Relax extension .c requirement for c-sources

haskell#9200 started filtering `.cc` files from `c-sources` and does not always issue a warning (there has to be at least one `.c` file present before it warns you it is ignoring the others).

Some packages in hackage (`double-conversion` for instance) rely on the ability to include `C++` source files in `c-sources`.  This is not ideal, but we should probably continue to support these packages.

This change will relax the filtering, so that only `.h` files are automatically excluded (with a warning).  It will also warn if other non `.c` files are present (suggesting the `cxx-sources` since C++ sources are the most likely to be used).

The bug that prevented warnings being displayed when no `.c` files were present is fixed.

Cabal/src/Distribution/Simple/GHC.hs

@@ -705,19 +705,12 @@ buildOrReplLib mReplFlags verbosity numJobs pkg_descr lbi lib clbi = do
       | filename <- cxxSources libBi]
 
   -- build any C sources
-  let (cSrcs', others) = partition (\filepath -> ".c"`isSuffixOf` filepath) (cSources libBi)
+  let libraryName = case libName lib of
+          LMainLibName -> "the main library"
+          LSubLibName name -> "library " <> prettyShow name
+  cSrcs' <- checkCSources verbosity libraryName (cSources libBi)
   unless (not has_code || null cSrcs') $ do
     info verbosity "Building C Sources..."
-    unless (null others) $ do
-      let files = intercalate ", " others
-      let libraryName = case libName lib of
-            LMainLibName -> "the main library"
-            LSubLibName name -> "library " <> prettyShow name
-      warn verbosity $ unlines
-        [ "The following files listed in " <> libraryName <> "'s c-sources will not be used: " <> files <> "."
-        , "Header files should be in the 'include' or 'install-include' stanza."
-        , "See https://cabal.readthedocs.io/en/3.10/cabal-package.html#pkg-field-includes"
-        ]
     forM_ cSrcs' $ \filename -> do
        let baseCcOpts    = Internal.componentCcGhcOptions verbosity implInfo
                            lbi libBi clbi relLibTargetDir filename
@@ -1537,17 +1530,9 @@ gbuild verbosity numJobs pkg_descr lbi bm clbi = do
      | filename <- cxxSrcs ]
 
   -- build any C sources
-  let (cSrcs', others) = partition (\filepath -> ".c"`isSuffixOf` filepath) cSrcs
+  cSrcs' <- checkCSources verbosity (gbuildName bm) cSrcs
   unless (null cSrcs') $ do
     info verbosity "Building C Sources..."
-    unless (null others) $ do
-      let files = intercalate ", " others
-      let currentComponentName = gbuildName bm
-      warn verbosity $ unlines
-        [ "The following files listed in " <> currentComponentName <> "'s c-sources will not be used: " <> files <> "."
-        , "Header files should be in the 'include' or 'install-include' stanza."
-        , "See https://cabal.readthedocs.io/en/3.10/cabal-package.html#pkg-field-includes"
-        ]
     forM_ cSrcs' $ \filename -> do
       let baseCcOpts    = Internal.componentCcGhcOptions verbosity implInfo
                               lbi bnfo clbi tmpDir filename
@@ -2184,3 +2169,23 @@ supportsDynamicToo = Internal.ghcLookupProperty "Support dynamic-too"
 
 withExt :: FilePath -> String -> FilePath
 withExt fp ext = fp <.> if takeExtension fp /= ('.':ext) then ext else ""
+
+checkCSources :: Verbosity -> String -> [String] -> IO [String]
+checkCSources verbosity name cSrcs = do
+  let (headers, cSrcs') = partition (\filepath -> ".h" `isSuffixOf` filepath) cSrcs
+      others = filter (\filepath -> not (".c" `isSuffixOf` filepath)) cSrcs'
+  unless (null headers) $ do
+    let files = intercalate ", " headers
+    warn verbosity $ unlines
+      [ "The following header files listed in " <> name <> "'s c-sources will not be used: " <> files <> "."
+      , "Header files should be in the 'include' or 'install-include' stanza."
+      , "See https://cabal.readthedocs.io/en/3.10/cabal-package.html#pkg-field-includes"
+      ]
+  unless (null others) $ do
+    let files = intercalate ", " others
+    warn verbosity $ unlines
+      [ "The following files listed in " <> name <> "'s c-sources do not have the expected '.c' extension " <> files <> "."
+      , "C++ files should be in the 'cxx-sources' stanza."
+      , "See https://cabal.readthedocs.io/en/3.10/cabal-package.html#pkg-field-cxx-sources"
+      ]
+  return cSrcs'

