There is no specification right now. Just reference implementation. The rustc team is working on it, but it will take a long time and it is not a priority.
There is a language reference, but it is far from the completeness you would need from specification.

BTW, I think it is funny.

On Tue, 9 Apr 2024, at 02:11, pinskia at gcc dot gnu.org wrote:
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114629
> 
> --- Comment #5 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
> (In reply to Pierre-Emmanuel Patry from comment #2)
> > I can see the problem with the message, but I don't think we can remove the
> > behavior as we have to match rustc's behavior. The compiler has to ICE when
> > meeting this identifier. I suggest changing the message to rustc's one "It
> > looks like you're trying to break rust; would you like some ICE?" and
> > removing the "gcc" identifier check.
> 
> Really this sounds like a bad idea to copy 100% of what rustc does. Instead we
> should follow the rust language specification (hopefully that actually exists).
> 
> While you are at it, it would be useful to add a link to the rust langauge
> specification (like there is for almost all other languages [I see objective-C
> is not listed]) to https://gcc.gnu.org/readings.html .
> 
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