core: Raise error for old map keys & list members referencing syntax

[radeksimko]

As @Djuke mentioned in #6322 (comment) the old syntax for referencing map keys & list members via dots stopped working silently since #6322 was merged.

While I agree that the new syntax w/ square brackets is a lot better and making the old syntax work would be probably waste of time I think many people may have the old, dotted references in many places of their codebase and it will confuse them (and make upgrades painful) if we just silently ignore these in 0.7.

Terraform Version

Built from master (fcc3736e6bb850e51c240fda5c033888f13dcb34).

Terraform Configuration Files

variable "minions" {
    default = ["first", "second"]
}
output "first_minion" {
    value = "${var.minions.0}"
}

variable "colour_codes" {
    default = {
        black = "000"
        white = "FFF"
    }
}
output "black_code" {
    value = "${var.colour_codes.black}"
}

Debug Output

https://gist.github.com/radeksimko/6a2126a75ea08ca2fd3540b6b899c14e

Expected Behavior

Either an error or warning should've been raised about wrong syntax or unknown variable.

Output interpolation "first_minion" failed: 1:3: unknown variable accessed: var.minions.0
Output interpolation "black_code" failed: 1:3: unknown variable accessed: var.colour_codes.black

We are actually able to detect these already, see the attached debug log, search for WARN.

Actual Behavior

$ terraform apply

Apply complete! Resources: 0 added, 0 changed, 0 destroyed.

Steps to Reproduce

1. terraform apply

References

• core: Support native lists in Terraform configuration #6322

---

New syntax w/ square brackets

This works as expected:

variable "minions" {
    default = ["first", "second"]
}
output "first_minion" {
    value = "${var.minions[0]}"
}

variable "colour_codes" {
    default = {
        black = "000"
        white = "FFF"
    }
}
output "black_code" {
    value = "${var.colour_codes["black"]}"
}

cc @jen20

[phinze]

Just confirmed that a nicely specific error is raised in both usages in this example:

Error reading config for output first_minion: Invalid dot index found: 'var.minions.0'. Values in maps and lists can be referenced using square bracket indexing, like: 'var.mymap["key"]' or 'var.mylist[1]'. in:

${var.minions.0}

Error reading config for output black_code: Invalid dot index found: 'var.colour_codes.black'. Values in maps and lists can be referenced using square bracket indexing, like: 'var.mymap["key"]' or 'var.mylist[1]'. in:

${var.colour_codes.black}

[ghost]

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