GCI: Imply outlives-bounds on free (generic) const items by fmease · Pull Request #143029 · rust-lang/rust

fmease · 2025-06-25T21:08:45Z

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Part of #113521.

fmease · 2025-06-25T21:09:02Z

@bors2 try @rust-timer queue

GCI: Imply outlives-bounds on free (generic) const items

rust-bors · 2025-06-25T21:09:07Z

⌛ Trying commit 5aa3d2a with merge eb427c5…

To cancel the try build, run the command @bors2 try cancel.

rust-bors · 2025-06-25T23:39:56Z

☀️ Try build successful (CI)
Build commit: eb427c5 (eb427c5414bee321329c635f289f46816100d758, parent: d14d202262d13df896b0c624b0cec6e4bfde631a)

rust-timer · 2025-06-27T06:55:56Z

Finished benchmarking commit (eb427c5): comparison URL.

Overall result: ❌ regressions - please read the text below

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@bors rollup=never
@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)	0.2%	[0.2%, 0.2%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.2%	[0.2%, 0.2%]	3

Max RSS (memory usage)

This benchmark run did not return any relevant results for this metric.

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 689.033s -> 689.561s (0.08%)
Artifact size: 372.09 MiB -> 372.11 MiB (0.01%)

fmease · 2025-06-27T20:12:37Z

(All of these failures are cycle errors that now occur when we try to recover from the user using _ in / omitting the type of consts, this "niceness" really is the bane of my existence)

fmease · 2025-06-27T20:29:18Z

@bors2 try parent=last @rust-timer queue

rust-bors · 2025-06-27T20:29:22Z

⌛ Trying commit c2b8a0b with merge 25444d3…

To cancel the try build, run the command @bors2 try cancel.

GCI: Imply outlives-bounds on free (generic) const items Part of #113521.

rust-bors · 2025-06-27T22:59:43Z

☀️ Try build successful (CI)
Build commit: 25444d3 (25444d3ddb1285d7ea04f0b89fdf4318a474d5df, parent: d14d202262d13df896b0c624b0cec6e4bfde631a)

rust-timer · 2025-06-28T00:40:08Z

Finished benchmarking commit (25444d3): comparison URL.

Overall result: no relevant changes - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (secondary 2.6%)

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)	2.6%	[2.5%, 2.7%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 689.033s -> 690.033s (0.15%)
Artifact size: 372.09 MiB -> 372.08 MiB (-0.00%)

rustbot · 2026-03-01T12:15:59Z

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol, @tshepang

fmease · 2026-03-01T12:19:55Z

compiler/rustc_hir_analysis/src/outlives/mod.rs

+    if tcx.generics_of(item_def_id).is_empty() {
+        // Fast path.
+        return &[];
    }


This fast path allowed me to turn a perf regression (#143029 (comment)) back into neutral state (#143029 (comment)).

fmease · 2026-03-01T12:31:56Z

Needs a trivial rebase but it's still ready for review. I'm afk now.

rustbot · 2026-03-01T18:23:04Z

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.

fmease · 2026-03-02T06:19:10Z

I forgot that I still had to break one more kind of query cycle. CI should be fixed now 🤞

fmease · 2026-03-02T06:22:43Z

tests/ui/generic-const-items/implied-outlives-bounds-1.rs

Right, I completely forgot about implementing that part of implied bounds back in June 2025. This PR should definitely be blocked on me implementing that but feel free to review the other parts already.

It should hopefully be straightforward to implement modulo perf, I just need to visit & handle ConstKind::Unevaluated in insert_outlives_predicate.

Edit: Actually it's not necessarily blocking since this is only observable under mGCA/GCE? I might just slap //@ known-bug: unknown on it 🤷

i think not even GCE as there you'll have anon consts with the same predicates as the item its inside of. mGCA should be the only way to get a const item directly used in a type signature which has clauses that are actual useful to derive implied bounds from

BoxyUwU · 2026-03-05T15:29:08Z

I am... unsure of this 🤔 I need to think a bit about this conceptually.

BoxyUwU · 2026-03-05T15:39:33Z

Is there a test that demonstrates the (lack of) implied bounds for associated consts?

fmease · 2026-03-05T15:42:47Z

Is there a test that demonstrates the (lack of) implied bounds for associated consts?

