[BUG][RUST][CLIENT] Generated default_api.rs doesn't import referenced type

[eliaperantoni]

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?
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Description

If an endpoint uses an enum as the path parameter, and the enum is defined in components rather than inline, then the generated default_api.rs uses the generated Rust enum without importing it first, resulting in a compilation error.

error[E0412]: cannot find type `Color` in this scope
  --> src/apis/default_api.rs:26:82
   |
26 | pub async fn demo_color_get(configuration: &configuration::Configuration, color: Color) -> Result<(), Error<DemoColorGetError>> {
   |                                                                                  ^^^^^ not found in this scope
   |
help: consider importing this enum through its public re-export
   |
12 + use crate::models::Color;

I've found that using the enum in the schema like this:

schema:
  allOf:
    - $ref: "#/components/schemas/Color"

instead of

schema:
  $ref: "#/components/schemas/Color"

fixes the issue. But still, it should work in both cases.

openapi-generator version

I'm using the Docker openapitools/openapi-generator-cli:latest image. openapi-generator-cli version prints 7.1.0-SNAPSHOT.

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: Demo
  version: 0.0.0
paths:
  /demo/{color}:
    get:
      parameters:
        - name: color
          required: true
          in: path
          schema:
            $ref: "#/components/schemas/Color"
      responses:
        200:
          description: OK
components:
  schemas:
    Color:
      type: string
      enum:
        - RED
        - GREEN
        - BLUE

Generation Details

$ openapi-generator generate -i openapi.yaml -g rust -o demo

[j-szulc]

I have started the work on #17503 . Together with #17504 (fixing linked issue #16975 ) it makes the provided openapi.yml generate a buildable Rust client.