Goal

Migrate scala-commons from Scala 2.13 to Scala 3 only.

Strategy: big bang + iterative restoration

Two-stage approach:

1. Phase 1 — single "comment-and-green" PR. Cut over the build to Scala 3 only. Anything that doesn't compile is
   wrapped in /* ... */ and tagged // TODO[scala3-port]: <feature>. CI ends green. Codebase is
   intentionally feature-poor; this PR is the cut-over, not a feature-complete port.

2. Phase 2+ — one PR per feature area. Each subsequent PR un-comments one block of TODO-tagged code, ships a Scala 3
   implementation, restores affected tests, and removes the entry from MIGRATION.md backlog.
   PRs stack on top of each other (base = prior phase's branch).

Why this shape

Previous attempt (closed PRs #856, #859; merged-but-not-on-scala-3: #857, #858) used a cross-build cherry-pick model —
scala/ + scala-2.13/ + scala-3/ source dirs and crossScalaVersions := Seq(scala3Version, scala2Version). The cross-axis was carrying ~70-80 files of dead 2.13 weight that the v2 release
won't ship anyway. Pivoting to Scala 3 only removes the maintenance cost and clarifies the
diff: every PR after Phase 1 is "implement feature X on Scala 3", not "keep both axes building".

Locked decisions

• Target branch: AVSystem/scala-commons:scala-3 (unchanged baseline 1561d8dc).
• Source layout: single source dir per module (src/main/scala, src/test/scala).
• Cherry-picking: halotukozak's fork master is reference only — its scala-3/ files are read for inspiration during restoration
  PRs.
• PR conventions:
  • title prefix [Scala 3]
  • assigned to this milestone
  • opened as draft (promoted to ready-for-review manually)
  • stacked chain — Phase 2 base = Phase 1 branch, Phase 3 base = Phase 2 branch, …
  • maintainer merges manually; CI must be green
• Code quality (carried from prior plans):
  • no @nowarn / -Wconf introduced — fix at source
  • no porting of @deprecated symbols when a stdlib replacement exists
• made dependency: added
• CI matrix single Scala 3 axis × Temurin 17 / 21 / 25 = 3 shards.
• MIGRATION.md: fresh start at repo root. Living backlog — entries added when commented, removed when
  restored.

Documentation deliverables

Every PR in this milestone updates MIGRATION.md so the doc captures the full Scala-3 transition contract for
downstream users:

• Will not migrate — symbols, modules, or features explicitly dropped. Listed with rationale (no longer needed,
  replaced by stdlib, replaced by external lib, obsolete API).
• Deprecated on Scala 3 — what we keep behind @deprecated with a pointer to the replacement. Includes since
  version and replaceWith where it makes sense.
• Source-compat breaks — every place a downstream import / call site needs to change to compile under Scala 3.
  Per-module list. Includes old → new pairings where the rename/move is non-obvious.
• Binary-compat breaks — separate section, picked up by MiMa once a Scala 3 baseline ships.
• Disabled tests / modules — anything intentionally not running, with a reason and a "restore in PR #N" pointer if
  known.

This doc is the public contract. Reviewers should be able to read MIGRATION.md and understand exactly what changes for
a consumer of the library.

Out of scope (this milestone)

• Keeping any Scala 2.13 artifact published. Last 2.13 release is whatever sits on master;
• Tasty-MiMa enforcement. Activated only once an initial Scala 3 release ships.
• jetty, analyzer, spring, RPC: Phase 1 disables; formal retirement decision deferred to a follow-up PR.