Generic Function to Produce Generic Struct

[wspeirs]

I would have thought this was super-simple, but struggling to write a function that produces a generic struct, with the same parameters: playground with below code

struct CapIt<I: Iterator<Item=u64>> {
    it: I
}

fn foo<I: Iterator<Item=u64>>() -> CapIt<I> {
    let v = vec![0u64, 1];
    
    CapIt {
        it: v.into_iter()
    }
}

Error:

error[E0308]: mismatched types
  --> src/lib.rs:10:13
   |
6  | fn foo<I: Iterator<Item=u64>>() -> CapIt<I> {
   |        - this type parameter
...
10 |         it: v.into_iter()
   |             ^^^^^^^^^^^^^ expected type parameter `I`, found struct `IntoIter`
   |
   = note: expected type parameter `I`
                      found struct `std::vec::IntoIter<u64>`

Am I missing a cast or something?

[jofas]

With your generic type argument I you let the caller decide what type to have in your output struct. The caller doesn't decide that though, because you do so in your function by calling v.into_iter(). Instead of letting your caller decide what type I is, decide it yourself with impl Trait in return position. This section of the reference describes what I'm trying to tell you better:

With impl Trait, unlike with a generic type parameter, the function chooses the return type, and the caller cannot choose the return type.

Here is your examle with impl Trait instead of a generic argument:

struct CapIt<I: Iterator<Item=u64>> {
    it: I
}

fn foo() -> CapIt<impl Iterator<Item=u64>> {
    let v = vec![0u64, 1];
    
    CapIt {
        it: v.into_iter()
    }
}

Playground.

[H2CO3]

Please next time search the forum for similar questions. This exact same problem has been asked several times in the past couple of days. Example.