Comment by mikepurvis

Interestingly despite being pretty hard-nosed about a lot of things, Nix does not insist on a read-only source directory at build time— the source is pulled into a read-only store path, but from there it is copied into the build sandbox, not bind-mounted.

I expect this is largely a concession to the reality that most autotools projects still expect an in-source build, not to mention Python wanting to spray pyc files and build/dist directories all over the place.