wfassoc: Core Rust library. Rust programmer can directly utilize it.
wfassoc_dylib: A dynamic library exposed for C/C++ and other languages users.
wfassoc_exec: A executable configuring file assocation according to user given profile and request.

If you are a programmer who just want to silently set your file association for user, wfassoc_exec would be your best choice. The only things you need to do is that write a TOML description file, and provide it and wfassoc_exec with your executable. By executing wfassoc_exec with this TOML file in your executable, you can simply achieve this goal and analyze its return value to check whether it success.

However, if you are prefering taht let user decide which file associations should be created, even have an UI displaying all current file association related with this program (like 7-Zip File Manager does), you can choose wfassoc_dylib for your solution. wfassoc_dylib expose all essential functions for this task. And you can build it with your own program.

At last, if all scenarios above can not cover your requirements, you can utilize wfassoc directly in Rust, change all essential code in your favor, and finally produce the application which onlt suit for yourself.