OutputIdentifier

/// An output_identifier can be build from either an input _or_ an output and
/// contains everything we need to uniquely identify an output being spent.
/// Needed because it is not sufficient to pass a commitment around.
pub struct OutputIdentifier {
    /// Output features (coinbase vs. regular transaction output)
    /// We need to include this when hashing to ensure coinbase maturity can be
    /// enforced.
    pub features: OutputFeatures,
    /// Output commitment
    pub commit: Commitment,
}

输入、输入结构有类似，甚至相同的地方，并且可用来做为标识。我们把它拿出来，这就是 OutputIdentifier.