Failed `get` in util::ppaux::re_scope_id_to_str, followed by ICE

[lkuper]

Gist of the backtrace: https://gist.github.com/2926927

This came up because I had set RUST_LOG=rustc::middle::typeck::infer=3 in order to debug an unrelated issue in infer, and turning logging on for infer caused this.

[nikomatsakis]

the problem here is the synthesized expressions created for predicates in the type check fn check_constraints(). they don't have a real expression id so there is no corresponding entry in the ast_map. this is pretty sketchy code to begin with, so I guess I'd be inclined to change that code rather than the print-out.

[nikomatsakis]

on the other hand, that'd be hard. so maybe I'll just make the printouts not fail when they find something unexpected but instead print out a less-than-useful string.

[nikomatsakis]

or maybe take the VERY EASY path and remove the preconditions from core::result::iter2

[nikomatsakis]

Fixed in commit 41a21f0, which removed current incarnation of typestate.