[mark-simulacrum]

For the initial release, NLL will be enabled in "migration" mode in 2018 only. `#![feature(nll)]` will continue to be available -- that hard-enables MIR borrowck instead of the migration mode. (edit: to be clear, the feature gate is not stabilized, but it will be available on nightly)

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.