I am writing an plugin that adds the source file + line number to every heading element.

Below you can see that the frontmatter is being parsed and then only the remaining markdown content is passed onwards.

const {data, content} = matter(fileString);
// ...
result = await mdx(content, options);

This means that in a rehype plugin the vfile.history[].position.start.line will not be correct, it not take into account the frontmatter lines.

I think a better approach would be to parse the frontmatter using a remark plugin so to retain the original line numbers. Like this: https://www.npmjs.com/package/remark-extract-frontmatter

This should also be corrected on https://mdxjs.com/guides/custom-loader#custom-loader page I think.

Workaround

I could use grey matter to parse the gray matter again in my plugin but that means a lot of duplicate file read operations.

Remark

In remark plugins, I am just using the first comment to apply config.

<!-- {tocMaxDepth: 1, tocMaxDepth: 4} -->

# foo

const plugin = (options = {}) => {
  const name = options.name || 'toc';

  const transformer = (node) => {

    const firstComment = node.children.find(node => node.type === 'comment')
    let config = {}
    if (firstComment) {
      config = JSON.parse(firstComment.value)
    }

	// stuff...

  }

}