I was trying to calculate multiplying a balance value by a ratio via Perbill, like Perbill::from_percent(90) * my_balance. But the mul operation of Perbill won't work for large values like when balance is u128 and 1 coin is 10^18.

As in the Mul impl of Perbill, rhs is converted to u64, and the output would be saturated to u64::MAX/1_000_000_000, which around 1.84 * 10^10. This make it impossible to multiply by a typical balance value.

Due to Currency::Balance :As<u64>, this implicit saturating is error prune for balances calculation, such as offline slash calculation in staking module if I don't misunderstand:

Expected behavior: Perbill should be capable of u64 and u128 multiplying, and saturate the output to be Output::max_value().