ascii character, else it will just indicate the appearance of one through the returned quoted string type. Note that if you set quoted_string_type to Utf8 the function still can returns a QuotedStringType::AsciiOnly which means only ascii characters where contained, even through non ascii character where allowed.

The quoting process can fail if characters are contained, which can not appear in a quoted string independent of quoted string type. This are chars which are neither qtext,vchar nor WS (' ' and '\t'). Which are basically only 0x7F (DEL) and the characters < 0x20 (' ') except 0x09 ('\t').

Note that if the valid_without_quoting function states a CTL char is valid without quoting then the algorithm will see it as such even through there shouldn't be any context in which a CTL char is valid without quoting.