Austin Seipp 3f7b5a75e7 github(nix): don't build everything twice ... When running the `nix build`, the `buildRustPackage` function -- which builds the `jj` crates -- calls `cargo build --release` with flags like `HOST_CXX` set. This is called the `buildPhase`. Then, it runs the `checkPhase`, which calls `cargo nextest`, in our case. However, it does not set `HOST_CXX`, for some reason. The intent of `buildRustPackage` is that the `buildPhase` and `checkPhase` have their compilation options fully aligned so that they reuse the local cargo `target/` cache directory, avoiding a full recompilation. However, the lack of `HOST_CXX` above among others causes a cache miss, and a bunch of cascading recompilations. The net impact is that we compile all of the codebase once in `buildPhase`, then again in `checkPhase`, making the Nix CI build 2x slower on average than the other Linux runners; 2-3 minutes versus 7 minutes, on average. Instead, re-introduce a 'check' into the Flake directly, which overrides the `jujustsu` package, but stubs out the `buildPhase`. This means it will only run `checkPhase`, which is what we want. Then, modify the CI to run `nix flake check` again, like it used to in the past. Unfortunately, this doesn't fix the cache miss when running `nix build` yourself, it recompiles from scratch in both phases still. That needs to be fixed in the future, but it's tolerable for now. This reverts most of 71a3045032. Signed-off-by: Austin Seipp <aseipp@pobox.com>		2024-02-24 13:56:03 -06:00
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