Understanding Source Filtering/Loading with Crane

[Gastove]

Hullo! I've been learning to build Rust projects with crane, and I'm having a really excellent experience. And, there's a thing I really don't understand in the workspace quickstart flake. It demonstrates source loading with cleanCargoSource:

   src = craneLib.cleanCargoSource ./.;

src is then inherit-ed into commonArgs, which in turn are shallow merged into individualCrateArgs, which are then shallow merged with each attribute set for building a given workspace. For ease of conversation, I'll copy one here:

        my-cli = craneLib.buildPackage (individualCrateArgs // {
          pname = "my-cli";
          cargoExtraArgs = "-p my-cli";
          src = fileSetForCrate ./my-cli;
        });

...except now we're overriding src entirely with the result of fileSetForCrate.

I'm just trying to understand: why? Why not "just" use the src from cleanCargoSource?

Thanks so much for the help, and the excellent project!

[ipetkov]

Hi @Gastove! It is totally fine to use the "entire", cleaned src for the workspace, but this has the effect that if you touch any file (kept by the filter) then all derivations which use that same src will need to be rebuilt. Imagine you have a workspace of three crates: foo, bar, and common, where common is a dependency of both foo and bar. Touching any of the source files for foo should not require rebuilding bar, so if you have a slow build it may be more useful to apply the fileSetForCrate technique to avoid accidental rebuilds (what this example illustrates is that only the top-level crate's source along with any common code is included for each leaf binary).

If you have a smaller project or don't care too much about the build invalidation, feel free to omit that extra code.

Hope that explains things!