immutable string literal?

[strtr]

Steven Schveighoffer Wrote:

> On Mon, 22 Feb 2010 08:32:20 -0500, strtr <strtr at spam.com> wrote:
> 
> > Daniel Keep Wrote:
> >
> >>
> >> strtr wrote:
> >> > On winXP (D1) I can compile/run this code without a problem.
> >> > Not even a warning.
> >> >
> >> > void main() {
> >> >   char[] s= "immutable literal?";
> >> >   s[$-1] = '!';
> >> >   writefln(s);
> >> > }
> >> > Codepad runs into a segmentation fault.
> >> > http://codepad.org/NQfsRoR5
> >> >
> >> > Why doesn't it result in a compiler error or warning?
> >> > If it did I would have noticed this quirk earlier.
> >>
> >> There's no compiler error because D1 doesn't have a const/immutable  
> >> system.
> >>
> >> There's no crash because Windows doesn't write-protect the data segment
> >> which contains the literal.
> >
> > But according to the specs, it does constitute an error and I suspect  
> > string literals are placed in a specific memory location.
> > Wouldn't it be possible to error on such code?
> 
> Yes, it is possible -- use D2 which has such errors :)  Without a const  
> system, D1 cannot distinguish between char[] that originated from a  
> literal (i.e. in the static data segment) or which originated from the  
> heap.  At the point where you see the line:
> 
> s[$-1] = '!';
> 
> You don't have the entire history of where s came from.  It can be even  
> harder to detect than you think.  For instance, should you allow the  
> following code to compile?
> 
> int main(char[][] args)
> {
>     char[] s;
>     if(args[1] == "y")
>        s = "immutable literal?";
>     else
>        s = "mutable literal?".dup;
>     s[$-1] = "!";
>     return 0;
> }
> 
> This program runs fine unless you pass the exact argument 'y' to the  
> program, and then it crashes.  How does the compiler know at the line  
> where s is modified that it could have possibly come from a literal?
> 
> If you still think it's possible, what about this?
> 
> void exclaim(char[] s)
> {
>     s[$-1] = '!';
> }
> 
> If this is in its own module, how can the compiler tell whether s is  
> immutable or not?
> 
> -Steve

Thanks, I understand. 
But, how about a runtime error?
Isn't a literal placed in easy to identify should-only-read memory?