Bazel is exactly this (and amazingly good at it), a cache of your entire build tree and all its dependencies. But it is a full blown build system that you need to buy into. You can’t bolt it on to whatever you are using like ccache.

This new zapcc thing seems to be more granular than ccache, it caches individual template instantiations. Bazel can't do that.

Bazel supports persistent compilers with its worker protocol [0]. It's already supported for javac. It may be worthwhile to investigate how well zapcc can integrate with Bazel.

[0] https://blog.bazel.build/2015/12/10/java-workers.html

Isn't that what ccache provides? Admittedly it's not perfect but it sounds like it might help your use case. https://ccache.samba.org

This new zapcc thing seems to be more granular than ccache, it caches individual template instantiations

No, it is close to what LTO, parse caches and precompiled headers provide. No need for daemons to get the same result.