I've seen people with functions returning `Box<Iterator>` in IRC decently often. The only alternative right now is to write a type signature that's probably too complex for anyone new to Rust to even figure out:

```rust
pub struct PhfMapEntries<'a, T> {
    priv iter: iter::FilterMap<'a,
                               &'a Option<(&'static str, T)>,
                               (&'static str, &'a T),
                               slice::Items<'a, Option<(&'static str, T)>>>,
}
```