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

[razww]

Problem

Changing a market's rate cap, rate floor, or fixed borrow rate took effect immediately, retroactively repricing all interest that had accrued since the market's last update at the new value. Lowering a cap re-priced past interest downward; raising a floor re-priced it upward.

Change

Both IRMs now call Moolah.accrueInterest for the market before writing the new value, so elapsed interest is settled at the bounds that were actually in effect and only future interest uses the new ones.

Contract	Entry points
InterestRateModel	updateRateCap, updateRateFloor
FixedRateIrm	updateRateCap, updateRateFloor, setBorrowRate

function _accrueInterest(Id id) private {
  if (IMoolah(MOOLAH).market(id).lastUpdate == 0) return;
  IMoolah(MOOLAH).accrueInterest(IMoolah(MOOLAH).idToMarketParams(id));
}

Two details worth reviewing closely:

• Placement is load-bearing. The accrual runs after the validation requires (so an invalid update doesn't pay for it) and before the storage write. Moolah calls back into the IRM's borrowRate during accrual, so the callback must still observe the old value — moving the call below the assignment would silently invert the fix.
• No-op when the market doesn't exist (lastUpdate == 0). Moolah.accrueInterest reverts with MARKET_NOT_CREATED otherwise, which would break pre-setting bounds ahead of market creation. FixedRateIrm.setBorrowRate needs this specifically — it is documented as running before the market is created.

updateMinCap is deliberately untouched: it is global across all markets, so there is no single market to accrue. Giving it the same guarantee needs a different design.

Breaking: FixedRateIrm constructor

FixedRateIrm held no reference to Moolah, so it gains a MOOLAH immutable and a constructor argument, matching InterestRateModel. All five construction sites were updated:

Site	Moolah
script/deploy_fixedRateIrm_impl.sol	BSC 0x8F73…5D8C
script/eth/deploy_fixedRateIrm.sol	ETH 0xf820…Fd70
test/interest-rate-model/AlphaIrmTest.sol	BSC mainnet (fork)
test/utils/PositionManager.t.sol	local moolah
test/interest-rate-model/FixedRateIrmTest.sol	address(this) + market(Id) stub

This ships as a fresh implementation deploy via the updated scripts, not an in-place proxy upgrade. The immutable lives in runtime bytecode, so it consumes no storage slot and does not disturb the existing layout — but a wrong address is only fixable by another upgrade. Please sanity-check the ETH address, taken from script/eth/deploy_irm_impl.sol.

Testing

• forge build — compiler run successful
• FixedRateIrmTest — 13/13 pass
• InterestRateModelTest — 23/23 pass

Not exercised: AlphaIrmTest and PositionManager.t.sol are fork tests — they compile, but running them needs an RPC.

🤖 Generated with Claude Code

[hashdit-bot]

Pull Request Review

This Solidity lending-protocol PR updates both interest-rate models to accrue pending market interest before changing rate caps, floors, or fixed borrow rates, preventing retroactive repricing. It also adds an immutable Moolah address to FixedRateIrm, updates deployment and test construction sites, and introduces Moolah stubs in unit tests.

Sensitive Content

No sensitive content detected.

Security Issues

No serious security issues detected.

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[qingyang-lista]

LGTM