cycle handling: mirror old solver by lcnr · Pull Request #163496 · rust-lang/rust

lcnr · 2026-09-29T16:41:57Z

I hate other people being right :<

When stabilizing the new solver, we really want to minimize the amount of things we're stabilizing. This far more closely matches the old solver while still being somewhat principled. I do think the approach we had before this PR is closer to the final state we want, but I am far from confident that this is the case and this gives us more room to experiment until then.

The rules are as follows: a cycle is coinductive if there's at least one "coinductive trait goal" involved and the only other involved goals are Projection goals.

r? types cc @jackh726

rustbot · 2026-09-29T16:42:00Z

Some changes occurred to the core trait solver

cc @rust-lang/initiative-trait-system-refactor

jackh726 · 2026-09-29T19:12:05Z

This seems fairly straightforward. The only minor comment: I wonder if it's worth to split this into two commits: one "prep" commit and one that actually changes the behavior.

r? jackh726
r=me

lcnr · 2026-09-30T06:56:52Z

The only minor comment: I wonder if it's worth to split this into two commits: one "prep" commit and one that actually changes the behavior.

would generally be yeah, rn that would require temporary changes to step_kind_to_nested, so it doesn't feel worth it to me. The only behavior change is in fn step_kind_to_nested.

Also added a regression test for one way the old and new solver diverge wrt cycle handling (found via an llm)

@bors r+ rollup

rust-bors · 2026-09-30T06:56:55Z

📌 Commit 14d09db has been approved by lcnr

It is now in the queue for this repository.

lcnr · 2026-09-30T06:56:59Z

@bors r=jackh726

rust-bors · 2026-09-30T06:57:03Z

📌 Commit 14d09db has been approved by jackh726

It is now in the queue for this repository.

rust-bors · 2026-09-30T06:57:51Z

⚠️ A new commit 582ece71ba43325ffd90211e5a8af97565e6e2b2 was pushed.

This pull request was unapproved.

lcnr · 2026-09-30T06:58:00Z

@bors r=jackh726 rollup

rust-bors · 2026-09-30T06:58:04Z

📌 Commit 582ece7 has been tentatively approved by jackh726

It will be put into the queue for this repository once PR CI succeeds.

rust-bors · 2026-09-30T07:26:32Z

⚠️ A new commit 7238eac0f61995d51d0fc5165535f13dbfc457d1 was pushed.

This pull request was unapproved.

lcnr · 2026-09-30T07:26:32Z

@bors r=jackh726 rollup

rust-bors · 2026-09-30T08:14:28Z

⚠️ A new commit 654d6642d918149742558744798c2cdce3996999 was pushed.

This pull request was unapproved.

lcnr · 2026-09-30T08:14:37Z

@bors r=jackh726 rollup

rust-bors · 2026-09-30T08:14:41Z

📌 Commit 654d664 has been tentatively approved by jackh726

It will be put into the queue for this repository once PR CI succeeds.

@jackh726

…h726 cycle handling: mirror old solver I hate other people being right :< When stabilizing the new solver, we really want to minimize the amount of things we're stabilizing. This far more closely matches the old solver while still being somewhat principled. I do think the approach we had before this PR is closer to the final state we want, but I am far from confident that this is the case and this gives us more room to experiment until then. The rules are as follows: a cycle is coinductive if there's at least one "coinductive trait goal" involved and the only other involved goals are `Projection` goals. r? types cc @jackh726

@jackh726

…h726 cycle handling: mirror old solver I hate other people being right :< When stabilizing the new solver, we really want to minimize the amount of things we're stabilizing. This far more closely matches the old solver while still being somewhat principled. I do think the approach we had before this PR is closer to the final state we want, but I am far from confident that this is the case and this gives us more room to experiment until then. The rules are as follows: a cycle is coinductive if there's at least one "coinductive trait goal" involved and the only other involved goals are `Projection` goals. r? types cc @jackh726

@jackh726

…h726 cycle handling: mirror old solver I hate other people being right :< When stabilizing the new solver, we really want to minimize the amount of things we're stabilizing. This far more closely matches the old solver while still being somewhat principled. I do think the approach we had before this PR is closer to the final state we want, but I am far from confident that this is the case and this gives us more room to experiment until then. The rules are as follows: a cycle is coinductive if there's at least one "coinductive trait goal" involved and the only other involved goals are `Projection` goals. r? types cc @jackh726

