[Issue 23573] New: std.bitmanip.bitfields doesn't respect native endianness

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=23573

          Issue ID: 23573
           Summary: std.bitmanip.bitfields doesn't respect native
                    endianness
           Product: D
           Version: D2
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P1
         Component: phobos
          Assignee: nobody at puremagic.com
          Reporter: ibuclaw at gdcproject.org

Given the following definition (-preview=bitfields)
```
union realbits
{
    real value;
    struct
    {
        version (LittleEndian)
        {
            private ubyte[6] padding;
            ulong significand;
            uint exp : 15;
            bool sign : 1;
        }
        else
        {
            bool sign : 1;
            uint exp : 15;
            ulong significand;
            private ubyte[6] padding;
        }
    }
}
```

You'd have thought that the equivalent std.bitmanip.bitfields would be
identical to the above, but no! You declare the bitfields in the same order,
regardless of the native endian in effect on the target.

```
import std.bitmanip : bitfields;
version (LittleEndian)
{
    // ...
    mixin(bitfields!(
        uint, "exponent", 15,
        bool, "sign"    , 1));
}
else
{
  version (THIS_GENERATES_WRONG_LAYOUT)
  {
    mixin(bitfields!(
        bool, "sign"    , 1,
        uint, "exponent", 15));
  }
  else
  {
    mixin(bitfields!(
        uint, "exponent", 15,
        bool, "sign"    , 1));
  }
    // ...
}
```

There could be a valid reason for this behaviour, and if so it should be
explicitly documented.

Otherwise, if this is indeed a bug, then it looks like the fix would be
something along the lines of foreach vs. foreach_reverse all bitfields
depending on little vs big endian respectively.

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