[BUG] rust-server codegen: enum parameters in form bodies incorrectly serialized as their enum variant names by clients, rather than the values described in the OpenAPI definition

[holdeg]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

For endpoints with request bodies of type form (e.g. application/x-www-form-urlencoded) and enum properties, the generated Rust client invokes format!("{:?}", param_foo) where param_foo is the model generated in models.rs for the enum property. {:?} is the debug formatter, which uses the derived impl Debug on the enum model, which resolves to the name of the enum variant (as per the rust docs) - however, we should instead be serializing this with the impl Display on the enum model, which produces the defined allowable values from the API definition. This incorrect serialization does not obey the OpenAPI spec and will cause any server policing the value of the enum to reject requests created by this client.

openapi-generator version

Present in all 7.x releases

OpenAPI declaration file content or url

openapi: "3.0.3"
info:
  title: Enum Form Parameter Example
  version: "1.0.0"
paths:
  /authentication-endpoint:
    post:
      operationId: authenticate
      requestBody:
        required: true
        content:
          application/x-www-form-urlencoded:
            schema:
              type: object
              properties:
                target_resource:
                  type: string
                  enum:
                    - resource_one
                    - resource_two
                  description: "Resource to authenticate for - must be one of `resource_one` or `resource_two`"
      responses:
        '200':
          description: Authorization successful
          content:
            application/json:
              schema:
                type: object
                properties:
                  token:
                    type: string

Generation Details

No specific generation is required.

Steps to reproduce

1. Generate Rust bindings for the above spec with rust-server
2. Open generated src/client/mod.rs bindings, find authenticate method
3. Observe the serialization of the enum form:

4. Call this method and note that target_resource is supplied as ResourceOne or ResourceTwo, not their correct serializations

Expected: it should be set to the serialized version, resource_one or resource_two.

Suggest a fix

The RustServerCodegen.java file should check if the parameter is an enum and set x-format-string to {} instead of {:?} - this is safe because all models impl Display and all enums have allowedValues in their impl Display.