I'm not sure what the proposal is -- not passing the proto in by const reference?
The alternative would be to pass-by-value and let the compiler optimize this. Not sure this is worth the effort.
I'm quite strongly against taking the parameter rvalue reference (&&) as that makes the api hard to use and complex.

If you really want to save a copy here, simply use emplace() instead of insert() on the map.

That said, I'm afraid we're looking into premature optimizations.

The two options proposed here are:

1- Pass-by-value so the caller can std::move().
2- Pass using std::unique_ptr<>.

P3 seems appropriate here but this one is also a very low-hanging fruit, because:
a) We don't have many callers to this API (4 calling sites).
b) Metadata loading happens during browser's startup and incurring in one copy (and I think today we may incur in two) may mean ~300 KB of RAM for a user with 1000 sync entities in total across all datatypes. Startup is precisely a time when resource optimization is worth.

I would personally prefer std::unique_ptr<> which we already do in other similar APIs.

sounds reasonable. I wish we could actually measure this.
sorry for the noise.