[rust client] Incomplete type in response generates code that fails to compile

Created by: lzybkr

Description

The Rust generated code fails to compile if the api defines a response type of object with no additional information. Other code generators like C# do generate code that compiles and runs even if it isn't a very useful response.

The errors look like:

error[E0412]: cannot find type `Value` in this scope
  --> src\apis\file_api.rs:34:44
   |
34 |     fn clone(&self, ) -> Box<Future<Item = Value, Error = Error<serde_json::Value>>>;
   |                                            ^^^^^ not found in this scope
help: possible candidates are found in other modules, you can import them into scope
   |
11 | use serde_json::Value;
   |
11 | use serde_json::value::Value;
   |

error[E0412]: cannot find type `Value` in this scope
  --> src\apis\file_api.rs:39:44
   |
39 |     fn clone(&self, ) -> Box<Future<Item = Value, Error = Error<serde_json::Value>>> {
   |                                            ^^^^^ not found in this scope
help: possible candidates are found in other modules, you can import them into scope
   |
11 | use serde_json::Value;
   |
11 | use serde_json::value::Value;

openapi-generator version

3.3.2

OpenAPI declaration file content or url

{
  "swagger": "2.0",
  "info": {
    "version": "v1.0",
    "title": "test api"
  },
  "paths": {
    "/api/clone": {
      "put": {
        "tags": [
          "File"
        ],
        "operationId": "Clone",
        "consumes": [],
        "produces": [
          "application/json",
          "text/json"
        ],
        "parameters": [],
        "responses": {
          "200": {
            "description": "OK",
            "schema": {
              "type": "object"
            }
          }
        }
      }
    }
  }
}

Command line used for generation

java -jar openapi-generator-cli.jar generate -i api.json -g rust

Steps to reproduce

java -jar openapi-generator-cli.jar generate -i api.json -g rust
cargo build

Related issues/PRs

Suggest a fix/enhancement

It looks like the return type could be serde_json::Value instead of Value, maybe as simple as changing this line.