[BUG][Rust] reqwest client does not compile for required nullable params, free-form object query params, inline object form fields and a param named configuration

[HardMax71]

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?

Description

The rust generator (default reqwest library) emits code that doesn't compile for four parameter shapes. They came up generating a client for GitLab's description (doc/api/openapi/openapi_v3.yaml, 19.5), where they account for 214 of the 219 compile errors. Four more come from path parameters GitLab uses without declaring, and one from .to_string() on an array-of-objects form field, which I haven't reduced. The spec below reduces the four to three operations.

1. A parameter that is required and nullable: true gets an Option type, but the template takes the required branch and uses it as a plain value. For a query array it iterates the Option and calls .to_string() on the whole Vec<String>; for a multipart file it passes Option<PathBuf> to TokioFile::open. This is 158 of GitLab's errors.
2. A free-form object query parameter (type: object, no properties) is typed models::serde_json::Value.
3. An inline object form field is typed models::models::CreateImportRequestConfiguration. Together with 2, that's 48 errors on GitLab.
4. A form field named configuration becomes a second parameter named configuration, next to the &configuration::Configuration argument, and the rest of the function then fails to type-check (8 errors on GitLab).

openapi-generator version

7.25.0.

OpenAPI declaration file content or url

openapi: 3.0.3
info:
  title: rust reqwest compile errors
  version: "1"
paths:
  /variables:
    delete:
      operationId: deleteVariables
      parameters:
        - name: filter
          in: query
          schema:
            type: object
            nullable: true
        - name: paths
          in: query
          required: true
          schema:
            type: array
            nullable: true
            items:
              type: string
      responses:
        "204":
          description: deleted
  /imports:
    post:
      operationId: createImport
      requestBody:
        content:
          application/x-www-form-urlencoded:
            schema:
              type: object
              properties:
                configuration:
                  type: object
                  properties:
                    url:
                      type: string
      responses:
        "201":
          description: created
  /uploads:
    post:
      operationId: upload
      requestBody:
        content:
          multipart/form-data:
            schema:
              type: object
              required:
                - file
              properties:
                file:
                  type: string
                  format: binary
                  nullable: true
      responses:
        "201":
          description: created

Generation Details

java -jar openapi-generator-cli-7.25.0.jar generate -g rust -i spec.yaml -o out --additional-properties=packageName=repro
cd out && cargo check --message-format=short

Steps to reproduce

src/apis/default_api.rs:42:74  error[E0415]: identifier `configuration` is bound more than once in this parameter list
src/apis/default_api.rs:42:104  error[E0433]: cannot find `models` in `models`
src/apis/default_api.rs:72:128  error[E0433]: cannot find `serde_json` in `models`
src/apis/default_api.rs:84:96  error[E0599]: `Vec<std::string::String>` doesn't implement `std::fmt::Display`
src/apis/default_api.rs:116:16  error[E0277]: the trait bound `std::option::Option<PathBuf>: AsRef<Path>` is not satisfied
src/apis/default_api.rs:118:33  error[E0599]: no method named `file_name` found for enum `std::option::Option<T>`

(plus follow-on errors from the duplicate configuration). The generated lines:

pub async fn create_import(configuration: &configuration::Configuration, configuration: Option<models::models::CreateImportRequestConfiguration>) -> Result<(), Error<CreateImportError>> {
pub async fn delete_variables(configuration: &configuration::Configuration, paths: Option<Vec<String>>, filter: Option<models::serde_json::Value>) -> Result<(), Error<DeleteVariablesError>> {
        "multi" => req_builder.query(&p_query_paths.into_iter().map(|p| ("paths".to_owned(), p.to_string())).collect::<Vec<(std::string::String, std::string::String)>>()),
    let file = TokioFile::open(&p_form_file).await?;

Suggest a fix

Treat required nullable parameters like optional ones in the query and multipart templates, keep serde_json::Value and already-qualified model types free of the extra models:: prefix, and escape a parameter named configuration the way other colliding names are escaped.