[BUG][Default][Rust] Free-form model generation

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?
• 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

Free-form models without validation which are not allOf included in any composed schema are not generated, though valid OpenAPI documents exist which make use of these objects. In larger, specification-based OpenAPI documents (such as [DMTF's Redfish schema|https://redfish.dmtf.org/]), these objects are leveraged to specify implementation-defined objects, which can contain any number of properties required by a valid implementation of the specification. This allows, for example, me to use a different request body for some requests than that which is required for a Redfish-compliant implementation provided by IBM or HP Enterprise hardware (etc).

Actual output: The model User in the below schema is not generated, though it is referenced in the requestBody of an action on a URL.
Expected output: The model User is generated.

openapi-generator version

Version 6.3.0, HEAD of master. This is not a regression.

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  description: "Optional multiline or single-line description in [CommonMark](http://commonmark.org/help/)\
    \ or HTML."
  title: Sample API
  version: 0.1.9
servers:
- description: "Optional server description, e.g. Main (production) server"
  url: http://api.example.com/v1
- description: "Optional server description, e.g. Internal staging server for testing"
  url: http://staging-api.example.com
paths:
  /users:
    get:
      description: Optional extended description in CommonMark or HTML.
      requestBody:
        content:
          application/json:
            schema:
              $ref: '#/components/schemas/User'
      responses:
        "200":
          description: A JSON array of user names
      summary: Returns a list of users.
components:
  schemas:
    User:
      additionalProperties: false
      description: A user
      properties: {}
      type: object

Generation Details

See steps to reproduce

Steps to reproduce

1. Attempt to generate the source using the rust-server generator.

openapi-generator -g rust-server -i openapi.yaml -o test-models

2. Attempt to compile the source (observe compilation errors about missing struct definitions)

cargo build
   Compiling openapi_client v0.1.9 (/home/edtwardy/Git/redfish/small-test/test-models)
error[E0412]: cannot find type `User` in module `models`
  --> src/lib.rs:34:30
   |
34 |         body: Option<models::User>,
   |                              ^^^^ not found in `models`

error[E0412]: cannot find type `User` in module `models`
  --> src/lib.rs:51:30
   |
51 |         body: Option<models::User>,
   |                              ^^^^ not found in `models`

error[E0412]: cannot find type `User` in module `models`
  --> src/lib.rs:82:30
   |
82 |         body: Option<models::User>,
   |                              ^^^^ not found in `models`

error[E0412]: cannot find type `User` in module `models`
   --> src/client/mod.rs:385:36
    |
385 |         param_body: Option<models::User>,
    |                                    ^^^^ not found in `models`

error[E0412]: cannot find type `User` in module `models`
   --> src/server/mod.rs:151:64
    |
151 | ...                   let param_body: Option<models::User> = if !body.is_empty() {
    |                                                      ^^^^ not found in `models`

For more information about this error, try `rustc --explain E0412`.
error: could not compile `openapi_client` due to 5 previous errors

Related issues/PRs

#5969, #5970: The same issue, observed for the same generator, a few years ago. Presumably, the PR was closed because isFreeFormObject was still provided by the Mustache object model at the time. This is no longer the case since commit #d4b8ff60

#7361, #7373: Implementation of a subset of use cases for free-form objects in OpenAPI documents.

Suggest a fix

I have a commit which fixes the issue on my fork. This commit allows templates to once again use {{#isFreeFormObject}} to specially handle free-form objects. Additionally, the third case of free-form objects not handled by #7373 is enabled if global property generateFreeFormModels is true.

Since CONTRIBUTING.md requests that an issue be opened first to enable discussion, I did not open a PR. testFreeFormSchemas(DefaultCodegenTest.java:2526) is currently failing. As soon as the community feels a PR is appropriate, I would be happy to open one and update the tests!

Start with the rust-server generator and DefaultCodegenTest.java:2526, which currently fails for free-form schemas. Review the linked fork commit and related issues #5969, #5970, #7361, and #7373 to understand the intended handling. Done means the User model is generated for the supplied schema, the rust-server output compiles, and the relevant test passes.