Add support for SignatureAtPos

[dbanck]

This PR adds the SignatureAtPos method to enable support for signature help. Function signatures must be present in the path context for it to work. We added the function signatures to terraform-schema in hashicorp/terraform-schema#145.

If you want to try it, there is a draft PR in terraform-ls, tying it all together: hashicorp/terraform-ls#1077

UX

Nested function calls with variadic parameters

Functions without any arguments

No signature help, if passing too many arguments

Multiline function calls

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• Part of Support built-in functions terraform-ls#37

[radeksimko]

Works pretty well for the happy path! 👍🏻 I just left some notes in-line about the implementation and the "negative cases".

[radeksimko]

This is smart, but it doesn't take the schema into account, which means that we provide signature help in all contexts/expressions, regardless of whether function calls are expected there or not.

In most contexts, users will less likely type the function name - where it would be nitpicking, but TypeDeclaration also uses FunctionCallExpr to express various types like list, set etc. and we do want to avoid the signature help there, or do we not? Otherwise we'd end up with signature help e.g. representing the list() function, rather than the type, wouldn't we?

I wouldn't expect you to account for nested expressions (although that would probably be a long-term ideal we can 1st solve that in AnyExpression and then maybe reuse here later) nor prevent signatureHelp for mismatching types.

I think that we could still check whether the outermost constraint for the expression is AnyExpression though?

[dbanck]

With the current implementation, the feature is decoupled from any expression work and can work stand-alone. Also, nested expressions/function calls should work out of the box, as we always try to find the specific FunctionCallExpr node.

There is no overlap between the functions in the schema and the type declarations. So, even though list( or tuple( is technically a FunctionCallExpr, no signature help should come up. Only for Terraform < 0.15 do we have the deprecated list and map functions which will trigger signature help. Is it worth extending the code to account for that? Adding schema and constraint checks while walking the AST sounds expensive.

[radeksimko]

I wouldn't think it's expensive in terms of performance (not significantly more than the implementation of hclsyntax.VisitAll). I can totally see it being expensive in terms of effort/complexity though and I can see why this is worth shipping as-is and how it can deliver immense value even if we don't account for these edge cases. 👍🏻

So with that in mind I'd just leave a note either in a comment, or in a new issue to help us remember to revisit this and/or explain why we took a slightly different approach here which doesn't take schema into account and call the PR done.