Can someone tell me what the compiler thought I was trying to do?

[mw]

> @mustuse as a function attribute was in the original version of 
> the DIP. It was vetoed by Walter. Thus, only the type attribute 
> remains in the accepted version.

Let's continue the discussion here:

https://forum.dlang.org/thread/nmornkxaxddfziqmqqld@forum.dlang.org

in general, it's about: command query separation principle