pub fn or_insert_with<F: FnOnce() -> V>(self, default: F) -> &'a mut V {
        match self {
            Entry::Vacant(entry) => entry.insert(default()),
            Entry::Occupied(entry) => entry.into_mut(),
        }
    }
Ensures a value is in the entry, computing a value if necessary.
Returns a mutable reference to the value.
crates/core/executor/src/memory.rs