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[NLL] Avoid borrowed value must be valid for lifetime '_#2r..." in errors Closes #48428 - [x] If NLL is enabled, [do not invoke `note_and_explain_region`](https://github.com/rust-lang/rust/issues/48428#issuecomment-367691123) - [x] Modify `-Zdump-nll-cause` to not print [the overwhelming debug output here](https://github.com/rust-lang/rust/blob/master/src/librustc_mir/borrow_check/nll/region_infer/mod.rs#L1288-L1299). This way we should I believe at least get nice-ish output for [our original example](https://github.com/rust-lang/rust/issues/48428#issue-299364536). - [x] Extend `explain_why_borrow_contains_point` to also work for "universal lifetimes" like the `'a` in [the example at the end of this comment](https://github.com/rust-lang/rust/issues/48428#issuecomment-367691123). - [ ] Figure out how to enable causal information all the time (but that is https://github.com/rust-lang/rust/issues/46590).