cabal: add empty_lib to stack snapshot components

Empty haskell libraries have been a longstanding problem. Add support
for an additional component type `empty_lib`, which is like `lib`,
except that an additional `haskell_cabal_args` target will be created
for the library, instructing Bazel not to look for any object files.

Remove the longstanding package blacklist for empty packages. Replace it
with the now possible correct entries for those libraries, indicating
that the main library is empty.

Author: iteratee

haskell/cabal.bzl

@@ -126,21 +126,6 @@ main = defaultMain
 
 _CABAL_TOOLS = ["alex", "c2hs", "cpphs", "doctest", "happy"]
 
-# Some old packages are empty compatibility shims. Empty packages
-# cause Cabal to not produce the outputs it normally produces. Instead
-# of detecting that, we blacklist the offending packages, on the
-# assumption that such packages are old and rare.
-#
-# TODO: replace this with a more general solution.
-_EMPTY_PACKAGES_BLACKLIST = [
-    "bytestring-builder",
-    "fail",
-    "ghc-byteorder",
-    "haskell-gi-overloading",
-    "mtl-compat",
-    "nats",
-]
-
 def _cabal_tool_flag(tool):
     """Return a --with-PROG=PATH flag if input is a recognized Cabal tool. None otherwise."""
     if tool.basename in _CABAL_TOOLS:
@@ -1133,8 +1118,10 @@ def _parse_components(package, components, spec):
         exe: list of string, List of executables.
     """
     lib = False
+    empty_lib = False
     exe = []
     sublibs = []
+    empty_sublibs = []
 
     for component in components:
         if component == "lib":
@@ -1144,6 +1131,15 @@ def _parse_components(package, components, spec):
                 lib = True
             else:
                 sublibs.append(component[4:])
+        elif component == "empty_lib":
+            lib = True
+            empty_lib = True
+        elif component.startswith("empty_lib:"):
+            if component == "empty_lib:%s" % package:
+                lib = True
+                empty_lib = True
+            else:
+                empty_sublibs.append(component[4:])
 
         elif component == "exe":
             exe.append(package)
@@ -1158,14 +1154,21 @@ def _parse_components(package, components, spec):
         if not lib or exe != []:
             fail("Invalid core package components: %s" % package, "components")
 
-    return struct(lib = lib, exe = exe, sublibs = sublibs)
+    return struct(lib = lib, empty_lib = empty_lib, exe = exe, sublibs = sublibs, empty_sublibs = empty_sublibs)
 
 _default_components = {
-    "alex": struct(lib = False, exe = ["alex"], sublibs = []),
-    "c2hs": struct(lib = False, exe = ["c2hs"], sublibs = []),
-    "cpphs": struct(lib = True, exe = ["cpphs"], sublibs = []),
-    "doctest": struct(lib = True, exe = ["doctest"], sublibs = []),
-    "happy": struct(lib = False, exe = ["happy"], sublibs = []),
+    "alex": struct(lib = False, empty_lib = False, exe = ["alex"], sublibs = [], empty_sublibs = []),
+    "c2hs": struct(lib = False, empty_lib = False, exe = ["c2hs"], sublibs = [], empty_sublibs = []),
+    "cpphs": struct(lib = True, empty_lib = False, exe = ["cpphs"], sublibs = [], empty_sublibs = []),
+    "doctest": struct(lib = True, empty_lib = False, exe = ["doctest"], sublibs = [], empty_sublibs = []),
+    "happy": struct(lib = False, empty_lib = False, exe = ["happy"], sublibs = [], empty_sublibs = []),
+    # Below are compatibility libraries that produce an empty cabal library.
+    "bytestring-builder": struct(lib = True, empty_lib = True, exe = [], sublibs = [], empty_sublibs = []),
+    "fail": struct(lib = True, empty_lib = True, exe = [], sublibs = [], empty_sublibs = []),
+    "ghc-byteorder": struct(lib = True, empty_lib = True, exe = [], sublibs = [], empty_sublibs = []),
+    "haskell-gi-overloading": struct(lib = True, empty_lib = True, exe = [], sublibs = [], empty_sublibs = []),
+    "mtl-compat": struct(lib = True, empty_lib = True, exe = [], sublibs = [], empty_sublibs = []),
+    "nats": struct(lib = True, empty_lib = True, exe = [], sublibs = [], empty_sublibs = []),
 }
 
 def _get_components(components, package):
@@ -1175,7 +1178,7 @@ def _get_components(components, package):
     will be taken from the `_default_components`. If it is not listed
     there then it will default to a library and no executable components.
     """
-    return components.get(package, _default_components.get(package, struct(lib = True, exe = [], sublibs = [])))
+    return components.get(package, _default_components.get(package, struct(lib = True, empty_lib = False, exe = [], sublibs = [], empty_sublibs = [])))
 
