Unsoundness: `JSValue` (and others) can outlive the `Context`

Hi,

JSValue, JSObject and others are not restricted in the lifetime, therefore are able to outlive the Context, in the best case just crashing the program and in the worst case causing a use-after-free.

The following script reproduces the problem and leads to a segfault (due to an safety check in jsc itself):

use javascriptcore::*;

fn main() {
    let v;
    {
        let ctx = JSContext::default();
        v = evaluate_script(&ctx, "'Hello' + ' ' + 'World'", None, "test.js", 1)
            .expect("Failed to evaluate script");
        println!("Value: {:?}", v.as_string());
    }
    println!("Value: {:?}", v.as_string());
}

Typically, you would add PhantomData to add a phantom lifetime. However, you already hold a raw pointer to the context in these structs and the context should be a widespread object in many real-world users of your bindings, ref-counting the context in Rust with Arc might be the best option.

Kind Regards
Tim

issue 内の Rust 再現コードから始め、JSContext、JSValue、JSObject、evaluate_script の定義を追跡してください。Context のスコープ終了後に use-after-free が発生することを確認し、そのうえで lifetime-safe な所有権アプローチを決定して文書化してください。再現コードが Context の破棄後に値へアクセスできず、関連する安全性の動作がテストでカバーされていれば完了です。