$ C:\dev\dmd\src>rdmd build.d -v
..\generated\windows\release\64\dmd.exe

It tries to get the canonical name of this path:
C:\dev\dmd\generated\windows\release\64\sc.ini

But it fails at this assertion:
> core.exception.AssertError@dmd\root\filename.d(1011): Assertion failure

This assert fails:
assert(length == fullPathLength);

I've read the docs here: https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-getfullpathnamew

It seems on the second call the count will actually not include the null character, specifically this note from MSDN:

> Note  Although the return value in this case is a length that includes the terminating null character, the return value on success does not include the terminating null character in the count.

I'll submit a PR.