A couple simple borrowck cleanups
This PR has a couple simple renamings:
- it's been a long time since the mapping from `Location`s to `PointIndex`es was extracted from `RegionElements` into the `DenseLocationMap`, but only the types were renamed at the time. borrowck still refers to this map as `elements`. That's confusing, especially since sometimes we also use the mapping via `LivenessValues`, and makes more sense as `location_map` instead.
- to clarify `LocationTable` is not as general as it sounds, and is only for datalog polonius. In this branch I didn't rename the handful of `location_table` fields and params to `polonius_table`, but can do that to differentiate it even more from `location_map`. I did try it locally and it looks worthwhile, so if you'd prefer I can also push it here. (Or we could even switch these datalog types and fields to even more explicit names)
- to clarify the incomprehensible `AllFacts`, it is renamed to `PoloniusFacts`. These can be referred to as `facts` within the legacy polonius module, but as `polonius_facts` outside of it to make it clear that they're not about NLLs (nor are they about in-tree polonius but that'll be magically fixed when they're removed in the future)
r? `@matthewjasper`
Its original naming hides the fact that it's related to datalog
polonius, and bound to be deleted in the near future.
It also conflicts with the expected name for the actual NLL location
map, and prefixing it with its use will make the differentiation
possible.
`best_blame_constraint`: Blame better constraints when the region graph has cycles from invariance or `'static`
This fixes#132749 by changing which constraint is blamed for region errors in several cases. `best_blame_constraint` had a heuristic that tried to pinpoint the constraint causing an error by filtering out any constraints where the outliving region is unified with the ultimate target region being outlived. However, it used the SCCs of the region graph to do this, which is unreliable; in particular, if the target region is `'static`, or if there are cycles from the presence of invariant types, it was skipping over the constraints it should be blaming. As is the case in that issue, this could lead to confusing diagnostics. The simplest fix seems to work decently, judging by test stderr: this makes `best_blame_constraint` no longer filter constraints by their outliving region's SCC.
There are admittedly some quirks in the test output. In many cases, subdiagnostics that depend on the particular constraint being blamed have either started or stopped being emitted. After starting at this for quite a while, I think anything too fickle about whether it outputs based on the particular constraint being blamed should instead be looking at the constraint path as a whole, similar to what's done for [the placeholder-from-predicate note](https://github.com/rust-lang/rust/compare/master...dianne:rust:better-blame-constraints-for-static#diff-3c0de6462469af483c9ecdf2c4b00cb26192218ef2d5c62a0fde75107a74caaeR506).
Very many tests involving invariant types gained a note pointing out the types' invariance, but in a few cases it was lost. A particularly illustrative example is [tests/ui/lifetimes/copy_modulo_regions.stderr](https://github.com/rust-lang/rust/compare/master...dianne:rust:better-blame-constraints-for-static?expand=1#diff-96e1f8b29789b3c4ce2f77a5e0fba248829b97ef9d1ce39e7d2b4aa57b2cf4f0); I'd argue the new constraint is a better one to blame, but it lacks the variance diagnostic information that's elsewhere in the constraint path. If desired, I can try making that note check the whole path rather than just the blamed constraint.
The subdiagnostic [`BorrowExplanation::add_object_lifetime_default_note`](https://doc.rust-lang.org/nightly/nightly-rustc/rustc_borrowck/diagnostics/explain_borrow/enum.BorrowExplanation.html#method.add_object_lifetime_default_note) depends on a `Cast` being blamed, so [a special case](364ca7f99c) was necessary to keep it from disappearing from tests specifically testing for it. However, see the FIXME comment in that commit; I think the special case should be removed once that subdiagnostic works properly, but it's nontrivial enough to warrant a separate PR. Incidentally, this removes the note from a test where it was being added erroneously: in [tests/ui/borrowck/two-phase-surprise-no-conflict.stderr](https://github.com/rust-lang/rust/compare/master...dianne:rust:better-blame-constraints-for-static?expand=1#diff-8cf085af8203677de6575a45458c9e6b03412a927df879412adec7e4f7ff5e14), the object lifetime is explicitly provided and it's not `'static`.
"Elements" are `RegionElement`s. The dense location mapping was removed
from the element containers a while ago but didn't rename its use-sites.
Most of the old naming only used the mapping, and are better named
`location_map`.
Convert typeck constraints in location-sensitive polonius
In this PR, we do a big chunk of the work of localizing regular outlives constraints.
The slightly annoying thing is handling effectful statements: usually the subset graph propagates loans at a single point between regions, and liveness propagates loans between points within a single region, but some statements have effects applied on exit.
This was also a problem before, in datalog polonius terms and Niko's solution at the time, this is about: the mid-point. The idea was to duplicate all MIR locations into two physical points, and orchestrate the effects with that. Somewhat easier to do, but double the CFG.
