[BUG][RUST] deepObject parameters don't compile

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

When using deepObjects in query strings the Rust client no longer compiles, this is for scenarios such as;

/mypath?page[size]=20&page[offset]=100

This is supported by the spec but, the struct that gets created can't be returned back as a string for the URI.

openapi-generator version

• 6.5.0

I have also tested in prior versions and it also failed, I check the mustache files and it looks like this never worked

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  title: Pets
  version: '1.0'
  description: Example of spec with an object query param
paths:
  '/pets':
    get:
      summary: List pets
      description: Paginated list of pets
      operationId: get-pets-list
      parameters:
        - $ref: '#/components/parameters/pagination-page'
      responses:
        '200':
          description: List pets
          content:
            application/json:
              schema:
                description: Pet schema
                type: object
                properties:
                  meta:
                    type: object
                    properties:
                      results:
                        type: number
                    required:
                      - results
                  data:
                    type: array
                    items:
                      type: object
                      properties:
                        name:
                          type: string
                      required:
                        - name
              examples:
                paginated-pets:
                  value:
                    meta:
                      results: 20
                      data:
                        - name: pet1
                        - name: pet2
      tags:
        - test

components:
  parameters:
    pagination-page:
      in: query
      name: page
      schema:
        type: object
        properties:
          before:
            type: integer
            example: 20
            description: Offset
          after:
            type: integer
            example: 20
            description: Offset
          size:
            type: integer
            example: 20
            description: Limit
      style: deepObject
      explode: true
      description: Query parameters for pagination


tags:
  - name: test

Generation Details

Place the above yaml in a file named object-params.yaml;

mkdir -p ./out/test-rust
docker run --rm -v "${PWD}:/local" openapitools/openapi-generator-cli:v6.5.0 generate \
  -i /local/object-params.yaml \
  -g rust \
  --library=hyper \
  --additional-properties=packageName=openapi-test-client,packageVersion=1.0.0 \
  -o /local/out/test-rust

Steps to reproduce

Running cargo build on the output produces;

   Compiling openapi-test-client v1.0.0 (/home/user/openapi-gen-rust-test/out/test-rust)
error[E0599]: the method `to_string` exists for reference `&GetPetsListPageParameter`, but its trait bounds were not satisfied
   --> src/apis/test_api.rs:48:33
    |
48  |             let query_value = s.to_string();
    |                                 ^^^^^^^^^ method cannot be called on `&GetPetsListPageParameter` due to unsatisfied trait bounds
    |
   ::: src/models/get_pets_list_page_parameter.rs:15:1
    |
15  | pub struct GetPetsListPageParameter {
    | -----------------------------------
    | |
    | doesn't satisfy `GetPetsListPageParameter: ToString`
    | doesn't satisfy `GetPetsListPageParameter: std::fmt::Display`
    |
    = note: the following trait bounds were not satisfied:
            `GetPetsListPageParameter: std::fmt::Display`
            which is required by `GetPetsListPageParameter: ToString`
            `&GetPetsListPageParameter: std::fmt::Display`
            which is required by `&GetPetsListPageParameter: ToString`
note: the trait `std::fmt::Display` must be implemented
   --> /home/andrew-s/.rustup/toolchains/stable-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/library/core/src/fmt/mod.rs:786:1
    |
786 | pub trait Display {
    | ^^^^^^^^^^^^^^^^^

For more information about this error, try `rustc --explain E0599`.
error: could not compile `openapi-test-client` due to previous error
warning: build failed, waiting for other jobs to finish...

Suggest a fix

It looks like the issue is within the model mustache file;

https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/rust/model.mustache

There's a toString for enums but not a way to put the struct back into a URI string as is needed by deep object parameters in the query string.

Start with modules/openapi-generator/src/main/resources/rust/model.mustache and the generated src/apis/test_api.rs and src/models/get_pets_list_page_parameter.rs from the supplied object-params.yaml. Run cargo build on the generated client to reproduce the failure. Done means the deepObject parameter can be converted into its URI query-string representation and the generated Rust client compiles.