Would you consider merging a patch that optionally splits pybind11 into declarations vs definitions in a backwards compatible opt-in manner?

Edit: PR at: https://github.com/pybind/pybind11/pull/2445

Hello!

TL;DR: a split into declarations/definitions would save a lot of time/money for the gem5 project which I work on. I already have a prototype which reduces build time by 40% from 25 minutes to 15 minutes.

If I were to send a patch that does this split, in an opt-in way that does not affect existing users at all, would you consider merging it?

I would like to know if this is reasonable before I sink time into something that won't get reviewed.

The proposed implementation is: I would just check if a macro if defined before inclusion of pybind11.hpp, say PYBIND11_HPP.

If the macro is defined, only declarations will be enabled, and therefore you won't get object redefinitions. So usage would be in a project that wants to opt-in would be like this.

In one single file of the project which gets linked at the end:

pybind11.cpp

#include <pybind11/pybind11.h>

This will be the only file to contain definitions of non-templated stuff that does not need to be defined in headers.

Then, on every other file of the project, include as:

#define PYBIND11_HPP 1
#include <pybind11/pybind11.h>

which will only expose declarations and not definitions, therefore preventing a ton of object redefinitions.

More details about the gem5 project can be found at: https://github.com/pybind/pybind11/issues/708#issuecomment-630422294 maybe this issue is a duplicate of that one, but I'm not sure, feel free to close if you think yes. I'm just waiting for another long build and decided to annoy you a bit :-) If you promise to review, I will send a patch.

Start by reviewing PR #2445 and the proposed inclusion pattern in pybind11/pybind11.h, including the example pybind11.cpp file. Check the related discussion in issue #708 for context. Done means an opt-in declarations-versus-definitions split that preserves existing users and reduces gem5 build time without object redefinitions.