We've always believed we didn't _need_ midpoints in principle, as we can represent changes on exit as on happening entry to the successor, but there's some difficulty in tracking the position information at sufficient granularity through outlives relation (especially since we also have bidirectional edges and time-traveling now).
Now, that is surely what we should be doing in the future. In the mean time, I infer this from the kind of statement/terminator where an outlives constraint arose. It's not particularly complicated but some explanation will help clarify the code.
Assignments (in their various forms) are the quintessential example of these crossover cases: loans that would flow into the LHS would not be visible on entry to the point but on exit -- so we'll localize these edges to the successor. Let's look at a real-world example, involving invariance for bidirectional edges:
```rust
let mut _1: HashMap<i32, &'7 i32>;
let mut _3: &'9 mut HashMap<i32, &'10 i32>;
...
/* at bb1[3]: */ _3 = &'3 mut _1;
```
Here, typeck expectedly produces 3 outlives constraints today:
1. `'3 -> '9`
2. `'7 -> '10`
3. `'10 -> '7`
And we localize them like so,
1. `'3 -> '9` flows into the LHS and becomes: `3_bb1_3 -> 9_bb1_4`
2. `'7 -> '10` flows into the LHS and becomes: `7_bb1_3 -> 10_bb1_4`
3. `'10 -> '7` flows from the LHS and becomes: `10_bb1_4 -> 7_bb1_3` (time traveling 👌)
---
r? ``@jackh726``
To keep you entertained during the holidays I also threw in a couple of small changes removing cruft in the borrow checker.
We're actually getting there. The next PR will be the last one needed to get end-to-end tests working.
remove unnecessary `eval_verify_bound`
This does not impact any tests. I feel like any cases where this could useful should instead be fixed by a general improvement to `eval_verify_bound` to avoid having to promote this `TypeTest` in the first place 🤔
r? types cc ``@nikomatsakis``
This renames variables named `str` to other names, to make sure `str`
always refers to a type.
It's confusing to read code where `str` (or another standard type name)
is used as an identifier. It also produces misleading syntax
highlighting.
The SCCs of the region graph are not a reliable heuristic to use for blaming an interesting
constraint for diagnostics. For region errors, if the outlived region is `'static`, or the involved
types are invariant in their lifetiems, there will be cycles in the constraint graph containing both
the target region and the most interesting constraints to blame. To get better diagnostics in these
cases, this commit removes that heuristic.
`ExtraConstraintInfo` was used only for a single subdiagnostic, so this moves the logic for that
to its own function and eliminates the indirection. In order to do so cleanly, this also changes
the arguments to `BorrowExplanation::add_explanation_to_diagnostic`, which happens to simplify its
call sites.
Some small nits to the borrowck suggestions for mutating a map through index
1. Suggesting users to either use `.insert` or `.get_mut` (which do totally different things) can be a bit of a footgun, so let's make that a bit more nuanced.
2. I find the suggestion of `.get_mut(|val| { *val = whatever; })` to be a bit awkward. I changed this to be an if-let instead.
3. Fix a bug which was suppressing the structured suggestion for some mutations via the index operator on `HashMap`/`BTreeMap`.
r? estebank or reassign
borrowck diagnostics: make `add_move_error_suggestions` use the HIR rather than `SourceMap`
This PR aims to fix#132806 by rewriting `add_move_error_suggestions`[^1]. Previously, it manually scanned the source text to find a leading `&`, which isn't always going to produce a correct result (see: that issue). Admittedly, the HIR visitor in this PR introduces a lot of boilerplate, but hopefully the logic at its core isn't too complicated (I go over it in the comments). I also tried a simpler version that didn't use a HIR visitor and suggested adding `ref` always, but the `&ref x` suggestions really didn't look good. As a bonus for the added complexity though, it's now able to produce nice `&`-removing suggestions in more cases.
I tried to do this such that it avoids edition-dependent checks and its suggestions can be applied together with those from the match ergonomics 2024 migration lint. I haven't added tests for that since the details of match ergonomics 2024 are still being sorted out, but I can try if desired once that's finalized.
[^1]: In brief, it fires on patterns where users try to bind by-value in such a way that moves out of a reference to a non-Copy type (including slice references with non-copy elements). The suggestions are to change the binding's mode to be by-reference, either by removing[^2] an enclosing `&`/`&mut` or adding `ref` to the binding.
[^2]: Incidentally, I find the terminology of "consider removing the borrow" a bit confusing for a suggestion to remove a `&` pattern in order to make bindings borrow rather than move. I'm not sure what a good, concise way to explain that would be though, and that should go in a separate PR anyway.
- add a FIXME when looking for the region variance of unexpected regions
- drive-by: fix a doc comment link
- drive-by: simplify the variance match using exported variants instead
