Addon-docs: Add legacy transitional support for MDX1

MIGRATION.md

@@ -52,6 +52,7 @@
       - [ArgsTable block](#argstable-block)
     - [Configuring Autodocs](#configuring-autodocs)
     - [MDX2 upgrade](#mdx2-upgrade)
+    - [Legacy MDX1 support](#legacy-mdx1-support)
     - [Default docs styles will leak into non-story user components](#default-docs-styles-will-leak-into-non-story-user-components)
     - [Explicit `<code>` elements are no longer syntax highlighted](#explicit-code-elements-are-no-longer-syntax-highlighted)
     - [Dropped source loader / storiesOf static snippets](#dropped-source-loader--storiesof-static-snippets)
@@ -978,11 +979,35 @@ export const MyDocsContainer = (props) => (
 
 Storybook 7 Docs uses MDXv2 instead of MDXv1. This means an improved syntax, support for inline JS expression, and improved performance among [other benefits](https://mdxjs.com/blog/v2/).
 
-If you use `.stories.mdx` files in your project, you may need to edit them since MDX2 contains [breaking changes](https://mdxjs.com/migrating/v2/#update-mdx-files).
+If you use `.stories.mdx` files in your project, you'll probably need to edit them since MDX2 contains [breaking changes](https://mdxjs.com/migrating/v2/#update-mdx-files). In general, MDX2 is stricter and more structured than MDX1.
 
-We will update this section with specific pointers based on user feedback during the prerelease period and probably add an codemod to help streamline the upgrade before final 7.0 release.
+We've provided an automigration, `mdx1to2` that makes a few of these changes automatically. For example, `mdx1to2` automatically converts MDX1-style HTML comments into MDX2-style JSX comments to save you time.
 
-As part of the upgrade we deleted the codemod `mdx-to-csf` and will be replacing it with a more sophisticated version prior to release.
+Unfortunately, the set of changes from MDX1 to MDX2 is vast, and many changes are subtle, so the bulk of the migration will be manual. You can use the [MDX Playground](https://mdxjs.com/playground/) to try out snippets interactively.
+
+#### Legacy MDX1 support
+
+If you get stuck with the [MDX2 upgrade](#mdx2-upgrade), we also provide opt-in legacy MDX1 support. This is intended as a temporary solution while you upgrade your Storybook; MDX1 will be discontinued in Storybook 8.0. The MDX1 library is no longer maintained and installing it results in `npm audit` security warnings.
+
+To process your `.stories.mdx` files with MDX1, first install the `@storybook/mdx1-csf` package in your project:
+
+```
+yarn add -D @storybook/mdx1-csf@next
+```
+
+Then enable the `legacyMdx1` feature flag in your `.storybook/main.js` file:
+
+```js
+export default {
+  features: {
+    legacyMdx1: true,
+  }
+}
+```
+
+NOTE: This only affects `.(stories|story).mdx` files. Notably, if you want to use Storybook 7's "pure" `.mdx` format, you'll need to use MDX2 for that.
+
+NOTE: Legacy MDX1 support is only for Webpack projects. There is currently no legacy support for Vite.
 
 #### Default docs styles will leak into non-story user components
 

code/addons/docs/package.json

@@ -126,9 +126,15 @@
     "typescript": "~4.9.3"
   },
   "peerDependencies": {
+    "@storybook/mdx1-csf": ">=1.0.0-0",
     "react": "^16.8.0 || ^17.0.0 || ^18.0.0",
     "react-dom": "^16.8.0 || ^17.0.0 || ^18.0.0"
   },
+  "peerDependenciesMeta": {
+    "@storybook/mdx1-csf": {
+      "optional": true
+    }
+  },
   "publishConfig": {
     "access": "public"
   },

