readGlobalConfig: Also consider cabal.config files in project root

If there is a cabal.config.local file in the project root directory,
readGlobalConfig reads it after ~/.cabal/config. This is intended to allow the
user to specify site-specific global options on a per-project basis. Global
options cannot be specified in cabal.project.local, which applies options only
to packages in the project.

See also: #3883 #4646

.gitignore

@@ -2,6 +2,7 @@
 .cabal-sandbox/
 cabal.sandbox.config
 cabal.project.local
+cabal.config.local
 .ghc.environment.*
 cabal-dev/
 .hpc/

Cabal/doc/nix-local-build.rst

@@ -467,11 +467,17 @@ following sources (later entries override earlier ones):
 
 1. ``~/.cabal/config`` (the user-wide global configuration)
 
-2. ``cabal.project`` (the project configuratoin)
+2. ``cabal.config.local`` (the project-wide global configuration)
 
-3. ``cabal.project.freeze`` (the output of ``cabal new-freeze``)
+3. ``cabal.project`` (the project configuration)
 
-4. ``cabal.project.local`` (the output of ``cabal new-configure``)
+4. ``cabal.project.freeze`` (the output of ``cabal new-freeze``)
+
+5. ``cabal.project.local`` (the output of ``cabal new-configure``)
+
+The project configuration files ``cabal.project`` and ``cabal.project.local``
+specify options for project packages only, while the global configuration files
+``~/.cabal/config`` and ``cabal.config.local`` affect *all* packages.
 
 
 Specifying the local packages

cabal-install/Distribution/Client/ProjectConfig.hs

@@ -65,7 +65,7 @@ import Distribution.Client.GlobalFlags
 import Distribution.Client.BuildReports.Types
          ( ReportLevel(..) )
 import Distribution.Client.Config
-         ( loadConfig, getConfigFilePath )
+         ( getConfigFilePath, loadConfig, loadExactConfig )
 
 import Distribution.Solver.Types.SourcePackage
 import Distribution.Solver.Types.Settings
@@ -410,7 +410,7 @@ renderBadProjectRoot (BadProjectRootExplicitFile projectFile) =
 --
 readProjectConfig :: Verbosity -> Flag FilePath -> DistDirLayout -> Rebuild ProjectConfig
 readProjectConfig verbosity configFileFlag distDirLayout = do
-    global <- readGlobalConfig             verbosity configFileFlag
+    global <- readGlobalConfig             verbosity distDirLayout configFileFlag
     local  <- readProjectLocalConfig       verbosity distDirLayout
     freeze <- readProjectLocalFreezeConfig verbosity distDirLayout
     extra  <- readProjectLocalExtraConfig  verbosity distDirLayout
@@ -539,13 +539,25 @@ writeProjectConfigFile file =
     writeFile file . showProjectConfig
 
 
--- | Read the user's @~/.cabal/config@ file.
---
-readGlobalConfig :: Verbosity -> Flag FilePath -> Rebuild ProjectConfig
-readGlobalConfig verbosity configFileFlag = do
-    config     <- liftIO (loadConfig verbosity configFileFlag)
-    configFile <- liftIO (getConfigFilePath configFileFlag)
-    monitorFiles [monitorFileHashed configFile]
+-- | Read the user's @~/.cabal/config@ file and any @cabal.config.local@ file in the
+-- project root directory.
+--
+readGlobalConfig
+    :: Verbosity
+    -> DistDirLayout
+    -> Flag FilePath
+    -> Rebuild ProjectConfig
+readGlobalConfig verbosity layout configFileFlag = do
+    userConfig     <- liftIO (loadConfig verbosity configFileFlag)
+    userConfigFile <- liftIO (getConfigFilePath configFileFlag)
+    let localConfigFile = distProjectRootDirectory layout </> "cabal.config.local"
+    localConfig    <- liftIO (loadExactConfig verbosity localConfigFile)
+    monitorFiles
+      [
+        monitorFileHashed userConfigFile,
+        monitorFileHashed localConfigFile
+      ]
+    let config = userConfig <> fromMaybe mempty localConfig
     return (convertLegacyGlobalConfig config)
 
 reportParseResult :: Verbosity -> String -> FilePath -> ParseResult a -> IO a