From c76510cc46cb5c38b289ba28663a42293f8276f5 Mon Sep 17 00:00:00 2001
From: Mark Probst <mark.probst@gmail.com>
Date: Mon, 20 Jul 2026 17:47:54 -0400
Subject: [PATCH] fix(rust): don't panic when there are no top-level types
 (#1629)

Rust's emitLeadingComments() assumed at least one top-level type exists to
name in its example-usage comment, panicking with "Defined value expected,
but got undefined" whenever this.topLevels was empty. This is legitimate,
reachable state: TypeScript sources without a #TopLevel marker (the common
case) fall back to uris: ["#/definitions/"], producing a non-empty type
graph with zero named top levels. Now the renderer simply skips the
top-level-specific example comment when there is nothing to name.

Co-Authored-By: gpt-5.6-sol via pi <noreply@openai.com>
---
 .../src/language/Rust/RustRenderer.ts         |  8 ++--
 test/unit/rust-empty-top-levels.test.ts       | 44 +++++++++++++++++++
 2 files changed, 48 insertions(+), 4 deletions(-)
 create mode 100644 test/unit/rust-empty-top-levels.test.ts

diff --git a/packages/quicktype-core/src/language/Rust/RustRenderer.ts b/packages/quicktype-core/src/language/Rust/RustRenderer.ts
index 3024d97d5b..4b1f4b1c36 100644
--- a/packages/quicktype-core/src/language/Rust/RustRenderer.ts
+++ b/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;
diff --git a/test/unit/rust-empty-top-levels.test.ts b/test/unit/rust-empty-top-levels.test.ts
new file mode 100644
index 0000000000..f5cc68f3fb
--- /dev/null
+++ b/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");
+});