While working on a library built for high efficiency, avoiding
unnecessary copies of structs became an issue. I had assumed
that `in` was doing this, but a bit of experimentation revealed
that it does not. However, `ref in` works great.
My question is, should `in` by default also imply `ref` for value
types like structs? Is there a reason not to do this?
This is the test program I used for reference:
```
import std.stdio;
struct Bob {
int a;
this(this) {
writeln("<Bob copied>");
}
}
void main()
{
Bob b = Bob(3);
writeln(" &b = ", &b);
void showAddrIn(in Bob b) {
writeln("(showAddrIn) &b = ", &b);
}
showAddrIn(b);
void showAddrRefIn(ref in Bob b) {
writeln("(showAddrRefIn) &b = ", &b);
}
showAddrRefIn(b);
}
```
The output is as follows:
```
&b = 7FFD9F526AD0
<Bob copied>
(showAddrIn) &b = 7FFD9F526AB0
(showAddrRefIn) &b = 7FFD9F526AD0
```
