> I would think the Err(err) => return Err(err) line needlessly constructs a copy of result

It sounds like this is coming from a C++ bias? So please forgive me if this is wrong.

Rust, in my experience, favors move semantics first, then copy semantics after.

I know in C++, we had implicit copy constructors, with move semantics after with rvalue references, where you need to use `std::move` in a lot of cases.

So what helps, in my opinion, is to think of Rust as using `std::move` as a default.