events: Enforce MessageContent to not be empty

crates/ruma-common/src/events/message.rs

@@ -47,7 +47,7 @@
 //! [MSC3245]: https://github.com/matrix-org/matrix-spec-proposals/pull/3245
 //! [MSC3381]: https://github.com/matrix-org/matrix-spec-proposals/pull/3381
 //! [`RoomMessageEventContent`]: super::room::message::RoomMessageEventContent
-use std::ops::Deref;
+use std::{convert::TryFrom, ops::Deref};
 
 use ruma_macros::EventContent;
 use serde::{Deserialize, Serialize};
@@ -123,11 +123,25 @@ impl MessageEventContent {
 }
 
 /// Text message content.
+///
+/// A `MessageContent` must contain at least one message to be used as a fallback text
+/// representation.
 #[derive(Clone, Debug, Deserialize)]
 #[serde(try_from = "MessageContentSerDeHelper")]
 pub struct MessageContent(pub(crate) Vec<Text>);
 
 impl MessageContent {
+    /// Create a `MessageContent` from an array of messages.
+    ///
+    /// Returns `None` if the array is empty.
+    pub fn new(messages: Vec<Text>) -> Option<Self> {
+        if messages.is_empty() {
+            None
+        } else {
+            Some(Self(messages))
+        }
+    }
+
     /// A convenience constructor to create a plain text message.
     pub fn plain(body: impl Into<String>) -> Self {
         Self(vec![Text::plain(body)])
@@ -178,9 +192,17 @@ impl MessageContent {
     }
 }
 
-impl From<Vec<Text>> for MessageContent {
-    fn from(variants: Vec<Text>) -> Self {
-        Self(variants)
+/// The error type returned when trying to construct an empty `MessageContent`.
+#[derive(Debug, Error)]
+#[non_exhaustive]
+#[error("MessageContent cannot be empty")]
+pub struct EmptyMessageContentError;
+
+impl TryFrom<Vec<Text>> for MessageContent {
+    type Error = EmptyMessageContentError;
+
+    fn try_from(messages: Vec<Text>) -> Result<Self, Self::Error> {
+        Self::new(messages).ok_or(EmptyMessageContentError)
     }
 }
 

crates/ruma-common/src/events/message/content_serde.rs

@@ -62,7 +62,7 @@ impl Serialize for MessageContent {
 }
 
 pub(crate) mod as_vec {
-    use serde::{ser::SerializeSeq, Deserialize, Deserializer, Serializer};
+    use serde::{de, ser::SerializeSeq, Deserialize, Deserializer, Serializer};
 
     use crate::events::message::{MessageContent, Text};
 
@@ -87,7 +87,10 @@ pub(crate) mod as_vec {
     where
         D: Deserializer<'de>,
     {
-        Option::<Vec<Text>>::deserialize(deserializer)
-            .map(|content| content.filter(|content| !content.is_empty()).map(Into::into))
+        Option::<Vec<Text>>::deserialize(deserializer).and_then(|content| {
+            content.map(MessageContent::new).ok_or_else(|| {
+                de::Error::invalid_value(de::Unexpected::Other("empty array"), &"a non-empty array")
+            })
+        })
     }
 }

crates/ruma-common/tests/events/message.rs

@@ -1,5 +1,7 @@
 #![cfg(feature = "unstable-msc1767")]
 
+use std::convert::TryFrom;
+
 use assign::assign;
 use js_int::uint;
 use matches::assert_matches;
@@ -7,7 +9,7 @@ use ruma_common::{
     event_id,
     events::{
         emote::EmoteEventContent,
-        message::MessageEventContent,
+        message::{MessageContent, MessageEventContent, Text},
         notice::NoticeEventContent,
         room::message::{
             EmoteMessageEventContent, InReplyTo, MessageType, NoticeMessageEventContent, Relation,
@@ -19,6 +21,19 @@ use ruma_common::{
 };
 use serde_json::{from_value as from_json_value, json, to_value as to_json_value};
 
+#[test]
+fn try_from_valid() {
+    assert_matches!(
+        MessageContent::try_from(vec![Text::plain("A message")]),
+        Ok(message) if message.len() == 1
+    );
+}
+
+#[test]
+fn try_from_invalid() {
+    assert_matches!(MessageContent::try_from(vec![]), Err(_));
+}
+
 #[test]
 fn html_content_serialization() {
     let message_event_content =
@@ -712,13 +727,12 @@ fn room_message_emote_unstable_deserialization() {
 #[test]
 #[cfg(feature = "unstable-msc3554")]
 fn lang_serialization() {
-    use ruma_common::events::message::{MessageContent, Text};
-
-    let content = MessageContent::from(vec![
+    let content = MessageContent::try_from(vec![
         assign!(Text::plain("Bonjour le monde !"), { lang: Some("fr".into()) }),
         assign!(Text::plain("Hallo Welt!"), { lang: Some("de".into()) }),
         assign!(Text::plain("Hello World!"), { lang: Some("en".into()) }),
-    ]);
+    ])
+    .unwrap();
 
     assert_eq!(
         to_json_value(&content).unwrap(),
@@ -735,8 +749,6 @@ fn lang_serialization() {
 #[test]
 #[cfg(feature = "unstable-msc3554")]
 fn lang_deserialization() {
-    use ruma_common::events::message::MessageContent;
-
     let json_data = json!({
         "org.matrix.msc1767.message": [
             { "body": "Bonjour le monde !", "mimetype": "text/plain", "lang": "fr"},