Remove the deriving-aeson dependency (#7871)

* Add tests for cost model JSON encoding stability

Compare the serialized form of BuiltinCostModelBase and CekMachineCostsBase
against the checked-in cost-model/data files for every semantics variant,
and cover the field-omitting instances used by generate-cost-model.
Differences are reported as a JSON patch so that a failure names the
offending entries instead of printing both cost models in full.

* Remove the deriving-aeson dependency

Replace the CustomJSON deriving-via JSON instances of the cost model
types with plain aeson generic instances (genericToJSON,
genericToEncoding, genericParseJSON with explicit Options). The JSON
format is unchanged. Drops the allow-newer exception for
deriving-aeson:aeson from cabal.project.

Issue: #7870

Author: Unisay

plutus-core/changelog.d/20260728_100010_yuriy.lazaryev_issue_7870_remove_deriving_aeson.md

@@ -0,0 +1,7 @@
+### Removed
+
+- `LowerInitialCharacter` is no longer exported from `PlutusCore.Evaluation.Machine.ExBudget`; it existed solely to support `deriving-aeson`, which `plutus-core` no longer depends on.
+
+### Changed
+
+- Replaced the `deriving-aeson`-based JSON instances of the cost model types with plain `aeson` generic instances. The JSON format is unchanged.

plutus-core/cost-model/CostModelGeneration.md

@@ -292,7 +292,7 @@ our case, the costing function is given by the
 type. The type prefix `ModelTwoArguments` is removed from the
 constructor name and the remaining `MinSize` is converted to
 `min_size` by the Aeson library's
-[`CamelToSnake`](https://hackage.haskell.org/package/deriving-aeson-0.2.8/docs/Deriving-Aeson.html#t:CamelToSnake)
+[`camelTo2`](https://hackage.haskell.org/package/aeson/docs/Data-Aeson.html#v:camelTo2)
 transformation.  Similarly, the names of the
 `modelMinSizeIntercept` and `modelMinSizeSlope` fields in the
 `ModelMinSize` type are converted to `slope` and `intercept`.  In

plutus-core/plutus-core.cabal

@@ -327,7 +327,6 @@ library
     , data-default-class
     , deepseq
     , dependent-sum               >=0.7.1.0
-    , deriving-aeson              >=0.2.3
     , deriving-compat
     , dlist
     , exceptions
@@ -382,6 +381,7 @@ test-suite plutus-core-test
     CBOR.DataStability
     Check.Spec
     CostModelInterface.Spec
+    CostModelJSON.Spec
     CostModelSafety.Spec
     Evaluation.Machines
     Evaluation.Spec
@@ -398,6 +398,9 @@ test-suite plutus-core-test
   default-language: Haskell2010
   build-depends:
     , aeson
+    , aeson-diff
+    , aeson-pretty
+    , barbies
     , base                             >=4.9     && <5
     , base16-bytestring                ^>=1.0
     , bytestring

plutus-core/plutus-core/src/PlutusCore/Evaluation/Machine/BuiltinCostModel.hs

@@ -1,6 +1,5 @@
 -- editorconfig-checker-disable-file
 {-# LANGUAGE ConstraintKinds #-}
-{-# LANGUAGE DataKinds #-}
 {-# LANGUAGE DeriveAnyClass #-}
 {-# LANGUAGE FlexibleInstances #-}
 {-# LANGUAGE StrictData #-}
@@ -57,13 +56,12 @@ import PlutusPrelude hiding (toList)
 
 import PlutusCore.Evaluation.Machine.CostingFun.Core
 import PlutusCore.Evaluation.Machine.CostingFun.JSON ()
-import PlutusCore.Evaluation.Machine.ExBudget
 
 import Barbies
 import Data.Aeson
 import Data.Kind qualified as Kind
+import Data.List (stripPrefix)
 import Data.Monoid
-import Deriving.Aeson
 import Language.Haskell.TH.Syntax hiding (Name, newName)
 
 type BuiltinCostModel = BuiltinCostModelBase CostingFun
@@ -203,18 +201,21 @@ data BuiltinCostModelBase f
   deriving stock (Generic)
   deriving anyclass (FunctorB, TraversableB, ConstraintsB)
 
-deriving via
-  CustomJSON
-    '[FieldLabelModifier (StripPrefix "param", LowerInitialCharacter)]
-    (BuiltinCostModelBase CostingFun)
-  instance
-    ToJSON (BuiltinCostModelBase CostingFun)
-deriving via
-  CustomJSON
-    '[FieldLabelModifier (StripPrefix "param", LowerInitialCharacter)]
-    (BuiltinCostModelBase CostingFun)
-  instance
-    FromJSON (BuiltinCostModelBase CostingFun)
+{-| JSON options for 'BuiltinCostModelBase': drop the @param@ prefix and lower the initial
+character, so that the names of the JSON fields are exactly the same as the names of the
+builtins. -}
+builtinCostModelOptions :: Options
+builtinCostModelOptions =
+  defaultOptions {fieldLabelModifier = lowerInitialChar . dropPrefix "param"}
+  where
+    dropPrefix prefix s = fromMaybe s (stripPrefix prefix s)
+
+instance ToJSON (BuiltinCostModelBase CostingFun) where
+  toJSON = genericToJSON builtinCostModelOptions
+  toEncoding = genericToEncoding builtinCostModelOptions
+
+instance FromJSON (BuiltinCostModelBase CostingFun) where
+  parseJSON = genericParseJSON builtinCostModelOptions
 
 {-| Same as 'CostingFun' but maybe missing.
 We could use 'Compose Maybe CostinFun' instead but we would then need an orphan ToJSON instance. -}
@@ -223,12 +224,12 @@ newtype MCostingFun a = MCostingFun (Maybe (CostingFun a))
   deriving (Semigroup, Monoid) via (Alt Maybe (CostingFun a)) -- for mempty == MCostingFun Nothing
 
 -- Omit generating JSON for any costing functions that have not been set (are missing).
-deriving via
-  CustomJSON
-    '[OmitNothingFields, FieldLabelModifier (StripPrefix "param", LowerInitialCharacter)]
-    (BuiltinCostModelBase MCostingFun)
-  instance
-    ToJSON (BuiltinCostModelBase MCostingFun)
+instance ToJSON (BuiltinCostModelBase MCostingFun) where
+  toJSON = genericToJSON mBuiltinCostModelOptions
+  toEncoding = genericToEncoding mBuiltinCostModelOptions
+
+mBuiltinCostModelOptions :: Options
+mBuiltinCostModelOptions = builtinCostModelOptions {omitNothingFields = True}
 
 -- Needed to help derive various instances for BuiltinCostModelBase
 type AllArgumentModels (constraint :: Kind.Type -> Kind.Constraint) f =

plutus-core/plutus-core/src/PlutusCore/Evaluation/Machine/CostingFun/Core.hs

@@ -59,8 +59,8 @@ import PlutusCore.Evaluation.Machine.ExMemoryUsage
 import Control.DeepSeq
 import Data.Default.Class
 import Data.Hashable
-import Deriving.Aeson
 import GHC.Exts
+import GHC.Generics (Generic)
 import Language.Haskell.TH.Syntax hiding
   ( Name
   , newName