Warn if builddir is going to be ignored (#8949)

* Unify the default project with the implicit one

Passing --ignore-project made cabal use `defaultProject` which is not
quite identical to the one used when cabal.project is missing
(`defaultImplifictProjectConfig`). There is no reason the two should be
different.

* Warn if builddir is going to be ignored.

builddir can only be specified from the command line.

Closes: #7941

* Clarify which project option can only be passed from the command line

* Add note about reading the project configuration

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

@@ -664,18 +664,15 @@ readProjectConfig
   -> Flag FilePath
   -> DistDirLayout
   -> Rebuild ProjectConfigSkeleton
-readProjectConfig verbosity httpTransport ignoreProjectFlag configFileFlag distDirLayout = do
-  let defaultProject =
-        mempty
-          { projectPackages = ["./"]
-          }
+readProjectConfig verbosity _ (Flag True) configFileFlag _ = do
+  global <- singletonProjectConfigSkeleton <$> readGlobalConfig verbosity configFileFlag
+  return (global <> singletonProjectConfigSkeleton defaultImplicitProjectConfig)
+readProjectConfig verbosity httpTransport _ configFileFlag distDirLayout = do
   global <- singletonProjectConfigSkeleton <$> readGlobalConfig verbosity configFileFlag
   local <- readProjectLocalConfigOrDefault verbosity httpTransport distDirLayout
   freeze <- readProjectLocalFreezeConfig verbosity httpTransport distDirLayout
   extra <- readProjectLocalExtraConfig verbosity httpTransport distDirLayout
-  if ignoreProjectFlag == Flag True
-    then return (global <> singletonProjectConfigSkeleton defaultProject)
-    else return (global <> local <> freeze <> extra)
+  return (global <> local <> freeze <> extra)
 
 -- | Reads an explicit @cabal.project@ file in the given project root dir,
 -- or returns the default project config for an implicitly defined project.
@@ -687,19 +684,21 @@ readProjectLocalConfigOrDefault
 readProjectLocalConfigOrDefault verbosity httpTransport distDirLayout = do
   let projectFile = distProjectFile distDirLayout ""
   usesExplicitProjectRoot <- liftIO $ doesFileExist projectFile
-  let defaultImplicitProjectConfig =
-        mempty
-          { -- We expect a package in the current directory.
-            projectPackages = ["./*.cabal"]
-          , projectConfigProvenance = Set.singleton Implicit
-          }
   if usesExplicitProjectRoot
     then do
       readProjectFileSkeleton verbosity httpTransport distDirLayout "" "project file"
     else do
       monitorFiles [monitorNonExistentFile projectFile]
       return (singletonProjectConfigSkeleton defaultImplicitProjectConfig)
 
+defaultImplicitProjectConfig :: ProjectConfig
+defaultImplicitProjectConfig =
+  mempty
+    { -- We expect a package in the current directory.
+      projectPackages = ["./*.cabal"]
+    , projectConfigProvenance = Set.singleton Implicit
+    }
+
 -- | Reads a @cabal.project.local@ file in the given project root dir,
 -- or returns empty. This file gets written by @cabal configure@, or in
 -- principle can be edited manually or by other tools.

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

@@ -331,11 +331,37 @@ sanityCheckElaboratedPackage
             `optStanzaSetIsSubset` pkgStanzasEnabled
         )
 
-------------------------------------------------------------------------------
-
--- * Deciding what to do: making an 'ElaboratedInstallPlan'
-
-------------------------------------------------------------------------------
+-- Note [reading project configuration]
+--
+-- The project configuration is assembled into a ProjectConfig as follows:
+--
+-- CLI arguments are converted using commandLineFlagsToProjectConfig in the
+-- v2 command entrypoints and passed to establishProjectBaseContext which
+-- then calls rebuildProjectConfig.
+--
+-- rebuildProjectConfig then calls readProjectConfig to read the project
+-- files. Because of conditionals, this output is in the form of a
+-- ProjectConfigSkeleton and will be resolved by rebuildProjectConfig using
+-- instantiateProjectConfigSkeletonFetchingCompiler.
+--
+-- readProjectConfig also loads the global configuration, which is read with
+-- loadConfig and convertd to a ProjectConfig with convertLegacyGlobalConfig.
+--
+-- *Important*
+--
+-- You can notice how some project config options are needed to read the
+-- project config! This is evident by the fact that rebuildProjectConfig
+-- takes HttpTransport and DistDirLayout as parameters. Two arguments are
+-- infact determined from the CLI alone (in establishProjectBaseContext).
+-- Consequently, project files (including global configuration) cannot
+-- affect those parameters.
+--
+-- Furthermore, the project configuration can specify a compiler to use,
+-- which we need to resolve the conditionals in the project configuration!
+-- To solve this, we configure the compiler from what is obtained by applying
+-- the CLI configuration over the the configuration obtained by "flattening"
+-- ProjectConfigSkeleton. This means collapsing all conditionals by taking
+-- both branches.
 
