Generating .di header files from ImportC files

[jmh530]

On Saturday, 21 October 2023 at 02:13:24 UTC, Walter Bright wrote:
> Bruce and Adam and I met at a Red Robin in Seattle to chat 
> about D, and they both made a case for the D compiler being 
> able to translate C files into .di "header" files.
>
> [...]

Sorry but I don’t think I’ve ever used .di files. What is the 
significance of this?