Generic associated type may not live long enough - rust

Question

Take the following example (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})
}
}
The GenericError has a lifetime to allow it to take Produce as a reference. However, this code doesn't compile. It gives me the error:
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
This error makes sense to me because the GenericError doesn't have any restriction telling it that T must be 'a. I'm having trouble figuring out how to solve the problem though. Perhaps my generic lifetimes are misplaced?
The feature of the trait I wish to capture is that any Produce::CustomError should be able to capture self in a return. Perhaps I am going about this in the wrong way?

Answer 1

The same trait without generic_associated_types takes the lifetime in the trait itself.
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})
}
}
This tells the impl upfront that T that it must be 'a.