Rip out old solver coherence by sjwang05 · Pull Request #161491 · rust-lang/rust

sjwang05 · 2026-08-22T00:28:45Z

cc #t-types/call-for-participation > rip out old solver coherence support

Probably best reviewed commit-by-commit with ignore-whitespace.

#160668 replaced the last remaining place where old solver was still used by default in coherence with new solver. This PR removes code that was only used during coherence by the old solver, so now new-solver coherence is the only way to do coherence. So we don't confuse users, passing -Znext-solver=coherence and =no now do the exact same thing.

We also remove tracking intercrate ambiguity causes, since afaict the new solver uses a completely different path.

Everything else is either removing code that is now unreachable, or removing a condition that is now always true.

r? lcnr

rustbot · 2026-08-22T00:28:48Z

Some changes occurred in src/tools/compiletest

cc @jieyouxu

rustc-dev-guide is developed in its own repository. If possible, consider making this change to rust-lang/rustc-dev-guide instead.

cc @BoxyUwU, @tshepang

sjwang05 · 2026-08-22T00:30:20Z

-    selcx: &mut SelectionContext<'cx, 'tcx>,
+#[instrument(level = "debug", skip(infcx), ret)]
+fn impl_intersection_has_impossible_obligation<'a, 'tcx>(
+    infcx: &InferCtxt<'tcx>,


The selcx was only ever used with the old solver, hence we just pass the infcx directly now.

View changes since the review

rustbot · 2026-08-22T05:35:02Z

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

miri is developed in its own repository. If the Miri part of this change can be broken out, consider making this change to rust-lang/miri instead. However, if Miri needs adjusting for rustc changes, just ignore this message.

cc @rust-lang/miri

Rollup of 18 pull requests Successful merges: - #158102 (When compiling without a specified `--edition`, emit a message) - #162761 (Lower attributes for functions without bodies) - #163161 (implement FCW for `rustc_allowed_through_unstable_modules` items) - #163613 (Tweak the rendering of "not general enough" errors on the old trait solver) - #162062 (core: fix the docs of PanicInfo::location) - #163140 (document safety requirements for atomic intrinsics) - #163342 (Don't imply incorrect things about `Global` in the docs of `System`) - #163445 (Add safety comments for alloc::str) - #163503 (Mark Rc strong/weak count methods must_use) - #163548 (fs::set_permissions_nofollow: Android support, test cleanup) - #163585 ([triagebot] Create `debugger_visualizer` assign group) - #163597 (Add `SplitPathsRef` implementation for motor to make std build) - #163602 (Move media & home dirs tests to fs tests.) - #163667 (Finalize changes on expect messages for library/core/src/fmt/mod.rs) - #163682 ([rustdoc] Correctly link to (imported) enum variants with "jump to def") - #163683 (Fix GCC codegen backend comment in bootstrap) - #163703 (Move more `rustdoc-html tests` in the right location) - #163725 (some crashes fixed with next-solver) Failed merges: - #161491 (Rip out old solver coherence)

rustbot · 2026-10-04T11:25:07Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

sjwang05 · 2026-10-04T11:25:41Z

@bors r=khyperia

this time, surely.

rust-bors · 2026-10-04T11:25:44Z

📌 Commit 3a7efa2 has been tentatively approved by khyperia

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

Rip out old solver coherence cc [#t-types/call-for-participation > rip out old solver coherence support](https://rust-lang.zulipchat.com/#narrow/channel/618216-t-types.2Fcall-for-participation/topic/rip.20out.20old.20solver.20coherence.20support/with/613938355) Probably best reviewed commit-by-commit with ignore-whitespace. rust-lang#160668 replaced the last remaining place where old solver was still used by default in coherence with new solver. This PR removes code that was only used during coherence by the old solver, so now new-solver coherence is the *only* way to do coherence. So we don't confuse users, passing `-Znext-solver=coherence` and `=no` now do the exact same thing. We also remove tracking intercrate ambiguity causes, since afaict the new solver uses a completely different path. Everything else is either removing code that is now unreachable, or removing a condition that is now always true. r? lcnr

…uwer Rollup of 6 pull requests Successful merges: - #161491 (Rip out old solver coherence) - #163223 (regression test for async handler normalization ICE) - #163653 (callconv: mips64: Match GCC for alignment of 16-byte scalars) - #163742 (Add the `movdir64b` and `movdiri` x86 target features) - #163752 (Revert "implement PartialEq<VecDeque<U>> for Vec<T>, &[T], &mut [T], [T; N], &[T; N] and &mut [T; N]") - #163755 (fix -Z track-diagnostics for errors and lints emitted from rustc_attr_parsing)

rust-bors · 2026-10-04T19:05:03Z

⌛ Testing commit 3a7efa2 with merge da0a774...

Workflow: https://github.com/rust-lang/rust/actions/runs/37226905815

Rip out old solver coherence cc [#t-types/call-for-participation > rip out old solver coherence support](https://rust-lang.zulipchat.com/#narrow/channel/618216-t-types.2Fcall-for-participation/topic/rip.20out.20old.20solver.20coherence.20support/with/613938355) Probably best reviewed commit-by-commit with ignore-whitespace. #160668 replaced the last remaining place where old solver was still used by default in coherence with new solver. This PR removes code that was only used during coherence by the old solver, so now new-solver coherence is the *only* way to do coherence. So we don't confuse users, passing `-Znext-solver=coherence` and `=no` now do the exact same thing. We also remove tracking intercrate ambiguity causes, since afaict the new solver uses a completely different path. Everything else is either removing code that is now unreachable, or removing a condition that is now always true. r? lcnr

JonathanBrouwer · 2026-10-04T19:29:10Z

@bors yield
Yielding to enclosing rollup

rust-bors · 2026-10-04T19:29:13Z

Auto build was cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/37226905815

The next pull request likely to be tested is #163767.

