Initialize $0 to a String as RubyGems expects it to

Author: eregon

CHANGELOG.md

@@ -5,6 +5,7 @@ Bug fixes:
 * The debugger now sees global variables as the global scope.
 * Temporary variables are no longer visible in the debugger.
 * Setting breakpoints on some lines has been fixed.
+* Initialize `$0` when not run from the 'ruby' launcher, which is needed to `require` gems (#1653).
 
 Compatibility:
 

src/main/java/org/truffleruby/core/CoreLibrary.java

@@ -849,6 +849,10 @@ private void afterLoadCoreLibrary() {
         assert Layouts.CLASS.isClass(eagainWaitWritable);
 
         findGlobalVariableStorage();
+
+        // Initialize $0 so it is set to a String as RubyGems expect, also when not run from the RubyLauncher
+        DynamicObject dollarZeroValue = StringOperations.createString(context, StringOperations.encodeRope("-", USASCIIEncoding.INSTANCE, CodeRange.CR_7BIT));
+        getContext().getCoreLibrary().getGlobalVariables().getStorage("$0").setValueInternal(dollarZeroValue);
     }
 
     @TruffleBoundary