We could add the extension to known extensions in package.json, but sourcekit-lsp would need to support the highlighting ideally

Do you think it's worth adding it as a TextMate grammar in the meantime?
Also, would adding GYB support in SourceKit-LSP be accepted or would it be out of scope?

Do you think it's worth adding it as a TextMate grammar in the meantime?

That would work for the syntax highlighting in the meantime, yeah. Not sure how difficult it would be to extend the existing Swift textmate grammar.

Also, would adding GYB support in SourceKit-LSP be accepted or would it be out of scope?

@ahoppen is this something you'd consider adding to SourceKit-LSP?

Not sure how difficult it would be to extend the existing Swift textmate grammar.

I recently wrote an extension that extends the HTML grammar to support Vapor's templating language Leaf, it's just a matter of writing good regular expressions, and given the structure of GYB I don't think it would be too difficult.

We are not planning to add gyb support to SourceKit-LSP. GYB can contain arbitrary Python, which can be nearly impossible to process for semantic information.

Not sure if it’s possible, but maybe a middle ground cold be to allow VS Code to use it’s grammar-based syntax highlighting for gyb files but not open them in SourceKit-LSP? That way you get at least some syntax coloring and we don’t try to provide semantic functionality that would only fail.

I opened a draft PR that adds syntax highlighting for GYB.

There are still known issues (which I mention in the PR).
I will continue to work on it, and would greatly appreciate an outside look.

If you want to know the reason behind the choices I made please ask me here or in the PR comments!

P.S. I would love to give GYB files a customised icon, I was thinking of Swift's icon but greyed out, what do you think?