Provide a context ClassLoader to macros

Install the macro ClassLoader as the context ClassLoader during macro
interpretation. This ClassLoader is used, for example, by Typesafe
Config for loading application.conf.

Currently macros see the default context ClassLoader which contains the
compiler's class path, not the compilation class path.

Author: szeiger

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

@@ -44,13 +44,19 @@ object Splicer {
       val macroOwner = newSymbol(ctx.owner, nme.MACROkw, Macro | Synthetic, defn.AnyType, coord = tree.span)
       try
         inContext(ctx.withOwner(macroOwner)) {
-          val interpreter = new Interpreter(pos, classLoader)
-
-          // Some parts of the macro are evaluated during the unpickling performed in quotedExprToTree
-          val interpretedExpr = interpreter.interpret[scala.quoted.QuoteContext => scala.quoted.Expr[Any]](tree)
-          val interpretedTree = interpretedExpr.fold(tree)(macroClosure => PickledQuotes.quotedExprToTree(macroClosure(QuoteContext())))
-
-          checkEscapedVariables(interpretedTree, macroOwner)
+          val oldContextClassLoader = Thread.currentThread().getContextClassLoader
+          Thread.currentThread().setContextClassLoader(classLoader)
+          try {
+            val interpreter = new Interpreter(pos, classLoader)
+
+            // Some parts of the macro are evaluated during the unpickling performed in quotedExprToTree
+            val interpretedExpr = interpreter.interpret[scala.quoted.QuoteContext => scala.quoted.Expr[Any]](tree)
+            val interpretedTree = interpretedExpr.fold(tree)(macroClosure => PickledQuotes.quotedExprToTree(macroClosure(QuoteContext())))
+
+            checkEscapedVariables(interpretedTree, macroOwner)
+          } finally {
+            Thread.currentThread().setContextClassLoader(oldContextClassLoader)
+          }
         }.changeOwner(macroOwner, ctx.owner)
       catch {
         case ex: CompilationUnit.SuspendException =>

tests/pos-macros/macro-classloaders/Caller.scala

@@ -0,0 +1,3 @@
+object Caller {
+  Macro.f
+}

tests/pos-macros/macro-classloaders/Macro.scala

@@ -0,0 +1,15 @@
+import java.net.URLClassLoader
+
+import scala.quoted._
+
+object Macro { self =>
+  inline def f: Any = ${ impl }
+
+  def impl(using QuoteContext): Expr[Any] = {
+    //println("======== "+self.getClass.getClassLoader.asInstanceOf[URLClassLoader].getURLs.mkString("; "))
+    //println("  ====== "+Thread.currentThread().getContextClassLoader.asInstanceOf[URLClassLoader].getURLs.mkString("; "))
+    assert(getClass.getClassLoader eq Thread.currentThread().getContextClassLoader,
+      "Macro ClassLoader should be available as context ClassLoader")
+    '{""}
+  }
+}