Fix stringify of param names with control characters

src/cases.spec.ts

@@ -221,6 +221,13 @@ export const STRINGIFY_TESTS: StringifyTestSet[] = [
     ]),
     expected: "\\\\:test",
   },
+  {
+    data: new TokenData([
+      { type: "text", value: "/" },
+      { type: "param", name: "a\nb" },
+    ]),
+    expected: '/:"a\nb"',
+  },
   {
     data: {
       tokens: [

src/index.ts

@@ -655,14 +655,25 @@ export function stringify(data: TokenData): string {
   return stringifyTokens(data.tokens, 0);
 }
 
+/**
+ * Wrap a parameter name in quotes, escaping only the characters the parser
+ * treats specially inside a quoted name (`"` and `\`). `JSON.stringify` can't
+ * be used here because it emits escapes like `\n` that the parser reads back as
+ * the literal character `n`, so names with control characters wouldn't survive
+ * a `parse` -> `stringify` -> `parse` round trip.
+ */
+function quoteName(name: string): string {
+  return `"${name.replace(/["\\]/g, "\\$&")}"`;
+}
+
 /**
  * Stringify a parameter name, escaping when it cannot be emitted directly.
  */
 function stringifyName(name: string, next: Token | undefined): string {
-  if (!ID.test(name)) return JSON.stringify(name);
+  if (!ID.test(name)) return quoteName(name);
 
   if (next?.type === "text" && ID_CONTINUE.test(next.value[0])) {
-    return JSON.stringify(name);
+    return quoteName(name);
   }
 
   return name;