rustdoc-json: Add assoc type ICE regression test

Author: Enselic

src/test/rustdoc-json/assoc_type.rs

@@ -0,0 +1,16 @@
+// Regression test for <https://github.com/rust-lang/rust/issues/98547>.
+
+// @has assoc_type.json
+// @has - "$.index[*][?(@.name=='Trait')]"
+// @has - "$.index[*][?(@.name=='AssocType')]"
+// @has - "$.index[*][?(@.name=='S')]"
+
+pub trait Trait {
+    type AssocType;
+}
+
+impl<T> Trait for T {
+    type AssocType = Self;
+}
+
+pub struct S;