fix union types

[cataggar]

This fixes a bug from #1414 which causes deserialization to fail.

• create the base type without the discriminator
• adds the description to the union type
• fixes union types for vector aliases
• fixes support for nested unions
• removes empty base types

[demoray]

I'm digging into this and found at least one issue that I don't think is right.

In services/svc/agrifood/src/package_2023_07_01_preview/models.rs, AuthCredentialsKind is one of the enums defined in the spec. It's generated, but never used. As is AuthCredentialsUnion.

From looking at the spec, these should be used in ApiKeyAuthCredentials, but that doesn't include it.

Ref: https://github.com/Azure/azure-rest-api-specs/blob/a26794fe26cdc68f4d18bc782e0b4f2f29e24b40/specification/agrifood/data-plane/Microsoft.AgFoodPlatform/preview/2023-07-01-preview/agfood.json#L13738-L13758

[cataggar]

Thanks for digging in. The generated code for that example looks fine to me. Here is the Union generated from #/definitions/AuthCredentials".

#[doc = "CredentialTypeEnum."]
#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
#[serde(tag = "kind")]
pub enum AuthCredentialsUnion {
    ApiKeyAuthCredentials(ApiKeyAuthCredentials),
    OAuthClientCredentials(OAuthClientCredentials),
}


#[doc = "Api Key Auth Credentials class for API Key based Auth."]
#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
pub struct ApiKeyAuthCredentials {
    #[doc = "Properties of the key vault."]
    #[serde(rename = "apiKey", default, skip_serializing_if = "Option::is_none")]
    pub api_key: Option<KeyVaultProperties>,
}

#[doc = "OAuthClientCredentials for clientId clientSecret auth."]
#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
pub struct OAuthClientCredentials {
    #[doc = "ClientId associated with the provider."]
    #[serde(rename = "clientId", default, skip_serializing_if = "Option::is_none")]
    pub client_id: Option<String>,
    #[doc = "Properties of the key vault."]
    #[serde(rename = "clientSecret", default, skip_serializing_if = "Option::is_none")]
    pub client_secret: Option<KeyVaultProperties>,
}

It is strange that that definition is not referenced anywhere else in the spec. Most specs then define the string enumeration inline, but the spec as defined it externally

 "AuthCredentials": {
      "description": "AuthCredentials abstract base class for Auth Purpose.",
      "required": [
        "kind"
      ],
      "type": "object",
      "properties": {
        "kind": {
          "$ref": "#/definitions/AuthCredentialsKind"
        }
      },
      "discriminator": "kind"
    },
    "AuthCredentialsKind": {
      "description": "CredentialTypeEnum.",
      "enum": [
        "OAuthClientCredentials",
        "ApiKeyAuthCredentials"
      ],
      "type": "string",
      "x-ms-enum": {
        "name": "AuthCredentialsKind",
        "modelAsString": true
      }
    },

Which is why there is a enum generated:

#[doc = "CredentialTypeEnum."]
#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
#[serde(remote = "AuthCredentialsKind")]
pub enum AuthCredentialsKind {
    OAuthClientCredentials,
    ApiKeyAuthCredentials,
    #[serde(skip_deserializing)]
    UnknownValue(String),
}

It is not used anywhere else in the spec, but it is not wrong to also generate it. Usually, you are going to reference it elsewhere.