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use Cache;
/// Trait used for updating the cache with a type.
///
/// This may be implemented on a type and used to update the cache via [`Cache::update`].
///
/// **Info**: You may not access the fields of the cache, as they are public for the crate only.
///
/// # Examples
///
/// Creating a custom struct implementation to update the cache with:
///
/// ```rust,ignore
/// use std::collections::hash_map::Entry;
///
/// use serenity::json::json;
/// use serenity::cache::{Cache, CacheUpdate};
/// use serenity::model::id::UserId;
/// use serenity::model::user::User;
///
/// // For example, an update to the user's record in the database was
/// // published to a pubsub channel.
/// struct DatabaseUserUpdate {
/// user_avatar: Option<String>,
/// user_discriminator: u16,
/// user_id: UserId,
/// user_is_bot: bool,
/// user_name: String,
/// }
///
/// #[serenity::async_trait]
/// impl CacheUpdate for DatabaseUserUpdate {
/// // A copy of the old user's data, if it existed in the cache.
/// type Output = User;
///
/// async fn update(&mut self, cache: &Cache) -> Option<Self::Output> {
/// // If an entry for the user already exists, update its fields.
/// match cache.users.entry(self.user_id) {
/// Entry::Occupied(entry) => {
/// let user = entry.get();
/// let old_user = user.clone();
///
/// user.bot = self.user_is_bot;
/// user.discriminator = self.user_discriminator;
/// user.id = self.user_id;
///
/// if user.avatar != self.user_avatar {
/// user.avatar = self.user_avatar.clone();
/// }
///
/// if user.name != self.user_name {
/// user.name = self.user_name.clone();
/// }
///
/// // Return the old copy for the user's sake.
/// Some(old_user)
/// },
/// Entry::Vacant(entry) => {
/// // We can convert a [`json::Value`] to a User for test
/// // purposes.
/// let user = from_value::<User>(json!({
/// "id": self.user_id,
/// "avatar": self.user_avatar.clone(),
/// "bot": self.user_is_bot,
/// "discriminator": self.user_discriminator,
/// "username": self.user_name.clone(),
/// })).expect("Error making user");
///
/// entry.insert(user);
///
/// // There was no old copy, so return None.
/// None
/// },
/// }
/// }
/// }
///
/// # async fn run() {
/// // Create an instance of the cache.
/// let mut cache = Cache::new();
///
/// // This is a sample pubsub message that you might receive from your
/// // database.
/// let mut update_message = DatabaseUserUpdate {
/// user_avatar: None,
/// user_discriminator: 6082,
/// user_id: UserId::new(379740138303127564),
/// user_is_bot: true,
/// user_name: "TofuBot".to_owned(),
/// };
///
/// // Update the cache with the message.
/// cache.update(&mut update_message).await;
/// # }
/// ```