feat(irm): accrue interest before changing borrow rate bounds

script/deploy_fixedRateIrm_impl.sol

@@ -9,14 +9,16 @@ import { FixedRateIrm } from "interest-rate-model/FixedRateIrm.sol";
 import { Moolah } from "moolah/Moolah.sol";
 
 contract FixedRateIrmDeploy is DeployBase {
+  address moolah = 0x8F73b65B4caAf64FBA2aF91cC5D4a2A1318E5D8C;
+
   function run() public {
     uint256 deployerPrivateKey = _deployerKey();
     address deployer = vm.addr(deployerPrivateKey);
     console.log("Deployer: ", deployer);
     vm.startBroadcast(deployerPrivateKey);
 
     // Deploy InterestRateModel implementation
-    FixedRateIrm impl = new FixedRateIrm();
+    FixedRateIrm impl = new FixedRateIrm(moolah);
     console.log("FixedRateIrm implementation: ", address(impl));
 
     vm.stopBroadcast();

script/eth/deploy_fixedRateIrm.sol

@@ -9,14 +9,16 @@ import { FixedRateIrm } from "interest-rate-model/FixedRateIrm.sol";
 import { Moolah } from "moolah/Moolah.sol";
 
 contract FixedRateIrmDeploy is DeployBase {
+  address moolah = 0xf820fB4680712CD7263a0D3D024D5b5aEA82Fd70;
+
   function run() public {
     uint256 deployerPrivateKey = _deployerKey();
     address deployer = vm.addr(deployerPrivateKey);
     console.log("Deployer: ", deployer);
     vm.startBroadcast(deployerPrivateKey);
 
     // Deploy InterestRateModel implementation
-    FixedRateIrm impl = new FixedRateIrm();
+    FixedRateIrm impl = new FixedRateIrm(moolah);
     console.log("FixedRateIrm implementation: ", address(impl));
     // Deploy InterestRateModel proxy
     ERC1967Proxy proxy = new ERC1967Proxy(

src/interest-rate-model/FixedRateIrm.sol

@@ -8,7 +8,7 @@ import { IIrm } from "moolah/interfaces/IIrm.sol";
 import { IFixedRateIrm } from "./interfaces/IFixedRateIrm.sol";
 
 import { MarketParamsLib } from "../moolah/libraries/MarketParamsLib.sol";
-import { Id, MarketParams, Market } from "moolah/interfaces/IMoolah.sol";
+import { IMoolah, Id, MarketParams, Market } from "moolah/interfaces/IMoolah.sol";
 import { ErrorsLib } from "./libraries/ErrorsLib.sol";
 
 /* ERRORS */
@@ -44,6 +44,11 @@ contract FixedRateIrm is UUPSUpgradeable, AccessControlEnumerableUpgradeable, IF
   /// @inheritdoc IFixedRateIrm
   int256 public constant MAX_BORROW_RATE = 8.0 ether / int256(365 days);
 
+  /* IMMUTABLES */
+
+  /// @inheritdoc IFixedRateIrm
+  address public immutable MOOLAH;
+
   /* STORAGE */
 
   /// @inheritdoc IFixedRateIrm
@@ -64,8 +69,11 @@ contract FixedRateIrm is UUPSUpgradeable, AccessControlEnumerableUpgradeable, IF
   /* CONSTRUCTOR */
 
   /// @custom:oz-upgrades-unsafe-allow constructor
-  constructor() {
+  /// @param moolah The address of the Moolah contract.
+  constructor(address moolah) {
+    require(moolah != address(0), ErrorsLib.ZERO_ADDRESS);
     _disableInitializers();
+    MOOLAH = moolah;
   }
 
   /// @notice Constructor.
@@ -96,6 +104,9 @@ contract FixedRateIrm is UUPSUpgradeable, AccessControlEnumerableUpgradeable, IF
       require(newBorrowRate >= int256(rateFloor[id]), "rate below floor");
     }
 
+    // settle interest accrued at the old rate before it changes
+    _accrueInterest(id);
+
     borrowRateStored[id] = newBorrowRate;
 
     emit SetBorrowRate(id, newBorrowRate);
@@ -107,6 +118,9 @@ contract FixedRateIrm is UUPSUpgradeable, AccessControlEnumerableUpgradeable, IF
     require(newRateCap >= minCap && newRateCap != oldCap && newRateCap <= uint256(MAX_BORROW_RATE), "invalid rate cap");
     require(rateFloor[id] <= newRateCap, "invalid new cap vs floor");
 
+    // settle interest accrued under the old cap before it changes
+    _accrueInterest(id);
+
     rateCap[id] = newRateCap;
 
     emit BorrowRateCapUpdate(id, oldCap, newRateCap);
@@ -122,11 +136,22 @@ contract FixedRateIrm is UUPSUpgradeable, AccessControlEnumerableUpgradeable, IF
     if (_cap < minCap) _cap = minCap;
     require(newRateFloor <= _cap, "invalid rate floor vs cap");
 
+    // settle interest accrued under the old floor before it changes
+    _accrueInterest(id);
+
     rateFloor[id] = newRateFloor;
 
     emit BorrowRateFloorUpdate(id, oldFloor, newRateFloor);
   }
 
+  /// @dev Accrues interest on Moolah for `id` so pending interest is settled at the current rate bounds.
+  /// @dev No-op when the market does not exist yet, so bounds can still be pre-set before market creation.
+  function _accrueInterest(Id id) private {
+    if (IMoolah(MOOLAH).market(id).lastUpdate == 0) return;
+
+    IMoolah(MOOLAH).accrueInterest(IMoolah(MOOLAH).idToMarketParams(id));
+  }
+
   /// @dev Updates the minimum borrow rate cap for all markets.
   function updateMinCap(uint256 newMinCap) external onlyRole(MANAGER) {
     uint256 oldMinCap = minCap;

src/interest-rate-model/InterestRateModel.sol

@@ -13,7 +13,7 @@ import { ExpLib } from "./libraries/ExpLib.sol";
 import { MathLib, WAD_INT as WAD } from "./libraries/MathLib.sol";
 import { ConstantsLib } from "./libraries/ConstantsLib.sol";
 import { MarketParamsLib } from "../moolah/libraries/MarketParamsLib.sol";
-import { Id, MarketParams, Market } from "moolah/interfaces/IMoolah.sol";
+import { IMoolah, Id, MarketParams, Market } from "moolah/interfaces/IMoolah.sol";
 import { MathLib as MoolahMathLib } from "moolah/libraries/MathLib.sol";
 
