terraform: disallow simple variables ("foo")

[mitchellh]

Fixes #5338 (and I'm sure many others)

There is no use case for "simple" variables in Terraform at all so
anytime one is found it should be an error.

There is a huge backwards incompatibility here that was not supposed
to be by design but I'm sure a lot of people are relying on: in the
template_file datasource, this bug allowed you to not escape your
interpolations and have the work. For example:

data "template_file" "foo" {
  template = "${a}"
  vars { a = 12 }
}

The above would work, but it shouldn't. The template should have to be
"$${a}" (to escape the interpolation).

Because of this BC, I recommend holding this until Terraform 0.8.0 and
documenting it carefully. As part of this PR, I've added some special
error message notes.

[apparentlymart]

I've seen questions about this particular instance of the bug with template_file lots of times on here, so glad to see a fix for it... it has in particular been causing confusion with people using template_file as a data resource, since it confuses the logic for deciding if the data source read needs to be deferred until apply time.

This fix looks plausible, but agreed about the BC concerns given how many people reported phantom bugs in the data sources code due to using template_file interpolations in this way.

[mitchellh]

@apparentlymart Yeah this is going to be paired with: hashicorp/hcl#150

That makes the HCL/HIL interaction a lot more sense at the expense of BC

[apparentlymart]

Some more issue references, just to make sure there are link trails from these other issues to here:

• I apparently filed a specific bug report about this and then forgot about it: Interpolating invalid variables should be an error #8077.
• Various questions and misunderstandings with the template_file antipattern shown in this issue's summary: template_file: whether or not changes show up in plans feels inconsistent #8021, Data Source template_file causes resource to be recreated each execution. #8316, Unnecessary template_file recreation when no template variables have changed #8871.

[ghost]

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