Log the expansion step in invocation policy.

This is only useful for debugging invocation policy itself, not as much for understanding its effect on a user command line, so this log statement will only occur at log levels fine and higher.

PiperOrigin-RevId: 152259362

src/main/java/com/google/devtools/build/lib/flags/InvocationPolicyEnforcer.java

diff --git a/src/main/java/com/google/devtools/build/lib/flags/InvocationPolicyEnforcer.java b/src/main/java/com/google/devtools/build/lib/flags/InvocationPolicyEnforcer.java
index 7e512c1..a01ecd7 100644
--- a/src/main/java/com/google/devtools/build/lib/flags/InvocationPolicyEnforcer.java
+++ b/src/main/java/com/google/devtools/build/lib/flags/InvocationPolicyEnforcer.java
@@ -15,6 +15,7 @@
 
 import com.google.common.base.Function;
 import com.google.common.base.Functions;
+import com.google.common.base.Joiner;
 import com.google.common.base.Verify;
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableSet;
@@ -235,6 +236,26 @@
             .build();
     boolean isExpansion = !desc.getExpansions().isEmpty();
 
+    if (!subflags.isEmpty() && log.isLoggable(Level.FINE)) {
+      // Log the expansion. Since this is logged regardless of user provided command line, it is
+      // only really useful for understanding the invocation policy itself. Most of the time,
+      // invocation policy does not change, so this can be a log level fine.
+      List<String> subflagNames = new ArrayList<>(subflags.size());
+      for (OptionValueDescription subflag : subflags) {
+        subflagNames.add("--" + subflag.getName());
+      }
+
+      log.logp(Level.FINE,
+          "InvocationPolicyEnforcer",
+          "expandPolicy",
+          String.format(
+            "Expanding %s on option %s to its %s: %s.",
+            originalPolicy.getOperationCase(),
+            originalPolicy.getFlagName(),
+            isExpansion ? "expansions" : "implied flags",
+            Joiner.on("; ").join(subflagNames)));
+    }
+
     // Create a flag policy for the child that looks like the parent's policy "transferred" to its
     // child. Note that this only makes sense for SetValue, when setting an expansion flag, or
     // UseDefault, when preventing it from being set.