Panic when generating an API for a struct with an enum serde(flatten)ed into a struct.

[blinsay]

Following up on #1244, I've been trying to use a struct with a serde(flatten) field as a return type, and I'm getting a panic when generating an openapi description. The panic seems to only happen when calling write on a generated stub description - I can serve traffic fine!

Here's a repro:

use dropshot::HttpError;
use dropshot::HttpResponseOk;
use dropshot::RequestContext;
use schemars::JsonSchema;
use serde::Deserialize;
use serde::Serialize;

fn main() {
    let description = example_api_mod::stub_api_description().unwrap();
    let openapi = description.openapi("example", "0.1.0".parse().unwrap());

    openapi.write(&mut std::io::stdout().lock()).unwrap();
}

#[derive(Serialize, Deserialize, JsonSchema)]
struct CoolStruct {
    #[serde(flatten)]
    cool_enum: CoolEnum,

    another_thing: u16,
}

#[derive(Serialize, Deserialize, JsonSchema)]
#[serde(tag = "type")]
enum CoolEnum {
    Foo(Foo),
    Bar(Bar),
}

#[derive(Serialize, Deserialize, JsonSchema)]
struct Foo {
    thing_one: String,
}

#[derive(Serialize, Deserialize, JsonSchema)]
struct Bar {
    thing_one: String,
    thing_two: String,
}

#[dropshot::api_description]
trait ExampleApi {
    type Context;

    #[endpoint {
        method = GET,
        path = "/hello",
    }]
    async fn hello(
        ctx: RequestContext<Self::Context>,
    ) -> Result<HttpResponseOk<CoolStruct>, HttpError>;
}

Here's the full panic

thread 'main' panicked at /Users/benl/.cargo/registry/src/index.crates.io-6f17d22bba15001f/dropshot-0.15.1/src/schema_util.rs:466:31:
a schema can't have both a type and subschemas:
{
  "type": "object",
  "oneOf": [
    {
      "type": "object",
      "required": [
        "thing_one",
        "type"
      ],
      "properties": {
        "thing_one": {
          "type": "string"
        },
        "type": {
          "type": "string",
          "enum": [
            "Foo"
          ]
        }
      }
    },
    {
      "type": "object",
      "required": [
        "thing_one",
        "thing_two",
        "type"
      ],
      "properties": {
        "thing_one": {
          "type": "string"
        },
        "thing_two": {
          "type": "string"
        },
        "type": {
          "type": "string",
          "enum": [
            "Bar"
          ]
        }
      }
    }
  ],
  "required": [
    "another_thing"
  ],
  "properties": {
    "another_thing": {
      "type": "integer",
      "format": "uint16",
      "minimum": 0.0
    }
  }
}