private static String renderToStringOrDump(ExpressionInfo expr) {
    return RenderUtil.toStringOrDump(expr.getValue());
  }
Returns a string representation of a value, or <tt>null</tt> if
the value should not be shown (because it does not add any valuable
information). Note that the method may also change rendered values
of child expression.

@param expr the expression whose value is to be rendered
@return a string representation of the value
spock-core/src/main/java/org/spockframework/runtime/ExpressionInfoValueRenderer.java