# 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 derive_more::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 derive_more::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.