Show interpolated expression values on hover

[radeksimko]

Use-cases

Terraform supports a number of expressions (such as function calls, arithmetic operators, references etc.).

While expressions help users achieve and increase readability of their configurations they can make it harder to tell what exact value a complex expression will interpolate into, until plan phase.

For example

resource "aws_vpc" "example" {
  count      = 3
  cidr_block = "10.0.${count.index}.0/24"
}

locals {
  number_of_subnets = length(aws_vpc.example) * 3
}

when hovering over number_of_subnets we could display

number_of_subnets number
length(aws_vpc.example) * 3 = 9

in the hover popup and when hovering over cidr_block we could try to enumerate over the first few indexes and display e.g.

cidr_block string

10.0.0.0/24 (count.index = 0)
10.0.1.0/24 (count.index = 1)
10.0.2.0/24 (count.index = 2)

Proposal

hcl-lang

• Recognize individual expressions when nested #496
• Support for local values (to increase chances that expressions are evaluated successfully)
  • Support self.* references in provisioner, connection and lifecycle blocks #859
  • Support count.index references in blocks w/ count #860
  • Support each.* references in blocks w/ for_each #861
• decoder: Update PathDecoder.HoverAtPos()) such that it attaches evaluated expression value (if successful) to returned HoverData

terraform-ls

• Update hcl-lang

Considerations for MVP

Initially we can consider just interpolation with context from within the configuration.

Later we can consider adding external sources, such as variables and state as reflecting these is likely going to involve more effort.

Blockers (upstream)

This is currently blocked as reimplementing whole interpolation logic within LS has quite high cost of maintenance cost and TF Core does not provide any way of doing interpolation on demand.

The best way would likely be some form of gRPC interface where TF Core acts as a gRPC server which LS can start on initialize and send arbitrary expressions for Core to evaluate and return back the result as gRPC response which can be reconstructed back into cty.Value on LS side. Such an interface doesn't exist yet and has not been RFC'd or planned in any level of detail yet.