Yeah, although you have a lot of flexibility regarding how much of your system will be rebuilt.
Like, if you add a little patch to libpng as an overlay over your entire system, then you'll recompile a lot of stuff, but you can also choose to only use that custom libpng with a specific package.
Even in the latter case, it's true that Nix will always recompile the package that depends on libpng, instead of using dynamic linking and only recompiling libpng. This is because Nix has no way of knowing how your libpng patch will affect its reverse dependency, and because link paths are always content-addressed in Nix for reproducibility.
It wasn’t apparent to me from the docs (back then?) how to use the custom libpng only with a specific package. Do you have a pointer to the documentation on that please?
Like, if you add a little patch to libpng as an overlay over your entire system, then you'll recompile a lot of stuff, but you can also choose to only use that custom libpng with a specific package.
Even in the latter case, it's true that Nix will always recompile the package that depends on libpng, instead of using dynamic linking and only recompiling libpng. This is because Nix has no way of knowing how your libpng patch will affect its reverse dependency, and because link paths are always content-addressed in Nix for reproducibility.