Warn about unused dependencies

[BeyondEvil]

Describe the enhancement

It'd be swell if there was some way, either when running init/plan/apply or any other validation command, if Terragrunt warned about unused dependency blocks.

RFC Not Needed

• I have evaluated the complexity of this enhancement, and I believe it does not require an RFC.

[mhulscher]

How would you define an unused dependency? If none of its outputs are used? Personally, I sometimes use (explicit) dependency blocks w/o outputs and I wouldn't need to be reminded of that 😅

[BeyondEvil]

How would you define an unused dependency? If none of its outputs are used? Personally, I sometimes use (explicit) dependency blocks w/o outputs and I wouldn't need to be reminded of that 😅

Interesting. What's the use-case for a dependency block that's not used? 🤔

Regardless, that's probably and edge case that could be solved with a "don't warn"-attribute in the block.

To answer your question, your use-case not withstanding, if there's a dependency block defined - but not called, a warning is issued.

Meaning, if we have:

dependency "foo" {
}

But no calls to dependency.foo, it warrants a warning.

[yhakbar]

Defining a dependency without using any outputs gives Terragrunt information on its relationship in the DAG.

It's useful, even though someone could theoretically do the same thing via a dependencies block, which can't define outputs.

I think what would be more useful here is some type of lint that can warn on that for users trying to discover dependencies without used outputs.

We don't have a dedicated Terragrunt linter right now, but we will have an hcl lint command after the CLI Redesign. It might be a good feature to add there.