PartialEq + "compare with" for two fields?

[Dushistov]

Describe the bug
I have type like this:

struct Foo {
  default_idx: Option<usize>,
  list: Vec<Boo>,
  //many other fields
}

I want generate PartialEq in such way that:

    fn eq(&self, o: &Foo) -> bool {
         self.default_idx.map(|idx| &self.list[idx]) == o.default_idx.map(|idx| &o.list[idx]) &&      
      // && self.other_fields == o.other_fields

It would be nice if there is way to generate code for comparing two fields at once.
The most simple way as I see will be syntax like this:

#[derivative(PartialEq(fn = compare_foo))]
struct Foo {
 #[derivative(PartialEq="ignore")]
default_idx: Option<usize>,
  list: Vec<Boo>,
 //many other fields
}

and it generate compare_foo function that compare only many other fields:

fn compare_foo(lhs: &Foo, rhs: &Foo) -> bool {
 self.other_fields == o.other_fields
}

so I can write PartialEq by hands and compare several special fields in the way I want,
and then use generated compare_foo for other fields.

• Version of derivative: 1.0.3