-preview=in might break code

[Steven Schveighoffer]

On 10/3/20 10:10 AM, Steven Schveighoffer wrote:
> Another option is to tag an `in` value as "I'm OK if this is passed by 
> reference". like `in ref` or something (that still binds to rvalues). 
> Then an optimization can say "actually, I'm going to pass this by 
> value", and nobody cares. This is different in that you have to 
> *declare* you're ok with a reference, not the other way around.

Thinking about this more, it seems my major problem is that `in` as it 
is without the switch is not a reference.

`in ref` is a reference, and it's OK if we make this not a reference in 
practice, because it's const. And code that takes something via `in ref` 
can already expect possible changes via other references, but should be 
OK if it doesn't change also.

Can we just change -preview=in so it affects `in ref` instead of `in`?

-Steve