schema: Implement EmptyCompletionData for AnyExpression

[radeksimko]

I'm not sure if this is the best solution, but it mostly mimics the existing behaviours of TraversalExpr (legacy Reference) and LiteralTypeExpr (legacy LiteralType).

That is, we produce pre-filling, when enabled:

the (perhaps undesirable?) side effect though is that we also pre-fill on attribute completion:

where it might be more appropriate to trigger completion and bring up functions/references:

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So one better solution I can think of would involve attaching another context value somewhere here

hcl-lang/decoder/attribute_candidates.go

Lines 21 to 24 in fba9786

	if attr.Constraint != nil {
	cData := attr.Constraint.EmptyCompletionData(ctx, 1, 0)
	snippet = fmt.Sprintf("%s = %s", name, cData.Snippet)
	triggerSuggest = cData.TriggerSuggest

and add that to the condition, such that for LiteralType-like behaviour (pre-filling empty values) we not only expect the prefilling to be toggled on, but also to not be providing completion for an attribute.

[dbanck]

👍 I think mimicking the existing behavior is fine for now