Gatsby: [gatsby-plugin-mdx] Remark plugins using mutateSource seems to be ignored

[Strajk]

Description

Remark plugins are able both to transform AST and mutate original source

exports.mutateSource = ({ markdownNode }) => {
  markdownNode.internal.content = markdownNode.internal.content.replace("foo", "bar")
}

exports.default = ({ markdownAST }) => {
  visit(markdownAST, ["heading"], (node) => { … })
  return markdownAST
}

Remark plugins in gatsby-plugin-mdx seem to ignore the mutateSource hook and only run AST transformation.
(or more precisely, it ignores plugins without default export && doesn't check for mutateSource on plugins)

• There seems to be some code prepared to handle this, but it's commented out
https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-mdx/utils/gen-mdx.js#L22
(Commented out by @ChristopherBiscardi so tagging, sry if not appropriate)

• Filtering out seems to be done here
https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby-plugin-mdx/utils/get-source-plugins-as-remark-plugins.js#L50

• There's an issue for this in old repo
https://github.com/ChristopherBiscardi/gatsby-mdx/issues/118
The proposed workaround in the last comment is to rewrite the plugin to use AST transformation instead of mutateSource, so not a real solution

Steps to reproduce

https://github.com/Strajk/jungle/tree/master/gatsby-replace-md-with-md

There are two local plugins:

• ./plugins/remark-ast – add ⭐ at the end of headings
• ./plugins/remark-mutate-source – replaces "dog" with dog emoji in html tag

Project uses both transformer-remark & plugin-mdx for easier reproducibility.
Both are configured to use both plugins https://github.com/Strajk/jungle/blob/master/gatsby-replace-md-with-md/gatsby-config.js

There are two sample posts to test the functionality

• ./content/blog/hello-world-md/index.md
• ./content/blog/hello-world-mdx/index.mdx

Expected result

Both posts (md & mdx) should be transformed with both plugins.

Actual result

• MD post is transformed with both plugins
• MDX post is transformed with just ./plugins/remark-ast plugin

Environment

Gatsby 2.20.25 – Expand for all info

  System:
    OS: macOS 10.15.4
    CPU: (8) x64 Intel(R) Core(TM) i7-4870HQ CPU @ 2.50GHz
    Shell: 5.7.1 - /bin/zsh
  Binaries:
    Node: 13.8.0 - ~/.nvm/versions/node/v13.8.0/bin/node
    Yarn: 1.19.1 - /usr/local/bin/yarn
    npm: 6.13.6 - ~/.nvm/versions/node/v13.8.0/bin/npm
  Languages:
    Python: 2.7.16 - /usr/bin/python
  Browsers:
    Chrome: 81.0.4044.122
    Firefox: 75.0
    Safari: 13.1
  npmPackages:
    gatsby: ^2.20.25 => 2.20.25 
    gatsby-image: ^2.3.4 => 2.3.4 
    gatsby-plugin-feed: ^2.4.3 => 2.4.3 
    gatsby-plugin-google-analytics: ^2.2.4 => 2.2.4 
    gatsby-plugin-manifest: ^2.3.5 => 2.3.5 
    gatsby-plugin-mdx: ^1.1.10 => 1.1.10 
    gatsby-plugin-offline: ^3.1.4 => 3.1.4 
    gatsby-plugin-react-helmet: ^3.2.4 => 3.2.4 
    gatsby-plugin-sharp: ^2.5.6 => 2.5.6 
    gatsby-plugin-typography: ^2.4.3 => 2.4.3 
    gatsby-remark-copy-linked-files: ^2.2.3 => 2.2.3 
    gatsby-remark-images: ^3.2.4 => 3.2.4 
    gatsby-remark-prismjs: ^3.4.3 => 3.4.3 
    gatsby-remark-responsive-iframe: ^2.3.3 => 2.3.3 
    gatsby-remark-smartypants: ^2.2.3 => 2.2.3 
    gatsby-source-filesystem: ^2.2.4 => 2.2.4 
    gatsby-transformer-remark: ^2.7.3 => 2.7.3 
    gatsby-transformer-sharp: ^2.4.6 => 2.4.6 
  npmGlobalPackages:
    gatsby-cli: 2.11.14

Tagging @mathieudutour and @vladar as I noticed they done some (maybe) similar work in https://github.com/gatsbyjs/gatsby/pull/21188
(sry if not appropriate 🙏)

[Strajk]

Real-world use case:

Remark plugin for finding and replacing, something like https://github.com/angeloashmore/gatsby-remark-find-replace, but with support for markdown and HTML in replacing text, eg:

{
  COMPANY: "The Company" // Supported in mention plugin
  NOTE: "This is a note with **markdown**" // Not Supported
  TOP: "Go to <a href="#top" style=''>top</a>" // Not Supported
}

[vladar]

Just to clarify - is this a recent break? Or was it working like this for a long time now?

[mathieudutour]

I don't think it ever worked

[Strajk]

@vladar @mathieudutour even though I don't use Gatsby for a long time, I agree with @mathieudutour that it probably never worked –– that commented out code is there from October 2018

Was not sure if this issue should be Feature request or Bug

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