std::string add nullptr attribute

[Segher Boessenkool]

On Mon, Feb 20, 2023 at 12:18:36PM +0100, Marc Glisse via Gcc-help wrote:
> On Mon, 20 Feb 2023, Gabriel Ravier via Gcc-help wrote:
> 
> >This is the kind of thing that makes me wonder why there isn't some kind 
> >of `__builtin_unreachable_do_not_optimize()` builtin that allows one to 
> >mark places in code that should never be reached and should thus be warned 
> >about if such a thing happens while at the same time never doing any 
> >optimization on the basis of the presence of the call.
> 
> -fsanitize=unreachable -fsanitize=null and others prevent the kind of 
> optimization you are worried about.

Or even just __builtin_trap(), or abort(), or similar.  Just a printf()
thing if you really want to just warn.

"Never doing any optimisation" based on <anything> is of course not a
reasonable expectation; but you *can* ask for reachable code not to be
optimised away.  This is the default, just don't mark reachable code as
unreachable :-)


Segher