Add test for opting out of Vec<Vec<T>> derive

tests/derive/occurrences.rs

@@ -17,6 +17,27 @@ fn test_vec_of_vec() {
     );
 }
 
+#[test]
+fn test_vec_of_vec_opt_out() {
+    fn parser(s: &str) -> Result<Vec<Vec<String>>, std::convert::Infallible> {
+        Ok(s.split(':').map(|v| v.split(',').map(str::to_owned).collect()).collect())
+    }
+
+    #[derive(Parser, PartialEq, Debug)]
+    struct Opt {
+        #[arg(value_parser = parser, short = 'p')]
+        arg: ::std::vec::Vec<Vec<String>>,
+    }
+
+    assert_eq!(
+        Opt {
+            arg: vec![vec!["1".into(), "2".into()], vec!["a".into(), "b".into()]],
+    },
+    Opt::try_parse_from(["test", "-p", "1,2:a,b"]).unwrap(),
+    );
+
+}
+
 #[test]
 fn test_vec_vec_empty() {
     #[derive(Parser, Debug, PartialEq)]