Add setting to restrict macro classpath

For better cacheability

src/compiler/scala/tools/nsc/settings/ScalaSettings.scala

@@ -265,6 +265,8 @@ trait ScalaSettings extends AbsScalaSettings
   val YcachePluginClassLoader  = CachePolicy.setting("plugin", "compiler plugins")
   val YcacheMacroClassLoader   = CachePolicy.setting("macro", "macros")
 
+  val YmacroClasspath = PathSetting       ("-Ymacro-classpath", "The classpath used to reflectively load macro implementations", "")
+
   val exposeEmptyPackage = BooleanSetting ("-Yexpose-empty-package", "Internal only: expose the empty package.").internalOnly()
   val Ydelambdafy        = ChoiceSetting  ("-Ydelambdafy", "strategy", "Strategy used for translating lambdas into JVM code.", List("inline", "method"), "method")
 

src/compiler/scala/tools/nsc/typechecker/Macros.scala

@@ -14,6 +14,8 @@ package scala.tools.nsc
 package typechecker
 
 import java.lang.Math.min
+import java.net.URL
+
 import symtab.Flags._
 import scala.reflect.internal.util.ScalaClassLoader
 import scala.reflect.runtime.ReflectionUtils
@@ -22,6 +24,7 @@ import scala.reflect.internal.TypesStats
 import scala.reflect.macros.util._
 import scala.util.control.ControlThrowable
 import scala.reflect.internal.util.ListOfNil
+import scala.reflect.io.AbstractFile
 import scala.reflect.macros.runtime.{AbortMacroException, MacroRuntimes}
 import scala.reflect.macros.compiler.DefaultMacroCompiler
 import scala.tools.reflect.FastTrack
@@ -75,7 +78,12 @@ trait Macros extends MacroRuntimes with Traces with Helpers {
    *  Mirrors with runtime definitions (e.g. Repl) need to adjust this method.
    */
   protected def findMacroClassLoader(): ClassLoader = {
-    val classpath = global.classPath.asURLs
+    val classpath: Seq[URL] = if (settings.YmacroClasspath.isSetByUser) {
+      for {
+        file <- scala.tools.nsc.util.ClassPath.expandPath(settings.YmacroClasspath.value, true)
+        af <- Option(AbstractFile getDirectory file)
+      } yield af.file.toURI.toURL
+    } else global.classPath.asURLs
     def newLoader = () => {
       macroLogVerbose("macro classloader: initializing from -cp: %s".format(classpath))
       ScalaClassLoader.fromURLs(classpath, self.getClass.getClassLoader)