Completion of references of matching complex types

[radeksimko]

Discovered originally in hashicorp/vscode-terraform#990 (comment)

Background

The language server currently only provides completion of references to targets which are "directly addressable" and have a matching type.

e.g.

data "aws_availability_zones" "available" {
}

data "aws_subnet" "private" {
  count             = var.subnet_count
  availability_zone = data.aws_availability_zones.available. # HERE
}

availability_zone field is of type string and available references targets under data.aws_availability_zones.available are the following:

• data.aws_availability_zones.available.all_availability_zones (ScopeId: "data", Type: "cty.Bool")
• data.aws_availability_zones.available.exclude_names (ScopeId: "data", Type: "cty.Set(cty.String)")
• data.aws_availability_zones.available.exclude_zone_ids (ScopeId: "data", Type: "cty.Set(cty.String)")
• data.aws_availability_zones.available.group_names (ScopeId: "data", Type: "cty.Set(cty.String)")
• data.aws_availability_zones.available.id (ScopeId: "data", Type: "cty.String")
• data.aws_availability_zones.available.names (ScopeId: "data", Type: "cty.List(cty.String)")
• data.aws_availability_zones.available.state (ScopeId: "data", Type: "cty.String")
• data.aws_availability_zones.available.zone_ids (ScopeId: "data", Type: "cty.List(cty.String)")

Therefore we end up picking just exact type matches data.aws_availability_zones.available.id and data.aws_availability_zones.available.state as both are of string type.

However, we have no way of knowing whether this is a final completion or just beginning and there are other relevant reference targets we could still offer. Sets are not directly addressable and there's no point of making them until #530 or #527 We can however try "descending" into lists, i.e. additionally suggest the following:

• data.aws_availability_zones.available.names (ScopeId: "data", Type: "cty.List(cty.String)")
• data.aws_availability_zones.available.zone_ids (ScopeId: "data", Type: "cty.List(cty.String)")

Proposal

Update type matching within hcl-lang to descend into lists and possibly maps and tuples, all of which are directly addressable via [] syntax.

https://github.com/hashicorp/hcl-lang/blob/d770b425fb2288fdcbf223b6955485a989dd3221/reference/targets.go#L78-L79