…uwer Rollup of 9 pull requests Successful merges: - #161491 (Rip out old solver coherence) - #163533 (Run cg_gcc tests with the correct compiler) - #163223 (regression test for async handler normalization ICE) - #163367 (simplify rustc_log a bit) - #163622 (x86: c-variadic functions don't use registers with `-Zregparm`) - #163653 (callconv: mips64: Match GCC for alignment of 16-byte scalars) - #163665 (Parser: Refactor & better document `should_continue_as_assoc_expr` & `can_continue_expr_unambiguously`) - #163742 (Add the `movdir64b` and `movdiri` x86 target features) - #163752 (Revert "implement PartialEq<VecDeque<U>> for Vec<T>, &[T], &mut [T], [T; N], &[T; N] and &mut [T; N]")

Rollup merge of #161491 - sjwang05:no-old-coherence, r=khyperia Rip out old solver coherence cc [#t-types/call-for-participation > rip out old solver coherence support](https://rust-lang.zulipchat.com/#narrow/channel/618216-t-types.2Fcall-for-participation/topic/rip.20out.20old.20solver.20coherence.20support/with/613938355) Probably best reviewed commit-by-commit with ignore-whitespace. #160668 replaced the last remaining place where old solver was still used by default in coherence with new solver. This PR removes code that was only used during coherence by the old solver, so now new-solver coherence is the *only* way to do coherence. So we don't confuse users, passing `-Znext-solver=coherence` and `=no` now do the exact same thing. We also remove tracking intercrate ambiguity causes, since afaict the new solver uses a completely different path. Everything else is either removing code that is now unreachable, or removing a condition that is now always true. r? lcnr

rust-timer · 2026-10-05T05:34:32Z

Note

This PR was benchmarked as part of triage of its containing rollup: triage URL.

Finished benchmarking commit (8e00340): comparison URL.

Overall result: ❌ regressions - please read:

Our benchmarks found a performance regression caused by this PR.
This might be an actual regression, but it can also be just noise.

Next Steps:

If the regression was expected or you think it can be justified,
please write a comment with sufficient written justification, and add
@rustbot label: +perf-regression-triaged to it, to mark the regression as triaged.
If you think that you know of a way to resolve the regression, try to create
a new PR with a fix for the regression.
If you do not understand the regression or you think that it is just noise,
you can ask the @rust-lang/wg-compiler-performance working group for help (members of this group
were already notified of this PR).

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.1%, 0.7%]	13
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.1%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[-0.1%, 0.7%]	15

Max RSS (memory usage)

This perf run didn't have relevant results for this metric.

Cycles

This perf run didn't have relevant results for this metric.

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: missing data
Artifact size: 408.65 MiB -> 408.69 MiB (0.01%)

JonathanBrouwer · 2026-10-05T07:05:02Z

This seems to have caused a perf regression, any idea why?

panstromek · 2026-10-05T07:12:06Z

This looks like some kind of inlining difference to me, most of the diff comes from these two:

  Ir______________________________  function:file

>  157,186,026  (1670.7%, 1670.7%)  <rustc_trait_selection::traits::select::SelectionContext>::candidate_from_obligation_no_cache:???

> -143,087,402 (-1520.8%,  149.8%)  <rustc_trait_selection::traits::select::SelectionContext>::assemble_candidates_from_impls:???

IIUC this touches some of the hot paths in obligation processing?

khyperia · 2026-10-05T08:56:39Z

The diff to candidate_from_obligation_no_cache in this PR is just deleting fully dead code (a LOT of dead code, and code that I'm guessing is dead in a way the optimizer cannot actually see is dead). Yet, it's +1670.7%.

assemble_candidates_from_impls is practically untouched in this PR (changes an enum variant match arm to panic instead of continuing), yet has -1520.8% - it does call candidate_from_obligation_no_cache though (a couple layers deep),

My guess would be inlining shenanigans too, my understanding is this PR doesn't change any behavior whatsoever - just nuking dead code, adding unreachable panics in unreachable places instead of continuing, etc.

JonathanBrouwer · 2026-10-05T12:27:03Z

changes an enum variant match arm to panic instead of continuing

This could increase code size, as a panic, especially one with a format str arg, is more instructions than a continue. This makes inlining less likely and worsens cache effiency.
Maybe this is related? This is just me theorizing

rustbot assigned lcnr Aug 22, 2026

sjwang05 commented Aug 22, 2026

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rust-bors Bot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Oct 4, 2026

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sjwang05 added 2 commits October 4, 2026 04:19

yeet old solver coherence

add63e4

bless tests

3a7efa2

sjwang05 force-pushed the no-old-coherence branch from 46d0705 to 3a7efa2 Compare October 4, 2026 11:25

rust-bors Bot added S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 4, 2026

JonathanBrouwer mentioned this pull request Oct 4, 2026

Rollup of 6 pull requests #163763

Closed

JonathanBrouwer mentioned this pull request Oct 4, 2026

Rollup of 9 pull requests #163767

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rust-bors Bot merged commit cd3e3d5 into rust-lang:main Oct 4, 2026
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rustbot added this to the 1.101.0 milestone Oct 4, 2026

rustbot added the perf-regression Performance regression. label Oct 5, 2026

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Overall result: ❌ regressions - please read:

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