[英]Why do I get a Rust compilation error when combining higher-rank trait bounds with associated types?
I am writing some Rust code that involves generic traits and non- 'static
types, and as a result I have come across the need to approximate generic associated types .我正在编写一些涉及泛型特征和非
'static
类型'static
Rust 代码,因此我遇到了近似泛型关联类型的需要。 I understand that GATs can't be emulated elegantly in current Rust, but I thought I had figured out an (inelegant) workaround that would work for my specific situation, using traits with lifetime parameters and higher-rank trait bounds.我知道在当前的 Rust 中无法优雅地模拟 GAT,但我认为我已经找到了一种(不优雅的)解决方法,它适用于我的特定情况,使用具有生命周期参数和更高等级特征边界的特征。 However, I am getting compiler errors that I don't understand, regarding missing trait implementations for associated types.
但是,关于缺少关联类型的特征实现,我遇到了我不明白的编译器错误。
The following code shows a minimal example that reproduces the error.以下代码显示了重现错误的最小示例。
use std::fmt::Debug;
trait Resource<'r> {
type Value;
}
struct ResourceImpl();
impl<'r> Resource<'r> for ResourceImpl {
type Value = u32;
}
fn test_generic<R>()
where
for<'r> R: Resource<'r>,
for<'r> <R as Resource<'r>>::Value: Debug,
{
}
fn test_specific() {
test_generic::<ResourceImpl>();
}
When I try to compile this code ( rustc
1.41.0), I get the following error message.当我尝试编译此代码 (
rustc
1.41.0) 时,收到以下错误消息。
error[E0277]: `<ResourceImpl as Resource<'r>>::Value` doesn't implement `std::fmt::Debug`
--> src/lib.rs:21:5
|
13 | fn test_generic<R>()
| ------------
...
16 | for<'r> <R as Resource<'r>>::Value: Debug,
| ----- required by this bound in `test_generic`
...
21 | test_generic::<ResourceImpl>();
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ `<ResourceImpl as Resource<'r>>::Value` cannot be formatted using `{:?}` because it doesn't implement `std::fmt::Debug`
|
= help: the trait `for<'r> std::fmt::Debug` is not implemented for `<ResourceImpl as Resource<'r>>::Value`
The error message sounds like it is saying u32
doesn't implement Debug
, which wouldn't make sense.错误消息听起来像是在说
u32
没有实现Debug
,这是没有意义的。 I must be misunderstanding what the error message means, but I can't figure out what the actual problem is.我一定误解了错误消息的含义,但我无法弄清楚实际问题是什么。
There is an open issue about this problem.有一个关于这个问题的公开问题。
In your case a workaround could be to bind Debug
to the associated type Resource::Value
?在您的情况下,解决方法可能是将
Debug
绑定到关联类型Resource::Value
?
trait Resource<'r> {
type Value: Debug;
}.
As attdona pointed out, this appears to be a compiler bug (with an open issue here ).正如 attdona 指出的那样,这似乎是一个编译器错误(这里有一个未解决的问题)。 The discussion on that issue points to this Stack Overflow question , which provides a workaround that worked for me.
关于这个问题的讨论指向了这个 Stack Overflow 问题,它提供了一个对我有用的解决方法。 The key point of the workaround is that the trait mentioned inside the higher-rank trait bound must have lifetime parameters matching those inside the
for<_>
.解决方法的关键点是在高阶 trait bound 中提到的 trait 必须具有与
for<_>
生命周期参数匹配的生命周期参数。 This can be achieved by creating a wrapper trait (in this case around Debug
) with the required lifetime parameters.这可以通过使用所需的生命周期参数创建包装特征(在这种情况下围绕
Debug
)来实现。
In the case of the minimal example given in the question, the workaround looks like this:在问题中给出的最小示例的情况下,解决方法如下所示:
use std::fmt::Debug;
trait Resource<'r> {
type Value;
}
struct ResourceImpl();
impl<'r> Resource<'r> for ResourceImpl {
type Value = u32;
}
trait DebugWithLifetime<'r>: Debug {}
impl<'r, T> DebugWithLifetime<'r> for T where T: Debug {}
fn test_generic<R>()
where
for<'r> R: Resource<'r>,
for<'r> <R as Resource<'r>>::Value: DebugWithLifetime<'r>,
{
}
fn test_specific() {
test_generic::<ResourceImpl>();
}
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