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+# What `#[derive(IndexMut)]` generates
+
+Deriving `IndexMut` only works for a single field of a struct.
+Furthermore it requires that the type also implements `Index`, so usually
+`Index` should also be derived.
+The result is that you will mutably index it's member directly.
+
+With `#[index_mut]` or `#[index_mut(ignore)]` it's possible to indicate the
+field that you want to derive `IndexMut` for.
+
+
+
+
+## Example usage
+
+```rust
+# use derive_more::{Index, IndexMut};
+#
+#[derive(Index, IndexMut)]
+struct MyVec(Vec<i32>);
+
+#[derive(Index, IndexMut)]
+struct Numbers {
+    #[index]
+    #[index_mut]
+    numbers: Vec<i32>,
+    useless: bool,
+}
+
+let mut myvec = MyVec(vec![5, 8]);
+myvec[0] = 50;
+assert_eq!(50, myvec[0]);
+
+let mut numbers = Numbers{numbers: vec![100, 200], useless: false};
+numbers[1] = 400;
+assert_eq!(400, numbers[1]);
+```
+
+
+
+
+## Regular structs
+
+When deriving `IndexMut` for a struct:
+
+```rust
+# use derive_more::{Index, IndexMut};
+#
+#[derive(Index, IndexMut)]
+struct Numbers {
+    #[index]
+    #[index_mut]
+    numbers: Vec<i32>,
+    useless: bool,
+}
+```
+
+Code like this will be generated to implement `IndexMut`:
+
+```rust
+# struct Numbers {
+#     numbers: Vec<i32>,
+#     useless: bool,
+# }
+# impl<__IdxT> ::core::ops::Index<__IdxT> for Numbers
+# where
+#     Vec<i32>: ::core::ops::Index<__IdxT>,
+# {
+#     type Output = <Vec<i32> as ::core::ops::Index<__IdxT>>::Output;
+#     #[inline]
+#     fn index(&self, idx: __IdxT) -> &Self::Output {
+#         <Vec<i32> as ::core::ops::Index<__IdxT>>::index(&self.numbers, idx)
+#     }
+# }
+impl<__IdxT> ::core::ops::IndexMut<__IdxT> for Numbers
+where
+    Vec<i32>: ::core::ops::IndexMut<__IdxT>,
+{
+    #[inline]
+    fn index_mut(&mut self, idx: __IdxT) -> &mut Self::Output {
+        <Vec<i32> as ::core::ops::IndexMut<__IdxT>>::index_mut(&mut self.numbers, idx)
+    }
+}
+```
+
+
+
+
+## Enums
+
+Deriving `IndexMut` is not supported for enums.