Doesn’t matter the license. If I modified this code and didn’t distribute the code, I have 0 obligation to publish/share my changes. These are copyright licenses and license agreements. Distribution is the trigger for copyright licenses.

(Assuming we’re talking about US law) Since the previous comment was talking about commercial use, there’s no entitlement to use someone else’s code for commercial purposes at all absent some sort of license agreement (or waiver, or fair use, or some other exception). If the license granting the right to use the code in the first place requires sharing changes, then someone using the code with modifications has to either share the changes or infringe copyright.

Standard dual license commercial and AGPL that most companies use. These are copyright licenses, Once you allow me to download the software, I can do anything I want to it but distribute. Licesense only invokes on the copy hence a copyright license. Copyright licenses can only grant rights not take them away, so they can't control how I use it privately.

Copyright law is concerned with the making of copies, not just with their distribution. Fair use covers making personal modifications to software for personal use, but does not cover modifications for commercial use.

fair use only covers distribution.

"Examples of fair use in United States copyright law include commentary, search engines, criticism, parody, news reporting, research, and scholarship. Fair use provides for the legal, unlicensed citation or incorporation of copyrighted material in another author's work under a four-factor test."

https://en.wikipedia.org/wiki/Fair_use

Copyleft which is the AGPL only cover distribution.

"Copyleft applies only when a person seeks to redistribute the program. Developers may make private modified versions with no obligation to divulge the modifications, as long as they do not distribute the modified software to anyone else. "

https://en.wikipedia.org/wiki/GNU_General_Public_License

> Could this be a way to make those improvements public by this replacing memcached over time?

Those who wish to share their contributions will, regardless of license. Those who do not, will stick to code with BSD-style licenses.

They do indeed have a CLA: https://cla-assistant.io/memc-rs/memc-rs

Ah, awesome. I did not see this mentioned in the contributing.md file (it may have been there, I just didn't notice it).