feat: Add zero IR strategy (#818)

* feat: Add zero IR strategy

* test: Add test cases for updating reserve interest rate strategy

* fix: Format zeroed IR

* fix: Use constant instead of immutable for zero values

* test: Add rebalance call in tests

* fix: Add additional rebalance in test with zero ir

contracts/misc/ZeroReserveInterestRateStrategy.sol

@@ -0,0 +1,105 @@
+// SPDX-License-Identifier: BUSL-1.1
+pragma solidity ^0.8.0;
+
+import {DataTypes} from '../protocol/libraries/types/DataTypes.sol';
+import {IDefaultInterestRateStrategy} from '../interfaces/IDefaultInterestRateStrategy.sol';
+import {IReserveInterestRateStrategy} from '../interfaces/IReserveInterestRateStrategy.sol';
+import {IPoolAddressesProvider} from '../interfaces/IPoolAddressesProvider.sol';
+
+/**
+ * @title ZeroReserveInterestRateStrategy contract
+ * @author Aave
+ * @notice Interest Rate Strategy contract, with all parameters zeroed.
+ * @dev It returns zero liquidity and borrow rate.
+ */
+contract ZeroReserveInterestRateStrategy is IDefaultInterestRateStrategy {
+  /// @inheritdoc IDefaultInterestRateStrategy
+  uint256 public constant OPTIMAL_USAGE_RATIO = 0;
+
+  /// @inheritdoc IDefaultInterestRateStrategy
+  uint256 public constant OPTIMAL_STABLE_TO_TOTAL_DEBT_RATIO = 0;
+
+  /// @inheritdoc IDefaultInterestRateStrategy
+  uint256 public constant MAX_EXCESS_USAGE_RATIO = 0;
+
+  /// @inheritdoc IDefaultInterestRateStrategy
+  uint256 public constant MAX_EXCESS_STABLE_TO_TOTAL_DEBT_RATIO = 0;
+
+  IPoolAddressesProvider public immutable ADDRESSES_PROVIDER;
+
+  // Base variable borrow rate when usage rate = 0. Expressed in ray
+  uint256 internal constant _baseVariableBorrowRate = 0;
+
+  // Slope of the variable interest curve when usage ratio > 0 and <= OPTIMAL_USAGE_RATIO. Expressed in ray
+  uint256 internal constant _variableRateSlope1 = 0;
+
+  // Slope of the variable interest curve when usage ratio > OPTIMAL_USAGE_RATIO. Expressed in ray
+  uint256 internal constant _variableRateSlope2 = 0;
+
+  // Slope of the stable interest curve when usage ratio > 0 and <= OPTIMAL_USAGE_RATIO. Expressed in ray
+  uint256 internal constant _stableRateSlope1 = 0;
+
+  // Slope of the stable interest curve when usage ratio > OPTIMAL_USAGE_RATIO. Expressed in ray
+  uint256 internal constant _stableRateSlope2 = 0;
+
+  // Premium on top of `_variableRateSlope1` for base stable borrowing rate
+  uint256 internal constant _baseStableRateOffset = 0;
+
+  // Additional premium applied to stable rate when stable debt surpass `OPTIMAL_STABLE_TO_TOTAL_DEBT_RATIO`
+  uint256 internal constant _stableRateExcessOffset = 0;
+
+  /**
+   * @dev Constructor.
+   * @param provider The address of the PoolAddressesProvider contract
+   */
+  constructor(IPoolAddressesProvider provider) {
+    ADDRESSES_PROVIDER = provider;
+  }
+
+  /// @inheritdoc IDefaultInterestRateStrategy
+  function getVariableRateSlope1() external pure returns (uint256) {
+    return _variableRateSlope1;
+  }
+
+  /// @inheritdoc IDefaultInterestRateStrategy
+  function getVariableRateSlope2() external pure returns (uint256) {
+    return _variableRateSlope2;
+  }
+
+  /// @inheritdoc IDefaultInterestRateStrategy
+  function getStableRateSlope1() external pure returns (uint256) {
+    return _stableRateSlope1;
+  }
+
+  /// @inheritdoc IDefaultInterestRateStrategy
+  function getStableRateSlope2() external pure returns (uint256) {
+    return _stableRateSlope2;
+  }
+
+  /// @inheritdoc IDefaultInterestRateStrategy
+  function getStableRateExcessOffset() external pure returns (uint256) {
+    return _stableRateExcessOffset;
+  }
+
+  /// @inheritdoc IDefaultInterestRateStrategy
+  function getBaseStableBorrowRate() public pure returns (uint256) {
+    return _variableRateSlope1 + _baseStableRateOffset;
+  }
+
+  /// @inheritdoc IDefaultInterestRateStrategy
+  function getBaseVariableBorrowRate() external pure override returns (uint256) {
+    return _baseVariableBorrowRate;
+  }
+
+  /// @inheritdoc IDefaultInterestRateStrategy
+  function getMaxVariableBorrowRate() external pure override returns (uint256) {
+    return _baseVariableBorrowRate + _variableRateSlope1 + _variableRateSlope2;
+  }
+
+  /// @inheritdoc IReserveInterestRateStrategy
+  function calculateInterestRates(
+    DataTypes.CalculateInterestRatesParams memory
+  ) public pure override returns (uint256, uint256, uint256) {
+    return (0, 0, 0);
+  }
+}