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Don't merge distinct impl candidates - #161386

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bit-aloo:2026-08-17-merge-impls
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Don't merge distinct impl candidates#161386
bit-aloo wants to merge 3 commits into
rust-lang:mainfrom
bit-aloo:2026-08-17-merge-impls

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@bit-aloo

@bit-aloo bit-aloo commented Aug 20, 2026

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@rustbot 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
@bit-aloo

bit-aloo commented Aug 20, 2026

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

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lcnr
lcnr reviewed Aug 20, 2026
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Comment thread compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
trait_def_id: I::TraitId,
candidates: &[Candidate<I>],
) -> Option<(CanonicalResponse<I>, MergeCandidateInfo)> {
let mut impls = candidates.iter().filter(|c| matches!(c.source, CandidateSource::Impl(_)));

@lcnr lcnr Aug 20, 2026 •
edited by rustbot
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should do the same with builtin impls 😁

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lcnr
lcnr reviewed Aug 20, 2026
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Comment thread compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
// remaining candidates.
let proven_via = if candidates
.iter()
.all(|c| matches!(c.source, CandidateSource::ParamEnv(ParamEnvSource::Global)))

@lcnr lcnr Aug 20, 2026 •
edited by rustbot
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effectively this match is a check whether we should merge candidates or not 😁

because these are all either impl, builtin impl, or global where-bounds

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lcnr
lcnr reviewed Aug 20, 2026
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Comment thread compiler/rustc_next_trait_solver/src/solve/trait_goals.rs
if impls.nth(1).is_some() && !self.cx().trait_is_marker(trait_def_id) {
return None;
}
self.try_merge_candidates(candidates)

@lcnr lcnr Aug 20, 2026 •
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why do we need to call try_merge_candidates if impls are Some?

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@lcnr

lcnr commented Aug 20, 2026

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

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

rust-log-analyzer commented Aug 20, 2026

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The job x86_64-gnu-next-trait-solver-polonius failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot) 
    |
275 |         let len = ZipImpl::size_hint(&self).0;
    |                   ^^^^^^^^^^^^^^^^^^ ----- type must be known at this point
    |                   |
    |                   cannot infer type of the type parameter `A` declared on the trait `ZipImpl`
    |
note: multiple `impl`s satisfying `Zip<A, B>: ZipImpl<_, _>` found
   --> library/core/src/iter/adapters/zip.rs:206:1
    |
206 | / impl<A, B> ZipImpl<A, B> for Zip<A, B>
207 | | where
208 | |     A: Iterator,
209 | |     B: Iterator,
    | |________________^
...
248 | / impl<A, B> ZipImpl<A, B> for Zip<A, B>
249 | | where
250 | |     A: TrustedRandomAccessNoCoerce + Iterator,
251 | |     B: TrustedRandomAccessNoCoerce + Iterator,
    | |______________________________________________^
help: consider specifying the generic arguments
    |
275 |         let len = ZipImpl::<A, B>::size_hint(&self).0;
    |                          ++++++++

error[E0283]: type annotations needed
   --> library/core/src/iter/adapters/zip.rs:281:39
    |
281 |                 accum = f(accum, self.get_unchecked(i));
    |                                       ^^^^^^^^^^^^^
    |
note: multiple `impl`s satisfying `Zip<A, B>: ZipImpl<_, _>` found
   --> library/core/src/iter/adapters/zip.rs:206:1
    |
206 | / impl<A, B> ZipImpl<A, B> for Zip<A, B>
207 | | where
208 | |     A: Iterator,
209 | |     B: Iterator,
    | |________________^
...
248 | / impl<A, B> ZipImpl<A, B> for Zip<A, B>
249 | | where
250 | |     A: TrustedRandomAccessNoCoerce + Iterator,
251 | |     B: TrustedRandomAccessNoCoerce + Iterator,
    | |______________________________________________^
help: try using a fully qualified path to specify the expected types
    |
281 -                 accum = f(accum, self.get_unchecked(i));
281 +                 accum = f(accum, <Zip<A, B> as ZipImpl<A, B>>::get_unchecked(&mut self, i));
    |

For more information about this error, try `rustc --explain E0283`.
[RUSTC-TIMING] core test:false 20.128
error: could not compile `core` (lib) due to 2 previous errors

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For more information how to resolve CI failures of this job, visit this link.

@lcnr

lcnr commented Aug 25, 2026

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@rustbot 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
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S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. 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)

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overlapping impl candidates are no longer an error

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