hclwrite: Allow blank quoted string block labels

[alisdair]

The hclsyntax package permits block labels to be blank quoted strings, and defers to the application to determine if this is valid or not. This commit updates hclwrite to allow the same behaviour.

This is in response to an upstream bug report on Terraform, which uses hclwrite to implement its fmt subcommand. Given a block with a blank quoted string label, it currently deletes the label when formatting. This is inconsistent with Terraform's behaviour when parsing HCL, which treats empty labels differently from missing labels:

provider "" {}

provider {}

$ terraform init
There are some problems with the configuration, described below.

The Terraform configuration must be valid before initialization so that
Terraform can determine which modules and providers need to be installed.

Error: Invalid provider local name

  on main.tf line 1:
   1: provider "" {}

 is an invalid provider local name: must have at least one character


Error: Missing name for provider

  on main.tf line 3, in provider:
   3: provider {}

All provider blocks must have 1 labels (name).

I've confirmed that incorporating this change into Terraform fixes the upstream bug.

[alisdair]

I'm not sure why this test is covering the parsing of foo "hoge" "" {} as foo "hoge" {}, but it is an example of the bug we are trying to fix with this change, so I've updated the test. @minamijoyo, do you have any concerns about this test update?

[minamijoyo]

@alisdair I'm not sure why 🤔
I checked the commit history and found that the test case was added by @apparentlymart
7e5b148

[alisdair]

🤦 Sorry, my mistake! I misread the commit history. Thanks!

[minamijoyo]

👌

[apparentlymart]

Hmm sorry about that! I imagine I must've misread the input while I was updating this (empty quotes in that position are unusual!) and managed to accidentally write a test that incorrectly passed, rather than a test that caught the bug. 😖

My main goal here was to test the handling of that comment in the middle, so I expect I was focused on it having removed that and not paying attention to how it handled the labels.

[mildwonkey]

This looks reasonable to me - I always favor consistency - but it might be a good idea to check in with @apparentlymart and see if there's a specific reason fmt handles it differently.