Simplify Mill cross-build to only include major version

This simplifies commands from, e.g. `mill chisel[2.13.18].compile` to
just `mill chisel[2.13].compile`. This makes it such that we no longer
have to change CI and documentation whenever we add support for a new
minor version of Scala.

Author: jackkoenig

.github/workflows/ci-circt-nightly.yml

@@ -49,7 +49,7 @@ jobs:
       matrix:
         system: ["ubuntu-24.04"]
         jvm: [21]
-        scala: ["2.13.18"]
+        scala: ["2.13"]
         espresso: ["2.4"]
         slang: ["7.0"]
         circt: ["nightly"]

.github/workflows/ci.yml

@@ -19,7 +19,7 @@ jobs:
       matrix:
         system: ["ubuntu-24.04"]
         jvm: [21]
-        scala: ["2.13.18"]
+        scala: ["2.13"]
         espresso: ["2.4"]
         slang: ["7.0"]
     uses: ./.github/workflows/test.yml

.github/workflows/test.yml

@@ -15,7 +15,7 @@ on:
         type: number
       scala:
         description: 'The Scala version to use'
-        default: '2.13.18'
+        default: '2.13'
         required: true
         type: string
       espresso:
@@ -265,7 +265,7 @@ jobs:
     runs-on: ubuntu-24.04
     strategy:
       matrix:
-        scala: ["2.13.18"]
+        scala: ["2.13"]
     steps:
       - name: Checkout
         uses: actions/checkout@v5

CONTRIBUTING.md

@@ -81,7 +81,7 @@ Most testing can be done on just the Chisel build unit:
 ```
 
 The `[]` exists because we are cross-compiling between Scala 2.13 and Scala 3.
-You can pick a specific version, e.g. `./mill chisel[2.13.18]`.
+You can pick a specific version, e.g. `./mill chisel[2.13]`.
 Using `[]` will pick the first version in the list of supported versions which one can think about as the "default" version.
 
 You can test everything with:

benchmark/package.mill

@@ -8,8 +8,8 @@ import mill.contrib.jmh.JmhModule
 import build._
 
 object `package` extends ScalaModule with JmhModule with ScalafmtModule {
-  def scalaVersion = v.scalaVersion
+  def scalaVersion = v.defaultScalaVersion
   def jmhCoreVersion = v.jmhVersion
 
-  override def moduleDeps = Seq(chisel(v.scalaVersion))
+  override def moduleDeps = Seq(chisel(v.defaultScalaVersion))
 }

build.mill

@@ -14,6 +14,12 @@ import mill.scalalib.scalafmt._
 
 object v extends Module {
 
+  // Add support for later Scala 2.13 by bumping this.
+  val scala213MinorVersion: Int = 18
+
+  // Bump Scala 3 version by bumping this.
+  val scala3MinorVersion: String = "3.4"
+
   val javaVersion = {
     val rawVersion = sys.props("java.specification.version")
     // Older versions of Java started with 1., e.g. 1.8 == 8
@@ -32,18 +38,19 @@ object v extends Module {
   // Only publish plugin for 2.13.11+ when using Java > 11, but still
   // publish all versions when Java version <= 11.
   val pluginScalaCrossVersions = {
-    val latest213 = 18
     val java21Min213 = 11
     val minVersion = if (javaVersion > 11) java21Min213 else 0
-    val versions = minVersion to latest213
+    val versions = minVersion to scala213MinorVersion
     val versionSeq = versions.map(v => s"2.13.$v").toSeq
-    versionSeq ++ Seq("3.3.4")
+    versionSeq :+ scalaCrossToVersion("3")
   }
 
-  val scalaCrossVersions = Seq(
-    "2.13.18",
-    "3.3.4"
-  )
+  val scalaCrossVersions = Seq("2.13", "3")
+
+  def scalaCrossToVersion(major: String): String = major match {
+    case "2.13" => s"2.13.$scala213MinorVersion"
+    case "3"    => s"3.$scala3MinorVersion"
+  }
 
   def isScala3(ver: String): Boolean = ver.startsWith("3.")
 
@@ -65,7 +72,10 @@ object v extends Module {
     }
   }
 
-  val scalaVersion = scalaCrossVersions.head
+  def defaultCrossVersion = scalaCrossVersions.head
+
+  def defaultScalaVersion = scalaCrossToVersion(defaultCrossVersion)
+
   val jmhVersion = "1.37"
   val osLib = mvn"com.lihaoyi::os-lib:0.10.7" // 0.11 requires Java 11
   val upickle = mvn"com.lihaoyi::upickle:3.3.1" // upickle 4.0 requires Scala 3.4 (we target Scala 3.3 LTS)
@@ -271,6 +281,8 @@ object circt extends Module {
 object chisel extends Cross[Chisel](v.scalaCrossVersions)
 
 trait Chisel extends CrossSbtModule with HasScala2MacroAnno with HasScalaPlugin with ScalafmtModule {
+  def scalaVersion = v.scalaCrossToVersion(crossScalaVersion)
+
   override def moduleDir = super.moduleDir / os.up
   def svsimModule = svsim.cross(crossScalaVersion)
   def coreModule = core.cross(crossScalaVersion)

core/package.mill

@@ -13,7 +13,7 @@ object `package` extends Module {
 }
 
 trait Core extends CrossSbtModule with HasScala2MacroAnno with HasCommonOptions with ScalafmtModule {
-  def scalaVersion = crossScalaVersion
+  def scalaVersion = v.scalaCrossToVersion(crossScalaVersion)
 
   def moduleDir = super.moduleDir / os.up
 

docs/package.mill

@@ -13,10 +13,10 @@ import build._
 /** MDoc project */
 object `package` extends SbtModule with HasScalaPlugin with HasCommonOptions {
 
-  override def scalaVersion = v.scalaVersion
+  override def scalaVersion = v.defaultScalaVersion
 
   // This not really a CrossModule but HasScala2Plugin and HasCommonOptions require it
-  override def crossValue = v.scalaVersion
+  override def crossValue = v.scalaCrossVersions.head
 
   def pluginModule = plugin.cross()
 
@@ -25,7 +25,7 @@ object `package` extends SbtModule with HasScalaPlugin with HasCommonOptions {
 
   override def mvnDeps = Task { Seq(v.mdoc, v.scalatest) }
 
-  override def moduleDeps = Seq(chisel(v.scalaVersion))
+  override def moduleDeps = Seq(chisel(crossValue))
 
   // Suppress missing interpolator warnings because mdoc appears to introduce them.
   override def extraWarnConf = Seq("msg=possible missing interpolator:s")

firrtl/package.mill

@@ -18,7 +18,7 @@ trait Firrtl
     with HasScala2MacroAnno
     with HasCommonOptions
     with ScalafmtModule {
-  def scalaVersion = crossScalaVersion
+  def scalaVersion = v.scalaCrossToVersion(crossScalaVersion)
 
   def moduleDir = super.moduleDir / os.up
 

macros/package.mill

@@ -13,6 +13,8 @@ object `package` extends Module {
 }
 
 trait Macros extends CrossSbtModule with HasScala2MacroAnno with HasCommonOptions with ScalafmtModule {
+  def scalaVersion = v.scalaCrossToVersion(crossScalaVersion)
+
   def moduleDir = super.moduleDir / os.up
 
   override def mvnDeps =