Reserve state and plan keywords in test files (#33846)

internal/addrs/parse_ref.go

@@ -90,6 +90,7 @@ func ParseRef(traversal hcl.Traversal) (*Reference, tfdiags.Diagnostics) {
 // scope and so should use this function to retrieve references.
 func ParseRefFromTestingScope(traversal hcl.Traversal) (*Reference, tfdiags.Diagnostics) {
 	root := traversal.RootName()
+	rootRange := traversal[0].SourceRange()
 
 	var diags tfdiags.Diagnostics
 	var reference *Reference
@@ -119,6 +120,17 @@ func ParseRefFromTestingScope(traversal hcl.Traversal) (*Reference, tfdiags.Diag
 			Remaining:   remain,
 		}
 		diags = runDiags
+	case "plan", "state":
+		// These names are all pre-emptively reserved in the hope of landing
+		// some version of referencing the plan and state files in test
+		// assertions.
+		diags = diags.Append(&hcl.Diagnostic{
+			Severity: hcl.DiagError,
+			Summary:  "Reserved symbol name",
+			Detail:   fmt.Sprintf("The symbol name %q is reserved for use in a future Terraform version. If you are using a provider that already uses this as a resource type name, add the prefix \"resource.\" to force interpretation as a resource type name.", root),
+			Subject:  rootRange.Ptr(),
+		})
+		return nil, diags
 	}
 
 	if reference != nil {