ADR-0006 — Mill 1.1.7 rather than the 0.12.x named in the style guide

• Status: accepted
• Date: 2026-08-01

Context

SCALA_CODE_STYLE.md §Tooling pins Mill at 0.12.x. CLAUDE.md §Toolchain lists the commands the project actually uses:

mill mill.scalalib.scalafmt/
mill mill.scalalib.scalafmt/checkFormatAll
mill __.fix

Both of those forms belong to the Mill 1.x line: the external.module/task slash syntax and the built-in fix (Scalafix) task do not exist in 0.12, where formatting is mill.scalalib.scalafmt.ScalafmtModule/… and Scalafix requires the third-party joan38/mill-scalafix plugin.

The two documents cannot both be satisfied, and the commands are the more load-bearing statement — they are what every contributor and CI job will run.

Decision

Pin Mill 1.1.7 in .mill-version, with the official bootstrap script committed as ./mill so the version is reproducible without a global install.

Consequences

• CLAUDE.md's command table works verbatim.
• Scalafix is built in; no third-party build plugin to track.
• mill-contrib-scoverage:1.1.7 matches the Mill version, so coverage needs no separate version pin.
• Mill 1.x renamed ivyDeps → mvnDeps, ivy"…" → mvn"…" and T[_] → Task. Any Mill snippet found online for 0.11/0.12 needs translating; that is a one-time cost paid in build.mill.
• The style guide's version table is now stale on this row. It is left unedited (it is derived from an upstream document) and this ADR records the divergence.

Note on .scalafmt.conf

project.git = true means Scalafmt only formats files Git already tracks. New files must be git add-ed before mill mill.scalalib.scalafmt/ will touch them, otherwise it fails with NoMatchingFiles. mill modules.__.reformat formats module sources regardless. This surprises everyone once.