rustc_hir_analysis: fix generics_of ICE on unresolved const arguments

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()))

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() {}

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`.