D's ImportC compiler already does this:

https://dlang.org/spec/importc.html#ctfe

It's a natural consequence of using D's semantic analysis for the C compiler.

I thought about removing it, because it's C, but then decided Vot De Heck and left it in. It's quite nice for the test suite for ImportC, too, as an executable doesn't have to be generated to check the semantics.

P.S. No keyword is required to make it work.

P.P.S. ImportC also allows forward references to declarations, another consequence of using D's semantic implementation.