为什么我得到“需要类型注释"?使用 Iterator::collect 时?

问题描述

我想得到一个我分割的字符串的长度:

I want to get a length of a string which I've split:

fn fn1(my_string: String) -> bool {
    let mut segments = my_string.split(".");
    segments.collect().len() == 55
}

error[E0282]: type annotations needed
 --> src/lib.rs:3:14
  |
3 |     segments.collect().len() == 55
  |              ^^^^^^^ cannot infer type for type parameter `B` declared on the associated function `collect`
  |
  = note: type must be known at this point

之前的编译器版本报错:

Previous compiler versions report the error:

error[E0619]: the type of this value must be known in this context
 --> src/main.rs:3:5
  |
3 |     segments.collect().len() == 55
  |     ^^^^^^^^^^^^^^^^^^^^^^^^

我该如何解决这个错误?

How can I fix this error?

推荐答案

关于迭代器，collect 方法 可以产生多种类型的集合:

On an iterator, the collect method can produce many types of collections:

fn collect<B>(self) -> B
where
    B: FromIterator<Self::Item>, 

实现 FromIterator 的类型包括 Vec、String 和 更多.因为有很多可能性，所以需要限制结果类型.您可以使用类似 .collect::<Vec<_>>() 或 let something: Vec<_> 来指定类型.= some_iter.collect().

Types that implement FromIterator include Vec, String and many more. Because there are so many possibilities, something needs to constrain the result type. You can specify the type with something like .collect::<Vec<_>>() or let something: Vec<_> = some_iter.collect().

在类型已知之前，您不能调用方法len()，因为无法知道未知类型是否具有特定方法.

Until the type is known, you cannot call the method len() because it's impossible to know if an unknown type has a specific method.

如果您纯粹想知道迭代器中有多少项，请使用 Iterator.count();为此目的创建向量的效率相当低.

If you’re purely wanting to find out how many items there are in an iterator, use Iterator.count(); creating a vector for the purpose is rather inefficient.

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