Derive a Trait for particular variants

Question

Let\'s say I have the following Enum

enum MyEnum {
  VariantA,
  VariantB,
  VariantC,
}

I can derive the PartialEq trait for the whole enum by

Answer 1

Assuming you have a setup like:

#[derive(PartialEq)]
struct VarB{
    pub value: u32,
}

#[derive(PartialEq)]
enum MyEnum{
    VarA(VarA),
    VarB(VarB)
}

VarA comes from a different crate and you can't compile due to it not having derived PartialEq (or any other external trait).

You can solve that with the newtype pattern (assuming you have access to the relevent fields / accessors)

struct MyVarA(VarA);

impl PartialEq for MyVarA{
    fn eq(&self, other: &MyVarA) -> bool {
        self.0.value == other.0.value
    }

    fn ne(&self, other: &MyVarA) -> bool {
        self.0.value != other.0.value
    }
}

#[derive(PartialEq)]
struct VarB{
    value: u32,
}

#[derive(PartialEq)]
enum MyEnum{
    VarA(MyVarA),
    VarB(VarB)
}

further informations: https://doc.rust-lang.org/rust-by-example/generics/new_types.html