[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 #5 from Steven Schveighoffer <schveiguy at gmail.com> ---
One further update, in Feb 2021, which is still there to this day, notes that
the `in` storage class without the preview=in switch means `const`:

https://github.com/dlang/dlang.org/commit/8f05cb00c99c90d88550972199c7941e2ea8ca21

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