Explain protected source destination collisions during Save As

crates/oa-desktop/src/app.rs

@@ -5605,6 +5605,7 @@ fn write_snapshot_inner(
                     .map_err(|errors| crate::document::DocumentError::Source(errors).to_string())?,
             );
         }
+        let has_original_destination = previous.is_some();
         let previous = previous.filter(|disk| {
             path.parent()
                 .filter(|p| !p.as_os_str().is_empty())
@@ -5615,14 +5616,33 @@ fn write_snapshot_inner(
                 == Some(&disk.root)
                 && entry == disk.bundle.entry
         });
+        // A missing-file expectation colliding with an existing destination is
+        // not evidence that an observed dependency changed outside the editor.
+        // Keep save_project's conflict reporting unchanged for that latter case.
+        let saving_existing = previous.is_some();
+        let destination_error = |error: source_files::SourceFileError| match error {
+            source_files::SourceFileError::Conflict(paths) => {
+                let guidance = if saving_existing {
+                    "Review the new included file's destination before saving."
+                } else if has_original_destination {
+                    "Choose a fresh directory for Save As, or use Save All to update the open project's current entry."
+                } else {
+                    "Choose a fresh directory for Save As."
+                };
+                format!(
+                    "Source destination files already exist: {paths:?}. Existing files are protected. {guidance}"
+                )
+            }
+            other => other.to_string(),
+        };
         let mut disk = match previous {
             Some(disk) => disk,
-            None => source_files::new_project(&path, LIMITS).map_err(|e| e.to_string())?,
+            None => source_files::new_project(&path, LIMITS).map_err(&destination_error)?,
         };
         for path in snapshot.bundle.files.keys() {
             if !disk.expectations.contains_key(path) {
                 source_files::expect_new_file(&mut disk, path, LIMITS)
-                    .map_err(|e| e.to_string())?;
+                    .map_err(&destination_error)?;
             }
         }
         Some(source_files::save_project(&disk, &snapshot, LIMITS).map_err(|e| e.to_string())?)