Macro static_key 

static_key!() { /* proc-macro */ }

Compute a StorageKey at compile time: sha256 of the string literal’s value (quotes stripped, escapes resolved).

static_key!("counter") hashes the 7 bytes counter, so the key can be reproduced off-chain (indexer, client, another language) as sha256("counter"). Source spelling does not matter: static_key!(r"a") == static_key!("a").