 def _parse_json_field(json, field, ty, errmsg):
     """Read and type-check a field from a JSON object.
@@ -2054,7 +2057,7 @@ packages = {
     # Write out the dependency graph as a BUILD file.
     build_file_builder = []
     build_file_builder.append("""
-load("@rules_haskell//haskell:cabal.bzl", "haskell_cabal_binary", "haskell_cabal_library")
+load("@rules_haskell//haskell:cabal.bzl", "haskell_cabal_args", "haskell_cabal_binary", "haskell_cabal_library")
 load("@rules_haskell//haskell:defs.bzl", "haskell_library", "haskell_toolchain_library")
 """)
     for (name, spec) in resolved.items():
@@ -2075,20 +2078,6 @@ alias(name = "{name}", actual = "{actual}", visibility = {visibility})
 haskell_toolchain_library(name = "{name}", visibility = {visibility})
 """.format(name = name, visibility = visibility),
             )
-        elif name in _EMPTY_PACKAGES_BLACKLIST:
-            build_file_builder.append(
-                """
-haskell_library(
-    name = "{name}",
-    version = "{version}",
-    visibility = {visibility},
-)
-""".format(
-                    name = name,
-                    version = version,
-                    visibility = visibility,
-                ),
-            )
         else:
             library_deps = [
                 dep
@@ -2119,6 +2108,26 @@ haskell_library(
                 )).relative(label))
                 for label in repository_ctx.attr.setup_deps.get(name, [])
             ]
+
+            wrote_cabal_args = False
+            cabal_args = "cabal_args = \"_{name}_cabal_args\",".format(name = name)
+            def add_cabal_args(written):
+                if not written:
+                    build_file_builder.append(
+                        """
+haskell_cabal_args(
+    name = "_{name}_cabal_args",
+    is_empty = True,
+)
+""".format(name = name)
+                    )
+                return True
+
+            lib_cabal_args = ""
+            if all_components[name].empty_lib:
+                wrote_cabal_args = add_cabal_args(wrote_cabal_args)
+                lib_cabal_args = cabal_args
+
             if all_components[name].lib:
                 build_file_builder.append(
                     """
@@ -2134,6 +2143,7 @@ haskell_cabal_library(
     visibility = {visibility},
     cabalopts = ["--ghc-option=-w", "--ghc-option=-optF=-w"],
     verbose = {verbose},
+    {lib_cabal_args}
     unique_name = True,
 )
 """.format(
@@ -2147,6 +2157,7 @@ haskell_cabal_library(
                         tools = library_tools,
                         visibility = visibility,
                         verbose = repr(repository_ctx.attr.verbose),
+                        lib_cabal_args = lib_cabal_args
                     ),
                 )
                 build_file_builder.append(
@@ -2224,6 +2235,46 @@ haskell_cabal_library(
                         visibility = visibility,
                     ),
                 )
+            for sublib in all_components[name].empty_sublibs:
+                sublib_component_deps = [
+                    _resolve_component_target_name(name, c)
+                    for c in package_components_dependencies.get("lib:{}".format(sublib), [])
+                ]
+                wrote_cabal_args = add_cabal_args(wrote_cabal_args)
+                build_file_builder.append(
+                    """
+haskell_cabal_library(
+    name = "_{name}_lib_{sublib}",
+    package_name = "{name}",
+    version = "{version}",
+    haddock = {haddock},
+    sublibrary_name = "{sublib}",
+    flags = {flags},
+    srcs = glob(["{dir}/**"]),
+    deps = {deps},
+    setup_deps = {setup_deps},
+    tools = {tools},
+    visibility = {visibility},
+    cabalopts = ["--ghc-option=-w", "--ghc-option=-optF=-w"],
+    verbose = {verbose},
+    {cabal_args},
+)
+""".format(
+                        name = name,
+                        version = version,
+                        haddock = repr(repository_ctx.attr.haddock),
+                        sublib = sublib,
+                        flags = repository_ctx.attr.flags.get(name, []),
+                        dir = package,
+                        deps = library_deps + sublib_component_deps,
+                        setup_deps = setup_deps,
+                        tools = library_tools,
+                        verbose = repr(repository_ctx.attr.verbose),
+                        visibility = visibility,
+                        cabal_args = cabal_args,
+                    ),
+                )
+
     build_file_content = "\n".join(build_file_builder)
     repository_ctx.file("BUILD.bazel", build_file_content, executable = False)