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