100 exercises to learn Rust

book/src/06_ticket_management/14_index_mut.md

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+# 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.
+
+```rust
+// 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.