Exercise 2.4.1.I Generic Index 's LocalStorageVecIndex seems unnecessary?

[fubupc]

The text says:

You've probably duplicated a lot of code in exercise 2.4.1.F. We can reduce the boilerplate by defining an empty trait:

trait LocalStorageVecIndex {}

First, implement this trait for usize, RangeTo, RangeFrom, and Range.

Next, replace the multiple implementations of Index with a single implementation. In English:

"For each type T, I and constant N of type usize, implement Index for LocalStorageVec<T, N>, where I implements LocalStorageVecIndex and [T] implements Index"

But it seems the trait bound LocalStorageVecIndex on I is unnecessary. Removing it also works, e.g.:

impl<T, I, const N: usize> std::ops::Index<I> for LocalStorageVec<T, N>
where
    [T]: std::ops::Index<I>,
{
    type Output = <[T] as std::ops::Index<I>>::Output;

    fn index(&self, index: I) -> &Self::Output {
        self.as_ref().index(index)
    }
}

or

impl<T, const N: usize, I> std::ops::Index<I> for LocalStorageVec<T, N>
where
    I: std::slice::SliceIndex<[T]>,
{
    type Output = I::Output;

    fn index(&self, index: I) -> &Self::Output {
        self.as_ref().index(index)
    }
}

Both compile and run without error.