[BUG] [rust] An empty enum variant becomes invalid code since v6.1.0

Created by: siketyan

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
Have you tested with the latest master to confirm the issue still exists?
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Description

Enum variants can have an "empty" variant, and in code generation it is transformed into some named variant like "SomeEnum::Empty" or something. Until v6.0.1, I confirmed the variant becomes valid code including the transformation for empty variants. Now since v6.1.0, the empty variant started to become invalid code without any transformation for that. See "generation details" section for details of code generation results.

openapi-generator version

It works < 6.1.0, and not working >= 6.1.0. I reproduced the issue on v6.1.0, and the latest using Docker image.

OpenAPI declaration file content or url

openapi: 3.0.0
servers:
  - url: 'http://example.com/'
info:
  version: 1.0.0
  title: Example
paths:
  /pet:
    post:
      summary: Add a new pet to the store
      operationId: addPet
      responses:
        '200':
          description: successful operation
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/Pet'
      requestBody:
        $ref: '#/components/requestBodies/Pet'
components:
  requestBodies:
    Pet:
      content:
        application/json:
          schema:
            $ref: '#/components/schemas/Pet'
      description: Pet object that needs to be added to the store
      required: true
  schemas:
    Pet:
      title: a Pet
      description: A pet for sale in the pet store
      type: object
      required:
        - name
      properties:
        name:
          type: string
          example: doggie
        status:
          type: string
          description: pet status in the store
          enum:
            - "" # This is an "empty" enum variant
            - available
            - pending
            - sold

Generation Details

docker run --rm -v $(pwd)/petstore.yaml:/petstore.yaml:ro -v $(pwd)/gen:/gen openapitools/openapi-generator-cli:v6.0.1 generate -g rust -i /petstore.yaml -o /gen

generates:

/// pet status in the store
#[derive(Clone, Copy, Debug, Eq, PartialEq, Ord, PartialOrd, Hash, Serialize, Deserialize)]
pub enum Status {
    #[serde(rename = "")]
    Empty,
    #[serde(rename = "available")]
    Available,
    #[serde(rename = "pending")]
    Pending,
    #[serde(rename = "sold")]
    Sold,
}

But,

docker run --rm -v $(pwd)/petstore.yaml:/petstore.yaml:ro -v $(pwd)/gen:/gen openapitools/openapi-generator-cli:v6.1.0 generate -g rust -i /petstore.yaml -o /gen

generates:

/// pet status in the store
#[derive(Clone, Copy, Debug, Eq, PartialEq, Ord, PartialOrd, Hash, Serialize, Deserialize)]
pub enum Status {
    #[serde(rename = "")]
    ,
    #[serde(rename = "available")]
    Available,
    #[serde(rename = "pending")]
    Pending,
    #[serde(rename = "sold")]
    Sold,
}

that is invalid Rust code of course.

Steps to reproduce

See "generation details" section.

Related issues/PRs

#13231 may cause this issue (I don't know details of the changes, so this is just my expectation).