Expose the default no-op transition to Starlark users.

This is the same transition as `cfg = "target"`.

RELNOTES: A no-op transition is now available as `config.target()`.

Part of #22248.

PiperOrigin-RevId: 633665060
Change-Id: I63f81d1378b6af7ed4fb4b81ea1df3db9de526ad

src/main/java/com/google/devtools/build/lib/analysis/BUILD

@@ -2167,6 +2167,7 @@ java_library(
         "//src/main/java/com/google/devtools/build/lib/events",
         "//src/main/java/com/google/devtools/build/lib/skyframe/serialization/autocodec",
         "//src/main/java/com/google/devtools/build/lib/skyframe/serialization/autocodec:serialization-constant",
+        "//src/main/java/com/google/devtools/build/lib/starlarkbuildapi/config",
         "//third_party:auto_value",
     ],
 )
@@ -2599,6 +2600,7 @@ java_library(
     srcs = ["starlark/StarlarkConfig.java"],
     deps = [
         ":config/execution_transition_factory",
+        ":config/transitions/no_transition",
         "//src/main/java/com/google/devtools/build/lib/packages",
         "//src/main/java/com/google/devtools/build/lib/starlarkbuildapi/config",
         "//src/main/java/net/starlark/java/eval",

src/main/java/com/google/devtools/build/lib/analysis/config/transitions/NoTransition.java

@@ -18,6 +18,7 @@
 import com.google.devtools.build.lib.analysis.config.BuildOptionsView;
 import com.google.devtools.build.lib.events.EventHandler;
 import com.google.devtools.build.lib.skyframe.serialization.autocodec.SerializationConstant;
+import com.google.devtools.build.lib.starlarkbuildapi.config.ConfigurationTransitionApi;
 
 /** No-op configuration transition. */
 public final class NoTransition implements PatchTransition {
@@ -47,7 +48,8 @@ public static <T extends TransitionFactory.Data> boolean isInstance(
 
   /** A {@link TransitionFactory} implementation that generates the no transition. */
   @AutoValue
-  abstract static class Factory<T extends TransitionFactory.Data> implements TransitionFactory<T> {
+  abstract static class Factory<T extends TransitionFactory.Data>
+      implements TransitionFactory<T>, ConfigurationTransitionApi {
     @Override
     public ConfigurationTransition create(T unused) {
       return INSTANCE;

src/main/java/com/google/devtools/build/lib/analysis/starlark/StarlarkConfig.java

@@ -20,7 +20,9 @@
 import static com.google.devtools.build.lib.packages.Types.STRING_LIST;
 
 import com.google.devtools.build.lib.analysis.config.ExecutionTransitionFactory;
+import com.google.devtools.build.lib.analysis.config.transitions.NoTransition;
 import com.google.devtools.build.lib.packages.BuildSetting;
+import com.google.devtools.build.lib.starlarkbuildapi.config.ConfigurationTransitionApi;
 import com.google.devtools.build.lib.starlarkbuildapi.config.StarlarkConfigApi;
 import net.starlark.java.eval.EvalException;
 import net.starlark.java.eval.Printer;
@@ -59,6 +61,11 @@ public ExecutionTransitionFactory exec(Object execGroupUnchecked) {
         : ExecutionTransitionFactory.createFactory((String) execGroupUnchecked);
   }
 
+  @Override
+  public ConfigurationTransitionApi target() {
+    return (ConfigurationTransitionApi) NoTransition.createFactory();
+  }
+
   @Override
   public void repr(Printer printer) {
     printer.append("<config>");

src/main/java/com/google/devtools/build/lib/starlarkbuildapi/config/StarlarkConfigApi.java

@@ -158,6 +158,14 @@ interface BuildSettingApi extends StarlarkValue {}
       })
   ExecTransitionFactoryApi exec(Object execGroupUnchecked);
 
