[BSP] Report compilation's completion percentage

bsp/worker/src/mill/bsp/worker/BspCompileProblemReporter.scala

@@ -154,6 +154,18 @@ private class BspCompileProblemReporter(
     client.onBuildTaskStart(taskStartParams)
   }
 
+  override def notifyProgress(percentage: Long, total: Long): Unit = {
+    val params = new TaskProgressParams(taskId).tap { it =>
+      it.setEventTime(System.currentTimeMillis())
+      it.setData(new CompileTask(targetId))
+      it.setDataKind("compile-progress")
+      it.setMessage(s"Compiling target ${targetDisplayName} ($percentage%)")
+      it.setProgress(percentage)
+      it.setTotal(total)
+    }
+    client.onBuildTaskProgress(params)
+  }
+
   override def finish(): Unit = {
     val taskFinishParams =
       new TaskFinishParams(taskId, if (errors > 0) StatusCode.ERROR else StatusCode.OK).tap { it =>

main/api/src/mill/api/CompileProblemReporter.scala

@@ -14,6 +14,9 @@ trait CompileProblemReporter {
   def fileVisited(file: os.Path): Unit
   def printSummary(): Unit
   def finish(): Unit
+  // TODO: cleanup once we break bin-compat in Mill 0.13
+  // Remove default implementation: `= ()`
+  def notifyProgress(percentage: Long, total: Long): Unit = ()
 }
 
 /**

scalalib/worker/src/mill/scalalib/worker/ZincWorkerImpl.scala

@@ -34,6 +34,7 @@ import xsbti.compile.{
   PreviousResult
 }
 import xsbti.{PathBasedFile, VirtualFile}
+import xsbti.compile.CompileProgress
 
 import java.io.File
 import java.net.URLClassLoader
@@ -589,6 +590,20 @@ class ZincWorkerImpl(
     val incOptions = IncOptions.of().withAuxiliaryClassFiles(
       auxiliaryClassFileExtensions.map(new AuxiliaryClassFileExtension(_)).toArray
     )
+    val compileProgress = reporter.map { reporter =>
+      new CompileProgress {
+        override def advance(
+            current: Int,
+            total: Int,
+            prevPhase: String,
+            nextPhase: String
+        ): Boolean = {
+          val percentage = current * 100 / total
+          reporter.notifyProgress(percentage = percentage, total = total)
+          true
+        }
+      }
+    }
 
     val inputs = ic.inputs(
       classpath = classpath,
@@ -608,7 +623,7 @@ class ZincWorkerImpl(
         cache = new FreshCompilerCache,
         incOptions = incOptions,
         reporter = newReporter,
-        progress = None,
+        progress = compileProgress,
         earlyAnalysisStore = None,
         extra = Array()
       ),