Don't merge distinct impl candidates by bit-aloo · Pull Request #161386 · rust-lang/rust

bit-aloo · 2026-08-20T07:29:56Z

View all comments

closes: rust-lang/trait-system-refactor-initiative#35

r? lcnr

bit-aloo · 2026-08-20T07:31:18Z

I am not very confident about fallout bless's: 994cca7

lcnr · 2026-08-20T07:52:19Z

this fails to bootstrap, probably something related to specialization 🤔 😁 please minimize and look into why

lcnr · 2026-08-25T12:45:14Z

@rustbot author

lcnr · 2026-09-17T11:16:25Z

        }
    }

+    fn has_distinct_impl_candidates(


maybe instead have a try_merge_impl_candidates which returns a Option<CanonicalResponse<I>> and is None if there are no impls. That might make fn merge_trait_candidates nicer to read 🤔

fn has_distinct_impl_candidates is definitely kinda meh as it checks trait_is_marker in whcih case we allow distinct candidates.

also, please add comments where applicable

the behavior itself lgtm

View changes since the review

Added try_merge_impl_candidates here: 76f0d90, and updated the docs. Thanks.

lcnr · 2026-09-21T14:31:55Z

-            self.flounder(&candidates).map(|r| (r, None))
+        match self.try_merge_impl_candidates(trait_def_id, &candidates) {
+            Some(response) => Ok((response, Some(proven_via))),
+            None => self.flounder(&candidates).map(|r| (r, None)),
        }


hmm, can you do another change to the structure to make it easier to read (for me)

use

self.try_merge_candidates(&alias_bounds) { Ok((response, Some(TraitGoalProvenVia::AliasBound))) } else { Ok((self.bail_with_ambiguity(&alias_bounds), None)) };

in the TraitGoalProvenVia::ParamEnv arm. Though, maybe change this pattern to use a new function, like merge_candidates_or_bail_with_ambiguity(&alias_bounds, TraitGoalProvenVia::AliasBound).

View changes since the review

Added helpers here: 6302baf

lcnr · 2026-09-21T14:35:13Z

+        if !self.cx().trait_is_marker(trait_def_id) {
+            let impl_count = candidates
+                .iter()
+                .filter(|candidate| matches!(candidate.source, CandidateSource::Impl(_)))
+                .count();
+            let has_builtin_impl = candidates
+                .iter()
+                .any(|candidate| matches!(candidate.source, CandidateSource::BuiltinImpl(_)));
+            if impl_count + usize::from(has_builtin_impl) > 1 {
+                return None;
+            }
+        }
+
+        self.try_merge_candidates(candidates).map(|(response, _)| response)


could you change this to

if is_marker || candidates .iter() .all(|candidate| matches!(candidate.source, CandidateSource::BuiltinImpl(_))) { self.try_merge_candidates(candidates).map(|(response, _)| response) } else if candidates.len() > 1 { None } else { candidates.first().copied() or sth like this }

View changes since the review

lcnr

some impl style nits, then r=me

sry 😅 i hope you at least agree with me that it makes the code a bit nicer to read

View changes since this review

lcnr · 2026-09-22T11:43:24Z

+        trait_def_id: I::TraitId,
+        candidates: &[Candidate<I>],
+    ) -> Result<(CanonicalResponse<I>, Option<TraitGoalProvenVia>), NoSolution> {
+        match self.try_merge_impl_candidates(trait_def_id, candidates) {


can you add a debug assert that all candidates are actually impl candidates here?

View changes since the review

lcnr · 2026-09-22T11:45:32Z

+        let only_global_where_bounds = !candidates.is_empty()
+            && candidates
+                .iter()
+                .all(|c| matches!(c.source, CandidateSource::ParamEnv(ParamEnvSource::Global)));
+        if only_global_where_bounds {
+            Ok(self
+                .merge_candidates_or_bail_with_ambiguity(&candidates, TraitGoalProvenVia::ParamEnv))


that !candidates.is_empty() is weird imo 🤔

do we just want to slap a single if candidates.is_empty() to the top of this function and then entirely rip out flounder?

