feat: allow py_cc_toolchain libs to be optional

CHANGELOG.md

@@ -28,6 +28,9 @@ A brief description of the categories of changes:
 * (gazelle): Update error messages when unable to resolve a dependency to be more human-friendly.
 * (flags) The {obj}`--python_version` flag now also returns
   {obj}`config_common.FeatureFlagInfo`.
+* (toolchains) {obj}`py_cc_toolchain.libs` and {obj}`PyCcToolchainInfo.libs` is
+  optional. This is to support situations where only the Python headers are
+  available.
 
 ### Fixed
 * (whl_library): Remove `--no-index` and add `--no-build-isolation` to the

python/private/py_cc_toolchain_info.bzl

@@ -41,9 +41,9 @@ Information about the header files, struct with fields:
     represents).
 """,
         "libs": """\
-:type: struct
+:type: struct | None
 
-Information about C libraries, struct with fields:
+If available, information about C libraries, struct with fields:
   * providers_map: A dict of string to provider instances. The key should be
     a fully qualified name (e.g. `@rules_foo//bar:baz.bzl#MyInfo`) of the
     provider to uniquely identify its type.

python/private/py_cc_toolchain_rule.bzl

@@ -23,19 +23,24 @@ load("@rules_cc//cc:defs.bzl", "CcInfo")
 load(":py_cc_toolchain_info.bzl", "PyCcToolchainInfo")
 
 def _py_cc_toolchain_impl(ctx):
+    if ctx.attr.libs:
+        libs = struct(
+            providers_map = {
+                "CcInfo": ctx.attr.libs[CcInfo],
+                "DefaultInfo": ctx.attr.libs[DefaultInfo],
+            },
+        )
+    else:
+        libs = None
+
     py_cc_toolchain = PyCcToolchainInfo(
         headers = struct(
             providers_map = {
                 "CcInfo": ctx.attr.headers[CcInfo],
                 "DefaultInfo": ctx.attr.headers[DefaultInfo],
             },
         ),
-        libs = struct(
-            providers_map = {
-                "CcInfo": ctx.attr.libs[CcInfo],
-                "DefaultInfo": ctx.attr.libs[DefaultInfo],
-            },
-        ),
+        libs = libs,
         python_version = ctx.attr.python_version,
     )
     extra_kwargs = {}
@@ -59,7 +64,6 @@ py_cc_toolchain = rule(
             doc = ("Target that provides the Python runtime libraries for linking. " +
                    "Typically this is a cc_library target of `.so` files."),
             providers = [CcInfo],
-            mandatory = True,
         ),
         "python_version": attr.string(
             doc = "The Major.minor Python version, e.g. 3.11",

tests/cc/py_cc_toolchain/py_cc_toolchain_tests.bzl

@@ -16,15 +16,16 @@
 
 load("@rules_testing//lib:analysis_test.bzl", "analysis_test", "test_suite")
 load("@rules_testing//lib:truth.bzl", "matching", "subjects")
+load("//python/cc:py_cc_toolchain.bzl", "py_cc_toolchain")
 load("//tests/support:cc_info_subject.bzl", "cc_info_subject")
 load("//tests/support:py_cc_toolchain_info_subject.bzl", "PyCcToolchainInfoSubject")
 
 _tests = []
 
-def _py_cc_toolchain_test(name):
+def _test_py_cc_toolchain(name):
     analysis_test(
         name = name,
-        impl = _py_cc_toolchain_test_impl,
+        impl = _test_py_cc_toolchain_impl,
         target = "//tests/support/cc_toolchains:fake_py_cc_toolchain_impl",
         attrs = {
             "header": attr.label(
@@ -34,7 +35,7 @@ def _py_cc_toolchain_test(name):
         },
     )
 
-def _py_cc_toolchain_test_impl(env, target):
+def _test_py_cc_toolchain_impl(env, target):
     env.expect.that_target(target).has_provider(platform_common.ToolchainInfo)
 
     toolchain = PyCcToolchainInfoSubject.new(
@@ -80,7 +81,26 @@ def _py_cc_toolchain_test_impl(env, target):
         matching.str_matches("/libpython3."),
     )
 
-_tests.append(_py_cc_toolchain_test)
+_tests.append(_test_py_cc_toolchain)
+
+def _test_libs_optional(name):
+    py_cc_toolchain(
+        name = name + "_subject",
+        libs = None,
+        headers = "//tests/support/cc_toolchains:fake_headers",
+        python_version = "4.5",
+    )
+    analysis_test(
+        name = name,
+        target = name + "_subject",
+        impl = _test_libs_optional_impl,
+    )
+
+def _test_libs_optional_impl(env, target):
+    libs = target[platform_common.ToolchainInfo].py_cc_toolchain.libs
+    env.expect.that_bool(libs == None).equals(True)
+
+_tests.append(_test_libs_optional)
 
 def py_cc_toolchain_test_suite(name):
     test_suite(