为何编译器声称泛型即使应有也不实现`Display`? [英] Why does the compiler claim that a generic doesn't implement `Display` even though it should?
问题描述
我正在建立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)
})
}
}
}
}
此特征实现无需进行任何编译即可编译.注意&'a C: IntoIterator
上的边界.许多容器实现IntoIterator
对其自身的引用,以允许遍历对所包含项目的引用(例如,Vec
实施IntoIterator
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).
但是,当我实际尝试使用Join
结构时,我得到了一个不满意的特征绑定:
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:
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
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
?我已经尝试过使用其他许多类型的方法,例如usize
和String
,但是它们都不起作用.它们都因相同的错误而失败.我知道这些引用类型不是直接 实现Display
,但是应该通过deref强制自动选择实现,对吧?
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
似乎像编译器的限制.您现在可以通过以表示终身显示"的私人帮助者特征来编写隐含绑定来解决此问题.这使编译器可以看到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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