Proxy edited messages if the message is the last one in the channel, and the edit introduces proxy tags where there were none previously

PluralKit.Bot/Services/LastMessageCacheService.cs

@@ -0,0 +1,25 @@
+using System.Collections.Concurrent;
+using System.Collections.Generic;
+
+namespace PluralKit.Bot
+{
+    // Doing things like this instead of enabling D.NET's message cache because the message cache is, let's face it,
+    // not particularly efficient? It allocates a dictionary *and* a queue for every single channel (500k in prod!)
+    // whereas this is, worst case, one dictionary *entry* of a single ulong per channel, and one dictionary instance
+    // on the whole instance, total. Yeah, much more efficient.
+    public class LastMessageCacheService
+    {
+        private IDictionary<ulong, ulong> _cache = new ConcurrentDictionary<ulong, ulong>();
+
+        public void AddMessage(ulong channel, ulong message)
+        {
+            _cache[channel] = message;
+        }
+
+        public ulong? GetLastMessage(ulong channel)
+        {
+            if (_cache.TryGetValue(channel, out var message)) return message;
+            return null;
+        }
+    }
+}