From a42643c2eee5c8e1246b33d6f9d47b632a09bfae Mon Sep 17 00:00:00 2001
From: PaleRoses <rosaliafialkova@gmail.com>
Date: Wed, 14 Oct 2026 23:57:46 -0700
Subject: [PATCH] Drop elabGPkgDescription from the elaborated plan

Every unit carried its package's GenericPackageDescription, so the plan
caches grew with the square of a package's component count. The
in-library configure now reads it from the package's .cabal file, as a
Setup script does.

Fixes #12463.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
---
 .../src/Distribution/Client/InLibrary.hs        | 17 ++++++++---------
 .../src/Distribution/Client/ProjectPlanning.hs  |  1 -
 .../Client/ProjectPlanning/Types.hs             |  2 --
 changelog.d/plan-cache-gpkg-description.md      |  9 +++++++++
 4 files changed, 17 insertions(+), 12 deletions(-)
 create mode 100644 changelog.d/plan-cache-gpkg-description.md

diff --git a/cabal-install/src/Distribution/Client/InLibrary.hs b/cabal-install/src/Distribution/Client/InLibrary.hs
index bda360b455b..acda3269700 100644
--- a/cabal-install/src/Distribution/Client/InLibrary.hs
+++ b/cabal-install/src/Distribution/Client/InLibrary.hs
@@ -30,6 +30,7 @@ import qualified Distribution.Simple.Configure as Cabal
 import Distribution.Simple.Haddock (haddock_setupHooks)
 import Distribution.Simple.Install (install_setupHooks)
 import Distribution.Simple.LocalBuildInfo (mbWorkDirLBI)
+import Distribution.Simple.PackageDescription (readGenericPackageDescription)
 import Distribution.Simple.PackageIndex (InstalledPackageIndex)
 import qualified Distribution.Simple.PreProcess as Cabal
 import Distribution.Simple.Program.Db
@@ -67,7 +68,6 @@ data LibraryConfigureInputs = LibraryConfigureInputs
   , localBuildConfig :: LBC.LocalBuildConfig
   , packageDBStack :: PackageDBStackCWD
   , packageDescription :: PD.PackageDescription
-  , gPackageDescription :: PD.GenericPackageDescription
   , flagAssignment :: PD.FlagAssignment
   , installedPkgIndex :: InstalledPackageIndex
   }
@@ -121,13 +121,11 @@ libraryConfigureInputsFromElabPackage
             }
       , packageDBStack = elabBuildPackageDBStack pkg
       , packageDescription = pkgDescr
-      , gPackageDescription = gpkgDescr
       , flagAssignment = elabFlagAssignment pkg
       , installedPkgIndex = ipi
       }
     where
       pkgDescr = elabPkgDescription pkg
-      gpkgDescr = elabGPkgDescription pkg
 
 configure
   :: LibraryConfigureInputs
@@ -143,7 +141,6 @@ configure
     , localBuildConfig = lbc0
     , packageDBStack = packageDBs
     , packageDescription = pkgDescr
-    , gPackageDescription = gpkgDescr
     , flagAssignment = flagAssgn
     , installedPkgIndex = ipi
     }
@@ -196,6 +193,13 @@ configure
               , benchmarksRequested = Cabal.fromFlag (Cabal.configBenchmarks cfg)
               }
 
+    -- Read the .cabal file, as a Setup script would; the plan does not carry it.
+    pkgDescrFilePath <-
+      case Cabal.flagToMaybe $ Cabal.configCabalFilePath cfg of
+        Just pkgFile -> return pkgFile
+        Nothing -> relativeSymbolicPath <$> tryFindPackageDesc verbosity mbWorkDir
+    gpkgDescr <- readGenericPackageDescription verbosity mbWorkDir pkgDescrFilePath
+
     -- NB: it's OK to discard constraints here: we already have a finalized PackageDescription
     -- in hand, and we are using exact UnitIds for all dependencies (this corresponds
     -- to using --exact-configuration and --dependency flags with the Setup CLI).
@@ -223,11 +227,6 @@ configure
         packageDBs'
         pkgInfo
 
-    -- Remember the .cabal filename if we know it.
-    pkgDescrFilePath <-
-      case Cabal.flagToMaybe $ Cabal.configCabalFilePath cfg of
-        Just pkgFile -> return pkgFile
-        Nothing -> relativeSymbolicPath <$> tryFindPackageDesc verbosity mbWorkDir
     return $ lbi1{pkgDescrFile = Just pkgDescrFilePath}
 
 --------------------------------------------------------------------------------
diff --git a/cabal-install/src/Distribution/Client/ProjectPlanning.hs b/cabal-install/src/Distribution/Client/ProjectPlanning.hs
index da185e990c0..f0767e8e302 100644
--- a/cabal-install/src/Distribution/Client/ProjectPlanning.hs
+++ b/cabal-install/src/Distribution/Client/ProjectPlanning.hs
@@ -2284,7 +2284,6 @@ elaborateInstallPlan
               gdesc of
               Right (desc, _) -> desc
               Left _ -> error "Failed to finalizePD in elaborateSolverToCommon"
-            elabGPkgDescription = gdesc
             elabFlagAssignment = flags
             elabFlagDefaults =
               PD.mkFlagAssignment
diff --git a/cabal-install/src/Distribution/Client/ProjectPlanning/Types.hs b/cabal-install/src/Distribution/Client/ProjectPlanning/Types.hs
index 67e10c46813..099ff4398dc 100644
--- a/cabal-install/src/Distribution/Client/ProjectPlanning/Types.hs
+++ b/cabal-install/src/Distribution/Client/ProjectPlanning/Types.hs
@@ -221,8 +221,6 @@ data ElaboratedConfiguredPackage = ElaboratedConfiguredPackage
   , elabFlagDefaults :: Cabal.FlagAssignment
   -- ^ The original default flag assignment, used only for reporting.
   , elabPkgDescription :: Cabal.PackageDescription
-  , elabGPkgDescription :: Cabal.GenericPackageDescription
-  -- ^ Original 'GenericPackageDescription' (just used to report errors/warnings)
   , elabPkgSourceLocation :: PackageLocation (Maybe FilePath)
   -- ^ Where the package comes from, e.g. tarball, local dir etc. This
   --   is not the same as where it may be unpacked to for the build.
diff --git a/changelog.d/plan-cache-gpkg-description.md b/changelog.d/plan-cache-gpkg-description.md
new file mode 100644
index 00000000000..31ec9919ada
--- /dev/null
+++ b/changelog.d/plan-cache-gpkg-description.md
@@ -0,0 +1,9 @@
+---
+synopsis: Plan caches no longer hold a GenericPackageDescription per component
+packages: [cabal-install]
+prs: 12464
+issues: 12463
+---
+
+The in-library configure reads it from the package's `.cabal` file. With 80 public
+sublibraries, `improved-plan` falls from 27 MB to 15 MB.