…uwer Rollup of 18 pull requests Successful merges: - #163532 (rustc_codegen_cranelift subtree update) - #163534 (miri subtree update) - #163279 (const and NonZero impl for clamp_magnitude()) - #162900 (Some refactorings around metadata encoding) - #163455 (Don't use the metadata based crate_hash for rustdoc runs) - #163483 (Bump bootstrap compiler to 1.100.0 beta) - #159798 (Attribute documentation for cfg_attr) - #163368 (Don't build format string suggestions from `concat!` offsets) - #163375 (Update expect messages in library/std/src/os/unix/net/ following Rust's `expect` guidance) - #163405 (Remove some #[linkage] options) - #163470 (Miscellaneous attr error stuff) - #163489 (expose Rc::is_unique) - #163492 (x86 and x86_64: cleanup some callconv code) - #163496 (cycle handling: mirror old solver) - #163509 (Use more default field values in `Resolver`) - #163519 (Forbid `Reborrow` impls for types with destructors) - #163520 (Cast cleanups) - #163524 (Document `Result` case for the `arena_cache` query modifier) Failed merges: - #163547 ([beta] rustfmt backport)

@jackh726

…h726 cycle handling: mirror old solver I hate other people being right :< When stabilizing the new solver, we really want to minimize the amount of things we're stabilizing. This far more closely matches the old solver while still being somewhat principled. I do think the approach we had before this PR is closer to the final state we want, but I am far from confident that this is the case and this gives us more room to experiment until then. The rules are as follows: a cycle is coinductive if there's at least one "coinductive trait goal" involved and the only other involved goals are `Projection` goals. r? types cc @jackh726

…uwer Rollup of 18 pull requests Successful merges: - #163532 (rustc_codegen_cranelift subtree update) - #163534 (miri subtree update) - #163279 (const and NonZero impl for clamp_magnitude()) - #162900 (Some refactorings around metadata encoding) - #163455 (Don't use the metadata based crate_hash for rustdoc runs) - #159798 (Attribute documentation for cfg_attr) - #162921 (make mips64 `Complex` GCC-compatible) - #163368 (Don't build format string suggestions from `concat!` offsets) - #163375 (Update expect messages in library/std/src/os/unix/net/ following Rust's `expect` guidance) - #163405 (Remove some #[linkage] options) - #163470 (Miscellaneous attr error stuff) - #163489 (expose Rc::is_unique) - #163492 (x86 and x86_64: cleanup some callconv code) - #163496 (cycle handling: mirror old solver) - #163509 (Use more default field values in `Resolver`) - #163519 (Forbid `Reborrow` impls for types with destructors) - #163520 (Cast cleanups) - #163524 (Document `Result` case for the `arena_cache` query modifier)

Rollup of 20 pull requests Successful merges: - #163279 (const and NonZero impl for clamp_magnitude()) - #163081 (Provide more context on "not general enough" error) - #163455 (Don't use the metadata based crate_hash for rustdoc runs) - #159798 (Attribute documentation for cfg_attr) - #162921 (make mips64 `Complex` GCC-compatible) - #163368 (Don't build format string suggestions from `concat!` offsets) - #163375 (Update expect messages in library/std/src/os/unix/net/ following Rust's `expect` guidance) - #163470 (Miscellaneous attr error stuff) - #163489 (expose Rc::is_unique) - #163492 (x86 and x86_64: cleanup some callconv code) - #163496 (cycle handling: mirror old solver) - #163509 (Use more default field values in `Resolver`) - #163519 (Forbid `Reborrow` impls for types with destructors) - #163520 (Cast cleanups) - #163524 (Document `Result` case for the `arena_cache` query modifier) - #163546 (PassWrapper: adapt for new PassPlugin load method) - #163551 (Add libs-nominated triagebot config) - #163559 (Suggest `#[unsafe(no_mangle)]` for entry points in `no_std` binaries) - #163564 (rustc-dev-guide subtree update) - #163568 (remove dead cfg_select! arm)

