Merge #2884

2884: Don't require all minted assets to be spent or burned r=jonathanknowles a=jonathanknowles

### Issue Number

ADP-1113

### Summary

This PR removes the restriction that all minted assets must be spent or burned.

Minted assets that are not spent or burned are now returned in the change outputs.

Co-authored-by: Jonathan Knowles <jonathan.knowles@iohk.io>

lib/core/src/Cardano/Wallet/Api/Server.hs

@@ -361,7 +361,6 @@ import Cardano.Wallet.Primitive.CoinSelection.Balance
     ( SelectionResult (..)
     , UnableToConstructChangeError (..)
     , balanceMissing
-    , missingOutputAssets
     , selectionDelta
     )
 import Cardano.Wallet.Primitive.Delegation.UTxO
@@ -3821,13 +3820,6 @@ instance IsServerError ErrSelectAssets where
                         , "must specify enough ada. Here are the problematic "
                         , "outputs:\n" <> pretty (indentF 2 $ blockListF xs)
                         ]
-                Balance.OutputsInsufficient e ->
-                    apiError err403 TokensMintedButNotSpentOrBurned $ mconcat
-                        [ "I can't process this transaction because some "
-                        , "minted values were not spent or burned. These "
-                        , "are the values that should be spent or burned: "
-                        , pretty . Flat $ missingOutputAssets e
-                        ]
                 Balance.UnableToConstructChange e ->
                     apiError err403 CannotCoverFee $ T.unwords
                         [ "I am unable to finalize the transaction, as there"

lib/core/src/Cardano/Wallet/Primitive/CoinSelection/Balance.hs

@@ -38,7 +38,6 @@ module Cardano.Wallet.Primitive.CoinSelection.Balance
     , SelectionResult (..)
     , SelectionError (..)
     , BalanceInsufficientError (..)
-    , OutputsInsufficientError (..)
     , SelectionInsufficientError (..)
     , InsufficientMinCoinValueError (..)
     , UnableToConstructChangeError (..)
@@ -115,7 +114,6 @@ module Cardano.Wallet.Primitive.CoinSelection.Balance
     , distance
     , mapMaybe
     , balanceMissing
-    , missingOutputAssets
     ) where
 
 import Prelude
@@ -507,56 +505,11 @@ data SelectionError
         SelectionInsufficientError
     | InsufficientMinCoinValues
         (NonEmpty InsufficientMinCoinValueError)
-    | OutputsInsufficient
-        OutputsInsufficientError
     | UnableToConstructChange
         UnableToConstructChangeError
     | EmptyUTxO
     deriving (Generic, Eq, Show)
 
--- | Indicates that a portion of the minted assets were not spent or burned.
---
--- This situation occurs if the following inequality does not hold:
---
--- >>> assetsToMint `leq` (assetsToBurn <> requestedOutputAssets)
---
--- The existence of this error reflects a deliberate design choice: all minted
--- assets must either be explicitly spent or explicitly burned by the caller.
--- In future, we could revise this design to allow excess minted assets (those
--- that are neither spent nor burned) to be returned to the wallet as change.
---
-data OutputsInsufficientError = OutputsInsufficientError
-    { assetsToMint
-        :: !TokenMap
-      -- ^ The assets to mint
-    , assetsToBurn
-        :: !TokenMap
-      -- ^ The assets to burn
-    , requestedOutputAssets
-        :: !TokenMap
-      -- ^ The complete set of assets found within the user-specified outputs
-    } deriving (Generic, Eq, Show)
-
--- | Computes the portion of minted assets that are not spent or burned.
---
-missingOutputAssets :: OutputsInsufficientError -> TokenMap
-missingOutputAssets e =
-    -- We use 'difference' which will show us the quantities in 'assetsToMint'
-    -- that are not in 'assetsToBurn <> requestedOutputAssets'.
-    --
-    -- Any asset quantity present in 'assetsToBurn <> requestedOutputAssets'
-    -- but not present in 'assetsToMint' will simply be zeroed out, which is
-    -- the behaviour we want for this error report.
-    --
-    assetsToMint `TokenMap.difference`
-        (assetsToBurn `TokenMap.add` requestedOutputAssets)
-  where
-    OutputsInsufficientError
-        { assetsToMint
-        , assetsToBurn
-        , requestedOutputAssets
-        } = e
-
 -- | Indicates that the balance of selected UTxO entries was insufficient to
 --   cover the balance required.
 --
@@ -684,11 +637,6 @@ performSelection
         -- ^ The selection goal to satisfy.
     -> m (Either SelectionError (SelectionResult TokenBundle))
 performSelection minCoinFor costFor bundleSizeAssessor criteria
-    -- Is the minted value all spent or burnt?
-    | not (assetsToMint `leq` (assetsToBurn <> requestedOutputAssets)) =
-        pure $ Left $ OutputsInsufficient $ OutputsInsufficientError
-            {assetsToMint, assetsToBurn, requestedOutputAssets}
-
     -- Is the total available UTXO balance sufficient?
     | not utxoBalanceSufficient =
         pure $ Left $ BalanceInsufficient $ BalanceInsufficientError
@@ -733,12 +681,6 @@ performSelection minCoinFor costFor bundleSizeAssessor criteria
             , utxoBalanceRequired
             }
 
-    requestedOutputBalance :: TokenBundle
-    requestedOutputBalance = F.foldMap (view #tokens) outputsToCover
-
-    requestedOutputAssets :: TokenMap
-    requestedOutputAssets = view #tokens requestedOutputBalance
-
     mkInputsSelected :: UTxOIndex -> NonEmpty (TxIn, TxOut)
     mkInputsSelected =
         fromMaybe invariantSelectAnyInputs . NE.nonEmpty . UTxOIndex.toList