Merge pull request #1 from balancer-labs/waToken

feat: add rate provider for Aave's StaticAToken contracts

contracts/StaticATokenRateProvider.sol

@@ -0,0 +1,39 @@
+// 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;
+
+import "./interfaces/IRateProvider.sol";
+import "./interfaces/IStaticAToken.sol";
+
+/**
+ * @title Wrapped aToken Rate Provider
+ * @notice Returns the value of a wrapped (static) aToken in terms of the underlying (dynamic) aToken
+ */
+contract StaticATokenRateProvider is IRateProvider {
+    IStaticAToken public immutable waToken;
+
+    constructor(IStaticAToken _waToken) {
+        waToken = _waToken;
+    }
+
+    /**
+     * @return The value of the wrapped aToken in terms of the underlying aToken
+     */
+    function getRate() external view override returns (uint256) {
+        // 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;
+    }
+}

contracts/interfaces/IStaticAToken.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 IStaticAToken {
+    /**
+     * @dev returns a 27 decimal fixed point 'ray' value so a rate of 1 is represented as 1e27
+     */
+    function rate() external view returns (uint256);
+}