cabal-testsuite/PackageTests/CSourcesSanitisation/build.out

@@ -5,21 +5,38 @@ In order, the following will be built:
  - repro-0.1.0.0 (lib) (first run)
  - repro-0.1.0.0 (exe:exec1) (first run)
  - repro-0.1.0.0 (lib:lib2) (first run)
+ - repro-0.1.0.0 (lib:lib3) (first run)
+ - repro-0.1.0.0 (lib:lib4) (first run)
 Configuring library for repro-0.1.0.0..
 Preprocessing library for repro-0.1.0.0..
 Building library for repro-0.1.0.0..
-Warning: The following files listed in the main library's c-sources will not be used: cbits/gwinsz.h.
+Warning: The following header files listed in the main library's c-sources will not be used: cbits/gwinsz.h.
 Header files should be in the 'include' or 'install-include' stanza.
 See https://cabal.readthedocs.io/en/3.10/cabal-package.html#pkg-field-includes
 Configuring executable 'exec1' for repro-0.1.0.0..
 Preprocessing executable 'exec1' for repro-0.1.0.0..
 Building executable 'exec1' for repro-0.1.0.0..
-Warning: The following files listed in exec1's c-sources will not be used: cbits/gwinsz.h.
+Warning: The following header files listed in exec1's c-sources will not be used: cbits/gwinsz.h.
 Header files should be in the 'include' or 'install-include' stanza.
 See https://cabal.readthedocs.io/en/3.10/cabal-package.html#pkg-field-includes
 Configuring library 'lib2' for repro-0.1.0.0..
 Preprocessing library 'lib2' for repro-0.1.0.0..
 Building library 'lib2' for repro-0.1.0.0..
-Warning: The following files listed in library lib2's c-sources will not be used: cbits/gwinsz.h.
+Warning: The following header files listed in library lib2's c-sources will not be used: cbits/gwinsz.h.
 Header files should be in the 'include' or 'install-include' stanza.
 See https://cabal.readthedocs.io/en/3.10/cabal-package.html#pkg-field-includes
+Configuring library 'lib3' for repro-0.1.0.0..
+Preprocessing library 'lib3' for repro-0.1.0.0..
+Building library 'lib3' for repro-0.1.0.0..
+Warning: The following header files listed in library lib3's c-sources will not be used: cbits/gwinsz.h.
+Header files should be in the 'include' or 'install-include' stanza.
+See https://cabal.readthedocs.io/en/3.10/cabal-package.html#pkg-field-includes
+Warning: The following files listed in library lib3's c-sources do not have the expected '.c' extension cbits/gwinsz.cc.
+C++ files should be in the 'cxx-sources' stanza.
+See https://cabal.readthedocs.io/en/3.10/cabal-package.html#pkg-field-cxx-sources
+Configuring library 'lib4' for repro-0.1.0.0..
+Preprocessing library 'lib4' for repro-0.1.0.0..
+Building library 'lib4' for repro-0.1.0.0..
+Warning: The following files listed in library lib4's c-sources do not have the expected '.c' extension cbits/gwinsz.cc.
+C++ files should be in the 'cxx-sources' stanza.
+See https://cabal.readthedocs.io/en/3.10/cabal-package.html#pkg-field-cxx-sources

cabal-testsuite/PackageTests/CSourcesSanitisation/repro.cabal

@@ -15,6 +15,17 @@ library lib2
                       cbits/gwinsz.c
     build-depends:    base
 
+library lib3
+    default-language: Haskell2010
+    c-sources:        cbits/gwinsz.h
+                      cbits/gwinsz.cc
+    build-depends:    base
+
+library lib4
+    default-language: Haskell2010
+    c-sources:        cbits/gwinsz.cc
+    build-depends:    base
+
 executable exec1
     main-is: Main.hs
     default-language: Haskell2010