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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.
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