Allow associated const equality constraints with GCA - #159506

bit-aloo · 2026-07-18T09:41:20Z

View all comments

closes: rust-lang/project-const-generics#77

r? @BoxyUwU

rustbot · 2026-07-18T09:41:23Z

HIR ty lowering was modified

cc @fmease

Some changes occurred in compiler/rustc_sanitizers

cc @rcvalle

rustbot · 2026-07-18T09:41:25Z

BoxyUwU is currently at their maximum review capacity.
They may take a while to respond.

fmease · 2026-07-18T11:14:52Z

                if !tcx.generics_of(item.def_id).is_own_empty() {
                    errors.push(AssocConstViolation::Generic);
-                } else if !is_type_const {
-                    errors.push(AssocConstViolation::NonType);


Isn't NonType now dead code and can be removed?

View changes since the review

~~Thanks, removed here: b464ecb~~, we need this for mgca, reverted the removal

fmease · 2026-07-18T11:22:55Z

-                                .filter(|item| item.is_type() || item.is_type_const())
+                                // Only associated items that support equality constraints can
+                                // be constrained in a trait object type via a binding.
+                                .filter(|item| item.can_have_equality_constraint(tcx))


Note that this change means that stabilizing mGCA will break stable code like tests/ui/type-alias/lack-of-wfcheck.rs.

View changes since the review

does that mean we can't stablize this? not sure how to go about this..

this change is supposed to be for generic_const_args not min_generic_const_args since mgca doesn't support const items in the type system but full GCA does. once you change that this'll go back to mgca being backwards compatible but with GCA being backwards incompatible (which is OK)

Ah! thanks, my bad 🥲 . Just made changes specific to GCA

BoxyUwU · 2026-07-22T13:03:45Z

                if !tcx.generics_of(item.def_id).is_own_empty() {
                    errors.push(AssocConstViolation::Generic);
-                } else if !is_type_const {
+                } else if !is_type_const && !tcx.features().generic_const_args() {


@fmease do you know if you added a test that type const FOO<const N: usize> makes a trait dyn incompatible? (because its a GAT)

regardless, @Shourya742 can you add a test that cosnt ASSOC<const N: usize>: usize makes a trait dyn incompatible under feature(generic_const_args)

View changes since the review

Added here: 6497008

BoxyUwU

hell yeah :) very cool thx for working on this. no worries about the mgca vs gca mixup, the feature names are confusing ✨

View changes since this review

BoxyUwU · 2026-07-22T13:13:26Z

Ah can you also add a variant of tests/ui/type-alias/lack-of-wfcheck.rs that breaks when generic_const_args is enabled so that it's obvious that this is a breaking change ✨

BoxyUwU · 2026-07-22T13:14:07Z

@rustbot author

bit-aloo · 2026-07-23T12:59:10Z

[0e7d4e2](/rust-lang/rust/pull/159506/commits/0e7d4e2819cf01929968701e9c6af6f7e12afa4b)

Added here: 0e7d4e2

BoxyUwU

test looks good just some nits about it

View changes since this review

BoxyUwU · 2026-07-24T14:33:44Z

+
+// * dyn incompatible due to GAT
+// * `'a: 'static`, `String: Copy` and `[u8]: Sized` unsatisfied, `loop {}` diverging
+type Several<'a> = dyn HasGenericAssocType<Type<'a, String, { loop {} }> = [u8]>;


can you split this into a separate test, it's also not actually showing an error right now

Added the variant for this, here: 2191fc1

BoxyUwU · 2026-07-24T14:34:07Z

+
+type DynIncompat0 = dyn Sized; // `Sized` axiomatically dyn incompatible
+// issue: <https://github.com/rust-lang/rust/issues/153731>
+type DynIncompat1 = dyn HasAssocConst;


this part of the test would ideally be revisioned so that it shows the behaviour with and without generic_const_args enabled :3 (to demonstrate the breaking nature of stabilization)

TIL, we have this pretty cool thing called revisions that helps feature gate compile tests, so I used it here. Added in 2191fc1.

BoxyUwU · 2026-08-06T08:12:50Z

@bors r+

rust-bors · 2026-08-06T08:12:53Z

📌 Commit 2191fc1 has been approved by BoxyUwU

It is now in the queue for this repository.

@BoxyUwU

…-non-typeconsts-in-GCA, r=BoxyUwU Allow associated const equality constraints with GCA closes: rust-lang/project-const-generics#77 r? @BoxyUwU

@BoxyUwU

…-non-typeconsts-in-GCA, r=BoxyUwU Allow associated const equality constraints with GCA closes: rust-lang/project-const-generics#77 r? @BoxyUwU

…uwer Rollup of 17 pull requests Successful merges: - #159530 (Cap socket send length to c_int::MAX on Apple targets) - #159506 (Allow associated const equality constraints with GCA) - #160006 (Account for desugaring in method call move errors) - #160415 (Split `aarch64-apple{,-macos-26}` => `aarch64-apple{,-macos-26}-{1,2}` jobs) - #160464 (fix: Check the fallback map before queueing child in `visible_parent_map` breadth-first search) - #160555 (Split `apply_primary_terminator_effect`) - #160592 (Suggest if-let chain continuation on unclosed delimiter) - #160600 (Avoid the std DLL copy alongside rustc) - #160156 (check_consts: exhaustively match on CastKind) - #160211 (Rename `#[unroll]` => `#[rustc_unroll]` to mitigate nameres ambiguity) - #160304 (Add tests for fixed new solver issues) - #160398 (rustc_abi: Add `LayoutData::is_variant_uninhabited` method) - #160546 (Update error message in documentation comments) - #160568 (Use `VisitorResult` helper macros) - #160571 (Add regression test for array type recovery in generic arguments) - #160588 (add a test showing polonius alpha is not a subset of datalog polonius) - #160617 (Add a suggestion to MissingUnsafeOnExtern diagnostic)

