For those wondering, the "new new borrow checker" is similar to its predecessor in that it operates on the compiler's MIR, but is a different and slightly more complete algorithm (it accepts a strict superset of code, thus backwards compatible). This two-step rollout was decided last year due to (at the time) some unacceptable compile-time regressions in the new-new algorithm's implementation. Read about it here: http://smallcultfollowing.com/babysteps/blog/2018/06/15/mir-...