Have a look at code needed to get OpenSSL working reliably across platforms:
Making that declarative seems pretty hard. Something has to know about quirks of OpenSSL in Homebrew, something has to know how to find it on FreeBSD, etc.
So where do you put these platform+package specific hacks? If you just tell cargo “I want OpenSSL, do your thing”, then all this mess would have to be built in into Cargo — every exception for every package on every platform.
Or you’d have to declaratively state “try pkg-config with this package, unless on macos, then try brew with that package, and fall back to that other package, and on freebsd search that dir, unless the header is busted or the version is wrong, then do…”, etc. You’d end up with a markup-based programming language.
Or you can say you don’t care about these hacks, and either the correct version is in /usr/lib or pkg-config or you get an error. Then we’re back to the pain of using dependencies in C, where every program has an INSTALL document with all the manual steps and workarounds for multiple platforms and the user has to fiddle with paths, config files, flags and env variables for every library.