Question

我正在编写一个在线算法，该算法由一系列带有迭代器并产生迭代器的函数实现。

当我这样编写函数时（内容更复杂但类型不同）

fn decode<'a, T>(input: T) -> impl Iterator<Item = i16> + 'a
where
    T: Iterator<Item = &'a u8> + 'a,
{
    input.map(|b| i16::from(*b)).filter(|i| *i != 0)
}

请参见playground。

但是，这使得调用函数不符合人体工程学：

let input: Vec<u8> = vec![1, 2, 3, 0];
let v: Vec<i16> = decode(input.iter()).collect();

我更愿意使用T: Into<Iterator<...，但不能。当我这样写签名时：

fn decode<'a, T>(input: T) -> impl Iterator<Item = i16> + 'a
where
    T: Into<Iterator<Item = &'a u8>> + 'a,

playground

我收到一条错误消息，指出在编译时不知道返回类型的大小：

error[E0277]: the size for values of type `(dyn std::iter::Iterator<Item=&'a u8> + 'static)` cannot be known at compilation time
 --> src/main.rs:1:1
  |
1 | / fn decode<'a, T>(input: T) -> impl Iterator<Item = i16> + 'a
2 | | where
3 | |     T: Into<Iterator<Item = &'a u8>> + 'a,
4 | | {
5 | |     input.into().map(|b| i16::from(*b)).filter(|i| *i != 0)
6 | | }
  | |_^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `(dyn std::iter::Iterator<Item=&'a u8> + 'static)`
  = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
  = note: required by `std::convert::Into`

这是为什么，还有更好的方法吗？

Answer 1

改为使用IntoIterator：

fn decode<'a>(input: impl IntoIterator<Item = &'a u8> + 'a) -> impl Iterator<Item = i16> + 'a {
    input.into_iter().map(|b| i16::from(*b)).filter(|i| *i != 0)
}

Iterator是一个特征，特征没有大小。这就是为什么您还不能（写）的原因：

fn example(x: Iterator<Item = ()>) {}

error[E0277]: the size for values of type `(dyn std::iter::Iterator<Item=()> + 'static)` cannot be known at compilation time
 --> src/lib.rs:1:12
  |
1 | fn example(x: Iterator<Item = ()>) {}
  |            ^ doesn't have a size known at compile-time
  |
  = help: the trait `std::marker::Sized` is not implemented for `(dyn std::iter::Iterator<Item=()> + 'static)`
  = note: to learn more, visit <https://doc.rust-lang.org/book/ch19-04-advanced-types.html#dynamically-sized-types-and-the-sized-trait>
  = note: all local variables must have a statically known size
  = help: unsized locals are gated as an unstable feature

Into定义为：

pub trait Into<T> {
    fn into(self) -> T;
}

实现Into<dyn Iterator>的东西必须具有函数fn into(self) -> dyn Iterator，并返回一个特征。由于特征没有大小，因此无法（尚未）将其返回，存储在变量中或作为参数接受。

另请参阅：

What does "Sized is not implemented" mean?

在线算法的Into <iterator>与Iterator

1 个答案: