Is there a better way to overload Index / IndexMut for a Rc<Refcell<_>>?

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@sfackler --- actually, if I remove Rc this seems to fix the issue with allowing two mut references --- at least I can't get rustc to allow two concurrent mutable references.

I can have an immutable and a mutable at the same time, though.

        let mut f = Cache::new();
        let mut g = f.0.borrow_mut();
        println!("f0={}", f[1]); // no complaint even though we have a borrow_mut
        g.0[1] = 1;
        f[1] = 1;               // rustc error b/c already did a borrow_mut()

The wording around UnsafeCell is confusing. I can't tell if this violation (immutable + mutable) at the same time is blessed or not. E.g.,