deserialization of discriminator models is broken

[cataggar]

In the OpenAPI spec, the definitions with discriminator act as a base type and a union. We are modeling the base type fine by using #[serde(flatten)] in the children. The problem is we are not modeling the union. In Rust, we should have both types.

For naming, here are a couple of options.

1. Add a new type with a Union suffix.
2. Append Base to the existing base type.

I manually tried out both in the internal PR and I prefer adding a Union type, but open to discussion.

Using specification\vmware\resource-manager\Microsoft.AVS\stable\2022-05-01\vmware.json as example. The relevant models would be:

#[doc = "A vSphere Distributed Resource Scheduler (DRS) placement policy"]
#[derive(Clone, Debug, PartialEq, Serialize, Deserialize, Default)]
pub struct PlacementPolicy {
    #[serde(flatten)]
    pub resource: Resource,
    #[doc = "Abstract placement policy properties"]
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub properties: Option<PlacementPolicyPropertiesUnion>,
}

#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
#[serde(tag = "type")]
pub enum PlacementPolicyPropertiesUnion {
    VmVm(VmVmPlacementPolicyProperties),
    VmHost(VmHostPlacementPolicyProperties),
}

#[doc = "Abstract placement policy properties"]
#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
pub struct PlacementPolicyProperties {
    #[doc = "Whether the placement policy is enabled or disabled"]
    #[serde(default, skip_serializing_if = "Option::is_none")]
    pub state: Option<placement_policy_properties::State>,
    #[doc = "Display name of the placement policy"]
    #[serde(rename = "displayName", default, skip_serializing_if = "Option::is_none")]
    pub display_name: Option<String>,
    #[doc = "The provisioning state"]
    #[serde(rename = "provisioningState", default, skip_serializing_if = "Option::is_none")]
    pub provisioning_state: Option<placement_policy_properties::ProvisioningState>,
}

#[doc = "VM-VM placement policy properties"]
#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
pub struct VmVmPlacementPolicyProperties {
    #[serde(flatten)]
    pub placement_policy_properties: PlacementPolicyProperties,
    #[doc = "Virtual machine members list"]
    #[serde(rename = "vmMembers")]
    pub vm_members: Vec<String>,
    #[doc = "Placement policy affinity type"]
    #[serde(rename = "affinityType")]
    pub affinity_type: AffinityType,
}

#[doc = "VM-Host placement policy properties"]
#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
pub struct VmHostPlacementPolicyProperties {
    #[serde(flatten)]
    pub placement_policy_properties: PlacementPolicyProperties,
    #[doc = "Virtual machine members list"]
    #[serde(rename = "vmMembers")]
    pub vm_members: Vec<String>,
    #[doc = "Host members list"]
    #[serde(rename = "hostMembers")]
    pub host_members: Vec<String>,
    #[doc = "Placement policy affinity type"]
    #[serde(rename = "affinityType")]
    pub affinity_type: AffinityType,
    #[doc = "VM-Host placement policy affinity strength (should/must)"]
    #[serde(rename = "affinityStrength", default, skip_serializing_if = "Option::is_none")]
    pub affinity_strength: Option<AffinityStrength>,
    #[doc = "Placement policy hosts opt-in Azure Hybrid Benefit type"]
    #[serde(rename = "azureHybridBenefitType", default, skip_serializing_if = "Option::is_none")]
    pub azure_hybrid_benefit_type: Option<AzureHybridBenefitType>,
}

I'm not sure what to do about unknown union variants, but may be that can be a future step.