Consolidate referneces to --experimental_exec_config.

Also more strongly verify that the Starlark exec transition is set for this build.

I thought about removing the generic `context` variable in `StarlarkExecTransitionLoadingException`'s
constructor. All uses today fill that out with `--experiemntal_exec_config`. But it seems nice to
keep that open in case we generalize uses with a wider API.

PiperOrigin-RevId: 588074546
Change-Id: I14f169e2e44d482c9acb34968a97dbb008149299

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

@@ -1820,6 +1820,7 @@ java_library(
     deps = [
         ":analysis_cluster",
         ":config/build_options",
+        ":config/common_options",
         ":config/core_options",
         ":config/starlark_defined_config_transition",
         "//src/main/java/com/google/devtools/build/lib/cmdline",

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

@@ -15,6 +15,7 @@
 
 import com.google.auto.value.AutoValue;
 import com.google.common.base.Splitter;
+import com.google.common.base.Verify;
 import com.google.devtools.build.lib.analysis.starlark.StarlarkAttributeTransitionProvider;
 import com.google.devtools.build.lib.cmdline.Label;
 import com.google.devtools.build.lib.cmdline.LabelSyntaxException;
@@ -67,15 +68,15 @@ public interface BzlFileLoader {
   public static Optional<StarlarkAttributeTransitionProvider> loadStarlarkExecTransition(
       @Nullable BuildOptions options, BzlFileLoader bzlFileLoader)
       throws StarlarkExecTransitionLoadingException, InterruptedException {
-    if (options == null) {
+    if (options == null || options.equals(CommonOptions.EMPTY_OPTIONS)) {
       return Optional.empty();
     }
-    String userRef = options.get(CoreOptions.class).starlarkExecConfig;
-    if (userRef == null) {
-      return Optional.empty(); // Use the native exec transition.
-    }
-    TransitionReference parsedRef =
-        TransitionReference.create(userRef, "--experimental_exec_config");
+    String userRef =
+        Verify.verifyNotNull(
+            options.get(CoreOptions.class).starlarkExecConfig,
+            "Cannot apply the exec transition since no transition is defined for this build.");
+    final String flagName = "--experimental_exec_config";
+    TransitionReference parsedRef = TransitionReference.create(userRef, flagName);
     BzlLoadValue bzlValue;
     try {
       bzlValue =
@@ -85,23 +86,20 @@ public static Optional<StarlarkAttributeTransitionProvider> loadStarlarkExecTran
                   ? BzlLoadValue.keyForBuiltins(parsedRef.bzlFile())
                   : BzlLoadValue.keyForBuild(parsedRef.bzlFile()));
     } catch (BzlLoadFailedException e) {
-      throw new StarlarkExecTransitionLoadingException(
-          "--experimental_exec_config", userRef, e.getMessage());
+      throw new StarlarkExecTransitionLoadingException(flagName, userRef, e.getMessage());
     }
     if (bzlValue == null) {
       return null;
     }
     Object transition = bzlValue.getModule().getGlobal(parsedRef.starlarkSymbolName());
     if (transition == null) {
       throw new StarlarkExecTransitionLoadingException(
-          "--experimental_exec_config",
+          flagName,
           userRef,
           String.format("%s not found in %s", parsedRef.starlarkSymbolName(), parsedRef.bzlFile()));
     } else if (!(transition instanceof StarlarkDefinedConfigTransition)) {
       throw new StarlarkExecTransitionLoadingException(
-          "--experimental_exec_config",
-          userRef,
-          parsedRef.starlarkSymbolName() + " is not a Starlark transition");
+          flagName, userRef, parsedRef.starlarkSymbolName() + " is not a Starlark transition");
     }
     return Optional.of(
         new StarlarkAttributeTransitionProvider((StarlarkDefinedConfigTransition) transition));