IdentifierName: support JDK 22 unnamed variables

This PR nominally fixes #4847 but it is perhaps more illustrative than usable.

Fixes #4848

FUTURE_COPYBARA_INTEGRATE_REVIEW=#4848 from commonquail:identifiername-accept-unnamed cc90fe1
PiperOrigin-RevId: 733740757

Author: commonquail

core/src/main/java/com/google/errorprone/bugpatterns/IdentifierName.java

@@ -278,6 +278,9 @@ private static boolean isConformant(Symbol symbol, String name) {
     if (isStaticVariable(symbol) && isConformantStaticVariableName(name)) {
       return true;
     }
+    if (name.isEmpty()) {
+      return true;
+    }
     return isConformantLowerCamelName(name);
   }
 

core/src/test/java/com/google/errorprone/bugpatterns/IdentifierNameTest.java

@@ -16,6 +16,8 @@
 
 package com.google.errorprone.bugpatterns;
 
+import static com.google.common.truth.TruthJUnit.assume;
+
 import com.google.errorprone.BugCheckerRefactoringTestHelper;
 import com.google.errorprone.CompilationTestHelper;
 import org.junit.Test;
@@ -670,4 +672,29 @@ public void enumName() {
             "}")
         .doTest();
   }
+
+  @Test
+  public void unnamedVariables() {
+    assume().that(Runtime.version().feature()).isAtLeast(21);
+
+    helper
+        .addSourceLines(
+            "Test.java",
+            """
+            import java.util.Scanner;
+            import java.util.function.Function;
+
+            class Test {
+              void unnamed() {
+                try (var _ = new Scanner("discarded")) {
+                  Function<String, String> f = _ -> "bar";
+                  String _ = f.apply("foo");
+                } catch (Exception _) {
+                }
+              }
+            }
+            """)
+        .setArgs("--enable-preview", "--release", Integer.toString(Runtime.version().feature()))
+        .doTest();
+  }
 }