[WalterBright]

The D compiler has an option to generate a "header file" from D modules. It's called a .di file. It's useful if you want to hide the implementation from a compiler, as you would with libraries.

As it turned out, though, people just found it too convenient to just import the .d file.

But as a very unexpected dividend, it was discovered that the D compiler would generate .di files from compiling .c files, and realized that D had an inherent ability to translate C code to D code!!!! This has become rather popular.