[MIR-borrowck] Two phase borrows
This adds limited support for two-phase borrows as described in
http://smallcultfollowing.com/babysteps/blog/2017/03/01/nested-method-calls-via-two-phase-borrowing/
The support is off by default; you opt into it via the flag `-Z two-phase-borrows`
I have written "*limited* support" above because there are simple variants of the simple `v.push(v.len())` example that one would think should work but currently do not, such as the one documented in the test compile-fail/borrowck/two-phase-reservation-sharing-interference-2.rs
(To be clear, that test is not describing something that is unsound. It is just providing an explicit example of a limitation in the implementation given in this PR. I have ideas on how to fix, but I want to land the work that is in this PR first, so that I can stop repeatedly rebasing this branch.)
rustc: unpack newtyped of #[repr(simd)] vector types.
Prerequisite for a `#[repr(transparent)]` implementation that works with SIMD vectors.
cc @rkruppe
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.
In reality the currently generated MIR has at least one of the activations
in a copy that occurs before the merge. But still, good to have a test,
in anticipation of that potentially changing...
trait alias infrastructure
This will be an implementation of trait aliases (RFC 1733, #41517).
Progress so far:
- [x] Feature gate
- [x] Add to parser
- [x] `where` clauses
- [x] prohibit LHS type parameter bounds via AST validation https://github.com/rust-lang/rust/pull/45047#discussion_r143575575
- [x] Add to AST and HIR
- [x] make a separate PathSource for trait alias contexts https://github.com/rust-lang/rust/pull/45047#discussion_r143353932
- [x] Stub out enough of typeck and resolve to just barely not ICE
Postponed:
- [ ] Actually implement the alias part
- [ ] #21903
- [ ] #24010
I need some pointers on where to start with that last one. The test currently does this:
```
error[E0283]: type annotations required: cannot resolve `_: CD`
--> src/test/run-pass/trait-alias.rs:34:16
|
34 | let both = foo();
| ^^^
|
= note: required by `foo`
```
incr.comp.: Speed up span hashing by caching expansion context hashes.
This PR fixes the performance regressions from https://github.com/rust-lang/rust/pull/46338.
r? @nikomatsakis
Validate miri against the HIR const evaluator
r? @eddyb
cc @alexcrichton @arielb1 @RalfJung
The interesting parts are the last few functions in `librustc_const_eval/eval.rs`
* We warn if miri produces an error while HIR const eval does not.
* We warn if miri produces a value that does not match the value produced by HIR const eval
* if miri succeeds and HIR const eval fails, nothing is emitted, but we still return the HIR error
* if both error, nothing is emitted and the HIR const eval error is returned
So there are no actual changes, except that miri is forced to produce the same values as the old const eval.
* This does **not** touch the const evaluator in trans at all. That will come in a future PR.
* This does **not** cause any code to compile that didn't compile before. That will also come in the future
It would be great if someone could start a crater run if travis passes
Point at whole method call instead of args
To avoid confusion in cases where the code is
```rust
fn foo() {}
/ foo(
| bar()
| ^^^ current diagnostics point here for arg count mismatch
| );
|_^ new diagnostic span points here
```
as this leads to confusion making people think that the diagnostic is
talking about `bar`'s arg count, not `foo`'s.
Point at `fn`s definition on arg mismatch, just like we do for closures.
Re #42855, Fix#45633.
make MIR type checker handle a number of other cases
The existing type checker was primarily used to verify types, but was skipping over a number of details. For example, it was not checking that the predicates on functions were satisfied and so forth. This meant that the NLL region checker was not getting a lot of the constraints it needed. This PR closes those gaps. It also includes a bit of refactoring for the way that we store region values, encapsulating the bit matrix over into its own module and improving the data structures in use.
This is mostly work by @spastorino being ported over from nll-master.
r? @arielb1 or @pnkfelix
Instead we are "just" careful to invoke it (which sets up a bunch of kill bits)
before we go into the code that sets up the gen bits.
That way, when the gen bits are set up, they will override any
previously set kill-bits for those reservations or activations.
Fix visible_parent_map to choose globally minimal paths
Fix#46112: visible_parent_map construction needs a BFS over whole crate forest to get globally minimal paths.
(There are other latent bugs that were e.g. causing this test case to have weirdness like `<unnamed>` in the diagnostic output. Those bugs are not fixed here, since they are issues long-standing in the stable channel.)
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.
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.