View changes since the review

This makes sense, the only thing flounder added over bail_with_ambiguity was the empty check, so I moved that to the top of merge_trait_candidates as a single early return. With that, every branch now ends in bail_with_ambiguity.

…nd add docs

…_or_flounder helpers

lcnr · 2026-09-24T06:24:28Z

nice, thanks

@bors r+ rollup

rust-bors · 2026-09-24T06:24:31Z

📌 Commit 0c27f4b has been approved by lcnr

It is now in the queue for this repository.

…uwer Rollup of 14 pull requests Successful merges: - #162976 (fix quadratic naming of duplicate sidebar links) - #161275 (Refactor `core::cmp::{smallest, largest}` & add `mir-opt` test) - #163143 (cg_llvm: Use fewer FFI calls to check the target CPU's features) - #163188 (Adjust for Arm64EC name mangling when checking for exported symbols) - #163211 (`rustc_builtin_macros` cleanup, part 6) - #161386 (Don't merge distinct impl candidates) - #162942 (Remove `StashKey::AssociatedTypeSuggestion`) - #163096 (Don't suggest `std::` rustfix paths in `#![no_std]` crates) - #163110 (Mark `std::os::wasip2` with correct doc-cfgs, mark as unstable) - #163185 (properly decrement available_depth on cycles and provisional cache hits) - #163214 (revert r14 register names for arm) - #163226 (miri subtree update) - #163228 (Add regression test for trait predicate with escaping bounds) - #163234 (`rustc_dump_symbol_name`: add demangling information as a note instead)

Rollup merge of #161386 - bit-aloo:2026-08-17-merge-impls, r=lcnr Don't merge distinct impl candidates closes: rust-lang/trait-system-refactor-initiative#35 r? lcnr

…uwer Rollup of 14 pull requests Successful merges: - rust-lang/rust#162976 (fix quadratic naming of duplicate sidebar links) - rust-lang/rust#161275 (Refactor `core::cmp::{smallest, largest}` & add `mir-opt` test) - rust-lang/rust#163143 (cg_llvm: Use fewer FFI calls to check the target CPU's features) - rust-lang/rust#163188 (Adjust for Arm64EC name mangling when checking for exported symbols) - rust-lang/rust#163211 (`rustc_builtin_macros` cleanup, part 6) - rust-lang/rust#161386 (Don't merge distinct impl candidates) - rust-lang/rust#162942 (Remove `StashKey::AssociatedTypeSuggestion`) - rust-lang/rust#163096 (Don't suggest `std::` rustfix paths in `#![no_std]` crates) - rust-lang/rust#163110 (Mark `std::os::wasip2` with correct doc-cfgs, mark as unstable) - rust-lang/rust#163185 (properly decrement available_depth on cycles and provisional cache hits) - rust-lang/rust#163214 (revert r14 register names for arm) - rust-lang/rust#163226 (miri subtree update) - rust-lang/rust#163228 (Add regression test for trait predicate with escaping bounds) - rust-lang/rust#163234 (`rustc_dump_symbol_name`: add demangling information as a note instead)

rustbot assigned lcnr Aug 20, 2026

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 Aug 20, 2026

This comment has been minimized.

Sign in to view

lcnr reviewed Aug 20, 2026

View reviewed changes

Comment thread compiler/rustc_next_trait_solver/src/solve/trait_goals.rs Outdated

lcnr reviewed Aug 20, 2026

View reviewed changes

Comment thread compiler/rustc_next_trait_solver/src/solve/trait_goals.rs Outdated

lcnr reviewed Aug 20, 2026

View reviewed changes

Comment thread compiler/rustc_next_trait_solver/src/solve/trait_goals.rs Outdated

rust-cloud-vms Bot force-pushed the 2026-08-17-merge-impls branch from 994cca7 to 36d20a8 Compare August 20, 2026 07:53

This comment has been minimized.

