External tagging for unit-like enum variants

[JakkuSakura]

I ran into this problem today.
I have a source code and I try to add serde support to deal with external data.

// no #derive[serde::Deserialize]
enum Foo {
   A,
   B
}

But the data I got is

{ "A": {} }

Is there a way to represent things like #[serde(external_tagged)]?
I can certainly change A to A{}, but it's a breaking change, and many other places need fixes.