The other issue with zlib, is that it’s not open and use of it involves additional license provisions that could block Rust adoption in many organisations. Facebook grants you patent indemnity over the patents involved in zlib (alone), but in return requires that you provide Facebook patent indemnity over any and every patent claim you might ever have against them (for any cause) in perpetuity. If the choice were to come between (a) use Rust and put all of a company’s patent IP at risk (if the zlib dependency were to be introduced), or, (b) Block Rust and protect the larger IP base; then almost all companies and many individuals, are going to choose option ‘b’ every time.
Correction: I was referring to libzstd, not zlib (as typed).
Facebook has done away with the patent issue, so that’s no longer a problem.