Improve incorrect classloader reporting in staging

* do not println staging crashes to stdout
* enrich the errors coming from the compiler with additional
hints about using the correct classloader

compiler/src/dotty/tools/dotc/Run.scala

@@ -661,4 +661,6 @@ object Run {
         report.enrichErrorMessage(errorMessage)
       else
         errorMessage
+    def doNotEnrichErrorMessage: Unit =
+      if run != null then run.myEnrichedErrorMessage = true
 }

staging/src/scala/quoted/staging/QuoteCompiler.scala

@@ -48,7 +48,9 @@ private class QuoteCompiler extends Compiler:
 
   override def newRun(implicit ctx: Context): ExprRun =
     reset()
-    new ExprRun(this, ctx.addMode(Mode.ReadPositions))
+    val run = new ExprRun(this, ctx.addMode(Mode.ReadPositions))
+    run.doNotEnrichErrorMessage
+    run
 
   def outputClassName: TypeName = "Generated$Code$From$Quoted".toTypeName
 

staging/src/scala/quoted/staging/QuoteDriver.scala

@@ -8,6 +8,7 @@ import dotty.tools.dotc.quoted.QuotesCache
 import dotty.tools.io.{AbstractFile, Directory, PlainDirectory, VirtualDirectory}
 import dotty.tools.repl.AbstractFileClassLoader
 import dotty.tools.dotc.reporting._
+import dotty.tools.dotc.config.Settings.Setting.value
 import dotty.tools.dotc.util.ClasspathFromClassloader
 import scala.quoted._
 import scala.quoted.staging.Compiler
@@ -40,7 +41,20 @@ private class QuoteDriver(appClassloader: ClassLoader) extends Driver:
       setCompilerSettings(ctx1.fresh.setSetting(ctx1.settings.outputDir, outDir), settings)
     }
 
-    new QuoteCompiler().newRun(ctx).compileExpr(exprBuilder) match
+    val compiledExpr = 
+      try
+        new QuoteCompiler().newRun(ctx).compileExpr(exprBuilder)
+      catch case ex: dotty.tools.FatalError =>
+        val enrichedMessage =
+          s"""An unhandled exception was thrown in the staging compiler.
+            |This might be caused by using an incorrect classloader
+            |when creating the `staging.Compiler` instance with `staging.Compiler.make`.
+            |For details, please refer to the documentation.
+            |For non-enriched exceptions, compile with -Yno-enrich-error-messages.""".stripMargin
+        if ctx.settings.YnoEnrichErrorMessages.value(using ctx) then throw ex
+        else throw new Exception(enrichedMessage, ex)
+
+    compiledExpr match
       case Right(value) =>
         value.asInstanceOf[T]
 

tests/run-staging/i19170c.check

@@ -0,0 +1 @@
+exception thrown, no additional printlns

tests/run-staging/i19170c.scala

@@ -0,0 +1,16 @@
+import scala.quoted.*
+
+given staging.Compiler =
+  staging.Compiler.make(getClass.getClassLoader.getParent) // different classloader that 19170b.scala
+class A(i: Int)
+
+def f(i: Expr[Int])(using Quotes): Expr[A] = { '{ new A($i) } }
+
+@main def Test = {
+  try
+    val g: Int => A = staging.run { '{ (i: Int) => ${ f('{i}) } } }
+    println(g(3))
+  catch case ex: Exception =>
+    assert(ex.getMessage().startsWith("An unhandled exception was thrown in the staging compiler."), ex.getMessage())
+    println("exception thrown, no additional printlns")
+}

tests/run-staging/i19176b.check

@@ -0,0 +1 @@
+exception thrown, no additional printlns

tests/run-staging/i19176b.scala

@@ -0,0 +1,14 @@
+import scala.quoted.*
+
+given staging.Compiler =
+  staging.Compiler.make(getClass.getClassLoader.getParent) // we want to make sure the classloader is incorrect
+
+class A
+
+@main def Test =
+  try
+    val f: (A, Int) => Int = staging.run { '{ (q: A, x: Int) => x } }
+    f(new A, 3)
+  catch case ex: Exception =>
+    assert(ex.getMessage().startsWith("An unhandled exception was thrown in the staging compiler."), ex.getMessage())
+    println("exception thrown, no additional printlns")