Update staging.Compiler.make documentation (scala#19428)

Fixes scala#19211
This addresses part of scala#19170, and scala#19176

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2432,9 +2432,14 @@ class UnqualifiedCallToAnyRefMethod(stat: untpd.Tree, method: Symbol)(using Cont
   def kind = MessageKind.PotentialIssue
   def msg(using Context) = i"Suspicious top-level unqualified call to ${hl(method.name.toString)}"
   def explain(using Context) =
+    val getClassExtraHint =
+      if method.name == nme.getClass_ && ctx.settings.classpath.value.contains("scala3-staging") then
+        i"""\n\n
+           |This class should not be used to get the classloader for `scala.quoted.staging.Compile.make`."""
+      else ""
     i"""Top-level unqualified calls to ${hl("AnyRef")} or ${hl("Any")} methods such as ${hl(method.name.toString)} are
        |resolved to calls on ${hl("Predef")} or on imported methods. This might not be what
-       |you intended."""
+       |you intended.$getClassExtraHint"""
 }
 
 class SynchronizedCallOnBoxedClass(stat: tpd.Tree)(using Context)

docs/_spec/TODOreference/metaprogramming/staging.md

@@ -108,7 +108,11 @@ to get a source-like representation of the expression.
 import scala.quoted.*
 
 // make available the necessary compiler for runtime code generation
-given staging.Compiler = staging.Compiler.make(getClass.getClassLoader)
+given staging.Compiler =
+  // We need an instance of a class that is defined in the current application (not the standard library)
+  // `this` can be used instead of an instance of `Dummy` if the Compiler is instantiated within one of the application classes.
+  object Dummy
+  staging.Compiler.make(Dummy.getClass.getClassLoader)
 
 val f: Array[Int] => Int = staging.run {
   val stagedSum: Expr[Array[Int] => Int] =

staging/src/scala/quoted/staging/Compiler.scala

@@ -13,15 +13,27 @@ object Compiler:
 
   /** Create a new instance of the compiler using the the classloader of the application.
    *
-   * Usage:
-   * ```
-   * import scala.quoted.staging._
-   * given Compiler = Compiler.make(getClass.getClassLoader)
-   * ```
+   *  Usage:
+   *  ```
+   *  import scala.quoted.staging._
+   *  given Compiler =
+   *    object Dummy
+   *    Compiler.make(Dummy.getClass.getClassLoader)
+   *  ```
    *
-   * @param appClassloader classloader of the application that generated the quotes
-   * @param settings compiler settings
-   * @return A new instance of the compiler
+   *  Note that we use an instance of `Dummy` to get the classloader that loaded the application.
+   *  Any other instance of a class defined in the application would also work.
+   *  Using a class defined in the standard library should be avoided as it might be loaded by a different classloader.
+   *
+   *  If the given compiler is defined in one of your classes (e.i. not as a top-level definition), then
+   *  the compiler can be instantiated with:
+   *  ```
+   *  given Compiler = Compiler.make(this.getClass.getClassLoader)
+   *  ```
+   *
+   *  @param appClassloader classloader of the application that generated the quotes
+   *  @param settings compiler settings
+   *  @return A new instance of the compiler
    */
   def make(appClassloader: ClassLoader)(implicit settings: Settings): Compiler =
     new Compiler:

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

@@ -53,7 +53,19 @@ private class QuoteDriver(appClassloader: ClassLoader) extends Driver:
         val method = clazz.getMethod("apply")
         val inst = clazz.getConstructor().newInstance()
 
-        method.invoke(inst).asInstanceOf[T]
+        try method.invoke(inst).asInstanceOf[T]
+        catch case ex: java.lang.reflect.InvocationTargetException =>
+          ex.getCause match
+            case ex: java.lang.NoClassDefFoundError =>
+              throw new Exception(
+                s"""`scala.quoted.staging.run` failed to load a class.
+                   |The classloader used for the `staging.Compiler` instance might not be the correct one.
+                   |Make sure that this classloader is the one that loaded the missing class.
+                   |Note that the classloader that loads the standard library might not be the same as
+                   |the one that loaded the application classes.""".stripMargin,
+                ex)
+
+            case _ => throw ex
     end match
 
   end run

tests/run-staging/i11162.scala

@@ -2,7 +2,7 @@ import scala.quoted.*
 
 object Test {
 
-  given staging.Compiler = staging.Compiler.make(getClass.getClassLoader)
+  given staging.Compiler = staging.Compiler.make(this.getClass.getClassLoader)
 
   def main(args: Array[String]): Unit =
     staging.run {

tests/run-staging/i19170.scala

@@ -0,0 +1,14 @@
+import scala.quoted.*
+
+given staging.Compiler =
+  object Dummy
+  staging.Compiler.make(Dummy.getClass.getClassLoader)
+
+class A(i: Int)
+
+def f(i: Expr[Int])(using Quotes): Expr[A] = { '{ new A($i) } }
+
+@main def Test = {
+  val g: Int => A = staging.run { '{ (i: Int) => ${ f('{i}) } } }
+  println(g(3))
+}

tests/run-staging/i19170b.scala

@@ -0,0 +1,16 @@
+import scala.quoted.*
+
+given staging.Compiler =
+  staging.Compiler.make(getClass.getClassLoader) // warn: Suspicious top-level unqualified call to getClass
+
+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("`scala.quoted.staging.run` failed to load a class."))
+}

tests/run-staging/i19176.scala

@@ -0,0 +1,10 @@
+import scala.quoted.*
+
+given staging.Compiler =
+  object Dummy
+  staging.Compiler.make(Dummy.getClass.getClassLoader)
+
+class A
+val f: (A, Int) => Int = staging.run { '{ (q: A, x: Int) => x } }
+
+@main def Test = f(new A, 3)