code/addons/docs/src/preset.ts

@@ -6,7 +6,9 @@ import { dedent } from 'ts-dedent';
 import type { IndexerOptions, StoryIndexer, DocsOptions, Options } from '@storybook/types';
 import type { CsfPluginOptions } from '@storybook/csf-plugin';
 import type { JSXOptions } from '@storybook/mdx2-csf';
+import { global } from '@storybook/global';
 import { loadCsf } from '@storybook/csf-tools';
+import { logger } from '@storybook/node-logger';
 import { ensureReactPeerDeps } from './ensure-react-peer-deps';
 
 async function webpack(
@@ -73,7 +75,12 @@ async function webpack(
     `);
   }
 
-  const mdxLoader = require.resolve('@storybook/mdx2-csf/loader');
+  const mdxVersion = global.FEATURES?.legacyMdx1 ? 'MDX1' : 'MDX2';
+  logger.info(`Addon-docs: using ${mdxVersion}`);
+
+  const mdxLoader = global.FEATURES?.legacyMdx1
+    ? require.resolve('@storybook/mdx1-csf/loader')
+    : require.resolve('@storybook/mdx2-csf/loader');
 
   let rules = module.rules || [];
   if (transcludeMarkdown) {
@@ -135,7 +142,9 @@ async function webpack(
 const storyIndexers = (indexers: StoryIndexer[] | null) => {
   const mdxIndexer = async (fileName: string, opts: IndexerOptions) => {
     let code = (await fs.readFile(fileName, 'utf-8')).toString();
-    const { compile } = await import('@storybook/mdx2-csf');
+    const { compile } = global.FEATURES?.legacyMdx1
+      ? await import('@storybook/mdx1-csf')
+      : await import('@storybook/mdx2-csf');
     code = await compile(code, {});
     return loadCsf(code, { ...opts, fileName }).parse();
   };

code/addons/docs/src/typings.d.ts

@@ -4,9 +4,20 @@ declare module 'remark-external-links';
 declare module 'babel-plugin-react-docgen';
 declare module 'acorn-jsx';
 declare module 'vue/dist/vue';
+declare module '@storybook/mdx1-csf';
 
 declare module 'sveltedoc-parser' {
   export function parse(options: any): Promise<any>;
 }
 
+declare var FEATURES:
+  | {
+      storyStoreV7?: boolean;
+      interactionsDebugger?: boolean;
+      breakingChangesV7?: boolean;
+      argTypeTargetsV7?: boolean;
+      legacyMdx1?: boolean;
+    }
+  | undefined;
+
 declare var LOGLEVEL: 'trace' | 'debug' | 'info' | 'warn' | 'error' | 'silent' | undefined;

code/lib/core-server/src/typings.d.ts

@@ -15,5 +15,6 @@ declare var FEATURES:
       interactionsDebugger?: boolean;
       breakingChangesV7?: boolean;
       argTypeTargetsV7?: boolean;
+      legacyMdx1?: boolean;
     }
   | undefined;

code/lib/preview-api/src/typings.d.ts

@@ -13,6 +13,7 @@ declare var FEATURES:
       interactionsDebugger?: boolean;
       breakingChangesV7?: boolean;
       argTypeTargetsV7?: boolean;
+      legacyMdx1?: boolean;
     }
   | undefined;
 

code/lib/types/src/modules/core-common.ts

@@ -317,6 +317,11 @@ export interface StorybookConfig {
      * Will be removed in 7.0.
      */
     warnOnLegacyHierarchySeparator?: boolean;
+
+    /**
+     * Use legacy MDX1, to help smooth migration to 7.0
+     */
+    legacyMdx1?: boolean;
   };
 
   /**

code/yarn.lock

@@ -5206,8 +5206,12 @@ __metadata:
     ts-dedent: ^2.0.0
     typescript: ~4.9.3
   peerDependencies:
+    "@storybook/mdx1-csf": ">=1.0.0-0"
     react: ^16.8.0 || ^17.0.0 || ^18.0.0
     react-dom: ^16.8.0 || ^17.0.0 || ^18.0.0
+  peerDependenciesMeta:
+    "@storybook/mdx1-csf":
+      optional: true
   languageName: unknown
   linkType: soft