mGCA: Add associated const type check #152146

zedddie · 2026-02-05T01:59:55Z

r? BoxyUwU

I didn't bless tests just yet as it only fixes the dyn arm

BoxyUwU · 2026-02-05T12:09:43Z

compiler/rustc_trait_selection/src/traits/wf.rs

+
+                for projection in data.projection_bounds() {
+                    let pred_binder = projection
+                        .with_self_ty(tcx, tcx.types.trait_object_dummy_self)


Suggested change

.with_self_ty(tcx, tcx.types.trait_object_dummy_self)

.with_self_ty(tcx, t)

I expect using trait_object_dummy_self might lead to code on the error-path crashing so we should use the "actual" self type. It also might just lead to weird diagnostics, such as Infer(FreshTy(0)) appearing in user facing text which we don't want.

Though eventually we'll also support assocaited constants such as const ASSOC: <Self as Other>::Assoc at which point Self will need to be a real type that implements Trait so that we can figure out the type of Assoc.

What's going on here is that we have the type dyn Trait<T, ASSOC = 10> (for example), and we want to construct [dyn Trait<T, ASSOC = 10>, T] as a list of generic arguments to <T as Trait<U>>::ASSOC for when we look at the type of ASSOC.

Oh I really missed we have the type in scope and can use it -_-, then it will be perfect here

BoxyUwU · 2026-02-05T12:15:58Z

tests/ui/const-generics/associated-const-bindings/wf-mismatch-1.rs

+
+trait Trait { #[type_const] const CT: bool; }
+
+// FIXME: this should yield a type mismatch (`bool` v `i32`)


This will get fixed once you add wf checks for T: Trait<ASSOC = N> where clauses.

You can do this by adding fairly similar logic to what you've already done, but in rustc_hir_analysis/src/check/wfcheck.rs in the check_where_clause function

You should be able to similarly iterate over all of the predicates and filter to only those that are associated const bindings

BoxyUwU · 2026-02-05T12:16:08Z

very nice :)

BoxyUwU · 2026-02-08T17:49:49Z

compiler/rustc_hir_analysis/src/check/wfcheck.rs

+        .filter_map(|(clause, sp)| clause.as_projection_clause().map(|proj| (proj, sp)))
+        .filter_map(|(proj, sp)| {


Suggested change

.filter_map(|(clause, sp)| clause.as_projection_clause().map(|proj| (proj, sp)))

.filter_map(|(proj, sp)| {

.filter_map(|(clause, sp)| {

let proj = clause.as_projection_clause().map(|proj| (proj, sp)))?;

does this work?

yes, and i simplified it a bit further now

BoxyUwU · 2026-02-08T17:52:33Z

compiler/rustc_trait_selection/src/traits/wf.rs

                }
+
+                for projection in data.projection_bounds() {
+                    if !t.has_escaping_bound_vars() {


can you move this out of the for loop? I think it'd read nicer as we don't care about anything to do with wf of the trait object if it has bound vars, whereas currently it sort of reads like this check is dependent on information about each project (which is not true)

BoxyUwU · 2026-02-08T17:53:31Z

compiler/rustc_hir_analysis/src/check/wfcheck.rs

+                })
+                .transpose();
+            pred_binder.map(|pred_binder| {
+                let pred: ty::Predicate<'tcx> = pred_binder.upcast(tcx);


Obligation::new accepts arbitrary T that implements Upcast you might be able to just do Obligation::new(... pred_binder) but I don't know for sure :)

BoxyUwU · 2026-02-08T17:59:39Z

Looks correct to me :) Just waiting on you to sort out the tests now I think

zedddie · 2026-02-09T10:14:36Z

compiler/rustc_hir_analysis/src/check/wfcheck.rs

+        .copied()
+        .zip(predicates.spans.iter().copied())
+        .filter_map(|(clause, sp)| {
+            let proj = clause.as_projection_clause()?;


in previous variant I didn't really use sp, just passed to the next filter_map, so can be simplified to this

zedddie · 2026-02-09T10:25:48Z

tests/ui/const-generics/associated-const-bindings/dyn-const-projection-escaping-bound-vars.rs

