feat: add support for the Mill build tool

This PR adds in support for the Mill build tool by utilizing
https://github.com/ckipp01/mill-scip. It's a pretty thin wrapper. You
can find more details about mill-scip in the repo but the general
workflow is:

- It's an external Mill plugin that gathers everything needed to do a
  compile via Mill, but then adds some extra plugins/scalacOptions to
  ensure semanticDB gets produced
- Once produced it uses `scip-java` as a library to actually produce the
  scip file.

The plugin isn't actually a dependency here which ensures we don't have
any cyclical dependency on anything, but rather just utilizes the
`--import` functionality of Mill and external plugins. I'll leave a
couple more comments on various parts of the PR.

_NOTE_ This only currently supports Scala and Scala 3 projects. There
are some issues with Java projects which will need some upstream work.
You can track that in com-lihaoyi/mill#1983.

Refs #306

Author: ckipp01

build.sbt

@@ -22,6 +22,8 @@ lazy val V =
     def semanticdbKotlinc = "0.2.0"
     def testcontainers = "0.39.3"
     def requests = "0.6.5"
+    def minimalMillVersion = "0.10.3"
+    def millScipVersion = "0.2.2"
   }
 
 inThisBuild(
@@ -166,7 +168,9 @@ lazy val cli = project
         "scala213" -> V.scala213,
         "scala3" -> V.scala3,
         "bloopVersion" -> V.bloop,
-        "bspVersion" -> V.bsp
+        "bspVersion" -> V.bsp,
+        "minimalMillVersion" -> V.minimalMillVersion,
+        "millScipVersion" -> V.millScipVersion
       ),
     buildInfoPackage := "com.sourcegraph.scip_java",
     libraryDependencies ++=

docs/getting-started.md

@@ -243,7 +243,7 @@ free to subscribe to the tracking issues to receive updates on your build tool.
 | Ant        | ❌   | ❌    | ❌     | [sourcegraph/scip-java#305](https://github.com/sourcegraph/scip-java/issues/305) |
 | Bazel      | ✅   | ✅    | ❌     |                                                                                  |
 | Buck       | ❌   | ❌    | ❌     | [sourcegraph/scip-java#99](https://github.com/sourcegraph/scip-java/issues/99)   |
-| Mill       | ❌   | ❌    | ❌     | [sourcegraph/scip-java#306](https://github.com/sourcegraph/scip-java/issues/306) |
+| Mill       | ❌   | ✅    | ❌     | 
 
 **✅**: automatic indexing is fully supported. Please report a bug if the
 `scip-java index` command does not work on your codebase.
@@ -292,13 +292,23 @@ The following Maven integrations are not yet supported:
 
 ### sbt
 
-The `scip-java index` build should be able to automatically index most Maven
+The `scip-java index` build should be able to automatically index most sbt
 projects, with the following caveats:
 
 | Integration   | Supported | Recommendation          |
 | ------------- | --------- | ----------------------- |
 | sbt <v0.13.17 | ❌        | Upgrade to sbt v0.13.17 |
 
+### Mill
+
+The `scip-java index` build should be able to automatically index most Mill 
+projects, with the following caveats:
+
+| Integration   | Supported | Recommendation             |
+| ------------- | --------- | -------------------------- |
+| Mill <v0.10.3 | ❌        | Upgrade to Mill >= v0.10.3 |
+
+
 ### Bazel
 
 Bazel is supported by scip-java but it requires custom configuration to work

scip-java/src/main/scala/com/sourcegraph/scip_java/buildtools/BuildTool.scala

@@ -24,7 +24,8 @@ object BuildTool {
       new GradleBuildTool(index),
       new MavenBuildTool(index),
       new ScipBuildTool(index),
-      new SbtBuildTool(index)
+      new SbtBuildTool(index),
+      new MillBuildTool(index)
     )
   def allNames: String =
     all(IndexCommand()).filterNot(_.isHidden).map(_.name).mkString(", ")

scip-java/src/main/scala/com/sourcegraph/scip_java/buildtools/MillBuildTool.scala

