Add error hint on local inline def used in quotes (#16572)

Closes #16532

Author: nicolasstucki

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

@@ -264,10 +264,16 @@ class PCPCheckAndHeal(@constructorOnly ictx: Context) extends TreeMapWithStages(
       if (!tp.isInstanceOf[ThisType]) sym.show
       else if (sym.is(ModuleClass)) sym.sourceModule.show
       else i"${sym.name}.this"
+    val hint =
+      if sym.is(Inline) && levelOf(sym) < level then
+        "\n\n" +
+        "Hint: Staged references to inline definition in quotes are only inlined after the quote is spliced into level 0 code by a macro. " +
+        "Try moving this inline definition in a statically accessible location such as an object (this definition can be private)."
+      else ""
     report.error(
       em"""access to $symStr from wrong staging level:
           | - the definition is at level ${levelOf(sym)},
-          | - but the access is at level $level.""", pos)
+          | - but the access is at level $level.$hint""", pos)
     tp
   }
 

tests/neg-macros/i16532.check

@@ -0,0 +1,8 @@
+-- Error: tests/neg-macros/i16532.scala:7:13 ---------------------------------------------------------------------------
+7 |    val x2 = recurseII($a, $b) // error
+  |             ^^^^^^^^^
+  |access to method recurseII from wrong staging level:
+  | - the definition is at level 0,
+  | - but the access is at level 1.
+  |
+  |Hint: Staged references to inline definition in quotes are only inlined after the quote is spliced into level 0 code by a macro. Try moving this inline definition in a statically accessible location such as an object (this definition can be private).

tests/neg-macros/i16532.scala

@@ -0,0 +1,9 @@
+import scala.quoted.*
+
+def power0Impl(a: Expr[Int], b: Expr[Int])(using Quotes): Expr[Int] =
+  inline def recurseII(a:Int, n:Int): Int = ???
+
+  '{
+    val x2 = recurseII($a, $b) // error
+    x2
+   }