[sbt-dotty] use sbt loader as parent of scala instance loader

Author: adpi2

project/Build.scala

@@ -337,7 +337,8 @@ object Build {
         scalaLibrary,
         dottyLibrary,
         dottyCompiler,
-        allJars
+        allJars,
+        appConfiguration.value
       )
     },
     // sbt-dotty defines `scalaInstance in doc` so we need to override it manually

sbt-bridge/src/xsbt/CompilerClassLoader.java

@@ -20,16 +20,9 @@
 
 public final class CompilerInterface {
   public CachedCompiler newCompiler(String[] options, Output output, Logger initialLog, Reporter initialDelegate) {
-    // The classloader that sbt uses to load the compiler bridge is broken
-    // (see CompilerClassLoader#fixBridgeLoader for details). To workaround
-    // this we construct our own ClassLoader and then run the following code
-    // with it:
-    //   new CachedCompilerImpl(options, output)
-
     try {
       ClassLoader bridgeLoader = this.getClass().getClassLoader();
-      ClassLoader fixedLoader = CompilerClassLoader.fixBridgeLoader(bridgeLoader);
-      Class<?> cciClass = fixedLoader.loadClass("xsbt.CachedCompilerImpl");
+      Class<?> cciClass = bridgeLoader.loadClass("xsbt.CachedCompilerImpl");
       return (CachedCompiler) cciClass.getConstructors()[0].newInstance(options, output);
     } catch (ClassNotFoundException | InstantiationException | IllegalAccessException | InvocationTargetException e) {
       throw new RuntimeException(e);

sbt-bridge/src/xsbt/CompilerInterface.java

@@ -8,11 +8,16 @@ import sbt.librarymanagement.{
   VersionNumber
 }
 import sbt.internal.inc.ScalaInstance
+import sbt.internal.inc.classpath.ClassLoaderCache
 import xsbti.compile._
+import xsbti.AppConfiguration
 import java.net.URLClassLoader
 import java.util.Optional
+import java.util.{Enumeration, Collections}
+import java.net.URL
 import scala.util.Properties.isJavaAtLeast
 
+
 object DottyPlugin extends AutoPlugin {
   object autoImport {
     val isDotty = settingKey[Boolean]("Is this project compiled with Dotty?")
@@ -521,15 +526,34 @@ object DottyPlugin extends AutoPlugin {
       scalaLibraryJar,
       dottyLibraryJar,
       compilerJar,
-      allJars
+      allJars,
+      appConfiguration.value
     )
   }
 
   // Adapted from private mkScalaInstance in sbt
   def makeScalaInstance(
-    state: State, dottyVersion: String, scalaLibrary: File, dottyLibrary: File, compiler: File, all: Seq[File]
+    state: State, dottyVersion: String, scalaLibrary: File, dottyLibrary: File, compiler: File, all: Seq[File], appConfiguration: AppConfiguration
   ): ScalaInstance = {
-    val libraryLoader = state.classLoaderCache(List(dottyLibrary, scalaLibrary))
+    /**
+      * The compiler bridge must load the xsbti classes from the sbt
+      * classloader, and similarly the Scala repl must load the sbt provided
+      * jline terminal. To do so we add the `appConfiguration` loader in
+      * the parent hierarchy of the scala 3 instance loader.
+      *
+      * The [[TopClassLoader]] ensures that the xsbti and jline classes
+      * only are loaded from the sbt loader. That is necessary because
+      * the sbt class loader contains the Scala 2.12 library and compiler
+      * bridge.
+      */
+    val topLoader = new TopClassLoader(appConfiguration.provider.loader)
+
+    val libraryJars = Array(dottyLibrary, scalaLibrary)
+    val libraryLoader = state.classLoaderCache.cachedCustomClassloader(
+      libraryJars.toList,
+      () => new URLClassLoader(libraryJars.map(_.toURI.toURL), topLoader)
+    )
+
     class DottyLoader
         extends URLClassLoader(all.map(_.toURI.toURL).toArray, libraryLoader)
     val fullLoader = state.classLoaderCache.cachedCustomClassloader(
@@ -540,10 +564,25 @@ object DottyPlugin extends AutoPlugin {
       dottyVersion,
       fullLoader,
       libraryLoader,
-      Array(dottyLibrary, scalaLibrary),
+      libraryJars,
       compiler,
       all.toArray,
       None)
+  }
+}
+
+private class TopClassLoader(sbtLoader: ClassLoader) extends ClassLoader(null) {
+  private val sharedPrefixes = List(
+    "xsbti.",
+    "org.jline."
+  )
 
+  override protected def loadClass(name: String, resolve: Boolean): Class[_] = {
+    if (sharedPrefixes.exists(name.startsWith(_))) {
+      val c = sbtLoader.loadClass(name)
+      if (resolve) resolveClass(c)
+      c
+    }
+    else super.loadClass(name, resolve)
   }
 }