Get a return value from arbitrary eval'd code

I have a requirement that the user can provide arbitrary statements which can be stored in a function and called later to get a return value. A simple example of this is that userInput might be

var x = 10;
x;

I would store this via

var callback = function() {
    return eval(userInput);
}

and then running callback() returns 10 as expected.

However, I also need to support the case with an explicit return statement, ie userInput might be

var x = 10;
return x;

In this case the eval method above will fail with SyntaxError: return not in function. Instead I could store callback as

var callback = new Function(userInput);

My issue is that I would like to combine these two approaches according the rule 'get explicit return value otherwise get the result of the last executed statement'. In particular this can't be done with analysis of the code at callback creation time as the user could potentially do something odd like

if(envVar < 10)
    return a;
b * 0.5;

which combines both.

Any thoughts on how to structure the creation of the callback function to accommodate these possible inputs? Unfortunately it is not possible to enforce one style or the other on the user.

UPDATE to answer some of the comments below.

One solution suggested is to search for a return token and choose between new Function and eval. This doesn't work for my last example, see http://jsfiddle.net/ZGb6z/2/ - out4 should be "no" but ends up being undefined as the last executed statement is not returned.

Another suggestion is to modify the user input to add an explicit return on the last line if one is not found. Unfortunately it's not possible to know which statement will be executed last. Consider

var x = 10;
switch(x) {
  case 10:
    100;
    break;
  default:
    200;
    break;
}

When called it should return 100, but it would take some serious analysis to determine where to put returns for arbitrary code.