Description of the bug:

After tripping over this flag for a bit, I finally was able to root-cause the usability issues:

1. The label passed to --experimental_propagate_custom_flag must be fully qualified. This means both using the canonical repo name, AND expanding any labels that match the name of their parent directory (e.g. //foo:foo).
2. If a nonexistent flag is passed, Bazel doesn't emit any warnings or errors.

The compounding of the two issues makes --experimental_propagate_custom_flag difficult to use since the only way to ensure you've done the right thing is by creating some kind of test that verifies the behavior change of the flag.

Which category does this issue belong to?

Configurability

What's the simplest, easiest way to reproduce this bug? Please provide a minimal example if possible.

https://github.com/armandomontanez/bazel_reproducers/tree/main/propagate_custom_flag_issues

Which operating system are you running Bazel on?

No response

What is the output of bazel info release?

8.0.0

If bazel info release returns development version or (@non-git), tell us how you built Bazel.

No response

What's the output of git remote get-url origin; git rev-parse HEAD ?

If this is a regression, please try to identify the Bazel commit where the bug was introduced with bazelisk --bisect.

No response

Have you found anything relevant by searching the web?

#22457

Any other information, logs, or outputs that you want to share?

No response