Marks a depends_on reference as optional.

A normal dependency on a group or step that this build didn't activate dangles — Buildkite rejects the upload because the key doesn't exist. That is the right default: a missing required dependency should fail loudly.

Some dependencies are legitimately conditional, though. In a change-scoped pipeline a group only renders when its files change, so a trigger that wants to wait for it when it runs can't require it. Wrapping the reference in optional/1 (see Pipette.Constructors) tells Pipette.run/2 to drop it when its target is defined in the pipeline but not active in this build:

import Pipette.Constructors, only: [optional: 1]

trigger :deploy_downstream do
  pipeline "production-deploy"
  depends_on [:api, optional(:web)]
end

An optional reference to a target that exists nowhere (a typo or a stale rename) is still kept and still fails the upload — only a defined-but-inactive target is dropped.