Error instead of StaleSymbol crash for certain cyclic macro dependencies (#19549)

If a suspended macro refers back to a symbol in a previously compiled
package object, report this as an error instead of crashing with a
StaleSymbol exception.

There's unfortunately not a lot of info available at the point where the
error is raised, so the error message is a bit vague. But it's better
than crashing.

Fixes #19351

Author: odersky

Diff for: compiler/src/dotty/tools/dotc/Driver.scala

@@ -54,8 +54,7 @@ class Driver {
       if (ctx.settings.XprintSuspension.value)
         report.echo(i"compiling suspended $suspendedUnits%, %")
       val run1 = compiler.newRun
-      for unit <- suspendedUnits do unit.suspended = false
-      run1.compileUnits(suspendedUnits)
+      run1.compileSuspendedUnits(suspendedUnits)
       finish(compiler, run1)(using MacroClassLoader.init(ctx.fresh))
 
   protected def initCtx: Context = (new ContextBase).initialCtx

Diff for: compiler/src/dotty/tools/dotc/Run.scala

@@ -358,6 +358,17 @@ class Run(comp: Compiler, ictx: Context) extends ImplicitRunInfo with Constraint
     compiling = false
   }
 
+  private var myCompilingSuspended: Boolean = false
+
+  /** Is this run started via a compilingSuspended? */
+  def isCompilingSuspended: Boolean = myCompilingSuspended
+
+  /** Compile units `us` which were suspended in a previous run */
+  def compileSuspendedUnits(us: List[CompilationUnit]): Unit =
+    myCompilingSuspended = true
+    for unit <- us do unit.suspended = false
+    compileUnits(us)
+
   /** Enter top-level definitions of classes and objects contained in source file `file`.
    *  The newly added symbols replace any previously entered symbols.
    *  If `typeCheck = true`, also run typer on the compilation unit, and set

Diff for: compiler/src/dotty/tools/dotc/core/Denotations.scala

@@ -954,7 +954,10 @@ object Denotations {
     }
 
     def staleSymbolError(using Context): Nothing =
-      throw new StaleSymbol(staleSymbolMsg)
+      if symbol.isPackageObject && ctx.run != null && ctx.run.nn.isCompilingSuspended then
+        throw TypeError(em"Cyclic macro dependency; macro refers to a toplevel symbol in ${symbol.source} from which the macro is called")
+      else
+        throw new StaleSymbol(staleSymbolMsg)
 
     def staleSymbolMsg(using Context): String = {
       def ownerMsg = this match {

Diff for: tests/neg/i19351.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i19351/A.scala:3:35 --------------------------------------------------------------------------------
+3 |  inline def myMacro(): x.type = ${myMacroExpr}  // error
+  |                                   ^
+  |Cyclic macro dependency; macro refers to a toplevel symbol in tests/neg/i19351/A.scala from which the macro is called

Diff for: tests/neg/i19351/A.scala

@@ -0,0 +1,5 @@
+//object A:
+  val x: Int = 1
+  inline def myMacro(): x.type = ${myMacroExpr}  // error
+  def test = myMacro()
+

Diff for: tests/neg/i19351/B.scala

@@ -0,0 +1,3 @@
+import scala.quoted.*
+//import A.*
+def myMacroExpr(using Quotes): Expr[x.type] = '{???}

Diff for: tests/neg/i19351a.check

@@ -0,0 +1,12 @@
+-- Error: tests/neg/i19351a/Test.scala:8:34 ----------------------------------------------------------------------------
+8 |inline def not(b: Bool): Bool = ${notMacro('b)} // error // error
+  |                                  ^
+  |Cyclic macro dependency; macro refers to a toplevel symbol in tests/neg/i19351a/Test.scala from which the macro is called
+-- [E046] Cyclic Error: tests/neg/i19351a/Test.scala:8:46 --------------------------------------------------------------
+8 |inline def not(b: Bool): Bool = ${notMacro('b)} // error // error
+  |                                              ^
+  |                                              Cyclic reference involving method $anonfun
+  |
+  |                                               Run with -explain-cyclic for more details.
+  |
+  | longer explanation available when compiling with `-explain`

Diff for: tests/neg/i19351a/Macro.scala

@@ -0,0 +1,13 @@
+// Macro class
+package test
+
+import scala.quoted.*
+
+def notMacro(bool: Expr[Bool])(using Quotes): Expr[Bool] =
+  '{$bool(False, True)}
+
+def showMacro(bool: Expr[Bool])(using Quotes): Expr[String] =
+  '{$bool("TRUE", "FALSE")}
+
+def foldMacro[T: Type](bool: Expr[Bool], t: Expr[T], f: Expr[T])(using Quotes): Expr[T] =
+  '{$bool($t, $f)}

Diff for: tests/neg/i19351a/Test.scala

@@ -0,0 +1,15 @@
+//Test class
+package test
+
+type Bool = [R] => (R, R) => R
+val True: Bool = [R] => (t: R, _: R) => t
+val False: Bool = [R] => (_: R, f: R) => f
+
+inline def not(b: Bool): Bool = ${notMacro('b)} // error // error
+inline def show(b: Bool): String = ${showMacro('b)}
+//inline def not(b: Bool): Bool = ${foldMacro('b, 'False, 'True)}
+//inline def show(b: Bool): String = ${foldMacro('b, '{"TRUE"}, '{"FALSE"})}
+
+
+@main def testing =
+  println(show(not(True)))