Let extensions add injection/highlight queries to existing languages

[slyannah]

Body

What are you proposing?

A way for extensions to add injection and/or highlight queries to an existing language such as Markdown, without having to replace that language.

The motivating case is Hugo shortcodes in Markdown content files:

{{< video_file >}}
{{< video_file id="20252026" >}}

Why does this matter?

These lines show as plain text in Zed, and I couldn't find a supported way to highlight them:

• An extension can't add queries to a built-in language like Markdown. It can only define a new language with its own grammar.
• tree-sitter-markdown has no node for shortcodes. They are text inside paragraphs, and queries match whole nodes, not part of a text run.
• tree-sitter-go-template can't parse {{< or {{%, so it isn't a fallback.
• The only workaround I see is writing a custom grammar and a separate "Hugo Markdown" language that injects Markdown into its text nodes. That means maintaining a grammar repo with a generated parser just to color a few delimiters.

The same limitation affects other Markdown-adjacent syntax:

• Jekyll/Liquid {% %} tags
• Obsidian-style wikilinks and callouts
• Pandoc-style fenced divs

Are there any examples or context?

VS Code extensions handle this with a TextMate injection grammar layered on top of Markdown. For example, the "Hugo Shortcode Syntax Highlighting" extension highlights {{< >}} and {{% %}} without replacing the Markdown language.

Possible approach

Allow an extension to contribute additional injections.scm and/or highlights.scm queries to an existing language, so they are layered on top of the built-in ones.

I understand tree-sitter queries can't match a sub-range of a text node, so this may need to pair with a small injected grammar or a lightweight pattern-based injection. I'm open to whatever fits Zed's design.

[N.B.: Generated with Claude AI assistance]