Aave v2. Zero interest rate strategy

Implementation of an interest rate strategy with all parameters zeroed.

Can it cause any problem in the Aave v2 Ethereum pool?

No, it can't.

Variable borrow rate

reserve.currentVariableBorrowRate gets used mainly in 2 internal functions of the protocol:

1. _updateIndexes(). Once the reserve.currentVariableBorrowRate on storage gets updated to 0, this will cause a multiplication by 0, making cumulatedVariableBorrowInterest == 0, and the variable borrow index to not change. CORRECT.
2. getNormalizedDebt(). This function returns a new "dynamics" variable debt index, adding the accrued growth since last update, based on time and the variable rate. As rate will be now 0, the effect will be that index will remain the same as before. Used by getReserveNormalizedVariableDebt(), that in turn gets used by balanceOf() and totalSupply() of the variable debt token. The final effect is that balances and supply of variable debt will be multiplied by the last updated index, not causing any growth, as expected. CORRECT.
3. getMaxVariableBorrowRate() (now returning 0) gets used in the protocol on validateRebalanceStableBorrowRate(). In this particular case, there is no effect because stable rate is not enabled. However, even if stable rate would be active, there would be no practical consequence for the protocol, because this require will always return false (as currentLiquidityRate would be > 0), which would mean no rebalance can be done, but rebalancing is non-logical, as the users will always close their stable rate positions to swap to variable at 0 rate. CORRECT

Stable borrow rate

The effect is the same as with variable debt, no problem happens, apart from the high-level economical impact of allowing an user to borrow at stable rate locking a 0 rate, which should probably be avoided. Furthermore, the first use case (renFIL on Aave v2 Ethereum) doesn't have stable rate mode enabled, so no impact

Setup

cp .env.example .env
forge install

Test

make test