chore: refactor VCs to use a watch channel internally (but I'm going to rework this in the next commit anyway)

src/command/leave.rs

@@ -37,9 +37,6 @@ pub enum GetGuildAndVoiceChannelIdError {
     /// there is no user who invoked this command
     NoUser,
 
-    /// there are no voice chats in this guild
-    NoVCsInGuild,
-
     /// the bot is not in a voice chat in this guild
     BotNotInVC,
 }
@@ -58,10 +55,10 @@ pub fn get_user_and_guild_and_voice_channel_id(
 
     let guild_id = interaction.guild_id.context(NotInGuildSnafu)?;
 
-    let guild_vcs = vcs.get(&guild_id).context(NoVCsInGuildSnafu)?;
-
-    let &voice_channel_id = guild_vcs
-        .get_left_for(&bot_user_id)
+    let voice_channel_id = vcs
+        .with_guild(guild_id, |guild_vcs| {
+            guild_vcs.get_left_for(&bot_user_id).copied()
+        })
         .context(BotNotInVCSnafu)?;
 
     Ok((user_id, guild_id, voice_channel_id))
@@ -75,9 +72,6 @@ fn get_guild_and_vc_error_to_embed(error: GetGuildAndVoiceChannelIdError) -> Emb
         GetGuildAndVoiceChannelIdError::NoUser => {
             EmbedBuilder::new().title("Not invoked by a user").description("This command works by joining the same VC as the user, but this bot didn't receive any user data. So did no user invoke it?! (This error should be impossible!)").validate().unwrap().build()
         },
-        GetGuildAndVoiceChannelIdError::NoVCsInGuild => {
-            EmbedBuilder::new().title("No VCs in this server").description("This bot can't leave VC because there aren't any in this server right now (therefore the bot must not be in any).").validate().unwrap().build()
-        },
         GetGuildAndVoiceChannelIdError::BotNotInVC => {
             EmbedBuilder::new().title("Not in a VC").description("This bot can't leave VC if it isn't in one in this server.").validate().unwrap().build()
         },