appendix crate implements a key-value mapping data structure called
Index<K, V> that is stored on disk. The crate allows for any type to inhabit
V type parameters and implements Send and Sync for them
Using a type that is not marked as
Index can allow it
to be used across multiple threads leading to data races. Additionally using
reference types for the keys or values will lead to the segmentation faults
in the crate's code.