test: move issue-1598 regression into rust-only codegen.rs

Author: mattwilkinsonn

crates/rust/tests/codegen.rs

@@ -235,3 +235,29 @@ mod method_chaining {
         enable_method_chaining: true
     });
 }
+
+// Regression: a named type brought in via `use interface.{T}` at world
+// level, embedded inside `future<result<T, _>>` on an exported function,
+// used to emit unresolved bare `T` references in the generated
+// `wit_future::vtable{N}` module. See issue #1598. The fix walks the
+// payload type tree and emits a `use super::super::...;` per named type
+// at the top of each vtable module.
+#[allow(unused, reason = "testing codegen, not functionality")]
+mod issue_1598_future_result_use {
+    wit_bindgen::generate!({
+        inline: r#"
+        package a:b;
+
+        world w {
+            use t.{r};
+            export f: func() -> future<result<r, string>>;
+        }
+
+        interface t {
+            record r {
+                x: u32,
+            }
+        }
+        "#,
+    });
+}