Do cross version checks on import qualifiers

 - keep all imports around until erasure
 - traverse import qualifiers to do cross version checks on their constituents
 - fix parts of the build that imported the deprecated package scala.collection.JavaConverters

Fixes #15479

Author: odersky

compiler/src/dotty/tools/backend/jvm/GenBCode.scala

@@ -8,7 +8,7 @@ import dotty.tools.dotc.ast.tpd
 import dotty.tools.dotc.core.Phases.Phase
 
 import scala.collection.mutable
-import scala.collection.JavaConverters._
+import scala.jdk.CollectionConverters._
 import dotty.tools.dotc.transform.SymUtils._
 import dotty.tools.dotc.interfaces
 import dotty.tools.dotc.report
@@ -605,13 +605,13 @@ class GenBCodePipeline(val int: DottyBackendInterface, val primitives: DottyPrim
       // Statistics.stopTimer(BackendStats.bcodeWriteTimer, writeStart)
     catch
       case e: MethodTooLargeException =>
-        val method = 
+        val method =
           s"${e.getClassName.replaceAll("/", ".")}.${e.getMethodName}"
-        val msg = 
-          s"Generated bytecode for method '$method' is too large. Size: ${e.getCodeSize} bytes. Limit is 64KB" 
+        val msg =
+          s"Generated bytecode for method '$method' is too large. Size: ${e.getCodeSize} bytes. Limit is 64KB"
         report.error(msg)
       case e: ClassTooLargeException =>
-        val msg = 
+        val msg =
           s"Class '${e.getClassName.replaceAll("/", ".")}' is too large. Constant pool size: ${e.getConstantPoolCount}. Limit is 64K entries"
         report.error(msg)
 

compiler/src/dotty/tools/dotc/classpath/DirectoryClassPath.scala

@@ -14,7 +14,7 @@ import dotty.tools.io.{AbstractFile, PlainFile, ClassPath, ClassRepresentation,
 import FileUtils._
 import PlainFile.toPlainFile
 
-import scala.collection.JavaConverters._
+import scala.jdk.CollectionConverters._
 import scala.collection.immutable.ArraySeq
 import scala.util.control.NonFatal
 

compiler/src/dotty/tools/dotc/config/CommandLineParser.scala

@@ -4,7 +4,7 @@ import scala.annotation.tailrec
 import scala.collection.mutable.ArrayBuffer
 import java.lang.Character.isWhitespace
 import java.nio.file.{Files, Paths}
-import scala.collection.JavaConverters._
+import scala.jdk.CollectionConverters._
 
 /** A simple enough command line parser.
  */

compiler/src/dotty/tools/dotc/config/WrappedProperties.scala

@@ -22,7 +22,7 @@ trait WrappedProperties extends PropertiesTrait {
   override def envOrNone(name: String): Option[String]       = wrap(super.envOrNone(name)).flatten
 
   def systemProperties: Iterator[(String, String)] = {
-    import scala.collection.JavaConverters._
+    import scala.jdk.CollectionConverters._
     wrap(System.getProperties.asScala.iterator) getOrElse Iterator.empty
   }
 }

compiler/src/dotty/tools/dotc/profile/Profiler.scala

@@ -81,7 +81,7 @@ private [profile] object NoOpProfiler extends Profiler {
   override def finished(): Unit = ()
 }
 private [profile] object RealProfiler {
-  import scala.collection.JavaConverters._
+  import scala.jdk.CollectionConverters._
   val runtimeMx: RuntimeMXBean = ManagementFactory.getRuntimeMXBean
   val memoryMx: MemoryMXBean = ManagementFactory.getMemoryMXBean
   val gcMx: List[GarbageCollectorMXBean] = ManagementFactory.getGarbageCollectorMXBeans.asScala.toList

compiler/src/dotty/tools/dotc/semanticdb/Tools.scala

@@ -2,7 +2,7 @@ package dotty.tools.dotc.semanticdb
 
 import java.nio.file._
 import java.nio.charset.StandardCharsets
-import scala.collection.JavaConverters._
+import scala.jdk.CollectionConverters._
 import dotty.tools.dotc.util.SourceFile
 import dotty.tools.dotc.semanticdb.Scala3.given
 

compiler/src/dotty/tools/dotc/transform/FirstTransform.scala

@@ -138,7 +138,6 @@ class FirstTransform extends MiniPhase with InfoTransformer { thisPhase =>
   }
 
   override def transformOther(tree: Tree)(using Context): Tree = tree match {
-    case tree: Import if untpd.languageImport(tree.expr).isEmpty => EmptyTree
     case tree: Export => EmptyTree
     case tree: NamedArg => transformAllDeep(tree.arg)
     case tree => if (tree.isType) toTypeTree(tree) else tree

compiler/src/dotty/tools/dotc/typer/CrossVersionChecks.scala

@@ -33,7 +33,7 @@ class CrossVersionChecks extends MiniPhase:
     val xMigrationValue = ctx.settings.Xmigration.value
     if xMigrationValue != NoScalaVersion then
       checkMigration(sym, pos, xMigrationValue)
-
+  end checkUndesiredProperties
 
   /** If @deprecated is present, and the point of reference is not enclosed
    * in either a deprecated member or a scala bridge method, issue a warning.
@@ -176,6 +176,15 @@ class CrossVersionChecks extends MiniPhase:
     tree
   }
 
+  override def transformOther(tree: Tree)(using Context): Tree = tree match
+    case tree: Import =>
+      tree.foreachSubTree {
+        case t: RefTree => checkUndesiredProperties(t.symbol, t.srcPos)
+        case _ =>
+      }
+      tree
+    case _ => tree
+
 end CrossVersionChecks
 
 object CrossVersionChecks:

compiler/src/dotty/tools/repl/ReplDriver.scala

@@ -36,7 +36,7 @@ import org.jline.reader._
 
 import scala.annotation.tailrec
 import scala.collection.mutable
-import scala.collection.JavaConverters._
+import scala.jdk.CollectionConverters._
 import scala.util.Using
 
 /** The state of the REPL contains necessary bindings instead of having to have

compiler/test/dotty/tools/TestSources.scala

@@ -5,7 +5,7 @@ import scala.language.unsafeNulls
 import java.io.File
 import java.nio.file._
 
-import scala.collection.JavaConverters._
+import scala.jdk.CollectionConverters._
 
 object TestSources {