build(bindgen): check for corresponding .zig file

src/codegen/bindgen-lib-internal.ts

@@ -4,6 +4,7 @@
 // always produce correct code, or bail with an error.
 import { expect } from "bun:test";
 import type { FuncOptions, Type, t } from "./bindgen-lib";
+import fs from "node:fs";
 import * as path from "node:path";
 import assert from "node:assert";
 
@@ -806,6 +807,12 @@ export function registerFunction(opts: FuncOptions) {
   const filename = stackTraceFileName(snapshot);
   expect(filename).toEndWith(".bind.ts");
   const zigFile = path.relative(src, filename.replace(/\.bind\.ts$/, ".zig"));
+  if (!fs.existsSync(zigFile)) {
+    const bindName = path.basename(filename);
+    throw new Error(
+      `$[bindName] is missing a corresponding Zig file at ${zigFile}. Please create it and make sure it matches signatures in ${bindName}.`,
+    );
+  }
   let file = files.get(zigFile);
   if (!file) {
     file = { functions: [], typedefs: [] };

src/codegen/bindgen-lib.ts

@@ -1,5 +1,9 @@
-// This is the public API for `bind.ts` files
-// It is aliased as `import {} from 'bindgen'`
+/**
+ * This is the public API for `bind.ts` files
+ * It is aliased as `import {} from 'bindgen'`
+ * @see https://bun.sh/docs/project/bindgen
+ */
+
 import {
   isType,
   dictionaryImpl,
@@ -196,6 +200,23 @@ export namespace t {
   export const ByteString = builtinType<string>()("ByteString");
   /**
    * DOMString but encoded as `[]const u8`
+   *
+   * @example
+   * ```ts
+   * // foo.bind.ts
+   * import { fn, t } from "bindgen";
+   *
+   * export const foo = fn({
+   *   args: { bar: t.UTF8String },
+   * })
+   * ```
+   *
+   * ```zig
+   * // foo.zig
+   * pub fn foo(bar: []const u8) void {
+   *   // ...
+   * }
+   * ```
    */
   export const UTF8String = builtinType<string>()("UTF8String");