Requirements for updating this repo about cassandra-sys-rs

[alexeiakimov]

Thank you @kw217 for the detailed explanation. It seems that there is a subtle problem here - the version of bindgen utility. Since it is not specified, the new Rust code can be incompatible with the old one. In particular

• in times of Cassandra driver 2.10 the bindgen mapped size_t to usize
• later after rust-lang/rust-bindgen#1671 bindgen start to map size_t to some other type depending on the target platform. On my machine it is u64
• it means that some functions from public API will have different signature, for example get_future_error_message
• formally the new generated code is not backward compatible, although the original C++ drivers are backward compatible

The old behavior can be restored by using the --size_t-is-usize option with bindgen. @kw217 do you think it is necessary to preserve the original size_t -> usize binding, otherwise a new major version of the library should be released? Thank you.

[kw217]

Thanks for the analysis. I think sticking with usize is the best approach. It will require a new major version anyway (since clients are forced to install a new libcassandra), but I'd prefer not to force them to make any other changes.

[kw217]

FTR I've updated CONTRIBUTING.md to include my notes from above.

[kw217]

Closing - #39 has upgraded the driver, and #41 has added the notes above to the docs.