Writing to any storage slot

On Starknet, a contract's storage is a map with $( 2^{251} )$ slots, where each slot is a felt252 which is initialized to 0. The address of storage variables is computed at compile time using the formula:

storage\_variable\_address~\text{:=}~\text{pedersen}(\text{keccak}(variable\_name),~keys)

Interactions with storage variables are commonly performed using the self.var.read() and self.var.write() functions.

Nevertheless, we can use the storage_write_syscall and storage_read_syscall syscalls, to write to and read from any storage slot. This is useful when writing to storage variables that are not known at compile time, or to ensure that even if the contract is upgraded and the computation method of storage variable addresses changes, they remain accessible.

In the following example, we use the Poseidon hash function to compute the address of a storage variable. Poseidon is a ZK-friendly hash function that is cheaper and faster than Pedersen, making it an excellent choice for onchain computations. Once the address is computed, we use the storage syscalls to interact with it.

#[starknet::interface]
pub trait IWriteToAnySlots<TContractState> {
    fn write_slot(ref self: TContractState, value: u32);
    fn read_slot(self: @TContractState) -> u32;
}
 
#[starknet::contract]
pub mod WriteToAnySlot {
    use starknet::syscalls::{storage_read_syscall, storage_write_syscall};
    use core::poseidon::poseidon_hash_span;
    use starknet::StorageAddress;
 
    #[storage]
    struct Storage {}
 
    const SLOT_NAME: felt252 = 'test_slot';
 
    #[abi(embed_v0)]
    impl WriteToAnySlot of super::IWriteToAnySlots<ContractState> {
        fn write_slot(ref self: ContractState, value: u32) {
            storage_write_syscall(0, get_address_from_name(SLOT_NAME), value.into()).unwrap();
        }
 
        fn read_slot(self: @ContractState) -> u32 {
            storage_read_syscall(0, get_address_from_name(SLOT_NAME)).unwrap().try_into().unwrap()
        }
    }
    pub fn get_address_from_name(variable_name: felt252) -> StorageAddress {
        let mut data: Array<felt252> = array![];
        data.append(variable_name);
        let hashed_name: felt252 = poseidon_hash_span(data.span());
        let MASK_250: u256 = 0x03ffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff;
        // By taking the 250 least significant bits of the hash output, we get a valid 250bits
        // storage address.
        let result: felt252 = (hashed_name.into() & MASK_250).try_into().unwrap();
        let result: StorageAddress = result.try_into().unwrap();
        result
    }
}