测试用例：单位明确

通过使用幻影类型参数实现 Add，可以研究一种有用的单位转换方法。下面将研究 Add trait

// This construction would impose: `Self + RHS = Output`
// where RHS defaults to Self if not specified in the implementation.
pub trait Add<RHS = Self> {
    type Output;

    fn add(self, rhs: RHS) -> Self::Output;
}

// `Output` must be `T<U>` so that `T<U> + T<U> = T<U>`.
impl<U> Add for T<U> {
    type Output = T<U>;
    ...
}

完整实现

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另请参阅

借用 (&), 约束 (X: Y), enum, impl & self, 重载, ref, 特征 (X for Y), 和 元组结构体。