Drop incremental compilation state for Mill modules when version chan…

…ges (#3884)

Mill used various tricks to map build scripts written by users to
generated enhanced versions, including package objects and other stuff.
Since some of these techniques may result in Zinc undercompilation, we
want avoid downstream issues,
especally in the `methodCodeHashSignatures` task, which is sensitive to
inconsistent bytecode.

We trait potentially longer compilation against incorrect or too coarse
code change detection, which may invalidate a lot more downstream tasks
than needed due to invalid data.

Fix #3874

Pull request: #3884

runner/src/mill/runner/MillBuildRootModule.scala

@@ -7,7 +7,7 @@ import mill.define.{Discover, Task}
 import mill.scalalib.{BoundDep, Dep, DepSyntax, Lib, ScalaModule}
 import mill.util.CoursierSupport
 import mill.util.Util.millProjectModule
-import mill.scalalib.api.Versions
+import mill.scalalib.api.{CompilationResult, Versions}
 import mill.main.client.OutFiles._
 import mill.main.client.CodeGenConstants.buildFileExtensions
 import mill.main.{BuildInfo, RootModule}
@@ -261,6 +261,49 @@ abstract class MillBuildRootModule()(implicit
 
   /** Used in BSP IntelliJ, which can only work with directories */
   def dummySources: Sources = Task.Sources(T.dest)
+
+  def millVersion = T.input { BuildInfo.millVersion }
+
+  override def compile: T[CompilationResult] = Task(persistent = true) {
+    val mv = millVersion()
+
+    val prevMillVersionFile = T.dest / s"mill-version"
+    val prevMillVersion = Option(prevMillVersionFile)
+      .filter(os.exists)
+      .map(os.read(_).trim)
+      .getOrElse("?")
+
+    if (prevMillVersion != mv) {
+      // Mill version changed, drop all previous incremental state
+      // see https://github.com/com-lihaoyi/mill/issues/3874
+      T.log.debug(
+        s"Detected Mill version change ${prevMillVersion} -> ${mv}. Dropping previous incremental compilation state"
+      )
+      os.remove.all(T.dest)
+      os.makeDir(T.dest)
+      os.write(prevMillVersionFile, mv)
+    }
+
+    // copied from `ScalaModule`
+    zincWorker()
+      .worker()
+      .compileMixed(
+        upstreamCompileOutput = upstreamCompileOutput(),
+        sources = Agg.from(allSourceFiles().map(_.path)),
+        compileClasspath = compileClasspath().map(_.path),
+        javacOptions = javacOptions() ++ mandatoryJavacOptions(),
+        scalaVersion = scalaVersion(),
+        scalaOrganization = scalaOrganization(),
+        scalacOptions = allScalacOptions(),
+        compilerClasspath = scalaCompilerClasspath(),
+        scalacPluginClasspath = scalacPluginClasspath(),
+        reporter = T.reporter.apply(hashCode),
+        reportCachedProblems = zincReportCachedProblems(),
+        incrementalCompilation = zincIncrementalCompilation(),
+        auxiliaryClassFileExtensions = zincAuxiliaryClassFileExtensions()
+      )
+  }
+
 }
 
 object MillBuildRootModule {