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+# What `#[derive(AddAssign)]` generates
+
+This code is very similar to the code that is generated for `#[derive(Add)]`.
+The difference is that it mutates the existing instance instead of creating a
+new one.
+
+
+
+
+## Tuple structs
+
+When deriving `AddAssign` for a tuple struct with two fields like this:
+
+```rust
+# use derive_more::AddAssign;
+#
+#[derive(AddAssign)]
+struct MyInts(i32, i32);
+```
+
+Code like this will be generated:
+
+```rust
+# struct MyInts(i32, i32);
+impl ::core::ops::AddAssign for MyInts {
+    fn add_assign(&mut self, rhs: MyInts) {
+        self.0.add_assign(rhs.0);
+        self.1.add_assign(rhs.1);
+    }
+}
+```
+
+The behaviour is similar with more or less fields.
+
+
+
+
+## Regular structs
+
+When deriving for a regular struct with two fields like this:
+
+```rust
+# use derive_more::AddAssign;
+#
+#[derive(AddAssign)]
+struct Point2D {
+    x: i32,
+    y: i32,
+}
+```
+
+Code like this will be generated:
+
+```rust
+# struct Point2D {
+#     x: i32,
+#     y: i32,
+# }
+impl ::core::ops::AddAssign for Point2D {
+    fn add_assign(&mut self, rhs: Point2D) {
+        self.x.add_assign(rhs.x);
+        self.y.add_assign(rhs.y);
+    }
+}
+```
+
+The behaviour is similar with more or less fields.
+
+
+
+
+## Enums
+
+Deriving `AddAssign` is not (yet) supported for enums.
+This is mostly due to the fact that it is not trivial convert the `Add`
+derivation code, because that returns a `Result<EnumType>` instead of an
+`EnumType`.
+Handling the case where it errors would be hard and maybe impossible.