This PR replaces the old gatsby-plugin-mdx with a new version that is powered by MDX v2.

We won't be 100% backwards compatible, but most use cases will be covered. Simplification and better performance is the overall goal.

Changes:

This is a complete rewrite of the plugin, so some old features might be gone.

New features

[x] Upgraded all dependencies including MDX to v2
[x] Per file tree-shaking and chunking via webpack
[x] MDXRenderer is no more needed.
[x] Similar or better performance than old plugin. (First tests on complex projects show similar performance)
[x] Extended e2e tests
[x] Simplified plugin configuration
[x] Via the mdxOptions config option you can pass any configuration to @mdx-js/js compile like remarkPlugins, rehypePlugins, recmaPlugins and more.
[x] Move away from react-test-renderer in tests
[x] Authored in TS

[x] .mdx files from src/pages are turned into real pages
[x] .mdx files loaded via gatsby-source-filesystem or as component via createPage can be turned into real pages
[x] support for frontmatter (via GraphQL and page context)
[x] excerpt & table of contents support https://github.com/gatsbyjs/gatsby/pull/35873
[x] support for remark, rehype and most gatsby-remark-* plugins #35751

[x] (compiled) MDX code won't be available anymore within GraphQL
[x] MD nodes are no more automatically turned into MDX
[x] config option defaultLayouts is gone
[x] config option lessBabel is gone
[x] config option mediaTypes is gone
[x] config option shouldBlockNodeFromTransformation is gone
[x] some GraphQL data is no more available in core and have to be implemented on project level (like time to read)

Investigate if we could/should replace gray-matter with the recommended approach by MDX via remark-mdx-frontmatter

Closes #25068 Closes #34421 Closes #35733 Closes #35732

May 13 '22 12:05 axe312ger

TODOs from other PR:

May 31 '22 13:05 LekoArts

I'm so excited about this! Great work @axe312ger 👏 👏

Jun 20 '22 11:06 alvinometric

I'm so excited about this! Great work @axe312ger 👏 👏

Thanks, not easy but we are getting there 🙈

Jun 28 '22 10:06 axe312ger

As some might follow just here: The RC is out! 🎉

https://github.com/gatsbyjs/gatsby/discussions/25068#discussioncomment-3100249

Jul 07 '22 16:07 axe312ger