[zeotroph]

That is the compiler itself sortof creating "ad hoc" modules to cache parts of the source code, ideally even inside a single compilation unit, which especially helped with template heavy code. This way completely new code could be sped up, which with ccache would never get a cache hit.

However that project is based on an old version of clang and the changes were never upstreamed (initially it was a commercial product), so sadly this project is practically dead.