[D-runtime] Why does druntime us .di files instead of .d?

[Jonathan M Davis]

Why does druntime use .di files reather than .d files? What do we gain by doing 
that? Phobos uses .d files. I don't understand why we use .di files in druntime. 
If they don't give us a real benefit, I would think that it would be better to 
just use .d files. Of course, if we get a real benefit from them, then we should 
probably use them, but the .di files are pointless from what I can see.

Apparently, there have been CTFE-related bugs where CTFE doesn't work because 
the compiler doesn't have druntime's full source when compiling Phobos. I 
don't know if the use of .di files is really the whole issue here or if it's 
something that needs to be fixed with regards to CTFE, but if using .di files 
for druntime is really causing a problem for CTFE, that would seem to be a 
good reason to stop using .di files for druntime.

So, my question is why we're using .di files for druntime. From what I can 
tell, we gain nothing from it, and it's causing us problems.

- Jonathan M Davis