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# What `#[derive(MulAssign)]` generates
This code is very similar to the code that is generated for `#[derive(Mul)]`.
The difference is that it mutates the existing instance instead of creating a
new one.
You can add the `#[mul_assign(forward)]` attribute if you don't want the same
semantics as `Mul`.
This will instead generate a `MulAssign` implementation with the same semantics
as `AddAssign`.
## Tuple structs
When deriving `MulAssign` for a tuple struct with two fields like this:
```rust
# use derive_more::MulAssign;
#
#[derive(MulAssign)]
struct MyInts(i32, i32);
```
Code like this will be generated:
```rust
# struct MyInts(i32, i32);
impl<__RhsT: ::core::marker::Copy> ::core::ops::MulAssign<__RhsT> for MyInts
where i32: ::core::ops::MulAssign<__RhsT>
{
fn mul_assign(&mut self, rhs: __RhsT) {
self.0.mul_assign(rhs);
self.1.mul_assign(rhs);
}
}
```
The behaviour is similar with more or less fields, except for the fact that
`__RhsT` does not need to implement `Copy` when only a single field is present.
## Regular structs
When deriving `MulAssign` for a regular struct with two fields like this:
```rust
# use derive_more::MulAssign;
#
#[derive(MulAssign)]
struct Point2D {
x: i32,
y: i32,
}
```
Code like this will be generated:
```rust
# struct Point2D {
# x: i32,
# y: i32,
# }
impl<__RhsT: ::core::marker::Copy> ::core::ops::MulAssign<__RhsT> for Point2D
where i32: ::core::ops::MulAssign<__RhsT>
{
fn mul_assign(&mut self, rhs: __RhsT) {
self.x.mul_assign(rhs);
self.y.mul_assign(rhs);
}
}
```
The behaviour is again similar with more or less fields, except that `Copy`
doesn't have to be implemented for `__Rhst` when the struct has only a single
field.
## Enums
Deriving `MulAssign` for enums is not (yet) supported.
This has two reason, the first being that deriving `Mul` is also not implemented
for enums yet.
The second reason is the same as for `AddAssign`.
Even if it would be deriving `Mul` was implemented it would likely return a
`Result<EnumType>` instead of an `EnumType`.
Handling the case where it errors would be hard and maybe impossible.