@BoxyUwU

Rollup merge of #159506 - bit-aloo:2026-07-14-dyn-compatible-non-typeconsts-in-GCA, r=BoxyUwU Allow associated const equality constraints with GCA closes: rust-lang/project-const-generics#77 r? @BoxyUwU

JonathanBrouwer · 2026-08-06T20:09:39Z

@rust-timer build e1373c7

rust-timer · 2026-08-06T20:50:16Z

Finished benchmarking commit (e1373c7): 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
Improvements ✅ (secondary)	-0.4%	[-0.5%, -0.2%]	6
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

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

Cycles

Results (primary 2.3%, secondary -5.3%)

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

	mean	range	count
Regressions ❌ (primary)	2.3%	[2.2%, 2.5%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.3%	[-5.3%, -5.3%]	1
All ❌✅ (primary)	2.3%	[2.2%, 2.5%]	2

Binary size

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

Bootstrap: 458.35s -> 474.78s (3.58%)
Artifact size: 398.64 MiB -> 398.61 MiB (-0.01%)

rustbot added PG-exploit-mitigations Project group: Exploit mitigations 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. labels Jul 18, 2026

rustbot assigned BoxyUwU Jul 18, 2026

fmease reviewed Jul 18, 2026

View reviewed changes

bit-aloo requested a review from fmease July 20, 2026 14:01

BoxyUwU mentioned this pull request Jul 21, 2026

Full Const Generics rust-lang/goals#100

Open

7 tasks

bit-aloo changed the title ~~Allow associated const equality constraints with MGCA~~ Allow associated const equality constraints with GCA Jul 21, 2026

bit-aloo requested a review from BoxyUwU July 22, 2026 02:45

BoxyUwU reviewed Jul 22, 2026

View reviewed changes

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 Jul 22, 2026

rust-cloud-vms Bot force-pushed the 2026-07-14-dyn-compatible-non-typeconsts-in-GCA branch from 79271ed to 0e7d4e2 Compare July 23, 2026 12:58

bit-aloo requested a review from BoxyUwU July 23, 2026 12:59

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 Jul 23, 2026

BoxyUwU reviewed Jul 24, 2026

View reviewed changes

bit-aloo added 6 commits July 25, 2026 05:37

add test which should pass

ea475ff

Add helper for equality-constrainable assoc items

62e3e3d

Allow associated const equality constraints with MGCA

f86e9f6

Correct the statments

cd57bc3

Remove NonType assocConstViolation variant, as we don't need it anymore

faebca4

Gate non-type assoc const equality on generic_const_args

a8087a3

bit-aloo added 3 commits July 25, 2026 05:37

add dyn generic non type assoc const

4077496

add lack_of_wfcheck for gca

ad7e5e2

Revise generic const args WF test and split GAT case

2191fc1

rust-cloud-vms Bot force-pushed the 2026-07-14-dyn-compatible-non-typeconsts-in-GCA branch from 0e7d4e2 to 2191fc1 Compare July 25, 2026 06:23

bit-aloo requested a review from BoxyUwU July 25, 2026 06:28

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 Aug 6, 2026

JonathanBrouwer mentioned this pull request Aug 6, 2026

Rollup of 17 pull requests #160615

Closed

This was referenced Aug 6, 2026

Rollup of 16 pull requests #160623

Closed

Rollup of 17 pull requests #160627

Merged

rust-bors Bot merged commit 237d7aa into rust-lang:main Aug 6, 2026
13 checks passed

rustbot added this to the 1.99.0 milestone Aug 6, 2026

bit-aloo deleted the 2026-07-14-dyn-compatible-non-typeconsts-in-GCA branch August 6, 2026 14:42

This comment has been minimized.

Sign in to view

Uh oh!

Uh oh!

Conversation

bit-aloo commented Jul 18, 2026 • edited by rustbot Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Uh oh!

rustbot commented Jul 18, 2026

Uh oh!

rustbot commented Jul 18, 2026

Uh oh!

Uh oh!

fmease Jul 18, 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 Jul 20, 2026 • edited Loading Uh oh! There was an error while loading. Please reload this page.

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Uh oh!

fmease Jul 18, 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!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

BoxyUwU Jul 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!

Choose a reason for hiding this comment

Uh oh!

BoxyUwU 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!

BoxyUwU commented Jul 22, 2026

Uh oh!

BoxyUwU commented Jul 22, 2026

Uh oh!

bit-aloo commented Jul 23, 2026

Uh oh!

BoxyUwU 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!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

Choose a reason for hiding this comment

Uh oh!

BoxyUwU commented Aug 6, 2026

Uh oh!

rust-bors Bot commented Aug 6, 2026

Uh oh!

Uh oh!

JonathanBrouwer commented Aug 6, 2026

Uh oh!

This comment has been minimized.

rust-timer commented Aug 6, 2026

Overall result: ✅ improvements - no action needed

Uh oh!

Reviewers

Assignees

Labels

Projects

Milestone

Development

Uh oh!

6 participants

bit-aloo commented Jul 18, 2026 •

edited by rustbot

Loading

fmease Jul 18, 2026 •

edited by rustbot

Loading

bit-aloo Jul 20, 2026 •

edited

Loading

fmease Jul 18, 2026 •

edited by rustbot

Loading

BoxyUwU Jul 22, 2026 •

edited by rustbot

Loading

BoxyUwU left a comment •

edited by rustbot

Loading

BoxyUwU left a comment •

edited by rustbot

Loading