ExEx calling contracts to track state

[0xalex88]

Hi,

would be nice to have an example on how to call smart contracts from within the EVM using an equivalent eth_call function.

I can add an example that tracks ERC20 balances by looking at addresses in the ERC20 transfer events, but I would need to know if this is the best way to call contracts:

let block_number = 123;
let db = StateBuilder::new()
    .with_database(StateProviderDatabase::new(
       self.provider
            .state_by_block_number_or_tag(block_number.into())?,
    ))
    .with_bundle_update()
    .build();
let evm_config = EthEvmConfig::new(self.provider.chain_spec());
let mut evm = evm_config.evm(db);

let mut balances = vec![];
for address in user_addresses {
let tx = Transaction::Legacy(TxLegacy {
    gas_limit: 50_000_000,
    to: TxKind::Call(self.farm.address),
    input: erc20::ERC20::ERC20Calls::balanceOf(erc20::ERC20::balanceOfCall {
        owner: address.clone(),
    })
    .abi_encode()
    .into(),
    ..Default::default()
});

let tx_env = evm.tx_mut();
tx_env.gas_limit = tx.gas_limit();
tx_env.nonce = None;
tx_env.caller = Address::ZERO;
tx_env.transact_to = TxKind::Call(tx.to().unwrap_or_default());
tx_env.value = tx.value();
tx_env.data = tx.input().clone();
tx_env.chain_id = tx.chain_id();

// evm_config.fill_tx_env(evm.tx_mut(), tx, Address::ZERO);
let result = evm.transact()?.result;
let output = result.output().expect("unable to execute balance check tx");

let balance = erc20::ERC20::balanceOfCall::abi_decode_returns(output, true)?;
balances.push(balance.value.into());
}

I came up with this looking at old reth issues/PRs and had to end up changing the tx_env manually because fill_tx_env doesn't support plain (unsigned) transactions anymore.
Also I'm not sure if there's an easier way to have an EVM running at a specific block by just using the provider instead of using StateBuilder, creating an EVM config etc.