I don't follow. I'd think that for any given header, you would only have to recompile it if the #include dependencies listed in that file (or their subdependencies) changed. I'd hope that for most projects this is far from every header. When would this happen?

It also seems like this would be a win even if only used for rarely-changing system headers. As shown in the slides, for small projects the lines of unchanging standard includes dwarf the project specific code. Wouldn't it be a big win to avoid parsing and compiling all of these?