Add a test confirming external #5151.

This seems completely consistent with JLS, but I was pretty surprised by it.

I feel like we could, in general, expand our definition of "constant" to include effectively final variables if we had the motivation to do so, and fancied writing something elaborate to track them.

PiperOrigin-RevId: 781521689

Author: graememorgan

core/src/test/java/com/google/errorprone/bugpatterns/formatstring/FormatStringTest.java

@@ -425,4 +425,37 @@ void f(String f) {
             """)
         .doTest();
   }
+
+  @Test
+  public void finalVariable_isConstant() {
+    compilationHelper
+        .addSourceLines(
+            "T.java",
+            """
+            class T {
+              public static void main() {
+                final String formatString = " %0$2s) %s";
+                // BUG: Diagnostic contains:
+                System.err.printf(formatString, "foo", "bar");
+              }
+            }
+            """)
+        .doTest();
+  }
+
+  @Test
+  public void effectivelyFinalVariable_isNotConstant() {
+    compilationHelper
+        .addSourceLines(
+            "T.java",
+            """
+            class T {
+              public static void main() {
+                String formatString = " %0$2s) %s";
+                System.err.printf(formatString, "foo", "bar");
+              }
+            }
+            """)
+        .doTest();
+  }
 }