Nialexsan/fusdev strategy

[nialexsan]

Closes: #???

Description

this PR contains bulk changes related only to FUSDEV strategy from #183
so it's easier to review just the strategy itself withoutt the second strategy

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[nialexsan]

@jribbink @jordanschalm
here's a claude generated explanation why pre-supplement and buffered swapper are required:

● Setup (UniV3 fee exaggerated to 1% for readability; FUSDEV redemption is free)

  ---
  Open position

  Borrow:                     100 MOET
  MOET→PYUSD0 (1% UniV3):     floor(100 × 0.99) = 99 PYUSD0
  Deposit into fresh FUSDEV:  receive 99 shares at 1:1

  Vault:  totalAssets = 99,  totalSupply = 99

  Yield accrues

  Vault:  totalAssets = 101,  totalSupply = 99  (2 PYUSD0 yield)

  ---
  Close — Step 1: check if yield covers debt

  quoteOut(99 shares):
    FUSDEV→PYUSD0 (free): floor(99 × 101 / 99) = 101 PYUSD0
    PYUSD0→MOET (1% fee): floor(101 × 0.99)    = 99 MOET

  99 < 100 debt  →  shortfall = 1 MOET

  The 2% round-trip fee exceeds the 2% yield by a rounding unit.

  ---
  Close — Step 2: pre-supplement

  buffered = 1 × 1.01 = 1.01 MOET
  Pull PYUSD0 collateral, swap → deposit 1 MOET to reduce debt

  Remaining debt:  99 MOET

  ---
  Close — Step 3a: repay with SwapSource — fails

  quoteIn(99 MOET):
    PYUSD0 needed: ceil(99 / 0.99)        = 100 PYUSD0
    shares needed: floor(100 × 99 / 101)  = floor(98.01) = 98 shares

  Redeem 98 shares (free ERC4626):
    floor(98 × 101 / 99) = floor(99.97) = 99 PYUSD0

  PYUSD0→MOET (1% fee):
    floor(99 × 0.99) = floor(98.01) = 98 MOET

  98 < 99 debt  ✗

  SwapSource computed 98 shares was enough to produce 100 PYUSD0 — it wasn't. ERC4626 floor
  division turned 100 into 99, and the UniV3 fee turned 99 into 98.

  ---
  Close — Step 3b: repay with BufferedSwapSource — succeeds

  Pull all 99 shares unconditionally:
    FUSDEV→PYUSD0: floor(99 × 101 / 99) = 101 PYUSD0
    PYUSD0→MOET:   floor(101 × 0.99)    = 99 MOET

  99 >= 99 debt  ✓

[zhangchiqing]

Can we also add assertion to check that after closing the yield vault, the debt is reduced.

We might want to assert that after opening and closing the yield vault, the total collateral amount should be very close to the original amount, there should be only a tiny amount loss due to running into the code path of expectedMOET < totalDebtAmount where some collateral has to be sold, right?

[zhangchiqing]

What is the rounding underestimate caused by? I

[zhangchiqing]

should we just assert extraMOET.balance > shortfall?

[zhangchiqing]

where is the ERC4626 connector?

  yieldToMoetSwapper = MultiSwapper containing:
    └── SequentialSwapper:
          1. yieldToUnderlying  ← MorphoERC4626SwapConnectors.Swapper (isReversed: true)
          2. underlyingToDebt   ← UniV3 swapper

  So the flow is:
  BufferedSwapSource
    ├── source: yieldTokenSource (AutoBalancer external source - just pulls yield tokens)
    └── swapper: yieldToMoetSwapper
                   └── yieldToken → [ERC4626 redeem] → underlying → [UniV3] → MOET

  The ERC4626 is inside the swapper — specifically yieldToUnderlying which redeems ERC4626 shares (FUSDEV) back to the underlying asset (PYUSD0), then underlyingToDebt swaps that to MOET via Uniswap.

I think the "at least two hops" Alex mentioned is the "yieldToken → [ERC4626 redeem] → underlying → [UniV3] → MOET" swap route, which is previewed with " FUSDEV → [ERC4626 previewRedeem, floor] → PYUSD0 → [UniV3 quote] → MOET"

But I think the shortfall is actually accurate (Meaning shortfall is always slightly bigger than the actual shortfall, rather than smaller). In other words, I think we don't need the 1% buffer. Why? because the expectedMOET is accurate.

The pre-supplyment already paid the shortfall (self.position.deposit(from: <-extraMOET)), which is slightly more than the actual shortfall , so closing the position with yield token wrapped with SwapSource should be enough to receive the remaining MOET after swap.

Note the pre-supplyment is converting to and paying debts with MOET rather than the yield token, so there is no more ERC4626 rounding error to be hit. The ERC4626 rounding error has been covered by the calculation of expectedMOET.

I think we can validate by removing the 1% buffer and add assertion for extraMOET.balance > shortfall. If closePosition didn't revert, then we should be good. Or did I miss something?

[nialexsan]

@zhangchiqing you're right, the 1% buffer is not required any more after all other roundings.
the BuffredSwapSource will be renamed to something like UnboundedSwapSource in the next contract version

[codecov]

Codecov Report

❌ Patch coverage is 26.47975% with 236 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
cadence/contracts/FlowYieldVaultsStrategiesV2.cdc	26.47%	236 Missing ⚠️

📢 Thoughts on this report? Let us know!

[zhangchiqing]

Would it be better to build a sequential swapper from yield -> moet -> collateral? That way we only need to do 1 quote (from yield to collateral), and decide whether to swap: if collQuote.outAmount == 0, we could just burn all the yield token without converting them to moet and then burn.

[nialexsan]

yes, sequential swapper is definitely way to go, I left it bare for now as we need to rip MOET out of the strategy, and I feel that with this it will be more obvious where it needs to be removed