What a mess.
There is [`testline.d`](https://github.com/D-Programming-Language/dmd/blob/master/test/runnable/testline.d#L14) which sets an empty filename. But because we're [now](https://github.com/D-Programming-Language/druntime/pull/688/files#diff-7eac7eb46e31907f148813e793155274R1368) checking for the length of the filename, this will no longer print a location.
Also the compiler will crash when it generates debug info for an empty filename ([Issue 11750](https://d.puremagic.com/issues/show_bug.cgi?id=11750 "ICE with debug info and empty #line Filespec")).