Use a frozen string for ENV['TZ']

Author: eregon

src/main/java/org/truffleruby/core/string/CoreStrings.java

@@ -29,7 +29,6 @@ public class CoreStrings {
     public final CoreString STACK_LEVEL_TOO_DEEP;
     public final CoreString TIME_INTERVAL_MUST_BE_POS;
     public final CoreString TOO_FEW_ARGUMENTS;
-    public final CoreString TZ;
     public final CoreString UNKNOWN;
     public final CoreString UTC;
     public final CoreString WRONG_ARGS_ZERO_PLUS_ONE;
@@ -52,7 +51,6 @@ public CoreStrings(RubyLanguage language) {
         STACK_LEVEL_TOO_DEEP = new CoreString(language, "stack level too deep");
         TIME_INTERVAL_MUST_BE_POS = new CoreString(language, "time interval must be positive");
         TOO_FEW_ARGUMENTS = new CoreString(language, "too few arguments");
-        TZ = new CoreString(language, "TZ");
         UNKNOWN = new CoreString(language, "(unknown)");
         UTC = new CoreString(language, "UTC");
         WRONG_ARGS_ZERO_PLUS_ONE = new CoreString(language, "wrong number of arguments (0 for 1+)");

src/main/java/org/truffleruby/core/string/FrozenStrings.java

@@ -35,6 +35,7 @@ public class FrozenStrings {
     public static final ImmutableRubyString SELF = createFrozenStaticBinaryString("self");
     public static final ImmutableRubyString SUPER = createFrozenStaticBinaryString("super");
     public static final ImmutableRubyString TRUE = createFrozenStaticBinaryString("true");
+    public static final ImmutableRubyString TZ = createFrozenStaticBinaryString("TZ");
 
     private static ImmutableRubyString createFrozenStaticBinaryString(String string) {
         // defined?(...) returns frozen strings with a binary encoding

src/main/java/org/truffleruby/core/time/GetTimeZoneNode.java

@@ -34,6 +34,7 @@
 import java.util.regex.Pattern;
 
 import com.oracle.truffle.api.CompilerDirectives;
+import org.truffleruby.core.string.FrozenStrings;
 import org.truffleruby.language.RubyBaseNode;
 import org.truffleruby.language.control.RaiseException;
 import org.truffleruby.language.dispatch.DispatchNode;
@@ -68,8 +69,7 @@ protected TimeZoneAndName getTimeZone(
 
     protected Object getTZ() {
         if (getContext().getEnv().isNativeAccessAllowed()) {
-            return lookupEnvNode.call(coreLibrary().getENV(), "[]",
-                    getLanguage().coreStrings.TZ.createInstance(getContext()));
+            return lookupEnvNode.call(coreLibrary().getENV(), "[]", FrozenStrings.TZ);
         } else {
             return nil;
         }