Understanding common std.Io.Reader/Writer mistakes (0.15.1)

[npc1054657282]

`@fieldParentPtr()` is innocent; what is dangerous is modifying a local copy of a structure

The new std.Io.Writer interface has faced a lot of misuse. Some attribute this misuse to @fieldParentPtr(). Some are eager to have Pinned struct accepted - by preventing this misuse through restricting copies of structures of ‘specific types’. In this regard, I would like to sing a different tune: @fieldParentPtr() is innocent! The real root cause of the mistake regarding the copying of structs is not the type, but the mutability of the copy target. Let us consider a very simple type: const S…

When you copy an interface and find that it needs to be var to use it, be careful: you are probably making a mistake and should use its pointer.