100-exercises-to-learn-rust/book/src/06_ticket_management/14_index_mut.md

Mutable indexing

Index allows read-only access. It doesn’t let you mutate the value you retrieved.

IndexMut

If you want to allow mutability, you need to implement the IndexMut trait.

// Slightly simplified
pub trait IndexMut<Idx>: Index<Idx>
{
    // Required method
    fn index_mut(&mut self, index: Idx) -> &mut Self::Output;
}

IndexMut can only be implemented if the type already implements Index, since it unlocks an additional capability.