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

[Walter Bright]

On 7/13/2011 3:19 PM, Jonathan M Davis wrote:
>
> Given its heavy use of templates, I'm not sure how much that'll buy us in
> terms of compilation speed, and since it's all open source, I don't know why
> it would matter about hiding implementation details.

Consider the garbage collector. Should every compilation have to recompile that, 
too?