Use stETH directly

src/wsteth-exchange-rate-adapter/WstEthEthExchangeRateChainlinkAdapter.sol

@@ -1,7 +1,7 @@
 // SPDX-License-Identifier: GPL-2.0-or-later
 pragma solidity 0.8.21;
 
-import {IWstEth} from "./interfaces/IWstEth.sol";
+import {IStEth} from "./interfaces/IStEth.sol";
 import {MinimalAggregatorV3Interface} from "./interfaces/MinimalAggregatorV3Interface.sol";
 
 import {ErrorsLib} from "./libraries/ErrorsLib.sol";
@@ -12,20 +12,21 @@ import {ErrorsLib} from "./libraries/ErrorsLib.sol";
 /// @notice wstETH/ETH exchange rate price feed.
 /// @dev This contract should only be used as price feed for `ChainlinkOracle`.
 contract WstEthEthExchangeRateChainlinkAdapter is MinimalAggregatorV3Interface {
+    // @dev The calculated price has 18 decimals precision, whatever the value of `decimals`.
     uint8 public constant decimals = 18;
     string public constant description = "wstETH/ETH exchange rate";
 
-    IWstEth public immutable WST_ETH;
+    IStEth public immutable ST_ETH;
 
-    constructor(address wstEth) {
-        require(wstEth != address(0), ErrorsLib.ZERO_ADDRESS);
+    constructor(address stEth) {
+        require(stEth != address(0), ErrorsLib.ZERO_ADDRESS);
 
-        WST_ETH = IWstEth(wstEth);
+        ST_ETH = IStEth(stEth);
     }
 
-    /// @dev Silently overflows if `stEthPerToken` is greater than `type(int256).max`.
+    /// @dev Silently overflows if `getPooledEthByShares`'s return value is greater than `type(int256).max`.
     function latestRoundData() external view returns (uint80, int256, uint256, uint256, uint80) {
-        // It is assumed that `stEthPerToken` returns a price with 18 decimals precision.
-        return (0, int256(WST_ETH.stEthPerToken()), 0, 0, 0);
+        // It is assumed that `getPooledEthByShares` returns a price with 18 decimals precision.
+        return (0, int256(ST_ETH.getPooledEthByShares(1 ether)), 0, 0, 0);
     }
 }

src/wsteth-exchange-rate-adapter/interfaces/IStEth.sol

@@ -0,0 +1,6 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+pragma solidity >=0.5.0;
+
+interface IStEth {
+    function getPooledEthByShares(uint256) external view returns (uint256);
+}

test/WstEthEthExchangeRateChainlinkAdapterTest.sol

@@ -7,14 +7,14 @@ import {ChainlinkOracle} from "../src/morpho-chainlink-v1/ChainlinkOracle.sol";
 import "../src/wsteth-exchange-rate-adapter/WstEthEthExchangeRateChainlinkAdapter.sol";
 
 contract WstEthEthExchangeRateChainlinkAdapterTest is Test {
-    IWstEth internal constant WST_ETH = IWstEth(0x7f39C581F595B53c5cb19bD0b3f8dA6c935E2Ca0);
+    IStEth internal constant ST_ETH = IStEth(0xae7ab96520DE3A18E5e111B5EaAb095312D7fE84);
 
     WstEthEthExchangeRateChainlinkAdapter internal oracle;
     ChainlinkOracle internal chainlinkOracle;
 
     function setUp() public {
         vm.createSelectFork(vm.envString("ETH_RPC_URL"));
-        oracle = new WstEthEthExchangeRateChainlinkAdapter(address(WST_ETH));
+        oracle = new WstEthEthExchangeRateChainlinkAdapter(address(ST_ETH));
         chainlinkOracle = new ChainlinkOracle(
             vaultZero, AggregatorV3Interface(address(oracle)), feedZero, feedZero, feedZero, 1, 18, 18
         );
@@ -37,7 +37,7 @@ contract WstEthEthExchangeRateChainlinkAdapterTest is Test {
         (uint80 roundId, int256 answer, uint256 startedAt, uint256 updatedAt, uint80 answeredInRound) =
             oracle.latestRoundData();
         assertEq(roundId, 0);
-        assertEq(uint256(answer), WST_ETH.stEthPerToken());
+        assertEq(uint256(answer), ST_ETH.getPooledEthByShares(1 ether));
         assertEq(startedAt, 0);
         assertEq(updatedAt, 0);
         assertEq(answeredInRound, 0);