Support folding in instruction file

[rebornix]

Does this issue occur when all extensions are disabled?: Yes/No

From what I read https://docs.github.com/en/copilot/customizing-copilot/adding-repository-custom-instructions-for-github-copilot#creating-a-repository-custom-instructions-file instruction and prompt files are markdown, however I don't seem to get folding for headers in instruction files.

[aeschli]

We defined our own language ids for prompts/instructions and modes. That gives some advantages, but we don't get all the markdown support that we already have in VS Code,.

We could decide to switch to markdown, or we could come up with something called derived Markdown

For JSON and HTML I come up with something called 'language participants'.
Languages could declare what features they want from the JSON/HTML language server
The language server would then also register the language support for these languages.

vscode/extensions/json-language-features/client/src/languageParticipants.ts

Line 48 in 10d3fb2

const jsonLanguageParticipants = extension.packageJSON?.contributes?.jsonLanguageParticipants as LanguageParticipantContribution[];

@mjbvz would you be interested?

[mjbvz]

Related: #249691

I wonder if we can use the normal markdown language but gate prompt features based on the file name. Which specific features do we want in prompt files but not regular markdown?

[aeschli]

What prompt files have in addition to markdown are the frontmatter headers. Depending on the prompt type (prompt file/instruction/mode) different properties are supported (applyTo, mode, tools, ...).
Front matter with markdown is very common, so IMO it would make sense for the markdown language service to support. IMO it would be good for add basic support such as for syntax highlighting and folding.

We want add rich language support for the header. That could be done in the markdown extension itself (driven by a contributed schema like in JSON), but that is the deluxe solution and likely is a lot of work as we can't just limit the support for our needs. E.g. it might mean to support full YAML syntax or other languages in the frontmatter header.

Therefore it's more pragmatic to provide that support in our own prompts extension. This is on my plan.

The advantage of having own language is that the editor mode is nicer (showing 'Instruction' file) and that we can use the fileAssociation setting to associate other file extensions. In a way these a details, so I would not mind using markdown as language
Or, as I suggest before, we add that concept of derived languages that can participate on the markdown language support

[mjbvz]

Thanks for the details. I'd be fine with changing our built-in markdown language features extension to be enabled for the instructions language too since they mostly just markdown. One thing to consider though is that every other markdown extension would also have to opt in to this language

Here's where you can change the file extensions and languages that we process:

vscode/extensions/markdown-language-features/src/client/client.ts

Line 63 in 57c041e

documentSelector: [{ language: 'markdown' }],

We should also already support yaml frontmatter syntax highlighting out of the box. What we don't support though is any more advanced features in the frontmatter

For instructions specific language feature and syntax highlighting, I think most of these could be contributed by a different extension using the normal VS Code apis. For example, suggestions and hovers should automatically get pulled in from multiple extensions

Give this a try and let me know if there are still any blockers