crater run to estimate impact of full NLL transition

[nikomatsakis]

In the @rust-lang/lang meeting today, we were saying it'd be a good idea to do a crater run against full NLL just to get a rough idea of how many affected crates there will be.

There are two ways we could do this and both have some value:

• change the "warning lints" to "deny" and then do the crater run. This has the benefit that if crate X fails to compile, dependencies of crate X will still be tested, giving a more accurate overall picture.
• change default borrowck mode to NLL. This has the advantage that some errors (notably regionck errors) that are no longer relevant are disabled. However, I think we will overall get an underestimate this way, plus there are still (maybe) some soundness holes.

[Centril]

I would suggest that we do both of the two ways since it's an important ticket.

[pnkfelix]

cc #43234

[pnkfelix]

triage: this is an important task, but not one that I find should be prioritized over the various bugs currently plaguing the compiler.

Marking as P-medium.

I'm not sure why @Centril nominated it. Its possible that they nominated it solely for prioritization. Or its possible they wanted it to get attention from the compiler team. I'm going to leave the nomination tag on for now, with the understanding, that if we do discuss it at the T-compiler meeting and manage to assign it to someone, then we can remove the nomination tag.

[lqd]

I will try to help @pnkfelix with this.

[pnkfelix]

@lqd looks like the easiest way to go is to open two distinct PR's corresponding to the two avenues outlined in the description, right?