ARC related bugfixes and refactorings (#12781)

lib/system.nim

@@ -3218,6 +3218,9 @@ proc `<`*[T: tuple](x, y: T): bool =
 const
   usesDestructors = defined(gcDestructors) or defined(gcHooks)
 
+when not usesDestructors:
+  {.pragma: nodestroy.}
+
 when not defined(nimscript) and hasAlloc:
   type
     GC_Strategy* = enum  ## The strategy the GC should use for the application.
@@ -3671,8 +3674,6 @@ when not defined(JS): #and not defined(nimscript):
         prev: PSafePoint # points to next safe point ON THE STACK
         status: int
         context: C_JmpBuf
-        hasRaiseAction: bool
-        raiseAction: proc (e: ref Exception): bool {.closure.}
       SafePoint = TSafePoint
 
   when declared(initAllocator):
@@ -3776,11 +3777,15 @@ when not defined(JS): #and not defined(nimscript):
       ## Retrieves the current exception; if there is none, `nil` is returned.
       result = currException
 
+    proc nimBorrowCurrentException(): ref Exception {.compilerRtl, inl, benign, nodestroy.} =
+      # .nodestroy here so that we do not produce a write barrier as the
+      # C codegen only uses it in a borrowed way:
+      result = currException
+
     proc getCurrentExceptionMsg*(): string {.inline, benign.} =
       ## Retrieves the error message that was attached to the current
       ## exception; if there is none, `""` is returned.
-      var e = getCurrentException()
-      return if e == nil: "" else: e.msg
+      return if currException == nil: "" else: currException.msg
 
     proc setCurrentException*(exc: ref Exception) {.inline, benign.} =
       ## Sets the current exception.