You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: lib.rs
    path of file: ./repos/wasmi/crates/arena/src
    
    the code of the file until where you have to start completion: //! Fast arena allocators for different usage purposes.
//!
//! They cannot deallocat
    
    pub fn get_pair_mut(&mut self, fst: Idx, snd: Idx) -> Option<(&mut T, &mut T)> {
        let fst_index = fst.into_usize();
        let snd_index = snd.into_usize();
        if fst_index == snd_index {
            return None;
        }
        if fst_index > snd_index {
            let (fst, snd) = self.get_pair_mut(snd, fst)?;
            return Some((snd, fst));
        }
        // At this point we know that fst_index < snd_index.
        let (fst_set, snd_set) = self.entities.split_at_mut(snd_index);
        let fst = fst_set.get_mut(fst_index)?;
        let snd = snd_set.get_mut(0)?;
        Some((fst, snd))
    }