From a previous comment:

> A copy of the source tree that exactly matches that test VS.

Why IDE unable to keep track versions of sources for me ?
Why not embed meta-information in PDB's on how and there to retrieve correct source file version ?

Actually, it does.  In VS2005, the debugger team added two features along these lines.  The first is "source file checksums", described here: http://blogs.msdn.com/andypennell/archive/2004/06/29/169002.aspx

The second is Source Server, described here: http://msdn.microsoft.com/msdnmag/issues/06/08/UseTheSource/default.aspx

However, the problem that I'm dealing with is a little more complex (and much less likely to occur in the wild).  On my dev machine, I need a copy of Visual Studio and the sources that built it, so that I can modify the sources, build one component, and try it out.  If the sources I start from don't match my copy of VS exactly, then I can't know which changes in behavior are due to my changes.  Furthermore, if interfaces have changed, then the product is likely to stop working entirely.

Does that explain things?