Assuming structs are cheap to copy?

[Peter Alexander]

I remember there was a discussion a little while back about how Phobos 
would assume that structs are cheap to copy, and would simply pass them 
by value.

Is this assumption now "the D way"? Should we all just pass structs by 
value, assuming cheap copy construction?

If so, I think this needs to be documented somewhere (assuming it isn't 
already), because it's a radical departure from C++, and also from most 
other languages (where everything is a reference type). People need to 
be aware of this.