Not your parent, but I do know the answer. extension() returns an Option<&OsStr>, because not every file has an extension https://doc.rust-lang.org/stable/std/

Not your parent, but I do know the answer. extension() returns an Option<&OsStr>, because not every file has an extension https://doc.rust-lang.org/stable/std/path/struct.Path.html#method.extension https://doc.rust-lang.org/stable/std/path/struct.Path.html#m... map on option says "run this closure on the value if it's Some, or do nothing if it's None https://doc.rust-lang.org/stable/std/option/enum.Option.html#method.map https://doc.rust-lang.org/stable/std/option/enum.Option.html... So after the map, we now have an Option<bool>. The unwrap_or method will say "give me the value if it's Some, and give me the argument if it's not https://doc.rust-lang.org/stable/std/option/enum.Option.html#method.unwrap_or https://doc.rust-lang.org/stable/std/option/enum.Option.html... That means we finally end up with a bool. (path.extension() == "bundle").unwrap_or(false) This code wouldn't work because you're comparing an Option<&OsStr> to a &str. Even if that worked, you'd have a boolean, and unwrap_or doesn't make sense on booleans, you'd just leave off that part.