Find struct not passed by reference

[jfondren]

On Monday, 2 August 2021 at 23:06:42 UTC, frame wrote:
> Is there a way to find a struct which should be passed by 
> reference but accidentally isn't? Maybe with copy constructors?

@disable postblit:

```d
struct NoCopy {
     int n;
     @disable this(this);
}

void modify(NoCopy nc) {
     nc.n++;
}

void main() {
     NoCopy x;
     x.modify; // Error: struct `catchcopy.NoCopy` is not copyable 
...
}
```

std.typecons.Unique:

```d
import std.typecons : Unique;

class Val {
     int n;
}

void incr(Unique!Val v) {
     v.n++;
}

void decr(ref Unique!Val v) {
     v.n--;
}

void show(Unique!Val v) {
     import std.stdio : writeln;

     writeln(v.n);
}

void main() {
     Unique!Val x = new Val;
     // x.incr; // Error: ... is not copyable
     x.decr;
     x.release.show;
}
```