Mark isolated extension usages as experimental

src/main/java/com/google/devtools/build/lib/bazel/bzlmod/ModuleExtensionContext.java

@@ -130,8 +130,11 @@ public boolean isDevDependency(TypeCheckedTag tag) {
       name = "is_isolated",
       doc =
           "Whether this particular usage of the extension had <code>isolate = True</code> "
-              + "specified and is thus isolated from all other usages.",
-      structField = true)
+              + "specified and is thus isolated from all other usages."
+              + "<p>This field is currently experimental and only available with the flag "
+              + "<code>--experimental_isolated_extension_usages</code>.",
+      structField = true,
+      enableOnlyWithFlag = "-experimental_isolated_extension_usages")
   public boolean isIsolated() {
     return extensionId.getIsolationKey().isPresent();
   }

src/main/java/com/google/devtools/build/lib/bazel/bzlmod/ModuleFileGlobals.java

@@ -441,10 +441,14 @@ public void registerToolchains(boolean devDependency, Sequence<?> toolchainLabel
                     + "usages, both in this and other modules. Tags created for this usage do not "
                     + "affect other usages and the repositories generated by the extension for "
                     + "this usage will be distinct from all other repositories generated by the "
-                    + "extension.",
+                    + "extension."
+                    + "<p>This parameter is currently experimental and only available with the "
+                    + "flag <code>--experimental_isolated_extension_usages</code>.",
             named = true,
             positional = false,
-            defaultValue = "False"),
+            defaultValue = "False",
+            enableOnlyWithFlag = "-experimental_isolated_extension_usages",
+            valueWhenDisabled = "False"),
       },
       useStarlarkThread = true)
   public ModuleExtensionProxy useExtension(

src/main/java/com/google/devtools/build/lib/packages/semantics/BuildLanguageOptions.java

@@ -215,6 +215,14 @@ public final class BuildLanguageOptions extends OptionsBase {
               + " WORKSPACE. See https://bazel.build/docs/bzlmod for more information.")
   public boolean enableBzlmod;
 
+  @Option(
+      name = "experimental_isolated_extension_usages",
+      defaultValue = "false",
+      documentationCategory = OptionDocumentationCategory.STARLARK_SEMANTICS,
+      effectTags = OptionEffectTag.LOADING_AND_ANALYSIS,
+      help = "foo")
+  public boolean experimentalIsolatedExtensionUsages;
+
   @Option(
       name = "experimental_java_proto_library_default_has_services",
       defaultValue = "true",
@@ -696,6 +704,7 @@ public StarlarkSemantics toStarlarkSemantics() {
                 EXPERIMENTAL_ENABLE_ANDROID_MIGRATION_APIS, experimentalEnableAndroidMigrationApis)
             .setBool(EXPERIMENTAL_ENABLE_SCL_DIALECT, experimentalEnableSclDialect)
             .setBool(ENABLE_BZLMOD, enableBzlmod)
+            .setBool(EXPERIMENTAL_ISOLATED_EXTENSION_USAGES, experimentalIsolatedExtensionUsages)
             .setBool(
                 EXPERIMENTAL_JAVA_PROTO_LIBRARY_DEFAULT_HAS_SERVICES,
                 experimentalJavaProtoLibraryDefaultHasServices)
@@ -791,6 +800,8 @@ public StarlarkSemantics toStarlarkSemantics() {
       "-experimental_enable_android_migration_apis";
   public static final String EXPERIMENTAL_ENABLE_SCL_DIALECT = "-experimental_enable_scl_dialect";
   public static final String ENABLE_BZLMOD = "-enable_bzlmod";
+  public static final String EXPERIMENTAL_ISOLATED_EXTENSION_USAGES =
+      "-experimental_isolated_extension_usages";
   public static final String EXPERIMENTAL_JAVA_PROTO_LIBRARY_DEFAULT_HAS_SERVICES =
       "+experimental_java_proto_library_default_has_services";
   public static final String INCOMPATIBLE_EXISTING_RULES_IMMUTABLE_VIEW =

src/test/java/com/google/devtools/build/lib/bazel/bzlmod/ModuleExtensionResolutionTest.java

@@ -271,7 +271,10 @@ public void setup() throws Exception {
 
     PrecomputedValue.STARLARK_SEMANTICS.set(
         differencer,
-        StarlarkSemantics.builder().setBool(BuildLanguageOptions.ENABLE_BZLMOD, true).build());
+        StarlarkSemantics.builder()
+            .setBool(BuildLanguageOptions.ENABLE_BZLMOD, true)
+            .setBool(BuildLanguageOptions.EXPERIMENTAL_ISOLATED_EXTENSION_USAGES, true)
+            .build());
     RepositoryDelegatorFunction.REPOSITORY_OVERRIDES.set(differencer, ImmutableMap.of());
     RepositoryDelegatorFunction.DEPENDENCY_FOR_UNCONDITIONAL_FETCHING.set(
         differencer, RepositoryDelegatorFunction.DONT_FETCH_UNCONDITIONALLY);

src/test/java/com/google/devtools/build/lib/bazel/bzlmod/ModuleFileFunctionTest.java

