Instead, filter out (non-)conflicts of activiations with themselves in
the same manner that we filter out non-conflict between an activation
and its reservation.
Since we are now checking activation points, I removed one of the
checks at the reservation point. (You can see the effect this had on
two-phase-reservation-sharing-interference-2.rs)
Also, since we now have checks at both the reservation point and the
activation point, we sometimes would observe duplicate errors (since
either one independently interferes with another mutable borrow). To
deal with this, I used a similar strategy to one used as discussed on
issue #45360: keep a set of errors reported (in this case for
reservations), and then avoid doing the checks for the corresponding
activations. (This does mean that some errors could get masked, namely
for conflicting borrows that start after the reservation but still
conflict with the activation, which is unchecked when there was an
error for the reservation. But this seems like a reasonable price to
pay.)
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.
Converting a `RegionElementIndex` to a `Location` is O(n) though could
trivially be O(log n), but we don't do it that much anyhow -- just on
error and debugging.
This has been bugging me. All the regions appear free in the source;
the real difference is that some of them are universally quantified
(those in the function signature) and some are existentially
quantified (those for which we are inferring values).
We now visit just the stuff in the CFG, and we add liveness
constraints for all the random types, regions etc that appear within
rvalues and statements.
