rust - 实现迭代器的另一个特性

Question

我正在尝试向所有Iterator添加另一个特征。但是我不明白为什么它不能编译。
这是代码:

use std::fmt::Display;
use std::iter::Sum;

trait Topla<T> {
    fn topla(&mut self)-> T;
}

impl<T, I> Topla<T> for I
where
    T: Sum + Display,
    I: Iterator<Item = T>,
{
    fn topla(&mut self) -> T {
        self.sum()
    }
}

fn main() {
    let data = vec![1,2,3,5,8];
    println!("{:?}", data.iter().topla());
}

Answer 1

如果我们使用完全限定的trait语法，则问题会自行解决:

fn main() {
    let data = vec![1,2,3,5,8u32];
    let mut iter = data.iter();
    println!("{:?}", Topla::<u32>::topla(&mut iter));
}

error[E0271]: type mismatch resolving `<std::slice::Iter<'_, u32> as std::iter::Iterator>::Item == u32`
  --> src/main.rs:22:22
   |
5  |     fn topla(&mut self) -> T;
   |     ------------------------- required by `Topla::topla`
...
22 |     println!("{:?}", Topla::<u32>::topla(&mut iter));
   |                      ^^^^^^^^^^^^^^^^^^^ expected reference, found `u32`
   |
   = note: expected reference `&u32`
                   found type `u32`
   = note: required because of the requirements on the impl of `Topla<u32>` for `std::slice::Iter<'_, u32>`

将Topla<u32>更改为Topla<&u32>使我们更加接近:

Compiling playground v0.0.1 (/playground)
error[E0277]: the trait bound `&u32: std::iter::Sum` is not satisfied
  --> src/main.rs:22:22
   |
5  |     fn topla(&mut self) -> T;
   |     ------------------------- required by `Topla::topla`
...
22 |     println!("{:?}", Topla::<&u32>::topla(&mut iter));
   |                      ^^^^^^^^^^^^^^^^^^^^ the trait `std::iter::Sum` is not implemented for `&u32`
   |
   = help: the following implementations were found:
             <u32 as std::iter::Sum<&'a u32>>
             <u32 as std::iter::Sum>
   = note: required because of the requirements on the impl of `Topla<&u32>` for `std::slice::Iter<'_, u32>`

问题是没有为Sum<&u32>实现&u32；它是针对u32实现的。由于我们需要返回一个u32而不是&u32，因此我们需要放宽对T的要求，从本身是迭代器类型变为仅具有T的Sum。

trait Topla<T> {
    fn topla(&mut self) -> T;
}

impl<T, I> Topla<T> for I
where
    T: Display + Sum<<I as Iterator>::Item>,
    I: Iterator,
{
    fn topla(&mut self) -> T {
        self.sum()
    }
}

fn main() {
    let data = vec![1,2,3,5,8u32];
    let mut iter = data.iter();
    println!("{:?}", Topla::<u32>::topla(&mut iter));
}

但是现在，如果我们返回原始语法，则会遇到类型推断错误，这会使我们的新API变得非常烦人。我们可以通过更加严格地使用API​​来解决此问题。
如果我们将Topla设为Iterator的子特征，则可以在topla方法的定义中引用项目类型，因此将输出的类型参数移至方法中而不是特征中。这将使我们像使用sum()一样使用turbofish语法。最后，我们有:

use std::fmt::Display;
use std::iter::Sum;

trait Topla: Iterator {
    fn topla<T>(self) -> T
    where
        Self: Sized,
        T: Sum<<Self as Iterator>::Item>;
}

impl<I> Topla for I
where
    I: Iterator,
    <I as Iterator>::Item: Display,
{
    fn topla<T>(self) -> T
    where
        Self: Sized,
        T: Sum<<Self as Iterator>::Item>,
    {
        self.sum()
    }
}

fn main() {
    let data = vec![1, 2, 3, 5, 8];
    println!("{:?}", data.iter().topla::<u32>());
}