为什么Cow的定义在Owned（<b as =“” toowned =“”> :: Owned）中使用“ as”？

Question

Cow的定义方式如下：

pub enum Cow<'a, B>
where
    B: 'a + ToOwned + ?Sized,
{
    Borrowed(&'a B),
    Owned(<B as ToOwned>::Owned),
}

为什么B as ToOwned在那儿？为了删除'a和?Sized的界限？

Answer 1

它只是通过fully qualified syntax被显式显示。同样可以将其定义为

pub enum Cow<'a, B>
where
    B: 'a + ToOwned + ?Sized,
{
    Borrowed(&'a B),
    Owned(B::Owned),
}

此语法是您访问B的{​​{1}}实现的关联类型的方式。