+  @StarlarkMethod(
+      name = "target",
+      doc =
+          "Creates a target transition. This is a no-op transition intended for the case where a"
+              + " transition object is needed, but doesn't want to actually change anything."
+              + " Equivalent to <code>cfg = \"target\"</code> in <code>attr.label()</code>.")
+  ConfigurationTransitionApi target();
+
   /** The api for exec transitions. */
   @StarlarkBuiltin(
       name = "ExecTransitionFactory",

src/main/java/com/google/devtools/build/skydoc/fakebuildapi/config/FakeConfigApi.java

@@ -14,6 +14,7 @@
 
 package com.google.devtools.build.skydoc.fakebuildapi.config;
 
+import com.google.devtools.build.lib.starlarkbuildapi.config.ConfigurationTransitionApi;
 import com.google.devtools.build.lib.starlarkbuildapi.config.StarlarkConfigApi;
 import com.google.devtools.build.skydoc.fakebuildapi.config.ConfigApiFakes.FakeBuildSettingDescriptor;
 import com.google.devtools.build.skydoc.fakebuildapi.config.ConfigApiFakes.FakeExecTransitionFactory;
@@ -47,6 +48,11 @@ public ExecTransitionFactoryApi exec(Object execGroup) {
     return new FakeExecTransitionFactory();
   }
 
+  @Override
+  public ConfigurationTransitionApi target() {
+    return new FakeConfigurationTransition();
+  }
+
   @Override
   public void repr(Printer printer) {}
 }

src/test/java/com/google/devtools/build/lib/starlark/StarlarkRuleClassFunctionsTest.java

@@ -1381,11 +1381,17 @@ public void testAttrCfgHostDisabled() throws Exception {
   }
 
   @Test
-  public void testAttrCfgTarget() throws Exception {
+  public void testAttrCfgTarget_string() throws Exception {
     Attribute attr = buildAttribute("a1", "attr.label(cfg = 'target', allow_files = True)");
     assertThat(NoTransition.isInstance(attr.getTransitionFactory())).isTrue();
   }
 
+  @Test
+  public void testAttrCfgTarget_object() throws Exception {
+    Attribute attr = buildAttribute("a1", "attr.label(cfg = config.target(), allow_files = True)");
+    assertThat(NoTransition.isInstance(attr.getTransitionFactory())).isTrue();
+  }
+
   private void writeRuleCfgTestRule(String cfg) throws Exception {
     scratch.file(
         "rule_testing/rule.bzl",

src/test/java/com/google/devtools/build/lib/starlark/util/BUILD

@@ -21,6 +21,7 @@ java_library(
         "//src/main/java/com/google/devtools/build/lib:runtime",
         "//src/main/java/com/google/devtools/build/lib:starlark_options_parser",
         "//src/main/java/com/google/devtools/build/lib/analysis:analysis_cluster",
+        "//src/main/java/com/google/devtools/build/lib/analysis:starlark/starlark_config",
         "//src/main/java/com/google/devtools/build/lib/cmdline",
         "//src/main/java/com/google/devtools/build/lib/events",
         "//src/main/java/com/google/devtools/build/lib/packages",

src/test/java/com/google/devtools/build/lib/starlark/util/BazelEvaluationTestCase.java

@@ -19,6 +19,7 @@
 import com.google.common.base.Preconditions;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
+import com.google.devtools.build.lib.analysis.starlark.StarlarkConfig;
 import com.google.devtools.build.lib.analysis.starlark.StarlarkGlobalsImpl;
 import com.google.devtools.build.lib.cmdline.BazelModuleContext;
 import com.google.devtools.build.lib.cmdline.Label;
@@ -181,6 +182,7 @@ private Object newModule(ImmutableMap.Builder<String, Object> predeclared) {
     predeclared.putAll(StarlarkGlobalsImpl.INSTANCE.getFixedBzlToplevels());
     predeclared.put("platform_common", new PlatformCommon());
     predeclared.put("config_common", new ConfigStarlarkCommon());
+    predeclared.put("config", new StarlarkConfig());
     Starlark.addMethods(predeclared, new ConfigGlobalLibrary());
 
     // Return the module's client data. (This one uses dummy values for tests.)