fix impl of size_in_words for ReaderArena

Author: dwrensha

capnp/src/private/arena.rs

@@ -157,7 +157,7 @@ where
         let mut result = 0;
         for ii in 0..self.segments.len() {
             if let Some(seg) = self.segments.get_segment(ii as u32) {
-                result += seg.len();
+                result += seg.len() / BYTES_PER_WORD;
             }
         }
         result

capnp/tests/message_size_in_words.rs

@@ -0,0 +1,22 @@
+#![cfg(feature = "alloc")]
+
+use capnp::message;
+use capnp::serialize;
+use capnp::text;
+
+#[test]
+fn message_size_in_words() {
+    let mut testdata = b"Hello, World!".to_vec();
+    let testdata = text::Builder::new(&mut testdata);
+
+    let mut message = message::Builder::new_default();
+    message.set_root(testdata.into_reader()).unwrap();
+    assert_eq!(message.size_in_words(), 3);
+
+    let buffer = serialize::write_message_to_words(&message);
+    assert_eq!(buffer.len(), 32);
+
+    let message =
+        serialize::read_message_from_flat_slice(&mut &*buffer, Default::default()).unwrap();
+    assert_eq!(message.size_in_words(), 3);
+}