[Issue 23352] New: `in` and `out` foreach variables

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23352

          Issue ID: 23352
           Summary: `in` and `out` foreach variables
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P1
         Component: dmd
          Assignee: nobody at puremagic.com
          Reporter: qs.il.paperinik at gmail.com

In Herb Sutter’s proposal for a brand new C++ syntax¹, I found this gem:
> “In this proposal, for(in auto x : rng) automatically
> is both efficient and guaranteed to be read-only.”

This is something D could do, too. Our `foreach` loops can be
foreach (         element; range) // copy
foreach (     ref element; range) // reference²
foreach (auto ref element; range) // copy/reference, depending on value
category
foreach (     in  element; range) // see next paragraph
foreach (     out element; range) // see next paragraph

`in` and `out` elements have the same binding as `in` and `out` function
parameters. They bind to
    opApply(int delegate(in Type));
and
    opApply(int delegate(out Type));
respectively.

¹ https://raw.githubusercontent.com/hsutter/708/main/708.pdf
² That `ref` is effectively `auto ref` is a known bug.

--