From 5faf78af96887420575c372daa795b8ba27e9820 Mon Sep 17 00:00:00 2001
From: Ethan Atkins <ethan.atkins@gmail.com>
Date: Thu, 30 May 2019 16:42:02 -0700
Subject: [PATCH] Add scala reflect layer

Not caching scala reflect is extremely painful if the build uses
scalatest. It adds O(1second) to my watch performance benchmarks. It
actually made sbt 1.3.0 much slower than 0.13.17
---
 .../main/scala/sbt/internal/ClassLoaders.scala  | 17 ++++++++++++-----
 1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/main/src/main/scala/sbt/internal/ClassLoaders.scala b/main/src/main/scala/sbt/internal/ClassLoaders.scala
index 3b4c607fc..773ec4813 100644
--- a/main/src/main/scala/sbt/internal/ClassLoaders.scala
+++ b/main/src/main/scala/sbt/internal/ClassLoaders.scala
@@ -105,7 +105,7 @@ private[sbt] object ClassLoaders {
   }
   /*
    * Create a layered classloader. There are up to four layers:
-   * 1) the scala instance class loader
+   * 1) the scala instance class loader (may actually be two layers if scala-reflect is used)
    * 2) the resource layer
    * 3) the dependency jars
    * 4) the rest of the classpath
@@ -135,11 +135,18 @@ private[sbt] object ClassLoaders {
         val scalaLibraryLayer = layer(si.libraryJars, interfaceLoader, cache, resources, tmp)
         val cpFiles = fullCP.map(_._1)
 
+        val scalaReflectJar = allDependencies.find(_.getName == "scala-reflect.jar")
+        val scalaReflectLayer = scalaReflectJar
+          .map { file =>
+            layer(file :: Nil, scalaLibraryLayer, cache, resources, tmp)
+          }
+          .getOrElse(scalaLibraryLayer)
+
         // layer 2 (resources)
         val resourceLayer =
           if (layerDependencies)
-            getResourceLayer(cpFiles, resourceCP, scalaLibraryLayer, cache, resources)
-          else scalaLibraryLayer
+            getResourceLayer(cpFiles, resourceCP, scalaReflectLayer, cache, resources)
+          else scalaReflectLayer
 
         // layer 3 (optional if in the test config and the runtime layer is not shared)
         val dependencyLayer =
@@ -148,8 +155,8 @@ private[sbt] object ClassLoaders {
 
         // layer 4
         val filteredSet =
-          if (layerDependencies) allDependencies.toSet ++ si.libraryJars
-          else Set(si.libraryJars: _*)
+          if (layerDependencies) allDependencies.toSet ++ si.libraryJars ++ scalaReflectJar
+          else Set(si.libraryJars ++ scalaReflectJar: _*)
         val dynamicClasspath = cpFiles.filterNot(filteredSet)
         new LayeredClassLoader(dynamicClasspath, dependencyLayer, resources, tmp)
     }