add compiler/debugutils.nim to help debugging compiler (#17652)

* add compiler/debugutils.nim

* address comments

compiler/debugutils.nim

@@ -0,0 +1,19 @@
+##[
+Utilities to help with debugging nim compiler.
+
+Experimental API, subject to change.
+]##
+
+import options
+
+var conf0: ConfigRef
+
+proc onNewConfigRef*(conf: ConfigRef) {.inline.} =
+  ## Caches `conf`, which can be retrieved with `getConfigRef`.
+  ## This avoids having to forward `conf` all the way down the call chain to
+  ## procs that need it during a debugging session.
+  conf0 = conf
+
+proc getConfigRef*(): ConfigRef =
+  ## nil, if -d:nimDebugUtils wasn't specified
+  result = conf0

compiler/options.nim

@@ -446,6 +446,10 @@ proc newProfileData(): ProfileData =
 const foreignPackageNotesDefault* = {
   hintProcessing, warnUnknownMagic, hintQuitCalled, hintExecuting, hintUser, warnUser}
 
+proc isDefined*(conf: ConfigRef; symbol: string): bool
+
+import debugutils
+
 proc newConfigRef*(): ConfigRef =
   result = ConfigRef(
     selectedGC: gcRefc,
@@ -504,16 +508,21 @@ proc newConfigRef*(): ConfigRef =
   # enable colors by default on terminals
   if terminal.isatty(stderr):
     incl(result.globalOptions, optUseColors)
+  when defined(nimDebugUtils):
+    onNewConfigRef(result)
 
 proc newPartialConfigRef*(): ConfigRef =
   ## create a new ConfigRef that is only good enough for error reporting.
-  result = ConfigRef(
-    selectedGC: gcRefc,
-    verbosity: 1,
-    options: DefaultOptions,
-    globalOptions: DefaultGlobalOptions,
-    foreignPackageNotes: foreignPackageNotesDefault,
-    notes: NotesVerbosity[1], mainPackageNotes: NotesVerbosity[1])
+  when defined(nimDebugUtils):
+    result = getConfigRef()
+  else:
+    result = ConfigRef(
+      selectedGC: gcRefc,
+      verbosity: 1,
+      options: DefaultOptions,
+      globalOptions: DefaultGlobalOptions,
+      foreignPackageNotes: foreignPackageNotesDefault,
+      notes: NotesVerbosity[1], mainPackageNotes: NotesVerbosity[1])
 
 proc cppDefine*(c: ConfigRef; define: string) =
   c.cppDefines.incl define