// test.d
enum x = __traits(compiles, (){ import doesNotExist; });

DMD works:
> dmd -c test.d

RDMD fails:
> rdmd -c test.d
Error: cannot read file doesNotExist.d

The problem is RDMD gets dependencies by reading DMD's -v output, which always includes doesNotExist (since DMD *does* attempt to import it). Since RDMD sees doesNotExist in the -v output, it mistakenly tries to build it.

This prevents optional imports from working with RDMD:

static if( __traits(compiles, (){ import foobar; }) )
    { import foobar; }
else
    /+ do something else +/

It might be better for RDMD to read the -deps output instead, and omit the files listed with "(???)" as the filename.