fix(rust): don't panic when there are no top-level types

packages/quicktype-core/src/language/Rust/RustRenderer.ts

@@ -13,7 +13,6 @@ import type { Name, Namer } from "../../Naming.js";
 import type { RenderContext } from "../../Renderer.js";
 import type { OptionValues } from "../../RendererOptions/index.js";
 import { type Sourcelike, maybeAnnotated } from "../../Source.js";
-import { defined } from "../../support/Support.js";
 import type { TargetLanguage } from "../../TargetLanguage.js";
 import {
     type ClassType,
@@ -351,9 +350,10 @@ export class RustRenderer extends ConvenienceRenderer {
             return;
         }
 
-        const topLevelName = defined(
-            mapFirst(this.topLevels),
-        ).getCombinedName();
+        const topLevel = mapFirst(this.topLevels);
+        if (topLevel === undefined) return;
+
+        const topLevelName = topLevel.getCombinedName();
         this.emitMultiline(
             `// Example code that deserializes and serializes the model.
 // extern crate serde;

test/unit/rust-empty-top-levels.test.ts

@@ -0,0 +1,44 @@
+import { expect, test } from "vitest";
+
+import {
+    InputData,
+    JSONSchemaInput,
+    quicktype,
+} from "../../packages/quicktype-core/src/index.js";
+
+async function renderTypeScriptSchema(definitions: object): Promise<string> {
+    const schemaInput = new JSONSchemaInput(undefined);
+    await schemaInput.addSource({
+        name: "",
+        schema: JSON.stringify({ definitions }),
+        uris: ["#/definitions/"],
+        isConverted: true,
+    });
+
+    const inputData = new InputData();
+    inputData.addInput(schemaInput);
+
+    const result = await quicktype({ inputData, lang: "rust" });
+    return result.lines.join("\n");
+}
+
+// TypeScript inputs without a #TopLevel marker use #/definitions/ as their
+// source URI.  If the compiler did not produce any definitions, the renderer
+// receives no top levels and must still emit Rust without crashing.
+test("Rust renders TypeScript schemas with no top levels", async () => {
+    const output = await renderTypeScriptSchema({});
+
+    expect(output).toContain("use serde::{Serialize, Deserialize};");
+});
+
+test("Rust still renders named TypeScript schema definitions", async () => {
+    const output = await renderTypeScriptSchema({
+        Person: {
+            type: "object",
+            properties: { age: { type: "integer" } },
+            required: ["age"],
+        },
+    });
+
+    expect(output).toContain("pub struct Person");
+});