In tests/ui/generic-const-items/reference-outlives-referent.rs I've added some cases. However, they only exercise RegionOutlives (because that test file is a preexisting regression test for a regionck issue). I should probably add a dedicated test for this that (also) exercises TypeOutlives.

BoxyUwU · 2026-03-13T15:08:17Z

compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs

+/// Infer outlives-predicates for the items in the local crate.
 pub(super) fn infer_predicates(
    tcx: TyCtxt<'_>,
 ) -> FxIndexMap<DefId, ty::EarlyBinder<'_, RequiredPredicates<'_>>> {


can we rename this stuff to be infer_outlives_predicates and RequiredOutlivesPredicates lol

BoxyUwU · 2026-03-13T15:09:59Z

compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs

+                //
+                // This is correct as we only ever imply outlives-predicates where the outlived
+                // region is early-bound ... for which the item must have generic parameters.
+                DefKind::Const { .. } if !tcx.generics_of(item_did).is_empty() => {


I could imagine for mgca wanting to imply bounds from the rhs too. e.g. const FOO<'a>: () = BAR::<'a> where BAR bounds 'a: 'static

BoxyUwU · 2026-03-13T15:16:16Z

compiler/rustc_hir_analysis/src/outlives/implicit_infer.rs

+                //
+                // This is correct as we only ever imply outlives-predicates where the outlived
+                // region is early-bound ... for which the item must have generic parameters.
+                DefKind::Const { .. } if !tcx.generics_of(item_did).is_empty() => {


where does that perf regression you were talking about actually come from? is it having this query return more stuff? would feature gating this match arm improve things too?

also kind of generally wondering if itd be "safer" to gate this behind GCI to avoid any accidents but that seems.. unlikely to happen

BoxyUwU · 2026-03-13T15:18:35Z

OK makes sense to me 🤔

BoxyUwU · 2026-03-13T15:19:23Z

can you record an item on the tracking issue for GCI that we should think about implied bounds before stabilization (i.e. what kinds we want)

rust-bors · 2026-03-15T20:48:57Z

☔ The latest upstream changes (presumably #153919) made this pull request unmergeable. Please resolve the merge conflicts.

fmease assigned BoxyUwU Jun 25, 2025

fmease added the F-generic_const_items `#![feature(generic_const_items)]` label Jun 25, 2025

fmease added this to Generic Const Items (GCI) Jun 25, 2025

rustbot added A-rustc-dev-guide Area: rustc-dev-guide T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 25, 2025

fmease moved this to In Progress in Generic Const Items (GCI) Jun 25, 2025

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rust-bors bot added a commit that referenced this pull request Jun 25, 2025

Auto merge of #143029 - fmease:gci-imply, r=<try>

eb427c5

GCI: Imply outlives-bounds on free (generic) const items

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 25, 2025

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rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Jun 27, 2025

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rust-bors bot added a commit that referenced this pull request Jun 27, 2025

Auto merge of #143029 - fmease:gci-imply, r=<try>

25444d3

GCI: Imply outlives-bounds on free (generic) const items Part of #113521.

rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Jun 27, 2025

fmease added the rla-silenced Silences rust-log-analyzer postings to the PR it's added on. label Jun 27, 2025

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rustbot removed S-waiting-on-perf Status: Waiting on a perf run to be completed. perf-regression Performance regression. labels Jun 28, 2025

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fmease force-pushed the gci-imply branch from c2b8a0b to d350426 Compare March 1, 2026 12:15

fmease marked this pull request as ready for review March 1, 2026 12:15

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 Mar 1, 2026

fmease force-pushed the gci-imply branch from d350426 to 99596aa Compare March 1, 2026 12:19

fmease commented Mar 1, 2026

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fmease removed the rla-silenced Silences rust-log-analyzer postings to the PR it's added on. label Mar 1, 2026

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fmease force-pushed the gci-imply branch from 99596aa to f85a44a Compare March 1, 2026 18:23

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fmease force-pushed the gci-imply branch from f85a44a to 7091062 Compare March 1, 2026 18:34

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GCI: Imply outlives-bounds on free (generic) const items

f1fd6a2

fmease force-pushed the gci-imply branch from 7091062 to f1fd6a2 Compare March 2, 2026 06:18

fmease commented Mar 2, 2026

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BoxyUwU reviewed Mar 13, 2026

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fmease 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 Mar 13, 2026

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rust-timer commented Jun 27, 2025

Overall result: ❌ regressions - please read the text below

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Overall result: no relevant changes - no action needed

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