Fix negative nonces failing to deserialize

Negative message nonces caused deserialization errors, as serde would
not deserialize integers into strings.

To fix this, change `Message::nonce` into an `Option<Snowflake>` from an
`Option<String>`. This new `Snowflake` is a wrapper around an `i64`. Use
a new `I64Visitor` to deserialize i64s, u64s, and strs into the wanted
i64.

src/model/channel/message.rs

@@ -48,7 +48,7 @@ pub struct Message {
     /// Array of users mentioned in the message.
     pub mentions: Vec<User>,
     /// Non-repeating number used for ensuring message order.
-    pub nonce: Option<String>,
+    pub nonce: Option<Snowflake>,
     /// Indicator of whether the message is pinned.
     pub pinned: bool,
     /// Array of reactions performed on the message.

src/model/mod.rs

@@ -53,7 +53,51 @@ use ::utils::Colour;
 
 fn default_true() -> bool { true }
 
-macro_rules! id {
+macro_rules! id_i64 {
+    ($(#[$attr:meta] $name:ident;)*) => {
+        $(
+            #[$attr]
+            #[derive(Copy, Clone, Debug, Eq, Hash, PartialOrd, Ord, Serialize)]
+            #[allow(derive_hash_xor_eq)]
+            pub struct $name(pub i64);
+
+            impl $name {
+                /// Retrieves the time that the Id was created at.
+                pub fn created_at(&self) -> NaiveDateTime {
+                    let offset = (self.0 >> 22) / 1000;
+
+                    NaiveDateTime::from_timestamp(1420070400 + offset, 0)
+                }
+            }
+
+            impl From<i64> for $name {
+                fn from(v: i64) -> $name {
+                    $name(v)
+                }
+            }
+
+            impl PartialEq for $name {
+                fn eq(&self, other: &Self) -> bool {
+                    self.0 == other.0
+                }
+            }
+
+            impl PartialEq<i64> for $name {
+                fn eq(&self, u: &i64) -> bool {
+                    self.0 == *u
+                }
+            }
+
+            impl<'de> Deserialize<'de> for $name {
+                fn deserialize<D: Deserializer<'de>>(deserializer: D) -> StdResult<Self, D::Error> {
+                    deserializer.deserialize_i64(I64Visitor).map($name)
+                }
+            }
+        )*
+    };
+}
+
+macro_rules! id_u64 {
     ($(#[$attr:meta] $name:ident;)*) => {
         $(
             #[$attr]
@@ -97,7 +141,7 @@ macro_rules! id {
     }
 }
 
-id! {
+id_u64! {
     /// An identifier for a Channel
     ChannelId;
     /// An identifier for an Emoji
@@ -116,6 +160,11 @@ id! {
     WebhookId;
 }
 
+id_i64! {
+    /// An identifier for a general-purpose signed snowflake.
+    Snowflake;
+}
+
 /// A container for guilds.
 ///
 /// This is used to differentiate whether a guild itself can be used or whether

src/model/utils.rs

@@ -240,3 +240,34 @@ impl<'de> Visitor<'de> for U64Visitor {
         Ok(v)
     }
 }
+
+pub struct I64Visitor;
+
+impl<'de> Visitor<'de> for I64Visitor {
+    type Value = i64;
+
+    fn expecting(&self, formatter: &mut Formatter) -> FmtResult {
+        formatter.write_str("identifier")
+    }
+
+    fn visit_str<E: DeError>(self, v: &str) -> StdResult<Self::Value, E> {
+        match v.parse::<i64>() {
+            Ok(v) => Ok(v),
+            Err(_) => {
+                let mut s = String::new();
+                s.push_str("Unknown i64 value: ");
+                s.push_str(v);
+
+                Err(DeError::custom(s))
+            },
+        }
+    }
+
+    fn visit_i64<E: DeError>(self, v: i64) -> StdResult<Self::Value, E> {
+        Ok(v)
+    }
+
+    fn visit_u64<E: DeError>(self, v: u64) -> StdResult<Self::Value, E> {
+        Ok(v as i64)
+    }
+}

tests/resources/message_create_2.json

@@ -0,0 +1,23 @@
+{
+  "type": 0,
+  "tts": false,
+  "timestamp": "2017-01-01T01:01:01.100000+00:00",
+  "pinned": false,
+  "nonce": -6000000000000000,
+  "mentions": [],
+  "mention_roles": [],
+  "mention_everyone": false,
+  "id": "300000000000000000",
+  "embeds": [],
+  "edited_timestamp": null,
+  "content": "fake",
+  "channel_id": "100000000000000000",
+  "author": {
+    "username": "fake",
+    "id": "300000000000000000",
+    "discriminator": "1234",
+    "bot": true,
+    "avatar": "f133549aac3208319a9fbc3c12345678"
+  },
+  "attachments": []
+}

tests/test_deser.rs

@@ -111,7 +111,11 @@ fn guild_update() {
 
 #[test]
 fn message_create() {
+    // standard
     p!(MessageCreateEvent, "message_create_1");
+
+    // negative nonce
+    p!(MessageCreateEvent, "message_create_2");
 }
 
 #[test]