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

Changing a market's rate cap, rate floor, or fixed borrow rate used to
take effect immediately, retroactively repricing all interest that had
accrued since the market's last update at the new value. Both IRMs now
call Moolah.accrueInterest for the market first, so elapsed interest is
settled at the bounds that were actually in effect and only future
interest uses the new ones.

Affected entry points:
- InterestRateModel: updateRateCap, updateRateFloor
- FixedRateIrm: updateRateCap, updateRateFloor, setBorrowRate

The accrual runs after the validation requires and before the storage
write. Ordering matters: Moolah calls back into the IRM's borrowRate
during accrual, so the callback must still observe the old value.

Accrual is a no-op when the market does not exist yet (lastUpdate == 0),
which keeps bounds settable ahead of market creation. FixedRateIrm needs
this in particular, since setBorrowRate is documented as running before
the market is created.

FixedRateIrm held no reference to Moolah, so it gains a MOOLAH immutable
and a constructor argument. This is a breaking constructor change: the
new implementation must be deployed via the updated scripts rather than
upgraded in place, and the address is baked into bytecode.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: razww

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 {

test/utils/PositionManager.t.sol

@@ -192,7 +192,7 @@ contract PositionManagerTest is Test {
     );
     irm = InterestRateModel(address(irmProxy));
 
-    FixedRateIrm fixedIrmImpl = new FixedRateIrm();
+    FixedRateIrm fixedIrmImpl = new FixedRateIrm(address(moolah));
     ERC1967Proxy fixedIrmProxy = new ERC1967Proxy(
       address(fixedIrmImpl),
       abi.encodeWithSelector(FixedRateIrm.initialize.selector, admin, manager)