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:
@@ -1166,6 +1151,16 @@ _default_components = {
     "cpphs": struct(lib = True, exe = ["cpphs"], sublibs = []),
     "doctest": struct(lib = True, exe = ["doctest"], sublibs = []),
     "happy": struct(lib = False, exe = ["happy"], sublibs = []),
+    # Below are compatibility libraries that produce an empty cabal library.
+}
+
+_default_components_args = {
+    "bytestring-builder:lib:bytestring-builder": "@rules_haskell//tools/cabal_args:empty_library",
+    "fail:lib:fail": "@rules_haskell//tools/cabal_args:empty_library",
+    "ghc-byteorder:lib:ghc-byteorder": "@rules_haskell//tools/cabal_args:empty_library",
+    "haskell-gi-overloading:lib:haskell-gi-overloading": "@rules_haskell//tools/cabal_args:empty_library",
+    "mtl-compat:lib:mtl-compat": "@rules_haskell//tools/cabal_args:empty_library",
+    "nats:lib:nats": "@rules_haskell//tools/cabal_args:empty_library",
 }
 
 def _get_components(components, package):
@@ -1177,6 +1172,9 @@ def _get_components(components, package):
     """
     return components.get(package, _default_components.get(package, struct(lib = True, exe = [], sublibs = [])))
 
+def _get_components_args(components_args, component):
+    return components_args.get(component, _default_components_args.get(component, None))
+
 def _parse_json_field(json, field, ty, errmsg):
     """Read and type-check a field from a JSON object.
 
@@ -1202,6 +1200,17 @@ def _parse_json_field(json, field, ty, errmsg):
         )))
     return json[field]
 
+def _parse_components_args_key(component):
+    pieces = component.split(':')
+    if len(pieces) == 1:
+        component = '{}:lib:{}'.format(component, component)
+    elif len(pieces) == 2 or (len(pieces) == 3 and pieces[2] == ''):
+        if pieces[1] == 'lib' or pieces[1] == 'exe':
+            component = '{}:{}:{}'.format(pieces[0], pieces[1], pieces[0])
+        else:
+            component = '{}:lib:{}'.format(pieces[0], pieces[1])
+    return component
+
 def _parse_package_spec(package_spec, enable_custom_toolchain_libraries, custom_toolchain_libraries):
     """Parse a package description from `stack ls dependencies json`.
 
@@ -1997,6 +2006,10 @@ def _stack_snapshot_impl(repository_ctx):
         for (name, components) in repository_ctx.attr.components.items()
     }
     all_components = {}
+    user_components_args = {
+        _parse_components_args_key(component): args
+        for (component, args) in repository_ctx.attr.components_args.items()
+    }
     for (name, spec) in resolved.items():
         all_components[name] = _get_components(user_components, name)
         user_components.pop(name, None)
@@ -2075,20 +2088,6 @@ 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 +2118,12 @@ haskell_library(
                 )).relative(label))
                 for label in repository_ctx.attr.setup_deps.get(name, [])
             ]
+
+            lib_args = _get_components_args(user_components_args, '{}:lib:{}'.format(name, name))
+            cabal_args = ""
+            if lib_args != None:
+                cabal_args = "cabal_args = \"{}\",".format(lib_args)
+
             if all_components[name].lib:
                 build_file_builder.append(
                     """
@@ -2134,6 +2139,7 @@ haskell_cabal_library(
     visibility = {visibility},
     cabalopts = ["--ghc-option=-w", "--ghc-option=-optF=-w"],
     verbose = {verbose},
+    {cabal_args}
     unique_name = True,
 )
 """.format(
@@ -2147,6 +2153,7 @@ haskell_cabal_library(
                         tools = library_tools,
                         visibility = visibility,
                         verbose = repr(repository_ctx.attr.verbose),
+                        cabal_args = cabal_args
                     ),
                 )
                 build_file_builder.append(
@@ -2162,6 +2169,10 @@ haskell_cabal_library(
                     for comp in ["exe:{}".format(exe)] + (["exe"] if exe == name else [])
                     for comp_dep in package_components_dependencies.get(comp, [])
                 ]
+                exe_args = _get_components_args(user_components_args, '{}:exe:{}'.format(name, exe))
+                cabal_args = ""
+                if exe_args != None:
+                    cabal_args = "cabal_args = \"{}\",".format(lib_args)
                 build_file_builder.append(
                     """
 haskell_cabal_binary(
@@ -2174,6 +2185,7 @@ haskell_cabal_binary(
     tools = {tools},
     visibility = ["@{workspace}-exe//{name}:__pkg__"],
     cabalopts = ["--ghc-option=-w", "--ghc-option=-optF=-w", "--ghc-option=-static"],
+    {cabal_args}
     verbose = {verbose},
 )
 """.format(
@@ -2185,6 +2197,7 @@ haskell_cabal_binary(
                         deps = library_deps + exe_component_deps + ([name] if all_components[name].lib else []),
                         setup_deps = setup_deps,
                         tools = library_tools,
+                        cabal_args = cabal_args,
                         verbose = repr(repository_ctx.attr.verbose),
                     ),
                 )
@@ -2193,6 +2206,10 @@ haskell_cabal_binary(
                     _resolve_component_target_name(name, c)
                     for c in package_components_dependencies.get("lib:{}".format(sublib), [])
                 ]
+                lib_args = _get_components_args(user_components_args, '{}:lib:{}'.format(name, sublib))
+                cabal_args = ""
+                if lib_args != None:
+                    cabal_args = "cabal_args = \"{}\",".format(lib_args)
                 build_file_builder.append(
                     """
 haskell_cabal_library(
@@ -2208,6 +2225,7 @@ haskell_cabal_library(
     tools = {tools},
     visibility = {visibility},
     cabalopts = ["--ghc-option=-w", "--ghc-option=-optF=-w"],
+    {cabal_args}
     verbose = {verbose},
 )
 """.format(
@@ -2222,8 +2240,10 @@ haskell_cabal_library(
                         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)
 
@@ -2273,6 +2293,7 @@ _stack_snapshot = repository_rule(
         "verbose": attr.bool(default = False),
         "custom_toolchain_libraries": attr.string_list(default = []),
         "enable_custom_toolchain_libraries": attr.bool(default = False),
+        "components_args": attr.string_dict(),
     },
 )
 
@@ -2484,6 +2505,7 @@ def stack_snapshot(
         netrc = "",
         toolchain_libraries = None,
         setup_stack = True,
+        components_args = {},
         label_builder = lambda l: Label(l),
         **kwargs):
     """Use Stack to download and extract Cabal source distributions.
@@ -2743,6 +2765,7 @@ def stack_snapshot(
         tools = tools,
         components = components,
         components_dependencies = components_dependencies,
+        components_args = components_args,
         verbose = verbose,
         custom_toolchain_libraries = toolchain_libraries,
         enable_custom_toolchain_libraries = toolchain_libraries != None,