为什么编译器声称来自更高等级特征绑定的关联类型没有实现 `Display`,即使它应该实现? [英] Why does the compiler claim that an associated type from a higher-ranked trait bound doesn't implement `Display` even though it should?
问题描述
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:
I'm building a library that implements string joins; that is, printing all the elements of a container separated by a separator. My basic design looks like this:
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)
})
}
}
}
}
这个 trait 实现编译时没有抱怨.注意 &'a C: IntoIterator
上的界限.许多容器实现了 IntoIterator
以作为对自身的引用,以允许迭代对包含项的引用(例如,Vec
实现了它这里).
This trait implementation compiles without complaint. Notice the bound on &'a C: IntoIterator
. Many containers implement IntoIterator
for a reference to themselves, to allow for iterating over references to the contained items (for instance, Vec
implements it here).
然而,当我真正尝试使用我的 Join
结构时,我得到了一个不满意的 trait bound:
However, when I actually try to use my Join
struct, I get an unsatisfied trait bound:
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`
关键线似乎是这样的:
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,而是基于我对 the docs,它似乎是 &T
,在这种情况下意味着 &&str
.
Unfortunately, the compiler doesn't actually tell me what the Item
type is, but based on my reading of the docs, it appears to be &T
, which in this case means &&str
.
为什么编译器不认为 &&str
实现了 Display
?我已经用许多其他类型尝试过这个,比如 usize
和 String
,但它们都不起作用;它们都因相同的错误而失败.我知道这些引用类型不直接实现Display
,但是实现应该通过deref强制自动获取,对吗?
Why doesn't the compiler think that &&str
implements Display
? I've tried this with many other types, like usize
and String
, and none of them work; they all fail with the same error. I know that these reference type don't directly implement Display
, but the implementation should be picked up automatically through deref coercion, right?
推荐答案
看起来像 a编译器限制.您现在可以通过根据表示显示与生命周期"的私有帮助程序特征编写 impl 边界来解决它.这使编译器能够看到 for<'a>private::Display<'a>
暗示 fmt::Display
.
Seems like a compiler limitation. You can work around it for now by writing the impl bound in terms of a private helper trait that represents "display with lifetime". This enables the compiler to see that for<'a> private::Display<'a>
implies 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: ", ",
}
);
}
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