Require cabal.project packages and/or optional-packages field

Currently, if a cabal.project file is present and the `packages` and
`optional-packages` fields do not exist, a warning will be issued, and
cabal will fail with a different, potentially misleading error message:

  "There is no <pkgname>.cabal package file or cabal.project file..."

This changes it so that the prior warning is now an error, hence will
cause cabal to fail before the misleading error is given. That at least
one of these fields is required is already specified in the docs, hence
this is not a specification change.

See #7401.

Author: tbidne

cabal-install/src/Distribution/Client/DistDirLayout.hs

@@ -9,6 +9,7 @@ module Distribution.Client.DistDirLayout
     DistDirLayout (..)
   , DistDirParams (..)
   , defaultDistDirLayout
+  , distProjectFileMain
 
     -- * 'ProjectRoot'
   , ProjectRoot (..)
@@ -316,3 +317,8 @@ mkCabalDirLayout mstoreDir mlogDir = do
   cabalLogsDirectory <-
     maybe defaultLogsDir pure mlogDir
   pure $ CabalDirLayout{..}
+
+-- | Given the 'DistDirLayout''s distProjectFile function, returns the
+-- main project file (i.e. cabal.project).
+distProjectFileMain :: (String -> FilePath) -> FilePath
+distProjectFileMain f = f ""

cabal-install/src/Distribution/Client/Errors.hs

@@ -192,6 +192,7 @@ data CabalInstallException
   | LegacyAndParsecParseResultsDiffer FilePath String String
   | CabalFileParseFailure CabalFileParseError
   | ProjectConfigParseFailure ProjectConfigParseError
+  | ProjectConfigNoPackages FilePath
   deriving (Show)
 
 exceptionCodeCabalInstall :: CabalInstallException -> Int
@@ -348,6 +349,7 @@ exceptionCodeCabalInstall e = case e of
   LegacyAndParsecParseResultsDiffer{} -> 7165
   CabalFileParseFailure{} -> 7166
   ProjectConfigParseFailure{} -> 7167
+  ProjectConfigNoPackages{} -> 7168
 
 exceptionMessageCabalInstall :: CabalInstallException -> String
 exceptionMessageCabalInstall e = case e of
@@ -885,6 +887,13 @@ exceptionMessageCabalInstall e = case e of
     renderCabalFileParseError cbfError
   ProjectConfigParseFailure pcfError ->
     renderProjectConfigParseError pcfError
+  ProjectConfigNoPackages configPath ->
+    concat
+      [ "The project config '"
+      , configPath
+      , "' requires at least one of the fields 'packages' "
+      , "or 'optional-packages', but neither was specified."
+      ]
 
 instance Exception (VerboseException CabalInstallException) where
   displayException :: VerboseException CabalInstallException -> [Char]

cabal-install/src/Distribution/Client/ProjectOrchestration.hs

@@ -307,8 +307,10 @@ establishProjectBaseContextWithRoot verbosity cliConfig projectRoot currentComma
           projectConfig
 
   -- https://github.com/haskell/cabal/issues/6013
+  -- https://github.com/haskell/cabal/issues/7401
+  let projPath = distProjectFileMain (distProjectFile distDirLayout)
   when (null (projectPackages projectConfig) && null (projectPackagesOptional projectConfig)) $
-    warn verbosity "There are no packages or optional-packages in the project"
+    dieWithException verbosity (ProjectConfigNoPackages projPath)
 
   return
     ProjectBaseContext

cabal-install/src/Distribution/Client/ProjectPlanning.hs

@@ -379,7 +379,7 @@ rebuildProjectConfig
       configPath <- getConfigFilePath verbosity projectConfigConfigFile
       return
         ( configPath
-        , distProjectFile ""
+        , distProjectFileMain distProjectFile
         , (projectConfigHcFlavor, projectConfigHcPath, projectConfigHcPkg)
         , projectConfigProjectFileParser
         , progsearchpath

cabal-install/tests/fixtures/configure/cabal.project

@@ -1,2 +1,4 @@
 ignore-project: False
 optimization: 2
+
+packages: .

cabal-install/tests/fixtures/configure/dummy.cabal

@@ -0,0 +1,17 @@
+cabal-version: 3.0
+
+-- Required by ./cabal.project, due to cabal.project requiring a valid
+-- packages or optional-packages field.
+
+name: main
+version: 0.1
+build-type: Simple
+category: Test
+maintainer: Dummy
+synopsis: Dummy
+description: Dummy
+license: BSD-3-Clause
+
+library
+  build-depends: base
+  default-language: Haskell2010

cabal-testsuite/PackageTests/ProjectPackages/Empty/cabal.out

@@ -0,0 +1,11 @@
+# cabal v2-build
+Error: [Cabal-7136]
+There is no <pkgname>.cabal package file or cabal.project file. To build packages locally you need at minimum a <pkgname>.cabal file. You can use 'cabal init' to create one.
+
+For non-trivial projects you will also want a cabal.project file in the root directory of your project. This file lists the packages in your project and all other build configuration. See the Cabal user guide for full details.
+
+# cabal v2-build
+Error: [Cabal-7136]
+There is no <pkgname>.cabal package file or cabal.project file. To build packages locally you need at minimum a <pkgname>.cabal file. You can use 'cabal init' to create one.
+
+For non-trivial projects you will also want a cabal.project file in the root directory of your project. This file lists the packages in your project and all other build configuration. See the Cabal user guide for full details.

cabal-testsuite/PackageTests/ProjectPackages/Empty/cabal.test.hs

@@ -0,0 +1,5 @@
+import Test.Cabal.Prelude
+
+main = cabalTest $ do
+  fails $ cabal "v2-build" ["--dry-run"]
+  fails $ cabal "v2-build" ["--dry-run", "all"]

cabal-testsuite/PackageTests/ProjectPackages/FieldNoCabal/cabal.out

@@ -0,0 +1,2 @@
+# cabal v2-build
+# cabal v2-build

cabal-testsuite/PackageTests/ProjectPackages/FieldNoCabal/cabal.project

@@ -0,0 +1 @@
+packages: .