Different semantic tokens for invalid references

[dbanck]

After updating the highlighting of constant keywords in hashicorp/syntax#24, semantic tokens should take care of highlighting broken references.

Use-cases

It would be helpful to get feedback when referencing a not-defined variable (or similar item).

Proposal

Instead of highlighting the var keyword differently (see screenshot above), it is proposed to change the highlighting of the reference name. In the example above, non_existing should be highlighted differently.

[Rabattkarte]

Instead of highlighting the var keyword differently (see screenshot above), it is proposed to change the highlighting of the reference name. In the example above, non_existing should be highlighted differently.

Other plugins (in this case Azure ARM Tools) do it with a squiggly underline like seen in the screenshot:

Some decoration like this screams at a user: "There's a problem!", personally I like this and see this behavior in multiple plugins.

[radeksimko]

@Rabattkarte Yes, this looks like diagnostics and we plan to do this via diagnostics. I think the question here was whether we could also leverage the semantic highlighting.

In retrospect - maybe it's stretching what semantic highlighting is designed for - at least with the current feature set. I'm assuming that diagnostics are pretty much always supported by the client - as it's quite an essential part of LSP - so maybe we just end up adding that to the list in hashicorp/hcl-lang#57

[jpogran]

The only method syntax/semantic tokens have to indicate 'invalid' is a red color which, in addition to being problematic for color-blind users, does not give any information for why it's invalid. We won't be able to do squiggly lines or text to say why the thing is incorrect.

This is truly where diagnostic warning and errors shine, so the effort is better spent in hashicorp/hcl-lang#57

[radeksimko]

I'm going to close this issue as there's nothing we can immediately in terms of semantic highlighting do and it's also unclear whether we should. hashicorp/hcl-lang#57 covers alternative solution involving diagnostics.

On the note of the clarity I opened microsoft/language-server-protocol#1415 upstream

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