 /// @title InterestRateModel
@@ -204,6 +204,9 @@ contract InterestRateModel is UUPSUpgradeable, AccessControlEnumerableUpgradeabl
     require(newRateCap >= minCap && newRateCap != oldCap, "invalid rate cap");
     require(rateFloor[id] <= newRateCap, "invalid new cap vs floor");
 
+    // settle interest accrued under the old cap before it changes
+    _accrueInterest(id);
+
     rateCap[id] = newRateCap;
 
     emit BorrowRateCapUpdate(id, oldCap, newRateCap);
@@ -219,11 +222,22 @@ contract InterestRateModel is UUPSUpgradeable, AccessControlEnumerableUpgradeabl
     if (_cap < minCap) _cap = minCap;
     require(newRateFloor <= _cap, "invalid rate floor vs cap");
 
+    // settle interest accrued under the old floor before it changes
+    _accrueInterest(id);
+
     rateFloor[id] = newRateFloor;
 
     emit BorrowRateFloorUpdate(id, oldFloor, newRateFloor);
   }
 
+  /// @dev Accrues interest on Moolah for `id` so pending interest is settled at the current rate bounds.
+  /// @dev No-op when the market does not exist yet, so bounds can still be pre-set before market creation.
+  function _accrueInterest(Id id) private {
+    if (IMoolah(MOOLAH).market(id).lastUpdate == 0) return;
+
+    IMoolah(MOOLAH).accrueInterest(IMoolah(MOOLAH).idToMarketParams(id));
+  }
+
   /// @dev Updates the minimum borrow rate cap for all markets.
   function updateMinCap(uint256 newMinCap) external onlyRole(MANAGER) {
     uint256 oldMinCap = minCap;

src/interest-rate-model/interfaces/IFixedRateIrm.sol

@@ -13,6 +13,9 @@ interface IFixedRateIrm is IIrm {
 
   /* EXTERNAL */
 
+  /// @notice The address of the Moolah contract.
+  function MOOLAH() external view returns (address);
+
   /// @notice Max settable borrow rate (800%).
   function MAX_BORROW_RATE() external returns (int256);
 

test/interest-rate-model/AlphaIrmTest.sol

@@ -13,6 +13,7 @@ contract AlpahIrmMainnetTest is Test {
   address admin = 0x07D274a68393E8b8a2CCf19A2ce4Ba3518735253; // timelock
   address manager = 0x8d388136d578dCD791D081c6042284CED6d9B0c6;
   address alphaIrm = 0x5F9f9173B405C6CEAfa7f98d09e4B8447e9797E6;
+  address moolah = 0x8F73b65B4caAf64FBA2aF91cC5D4a2A1318E5D8C;
 
   FixedRateIrm fixedRateIrm = FixedRateIrm(alphaIrm);
 
@@ -21,7 +22,7 @@ contract AlpahIrmMainnetTest is Test {
   function setUp() external {
     vm.createSelectFork("https://bsc-dataseed.bnbchain.org");
 
-    address impl = address(new FixedRateIrm());
+    address impl = address(new FixedRateIrm(moolah));
 
     vm.startPrank(admin);
     UUPSUpgradeable proxy = UUPSUpgradeable(alphaIrm);

test/interest-rate-model/FixedRateIrmTest.sol

@@ -18,7 +18,8 @@ contract FixedRateIrmTest is Test {
   address bot = makeAddr("bot");
 
   function setUp() external {
-    FixedRateIrm impl = new FixedRateIrm();
+    // this contract stands in as MOOLAH; see the `market` stub below
+    FixedRateIrm impl = new FixedRateIrm(address(this));
     ERC1967Proxy proxy = new ERC1967Proxy(
       address(impl),
       abi.encodeWithSelector(impl.initialize.selector, admin, manager)
@@ -144,4 +145,10 @@ contract FixedRateIrmTest is Test {
 
     assertEq(fixedRateIrm.borrowRateView(marketPrams, market), uint256(newBorrowRate));
   }
+
+  /* MOOLAH STUB */
+
+  /// @dev This contract is deployed as the IRM's MOOLAH. `updateRateCap`/`updateRateFloor` probe the market
+  /// before accruing interest; returning an empty market (lastUpdate == 0) makes the accrual a no-op.
+  function market(Id) external pure returns (Market memory m) {}
 }

test/interest-rate-model/InterestRateModelTest.sol

@@ -380,6 +380,12 @@ contract InterestRateModelTest is Test {
     assertEq(irm.rateFloor(id), newFloor);
   }
 
+  /* MOOLAH STUB */
+
+  /// @dev This contract is deployed as the IRM's MOOLAH. `updateRateCap`/`updateRateFloor` probe the market
+  /// before accruing interest; returning an empty market (lastUpdate == 0) makes the accrual a no-op.
+  function market(Id) external pure returns (Market memory m) {}
+
   /* HANDLERS */
 
   function handleBorrowRate(uint256 totalSupplyAssets, uint256 totalBorrowAssets, uint256 elapsed) external {