Fixes #5880. (#7229)

changelog.md

@@ -50,6 +50,7 @@
 - For string inputs, ``strutils.isUpperAscii`` and ``strutils.isLowerAscii`` now
   require a second mandatory parameter ``skipNonAlpha``.
 
+- ``osLastError`` is now marked with ``sideEffect``
 - The procs ``parseHexInt`` and ``parseOctInt`` now fail on empty strings
     and strings containing only valid prefixes, e.g. "0x" for hex integers.
 

lib/pure/includes/oserr.nim

@@ -66,7 +66,7 @@ proc raiseOSError*(errorCode: OSErrorCode; additionalInfo = "") {.noinline.} =
   raise e
 
 {.push stackTrace:off.}
-proc osLastError*(): OSErrorCode =
+proc osLastError*(): OSErrorCode {.sideEffect.} =
   ## Retrieves the last operating system error code.
   ##
   ## This procedure is useful in the event when an OS call fails. In that case