pub fn expand<'a>(text: &'a str, source: &str) -> Cow<'a, str>Expand description
Expand $VAR, ${VAR}, ${VAR:-default} and $$ in one string.
source names the file for the warning an unset variable produces; it has
no effect on the result.
Borrows when there is nothing to do, which is the overwhelmingly common
case — most strings in a TOML file contain no $ at all.