rust/clippy: allow derivable impls

The latest Rust will automatically "fix" derivable default
implementation, which is nice, but makes changes that don't meet our
current MSRV, so allow derivable impls for now.

rust/src/lib.rs

@@ -33,6 +33,10 @@
 // is unavoidable at this time.
 #![allow(clippy::too_many_arguments)]
 
+// This would be nice, but having this lint enables causes
+// clippy --fix to make changes that don't meet our MSRV.
+#![allow(clippy::derivable_impls)]
+
 // TODO: All unsafe functions should have a safety doc, even if its
 // just due to FFI.
 #![allow(clippy::missing_safety_doc)]