Rust之type后面的冒号:是什么意思

	#[pallet::config]
	pub trait Config<I: 'static = ()>: frame_system::Config {
		/// The balance of an account.
		type Balance: Parameter + Member + AtLeast32BitUnsigned + Codec + Default + Copy +
			MaybeSerializeDeserialize + Debug;

		/// Handler for the unbalanced reduction when removing a dust account.
		type DustRemoval: OnUnbalanced<NegativeImbalance<Self, I>>;

		/// The overarching event type.
		type Event: From<Event<Self, I>> + IsType<<Self as frame_system::Config>::Event>;

		/// The minimum amount required to keep an account open.
		#[pallet::constant]
		type ExistentialDeposit: Get<Self::Balance>;

		/// The means of storing the balances of an account.
		type AccountStore: StoredMap<Self::AccountId, AccountData<Self::Balance>>;

		/// Weight information for extrinsics in this pallet.
		type WeightInfo: WeightInfo;

		/// The maximum number of locks that should exist on an account.
		/// Not strictly enforced, but used for weight estimation.
		type MaxLocks: Get<u32>;
	}