Add documentation to debug missing position crash

compiler/src/dotty/tools/dotc/ast/Positioned.scala

@@ -10,6 +10,9 @@ import core.Flags.{JavaDefined, Extension}
 import core.StdNames.nme
 import annotation.constructorOnly
 import annotation.internal.sharable
+import reporting.Reporter
+
+import java.io.{ PrintWriter }
 
 /** A base class for things that have positions (currently: modifiers and trees)
  */
@@ -24,13 +27,11 @@ abstract class Positioned(implicit @constructorOnly src: SourceFile) extends Pro
   def uniqueId: Int = myUniqueId
 
   def uniqueId_=(id: Int): Unit = {
-    if (Positioned.debugId == id) {
-      def printTrace() = {
-        val stack = Thread.currentThread().getStackTrace().map(">   " + _)
-        System.err.println(stack.mkString(s"> Debug tree (id=${Positioned.debugId}) creation \n> $this\n", "\n", "\n"))
-      }
-      printTrace()
+    def printTrace() = {
+      println(s"Debug tree (id=${Positioned.debugId}) creation \n$this\n")
+      Reporter.displayPrompt(Console.in, new PrintWriter(Console.err, true))
     }
+    if (Positioned.debugId == id) printTrace()
     myUniqueId = id
   }
 

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

@@ -25,7 +25,7 @@ class ConsoleReporter(
     val didPrint = m match {
       case m: Error =>
         printMessage(messageAndPos(m.contained(), m.pos, diagnosticLevel(m)))
-        if (ctx.settings.Xprompt.value) displayPrompt()
+        if (ctx.settings.Xprompt.value) Reporter.displayPrompt(reader, writer)
         true
       case m: ConditionalWarning if !m.enablingOption.value =>
         false
@@ -40,28 +40,5 @@ class ConsoleReporter(
       printMessage("\nlonger explanation available when compiling with `-explain`")
   }
 
-  /** Show prompt if `-Xprompt` is passed as a flag to the compiler */
-  def displayPrompt(): Unit = {
-    writer.println()
-    writer.print("a)bort, s)tack, r)esume: ")
-    writer.flush()
-    if (reader != null) {
-      def loop(): Unit = reader.read match {
-        case 'a' | 'A' =>
-          new Throwable().printStackTrace(writer)
-          System.exit(1)
-        case 's' | 'S' =>
-          new Throwable().printStackTrace(writer)
-          writer.println()
-          writer.flush()
-        case 'r' | 'R' =>
-          ()
-        case _ =>
-          loop()
-      }
-      loop()
-    }
-  }
-
   override def flush()(implicit ctx: Context): Unit = { writer.flush() }
 }

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

@@ -8,14 +8,17 @@ import core.Contexts._
 import util.{SourcePosition, NoSourcePosition}
 import core.Decorators.PhaseListDecorator
 import collection.mutable
-import java.lang.System.currentTimeMillis
 import core.Mode
 import dotty.tools.dotc.core.Symbols.{Symbol, NoSymbol}
 import diagnostic.messages._
 import diagnostic._
 import ast.{tpd, Trees}
 import Message._
 
+import java.lang.System.currentTimeMillis
+import java.io.{ BufferedReader, PrintWriter }
+
+
 object Reporter {
   /** Convert a SimpleReporter into a real Reporter */
   def fromSimpleReporter(simple: interfaces.SimpleReporter): Reporter =
@@ -37,6 +40,29 @@ object Reporter {
 
   private val defaultIncompleteHandler: ErrorHandler =
     (mc, ctx) => ctx.reporter.report(mc)(ctx)
+
+  /** Show prompt if `-Xprompt` is passed as a flag to the compiler */
+  def displayPrompt(reader: BufferedReader, writer: PrintWriter): Unit = {
+    writer.println()
+    writer.print("a)bort, s)tack, r)esume: ")
+    writer.flush()
+    if (reader != null) {
+      def loop(): Unit = reader.read match {
+        case 'a' | 'A' =>
+          new Throwable().printStackTrace(writer)
+          System.exit(1)
+        case 's' | 'S' =>
+          new Throwable().printStackTrace(writer)
+          writer.println()
+          writer.flush()
+        case 'r' | 'R' =>
+          ()
+        case _ =>
+          loop()
+      }
+      loop()
+    }
+  }
 }
 
 trait Reporting { this: Context =>

docs/docs/internals/debug-macros.md

@@ -0,0 +1,32 @@
+---
+layout: doc-page
+title: "Debug Macros"
+---
+
+Complex macros may break invariants of the compiler, which leads to compiler crashes.
+Here we lists common compiler crashes and how to deal with them.
+
+## position not set
+
+For this problem, here is the log that is usually shown:
+
+```
+[error] assertion failed: position not set for org.scalactic.anyvals.PosZInt.+$extension3(SizeParam.this.minSize)(
+[error]   org.scalactic.anyvals.PosZInt.widenToInt(SizeParam.this.sizeRange)
+[error] ) # 2326942 of class dotty.tools.dotc.ast.Trees$Apply in library/src-bootstrapped/scala/tasty/reflect/utils/TreeUtils.scala
+```
+
+To debug why the position is not set, note the tree id `2326942`, and enable
+the following compiler option:
+
+```
+-Ydebug-tree-with-id 2326942
+```
+
+With the option above, the compiler will crash when the tree is created. From
+the stack trace, we will be able to figure out where the tree is created.
+
+If the position is in the compiler, then either report a compiler bug or
+fix the problem with `.withSpan(tree.span)`. The following fix is an example:
+
+- https://github.com/lampepfl/dotty/pull/6581

docs/sidebar.yml

@@ -183,6 +183,8 @@ sidebar:
           url: docs/internals/type-system.html
         - title: "Dotty Internals 1: Trees & Symbols (Meeting Notes)"
           url: docs/internals/dotty-internals-1-notes.html
+        - title: Debug Macros
+          url: docs/internals/debug-macros.html
     - title: Resources
       subsection:
         - title: Talks