Use special DefIds for aliases by ChayimFriedman2 · Pull Request #155981 · rust-lang/rust

ChayimFriedman2 · 2026-04-30T03:16:50Z

Renewal of #155025, after AliasTermKind was also ported.

Like we do for other things for better experience in rust-analyzer.

It's possible now that the AliasTyKind and AliasTermKind contains the DefId.

It does require a few try_into().unwrap()s since in the solver's consider_X_candidate() only get an untyped DefId. It's possible to reduce that considerably if we'd pass them the typed def id as a parameter, but I don't know what will be the impact on perf. Should I try to pursue that?

r? lcnr

lcnr · 2026-04-30T07:49:17Z

+            Err(TypeError::ProjectionMismatched(ExpectedFound::new(
+                a.def_id.into(),
+                b.def_id.into(),
+            )))


can you update TypeError instead?

View changes since the review

I can't because unlike the name the def id can be any alias; see <AliasTy as Relate>::relate().

ah, we also need to update the Relate<I> for AliasTy impl 👍 #154758

rustbot · 2026-05-04T03:27:29Z

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.

ChayimFriedman2 · 2026-05-04T03:30:28Z

@bors try @rust-timer queue

Use special DefIds for aliases

rust-bors · 2026-05-04T04:51:29Z

❗ There is currently no auto build in progress on this PR.

Hint: There is a pending try build on this PR. Maybe you meant to cancel it? You can do that using @bors try cancel.

ChayimFriedman2 · 2026-05-04T04:52:00Z

@bors try cancel

rust-bors · 2026-05-04T04:52:03Z

Try build cancelled. Cancelled workflows:

https://github.com/rust-lang/rust/actions/runs/25299606130

ChayimFriedman2 · 2026-05-04T04:53:39Z

How do I request a perf run with -Znext-solver=globally?

ChayimFriedman2 · 2026-05-04T14:52:18Z

Seems like there is no way to do that, so at least...

@bors try @rust-timer queue

Use special DefIds for aliases

rust-bors · 2026-05-04T17:00:27Z

☀️ Try build successful (CI)
Build commit: a2fdd5d (a2fdd5dc4e99453080f5bb2070e5875c96f113b2, parent: 1d72d7e8136faaebad3a85eeed432e6ea1b2ffab)

rust-timer · 2026-05-04T17:43:28Z

Finished benchmarking commit (a2fdd5d): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking means the PR may be perf-sensitive. Consider adding rollup=never if this change is not fit for rolling up.

@rustbot label: -S-waiting-on-perf -perf-regression

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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.1%	[-0.1%, -0.1%]	1

Max RSS (memory usage)

Results (secondary 3.0%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	3.0%	[0.4%, 8.2%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary -5.1%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.9%	[0.7%, 1.2%]	3
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-8.1%	[-23.7%, -0.4%]	6
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 494.198s -> 495.332s (0.23%)
Artifact size: 394.42 MiB -> 394.45 MiB (0.01%)

ChayimFriedman2 · 2026-05-04T18:58:32Z

Converted to draft because alongside this PR I'm also porting r-a to it, so I want to make sure I don't miss anything.

ChayimFriedman2 · 2026-05-04T21:20:29Z

Okay, I've verified that the changes here work for r-a.

lcnr · 2026-05-05T09:02:52Z

    pub(super) fn normalize_anon_const(
        &mut self,
        goal: Goal<I, ty::NormalizesTo<I>>,
+        def_id: I::UnevaluatedConstId,


hmm, passing the DefId in separately feels redundant 🤔

could you do

pub struct NormalizesTo<I, K = AliasTermKind<I>> { pub alias: AliasTerm<I, K>, pub term: Term<'tcx>, }

oh actually, that's just a very big change xd

we should have a single

struct Alias<I: Interner, K> { kind: K, args: I::GenericArgs, }

this is not something we should do in this PR but as a separate cleanup. so this is fine for now

View changes since the review

opened #156181 for this

ChayimFriedman2 · 2026-05-05T18:59:08Z

@lcnr Changed according to comments.

lcnr

some final nits 🙏

View changes since this review

lcnr · 2026-05-05T19:52:49Z

+        self.parent(def_id)
+    }
+
+    fn impl_or_trait_assoc_term_parent(self, def_id: Self::ImplOrDefAssocTyId) -> DefId {


could have a Self::ImplOrTraitId?

Could, but this is not needed.

lcnr · 2026-05-05T19:56:04Z


 #[rust_analyzer::prefer_underscore_import]
-pub trait DefId<I: Interner>: Copy + Debug + Hash + Eq + TypeFoldable<I> {
+pub trait DefId<I: Interner, Local = <I as Interner>::LocalDefId>:


feel like Local should be an associated type?

Yes but associated type aliases are unstable 😢

what do you mean with "associated type aliases" here?

Sorry, I meant associated type defaults.

Like we do for other things for better experience in rust-analyzer. It's possible now that the `AliasTyKind` and `AliasTermKind` contains the DefId. It does require a few `try_into().unwrap()`s since in the solver's `consider_X_candidate()` only get an untyped `DefId`. It's possible to reduce that considerably if we'd pass them the typed def id as a parameter, but I don't know what will be the impact on perf.

rustbot assigned lcnr Apr 30, 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 Apr 30, 2026

lcnr reviewed Apr 30, 2026

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Comment thread compiler/rustc_next_trait_solver/src/solve/normalizes_to/inherent.rs Outdated

lcnr reviewed Apr 30, 2026

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Comment thread compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs

lcnr reviewed Apr 30, 2026

View reviewed changes

Comment thread compiler/rustc_next_trait_solver/src/solve/normalizes_to/mod.rs

lcnr reviewed Apr 30, 2026

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Comment thread compiler/rustc_next_trait_solver/src/delegate.rs Outdated

ChayimFriedman2 force-pushed the alias-def-id branch from 81000a1 to 5e8cb76 Compare May 4, 2026 03:27

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rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label May 4, 2026

rust-bors Bot pushed a commit that referenced this pull request May 4, 2026

Auto merge of #155981 - ChayimFriedman2:alias-def-id, r=<try>

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Use special DefIds for aliases

ChayimFriedman2 removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label May 4, 2026

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rust-bors Bot pushed a commit that referenced this pull request May 4, 2026

Auto merge of #155981 - ChayimFriedman2:alias-def-id, r=<try>

a2fdd5d

Use special DefIds for aliases

ChayimFriedman2 added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label May 4, 2026

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rustbot removed the S-waiting-on-perf Status: Waiting on a perf run to be completed. label May 4, 2026

ChayimFriedman2 force-pushed the alias-def-id branch from 5e8cb76 to da673e8 Compare May 4, 2026 18:41

ChayimFriedman2 marked this pull request as draft May 4, 2026 18:58

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 May 4, 2026

ChayimFriedman2 force-pushed the alias-def-id branch 2 times, most recently from fa67dc2 to 598bf01 Compare May 4, 2026 21:20

ChayimFriedman2 marked this pull request as ready for review May 4, 2026 21:20

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 May 4, 2026