Scalable Polonius support on nightly
| Metadata | |
|---|---|
| Owner(s) | Rémy Rakic |
| Teams | types |
| Status | Proposed |
Summary
Keep working on implementing a native rustc version of the Polonius next generation borrow checking algorithm, that would scale better than the previous datalog implementation, continuing from the 2024h2 goal.
Motivation
Polonius is an improved version of the borrow checker that resolves common limitations of the borrow checker and which is needed to support future patterns such as "lending iterators" (see #92985). Its model also prepares us for further improvements in the future.
Some support exists on nightly, but this older prototype has no path to stabilization due to scalability issues. We need an improved architecture and implementation to fix these issues.
The next six months
- Complete the ongoing work to land polonius on nightly
The "shiny future" we are working towards
Stable support for Polonius.
Ownership and team asks
Owner: lqd
Other support provided by Amanda Stjerna as part of her PhD.
| Subgoal | Owner(s) or team(s) | Notes |
|---|---|---|
| Design review | Niko Matsakis | |
| Implementation | Rémy Rakic, Amanda Stjerna | |
| Standard reviews |  types | Matthew Jasper |
Support needed from the project
We expect most support to be needed from the types team, for design, reviews, interactions with the trait solver, and so on. We expect Niko Matsakis, leading the polonius working group and design, to provide guidance and design time, and Michael Goulet and Matthew Jasper to help with reviews.
Outputs and milestones
Outputs
Nightly implementation of polonius that passes NLL problem case #3 and accepts lending iterators (#92985).
Performance should be reasonable enough that we can run the full test suite, do crater runs, and test it on CI, without significant slowdowns. We do not expect to be production-ready yet by then, and therefore the implementation would still be gated under a nightly -Z feature flag.
As our model is a superset of NLLs, we expect little to no diagnostics regressions, but improvements would probably still be needed for the new errors.
Milestones
Note: some of these are currently being worked on as part of the 2024h2 goal, and could be completed before the 2025h1 period.
| Milestone | Contributor | Notes |
|---|---|---|
| Factoring out higher-ranked concerns from the main path | Amanda Stjerna | |
↳ [x] rewrite invalid universe constraints with outlives 'static constraints | PR 123720 | |
| ↳ [ ] completely remove placeholders | in progress PR 130227 | |
| Location-sensitive prototype on nightly | Rémy Rakic | in progress |
| ↳ [x] create structures for location-dependent outlives constraints | ||
| ↳ [x] build new constraint graph from typeck constraints and liveness constraints | ||
| ↳ [x] update NLLs for required changes to local & region liveness, loan liveness & loan scopes, (possibly unreachable) kills, bidirectional traversal & active loans | ||
| ↳ [ ] limit regressions about diagnostics when using the new constraints on diagnostics tailored to the old constraints | ||
↳ [ ] land on nightly under a -Z flag | ||
| [x] Debugging / dump tool for analysis of location-sensitive analysis | Rémy Rakic | |
| [ ] Tests and validation of location-sensitive Polonius | Rémy Rakic | |
| ↳ [ ] make the full test suite pass | in progress | |
| ↳ [ ] do a crater run for assertions and backwards-compatibility | ||
| ↳ [ ] expand test suite with tests about the new capabilities | ||
| [ ] Location-sensitive pass on nightly, tested on CI | Rémy Rakic |