allow echoing internal stacktrace if compiling with `d:debug`

`internalError` and `yAssert` now allow to echo the internal stack
traces if the `d:debug` compile flag is set.

README.md

@@ -44,6 +44,8 @@ nim build # build a library
 NimYAML needs at least Nim 0.17.0 in order to compile. Version 0.9.1
 is the last release to support 0.15.x and 0.16.0.
 
+When debugging crashes in this library, use the `d:debug` compile flag to enable printing of the internal stack traces for calls to `internalError` and `yAssert`.
+
 ## License
 
 [MIT][2]

yaml/private/internal.nim

@@ -5,16 +5,24 @@
 #    distribution, for details about the copyright.
 
 template internalError*(s: string) =
+  # Note: to get the internal stacktrace that caused the error
+  # compile with the `d:debug` flag.
   when not defined(release):
     let ii = instantiationInfo()
     echo "[NimYAML] Error in file ", ii.filename, " at line ", ii.line, ":"
     echo s
     when not defined(JS):
       echo "[NimYAML] Stacktrace:"
-      try: writeStackTrace()
+      try:
+        writeStackTrace()
+        when defined(debug):
+          echo "Internal stacktrace:"
+          let exc = getCurrentException()
+          echo getStackTrace(exc.parent)
       except: discard
     echo "[NimYAML] Please report this bug."
     quit 1
+
 template yAssert*(e: typed) =
   when not defined(release):
     if not e:
@@ -23,7 +31,12 @@ template yAssert*(e: typed) =
       echo "assertion failed!"
       when not defined(JS):
         echo "[NimYAML] Stacktrace:"
-        try: writeStackTrace()
+        try:
+          writeStackTrace()
+          when defined(debug):
+            echo "Internal stacktrace:"
+            let exc = getCurrentException()
+            echo getStackTrace(exc.parent)
         except: discard
       echo "[NimYAML] Please report this bug."
       quit 1