WIP: define rust/cargo-build pipeline

[imjasonh]

This demonstrates a rust/cargo-build pipeline that shares come very common workflows we have for Rust builds in the codebase.

To make it easier to review and iterate, the pipeline is defined in Wolfi, but it should probably go in Melange itself like go/build etc. If this looks good we can move it to Melange, wait for a digest bump, and remove it from pipelines/.

[joshrwolf]

I am not a rust expert, but should we split this into cargo/fetch and cargo/build (with --offline or --locked) as the default?

[kaniini]

Yes, I would like to split it, so that we can fetch in a prepare phase, and build in a later phase. This allows us to make the build process more hermetic in the future.

[kaniini]

I wonder if we should have a pipeline node which defines a value. Something like:

pipeline:
  - set:
      target: ${{targets.destdir}}
  - if: ${{inputs.subpackage}} == 'true'
    set:
      target: ${{targets.subpkgdir}}
  - runs: |
       mkdir -p ${{vars.target}}
       mv target/release/${{inputs.target}} ${{vars.target}}

But I also wonder if it would be helpful to have a context-specific ${{targets.foo}} which evaluates to the correct location based on package vs. subpackage. WDYT?

[kaniini]

I went ahead and implemented this in chainguard-dev/melange#622.