Cherry-pick missing things.

src/main/java/com/google/devtools/build/lib/rules/proto/ProtoCommon.java

@@ -31,10 +31,15 @@
 import com.google.devtools.build.lib.collect.nestedset.NestedSet;
 import com.google.devtools.build.lib.collect.nestedset.NestedSetBuilder;
 import com.google.devtools.build.lib.concurrent.BlazeInterners;
+import com.google.devtools.build.lib.packages.BazelModuleContext;
 import com.google.devtools.build.lib.packages.BuildType;
 import com.google.devtools.build.lib.vfs.FileSystemUtils;
 import com.google.devtools.build.lib.vfs.PathFragment;
 import javax.annotation.Nullable;
+import net.starlark.java.eval.EvalException;
+import net.starlark.java.eval.Module;
+import net.starlark.java.eval.Starlark;
+import net.starlark.java.eval.StarlarkThread;
 
 /** Utility functions for proto_library and proto aspect implementations. */
 public class ProtoCommon {
@@ -518,4 +523,14 @@ public static boolean areDepsStrict(RuleContext ruleContext) {
     StrictDepsMode getBool = ruleContext.getFragment(ProtoConfiguration.class).strictProtoDeps();
     return getBool != StrictDepsMode.OFF && getBool != StrictDepsMode.DEFAULT;
   }
+
+  public static void checkPrivateStarlarkificationAllowlist(StarlarkThread thread)
+      throws EvalException {
+    Label label =
+        ((BazelModuleContext) Module.ofInnermostEnclosingStarlarkFunction(thread).getClientData())
+            .label();
+    if (!label.getPackageIdentifier().getRepository().toString().equals("@_builtins")) {
+      throw Starlark.errorf("Rule in '%s' cannot use private API", label.getPackageName());
+    }
+  }
 }

src/main/java/com/google/devtools/build/lib/rules/proto/ProtoInfo.java

@@ -25,6 +25,8 @@
 import com.google.devtools.build.lib.starlarkbuildapi.ProtoInfoApi;
 import com.google.devtools.build.lib.starlarkbuildapi.proto.ProtoBootstrap;
 import com.google.devtools.build.lib.vfs.PathFragment;
+import net.starlark.java.eval.EvalException;
+import net.starlark.java.eval.StarlarkThread;
 
 /**
  * Configured target classes that implement this class can contribute .proto files to the
@@ -133,6 +135,12 @@ public String getDirectProtoSourceRoot() {
     return Depset.of(Artifact.TYPE, getTransitiveProtoSources());
   }
 
+  @Override
+  public Depset getTransitiveSourcesForStarlark(StarlarkThread thread) throws EvalException {
+    ProtoCommon.checkPrivateStarlarkificationAllowlist(thread);
+    return Depset.of(ProtoSource.TYPE, transitiveSources);
+  }
+
   /**
    * The {@code .proto} source files in this {@code proto_library}'s {@code srcs} and all of its
    * transitive dependencies.
@@ -222,6 +230,12 @@ public NestedSet<ProtoSource> getStrictImportableSources() {
     return strictImportableSources;
   }
 
+  @Override
+  public Depset getStrictImportableSourcesForStarlark(StarlarkThread thread) throws EvalException {
+    ProtoCommon.checkPrivateStarlarkificationAllowlist(thread);
+    return Depset.of(ProtoSource.TYPE, strictImportableSources);
+  }
+
   /**
    * Returns a set of {@code .proto} sources that may be re-exported by this {@code proto_library}'s
    * direct sources.

src/main/java/com/google/devtools/build/lib/rules/proto/ProtoSource.java

@@ -15,6 +15,7 @@
 package com.google.devtools.build.lib.rules.proto;
 
 import com.google.devtools.build.lib.actions.Artifact;
+import com.google.devtools.build.lib.collect.nestedset.Depset;
 import com.google.devtools.build.lib.concurrent.ThreadSafety.Immutable;
 import com.google.devtools.build.lib.skyframe.serialization.autocodec.AutoCodec;
 import com.google.devtools.build.lib.vfs.PathFragment;
@@ -24,6 +25,8 @@
 @Immutable
 @AutoCodec
 class ProtoSource implements StarlarkValue {
+  public static final Depset.ElementType TYPE = Depset.ElementType.of(ProtoSource.class);
+
   private final Artifact sourceFile;
   private final Artifact originalSourceFile;
   private final PathFragment sourceRoot;

src/main/java/com/google/devtools/build/lib/starlarkbuildapi/ProtoInfoApi.java

@@ -21,6 +21,8 @@
 import com.google.devtools.build.lib.starlarkbuildapi.core.StructApi;
 import net.starlark.java.annot.StarlarkBuiltin;
 import net.starlark.java.annot.StarlarkMethod;
+import net.starlark.java.eval.EvalException;
+import net.starlark.java.eval.StarlarkThread;
 
 /** Info object propagating information about protocol buffer sources. */
 @StarlarkBuiltin(
@@ -105,4 +107,10 @@ interface ProtoInfoProviderApi extends ProviderApi {
               + " as a source, that source file would be imported as 'import c/d.proto'",
       structField = true)
   String getDirectProtoSourceRoot();
+
+  @StarlarkMethod(name = "strict_importable_sources", documented = false, useStarlarkThread = true)
+  Depset getStrictImportableSourcesForStarlark(StarlarkThread thread) throws EvalException;
+
+  @StarlarkMethod(name = "transitive_proto_sources", documented = false, useStarlarkThread = true)
+  Depset getTransitiveSourcesForStarlark(StarlarkThread thread) throws EvalException;
 }

src/main/starlark/builtins_bzl/common/exports.bzl

@@ -39,7 +39,7 @@ exported_rules = {
     "+apple_static_library": apple_static_library,
     "+cc_shared_library": cc_shared_library,
     "+cc_shared_library_permissions": cc_shared_library_permissions,
-    "+proto_lang_toolchain": proto_lang_toolchain,
+    "proto_lang_toolchain": proto_lang_toolchain,
 }
 exported_to_java = {
     "register_compile_and_archive_actions_for_j2objc": compilation_support.register_compile_and_archive_actions_for_j2objc,