Related to #314

We’ve encountered a persistent issue with the zinkc dispatcher when using the #[derive(Storage)] macro (introduced in PR #314, resolving #288) for unified structs with multiple fields. While single-field structs (e.g., getter.rs) work correctly, multi-field structs (e.g., contract.rs) exhibit runtime dispatch failures, preventing external method calls from functioning as expected. This blocks core functionality like transfer() and approve() in ERC20 contracts, critical for practical use.

For example:
getter.rs (Single-field):
Struct: Getter { value: Bytes32 }
Method: #[zink::external] get_value(&self) -> Bytes32
Result: Test passes—get_value() returns the stored value.

let value_info = evm.calldata(&contract.encode(&[b"get_value()".to_vec()])?).call(address)?;
assert_eq!(value_info.ret, value_array.to_vec(), "Getter failed");

contract.rs (Multi-field, ERC20):
Struct:

ERC20 {
 name: Bytes32,
 symbol: Bytes32,
 total_supply: U256,
 balances: Mapping<Address, U256>,
 allowances: DoubleKeyMapping<Address, Address, U256>
}

Methods: Multiple #[zink::external] (e.g., init, name, transfer) and internal #[no_mangle] (e.g., _transfer)
Result: Storage tests pass, runtime calls fail:
name() reverts with "Empty from address" from _transfer.

// Failed runtime call
let name_ret = evm.calldata(&contract.encode(&[b"name()".to_vec()])?).call(address)?;
assert_eq!(name_ret.ret, name_array.to_vec(), "Name mismatch: {name_ret:?}");
// Output: revert: Some("Empty from address")

Cause: Dispatcher misroutes name() (0x06fdde03) to _transfer (0xa9059cbb).

Tracking List for Resolution

• Reliable Runtime Calls: return correct storage values in multi-field structs.
• ERC20 Functionality: Allow transfer(), approve(), and transfer_from() in contract.rs and erc20.rs to execute without reverts.