 -- | Return the up-to-date project config and information about the local
 -- packages within the project.
@@ -390,6 +416,9 @@ 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)
 
@@ -510,6 +539,11 @@ configureCompiler
             )
           $ defaultProgramDb
 
+
+------------------------------------------------------------------------------
+-- * Deciding what to do: making an 'ElaboratedInstallPlan'
+------------------------------------------------------------------------------
+
 -- | Return an up-to-date elaborated install plan.
 --
 -- Two variants of the install plan are returned: with and without packages

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

changelog.d/pr-8949

@@ -0,0 +1,11 @@
+synopsis: Warn when project configuration options are going to be ignored.
+packages: cabal-install
+prs: #8949
+
+description: {
+
+Some project configuration options can only be specified from the command line.
+If the user specified those options from a project file, cabal-install would
+silently ignore them. Now cabal-install will emit a warning.
+
+}

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
@@ -297,9 +297,9 @@ package, and thus apply globally:
     :ref:`project-file<cmdoption-project-file>` path is also specified,
     it will be resolved relative to this directory.
 
-    The project directory need not contain a ``cabal.project`` file.
+    The project directory does not need to contain a ``cabal.project`` file.
 
-    This option cannot be specified via a ``cabal.project`` file.
+    This option can only be specified from the command line.
 
 .. _cmdoption-project-file:
 .. option:: --project-file=FILE
@@ -318,7 +318,7 @@ package, and thus apply globally:
     and then for the parent directory, until the project file is
     found or we have hit the top of the user's home directory.
 
-    This option cannot be specified via a ``cabal.project`` file.
+    This option can only be specified from the command line.
 
 .. option:: --ignore-project
 
@@ -880,21 +880,7 @@ feature was added.
     The command line variant of this flag is ``--enable-benchmarks`` and
     ``--disable-benchmarks``.
 
-.. cfg-field:: multi-repl: boolean
-               --enable-multi-repl
-               --disable-multi-repl
-    :synopsis: Enable starting a repl with multiple targets.
-
-    :default: ``False``
-
-    Allow starting GHCi with multiple targets. This requires GHC with multiple
-    home unit support (GHC-9.4+).
-
-    The closure of required components will be loaded.
-
-    The command line variant of this flag is ``--enable-multi-repl`` and
-    ``--disable-multi-repl``.
-
+.. _cmdoption-extra-prog-path:
 .. cfg-field:: extra-prog-path: paths (newline or comma separated)
                --extra-prog-path=PATH
     :synopsis: Add directories to program search path.
@@ -909,6 +895,10 @@ feature was added.
     The command line variant of this flag is ``--extra-prog-path=PATH``,
     which can be specified multiple times.
 
+    When specifying :ref:`--http-transport<cmdoption-http-transport>` from the
+    command line, only extra-prog-path from the command line are added to the
+    program search path.
+
 .. cfg-field:: run-tests: boolean
                --run-tests
     :synopsis: Run package test suite upon installation.
@@ -1587,7 +1577,7 @@ Advanced global configuration options
         The format and fields of the generated build information is currently experimental,
         in the future we might add or remove fields, depending on the needs of other tooling.
 
-
+.. _cmdoption-http-transport:
 .. cfg-field:: http-transport: curl, wget, powershell, or plain-http
                --http-transport=transport
     :synopsis: Transport to use with http(s) requests.
@@ -1598,6 +1588,12 @@ Advanced global configuration options
 
     The command line variant of this field is ``--http-transport=curl``.
 
+    If the project configuration imports remote urls, the user can only specify
+    the http-transport option from the command line.
+
+    When specifying the http-transport from the command line, the program
+    search path can only be influenced using :ref:`--extra-prog-path<cmdoption-extra-prog-path>`.
+
 .. cfg-field:: ignore-expiry: boolean
                --ignore-expiry
     :synopsis: Ignore Hackage expiration dates.