Don't complain about unused variables named _

See https://openjdk.org/jeps/456

Fixes #4451

PiperOrigin-RevId: 652904059

Author: cushon

check_api/src/main/java/com/google/errorprone/util/RuntimeVersion.java

@@ -79,6 +79,11 @@ public static boolean isAtLeast21() {
     return FEATURE >= 21;
   }
 
+  /** Returns true if the current runtime is JDK 22 or newer. */
+  public static boolean isAtLeast22() {
+    return FEATURE >= 22;
+  }
+
   /**
    * Returns the latest {@code --release} version.
    *

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

@@ -198,6 +198,7 @@ public final class UnusedVariable extends BugChecker implements CompilationUnitT
     this.exemptNames =
         ImmutableSet.<String>builder()
             .add("ignored")
+            .add("") // `var _ = ...` is handled as an empty variable name
             .addAll(flags.getListOrEmpty("Unused:exemptNames"))
             .build();
 

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

@@ -1679,4 +1679,19 @@ public void parameterUsedInOverride() {
         .expectUnchanged()
         .doTest();
   }
+
+  @Test
+  public void underscoreVariable() {
+    assumeTrue(RuntimeVersion.isAtLeast22());
+    refactoringHelper
+        .addInputLines(
+            "Test.java",
+            "class Test {",
+            "  public static void main(String[] args) {",
+            "    var _ = new Object();",
+            "  }",
+            "}")
+        .expectUnchanged()
+        .doTest();
+  }
 }