feat(module-source): Introduce a shim that composes with lockdown

packages/module-source/package.json

@@ -23,6 +23,7 @@
   "main": "./index.js",
   "exports": {
     ".": "./index.js",
+    "./shim.js": "./shim.js",
     "./package.json": "./package.json"
   },
   "scripts": {

packages/module-source/shim.js

@@ -0,0 +1,7 @@
+import { ModuleSource } from '@endo/module-source';
+Object.defineProperty(globalThis, 'ModuleSource', {
+  value: ModuleSource,
+  enumerable: false,
+  writable: true,
+  configurable: true,
+});

packages/module-source/src/module-source.js

@@ -55,6 +55,19 @@ const analyzeModule = makeModuleAnalyzer();
  * @property {SourceMapHook} [sourceMapHook]
  */
 
+// https://github.com/tc39/proposal-source-phase-imports?tab=readme-ov-file#js-module-source
+function AbstractModuleSource() {
+  // no-op, safe to super()
+}
+
+// WebAssembly and ModuleSource are both in motion.
+// The Source Phase Imports proposal implies an additional AbstractModuleSource
+// layer above the existing WebAssembly.Module that would be shared by
+// the JavaScript ModuleSource prototype chain.
+// At time of writing, no version of WebAssembly provides the shared base class,
+// and the shimmed ModuleSource gains nothing from sharing that prototype when
+// it comes into being.
+
 // XXX implements import('ses').PrecompiledModuleSource but adding
 // `@implements` errors that this isn't a class and `@returns` errors that
 // there's no value returned.
@@ -100,3 +113,5 @@ export function ModuleSource(source, opts = {}) {
   this.__needsImportMeta__ = needsImportMeta;
   freeze(this);
 }
+
+Object.setPrototypeOf(ModuleSource.prototype, AbstractModuleSource.prototype);

packages/ses/test/module-source.test.js

@@ -1,6 +1,8 @@
+/// <reference types="ses">
+
 import test from 'ava';
 import '../index.js';
-import { ModuleSource } from '@endo/module-source';
+import '@endo/module-source/shim.js';
 
 lockdown();
 
@@ -42,3 +44,7 @@ test('module source constructor', t => {
     'ModuleSource imports should be frozen',
   );
 });
+
+test('ModuleSource is a shared intrinsic', t => {
+  t.truthy(ModuleSource === new Compartment().globalThis.ModuleSource);
+});