From d5fcdffb29e75b7f7513de12c2206e32b2f4ffa4 Mon Sep 17 00:00:00 2001
From: "Mark Z. Ding" <invalidpathexception@gmail.com>
Date: Tue, 11 Nov 2025 17:43:14 -0500
Subject: [PATCH] Handle Node::ConstArg in generics_of by reusing the parent's
 generics

---
 .../src/collect/generics_of.rs                | 28 +++++++++++++++++++
 .../ui/const-generics/const-arg-unresolved.rs | 10 +++++++
 .../const-arg-unresolved.stderr               | 27 ++++++++++++++++++
 3 files changed, 65 insertions(+)
 create mode 100644 tests/ui/const-generics/const-arg-unresolved.rs
 create mode 100644 tests/ui/const-generics/const-arg-unresolved.stderr

diff --git a/compiler/rustc_hir_analysis/src/collect/generics_of.rs b/compiler/rustc_hir_analysis/src/collect/generics_of.rs
index 3d2f0466cad08..fb8cc80e8e2ee 100644
--- a/compiler/rustc_hir_analysis/src/collect/generics_of.rs
+++ b/compiler/rustc_hir_analysis/src/collect/generics_of.rs
@@ -166,6 +166,34 @@ pub(super) fn generics_of(tcx: TyCtxt<'_>, def_id: LocalDefId) -> ty::Generics {
                 ty::AnonConstKind::NonTypeSystem => None,
             }
         }
+        Node::ConstArg(_) => {
+            let parent_did = tcx.parent(def_id.to_def_id());
+            debug!(?parent_did);
+
+            if let Node::GenericParam(hir::GenericParam {
+                def_id: param_id,
+                kind: hir::GenericParamKind::Const { .. },
+                ..
+            }) = tcx.parent_hir_node(hir_id)
+            {
+                let generics = tcx.generics_of(parent_did);
+                let param_def_idx = generics.param_def_id_to_index[&param_id.to_def_id()];
+                let own_params = generics.params_to(param_def_idx as usize, tcx).to_owned();
+                let param_def_id_to_index =
+                    own_params.iter().map(|param| (param.def_id, param.index)).collect();
+
+                return ty::Generics {
+                    parent: generics.parent,
+                    parent_count: generics.parent_count,
+                    own_params,
+                    param_def_id_to_index,
+                    has_self: generics.has_self,
+                    has_late_bound_regions: generics.has_late_bound_regions,
+                };
+            }
+
+            Some(parent_did)
+        }
         Node::ConstBlock(_)
         | Node::Expr(&hir::Expr { kind: hir::ExprKind::Closure { .. }, .. }) => {
             Some(tcx.typeck_root_def_id(def_id.to_def_id()))
diff --git a/tests/ui/const-generics/const-arg-unresolved.rs b/tests/ui/const-generics/const-arg-unresolved.rs
new file mode 100644
index 0000000000000..ea2dccec76878
--- /dev/null
+++ b/tests/ui/const-generics/const-arg-unresolved.rs
@@ -0,0 +1,10 @@
+#![feature(min_generic_const_args)]
+#![feature(generic_const_exprs)]
+#![expect(incomplete_features)]
+
+struct Both<const is_123: u32 = 3, T> { //~ ERROR generic parameters with a default must be trailing
+    a: A<{ B::<1>::M }>, //~ ERROR cannot find type `A` in this scope
+    //~| ERROR failed to resolve: use of undeclared type `B`
+}
+
+fn main() {}
diff --git a/tests/ui/const-generics/const-arg-unresolved.stderr b/tests/ui/const-generics/const-arg-unresolved.stderr
new file mode 100644
index 0000000000000..276b84b1f86ac
--- /dev/null
+++ b/tests/ui/const-generics/const-arg-unresolved.stderr
@@ -0,0 +1,27 @@
+error: generic parameters with a default must be trailing
+  --> $DIR/const-arg-unresolved.rs:5:19
+   |
+LL | struct Both<const is_123: u32 = 3, T> {
+   |                   ^^^^^^
+
+error[E0412]: cannot find type `A` in this scope
+  --> $DIR/const-arg-unresolved.rs:6:8
+   |
+LL | struct Both<const is_123: u32 = 3, T> {
+   |                                    - similarly named type parameter `T` defined here
+LL |     a: A<{ B::<1>::M }>,
+   |        ^ help: a type parameter with a similar name exists: `T`
+
+error[E0433]: failed to resolve: use of undeclared type `B`
+  --> $DIR/const-arg-unresolved.rs:6:12
+   |
+LL |     a: A<{ B::<1>::M }>,
+   |            ^
+   |            |
+   |            use of undeclared type `B`
+   |            help: a type parameter with a similar name exists: `T`
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0412, E0433.
+For more information about an error, try `rustc --explain E0412`.