crane/docs/advanced/overriding-function-behavior.md

Overriding function behavior

At it's core, crane is instantiated via pkgs.lib.newScope which allows any internal definition to be changed or replaced via .overrideScope' (which behaves very much like applying overlays to nixpkgs). Although this mechanism is incredibly powerful, care should be taken to avoid creating confusing or brittle integrations built on undocumented details.

Note that crane's stability guarantees (with respect to semantic versioning) only apply to what has been documented at the API level. For example, buildPackage is documented to delegate to mkCargoDerivation, so any changes or overrides to mkCargoDerivation's behavior will be observed by buildPackage. Other non-documented internal details, however, may change at any time, so take care when reaching this deep into the internals.

Here is an example:

let
  craneLib = (inputs.crane.mkLib pkgs).overrideScope' (final: prev: {
    # We override the behavior of `mkCargoDerivation` by adding a wrapper which
    # will set a default value of `CARGO_PROFILE` when not set by the caller.
    # This change will automatically be propagated to any other functions built
    # on top of it (like `buildPackage`, `cargoBuild`, etc.)
    mkCargoDerivation = args: prev.mkCargoDerivation ({
      CARGO_PROFILE = "bench"; # E.g. always build in benchmark mode unless overridden
    } // args);
  });
in
{
    # Build two different workspaces with the modified behavior above

    foo = craneLib.buildPackage {
      src = craneLib.cleanCargoSource (craneLib.path ./foo);
    };

    bar = craneLib.buildPackage {
      src = craneLib.cleanCargoSource (craneLib.path ./bar);
    };
}