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

@@ -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

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.`,

terraform/testdata/static-validate-refs/static-validate-refs.tf

@@ -18,3 +18,6 @@ resource "boop_instance" "yep" {
 
 resource "boop_whatever" "nope" {
 }
+
+data "beep" "boop" {
+}