Add a function for parsing a channel ID from a channel URL (#3075)

Similar to message URLs, Discord also provides URLs for guild channels.

Additionally, the function is added as an alternative for parsing a `Channel` from a string. 
Private channels are not affected by this change.

src/utils/argument_convert/channel.rs

@@ -45,7 +45,12 @@ async fn lookup_channel_global(
     guild_id: Option<GuildId>,
     s: &str,
 ) -> Result<Channel, ChannelParseError> {
-    if let Some(channel_id) = s.parse().ok().or_else(|| crate::utils::parse_channel_mention(s)) {
+    if let Some(channel_id) = s
+        .parse()
+        .ok()
+        .or_else(|| crate::utils::parse_channel_mention(s))
+        .or_else(|| crate::utils::parse_channel_url(s).map(|(_, channel_id)| channel_id))
+    {
         return channel_id.to_channel(ctx, guild_id).await.map_err(ChannelParseError::Http);
     }
 

src/utils/argument_convert/mod.rs

@@ -140,6 +140,44 @@ pub fn parse_message_url(s: &str) -> Option<(GuildId, ChannelId, MessageId)> {
     None
 }
 
+/// Retrieves guild, and channel ID from a channel URL.
+///
+/// If the URL is malformed, None is returned.
+///
+/// # Examples
+/// ```rust
+/// use serenity::model::prelude::*;
+/// use serenity::utils::parse_channel_url;
+///
+/// assert_eq!(
+///     parse_channel_url("https://discord.com/channels/381880193251409931/381880193700069377"),
+///     Some((GuildId::new(381880193251409931), ChannelId::new(381880193700069377),)),
+/// );
+/// assert_eq!(
+///     parse_channel_url(
+///         "https://canary.discord.com/channels/381880193251409931/381880193700069377"
+///     ),
+///     Some((GuildId::new(381880193251409931), ChannelId::new(381880193700069377),)),
+/// );
+/// assert_eq!(parse_channel_url("https://google.com"), None);
+/// ```
+#[must_use]
+pub fn parse_channel_url(s: &str) -> Option<(GuildId, ChannelId)> {
+    use aformat::{aformat, CapStr};
+
+    for domain in DOMAINS {
+        let prefix = aformat!("https://{}/channels/", CapStr::<MAX_DOMAIN_LEN>(domain));
+        if let Some(parts) = s.strip_prefix(prefix.as_str()) {
+            let mut parts = parts.splitn(2, '/');
+
+            let guild_id = parts.next()?.parse().ok()?;
+            let channel_id = parts.next()?.parse().ok()?;
+            return Some((guild_id, channel_id));
+        }
+    }
+    None
+}
+
 const MAX_DOMAIN_LEN: usize = {
     let mut max_len = 0;
     let mut i = 0;