// How to add an element to the accumulator acc and move out?

.fold(|| HashMap::new(), | mut acc: HashMap<Hash, Group>, (idx, hash) | {
            {
                let e = acc.entry(hash)
                        .or_insert(Group {value: Vec::new(), pos: group.pos + CHUNK_SIZE as u64, size: group.size });
                    (*e).value.push(*idx);
            }
            acc
    })

// compiler output
error[E0505]: cannot move out of `acc` because it is borrowed
   --> src/main.rs:114:13
    |
111 |             let e = acc.entry(hash)
    |                     --- borrow of `acc` occurs here
...
114 |             acc
    |             ^^^ move out of `acc` occurs here

error: aborting due to previous error