Function deserialize_and_check 

fn deserialize_and_check<R: Read>(
    reader: R,
    branch_id: BranchId,
) -> Result<Transaction, SerializationError>

Parses a transaction and runs the parse-time consensus checks that zcash_primitives does not enforce. Both deserialization impls go through this function, so a transaction cannot reach a Transaction value without passing the checks.