context: Introduce LanguageId

[radeksimko]

Context

This is expected to be no-op for end-users but it is a small piece of code which will allow us to make further optimisations within #1358 and enable some future feature work.

Specifically:

• Provide diagnostics in tfvars files #1410
• Update ParseModuleConfiguration to only parse changed file (*.tf) #1404
• We could also use this to avoid scheduling some jobs conditionally. e.g. if the job is being scheduled because *.tfvars/*.tfvars.json file has been opened or changed, we don't need to run any module (*.tf/*.tf.json) related jobs.
  • Note that the situation isn't as simple in the other direction though - i.e. we still do need to (re)run jobs related to *.tfvars/*.tfvars.json when *.tf/*.tf.json files change, because these inform the schema of the variable files.

Implementation Notes

You may ask Why IsLanguageId() bool interface specifically?

• We could have gone with (string, bool) or (string, error) return values.
• It is expected that the language ID will only be available in textDocument/didOpen and textDocument/didChange, rather than in all requests, hence the simple single string return value doesn't seem appropriate, whether we return empty string or panic in the edge case.
• Having two return values means more LOC on the consumer side, i.e. one has to first check the ok/error and then compare for equality. I am assuming that we can treat other requests the same as mismatch and that the additional LOC aren't therefore useful.

[dbanck]

Why do you use a string instead of ilsp.LanguageID in the function signatures? I guess it's unlikely that the string "terraform" will ever change, but it feels cleaner to me to use the constant.

With the new context methods here, can we get rid of https://github.com/hashicorp/terraform-ls/blob/main/internal/decoder/context.go?

[radeksimko]

@dbanck I'm not against it but it goes much further than I was hoping to go with the PR due to conflicts.

Meaning, that we should update it here

terraform-ls/internal/document/document.go

Line 18 in f8445e8

LanguageID string

and anywhere downstream from there + in the didOpen handler we should also add a switch statement rather than be casting an arbitrary string to the desired type.

That is to say it's the right thing to do but I'm not sure it's worth doing right now due to the long-running branch. 😅

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