Function join 

pub fn join(left: &str, right: &str, fmt: PathFormat) -> String

Join two path strings using the separator and absoluteness rules for fmt.

If right is absolute (according to fmt), it replaces left entirely. On Windows, if right starts with a single / or \ (root-relative), the drive letter from left is preserved (matching ntpath.join behavior). Otherwise, right is appended to left with the appropriate separator.