通用关联类型可能寿命不够长

Question

举个例子(Playground):

#![feature(generic_associated_types)]
#![allow(incomplete_features)]

trait Produce {
    type CustomError<'a>;

    fn produce<'a>(&'a self) -> Result<(), Self::CustomError<'a>>;
}

struct GenericProduce<T> {
    val: T,
}

struct GenericError<'a, T> {
    producer: &'a T,
}

impl<T> Produce for GenericProduce<T> {
    type CustomError<'a> = GenericError<'a, T>;

    fn produce<'a>(&'a self) -> Result<(), Self::CustomError<'a>> {
        Err(GenericError{producer: &self.val})
    }
}

GenericError有生命周期允许它以Produce作为参考。但是，此代码无法编译。它给了我错误：

error[E0309]: the parameter type `T` may not live long enough
  --> src/lib.rs:19:5
   |
18 | impl<T> Produce for GenericProduce<T> {
   |      - help: consider adding an explicit lifetime bound...: `T: 'a`
19 |     type CustomError<'a> = GenericError<'a, T>;
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ ...so that the type `T` will meet its required lifetime bounds

这个错误对我来说很有意义，因为 GenericError 没有任何限制告诉它 T 必须是 'a。我无法弄清楚如何解决问题。也许我的通用生命周期错位了？

我希望捕获的特征是任何 Produce::CustomError 都应该能够捕获 return 中的 self。也许我以错误的方式解决这个问题？

Answer 1

没有 generic_associated_types 的相同特征会占用特征本身的生命周期。

trait Produce<'a> {
    type CustomError;

    fn produce(&'a self) -> Result<(), Self::CustomError>;
}

struct GenericProduce<T> {
    val: T,
}

struct GenericError<'a, T> {
    producer: &'a T,
}

impl<'a, T: 'a> Produce<'a> for GenericProduce<T> {
    type CustomError = GenericError<'a, T>;

    fn produce(&'a self) -> Result<(), Self::CustomError> {
        Err(GenericError{producer: &self.val})
    }
}

这预先告诉 impl T 它必须是 'a.

通用关联类型可能寿命不够长

Generic associated type may not live long enough

traits

rust

generic-associated-types