support virtual_modules

the same as Cabal virtual-modules. This is necessary to support ghc-prim compilation.

decls/haskell_common.bzl

@@ -76,6 +76,13 @@ def _use_argsfile_at_link_arg():
 """),
     }
 
+def _virtual_modules_arg():
+    return {
+        "virtual_modules": attrs.named_set(attrs.string(), sorted = True, default = [], doc = """
+    A list of virtual Haskell modules. See Cabal virtual-modules.
+"""),
+    }
+
 haskell_common = struct(
     srcs_arg = _srcs_arg,
     deps_arg = _deps_arg,
@@ -85,4 +92,5 @@ haskell_common = struct(
     external_tools_arg = _external_tools_arg,
     srcs_envs_arg = _srcs_envs_arg,
     use_argsfile_at_link_arg = _use_argsfile_at_link_arg,
+    virtual_modules_arg = _virtual_modules_arg,
 )

decls/haskell_rules.bzl

@@ -170,6 +170,7 @@ haskell_library = prelude_rule(
         haskell_common.external_tools_arg() |
         haskell_common.srcs_envs_arg() |
         haskell_common.use_argsfile_at_link_arg() |
+        haskell_common.virtual_modules_arg() |
         haskell_common.compiler_flags_arg() |
         haskell_common.deps_arg() |
         haskell_common.scripts_arg() |

haskell/haskell.bzl

@@ -455,6 +455,7 @@ def _make_package(
             for module in md["module_graph"].keys()
             if not module.endswith("-boot")
         ]
+        virtual_modules = ctx.attrs.virtual_modules
 
         # XXX use a single import dir when this package db is used for resolving dependencies with ghc -M,
         #     which works around an issue with multiple import dirs resulting in GHC trying to locate interface files
@@ -469,7 +470,7 @@ def _make_package(
             "id: " + pkgname,
             "key: " + pkgname,
             "exposed: False",
-            "exposed-modules: " + ", ".join(modules),
+            "exposed-modules: " + ", ".join(modules + virtual_modules),
             "import-dirs:" + ", ".join(import_dirs),
             "depends: " + ", ".join([lib.id for lib in hlis]),
         ]