Why has Index trait has an associated type Output and not IndexMut?

Here: IndexMut in std::ops - Rust

and here: Index in std::ops - Rust

Am I missing something?

Notice that it's dependent, IndexMut: Index, so it reuses the same Output type. So here:

    fn index_mut(&mut self, index: Idx) -> &mut Self::Output;

It really means -> &mut <Self as Index>::Output.

Thanks a lot, I was definitely missing something !