From 358fb54f75709c4c630a843dfc055c0b41102338 Mon Sep 17 00:00:00 2001
From: Martin Atkins <mart@degeneration.co.uk>
Date: Fri, 14 May 2021 09:58:28 -0700
Subject: [PATCH] core: "Did you mean" hint for missing data. prefix in
 references

It's a relatively common mistake to try to refer to a data resource
without including the data. prefix, making Terraform understand it as a
reference to a managed resource.

To help with that case, we'll include an additonal suggestion if we can
see that there's a data resource declared with the same type and name as
in the given address.
---
 terraform/evaluate_valid.go                         | 13 ++++++++++++-
 terraform/evaluate_valid_test.go                    |  6 ++++++
 .../static-validate-refs/static-validate-refs.tf    |  3 +++
 3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/terraform/evaluate_valid.go b/terraform/evaluate_valid.go
index 0746ce507..3a8323ae5 100644
--- a/terraform/evaluate_valid.go
+++ b/terraform/evaluate_valid.go
@@ -203,10 +203,21 @@ func (d *evaluationStateData) staticValidateResourceReference(modCfg *configs.Co
 
 	cfg := modCfg.Module.ResourceByAddr(addr)
 	if cfg == nil {
+		var suggestion string
+		// A common mistake is omitting the data. prefix when trying to refer
+		// to a data resource, so we'll add a special hint for that.
+		if addr.Mode == addrs.ManagedResourceMode {
+			candidateAddr := addr // not a pointer, so this is a copy
+			candidateAddr.Mode = addrs.DataResourceMode
+			if candidateCfg := modCfg.Module.ResourceByAddr(candidateAddr); candidateCfg != nil {
+				suggestion = fmt.Sprintf("\n\nDid you mean the data resource %s?", candidateAddr)
+			}
+		}
+
 		diags = diags.Append(&hcl.Diagnostic{
 			Severity: hcl.DiagError,
 			Summary:  `Reference to undeclared resource`,
-			Detail:   fmt.Sprintf(`A %s resource %q %q has not been declared in %s.`, modeAdjective, addr.Type, addr.Name, moduleConfigDisplayAddr(modCfg.Path)),
+			Detail:   fmt.Sprintf(`A %s resource %q %q has not been declared in %s.%s`, modeAdjective, addr.Type, addr.Name, moduleConfigDisplayAddr(modCfg.Path), suggestion),
 			Subject:  rng.ToHCL().Ptr(),
 		})
 		return diags
diff --git a/terraform/evaluate_valid_test.go b/terraform/evaluate_valid_test.go
index 086ca3037..9267aee65 100644
--- a/terraform/evaluate_valid_test.go
+++ b/terraform/evaluate_valid_test.go
@@ -32,6 +32,12 @@ func TestStaticValidateReferences(t *testing.T) {
 			"aws_instance.nonexist",
 			`Reference to undeclared resource: A managed resource "aws_instance" "nonexist" has not been declared in the root module.`,
 		},
+		{
+			"beep.boop",
+			`Reference to undeclared resource: A managed resource "beep" "boop" has not been declared in the root module.
+
+Did you mean the data resource data.beep.boop?`,
+		},
 		{
 			"aws_instance.no_count[0]",
 			`Unexpected resource instance key: Because aws_instance.no_count does not have "count" or "for_each" set, references to it must not include an index key. Remove the bracketed index to refer to the single instance of this resource.`,
diff --git a/terraform/testdata/static-validate-refs/static-validate-refs.tf b/terraform/testdata/static-validate-refs/static-validate-refs.tf
index e9f9344a8..3667a4e11 100644
--- a/terraform/testdata/static-validate-refs/static-validate-refs.tf
+++ b/terraform/testdata/static-validate-refs/static-validate-refs.tf
@@ -18,3 +18,6 @@ resource "boop_instance" "yep" {
 
 resource "boop_whatever" "nope" {
 }
+
+data "beep" "boop" {
+}