rustc_mir: don't move temporaries that are still used later.
This should unbreak using the MIR borrow-checker on `libcore` (assuming #46268 is merged).
NLL: improve inference with flow results, represent regions with bitsets, and more
This PR begins with a number of edits to the NLL code and then includes a large number of smaller refactorings (these refactorings ought not to change behavior). There are a lot of commits here, but each is individually simple. The goal is to land everything up to but not including the changes to how we handle closures, which are conceptually more complex.
The NLL specific changes are as follows (in order of appearance):
**Modify the region inferencer's approach to free regions.** Previously, for each free region (lifetime parameter) `'a`, it would compute the set of other free regions that `'a` outlives (e.g., if we have `where 'a: 'b`, then this set would be `{'a, 'b}`). Then it would mark those free regions as "constants" and report an error if inference tried to extend `'a` to include any other region (e.g., `'c`) that is not in that outlives set. In this way, the value of `'a` would never grow beyond the maximum that could type check. The new approach is to allow `'a` to grow larger. Then, after the fact, we check over the value of `'a` and see what other free regions it is required to outlive, and we check that those outlives relationships are justified by the where clauses in scope etc.
**Modify constraint generation to consider maybe-init.** When we have a "drop-live" variable `x` (i.e., a variable that will be dropped but will not be otherwise used), we now consider whether `x` is "maybe initialized" at that point. If not, then we know the drop is a no-op, and we can allow its regions to be dead. Due to limitations in the fragment code, this currently only works at the level of entire variables.
**Change representation of regions to use a `BitMatrix`.** We used to use a `BTreeSet`, which was rather silly. We now use a MxN matrix of bits, where `M` is the number of variables and `N` is the number of possible elements in each set (size of the CFG + number of free regions).
The remaining commits (starting from
extract the `implied_bounds` code into a helper function ") are all "no-op" refactorings, I believe.
~~One concern I have is with the commit "with -Zverbose, print all details of closure substs"; this commit seems to include some "internal" stuff in the mir-dump files, such as internal interner numbers, that I fear may vary by platform. Annoying. I guess we will see.~~ (I removed this commit.)
As for reviewer, @arielb1 has been reviewing the PRs, and they are certainly welcome to review this one too. But I figured it'd maybe be good to have more people taking a look and being familiar with this code, so I'll "nominate" @pnkfelix .
r? @pnkfelix
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.
In particular, if we see a variable is DROP-LIVE, but it is not
MAYBE-INIT, then we can ignore the drop. This leavess attempt to use
more complex refinements of the idea (e.g., for subpaths or subfields)
to future work.
Rather than declaring some region variables to be constant, and
reporting errors when they would have to change, we instead populate
each free region X with a minimal set of points (the CFG plus end(X)),
and then we let inference do its thing. This may add other `end(Y)`
points into X; we can then check after the fact that indeed `X: Y`
holds.
This requires a bit of "blame" detection to find where the bad
constraint came from: we are currently using a pretty dumb
algorithm. Good place for later expansion.
Fix CopyPropagation regression (2)
Remaining part of MIR copyprop regression by (I think) #45380, which I missed in #45753.
```rust
fn foo(mut x: i32) -> i32 {
let y = x;
x = 123; // `x` is assigned only once in MIR, but cannot be propagated to `y`
y
}
```
So any assignment to an argument cannot be propagated.
create a drop ladder for an array if any value is moved out
r? @arielb1
first commit for fix https://github.com/rust-lang/rust/issues/34708 (note: this still handles the subslice case in a very broken manner)
This simplifies analysis and borrow-checking because liveness at the
resume point can always be simply propagated.
Later on, the "dead" Resumes are removed.
Region inference can create borrows for an empty region if the borrow is
dead. In that case, there's no reason to track the borrow, but because
there's no such thing as an EndRegion(ReEmpty) these borrows used to live
for the entire function.
Fixes#46161.
incr.comp.: Some preparatory work for caching more query results.
This PR
* adds and updates some encoding/decoding routines for various query result types so they can be cached later, and
* adds missing `[input]` annotations for a few `DepNode` variants.
The situation around having to explicitly mark dep-nodes/queries as inputs is not really satisfactory. I hope we can find a way of making this more fool-proof in the future.
r? @nikomatsakis
avoid type-live-for-region obligations on dummy nodes
Type-live-for-region obligations on DUMMY_NODE_ID cause an ICE, and it
turns out that in the few cases they are needed, these obligations are not
needed anyway because they are verified elsewhere.
Fixes#46069.
Beta-nominating because this is a regression for our new beta.
r? @nikomatsakis
