It's not just the borrow checker. One interesting way to see how much lifting is needed is to examine the "Circle C++" compiler. Sean Baxter's language / C++ experiments takes the same approach and you'll see Sean needed not only an equivalent to Rust's borrow checker, but its trait system (roughly equivalent to the Concepts proposal in C++ 0x, not the "Concepts Lite" which is modern C++ 20 Concepts) including Rust's auto traits Send and Sync, and a whole bunch of stdlib infrastructure.
If you try to do without Send and Sync you have to choose, either your language has data races and they induce UB (as in, say, Go and several popular garbage collected languages) or your language doesn't have real multi-threading. Rust needs these traits to be able to provide a data rate free safe language with multi-threading. Could other approaches exist? Well for one thing after Rust 1.0 there has been considerable work on theory and practice for Java-like loss of sequential consistency without UB. But there isn't today any good "This definitely just works" alternative. For a future Rust-like language, an industrialization of best practices, this is the best we currently know although it's reasonable to expect that one make a decade from now will have better options.
If you try to do without Send and Sync you have to choose, either your language has data races and they induce UB (as in, say, Go and several popular garbage collected languages) or your language doesn't have real multi-threading. Rust needs these traits to be able to provide a data rate free safe language with multi-threading. Could other approaches exist? Well for one thing after Rust 1.0 there has been considerable work on theory and practice for Java-like loss of sequential consistency without UB. But there isn't today any good "This definitely just works" alternative. For a future Rust-like language, an industrialization of best practices, this is the best we currently know although it's reasonable to expect that one make a decade from now will have better options.