Nix is different, yeah, and it won’t wire together a build cache for you. Nix is great for many things of course, it’s just not a replacement for sccache per se
Nix + sccache would probably be pretty great for preserving paths and environment, which is really healthy for build caching in general.
Properly handling cargo and bazel builds in nix is very much not a solved problem, and Nix's resistance to allowing ccache into the sandbox for purity reasons definitely exacerbates the problem.
sccache should not be allowed in the sandbox, at least not just bolted on; realistically such cases are probably better handled in the long run by Recursive Nix, so that you can build new derivations inside of existing ones, and the results are cached in the same (outer) store. This means there won't be duplicate caches; /nix/store and where-ever sccache puts results.
Cargo is a good example. For a number of (practical) reasons, cargo -> nix translators are a lot of effort and often have bugs, so for "simplicity" all upstream crates just compile every crate dependency every time. That means if two crates use the same dependency, it gets compiled twice. It's important to understand this is no worse than the way Cargo works already for most people. Cargo does not have a content address storage model, unfortunately. But it's pretty annoying for Nix users and costs a lot.
In theory, we could wrap rustc with a recursive-nix enabled wrapper so that rlib's etc each get a granular derivation and then get put into the host store. So assuming a crate gets built with the same flags, between two "outer" expressions, they'll get to share the work and it will go into the store. A working example of this for C++ code is nix-ccache, but a fully robust implementation is a bit of work.[1]
Recursive Nix is still experimental but there is some use of it (privately and publicly) that I know of for these purposes.
It sounds like you’re really experienced with Nix or perhaps a contributor. We’re new to Nix at Elide (makers of Buildless) but we’d love to collaborate :)
I just tried it for the first time the other day and although I’m not ready to move to it yet, I can already see the brilliance.
As a user (and occasional recompiler) of the tensorflow derivation in NixOS, I'd love to see Nix able to somehow do a poetry2nix style transformation on bazel dependencies so they would be properly cached in individual store paths.
Nix + sccache would probably be pretty great for preserving paths and environment, which is really healthy for build caching in general.