-preview=in might break code

[kinke]

On Sunday, 4 October 2020 at 23:40:29 UTC, Ola Fosheim Grøstad 
wrote:
> It might help some if compilers would run unit tests 3 times 
> with different 'in' implementations.
>
> 1 mixed value/ref
> 2 value
> 3 ref

I was about to propose something like this (restricted to POD 
types), possibly augmented by some indeterministic fuzzing. Code 
ported to the new `in` semantics could then even show some 
previously unintended aliasing issues; getting to the root of the 
problem would probably still be non-trivial though.

A compiler mode enforcing by-ref, coupled with some sort of 
runtime sanitizer detecting invalid writes to live in-params, 
would probably be a very valuable tool for validation & 
troubleshooting.