`in` parameters made useful

[Kagamin]

On Friday, 31 July 2020 at 21:49:25 UTC, Mathias LANG wrote:
> https://github.com/dlang/dmd/pull/11000

1. Deprecation of `in ref` makes no sense. Why? I assume it's due 
to a bug in the proposed change. How the compiler should know 
that the argument should be passed by ref? I doesn't necessarily 
know how to load the argument, it may have alignment and 
synchronization requirements. And more importantly how the 
programmer can know whether the argument is passed by ref, now 
that it varies by platform?
2. Dependence on calling convention. AIU ref semantics depends on 
parameter position?
3. Runtime hooks don't go through semantic checks. Is this a 
theoretical concern or did you introduce some new behavior that 
causes problem with this?