Sign in to view

rustbot 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-review Status: Awaiting review from the assignee but also interested parties. labels Aug 25, 2026

add trait_is_marker to Interner

7e43fd2

rust-cloud-vms Bot force-pushed the 2026-08-17-merge-impls branch from 36d20a8 to 083fa89 Compare September 3, 2026 12:52

bit-aloo requested a review from lcnr September 8, 2026 02:12

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Sep 8, 2026

lcnr reviewed Sep 17, 2026

View reviewed changes

Comment thread tests/ui/traits/next-solver/assembly/merge-marker-trait-impls.rs Outdated

lcnr reviewed Sep 17, 2026

View reviewed changes

rust-cloud-vms Bot force-pushed the 2026-08-17-merge-impls branch 2 times, most recently from 4fee497 to 76f0d90 Compare September 19, 2026 15:54

bit-aloo requested a review from lcnr September 19, 2026 15:57

rust-cloud-vms Bot force-pushed the 2026-08-17-merge-impls branch from 76f0d90 to 53e9bd8 Compare September 19, 2026 15:58

lcnr reviewed Sep 21, 2026

View reviewed changes

don't merge distinct impl candidates

819623a

drop specialized impls with equal inference constraints

f002f33

rust-cloud-vms Bot force-pushed the 2026-08-17-merge-impls branch from 53e9bd8 to 6302baf Compare September 22, 2026 03:33

bit-aloo requested a review from lcnr September 22, 2026 03:33

lcnr reviewed Sep 22, 2026

View reviewed changes

bit-aloo added 5 commits September 22, 2026 15:19

update fallout tests

d86cf85

add tests

0adc046

replace has_distinct_impl_candidates with try_merge_impl_candidates a…

781bc2b

…nd add docs

add merge_candidates_or_bail_with_ambiguity and merge_impl_candidates…

b49ec62

…_or_flounder helpers

Remove flounder from merge_trait_candidates

0c27f4b

rust-cloud-vms Bot force-pushed the 2026-08-17-merge-impls branch from 6302baf to 0c27f4b Compare September 22, 2026 15:19

bit-aloo requested a review from lcnr September 22, 2026 15:22

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-review Status: Awaiting review from the assignee but also interested parties. labels Sep 24, 2026

JonathanBrouwer mentioned this pull request Sep 24, 2026

Rollup of 14 pull requests #163244

Merged

rust-bors Bot merged commit 795a915 into rust-lang:main Sep 24, 2026
13 checks passed

rustbot added this to the 1.100.0 milestone Sep 24, 2026

rust-bors Bot pushed a commit that referenced this pull request Sep 24, 2026

Unrolled build for #161386 in rollup 163244

e7df56e

Rollup merge of #161386 - bit-aloo:2026-08-17-merge-impls, r=lcnr Don't merge distinct impl candidates closes: rust-lang/trait-system-refactor-initiative#35 r? lcnr

bit-aloo deleted the 2026-08-17-merge-impls branch September 24, 2026 14:22

Uh oh!

Conversation

bit-aloo commented Aug 20, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

bit-aloo commented Aug 20, 2026

Uh oh!

This comment has been minimized.

Uh oh!

Uh oh!

Uh oh!

lcnr commented Aug 20, 2026

Uh oh!

This comment has been minimized.

lcnr commented Aug 25, 2026

Uh oh!

Uh oh!

lcnr Sep 17, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

bit-aloo Sep 19, 2026

Choose a reason for hiding this comment

Uh oh!

lcnr Sep 21, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

bit-aloo Sep 22, 2026

Choose a reason for hiding this comment

Uh oh!

lcnr Sep 21, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

lcnr left a comment • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

lcnr Sep 22, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

lcnr Sep 22, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

bit-aloo Sep 22, 2026

Choose a reason for hiding this comment

Uh oh!

lcnr commented Sep 24, 2026

Uh oh!

rust-bors Bot commented Sep 24, 2026

Uh oh!

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

4 participants

bit-aloo commented Aug 20, 2026 •

edited by rustbot

Loading

lcnr Sep 17, 2026 •

edited by rustbot

Loading

lcnr Sep 21, 2026 •

edited by rustbot

Loading

lcnr Sep 21, 2026 •

edited by rustbot

Loading

lcnr left a comment •

edited by rustbot

Loading

lcnr Sep 22, 2026 •

edited by rustbot

Loading

lcnr Sep 22, 2026 •

edited by rustbot

Loading