The reason I said to use rvalue references is because otherwise it is an apples-to-oranges comparison: in the C++ code you have implicit ABI decisions around the call convention and whose responsibility it is to destroy the temporary.

Yes, sure, compiling in one translation unit helps, but as I mentioned above, passing an owning pointer between translation units shouldn’t be inherently inefficient

https://godbolt.org/z/9875qMM6Y (or alternatively: https://godbolt.org/z/9xehs3sYP)

The assembly is identical, the ownership is clearly transferred, and this doesn't need LTO or looking at the function bodies and is entirely done by the C++ compiler. It involves using (when available) a vendor attribute (see trivial_abi, shouldn't be an issue given Rust devs are fine with having only one compiler anyway) and writing a UniquePtr class (shouldn't be used in production code, what I've given there is only for illustration purposes) that assumes that the custom deleter cannot have an internal state.

This is a zero-runtime-cost abstraction. Now whether the zeroing of that cost can depend on what ABI assumptions you're ready to make, or whether you want to depend on LTO is another thing. We're literally discussing a "problem" that is not really a problem because Rust doesn't have the luxury yet to have that problem: you're easily forgetting that Rust has only one compiler.

Carbon was announced in 2022

A project like that usually takes years, so again, very likely that they began working on it years before that. For instance, Google designed Go in 2007 and announced it in November 2009.