Include the top of the stack StackTraceElement in the SystemStackError message

* Helpful to debug if the StackOverflowError happens in implementation code.

Author: eregon

src/main/java/org/truffleruby/core/exception/CoreExceptions.java

@@ -236,7 +236,12 @@ public DynamicObject runtimeError(String fullMessage, Node currentNode, Throwabl
     @TruffleBoundary
     public DynamicObject systemStackErrorStackLevelTooDeep(Node currentNode, StackOverflowError javaThrowable) {
         DynamicObject exceptionClass = context.getCoreLibrary().getSystemStackErrorClass();
-        return ExceptionOperations.createRubyException(context, exceptionClass, coreStrings().STACK_LEVEL_TOO_DEEP.createInstance(), currentNode, javaThrowable);
+        StackTraceElement[] stackTrace = javaThrowable.getStackTrace();
+        String topOfTheStack = stackTrace.length > 0 ? BacktraceFormatter.formatJava(stackTrace[0]) : "<empty Java stacktrace>";
+        final String message = coreStrings().STACK_LEVEL_TOO_DEEP + "\n\tfrom " + topOfTheStack;
+        return ExceptionOperations.createRubyException(context, exceptionClass,
+                StringOperations.createString(context, StringOperations.encodeRope(message, UTF8Encoding.INSTANCE)),
+                currentNode, javaThrowable);
     }
 
     // NoMemoryError

src/main/java/org/truffleruby/language/backtrace/BacktraceFormatter.java

@@ -244,6 +244,11 @@ private String formatLineInternal(Activation[] activations, int n, DynamicObject
         return builder.toString();
     }
 
+    public static String formatJava(StackTraceElement stackTraceElement) {
+        return stackTraceElement.getFileName() + ":" + stackTraceElement.getLineNumber() +
+                ":in `" + stackTraceElement.getClassName() + "." + stackTraceElement.getMethodName() + "'";
+    }
+
     private String formatForeign(Node callNode, String methodName) {
         final StringBuilder builder = new StringBuilder();
         final SourceSection sourceSection = callNode == null ? null : callNode.getEncapsulatingSourceSection();