Merge pull request #3 from balancer-labs/optimised-atoken-provider

feat: update StaticATokenRateProvider to bypass waToken for gas savings

contracts/StaticATokenRateProvider.sol

@@ -22,18 +22,26 @@ import "./interfaces/IStaticAToken.sol";
  * @notice Returns the value of a wrapped (static) aToken in terms of the underlying (dynamic) aToken
  */
 contract StaticATokenRateProvider is IRateProvider {
-    IStaticAToken public immutable waToken;
+    ILendingPool public immutable lendingPool;
+    address public immutable asset;
 
     constructor(IStaticAToken _waToken) {
-        waToken = _waToken;
+        lendingPool = _waToken.LENDING_POOL();
+        asset = _waToken.ASSET();
     }
 
     /**
      * @return The value of the wrapped aToken in terms of the underlying aToken
      */
     function getRate() external view override returns (uint256) {
+        // This pulls the implementation of used in the StaticAToken contract
+        // except avoiding storing relevant variables in storage for gas reasons.
+        // solhint-disable-next-line max-line-length
+        // see: https://github.com/aave/protocol-v2/blob/ac58fea62bb8afee23f66197e8bce6d79ecda292/contracts/protocol/tokenization/StaticATokenLM.sol#L255-L257
+        uint256 rate = lendingPool.getReserveNormalizedIncome(asset);
+
         // getRate returns a 18 decimal fixed point number, but `rate` has 27 decimals (i.e. a 'ray' value)
         // so we need to convert it.
-        return waToken.rate() / 10**9;
+        return rate / 10**9;
     }
 }

contracts/interfaces/ILendingPool.sol

@@ -0,0 +1,22 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+pragma solidity ^0.8.0;
+
+interface ILendingPool {
+    /**
+     * @dev returns a 27 decimal fixed point 'ray' value so a rate of 1 is represented as 1e27
+     */
+    function getReserveNormalizedIncome(address asset) external view returns (uint256);
+}

contracts/interfaces/IStaticAToken.sol

@@ -14,7 +14,19 @@
 
 pragma solidity ^0.8.0;
 
+import "./ILendingPool.sol";
+
 interface IStaticAToken {
+    /**
+     * @dev returns the address of the staticAToken's underlying asset
+     */
+    function ASSET() external view returns (address);
+
+    /**
+     * @dev returns the address of the staticAToken's lending pool
+     */
+    function LENDING_POOL() external view returns (ILendingPool);
+
     /**
      * @dev returns a 27 decimal fixed point 'ray' value so a rate of 1 is represented as 1e27
      */