pub fn insert(&self, key: &K, value: V) -> Result<&V, UnknownKey> {
        let item_index = self
            .inner
            .binary_search_by(|(k, _)| k.cmp(key))
            .map_err(|_| UnknownKey)?;
        let (_, value_slot) = &self.inner[item_index];
        // We do not care about if this set was successful or if another call won out.
        // The end result is that we have a value for the key.
        let _ = value_slot.set(value);
        Ok(value_slot
            .get()
            .expect("OnceLock::set will always result in a value being present in the lock"))
    }
Insert a value for a key.

There is no guarantee that the provided value will be the one returned.
crates/turborepo-fixed-map/src/lib.rs