add #![feature(nll)] to supplement -Znll

[nikomatsakis]

As part of the NLL development, the plan is to allow people on nightly to "opt-in" to the new checker. My plan is to allow one of two things. Either you can use the -Znll compilation flag (as we do today) or you can use #![feature(nll)] in your code.

I think it should work like this:

• -Znll implies -Zborrowck=mir. This is not true today and requires a small amount of tweaking.
• #![feature(nll)] implies -Znll.

[pnkfelix]

(-Z nll should also imply -Z two-phase-borrows; that is, #46537 is part of the game plan here.)

[spastorino]

I'm working on this task, it's almost done. Just some things are taking ages because of compile times :(.

[pnkfelix]

This is probably resolved by #46862 (though I think we still want to have #![feature(nll)] imply -Z two-phase-borrows, and I don't think that PR included that).

[spastorino]

@pnkfelix it is included, it's this commit 2019d69

[pnkfelix]

ah great, then I'm closing this as finished.