Warn if builddir is going to be ignored.

builddir can only be specified from the command line.

Closes: #7941

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

@@ -390,6 +390,9 @@ rebuildProjectConfig
                 pure (os, arch, compilerInfo compiler)
 
           projectConfig <- instantiateProjectConfigSkeletonFetchingCompiler fetchCompiler mempty projectConfigSkeleton
+          when (projectConfigDistDir (projectConfigShared $ projectConfig) /= NoFlag) $
+            liftIO $
+              warn verbosity "The builddir option is not supported in project and config files. It will be ignored."
           localPackages <- phaseReadLocalPackages (projectConfig <> cliConfig)
           return (projectConfig, localPackages)
 

cabal-testsuite/PackageTests/Project/WarnBuilddir/cabal.out

@@ -0,0 +1,6 @@
+# cabal v2-build
+Warning: The builddir option is not supported in project and config files. It will be ignored.
+Resolving dependencies...
+Build profile: -w ghc-<GHCVER> -O1
+In order, the following would be built:
+ - main-0.1 (lib) (first run)

cabal-testsuite/PackageTests/Project/WarnBuilddir/cabal.project

@@ -0,0 +1,2 @@
+packages: .
+builddir: something

cabal-testsuite/PackageTests/Project/WarnBuilddir/cabal.test.hs

@@ -0,0 +1,4 @@
+import Test.Cabal.Prelude
+
+main = cabalTest $ do
+  cabal "v2-build" ["--dry-run", "all"]

cabal-testsuite/PackageTests/Project/WarnBuilddir/main.cabal

@@ -0,0 +1,14 @@
+cabal-version: 3.0
+
+name: main
+version: 0.1
+build-type: Simple
+category: Test
+maintainer: Joe
+synopsis: Test input
+description: Test input
+license: BSD-3-Clause
+
+library
+  build-depends: base
+  default-language: Haskell2010

doc/cabal-project.rst

@@ -288,7 +288,7 @@ package, and thus apply globally:
     to the root of the project (i.e., where the ``cabal.project``
     file lives.)
 
-    This option cannot be specified via a ``cabal.project`` file.
+    This option can only be specified from the command line.
 
 .. _cmdoption-project-dir:
 .. option:: --project-dir=DIR