ignore lang.ReferencesInExpr errors when walking all expressions

[bendrucker]

When visiting all expressions, we might encounter special attributes/blocks, where normal reference rules no longer apply:

https://www.terraform.io/docs/configuration/resources.html#meta-arguments

This currently returns an error for the unused_declarations and workspace_remote rules:

Error: Invalid reference: A reference to a resource type must be followed by at least one attribute access, specifying the resource name.

Detecting these attributes seems too brittle/awkward, so simply exiting if an expression cannot be evaluated for references seems like the best fix

[wata727]

Umm... Good catch, @bendrucker

Terraform doesn't seem to have this problem because it treats meta-arguments specially.
https://github.com/hashicorp/terraform/blob/v0.12.25/configs/resource.go#L117-L144

Ideally, I feel like we need lang.ReferencesInExpr that can handle non-evaluable expressions, but it certainly seems difficult. Or maybe we need a WalkExpressions that ignores the meta-arguments.

I leave only one comment, but at this point, the basic policy seems to be fine.

[bendrucker]

Totally, I'm open to addressing this in WalkExpressions. Not sure we can deal with it once we're at the expression level, since we need to know that we're in a special meta-argument.

[bendrucker]

Wanted to get this fixed ASAP but I can try to follow up so we don't have to ignore these errors in rules.

[wata727]

Great 👍