Function validate_fee_payer 

pub fn validate_fee_payer(
    payer_address: &Pubkey,
    payer_account: &mut AccountSharedData,
    payer_index: IndexOfAccount,
    error_metrics: &mut TransactionErrorMetrics,
    rent: &Rent,
    fee: u64,
) -> TransactionResult<()>

👎Deprecated since 3.1.0: This crate has been marked for formal inclusion in the Agave Unstable API. From v4.0.0 onward, the agave-unstable-api crate feature must be specified to acknowledge use of an interface that may break without warning.

Check whether the payer_account is capable of paying the fee. The side effect is to subtract the fee amount from the payer_account balance of lamports. If the payer_account is not able to pay the fee, the error_metrics is incremented, and a specific error is returned.