Trait to store structs with different generic parameters - rust

Question

I need to store in the same Vec instances of the same struct, but with different generic parameters. This is the struct definition:
struct Struct<'a, T: 'a> {
items: Vec<&'a T>
}
The struct has a method returning an iterator to a type that does not depend on the generic type parameter T:
impl<'a, T: 'a> Struct<'a, T> {
fn iter(&self) -> slice::Iter<&i32> {
unimplemented!()
}
}
I need to access this method for those different structs in the vector, so I've implemented this trait:
type Iter<'a> = Iterator<Item=&'a i32>;
trait Trait {
fn iter(&self) -> Box<Iter>;
}
And I've implemented the trait for Struct:
impl<'a, T: 'a> Trait for Struct<'a, T> {
fn iter(&self) -> Box<Iter> {
Box::new(self.iter())
}
}
But the compiler complains:
<anon>:21:9: 21:30 error: type mismatch resolving `<core::slice::Iter<'_, &i32> as core::iter::Iterator>::Item == &i32`:
expected &-ptr,
found i32 [E0271]
<anon>:21 Box::new(self.iter())
^~~~~~~~~~~~~~~~~~~~~
<anon>:21:9: 21:30 help: see the detailed explanation for E0271
<anon>:21:9: 21:30 note: required for the cast to the object type `core::iter::Iterator<Item=&i32> + 'static`
<anon>:21 Box::new(self.iter())
^~~~~~~~~~~~~~~~~~~~~
I've tried different possibilities for lifetime parameters in the trait, but none of them work. How can I make this work?
Rust Playground snippet
Edit
As pointed out by #MatthieuM. one problem is that the type alias is not working properly. Here's another example demonstrating this:
use std::slice;
type Iter<'a> = Iterator<Item=&'a i32>;
struct Struct<'a> { _phantom: std::marker::PhantomData<&'a i32> }
impl<'a> Struct<'a> {
fn direct<'b>(i: &'b slice::Iter<'a, i32>) -> &'b Iterator<Item=&'a i32>
{ i }
fn aliased<'b>(i: &'b slice::Iter<'a, i32>) -> &'b Iter<'a>
{ i }
}
In this example, direct compiles, but aliased not, with the error:
<anon>:12:7: 12:8 error: the type `core::slice::Iter<'a, i32>` does not fulfill the required lifetime
<anon>:12 { i }
^
note: type must outlive the static lifetime
But they seem to be the same thing. What's happening?

Answer 1

Problem 1 — slice::Iter<T> has an Iterator::Item of &T, thus your reference levels are mismatched. Change your method to be
fn iter(&self) -> slice::Iter<i32>
Problem 2 — Box<SomeTrait> is equivalent to Box<SomeTrait + 'static>, but your iterator does not live for the 'static lifetime. You need to explicitly bring in a lifetime:
Box<SomeTrait + 'a>
Problem 3 — I don't understand how you can create a type alias for a trait, that seems very odd. You probably don't want it anyway. Instead, create a type alias for the whole boxed version:
type IterBox<'a> = Box<Iterator<Item=&'a i32> + 'a>;
Problem 4 — Rearrange your main so that references will live long enough and add mutability:
fn main() {
let i = 3;
let v = vec![&i];
let mut traits : Vec<Box<Trait>> = Vec::new();
traits.push(Box::new(Struct{ items: v }));
}
All together:
use std::slice;
type IterBox<'a> = Box<Iterator<Item=&'a i32> + 'a>;
trait Trait {
fn iter<'a>(&'a self) -> IterBox;
}
struct Struct<'a, T: 'a> {
items: Vec<&'a T>
}
impl<'a, T: 'a> Struct<'a, T> {
fn iter(&self) -> slice::Iter<i32> {
unimplemented!()
}
}
impl<'a, T: 'a> Trait for Struct<'a, T> {
fn iter(&self) -> IterBox {
Box::new(self.iter())
}
}
fn main() {
let i = 3;
let v = vec![&i];
let mut traits: Vec<Box<Trait>> = Vec::new();
traits.push(Box::new(Struct { items: v }));
}