fix(ups): formalize subjects

[MasterPtato]

Description

Please include a summary of the changes and the related issue. Please also include relevant motivation and context.

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[MasterPtato]

This change is part of the following stack:

• chore: deploy patch #4324
  • chore: flatten runtime config #4326
    • fix(api): make actor destroy wait for ack #4327
      • fix(udb): add txn tagging #4328
        • chore: bump rivetkit protocol version #4329
          • fix(ups): formalize subjects #4334 ◀
            • fix(api): properly dedup name list responses #4337
              • fix(pb): clear actor reschedule ts on alloc #4338

_{Change managed by git-spice.}

[railway-app]

🚅 Deployed to the rivet-pr-4334 environment in rivet-frontend

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commit: f076166

[claude]

PR Review: fix(ups): formalize subjects

Overall this is a clean, well-scoped refactor. The Subject trait is a natural improvement over passing raw &strs everywhere, and the MsgSubject struct is a nice way to defer string formatting while keeping type safety.

---

Issues

1. root() is unused dead code

subject.rs:5

The comment says "Used for metrics" but nothing calls it anywhere in the codebase. As an associated function (not &self), it also cannot be used with trait objects (dyn Subject). Either wire it up to the metrics code or remove it until it is actually needed.

2. subject_cow is computed unconditionally but only used in the local path

pubsub.rs:165

For MsgSubject on the non-local path, this means two separate String allocations per publish: one from subject_cow (unused in that branch) and one inside publish_with_backoff. The subject_cow computation could be deferred into the if use_local branch, or publish_with_backoff could accept a Cow<str> directly to share it.

3. Missing impl Subject for String

There are implementations for &str and &String but not for owned String. Callers with owned subjects must borrow at the call site (&my_string), which may not be obvious. This matters since M::subject() returns String, not &str. If Subject is intended as a general-purpose API, String should be covered too.

4. subscribe_opt still uses a manual format string

message.rs:201

The publish path uses the new typed MsgSubject, but subscribe still constructs a raw string via format!. Functionally equivalent, but the inconsistency is confusing. If MsgSubject is the canonical way to express this subject, subscribe should use it too (or at minimum share a helper).

5. Unrelated file in PR

.claude/skills/git-spice-merge-my-stack/SKILL.md has nothing to do with UPS subject formalization and should be in a separate commit.

---

Minor Notes

• The error handling improvement in message() -- propagating spawn failures as WorkflowError::PublishMessage instead of silently logging -- is a genuine improvement, though spawn only fails on runtime shutdown so the practical impact is minimal.
• MsgSubject::root() calls M::subject() which allocates a String each time. If this is ever wired to metrics on a hot path, the allocation will add up.
• Copy could be derived for MsgSubject since it contains only &str and PhantomData, avoiding the need for explicit Clone in the publish loop.