Just out of curiosity : what are the next big features related to the borrow / ownership management ? I've always heard that was rust main pain point, so i'm wondering what are the Rust devs main long-term improvements on that matter.
After that, there's still some work that can be done towards NLL, specifically some details around how borrowing across functions works when the compiler "should" know that there actually isn't a borrow. This is part of some work codenamed "Polonius" [1], which should also improve the compiler time required to apply the borrow checker's rules.
Migration mode means that the MIR borrow check will run, and if it errors, then AST borrow check will run, and if that compiles successfully then the MIR borrow check errors will be downgraded to warnings. If MIR borrow check is successful then AST borrow check does not run and its errors will not impact the results.
Eventually we will enable MIR borrowck fully on all editions; I personally hope this happens sooner rather than later so we can delete the old code, but we have no concrete timeline just yet.