Add missing IThreadUser interface (#2055)

* Implement

* Add IMentionable to RestThreadUser

* Rather move mentionable to interface for consistency.

* Further consistency

src/Discord.Net.Core/Entities/Users/IThreadUser.cs

@@ -0,0 +1,25 @@
+using System;
+
+namespace Discord
+{
+    /// <summary>
+    ///     Represents a Discord thread user.
+    /// </summary>
+    public interface IThreadUser : IMentionable
+    {
+        /// <summary>
+        ///     Gets the <see cref="IThreadChannel"/> this user is in.
+        /// </summary>
+        IThreadChannel Thread { get; }
+
+        /// <summary>
+        ///     Gets the timestamp for when this user joined this thread.
+        /// </summary>
+        DateTimeOffset ThreadJoinedAt { get; }
+
+        /// <summary>
+        ///     Gets the guild this thread was created in.
+        /// </summary>
+        IGuild Guild { get; }
+    }
+}

src/Discord.Net.Rest/Entities/Users/RestThreadUser.cs

@@ -7,23 +7,20 @@ namespace Discord.Rest
     /// <summary>
     ///     Represents a thread user received over the REST api.
     /// </summary>
-    public class RestThreadUser : RestEntity<ulong>
+    public class RestThreadUser : RestEntity<ulong>, IThreadUser
     {
-        /// <summary>
-        ///     Gets the <see cref="RestThreadChannel"/> this user is in.
-        /// </summary>
+        /// <inheritdoc/>
         public IThreadChannel Thread { get; }
 
-        /// <summary>
-        ///     Gets the timestamp for when this user joined this thread.
-        /// </summary>
-        public DateTimeOffset JoinedAt { get; private set; }
+        /// <inheritdoc/>
+        public DateTimeOffset ThreadJoinedAt { get; private set; }
 
-        /// <summary>
-        ///     Gets the guild this user is in.
-        /// </summary>
+        /// <inheritdoc/>
         public IGuild Guild { get; }
 
+        /// <inheritdoc/>
+        public string Mention => MentionUtils.MentionUser(Id);
+
         internal RestThreadUser(BaseDiscordClient discord, IGuild guild, IThreadChannel channel, ulong id)
             : base(discord, id)
         {
@@ -40,7 +37,7 @@ namespace Discord.Rest
 
         internal void Update(Model model)
         {
-            JoinedAt = model.JoinTimestamp;
+            ThreadJoinedAt = model.JoinTimestamp;
         }
 
         /// <summary>

src/Discord.Net.WebSocket/Entities/Users/SocketThreadUser.cs

@@ -10,16 +10,14 @@ namespace Discord.WebSocket
     /// <summary>
     ///     Represents a thread user received over the gateway.
     /// </summary>
-    public class SocketThreadUser : SocketUser, IGuildUser
+    public class SocketThreadUser : SocketUser, IThreadUser, IGuildUser
     {
         /// <summary>
         ///     Gets the <see cref="SocketThreadChannel"/> this user is in.
         /// </summary>
         public SocketThreadChannel Thread { get; private set; }
 
-        /// <summary>
-        ///     Gets the timestamp for when this user joined this thread.
-        /// </summary>
+        /// <inheritdoc/>
         public DateTimeOffset ThreadJoinedAt { get; private set; }
 
         /// <summary>
@@ -180,8 +178,12 @@ namespace Discord.WebSocket
 
         /// <inheritdoc/>
         public Task RemoveTimeOutAsync(RequestOptions options = null) => GuildUser.RemoveTimeOutAsync(options);
+
         /// <inheritdoc/>
-        GuildPermissions IGuildUser.GuildPermissions => GuildUser.GuildPermissions;
+        IThreadChannel IThreadUser.Thread => Thread;
+
+        /// <inheritdoc/>
+        IGuild IThreadUser.Guild => Guild;
 
         /// <inheritdoc/>
         IGuild IGuildUser.Guild => Guild;
@@ -189,6 +191,9 @@ namespace Discord.WebSocket
         /// <inheritdoc/>
         ulong IGuildUser.GuildId => Guild.Id;
 
+        /// <inheritdoc/>
+        GuildPermissions IGuildUser.GuildPermissions => GuildUser.GuildPermissions;
+
         /// <inheritdoc/>
         IReadOnlyCollection<ulong> IGuildUser.RoleIds => GuildUser.Roles.Select(x => x.Id).ToImmutableArray();