introduce --laxstrings switch for backwards compatibility

compiler/commands.nim

@@ -499,6 +499,7 @@ proc processSwitch(switch, arg: string, pass: TCmdLinePass, info: TLineInfo;
       undefSymbol("nimOldNewlines")
     else:
       localError(info, errOnOrOffExpectedButXFound, arg)
+  of "laxstrings": processOnOffSwitch({optLaxStrings}, arg, pass, info)
   of "checks", "x": processOnOffSwitch(ChecksOptions, arg, pass, info)
   of "floatchecks":
     processOnOffSwitch({optNaNCheck, optInfCheck}, arg, pass, info)

compiler/options.nim

@@ -37,7 +37,8 @@ type                          # please make sure we have under 32 options
                               # evaluation
     optPatterns,              # en/disable pattern matching
     optMemTracker,
-    optHotCodeReloading
+    optHotCodeReloading,
+    optLaxStrings
 
   TOptions* = set[TOption]
   TGlobalOption* = enum       # **keep binary compatible**

doc/advopt.txt

@@ -66,6 +66,8 @@ Advanced options:
   --excessiveStackTrace:on|off
                             stack traces use full file paths
   --oldNewlines:on|off      turn on|off the old behaviour of "\n"
+  --laxStrings:on|off       when turned on, accessing the zero terminator in
+                            strings is allowed; only for backwards compatibility
   --skipCfg                 do not read the general configuration file
   --skipUserCfg             do not read the user's configuration file
   --skipParentCfg           do not read the parent dirs' configuration files