This is a tracking issue for -Znext-solver=coherence, and enabling the new solver in coherence mode. The -Znext-solver=coherence flag can be used to only enable the new solver for coherence, or -Znext-solver can be used to enable the new solver everywhere (see #107374 instead). This work is lead by the @rust-lang/initiative-trait-system-refactor.

About tracking issues

Tracking issues are used to record the overall progress of implementation. They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions. A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature. Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Implementation history

• new solver cleanup + implement coherence #109447
• Warn on inductive cycle in coherence leading to impls being considered not overlapping #114023
• Add -Ztrait-solver=next-coherence #112122
• stabilize -Znext-solver=coherence #121848
• Revert "Stabilize -Znext-solver=coherence" #130249

Crater runs

• [Experiment] Crater run with -Ztrait-solver=next-coherence #113895
• [experiment] -Ztrait-solver=next-coherence but hacks that make things not fail anymore #113991
• [crater] next-coherence crater part 2 #116357

Issues

• incorrect occurs check in projections generalization is incomplete for aliases trait-system-refactor-initiative#8 (fixed by generalize: handle occurs check failure in aliases #117088)
• intercrate ambiguity causes/proof tree work
• overflow handling
  • test breakage -Ztrait-solver=next-coherence: issue-90662-projection-caching.rs breaks trait-system-refactor-initiative#70
  • typenum perrrf typenum requires non-fatal overflow trait-system-refactor-initiative#73
• normalize for diagnostics, add new folder to TypeErrCtxt or sth
• experiment normalizing both sides in AliasRElate before equating (perf)
• Blog post asking for testing and experimentation https://blog.rust-lang.org/inside-rust/2023/12/22/trait-system-refactor-initiative.html

• Investigate all the issues of the crater runs
• opaque auto trait leakage in coherence trait-system-refactor-initiative#65
• ambiguity on inductive cycles trait-system-refactor-initiative#20
  • Warn on inductive cycle in coherence leading to impls being considered not overlapping #114023
  • Make inductive cycles in coherence ambiguous always #118649
• Ambiguity in assemble_candidates_after_normalizing_self_ty causes coherence error trait-system-refactor-initiative#52 accepted breakage, caused by a bug of the old solver coherence incorrectly considers unnormalizable_projection: Trait to not hold even if it could #114061
• Trait ref is not considered knowable due to normalization trait-system-refactor-initiative#51
  • normalize in trait_ref_is_knowable in new solver #114457
• work towards stabilization, writing an FCP proposal and documenting the solver https://rust-lang.zulipchat.com/#narrow/stream/364551-t-types.2Ftrait-system-refactor/topic/coherence.20stabilization
• stabilization PR in stabilize -Znext-solver=coherence #121848
• reverted in Revert "Stabilize -Znext-solver=coherence" #130249 due to Regression in nightly-2024-09-07: hang affecting nalgebra #130056
• restabilize coherence