为什么编译器声称来自更高级别特征边界的关联类型没有实现“Display”，即使它应该实现？

Question

我正在构建一个实现字符串连接的库；也就是说，打印由分隔符分隔的容器的所有元素。我的基本设计如下所示：

use std::fmt;

#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Join<Container, Sep> {
    container: Container,
    sep: Sep,
}

impl<Container, Sep> fmt::Display for Join<Container, Sep>
where
    for<'a> &'a Container: IntoIterator,
    for<'a> <&'a Container as IntoIterator>::Item: fmt::Display,
    Sep: fmt::Display,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let mut iter = self.container.into_iter();

        match iter.next() {
            None => Ok(()),
            Some(first) => {
                first.fmt(f)?;

                iter.try_for_each(move |element| {
                    self.sep.fmt(f)?;
                    element.fmt(f)
                })
            }
        }
    }
}

此特征实现编译无怨言。注意 &'a C: IntoIterator 上的界限。许多容器实现 IntoIterator 以引用自身，以允许迭代对包含项的引用（例如，Vec 实现它 here）。

然而，当我实际尝试使用我的 Join 结构时，我得到了一个不满足的特征界限：

fn main() {
    let data = vec!["Hello", "World"];
    let join = Join {
        container: data,
        sep: ", ",
    };
    println!("{}", join);
}

此代码产生编译错误：

error[E0277]: `<&'a std::vec::Vec<&str> as std::iter::IntoIterator>::Item` doesn't implement `std::fmt::Display`
  --> src/main.rs:38:20
   |
38 |     println!("{}", join);
   |                    ^^^^ `<&'a std::vec::Vec<&str> as std::iter::IntoIterator>::Item` cannot be formatted with the default formatter
   |
   = help: the trait `for<'a> std::fmt::Display` is not implemented for `<&'a std::vec::Vec<&str> as std::iter::IntoIterator>::Item`
   = note: in format strings you may be able to use `{:?}` (or {:#?} for pretty-print) instead
   = note: required because of the requirements on the impl of `std::fmt::Display` for `Join<std::vec::Vec<&str>, &str>`
   = note: required by `std::fmt::Display::fmt`

关键的一行好像是这样的：

the trait `for<'a> std::fmt::Display` is not implemented for `<&'a std::vec::Vec<&str> as std::iter::IntoIterator>::Item`

不幸的是，编译器实际上并没有告诉我 Item 类型是什么，但根据我对 the docs 的阅读，它似乎是 &T，在本文中case 表示 &&str。

为什么编译器不认为 &&str 实现了 Display？我已经对许多其他类型进行了尝试，例如 usize 和 String，其中 none 有效；他们都因同样的错误而失败。我知道这些引用类型不会直接实现Display，但是应该通过解引用强制自动获取实现，对吗？

Answer 1

好像是 a compiler limitation。您现在可以通过根据表示“显示生命周期”的私有助手特征来编写 impl 绑定来解决它。这使编译器能够看到 for<'a> private::Display<'a> 意味着 fmt::Display.

use std::fmt;

pub struct Join<Container, Sep> {
    container: Container,
    sep: Sep,
}

mod private {
    use std::fmt;
    pub trait Display<'a>: fmt::Display {}
    impl<'a, T> Display<'a> for T where T: fmt::Display {}
}

impl<Container, Sep> fmt::Display for Join<Container, Sep>
where
    for<'a> &'a Container: IntoIterator,
    for<'a> <&'a Container as IntoIterator>::Item: private::Display<'a>,
    Sep: fmt::Display,
{
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        let mut iter = self.container.into_iter();

        match iter.next() {
            None => Ok(()),
            Some(first) => {
                first.fmt(f)?;

                iter.try_for_each(move |element| {
                    self.sep.fmt(f)?;
                    element.fmt(f)
                })
            }
        }
    }
}

fn main() {
    println!(
        "{}",
        Join {
            container: vec!["Hello", "World"],
            sep: ", ",
        }
    );
}

为什么编译器声称来自更高级别特征边界的关联类型没有实现“Display”，即使它应该实现？

Why does the compiler claim that an associated type from a higher-ranked trait bound doesn't implement `Display` even though it should?

formatting

traits

rust