Applied on fields of ink! event types to indicate that they are topics.

Tells the ink! codegen to provide a topic hash for the given field. Every ink! event can only have a limited number of such topic field. The semantics are similar to indexed event arguments in Solidity.

Example

#[ink(event)]
pub struct Transferred {
    #[ink(topic)]
    from: Option<AccountId>,

    #[ink(topic)]
    to: Option<AccountId>,

    amount: Balance
}