DIP 1021--Argument Ownership and Function Calls--Final Review

[IGotD-]

On Monday, 16 September 2019 at 10:06:55 UTC, Walter Bright wrote:
> DIP 1021 can be experimented with as the code is now in the 
> compiler and enabled with -preview=useDIP1021. I'd expect some 
> trouble with it as it doesn't have much wear on the tires.

A few questions:

What would the default behaviour of the compiler be, is the 
ownership check on or off.
Can it be switched on/off by a compiler switch like now, in order 
to make existing code pass instead of adding @system or @trusted?
If an ownership problem is encountered, what is the action, 
compiler warning or error?