[rust] Fix generation for optional and nullable fields (double option pattern) (#13177)

* Fix generation for optional and nullable fields (double option pattern)

* Only import serde_with if necessary

samples/client/petstore/rust/hyper/petstore/.openapi-generator/FILES

@@ -7,6 +7,7 @@ docs/ApiResponse.md
 docs/Baz.md
 docs/Category.md
 docs/FakeApi.md
+docs/OptionalTesting.md
 docs/Order.md
 docs/Pet.md
 docs/PetApi.md
@@ -35,6 +36,7 @@ src/models/baz.rs
 src/models/category.rs
 src/models/mod.rs
 src/models/model_return.rs
+src/models/optional_testing.rs
 src/models/order.rs
 src/models/pet.rs
 src/models/property_test.rs

samples/client/petstore/rust/hyper/petstore/Cargo.toml

@@ -7,6 +7,7 @@ edition = "2018"
 [dependencies]
 serde = "^1.0"
 serde_derive = "^1.0"
+serde_with = "^2.0"
 serde_json = "^1.0"
 url = "^2.2"
 uuid = { version = "^1.0", features = ["serde"] }

samples/client/petstore/rust/hyper/petstore/README.md

@@ -56,6 +56,7 @@ Class | Method | HTTP request | Description
  - [ApiResponse](docs/ApiResponse.md)
  - [Baz](docs/Baz.md)
  - [Category](docs/Category.md)
+ - [OptionalTesting](docs/OptionalTesting.md)
  - [Order](docs/Order.md)
  - [Pet](docs/Pet.md)
  - [PropertyTest](docs/PropertyTest.md)

samples/client/petstore/rust/hyper/petstore/docs/OptionalTesting.md

@@ -0,0 +1,14 @@
+# OptionalTesting
+
+## Properties
+
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**optional_nonnull** | Option<**String**> |  | [optional]
+**required_nonnull** | **String** |  | 
+**optional_nullable** | Option<**String**> |  | [optional]
+**required_nullable** | Option<**String**> |  | 
+
+[[Back to Model list]](../README.md#documentation-for-models) [[Back to API list]](../README.md#documentation-for-api-endpoints) [[Back to README]](../README.md)
+
+

samples/client/petstore/rust/hyper/petstore/src/models/mod.rs

@@ -6,6 +6,8 @@ pub mod baz;
 pub use self::baz::Baz;
 pub mod category;
 pub use self::category::Category;
+pub mod optional_testing;
+pub use self::optional_testing::OptionalTesting;
 pub mod order;
 pub use self::order::Order;
 pub mod pet;

samples/client/petstore/rust/hyper/petstore/src/models/optional_testing.rs

@@ -0,0 +1,39 @@
+/*
+ * OpenAPI Petstore
+ *
+ * This is a sample server Petstore server. For this sample, you can use the api key `special-key` to test the authorization filters.
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ * Generated by: https://openapi-generator.tech
+ */
+
+/// OptionalTesting : Test handling of optional and nullable fields
+
+
+
+#[derive(Clone, Debug, PartialEq, Default, Serialize, Deserialize)]
+pub struct OptionalTesting {
+    #[serde(rename = "optional_nonnull", skip_serializing_if = "Option::is_none")]
+    pub optional_nonnull: Option<String>,
+    #[serde(rename = "required_nonnull")]
+    pub required_nonnull: String,
+    #[serde(rename = "optional_nullable", default, with = "::serde_with::rust::double_option", skip_serializing_if = "Option::is_none")]
+    pub optional_nullable: Option<Option<String>>,
+    #[serde(rename = "required_nullable", deserialize_with = "Option::deserialize")]
+    pub required_nullable: Option<String>,
+}
+
+impl OptionalTesting {
+    /// Test handling of optional and nullable fields
+    pub fn new(required_nonnull: String, required_nullable: Option<String>) -> OptionalTesting {
+        OptionalTesting {
+            optional_nonnull: None,
+            required_nonnull,
+            optional_nullable: None,
+            required_nullable,
+        }
+    }
+}
+
+