config: variable names and outputs must be unique

[mitchellh]

Fixes #5052
Fixes #5144

This adds new config validation to verify output names and variable names are unique.

[darrensony]

Regarding #5144, and looking at your test code, this may or may not be the right solution.

Say I have two .tf files, A.tf and B.tf

A.tf sets the value of the variable
variable "foo" { default = 1 }

B.tf needs to use the value of "foo" but not define it.
variable "foo" {}

The core issue of #5144 was that the above worked fine, but if A.tf was renamed to C.tf, it no longer worked because Terraform reads files in alphabetical order and thus sees the reference before the declaration.

Throwing an error because there is both a declaration and a reference is not the correct behaviour.

[mitchellh]

There is no such thing as a "declaration" and a "reference". Every variable is a declaration, you don't need to "reference" anything (and that doesn't exist in Terraform). Every Terraform file in the same directory is loaded into the same namespace.

Unfortunately what you were doing before should've never worked. Terraform loads and merges all files together. Having a config in A.tf and B.tf is the same as concatenating B.tf to A.tf. Terraform currently happens to load files in alphabetical order which caused your behavior.

We never intended two variables to be defined, as they're all in the same namespace, so this is the correct behavior.

Actually, to fight against this in the future, we should probably load all the files in a random order in each load. The declarative nature of Terraform means the load order shouldn't matter. I'll probably do that in 0.8...

[mitchellh]

@jen20 brought up a good point in our Slack channel that I need to test how this interacts with override files. I'm going to add test cases for this since its currently an unknown.

[jen20]

LGTM pending Travis with the new test.

[ghost]

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