Adds virtual-modules to handle cases like GHC.Prim from ghc-prim.

Cabal/Distribution/PackageDescription/FieldGrammar.hs

@@ -375,6 +375,7 @@ buildInfoFieldGrammar = BuildInfo
     <*> monoidalFieldAla "js-sources"           (alaList' VCat FilePathNT)    L.jsSources
     <*> hsSourceDirsGrammar
     <*> monoidalFieldAla "other-modules"        (alaList' VCat MQuoted)       L.otherModules
+    <*> monoidalFieldAla "virtual-modules"      (alaList' VCat MQuoted)       L.virtualModules
     <*> monoidalFieldAla "autogen-modules"      (alaList' VCat MQuoted)       L.autogenModules
     <*> optionalFieldAla "default-language"     MQuoted                       L.defaultLanguage
     <*> monoidalFieldAla "other-languages"      (alaList' FSep MQuoted)       L.otherLanguages

Cabal/Distribution/PackageDescription/Parse.hs

@@ -523,6 +523,9 @@ binfoFieldDescrs =
  , listFieldWithSep vcat "other-modules"
            disp               parseModuleNameQ
            otherModules       (\val binfo -> binfo{otherModules=val})
+ , listFieldWithSep vcat "virtual-modules"
+           disp               parseModuleNameQ
+           virtualModules       (\val binfo -> binfo{virtualModules=val})
  , listFieldWithSep vcat "autogen-modules"
            disp               parseModuleNameQ
            autogenModules       (\val binfo -> binfo{autogenModules=val})

Cabal/Distribution/Simple/Register.hs

@@ -420,7 +420,10 @@ generalInstalledPackageInfo adjustRelIncDirs pkg abi_hash lib lbi clbi installDi
     IPI.abiHash            = abi_hash,
     IPI.indefinite         = componentIsIndefinite clbi,
     IPI.exposed            = libExposed  lib,
-    IPI.exposedModules     = componentExposedModules clbi,
+    IPI.exposedModules     = componentExposedModules clbi
+                             -- add virtual modules into the list of exposed modules for the
+                             -- package database as well.
+                             ++ map (\name -> IPI.ExposedModule name Nothing) (virtualModules bi),
     IPI.hiddenModules      = otherModules bi,
     IPI.trusted            = IPI.trusted IPI.emptyInstalledPackageInfo,
     IPI.importDirs         = [ libdir installDirs | hasModules ],

Cabal/Distribution/Types/BuildInfo.hs

@@ -66,6 +66,7 @@ data BuildInfo = BuildInfo {
         jsSources         :: [FilePath],
         hsSourceDirs      :: [FilePath], -- ^ where to look for the Haskell module hierarchy
         otherModules      :: [ModuleName], -- ^ non-exposed or non-main modules
+        virtualModules    :: [ModuleName], -- ^ exposed modules that do not have a source file (e.g. @GHC.Prim@ from @ghc-prim@ package)
         autogenModules    :: [ModuleName], -- ^ not present on sdist, Paths_* or user-generated with a custom Setup.hs
 
         defaultLanguage   :: Maybe Language,-- ^ language used when not explicitly specified
@@ -126,6 +127,7 @@ instance Monoid BuildInfo where
     jsSources           = [],
     hsSourceDirs        = [],
     otherModules        = [],
+    virtualModules      = [],
     autogenModules      = [],
     defaultLanguage     = Nothing,
     otherLanguages      = [],
@@ -171,6 +173,7 @@ instance Semigroup BuildInfo where
     jsSources           = combineNub jsSources,
     hsSourceDirs        = combineNub hsSourceDirs,
     otherModules        = combineNub otherModules,
+    virtualModules      = combineNub virtualModules,
     autogenModules      = combineNub autogenModules,
     defaultLanguage     = combineMby defaultLanguage,
     otherLanguages      = combineNub otherLanguages,

Cabal/Distribution/Types/BuildInfo/Lens.hs

@@ -99,6 +99,10 @@ class HasBuildInfo a where
    otherModules = buildInfo . otherModules
    {-# INLINE otherModules #-}
 
+   virtualModules :: Lens' a [ModuleName]
+   virtualModules = buildInfo . virtualModules
+   {-# INLINE virtualModules #-}
+
    autogenModules :: Lens' a [ModuleName]
    autogenModules = buildInfo . autogenModules
    {-# INLINE autogenModules #-}
@@ -245,6 +249,9 @@ instance HasBuildInfo BuildInfo where
     otherModules f s = fmap (\x -> s { T.otherModules = x }) (f (T.otherModules s))
     {-# INLINE otherModules #-}
 
+    virtualModules f s = fmap (\x -> s { T.virtualModules = x }) (f (T.virtualModules s))
+    {-# INLINE virtualModules #-}
+
     autogenModules f s = fmap (\x -> s { T.autogenModules = x }) (f (T.autogenModules s))
     {-# INLINE autogenModules #-}
 

Cabal/changelog

@@ -1,6 +1,8 @@
 -*-change-log-*-
 
 2.2.0.0 (current development version)
+	* Added `virtual-module` field, to allow modules that are not built
+	  but registered (#4875).
 	* `copyComponent` and `installIncludeFiles` will look for include
 	  headers in the build directory ('dist/build/...' by default)
 	  as well (#4866).

Cabal/doc/developing-packages.rst

@@ -1010,6 +1010,15 @@ The library section should contain the following fields:
 
     A list of modules added by this package.
 
+.. pkg-field:: virtual-modules: identifier list
+
+    A list of virtual modules provided by this package.  Virtual modules
+    are modules without a source file.  See for example the ``GHC.Prim``
+    module from the ``ghc-prim`` package.  Modules listed here will not be
+    built, but still end up in the list of ``exposed-modules`` in the
+    installed package info when the package is registered in the package
+    database.
+
 .. pkg-field:: exposed: boolean
 
     :default: ``True``