Whitespaces in path to macro inside derive block

"cargo fmt" command does not remove white spaces in derive block. For example i expected this

#[derive(serde :: Serialize)]
enum Something {
    Variant
}

to become this

#[derive(serde::Serialize)]
enum Something {
    Variant
}

cannot find anything similar in https://rust-lang.github.io/rustfmt/?version=master&search= tried to find by words "space" or "derive" (default config)