Function quote_ascii_string_if_required 

pub fn quote_ascii_string_if_required(value: &str) -> Cow<'_, str>

Available on crate feature api only.

If the US-ASCII field value does not contain only token chars, convert it to a quoted string.

The string should be sanitized with sanitize_for_ascii_quoted_string() or should only contain characters that pass is_ascii_string_quotable().