Make cc_toolchain.(static|dynamic)_runtime_libs attributes optional

Issue for the --incompatible_disable_runtimes_filegroups incompatible flag
(which this cl is a step towards to): #6942
Tracking issue for legacy crosstool fields removal: #5883

RELNOTES: cc_toolchain.(static|dynamic)_runtime_libs attributes are now optional
PiperOrigin-RevId: 225990391

src/main/java/com/google/devtools/build/lib/rules/cpp/CcToolchainProviderHelper.java

@@ -477,6 +477,11 @@ static CcToolchainProvider getCcToolchainProvider(
     final NestedSet<Artifact> staticRuntimeLinkInputs;
     final Artifact staticRuntimeLinkMiddleman;
     if (toolchainInfo.supportsEmbeddedRuntimes()) {
+      if (staticRuntimeLibDep == null) {
+        throw ruleContext.throwWithRuleError(
+            "Toolchain supports embedded runtimes, but didn't "
+                + "provide static_runtime_libs attribute.");
+      }
       staticRuntimeLinkInputs =
           staticRuntimeLibDep.getProvider(FileProvider.class).getFilesToBuild();
     } else {
@@ -505,6 +510,11 @@ static CcToolchainProvider getCcToolchainProvider(
     List<Artifact> dynamicRuntimeLinkInputs = new ArrayList<>();
     Artifact dynamicRuntimeLinkMiddleman;
     if (toolchainInfo.supportsEmbeddedRuntimes()) {
+      if (dynamicRuntimeLibDep == null) {
+        throw ruleContext.throwWithRuleError(
+            "Toolchain supports embedded runtimes, but didn't "
+                + "provide dynamic_runtime_libs attribute.");
+      }
       NestedSetBuilder<Artifact> dynamicRuntimeLinkSymlinksBuilder = NestedSetBuilder.stableOrder();
       for (Artifact artifact :
           dynamicRuntimeLibDep.getProvider(FileProvider.class).getFilesToBuild()) {
@@ -835,6 +845,9 @@ private static CppModuleMap createCrosstoolModuleMap(CcToolchainAttributesProvid
 
   static TransitiveInfoCollection selectDep(
       ImmutableList<? extends TransitiveInfoCollection> deps, Label label) {
+    if (deps.isEmpty()) {
+      return null;
+    }
     for (TransitiveInfoCollection dep : deps) {
       if (dep.getLabel().equals(label)) {
         return dep;

src/main/java/com/google/devtools/build/lib/rules/cpp/CcToolchainRule.java

@@ -221,15 +221,15 @@ Currently unused (<a href="https://github.com/bazelbuild/bazel/issues/6928">#692
         <p>cc_toolchain will select one of these libraries based on the label from
         crosstool_proto.static_runtimes_filegroup field.</p>
         <!-- #END_BLAZE_RULE.ATTRIBUTE -->*/
-        .add(attr("static_runtime_libs", LABEL_LIST).legacyAllowAnyFileType().mandatory())
+        .add(attr("static_runtime_libs", LABEL_LIST).legacyAllowAnyFileType())
         /* <!-- #BLAZE_RULE(cc_toolchain).ATTRIBUTE(dynamic_runtime_libs) -->
         A collection of artifacts for dynamic libraries for the C++ runtime library
         (e.g. libstdc++.so).
 
         <p>cc_toolchain will select one of these libraries based on the label from
         crosstool_proto.dynamic_runtimes_filegroup field.</p>
         <!-- #END_BLAZE_RULE.ATTRIBUTE -->*/
-        .add(attr("dynamic_runtime_libs", LABEL_LIST).legacyAllowAnyFileType().mandatory())
+        .add(attr("dynamic_runtime_libs", LABEL_LIST).legacyAllowAnyFileType())
         /* <!-- #BLAZE_RULE(cc_toolchain).ATTRIBUTE(module_map) -->
         Module map artifact to be used for modular builds.
         <!-- #END_BLAZE_RULE.ATTRIBUTE -->*/

src/test/java/com/google/devtools/build/lib/rules/cpp/CcToolchainProviderTest.java

