I wasn't really thinking about language-level runtimes, like, I don't want to solve the problem of only having one global scope in python and running into module name collisions. I'm not sure how concerned rez is with shell environments beyond what (eg.) python packages are available in it, but I was wondering about having a shell where a program foo is available that, say, requires a lib compiled with gcc-4, and another program bar that requires the same lib at the same version but compiled with gcc-3.
Nix expends a bunch of complexity into fucking with dynamic linking to make that work transparently most of the time and deploying wrapper scripts to cover other cases, so I was curious if rez has a cleverer solution there.