[Rust] Add support for discriminator

[gferon]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\. If contributing template-only or documentation-only changes which will change sample output, be sure to build the project first.
• Filed the PR against the correct branch: master, 4.1.x, 5.0.x. Default: master.
• Copied the technical committee to review the pull request if your PR is targeting a particular programming language.

Description of the PR

• Add support for the discriminator feature of OpenAPI 3, and implement it with enum in Rust
• Add all missing explicit dyn to trait types to remove warnings
• Add missing re-export for properties that are enum (was missing from Add enum support to rust and skip none option serialization in clients #2244).

Example

A schema that looks like:

components:
  schemas:
    Pet:
      type: object
      required:
      - pet_type
      properties:
        pet_type:
          type: string
      discriminator:
        propertyName: pet_type
        mapping:
          cachorro: Dog
          reptilian: Lizard
    Cat:
      allOf:
      - $ref: '#/components/schemas/Pet'
      - type: object
        # all other properties specific to a `Cat`
        properties:
          name:
            type: string
    Dog:
      allOf:
      - $ref: '#/components/schemas/Pet'
      - type: object
        # all other properties specific to a `Dog`
        properties:
          bark:
            type: string
    Lizard:
      allOf:
      - $ref: '#/components/schemas/Pet'
      - type: object
        # all other properties specific to a `Lizard`
        properties:
          lovesRocks:
            type: boolean

would generate a Rust enum like:

#[derive(Debug, PartialEq, Serialize, Deserialize)]
#[serde(tag = "pet_type")]
pub enum Pet {
    #[serde(rename="cachorro")]
    Dog {
        #[serde(rename = "bark", skip_serializing_if = "Option::is_none")]
        bark: Option<String>,
    },
    #[serde(rename="reptilian")]
    Lizard {
        #[serde(rename = "lovesRocks", skip_serializing_if = "Option::is_none")]
        loves_rocks: Option<bool>,
    },
}

[wing328]

Errors reported by the Travis CI:

   Checking tokio-threadpool v0.1.15
    Checking tokio-udp v0.1.5
    Checking tokio-uds v0.2.5
    Checking tokio-tcp v0.1.3
    Checking tokio-fs v0.1.6
    Checking tokio v0.1.22
    Checking tokio-core v0.1.17
   Compiling serde_derive v1.0.101
    Checking tokio-proto v0.1.1
    Checking hyper v0.11.27
    Checking petstore_client v1.0.0 (/home/travis/build/OpenAPITools/openapi-generator/samples/client/petstore/rust)
error[E0252]: the name `Status` is defined multiple times
  --> src/models/mod.rs:10:9
   |
7  | pub use self::order::Status;
   |         ------------------- previous import of the type `Status` here
...
10 | pub use self::pet::Status;
   |         ^^^^^^^^^^^^^^^^^ `Status` reimported here
   |
   = note: `Status` must be defined only once in the type namespace of this module
help: you can use `as` to change the binding name of the import
   |
10 | pub use self::pet::Status as OtherStatus;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: unreachable expression
   --> src/apis/pet_api.rs:107:9
    |
107 | /         __internal_request::Request::new(hyper::Method::Post, "/pet/{petId}/uploadImage".to_string())
108 | |             .with_auth(__internal_request::Auth::Oauth)
109 | |             .with_path_param("petId".to_string(), pet_id.to_string())
110 | |             .with_form_param("additionalMetadata".to_string(), additional_metadata.to_string())
111 | |             .with_form_param("file".to_string(), unimplemented!())
    | |__________________________________________________________________^
    |
    = note: #[warn(unreachable_code)] on by default
error: aborting due to previous error
For more information about this error, try `rustc --explain E0252`.
error: Could not compile `petstore_client`.

ref: https://travis-ci.org/OpenAPITools/openapi-generator/builds/585652775

[gferon]

Errors reported by the Travis CI:

   Checking tokio-threadpool v0.1.15
    Checking tokio-udp v0.1.5
    Checking tokio-uds v0.2.5
    Checking tokio-tcp v0.1.3
    Checking tokio-fs v0.1.6
    Checking tokio v0.1.22
    Checking tokio-core v0.1.17
   Compiling serde_derive v1.0.101
    Checking tokio-proto v0.1.1
    Checking hyper v0.11.27
    Checking petstore_client v1.0.0 (/home/travis/build/OpenAPITools/openapi-generator/samples/client/petstore/rust)
error[E0252]: the name `Status` is defined multiple times
  --> src/models/mod.rs:10:9
   |
7  | pub use self::order::Status;
   |         ------------------- previous import of the type `Status` here
...
10 | pub use self::pet::Status;
   |         ^^^^^^^^^^^^^^^^^ `Status` reimported here
   |
   = note: `Status` must be defined only once in the type namespace of this module
help: you can use `as` to change the binding name of the import
   |
10 | pub use self::pet::Status as OtherStatus;
   |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
warning: unreachable expression
   --> src/apis/pet_api.rs:107:9
    |
107 | /         __internal_request::Request::new(hyper::Method::Post, "/pet/{petId}/uploadImage".to_string())
108 | |             .with_auth(__internal_request::Auth::Oauth)
109 | |             .with_path_param("petId".to_string(), pet_id.to_string())
110 | |             .with_form_param("additionalMetadata".to_string(), additional_metadata.to_string())
111 | |             .with_form_param("file".to_string(), unimplemented!())
    | |__________________________________________________________________^
    |
    = note: #[warn(unreachable_code)] on by default
error: aborting due to previous error
For more information about this error, try `rustc --explain E0252`.
error: Could not compile `petstore_client`.

ref: https://travis-ci.org/OpenAPITools/openapi-generator/builds/585652775

good catch, sorry for that. I've adapted my changes.

[gferon]

Pinging @bjgill and @richardwhiuk for a review

[bjgill]

Thanks. I'm quite busy this week, so will take a look next week unless this is urgent.

[bjgill]

Looks like a sensible change. Just a couple of questions.

FYI - I'm about to merge #3543, which will conflict with this. You should just need to regenerate the samples to sort out the conflicts.

[bjgill]

I've just merged the conflicting PR. You should just need to rebase onto master and re-generate the samples again.

[gferon]

I've just merged the conflicting PR. You should just need to rebase onto master and re-generate the samples again.

Done, I added a small change to the template files to remove some clippy warnings as well.

[bjgill]

Thanks. I'll merge this, which should fix #4079

[wing328]

@gferon thanks for the PR, which has been included in v4.2.0 release: https://twitter.com/oas_generator/status/1189824932345069569