Improve depends_on error-reporting when used with interpolated resources

[dekimsey]

Current Terraform Version

terraform -v
Terraform v0.11.11
+ provider.aws v1.60.0
+ provider.azuread v0.2.0
+ provider.null v1.0.0
+ provider.random v2.0.0
+ provider.template v1.0.0
+ provider.tls v1.2.0

Use-cases

Improve error-reporting when depends_on, which has a somewhat special syntax, is used incorrectly. This is to provide helpful hint to the editor as to how depends_on must be used. I know I stared at this blankly for a bit before I realized my mistake.

Proposal

Given an input like ...

resource "aws_lb" "foo" {
   ...
   depends_on = ["${aws_s3_bucket.lb-access-logs}"]
}

We encounter the following error during apply:

$ terraform apply

Error: aws_lb.foo: resource depends on non-existent resource '${aws_s3_bucket.lb-access-logs}'

The depends_on syntax is unusual in that it requires un-interpolated values. It's one of the few places that references another resource by it's literal name.
This request is that the error message be improved to something like:

$ terraform apply

Error: aws_lb.foo: resource depends on non-existent value '${aws_s3_bucket.lb-access-logs}'. Note, interpolated values cannot be used in depends_on, did you mean 'aws_s3_bucket.lb-access-logs'?

References

• resource depends on non-existent resource when using depends_on #985

None.

[jbardin]

Hi @dekimsey,

Thanks for filing the issue!

This is covered by the configuration enhancements in the next release.
The new error output would look like this if you attempt to reference an attribute:

Error: Invalid depends_on reference

  on main.tf line 5, in resource "aws_lb" "foo":
   5:   depends_on = [aws_s3_bucket.lb-access-logs.id]
References in depends_on must be to a whole object (resource, etc), not to an
attribute of an object.

Since the "${} interpolation strings are no longer needed, and there shouldn't be any existing working configuration with that value, the exact case above shouldn't be encountered in the new version or by the upgrade tool.

[dekimsey]

That's perfect, thank you! I looked around and didn't see any mention of it.

[ghost]

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