test(rust): Add test for anyOf support

This commit adds a test case to verify that anyOf schemas generate
proper untagged enums instead of empty structs in the Rust client
generator.

The test includes:
- A test OpenAPI spec with anyOf schemas
- Unit test that verifies the generated code structure
- Assertions to ensure enums are created instead of empty structs

Author: timvw

modules/openapi-generator/src/test/java/org/openapitools/codegen/rust/RustClientCodegenTest.java

@@ -271,4 +271,37 @@ public void testMultipleArrayTypesEnum() throws IOException {
         TestUtils.assertFileExists(outputPath);
         TestUtils.assertFileContains(outputPath, enumSpec);
     }
+
+    @Test
+    public void testAnyOfSupport() throws IOException {
+        Path target = Files.createTempDirectory("test-anyof");
+        final CodegenConfigurator configurator = new CodegenConfigurator()
+                .setGeneratorName("rust")
+                .setInputSpec("src/test/resources/3_0/rust/rust-anyof-test.yaml")
+                .setSkipOverwrite(false)
+                .setOutputDir(target.toAbsolutePath().toString().replace("\\", "/"));
+        List<File> files = new DefaultGenerator().opts(configurator.toClientOptInput()).generate();
+        files.forEach(File::deleteOnExit);
+        
+        // Test that ModelIdentifier generates an untagged enum, not an empty struct
+        Path modelIdentifierPath = Path.of(target.toString(), "/src/models/model_identifier.rs");
+        TestUtils.assertFileExists(modelIdentifierPath);
+        
+        // Should generate an untagged enum
+        String enumDeclaration = linearize("#[serde(untagged)] pub enum ModelIdentifier");
+        TestUtils.assertFileContains(modelIdentifierPath, enumDeclaration);
+        
+        // Should have String variant (for anyOf with string types)
+        TestUtils.assertFileContains(modelIdentifierPath, "String(String)");
+        
+        // Should NOT generate an empty struct
+        TestUtils.assertFileNotContains(modelIdentifierPath, "pub struct ModelIdentifier {");
+        TestUtils.assertFileNotContains(modelIdentifierPath, "pub fn new()");
+        
+        // Test AnotherAnyOfTest with mixed types
+        Path anotherTestPath = Path.of(target.toString(), "/src/models/another_any_of_test.rs");
+        TestUtils.assertFileExists(anotherTestPath);
+        TestUtils.assertFileContains(anotherTestPath, "#[serde(untagged)]");
+        TestUtils.assertFileContains(anotherTestPath, "pub enum AnotherAnyOfTest");
+    }
 }

modules/openapi-generator/src/test/resources/3_0/rust/rust-anyof-test.yaml

@@ -0,0 +1,42 @@
+openapi: 3.0.0
+info:
+  title: Rust anyOf Test
+  version: 1.0.0
+paths:
+  /model:
+    get:
+      responses:
+        '200':
+          description: OK
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/TestResponse'
+components:
+  schemas:
+    TestResponse:
+      type: object
+      properties:
+        model:
+          $ref: '#/components/schemas/ModelIdentifier'
+        status:
+          type: string
+    ModelIdentifier:
+      description: Model identifier that can be a string or specific enum value
+      anyOf:
+        - type: string
+          description: Any model name as string
+        - type: string
+          enum:
+            - gpt-4
+            - gpt-3.5-turbo
+            - dall-e-3
+          description: Known model enum values
+    AnotherAnyOfTest:
+      description: Another test case with different types
+      anyOf:
+        - type: string
+        - type: integer
+        - type: array
+          items:
+            type: string