I'm investigating the machine code that is generated when RefCells are involved. RefMut contains a NonNull, with a comment explaining its purpose:

pub struct RefMut<'b, T: ?Sized + 'b> {
    // NB: we use a pointer instead of `&'b mut T` to avoid `noalias` violations, because a
    // `RefMut` argument doesn't hold exclusivity for its whole scope, only until it drops.
    value: NonNull<T>,
    borrow: BorrowRefMut<'b>,
    // `NonNull` is covariant over `T`, so we need to reintroduce invariance.
    marker: PhantomData<&'b mut T>,
}

The comment isn't clear to me - how could a RefMut's whole scope extend past the time it is dropped? What sort of code would fail if &'b mut T was used instead of NonNull<T>?

The NonNull<T> is preventing machine code optimizations that I hoped would be present in my project that uses RefCells.

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