`in` on function parameters: const, scope const, ref scope const ?

[Mathias LANG]

There's been a discussion recently about `-preview` switches 
(https://forum.dlang.org/thread/r627m8$2p6c$1@digitalmars.com).

It turns out one of the preview switch that is to be introduced 
next release is `-preview=in`, meant to make `in` on parameters 
(not to be confused with opIn) means `scope const` instead of 
just `const`.
Now I wasn't particularly fond of this PR, since it doesn't seem 
logical to me to have 2 switches for the same thing (`in` was 
changed to mean `const` when DIP1000 was a PR, aiming to reduce 
the breakage, but before it was behind a switch, which completely 
nullified the need for changing it in the first place).

But since leadership decided to go with the switch anyway, why 
not go all the way and make it means what it was actually 
supposed to mean in the first place ? My experience with D is 
that I had to stick quite a few `const ref scope` (in varying 
order) on my parameters to mean "this is an input parameter. I'm 
going to read it, not modify it. And I will not keep a reference 
to it, because I just need it for the processing of this 
function". However I couldn't use `in` because it would pass 
things by value, meaning my `struct`s would get blitted over and 
over.

I have submitted a PR for it: 
https://github.com/dlang/dmd/pull/11000 and would like to know 
how other users of `in` expect it to behave, or would want it to 
behave.
Bear in mind this is a `-preview` switch: it won't break your 
code.
One of the downside I'm aware of is passing rvalue. But luckily, 
we got another preview for this (`-preview=rvaluerefparam`).