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.
Download crosstool-ng from GitHub
Workaround the current problem where http://crosstool-ng.org was done, causing all non-x86 jobs to fail spuriously (cc #40474).
If http://crosstool-ng.org becomes online before this PR is merged, this PR should be closed and the tree should be reopened.
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.
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.
macros: hygienize use of `core`/`std` in builtin macros
Today, if a builtin macro wants to access an item from `core` or `std` (depending `#![no_std]`), it generates `::core::path::to::item` or `::std::path::to::item` respectively (c.f. `fn std_path()` in `libsyntax/ext/base.rs`).
This PR refactors the builtin macros to instead always emit `$crate::path::to::item` here. That is, the def site of builtin macros is taken to be in `extern crate core;` or `extern crate std;`. Since builtin macros are macros 1.0 (i.e. mostly unhygienic), changing the def site can only effect the resolution of `$crate`.
r? @nrc
Record all imports (`use`, `extern crate`) in the crate metadata
This PR adds non-`pub` `use` and `extern crate` imports in the crate metadata since hygienic macros invoked in other crates may use them. We already include all other non-`pub` items in the crate metadata. This improves import suggestions in some cases.
Fixes#42337.
r? @nrc
Having the HIR local id is useful for cases like understanding the
ReScope identifiers, which are now derived from the HIR local id, and
thus one can map an ReScope back to the HIR node, once one knows what
those local ids are.
Implement impl Trait lifetime elision
Fixes#43396.
There's one weird ICE in the interaction with argument-position `impl Trait`. I'm still debugging it-- I've left a test for it commented out with a FIXME.
Also included a FIXME to ensure that `impl Trait` traits are caught under the lint in https://github.com/rust-lang/rust/issues/45992.
r? @nikomatsakis
