[NLL] Dangly paths for box #52782

This should address issue 45696. Since we know dropping a box will not access any `&mut` or `&` references, it is safe to model its destructor as only touching the contents *owned* by the box. Note: At some point we may want to generalize this machinery to other reference and collection types that are "pure" in the same sense as box. If we add a `&move` reference type, it would probably also fall into this branch of code. But for the short term, we will be conservative and restrict this change to `Box<T>` alone. The code works by recursively descending a deref of the `Box`. We prevent `visit_terminator_drop` infinite-loop (which can arise in a very obscure scenario) via a linked-list of seen types. Note: A similar style stack-only linked-list definition can be found in `rustc_mir::borrow_check::places_conflict`. It might be good at some point in the future to unify the two types and put the resulting definition into `librustc_data_structures/`. ---- One final note: Review feedback led to significant simplification of logic here. During review, eddyb RalfJung and I uncovered the heart of why I needed a so-called "step 2" aka the Shallow Write to the Deref of the box. It was because the `visit_terminator_drop`, in its base case, will not emit any write at all (shallow or deep) to a place unless that place has a need_drop. So I was encoding a Shallow Write by hand for a `Box<T>`, as a separate step from recursively descending through `*a_box` (which was at the time known as "step 1"; it is now the *only* step, apart from the change to the base case for `visit_terminator_drop` that this commit now has encoded). eddyb aruged that *something* should be emitting some sort of write in the base case here (even a shallow one), of the dropped place, since by analogy we also emit a write when you *move* a place. That led to the revision here in this commit. * (Its possible that this desired write should be attached in some manner to StorageDead instead of Drop. But in this PR, I tried to leave the StorageDead logic alone and focus my attention solely on how Drop(x) is modelled in MIR-borrowck.)

(Presumably the place that borrow_check ends up reporting for the error about is no longer the root `Local` itself, and thus the note diagnostic here stops firing.)

…per.

…ve cases. After talking about the PR with eddyb, I decided it was best to try to have some test cases that simplify the problem down to its core, so that people trying to understand what the issue is here will see those core examples first.

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[NLL] Dangly paths for box #52782

[NLL] Dangly paths for box #52782

Commits on Aug 1, 2018