@jackh726

Rollup merge of #163496 - lcnr:weaken-cycle-handling, r=jackh726 cycle handling: mirror old solver I hate other people being right :< When stabilizing the new solver, we really want to minimize the amount of things we're stabilizing. This far more closely matches the old solver while still being somewhat principled. I do think the approach we had before this PR is closer to the final state we want, but I am far from confident that this is the case and this gives us more room to experiment until then. The rules are as follows: a cycle is coinductive if there's at least one "coinductive trait goal" involved and the only other involved goals are `Projection` goals. r? types cc @jackh726

Rollup of 20 pull requests Successful merges: - rust-lang/rust#163279 (const and NonZero impl for clamp_magnitude()) - rust-lang/rust#163081 (Provide more context on "not general enough" error) - rust-lang/rust#163455 (Don't use the metadata based crate_hash for rustdoc runs) - rust-lang/rust#159798 (Attribute documentation for cfg_attr) - rust-lang/rust#162921 (make mips64 `Complex` GCC-compatible) - rust-lang/rust#163368 (Don't build format string suggestions from `concat!` offsets) - rust-lang/rust#163375 (Update expect messages in library/std/src/os/unix/net/ following Rust's `expect` guidance) - rust-lang/rust#163470 (Miscellaneous attr error stuff) - rust-lang/rust#163489 (expose Rc::is_unique) - rust-lang/rust#163492 (x86 and x86_64: cleanup some callconv code) - rust-lang/rust#163496 (cycle handling: mirror old solver) - rust-lang/rust#163509 (Use more default field values in `Resolver`) - rust-lang/rust#163519 (Forbid `Reborrow` impls for types with destructors) - rust-lang/rust#163520 (Cast cleanups) - rust-lang/rust#163524 (Document `Result` case for the `arena_cache` query modifier) - rust-lang/rust#163546 (PassWrapper: adapt for new PassPlugin load method) - rust-lang/rust#163551 (Add libs-nominated triagebot config) - rust-lang/rust#163559 (Suggest `#[unsafe(no_mangle)]` for entry points in `no_std` binaries) - rust-lang/rust#163564 (rustc-dev-guide subtree update) - rust-lang/rust#163568 (remove dead cfg_select! arm)

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. WG-trait-system-refactor The Rustc Trait System Refactor Initiative (-Znext-solver) labels Sep 29, 2026

rustbot assigned spastorino Sep 29, 2026

cycle handling: mirror old solver

14d09db

lcnr force-pushed the weaken-cycle-handling branch from 9ed2cc0 to 14d09db Compare September 29, 2026 16:49

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lcnr force-pushed the weaken-cycle-handling branch from 582ece7 to 7238eac Compare September 30, 2026 07:26

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lcnr force-pushed the weaken-cycle-handling branch from 7238eac to 654d664 Compare September 30, 2026 08:14

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JonathanBrouwer mentioned this pull request Sep 30, 2026

Rollup of 14 pull requests #163540

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JonathanBrouwer mentioned this pull request Sep 30, 2026

Rollup of 13 pull requests #163542

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JonathanBrouwer mentioned this pull request Sep 30, 2026

Rollup of 18 pull requests #163560

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JonathanBrouwer mentioned this pull request Sep 30, 2026

Rollup of 18 pull requests #163566

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jhpratt mentioned this pull request Oct 1, 2026

Rollup of 20 pull requests #163583

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rust-bors Bot merged commit 77efc48 into rust-lang:main Oct 1, 2026
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rustbot added this to the 1.101.0 milestone Oct 1, 2026

amirHdev mentioned this pull request Oct 3, 2026

Handle recursive TAIT item bounds #160711

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github-actions Bot mentioned this pull request Oct 3, 2026

Toolchain upgrade to nightly-2026-10-09 flux-rs/flux#1811

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LorrensP-2158466 mentioned this pull request Oct 6, 2026

Do not retain Normalization goal errors in nested goals for BestObligationVisitor:: non_trivial_candidates #162443

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lcnr mentioned this pull request Oct 8, 2026

Stabilize the next-generation trait solver rust-lang/goals#113

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