@@ -409,4 +409,107 @@ public void testFissionConfigWithMissingDwp() throws Exception {
     getConfiguredTarget("//a:a");
     assertContainsEvent("Tool path for 'dwp' is missing");
   }
+
+  @Test
+  public void testRuntimeLibsAttributesAreNotObligatory() throws Exception {
+    scratch.file(
+        "a/BUILD",
+        "filegroup(name='empty') ",
+        "cc_toolchain_suite(",
+        "    name = 'a',",
+        "    toolchains = { 'k8': ':b' },",
+        ")",
+        "cc_toolchain(",
+        "    name = 'b',",
+        "    cpu = 'banana',",
+        "    all_files = ':empty',",
+        "    ar_files = ':empty',",
+        "    as_files = ':empty',",
+        "    compiler_files = ':empty',",
+        "    dwp_files = ':empty',",
+        "    linker_files = ':empty',",
+        "    strip_files = ':empty',",
+        "    objcopy_files = ':empty',",
+        "    proto = \"\"\"",
+        "      toolchain_identifier: \"a\"",
+        "      host_system_name: \"a\"",
+        "      target_system_name: \"a\"",
+        "      target_cpu: \"a\"",
+        "      target_libc: \"a\"",
+        "      compiler: \"a\"",
+        "      abi_version: \"a\"",
+        "      abi_libc_version: \"a\"",
+        "\"\"\")");
+    reporter.removeHandler(failFastHandler);
+    useConfiguration("--cpu=k8", "--host_cpu=k8");
+    getConfiguredTarget("//a:a");
+    assertNoEvents();
+  }
+
+  @Test
+  public void testWhenRuntimeLibsAtttributesMandatoryWhenSupportsEmbeddedRuntimes()
+      throws Exception {
+    scratch.file(
+        "a/BUILD",
+        "filegroup(name='empty') ",
+        "cc_toolchain_suite(",
+        "    name = 'a',",
+        "    toolchains = { 'k8': ':b', 'k9': ':c' },",
+        ")",
+        "cc_toolchain(",
+        "    name = 'b',",
+        "    cpu = 'banana',",
+        "    all_files = ':empty',",
+        "    ar_files = ':empty',",
+        "    as_files = ':empty',",
+        "    compiler_files = ':empty',",
+        "    dwp_files = ':empty',",
+        "    linker_files = ':empty',",
+        "    strip_files = ':empty',",
+        "    objcopy_files = ':empty',",
+        "    proto = \"\"\"",
+        "      toolchain_identifier: \"a\"",
+        "      host_system_name: \"a\"",
+        "      target_system_name: \"a\"",
+        "      target_cpu: \"a\"",
+        "      target_libc: \"a\"",
+        "      compiler: \"a\"",
+        "      abi_version: \"a\"",
+        "      abi_libc_version: \"a\"",
+        "      supports_embedded_runtimes: true,",
+        "\"\"\")",
+        "cc_toolchain(",
+        "    name = 'c',",
+        "    cpu = 'banana',",
+        "    all_files = ':empty',",
+        "    ar_files = ':empty',",
+        "    as_files = ':empty',",
+        "    compiler_files = ':empty',",
+        "    dwp_files = ':empty',",
+        "    linker_files = ':empty',",
+        "    strip_files = ':empty',",
+        "    objcopy_files = ':empty',",
+        "    static_runtime_libs = [ ':yolo' ],",
+        "    proto = \"\"\"",
+        "      toolchain_identifier: \"a\"",
+        "      host_system_name: \"a\"",
+        "      target_system_name: \"a\"",
+        "      target_cpu: \"a\"",
+        "      target_libc: \"a\"",
+        "      compiler: \"a\"",
+        "      abi_version: \"a\"",
+        "      abi_libc_version: \"a\"",
+        "      supports_embedded_runtimes: true,",
+        "\"\"\")");
+    reporter.removeHandler(failFastHandler);
+    useConfiguration("--cpu=k8", "--host_cpu=k8");
+    getConfiguredTarget("//a:a");
+    assertContainsEvent(
+        "Toolchain supports embedded runtimes, but didn't provide static_runtime_libs attribute.");
+
+    useConfiguration("--cpu=k9", "--host_cpu=k9");
+    getConfiguredTarget("//a:a");
+    assertContainsEvent(
+        "Toolchain supports embedded runtimes, but didn't provide dynamic_runtime_libs attribute.");
+  }
 }