Propogate error types in mir type projections

crates/ide-diagnostics/src/handlers/unused_variables.rs

@@ -260,6 +260,19 @@ fn main() {
     let arr = [1, 2, 3, 4, 5];
     let [_x, _y @ ..] = arr;
 }
+"#,
+        );
+    }
+
+    // regression test as we used to panic in this scenario
+    #[test]
+    fn unknown_struct_pattern_param_type() {
+        check_diagnostics(
+            r#"
+struct S { field : u32 }
+fn f(S { field }: error) {
+      // ^^^^^ 💡 warn: unused variable
+}
 "#,
         );
     }