iterator - 如何将实现特征的类型的迭代器的内容装箱？

Question

我正在使用某种类型的迭代器，它必须实现 A 特征，并尝试将其转换为 Box 的 Vec该特征的 es:

trait A {}

fn test2<'a, I>(iterator: I) -> Vec<Box<A + 'a>>
where
    I: IntoIterator,
    I::Item: A + 'a,
{
    iterator
        .into_iter()
        .map(|a| Box::new(a))
        .collect::<Vec<Box<A + 'a>>>()
}

然而，这编译失败，说:

error[E0277]: the trait bound `std::vec::Vec<std::boxed::Box<A + 'a>>: std::iter::FromIterator<std::boxed::Box<<I as std::iter::IntoIterator>::Item>>` is not satisfied
  --> src/main.rs:11:10
   |
11 |         .collect::<Vec<Box<A + 'a>>>()
   |          ^^^^^^^ a collection of type `std::vec::Vec<std::boxed::Box<A + 'a>>` cannot be built from an iterator over elements of type `std::boxed::Box<<I as std::iter::IntoIterator>::Item>`
   |
   = help: the trait `std::iter::FromIterator<std::boxed::Box<<I as std::iter::IntoIterator>::Item>>` is not implemented for `std::vec::Vec<std::boxed::Box<A + 'a>>`
   = help: consider adding a `where std::vec::Vec<std::boxed::Box<A + 'a>>: std::iter::FromIterator<std::boxed::Box<<I as std::iter::IntoIterator>::Item>>` bound

这个错误是有道理的，但后来我不明白为什么以下没有问题:

fn test<'a, T: A + 'a>(t: T) -> Box<A + 'a> {
    Box::new(t)
}

这有什么不同？我如何表达我想将它们 Box 为 A，而不是它们可能是什么类型？

Answer 1

您需要转换 Box<I::Item>进入 Box<A> :

fn test2<'a, I>(iterator: I) -> Vec<Box<dyn A + 'a>>
where
    I: IntoIterator,
    I::Item: A + 'a,
{
    iterator
        .into_iter()
        .map(|a| Box::new(a) as Box<dyn A>)
        .collect()
}

How is [returning Box::new directly] any different?

作为Sven Marnach points out :

The reason why you don't need an explicit cast in the function is that the last statement of a block is a coercion site and coercions happen implicitly at these sites. See the chapter on coercions in the nomicon for further details.