Transitions to new language semantics

[WebFreak001]

On Friday, 11 June 2021 at 11:27:03 UTC, Ola Fosheim Grøstad 
wrote:
> On Friday, 11 June 2021 at 07:36:47 UTC, Sönke Ludwig wrote:
>> This is something that should have been discussed already, but 
>> I can't remember whether that was actually the case, and it 
>> always bothers me every time there is friction with new DIP 
>> switches.
>>
>> Right now, new language semantics are introduced using 
>> `-preview` and `-revert` switches, which unfortunately has a 
>> massive drawback:
>
> I am troubled in general by the implementation of incomplete 
> solutions and making them gradually available.
>
> I would find it much more reassuring if a comprehensive 
> solution was developed as a completely separate compiler 
> branch. Basically have a stable branch (as is), and then a 
> future branch that is considered unstable until all the corner 
> cases have been ironed out. This also allows more heavy 
> restructuring of compiler internals, like introducing an 
> appropriate IR (which is needed for things like borrowing or 
> ARC, if you want something solid).
>
> The cost of moving to a more complete solution after something 
> incomplete has been made official could break the camel's back.
>
> The piece-by-piece approach is a slippery slope.

I think this is a good point.

Having an unstable compiler would allow removing things that 
don't work so well again, while having the advantage of having a 
big user base trying it out (and complaining about bugs) - though 
this could also be seen as inconvenience for users and authors 
e.g. when libraries are only working with an unstable branch or 
change in behavior with the unstable compiler.