Fix guild deserialisation regression (#640)

A regression in 'model::guild::Guild' deserialization was introduced by
commit 4541935[1].

The bug introduced was that deserialisation of
'Guild::premium_subscription_count'[2] would fail. The
'premium_subscription_count' field isn't always present in the JSON
provided as noted by the REST API documentation[3], so a default value
needs to be set for it, in this case 0.

Additionally, set a default value for 'Guild::premium_tier'[4] in case this
is ever nulled in the API too, defaulting to 'PremiumTier::Tier0'.

[1]: 4541935
[2]: 4541935#diff-e1c86b244465a2bb338d53758aa2c7efR154
[3]: discord/discord-api-docs@3d8cca0#diff-c4e1390fca89ecc96b01a093f988fa1cR48
[4]: 4541935#diff-e1c86b244465a2bb338d53758aa2c7efR156

Signed-off-by: Zeyla Hellyer <zeyla@hellyer.dev>

src/model/guild/mod.rs

@@ -153,8 +153,10 @@ pub struct Guild {
     /// The server's description
     pub description: Option<String>,
     /// The server's premium boosting level
+    #[serde(default)]
     pub premium_tier: PremiumTier,
     /// The total number of users currently boosting this server
+    #[serde(default)]
     pub premium_subscription_count: u64,
     /// The server's banner
     pub banner: Option<String>,
@@ -1763,14 +1765,14 @@ impl<'de> Deserialize<'de> for Guild {
             Some(v) => Option::<String>::deserialize(v).map_err(DeError::custom)?,
             None => None,
         };
-        let premium_tier = map.remove("premium_tier")
-            .ok_or_else(|| DeError::custom("expected guild premium_tier"))
-            .and_then(PremiumTier::deserialize)
-            .map_err(DeError::custom)?;
-        let premium_subscription_count = map.remove("premium_subscription_count")
-            .ok_or_else(|| DeError::custom("expected guild premium_subscription_count"))
-            .and_then(u64::deserialize)
-            .map_err(DeError::custom)?;
+        let premium_tier = match map.remove("premium_tier") {
+            Some(v) => PremiumTier::deserialize(v).map_err(DeError::custom)?,
+            None => PremiumTier::default(),
+        };
+        let premium_subscription_count = match map.remove("premium_subscription_count") {
+            Some(Value::Null) | None => 0,
+            Some(v) => u64::deserialize(v).map_err(DeError::custom)?,
+        };
         let banner = match map.remove("banner") {
             Some(v) => Option::<String>::deserialize(v).map_err(DeError::custom)?,
             None => None,
@@ -1780,7 +1782,6 @@ impl<'de> Deserialize<'de> for Guild {
             None => None,
         };
 
-
         Ok(Self {
             afk_channel_id,
             application_id,

src/model/guild/premium_tier.rs

@@ -31,3 +31,9 @@ impl PremiumTier {
         }
     }
 }
+
+impl Default for PremiumTier {
+    fn default() -> Self {
+        PremiumTier::Tier0
+    }
+}