For those unfamiliar, Nix is a powerful package manager and build system that ensures reproducible builds via strict dependency isolation. However, one of its limitations has been that it builds packages as monolithic units - if you change one source file, Nix rebuilds the entire package from scratch.

nix-ninja solves this by bringing compilation unit level granularity to Nix builds. It targets ninja build files, a format for describing build graphs output by popular build systems like CMake and meson. This means that when you modify a single source file, only the affected compilation units need to be rebuilt, significantly reducing build times.

We're excited because this gives Nix fine-grained caching with early cutoff optimisation (see the Build Systems à la Carte paper). Combined with remote build farms like Nixbuild.net, this moves Nix into the incremental cloud build systems arena, alongside Google's Bazel and Meta's Buck2.

Dynamic derivations and content-addressed derivations are still experimental features in Nix, so we're hoping to accelerate their maturation by providing a compelling use-case and implementation. Our north star is to have Hydra (nixpkgs' CI runner) support incremental compilation in nixpkgs for slow builds like LLVM.

nix-ninja can compile Nix itself today, but we're still very early in its development. Given community interest in dynamic derivations and incremental compilation in Nix, we decided to open source it in an pre-alpha state to involve the community in its design.

Please take a look. We'll be available in the comments to answer any questions: https://github.com/pdtpartners/nix-ninja