@@ -0,0 +1,103 @@
+package com.sourcegraph.scip_java.buildtools
+
+import java.nio.file.Files
+import java.nio.file.StandardCopyOption
+
+import scala.jdk.CollectionConverters._
+
+import com.sourcegraph.scip_java.commands.IndexCommand
+import com.sourcegraph.scip_java.BuildInfo
+
+class MillBuildTool(index: IndexCommand) extends BuildTool("mill", index) {
+
+  override def usedInCurrentDirectory(): Boolean =
+    Files.isRegularFile(index.workingDirectory.resolve("build.sc"))
+
+  override def generateScip(): Int =
+    millVersion() match {
+      case Some(version) if isSupportedMillVersion(version) =>
+        unconditionallyGenerateScip()
+      case Some(version) =>
+        failFast(
+          s"Unsupported Mill version '${version}'. " +
+            s"To fix this problem, upgrade Mill to at least ${minimalMillVersion} and try again."
+        )
+      case None =>
+        failFast(
+          s"No Mill version detected. " +
+            s"To fix this problem, run the following command and try again: " +
+            s"echo '${minimalMillVersion}' >> .mill-version"
+        )
+    }
+
+  private def failFast(message: String): Int = {
+    index.app.error(message)
+    1
+  }
+
+  private def unconditionallyGenerateScip(): Int = {
+    val localMill = Files.isRegularFile(millFile)
+    val command =
+      if (localMill) {
+        "./mill"
+      } else {
+        "mill"
+      }
+    val millProcess = index.process(
+      List(
+        command,
+        "--import",
+        s"ivy:io.chris-kipp::mill-scip::${BuildInfo.millScipVersion}",
+        "io.kipp.mill.scip.Scip/generate"
+      )
+    )
+    val scipFile = index
+      .workingDirectory
+      .resolve("out")
+      .resolve("io")
+      .resolve("kipp")
+      .resolve("mill")
+      .resolve("scip")
+      .resolve("Scip")
+      .resolve("generate.dest")
+      .resolve("index.scip")
+
+    if (millProcess.exitCode == 0 && Files.isRegularFile(scipFile)) {
+      val output = index.workingDirectory.resolve("index.scip")
+      Files.copy(scipFile, output, StandardCopyOption.REPLACE_EXISTING)
+      index.app.info(output.toString)
+    }
+    millProcess.exitCode
+  }
+
+  private lazy val minimalMillVersion = BuildInfo.minimalMillVersion
+
+  private lazy val millFile = index.workingDirectory.resolve("mill")
+
+  private def isSupportedMillVersion(version: String): Boolean =
+    if (version.startsWith("0.1"))
+      true
+    else
+      false
+
+  /**
+   * Try to grab the Mill version from the .mill-version file. If not found we
+   * fall back to the mill file which could be the official mill launcher or the
+   * millw launcher renamed as mill, both which will have a DEFAULT_MILL_VERSION
+   * line.
+   */
+  private def millVersion(): Option[String] = {
+    val millVersionFile = index.workingDirectory.resolve(".mill-version")
+    if (Files.isRegularFile(millVersionFile)) {
+      Files.readAllLines(millVersionFile).asScala.headOption
+    } else if (Files.isRegularFile(millFile)) {
+      Files
+        .readAllLines(millFile)
+        .asScala
+        .find(_.startsWith("DEFAULT_MILL_VERSION"))
+        .map(line => line.dropWhile(!_.isDigit))
+    } else {
+      None
+    }
+  }
+}

tests/buildTools/src/test/scala/tests/MillBuildToolSuite.scala

@@ -0,0 +1,32 @@
+package tests
+
+class MillBuildToolSuite extends BaseBuildToolSuite {
+  checkBuild(
+    s"minimal",
+    s"""|/.mill-version
+        |0.10.5
+        |/build.sc
+        |import mill._, scalalib._
+        |object minimal extends ScalaModule {
+        |  def scalaVersion = "2.13.8"
+        |  object test extends Tests with TestModule.Munit {
+        |    def ivyDeps = Agg(ivy"org.scalameta::munit:1.0.0-M6")
+        | }
+        |}
+        |/minimal/src/Main.scala
+        |package minimal
+        |object Main extends App
+        |/minimal/test/src/MainSuite.scala
+        |package minimal
+        |class MainSpec extends munit.FunSuite {
+        |  test("numbers") {
+        |    assertEquals(1, 1)
+        |  }
+        |}
+        |""".stripMargin,
+    expectedSemanticdbFiles = 2,
+    expectedPackages =
+      """|maven:munit:munit:1.0.0-M6
+         |""".stripMargin
+  )
+}