@@ -1049,6 +1049,13 @@ public void module_calledLate() throws Exception {
 
   @Test
   public void isolatedUsageWithoutImports() throws Exception {
+    PrecomputedValue.STARLARK_SEMANTICS.set(
+        differencer,
+        StarlarkSemantics.builder()
+            .setBool(BuildLanguageOptions.ENABLE_BZLMOD, true)
+            .setBool(BuildLanguageOptions.EXPERIMENTAL_ISOLATED_EXTENSION_USAGES, true)
+            .build());
+
     scratch.file(
         rootDirectory.getRelative("MODULE.bazel").getPathString(),
         "isolated_ext = use_extension('//:extensions.bzl', 'my_ext', isolate = True)");
@@ -1069,6 +1076,13 @@ public void isolatedUsageWithoutImports() throws Exception {
 
   @Test
   public void isolatedUsageNotExported() throws Exception {
+    PrecomputedValue.STARLARK_SEMANTICS.set(
+        differencer,
+        StarlarkSemantics.builder()
+            .setBool(BuildLanguageOptions.ENABLE_BZLMOD, true)
+            .setBool(BuildLanguageOptions.EXPERIMENTAL_ISOLATED_EXTENSION_USAGES, true)
+            .build());
+
     scratch.file(
         rootDirectory.getRelative("MODULE.bazel").getPathString(),
         "use_extension('//:extensions.bzl', 'my_ext', isolate = True)");
@@ -1081,4 +1095,20 @@ public void isolatedUsageNotExported() throws Exception {
         "Isolated extension usage at /workspace/MODULE.bazel:1:14 must be assigned to a "
             + "top-level variable");
   }
+
+  @Test
+  public void isolatedUsage_notEnabled() throws Exception {
+    scratch.file(
+        rootDirectory.getRelative("MODULE.bazel").getPathString(),
+        "use_extension('//:extensions.bzl', 'my_ext', isolate = True)");
+    FakeRegistry registry = registryFactory.newFakeRegistry("/foo");
+    ModuleFileFunction.REGISTRIES.set(differencer, ImmutableList.of(registry.getUrl()));
+
+    reporter.removeHandler(failFastHandler); // expect failures
+    evaluator.evaluate(ImmutableList.of(ModuleFileValue.KEY_FOR_ROOT_MODULE), evaluationContext);
+    assertContainsEvent(
+        "Error in use_extension: in call to use_extension(), parameter 'isolate' is experimental "
+            + "and thus unavailable with the current flags. It may be enabled by setting "
+            + "--experimental_isolated_extension_usages");
+  }
 }

src/test/java/com/google/devtools/build/lib/packages/semantics/ConsistencyTest.java

@@ -128,6 +128,7 @@ private static BuildLanguageOptions buildRandomOptions(Random rand) throws Excep
         "--experimental_bzl_visibility=" + rand.nextBoolean(),
         "--experimental_enable_android_migration_apis=" + rand.nextBoolean(),
         "--enable_bzlmod=" + rand.nextBoolean(),
+        "--experimental_isolated_extension_usages=" + rand.nextBoolean(),
         "--experimental_google_legacy_api=" + rand.nextBoolean(),
         "--experimental_platforms_api=" + rand.nextBoolean(),
         "--incompatible_allow_tags_propagation=" + rand.nextBoolean(), // flag, Java names differ
@@ -170,6 +171,7 @@ private static StarlarkSemantics buildRandomSemantics(Random rand) {
         .setBool(
             BuildLanguageOptions.EXPERIMENTAL_ENABLE_ANDROID_MIGRATION_APIS, rand.nextBoolean())
         .setBool(BuildLanguageOptions.ENABLE_BZLMOD, rand.nextBoolean())
+        .setBool(BuildLanguageOptions.EXPERIMENTAL_ISOLATED_EXTENSION_USAGES, rand.nextBoolean())
         .setBool(BuildLanguageOptions.EXPERIMENTAL_GOOGLE_LEGACY_API, rand.nextBoolean())
         .setBool(BuildLanguageOptions.EXPERIMENTAL_PLATFORMS_API, rand.nextBoolean())
         .setBool(BuildLanguageOptions.INCOMPATIBLE_ALLOW_TAGS_PROPAGATION, rand.nextBoolean())

src/test/py/bazel/bzlmod/bazel_lockfile_test.py

@@ -45,6 +45,7 @@ def setUp(self):
             # In ipv6 only network, this has to be enabled.
             # 'startup --host_jvm_args=-Djava.net.preferIPv6Addresses=true',
             'build --enable_bzlmod',
+            'build --experimental_isolated_extension_usages',
             'build --registry=' + self.main_registry.getURL(),
             # We need to have BCR here to make sure built-in modules like
             # bazel_tools can work.