pub fn keccak256_concat(hash1: H256, hash2: H256) -> H256 {
    let mut bytes = [0_u8; 64];
    bytes[..32].copy_from_slice(hash1.as_bytes());
    bytes[32..].copy_from_slice(hash2.as_bytes());
    H256(keccak256(&bytes))
}
Hashes concatenation of the two provided hashes using `keccak256`.
core/lib/basic_types/src/web3/mod.rs