Ensure that pruned Instances aren't treated as existing in syncback (#1179)

Closes #1178.

Author: Dekkonot

CHANGELOG.md

@@ -30,6 +30,9 @@ Making a new release? Simply add the new header with the version and date undern
 -->
 
 ## Unreleased
+* Fixed a bug caused by having reference properties (such as `ObjectValue.Value`) that point to an Instance not included in syncback. ([#1179])
+
+[#1179]: https://github.com/rojo-rbx/rojo/pull/1179
 
 ## [7.7.0-rc.1] (November 27th, 2025)
 

rojo-test/syncback-test-snapshots/end_to_end__rojo_test__syncback_util__ref_properties_pruned-stdout.snap

@@ -0,0 +1,7 @@
+---
+source: tests/rojo_test/syncback_util.rs
+expression: "String::from_utf8_lossy(&output.stdout)"
+---
+Writing src/Pointer1.model.json
+Writing src/Pointer2.model.json
+Writing src/Pointer3.model.json

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__ref_properties_pruned-src__Pointer1.model.json.snap

@@ -0,0 +1,7 @@
+---
+source: tests/tests/syncback.rs
+expression: src/Pointer1.model.json
+---
+{
+  "className": "ObjectValue"
+}

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__ref_properties_pruned-src__Pointer2.model.json.snap

@@ -0,0 +1,7 @@
+---
+source: tests/tests/syncback.rs
+expression: src/Pointer2.model.json
+---
+{
+  "className": "ObjectValue"
+}

rojo-test/syncback-test-snapshots/end_to_end__tests__syncback__ref_properties_pruned-src__Pointer3.model.json.snap

@@ -0,0 +1,7 @@
+---
+source: tests/tests/syncback.rs
+expression: src/Pointer3.model.json
+---
+{
+  "className": "ObjectValue"
+}

rojo-test/syncback-tests/ref_properties_pruned/input-project/default.project.json

@@ -0,0 +1,9 @@
+{
+  "name": "ref_properties_pruned",
+  "tree": {
+    "$className": "DataModel",
+    "ServerScriptService": {
+      "$path": "src"
+    }
+  }
+}

rojo-test/syncback-tests/ref_properties_pruned/input-project/src/.gitkeep

@@ -8,7 +8,10 @@ use rbx_dom_weak::{
     ustr, Instance, Ustr, WeakDom,
 };
 
-use crate::{multimap::MultiMap, REF_ID_ATTRIBUTE_NAME, REF_POINTER_ATTRIBUTE_PREFIX};
+use crate::{
+    multimap::MultiMap, syncback::snapshot::inst_path, REF_ID_ATTRIBUTE_NAME,
+    REF_POINTER_ATTRIBUTE_PREFIX,
+};
 
 pub struct RefLinks {
     /// A map of referents to each of their Ref properties.
@@ -50,6 +53,22 @@ pub fn collect_referents(dom: &WeakDom) -> RefLinks {
                 continue;
             }
 
+            let target = match dom.get_by_ref(*prop_value) {
+                Some(inst) => inst,
+                None => {
+                    // Properties that are Some but point to nothing may as
+                    // well be `nil`. Roblox and us never produce these values
+                    // but syncback prunes trees without adjusting ref
+                    // properties for performance reasons.
+                    log::warn!(
+                        "Property {}.{} will be `nil` on the disk because the actual value is not being included in syncback",
+                        inst_path(dom, inst_ref),
+                        prop_name
+                    );
+                    continue;
+                }
+            };
+
             links.insert(
                 inst_ref,
                 RefLink {
@@ -58,10 +77,6 @@ pub fn collect_referents(dom: &WeakDom) -> RefLinks {
                 },
             );
 
-            let target = dom
-                .get_by_ref(*prop_value)
-                .expect("Refs in DOM should point to valid Instances");
-
             // 1. Check if target has an ID
             if let Some(id) = get_existing_id(target) {
                 // If it does, we need to check whether that ID is a duplicate

rojo-test/syncback-tests/ref_properties_pruned/input.rbxl

@@ -71,8 +71,13 @@ syncback_tests! {
     ref_properties_conflict => ["src/Pointer_2.model.json", "src/Target_2.model.json"],
     // Ensures that having multiple pointers that are aimed at the same target doesn't trigger ref rewrites.
     ref_properties_duplicate => [],
+    // Ensures that ref properties that point to nothing after the prune both
+    // do not leave any trace of themselves
+    ref_properties_pruned => ["src/Pointer1.model.json", "src/Pointer2.model.json", "src/Pointer3.model.json"],
     // Ensures that the old middleware is respected during syncback
     respect_old_middleware => ["default.project.json", "src/model_json.model.json", "src/rbxm.rbxm", "src/rbxmx.rbxmx"],
+    // Ensures that the `$schema` field roundtrips with syncback
+    schema_roundtrip => ["default.project.json", "src/model.model.json", "src/init/init.meta.json", "src/adjacent.meta.json"],
     // Ensures that StringValues inside project files are written to the
     // project file, but only if they don't have `$path` set
     string_value_project => ["default.project.json"],
@@ -81,5 +86,4 @@ syncback_tests! {
     sync_rules => ["src/module.modulescript", "src/text.text"],
     // Ensures that the `syncUnscriptable` setting works
     unscriptable_properties => ["default.project.json"],
-    schema_roundtrip => ["default.project.json", "src/model.model.json", "src/init/init.meta.json", "src/adjacent.meta.json"]
 }