Sometimes a simple goto misses the cleanup/unwind edges. Specifically, in the
case of infinite loops such as those introduced by a loop statement without any
other out edges. Analogous to TerminatorKind::FalseEdges; this new terminator
kind is used when we want borrowck to consider an unwind path, but real control
flow should never actually take it.

The Abort Terminatorkind will cause an llvm.trap function call to be
emitted.
Signed-off-by: NDavid Henningsson <diwic@ubuntu.com>

High-level picture: The old `Borrows` analysis is now called
`Reservations` (implemented as a newtype wrapper around `Borrows`);
this continues to compute whether a `Rvalue::Ref` can reach a
statement without an intervening `EndRegion`. In addition, we also
track what `Place` each such `Rvalue::Ref` was immediately assigned
to in a given borrow (yay for MIR-structural properties!).

The new `ActiveBorrows` analysis then tracks the initial use of any of
those assigned `Places` for a given borrow. I.e. a borrow becomes
"active" immediately after it starts being "used" in some way. (This
is conservative in the sense that we will treat a copy `x = y;` as a
use of `y`; in principle one might further delay activation in such
cases.)

The new `ActiveBorrows` analysis needs to take the `Reservations`
results as an initial input, because the reservation state influences
the gen/kill sets for `ActiveBorrows`. In particular, a use of `a`
activates a borrow `a = &b` if and only if there exists a path (in the
control flow graph) from the borrow to that use. So we need to know if
the borrow reaches a given use to know if it really gets a gen-bit or
not.

 * Incorporating the output from one dataflow analysis into the input
   of another required more changes to the infrastructure than I had
   expected, and even after those changes, the resulting code is still
   a bit subtle.

 * In particular, Since we need to know the intrablock reservation
   state, we need to dynamically update a bitvector for the
   reservations as we are also trying to compute the gen/kills
   bitvector for the active borrows.

 * The way I ended up deciding to do this (after also toying with at
   least two other designs) is to put both the reservation state and
   the active borrow state into a single bitvector. That is why we now
   have separate (but related) `BorrowIndex` and
   `ReserveOrActivateIndex`: each borrow index maps to a pair of
   neighboring reservation and activation indexes.

As noted above, these changes are solely adding the active borrows
dataflow analysis (and updating the existing code to cope with the
switch from `Borrows` to `Reservations`). The code to process the
bitvector in the borrow checker currently just skips over all of the
active borrow bits.

But atop this commit, one *can* observe the analysis results by
looking at the graphviz output, e.g. via

```rust
 #[rustc_mir(borrowck_graphviz_preflow="pre_two_phase.dot",
             borrowck_graphviz_postflow="post_two_phase.dot")]
```

Includes doc for `FindPlaceUses`, as well as `Reservations` and
`ActiveBorrows` structs, which are wrappers are the `Borrows` struct
that dictate which flow analysis should be performed.

(Still musing about whether it could make sense to revise the design
here to make these constraints on usage explicit.)

This is meant to ease development of multi-stage dataflow analyses
where the output from one analysis is used to initialize the state
for the next; in such a context, you cannot start with `bottom_value`
for all the bits.

Made `do_dataflow` and related API `pub(crate)`.

If the gen/kill bits are set there, the effects of `start_block_effects`
will not be seen when using `FlowAtLocation` etc. to go over the MIR.

EverInitializedLvals is the only pass that got this wrong, but this
fixes the footgun for everyone.

We can now use it in e.g. drop elaboration if we want to.

* Used for new dataflow to track if a variable has every been initialized
* Used for other dataflows that need to be updated for initializations

Also, factor out `do_mir_borrowck`, which is the code that actually
performs the MIR borrowck from within the scope of an inference context.

This change should be a pure refactoring.

Post-rebase: ariel confirmed `SetDiscriminant` should indeed be a mutate.

One can either use `-Z borrowck-mir` or add the `#[rustc_mir_borrowck]` attribute
to opt into MIR based borrow checking.

Note that regardless of whether one opts in or not, AST-based borrow
check will still run as well.  The errors emitted from AST-based
borrow check will include a "(Ast)" suffix in their error message,
while the errors emitted from MIR-based borrow check will include a
"(Mir)" suffix.

post-rebase: removed check for intra-statement mutual conflict;
replaced with assertion checking that at most one borrow is generated
per statement.

post-rebase: removed dead code: `IdxSet::pairs` and supporting stuff.

Refactored `each_bit`, which traverses a `IdxSet`, so that the bulk of
its implementation lives in `rustc_data_structures`.

(This code is more general purpose than just supporting drop flag elaboration.)

post-rebase: addressed review comment: rename `loc_map`/`location_map` and `rgn_map`/`region_map`.

post-rebase: remove now unnecessary `mut` decl.

post-rebase: address comments: bind iterator expr, and alpha-rename `loc`/`location` and `idx`/`index`.

Like #43008 (f6689991), but _much more aggressive_.

Turn `elaborate_drops` and `rustc_peek` implementations into MIR
passes that also live in `rustc_mir` crate.

Rewire things so `rustc_driver` uses the `ElaborateDrops` from
`rustc_mir` crate.

This avoids creating drop flags in many unnecessary situations.

Fixes #41110.

This removes another special case of Switch by replacing it with the more general SwitchInt. While
this is more clunky currently, there’s no reason we can’t make it nice (and efficient) to use.

I no longer remember why I needed this (or thought I did). The way
that the `BitDenotation` is passed around in all existing use cases
(and planned future ones), the thing that were in the `Ctxt` can just
be part of `Self` instead.

(I think ariel had been pushing me to do this back when I first put in
this infrastructure; it took me a while to see how much of pain the
`Ctxt` was causing.)