@@ -0,0 +1,11 @@
+//! Check associated const binding with escaping bound vars doesn't cause ICE


was not sure on adding this, but i guess this works as regression test for the check logic :"

BoxyUwU · 2026-02-09T17:21:15Z

I think this looks good to me, you can undraft and I'll approve this after #152324 has landed (though you'll have to rebase after that to handle the merge conflicts)

rustbot · 2026-02-10T03:05:43Z

changes to the core type system

cc @lcnr

BoxyUwU · 2026-02-10T12:21:28Z

@bors r+ rollup

thanks for working on this :3

rust-bors · 2026-02-10T12:21:31Z

📌 Commit ec03e39 has been approved by BoxyUwU

It is now in the queue for this repository.

…s-wfck, r=BoxyUwU mGCA: Add associated const type check rust-lang#151642 r? BoxyUwU I didn't bless tests just yet as it only fixes the dyn arm

…uwer Rollup of 13 pull requests Successful merges: - #149937 (try enabling `linker-messages` by default again) - #151733 (Use function shims to make sure EII works on apple targets) - #152120 (Don't ICE on layout error in vtable computation) - #152419 (Move more query system code) - #152431 (Restrict the set of things that const stability can be applied to) - #152436 (Reenable a GCI+mGCA+GCPT test case) - #151142 (Support ADT types in type info reflection) - #152021 (Bump tvOS, visionOS and watchOS Aarch64 targets to tier 2) - #152146 (mGCA: Add associated const type check) - #152372 (style: remove unneeded trailing commas) - #152383 (BikeshedGuaranteedNoDrop trait: add comments indicating that it can be observed on stable) - #152397 (Update books) - #152441 (Fix typos and grammar in top-level and src/doc documentation)

…s-wfck, r=BoxyUwU mGCA: Add associated const type check rust-lang#151642 r? BoxyUwU I didn't bless tests just yet as it only fixes the dyn arm

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zedddie force-pushed the mgca-improve-const-bindings-wfck branch from 93d0247 to b2f6570 Compare February 7, 2026 06:28

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zedddie force-pushed the mgca-improve-const-bindings-wfck branch from b2f6570 to c290611 Compare February 7, 2026 22:40

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zedddie force-pushed the mgca-improve-const-bindings-wfck branch from c290611 to f5d85b4 Compare February 9, 2026 10:11

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zedddie added 2 commits February 10, 2026 03:49

mGCA: add associated const type check in dyn arm

b6b6dc0

mGCA: add associated const obligations in wfck

963f82d

zedddie force-pushed the mgca-improve-const-bindings-wfck branch from f5d85b4 to f670e0a Compare February 10, 2026 03:04

zedddie marked this pull request as ready for review February 10, 2026 03:05

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bless tests

ec03e39

zedddie force-pushed the mgca-improve-const-bindings-wfck branch from f670e0a to ec03e39 Compare February 10, 2026 05:28

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 Feb 10, 2026

JonathanBrouwer mentioned this pull request Feb 10, 2026

Rollup of 10 pull requests #152442

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JonathanBrouwer mentioned this pull request Feb 10, 2026

Rollup of 13 pull requests #152451

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matthiaskrgr mentioned this pull request Feb 10, 2026

Rollup of 12 pull requests #152454

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	.with_self_ty(tcx, tcx.types.trait_object_dummy_self)
	.with_self_ty(tcx, t)


		trait Trait { #[type_const] const CT: bool; }

		// FIXME: this should yield a type mismatch (`bool` v `i32`)

		.filter_map(\|(clause, sp)\| clause.as_projection_clause().map(\|proj\| (proj, sp)))
		.filter_map(\|(proj, sp)\| {

		@@ -0,0 +1,11 @@
		//! Check associated const binding with escaping bound vars doesn't cause ICE

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mGCA: Add associated const type check #152146

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mGCA: Add associated const type check #152146

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