Derived default on enums ignored

[JakeStanger]

Although using #[serde(default)] does work, it's usually cleaner for enums to use the Rust built-in Default derive, with #[default] on a key. Schemars does not currently detect this:

use schemars::JsonSchema;

#[derive(Debug, JsonSchema, Default)]
enum Enum {
    Foo,
    #[default]
    Bar
}

fn main() {
    let val = Enum::default();
    println!("{:?}", val); // "Bar"

    let schema = schemars::schema_for!(Enum);
    println!(
        "{}",
        serde_json::to_string_pretty(&schema).expect("to be serializable")
    );
}

This results in the following schema, with no default key present:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "title": "Enum",
  "type": "string",
  "enum": [
    "Foo",
    "Bar"
  ]
}