feat: Add `extra-args` config option to pixi-build-rattler-build backend

[jackm97]

Summary

The pixi-build-rattler-build backend currently only supports debug-dir and extra-input-globs configuration options. It would be very useful to add an extra-args option (similar to pixi-build-cmake) to pass arbitrary flags to the underlying rattler-build invocation.

Use Case

I'm building a multi-output conda package using the cache: feature in my recipe.yaml. This feature requires the --experimental flag to be passed to rattler-build:

# recipe.yaml
schema_version: 1

cache:
  source:
    path: ../repo
  # ... shared build configuration

outputs:
  - package:
      name: my-package-libs
      # ...
  - package:
      name: my-package-devel
      # ...

When running rattler-build build --experimental -r recipe.yaml directly, this works fine. However, when using pixi build, there's no way to pass the --experimental flag through the backend.

Current Behavior

# pixi.toml
[package.build]
backend = { name = "pixi-build-rattler-build", version = "*" }

[package.build.config]
extra-args = ["--experimental"]  # ❌ Not supported

Results in:

Error: unknown field `extra-args`, expected one of `debug-dir`, `debug_dir`, `extra-input-globs`

Proposed Solution

Add an extra-args configuration option to pixi-build-rattler-build, similar to how pixi-build-cmake handles it:

[package.build.config]
extra-args = ["--experimental"]

This would allow users to pass any rattler-build CLI flags through pixi, including:

• --experimental - for experimental features like cache:
• --no-test - to skip tests
• --keep-build - to preserve build directory
• Any future flags

Alternatives Considered

1. Add only an experimental = true option: This would solve my immediate problem but wouldn't be as flexible for other use cases.

2. Restructure the recipe to avoid cache:: This would require either building multiple times (very slow for large projects like LLVM) or using a single monolithic package.

3. Use rattler-build directly instead of pixi build: Loses the benefits of pixi's dependency management and workspace features.

Additional Context

• The CMake backend already supports extra-args: https://github.com/prefix-dev/pixi-build-backends/blob/main/docs/backends/pixi-build-cmake.md
• Related pixi PR that added [package.build.configuration]: feat: implement package.build.configuration parsing pixi#3115

[baszalmstra]

Maybe we can just add an experimental configuration option instead?

[jackm97]

Maybe we can just add an experimental configuration option instead?

That works! I only mentioned extra-args because it would allow for more than just experimental flag (e.g. specifying a variants.yaml).

But an experimental config option definitely solves this particular issue :)

[baszalmstra]

You can already pass variant files through pixi! See https://pixi.sh/latest/reference/pixi_manifest/#build-variants-files-optional

[jackm97]

How'd I miss that! That's amazing, thanks for the quick responses.