types - 在线算法的 Into vs Iterator

Question

我正在编写一个在线算法，使用一系列函数实现，这些函数采用迭代器并生成迭代器。

当我这样写函数时(内容更复杂但类型没有区别):

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)
}

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<... ，但我不能。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，指出返回类型的大小在编译时未知:

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，有没有更好的办法？

Answer 1

使用 IntoIterator 相反:

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，而特征没有大小。这就是为什么你不能(还)写:

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 , 返回一个特征。由于 trait 没有大小，因此(目前)不能将其返回、存储在变量中或作为参数接受。

另见:

• What does "Sized is not implemented" mean?