Mutable and immutable reference issue.

[iFaceless]

In order to pass borrow checking, every key is cloned and pushed into a new vector, which is a little hacky and inefficient.

pub fn for_each<F>(&mut self, f: F) -> Result<()>
where
    F: Fn(&[u8], &[u8]),
{
    // too stupid, just in order to pass borrow checking. 
    // FIXME: find a better way to implement this feature?
    let mut keys = vec![];
    for key in self.keys() {
        keys.push(key.clone());
    }
    
    for key in keys {
        let r = self.get(&key)?;
        if let Some(value) = r {
            f(&key, &value);
        }
    }
    Ok(())
}

Just wonder is there a better way to implement the above feature? HOW?

Following implementation cannot be compiled, we will get error cannot borrow *self as mutable because it is also borrowed as immutable.

pub fn for_each<F>(&mut self, f: F) -> Result<()>
where
    F: Fn(&[u8], &[u8]),
{
    for key in self.keys() {
        let r = self.get(&key)?;
        if let Some(value) = r {
            f(&key, &value);
        }
    }
    Ok(())
}