Insert Model.NeedsPivotMigration in insert_instance when missing (#865)

rojo-test/build-test-snapshots/end_to_end__tests__build__rbxmx_in_folder.snap

@@ -1,7 +1,6 @@
 ---
 source: tests/tests/build.rs
 expression: contents
-
 ---
 <roblox version="4">
   <Item class="Folder" referent="0">
@@ -25,6 +24,7 @@ expression: contents
           <R21>0</R21>
           <R22>1</R22>
         </CoordinateFrame>
+        <bool name="NeedsPivotMigration">false</bool>
         <Ref name="PrimaryPart">null</Ref>
         <BinaryString name="Tags"></BinaryString>
       </Properties>

rojo-test/build-test-snapshots/end_to_end__tests__build__unresolved_values.snap

@@ -1,7 +1,6 @@
 ---
 source: tests/tests/build.rs
 expression: contents
-
 ---
 <roblox version="4">
   <Item class="DataModel" referent="0">
@@ -22,6 +21,7 @@ expression: contents
     <Item class="Workspace" referent="2">
       <Properties>
         <string name="Name">Workspace</string>
+        <bool name="NeedsPivotMigration">false</bool>
       </Properties>
       <Item class="BoolValue" referent="3">
         <Properties>

rojo-test/serve-test-snapshots/end_to_end__tests__serve__empty_json_model_all-2.snap

@@ -1,7 +1,6 @@
 ---
 source: tests/tests/serve.rs
 expression: "read_response.intern_and_redact(&mut redactions, root_id)"
-
 ---
 instances:
   id-2:
@@ -22,7 +21,9 @@ instances:
       ignoreUnknownInstances: false
     Name: test
     Parent: id-2
-    Properties: {}
+    Properties:
+      NeedsPivotMigration:
+        Bool: false
 messageCursor: 1
 sessionId: id-1
 

rojo-test/serve-test-snapshots/end_to_end__tests__serve__empty_json_model_subscribe.snap

@@ -1,7 +1,6 @@
 ---
 source: tests/tests/serve.rs
 expression: "subscribe_response.intern_and_redact(&mut redactions, ())"
-
 ---
 messageCursor: 1
 messages:
@@ -14,7 +13,9 @@ messages:
           ignoreUnknownInstances: false
         Name: test
         Parent: id-2
-        Properties: {}
+        Properties:
+          NeedsPivotMigration:
+            Bool: false
     removed: []
     updated: []
 sessionId: id-1

src/snapshot/tree.rs

@@ -87,10 +87,28 @@ impl RojoTree {
     }
 
     pub fn insert_instance(&mut self, parent_ref: Ref, snapshot: InstanceSnapshot) -> Ref {
+        // !!!!!!!!!! UGLY HACK !!!!!!!!!!
+        //
+        // This is a set of special cases working around a more general problem upstream
+        // in rbx-dom that causes pivots to not build to file correctly, described in
+        // github.com/rojo-rbx/rojo/issues/628.
+        //
+        // We need to insert the NeedsPivotMigration property with a value of false on
+        // every instance that inherits from Model for pivots to build correctly.
+        let hack_needs_pivot_migration = match snapshot.class_name.as_ref() {
+            "Model" | "Actor" | "Tool" | "HopperBin" | "Flag" | "WorldModel" | "Workspace"
+                if !snapshot.properties.contains_key("NeedsPivotMigration") =>
+            {
+                vec![("NeedsPivotMigration", Variant::Bool(false))]
+            }
+            _ => Vec::new(),
+        };
+
         let builder = InstanceBuilder::empty()
             .with_class(snapshot.class_name.into_owned())
             .with_name(snapshot.name.into_owned())
-            .with_properties(snapshot.properties);
+            .with_properties(snapshot.properties)
+            .with_properties(hack_needs_pivot_migration);
 
         let referent = self.inner.insert(parent_ref, builder);
         self.insert_metadata(referent, snapshot.metadata);