[Issue 23175] -preview=in silently adds possible stack memory escape

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23175

--- Comment #9 from Mathias LANG <pro.mathias.lang at gmail.com> ---
> I do not think we need to change the semantics of preview=in, or to disable optimizations based on it. What we need is a warning to tell the user that what he wrote before, while valid with the previous compiler, has memory corruption implications now that the semantics have changed.

You should have led with this. That's what `-transition` is for. It lists all
instances of `in` usage so you can inspect them.

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