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* Emit `EndRegion` for every code-extent for which we observe a
borrow. To do this, we needed to thread source info back through
to `fn in_scope`, which makes this commit a bit more painful than
one might have expected.
* There is `end_region` emission in `Builder::pop_scope` and in
`Builder::exit_scope`; the first handles falling out of a scope
normally, the second handles e.g. `break`.
* Remove `EndRegion` statements during the erase_regions mir
transformation.
* Preallocate the terminator block, and throw an `Unreachable` marker
on it from the outset. Then overwrite that Terminator as necessary
on demand.
* Instead of marking the scope as needs_cleanup after seeing a
borrow, just treat every scope in the chain as being part of the
diverge_block (after any *one* of them has separately signalled
that it needs cleanup, e.g. due to having a destructor to run).
* Allow for resume terminators to be patched when looking up drop flags.
(In particular, `MirPatch::new` has an explicit code path,
presumably previously unreachable, that patches up such resume
terminators.)
* Make `Scope` implement `Debug` trait.
* Expanded a stray comment: we do not emit StorageDead on diverging
paths, but that end behavior might not be desirable.
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