diff --git a/libs/binder/rust/src/parcel.rs b/libs/binder/rust/src/parcel.rs
index 3c615ed..f9f135d 100644
--- a/libs/binder/rust/src/parcel.rs
+++ b/libs/binder/rust/src/parcel.rs
@@ -54,6 +54,10 @@
 
 /// Safety: This type guarantees that it owns the AParcel and that all access to
 /// the AParcel happens through the Parcel, so it is ok to send across threads.
+///
+/// It would not be okay to implement Sync, because that would allow you to call
+/// the reading methods from several threads in parallel, which would be a data
+/// race on the cursor position inside the AParcel.
 unsafe impl Send for Parcel {}
 
 /// Container for a message (data and object references) that can be sent