The rust builds using rustPlatform.buildRustPackage always re-build all dependencies, which makes builds rather slow. I’ve been trying to use sccache to speed it up, but my possibly quite naive approach doesn’t work:
When building a shell.nix on the above it works as expected, i.e. envvars are set and the cache is used. When building using nix-build however, the cache isn’t created where I hoped it would be and a second build takes just as long as the first. I’m guessing there’s some sort of “sandboxing” by nix-build. Maybe that completely prevents the use of sccache.
Is there some other way of speeding up builds of rust packages?
I had a quick look and I can’t find any documentation on where the cache ends up. I need to know since I want TravisCI to cache it for the next build. (Asking before I dig deeper to find the answer for myself.)
No I don’t use CI atm. I used to use Hydra though.
There is no dedicated cache directory with Nix builds. Everything that gets built ends up in the Nix store. I never used Travis, but as long as you can keep /nix/store/ through CI invocations, your individual crates will be cached.
You might want to read a bit into the concept of the Nix store and the Nix binary cache. Nix has a very different model compared to traditional build systems.
I use crate2nix/buildRustCrate. Caching /nix with GitHub Actions or Travis CI always seemed a bit fragile to me, so I use Cachix or a self-hosted cache across some projects (S3-compatible object storage). Both work very nicely, though uploads to the self-hosted cache are a bit slow, because Nix uses LZMA for the NAR files.
For private caches, I use something along the lines of: