invariant tests

src/test/invariant/xERC4626.invariant.t.sol

@@ -0,0 +1,37 @@
+// SPDX-License-Identifier: AGPL-3.0-only
+pragma solidity 0.8.10;
+
+import {DSTestPlus} from "solmate/test/utils/DSTestPlus.sol";
+import {MockERC20} from "solmate/test/utils/mocks/MockERC20.sol";
+import {MockxERC4626} from "../mocks/MockxERC4626.sol";
+
+contract xERC4626Test is DSTestPlus {
+    MockERC20 token;
+    MockxERC4626 xToken;
+    function setUp() public {
+        token = new MockERC20("token", "TKN", 18);
+        xToken = new MockxERC4626(token, "xToken", "xTKN", 1000);
+    }
+
+    function invariant_totalAssets() public {
+        require(xToken.totalAssets() <= token.balanceOf(address(xToken)));
+    }
+
+    function invariant_convert() public {
+        require(xToken.convertToShares(xToken.totalAssets()) == xToken.totalSupply());
+        require(xToken.convertToAssets(xToken.totalSupply()) == xToken.totalAssets());
+    }
+
+    function invariant_increase() public {
+        uint256 previousTotal = xToken.totalAssets();
+        hevm.warp(block.timestamp + 100);
+        require(xToken.totalAssets() >= previousTotal);
+    }
+
+    function invariant_cycleEnd() public {
+        hevm.warp(xToken.rewardsCycleEnd());
+        xToken.syncRewards();
+        hevm.warp(xToken.rewardsCycleEnd());
+        require(xToken.totalAssets() == token.balanceOf(address(xToken)));
+    }
+}

src/test/mocks/MockxERC4626.sol

@@ -3,11 +3,27 @@ pragma solidity >=0.8.0;
 
 import {ERC20, xERC4626, ERC4626} from "../../xERC4626.sol";
 
+import {Hevm} from "solmate/test/utils/Hevm.sol";
+
+interface Mintable {
+    function mint(address,uint) external;
+}
+
 contract MockxERC4626 is xERC4626 {
+    Hevm internal constant hevm = Hevm(0x7109709ECfa91a80626fF3989D68f67F5b1DD12D);
+
     constructor(
         ERC20 _underlying,
         string memory _name,
         string memory _symbol,
         uint32 _rewardsCycleLength
     ) ERC4626(_underlying, _name, _symbol) xERC4626(_rewardsCycleLength) {}
+
+    function warpInCycle(uint256 warp) public {
+        hevm.warp(block.timestamp + (warp % rewardsCycleLength));
+    }
+
+    function reward(uint256 amount) public {
+        Mintable(address(asset)).mint(address(this), amount);
+    }
 }