Why am I getting “parameter is never used [E0392]”?

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I don’t believe you want another generic here, you want an associated type:

pub trait Generable {
    type From;
    fn generate_children(&self, data: &Self::From) -> Vec<Option<Self>>
    where
        Self: Sized;
}

pub enum Octree<T>
where
    T: Generable,
{
    Node {
        data: T,
        children: Vec<Box<Octree<T>>>,
    },
    Empty,
    Uninitialized,
}

fn main() {}

As an aside, Vec<Box<Octree<T>>> is probably one level extra of indirection — you can just use Vec<Octree<T>>.

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