Error: overflow evaluating the requirement ... in Serde, raising recursion limit doesn't help

[LPGhatguy]

I was working through a couple Serde issues with @dtolnay when I hit a bad error:

error[E0275]: overflow evaluating the requirement `<&mut serde_json::Serializer<&mut std::vec::Vec<u8>> as serde::Serializer>::Error`
  |
  = help: consider adding a `#![recursion_limit="128"]` attribute to your crate

Minimal playground example:
https://play.rust-lang.org/?gist=c19a458d3246d211beb1ae7d88c0ddee&version=stable

Raising the recursion limit causes the compiler to ask me to raise the recursion limit again and again, out forever.

@dtolnay recommended I file an issue here after minimizing the error case:

<dtolnay> wow that's crazy
<dtolnay> this part looks like a compiler bug -- that associated type should be easy to figure out
<dtolnay> lpg: see how much you can minimize this "overflow evaluating the requirement" error and file an issue in github.com/rust-lang/rust

[dtolnay]

#[derive(Serialize)]
#[serde(tag = "type")]
enum Expression {
    Foo,
    Baz(Box<Expression>),
}

After taking a closer look, I think this is a case of the compiler catching something that legitimately doesn't make sense (albeit with not a great error message). In this Expression type how would you expect Baz(Box::new(Baz(Box::new(Foo)))) to be represented in JSON?

{
  "type": "Baz",
  ??? "type": "Baz",
  ??? ??? "type": "Foo"
}

If your expression is a Baz then the "type" tag needs to be inside the nested expression, but if that one is also a Baz then both your tag and its tag need to be inside the next nested expression, but if that one is a Baz as well then etc, overflow.

I filed #50043 to detect this particular type of overflow and handle it with a better error message.

[Mark-Simulacrum]

It seems like there's nothing in this issue that needs to be fixed? That is, #50043 is the underlying issue. So I'm going to close this.