Deprecation Warning For Outputs

[dbanck]

In this PR @DanielMSchmidt and I take a first stab at #18381 and add a new attribute to the output block to allow it to be marked as deprecated.

UX

If this is the right approach, my suggestion would be to deal with the deprecation of inputs in a separate PR.

Partially resolves #18381

Target Release

1.11.x

[jbardin]

The reason we don't have deprecation warnings in outputs is really the same reason we don't have deprecation warnings for resource attributes, there is no way to track value provenance. The approach taken here will flag every use of the module value as triggering the deprecation, rather than just use of the deprecated output value.

When references are evaluated in Terraform, they are always evaluated by getting the entire container of values because any indices need to be evaluated as part of the larger expression. So when you reference a module output, you actually obtain an object containing all outputs; and further if the module is expanded, you get a container of all module objects.

The easiest problem to see may be a reference like this:

attr = module.test[*].not_deprecated

The base reference is to the entire tuple of module objects (module.test), even though you only assign the not_deprecated output.

There are a lot of other similar use cases, like passing module object around as arguments themselves should not trigger deprecation warnings if the deprecated output values are never used.