interpret: make identity upcasts a NOP again to avoid them generating a new random vtable

Author: RalfJung

compiler/rustc_const_eval/src/interpret/cast.rs

@@ -400,6 +400,12 @@ impl<'tcx, M: Machine<'tcx>> InterpCx<'tcx, M> {
             }
             (ty::Dynamic(data_a, _, ty::Dyn), ty::Dynamic(data_b, _, ty::Dyn)) => {
                 let val = self.read_immediate(src)?;
+                // MIR building generates odd NOP casts, prevent them from causing unexpected trouble.
+                // See <https://github.com/rust-lang/rust/issues/128880>.
+                // FIXME: ideally we wouldn't have to do this.
+                if data_a == data_b {
+                    return self.write_immediate(*val, dest);
+                }
                 // Take apart the old pointer, and find the dynamic type.
                 let (old_data, old_vptr) = val.to_scalar_pair();
                 let old_data = old_data.to_pointer(self)?;