Add $libexecsubdir InstallDirs variable

Cabal/Distribution/Simple/InstallDirs.hs

@@ -85,6 +85,7 @@ data InstallDirs dir = InstallDirs {
         dynlibdir    :: dir,
         flibdir      :: dir, -- ^ foreign libraries
         libexecdir   :: dir,
+        libexecsubdir:: dir,
         includedir   :: dir,
         datadir      :: dir,
         datasubdir   :: dir,
@@ -116,6 +117,7 @@ combineInstallDirs combine a b = InstallDirs {
     dynlibdir    = dynlibdir a  `combine` dynlibdir b,
     flibdir      = flibdir a    `combine` flibdir b,
     libexecdir   = libexecdir a `combine` libexecdir b,
+    libexecsubdir= libexecsubdir a `combine` libexecsubdir b,
     includedir   = includedir a `combine` includedir b,
     datadir      = datadir a    `combine` datadir b,
     datasubdir   = datasubdir a `combine` datasubdir b,
@@ -129,8 +131,10 @@ combineInstallDirs combine a b = InstallDirs {
 appendSubdirs :: (a -> a -> a) -> InstallDirs a -> InstallDirs a
 appendSubdirs append dirs = dirs {
     libdir     = libdir dirs `append` libsubdir dirs,
+    libexecdir = libexecdir dirs `append` libexecsubdir dirs,
     datadir    = datadir dirs `append` datasubdir dirs,
     libsubdir  = error "internal error InstallDirs.libsubdir",
+    libexecsubdir = error "internal error InstallDirs.libexecsubdir",
     datasubdir = error "internal error InstallDirs.datasubdir"
   }
 
@@ -200,6 +204,7 @@ defaultInstallDirs' False comp userInstall _hasLibs = do
            LHC    -> "$compiler"
            UHC    -> "$pkgid"
            _other -> "$abi",
+      libexecsubdir= "$abi" </> "$pkgid",
       flibdir      = "$libdir",
       libexecdir   = case buildOS of
         Windows   -> "$prefix" </> "$libname"
@@ -244,6 +249,7 @@ substituteInstallDirTemplates env dirs = dirs'
       dynlibdir  = subst dynlibdir  [prefixVar, bindirVar, libdirVar],
       flibdir    = subst flibdir    [prefixVar, bindirVar, libdirVar],
       libexecdir = subst libexecdir prefixBinLibVars,
+      libexecsubdir = subst libexecsubdir [],
       includedir = subst includedir prefixBinLibVars,
       datadir    = subst datadir    prefixBinLibVars,
       datasubdir = subst datasubdir [],

Cabal/Distribution/Simple/Setup.hs

@@ -927,6 +927,11 @@ installDirsOptions =
       libexecdir (\v flags -> flags { libexecdir = v })
       installDirArg
 
+  , option "" ["libexecsubdir"]
+      "subdirectory of libexecdir in which private executables are installed"
+      libexecsubdir (\v flags -> flags { libexecsubdir = v })
+      installDirArg
+
   , option "" ["datadir"]
       "installation directory for read-only data"
       datadir (\v flags -> flags { datadir = v })

Cabal/doc/installing-packages.rst

@@ -656,6 +656,14 @@ path options:
     ``$pkg``, ``$version``, ``$compiler``, ``$os``, ``$arch``, ``$abi``,
     ``$abitag``
 
+.. option:: --libexecsubdir=dir
+
+    A subdirectory of *libexecdir* in which private executables are installed.
+
+    *dir* may contain the following path variables: ``$pkgid``,
+    ``$pkg``, ``$version``, ``$compiler``, ``$os``, ``$arch``, ``$abi``,
+    ``$abitag``
+
 .. option:: --datasubdir=dir
 
     A subdirectory of *datadir* in which data files are actually

cabal-install/Distribution/Client/ProjectPlanning.hs

@@ -1420,9 +1420,12 @@ elaborateInstallPlan verbosity platform compiler compilerprogdb pkgConfigDB
                    platform
                    defaultInstallDirs) {
 
-                  InstallDirs.libsubdir  = "", -- absoluteInstallDirs sets these as
-                  InstallDirs.datasubdir = ""  -- 'undefined' but we have to use
-                }                              -- them as "Setup.hs configure" args
+                  -- absoluteInstallDirs sets these as 'undefined' but we have
+                  -- to use them as "Setup.hs configure" args
+                  InstallDirs.libsubdir  = "",
+                  InstallDirs.libexecsubdir  = "",
+                  InstallDirs.datasubdir = ""
+                }
 
               | otherwise
               -- use special simplified install dirs
@@ -1567,9 +1570,12 @@ elaborateInstallPlan verbosity platform compiler compilerprogdb pkgConfigDB
                platform
                defaultInstallDirs) {
 
-              InstallDirs.libsubdir  = "", -- absoluteInstallDirs sets these as
-              InstallDirs.datasubdir = ""  -- 'undefined' but we have to use
-            }                              -- them as "Setup.hs configure" args
+              -- absoluteInstallDirs sets these as 'undefined' but we have to
+              -- use them as "Setup.hs configure" args
+              InstallDirs.libsubdir  = "",
+              InstallDirs.libexecsubdir = "",
+              InstallDirs.datasubdir = ""
+            }
 
           | otherwise
           -- use special simplified install dirs
@@ -2930,6 +2936,7 @@ storePackageInstallDirs StoreDirLayout{storePackageDirectory}
     dynlibdir    = libdir
     flibdir      = libdir
     libexecdir   = prefix </> "libexec"
+    libexecsubdir= ""
     includedir   = libdir </> "include"
     datadir      = prefix </> "share"
     datasubdir   = ""

cabal-install/tests/UnitTests/Distribution/Client/ProjectConfig.hs

@@ -550,7 +550,7 @@ instance Arbitrary a => Arbitrary (InstallDirs a) where
         <$> arbitrary <*> arbitrary <*> arbitrary <*> arbitrary --  4
         <*> arbitrary <*> arbitrary <*> arbitrary <*> arbitrary --  8
         <*> arbitrary <*> arbitrary <*> arbitrary <*> arbitrary -- 12
-        <*> arbitrary <*> arbitrary <*> arbitrary               -- 15
+        <*> arbitrary <*> arbitrary <*> arbitrary <*> arbitrary -- 16
 
 instance Arbitrary PackageDB where
     arbitrary = oneof [ pure GlobalPackageDB