test: derived encoded_len, only checked fields

Add a test Protobuf message that only has unlimited length fields,
to check that the derived encoded_len method implementation is correct
in this case, confirmed with a few test cases.

tests/overflow/src/encoded_len.proto

@@ -33,6 +33,11 @@ message Testbed {
     map<uint32, int32> map = 19;
 }
 
+message TwoUnlimited {
+    repeated Empty repeated_empty = 536870911; // MAX_TAG
+    string string = 16;
+}
+
 message Empty {}
 
 enum BadEnum {

tests/overflow/src/encoded_len/overflow.rs

@@ -252,6 +252,21 @@ mod derived {
         assert!(verify_overflowing_encoded_len(encoded_len, expected_len));
     }
 
+    #[test]
+    #[cfg_attr(target_pointer_width = "32", should_panic)]
+    fn encoded_len_can_overflow_u32_all_checked_with_repeated_field() {
+        let filler = proto::Empty {};
+        let filler_len = message::encoded_len(MAX_TAG, &filler);
+        let supercritical = vec![filler; u32::MAX as usize / filler_len + 1];
+        let expected_len = supercritical.len() as u64 * filler_len as u64;
+        let bomb32 = proto::TwoUnlimited {
+            repeated_empty: supercritical,
+            ..Default::default()
+        };
+        let encoded_len = bomb32.encoded_len();
+        assert!(verify_overflowing_encoded_len(encoded_len, expected_len));
+    }
+
     #[test]
     #[cfg_attr(target_pointer_width = "32", should_panic)]
     fn encoded_len_can_overflow_u32_with_string() {
@@ -271,6 +286,22 @@ mod derived {
         assert!(verify_overflowing_encoded_len(encoded_len, expected_len));
     }
 
+    #[test]
+    #[cfg_attr(target_pointer_width = "32", should_panic)]
+    fn encoded_len_can_overflow_u32_all_checked_with_string() {
+        let filler = proto::Empty {};
+        let filler_len = message::encoded_len(MAX_TAG, &filler);
+        let padding = vec![filler; u32::MAX as usize / filler_len];
+        let padding_len = padding.len() * filler_len;
+        let bomb32 = proto::TwoUnlimited {
+            repeated_empty: padding,
+            string: " ".repeat(filler_len - 2 - 1),
+        };
+        let encoded_len = bomb32.encoded_len();
+        let expected_len = padding_len as u64 + filler_len as u64;
+        assert!(verify_overflowing_encoded_len(encoded_len, expected_len));
+    }
+
     #[test]
     #[cfg_attr(target_pointer_width = "32", should_panic)]
     fn encoded_len_can_overflow_u32_with_bytes() {