identifiers: Don't require room IDs to contain a server name

Room IDs being splittable into localpart and servername does not have
much inherent value and there are proposals like MSC4051¹ that propose
changing the format. Relaxing the rules makes Ruma forwards-compatible
with those proposals. The server_name accessor is kept because it is
used by at least one downstream, but is updated to return an `Option`.

¹ matrix-org/matrix-spec-proposals#4051

crates/ruma-common/src/identifiers/room_id.rs

@@ -33,14 +33,10 @@ impl RoomId {
         Self::from_borrowed(&format!("!{}:{server_name}", super::generate_localpart(18))).to_owned()
     }
 
-    /// Returns the rooms's unique ID.
-    pub fn localpart(&self) -> &str {
-        &self.as_str()[1..self.colon_idx()]
-    }
-
     /// Returns the server name of the room ID.
-    pub fn server_name(&self) -> &ServerName {
-        ServerName::from_borrowed(&self.as_str()[self.colon_idx() + 1..])
+    pub fn server_name(&self) -> Option<&ServerName> {
+        let colon_idx = self.as_str().find(':')?;
+        Some(ServerName::from_borrowed(&self.as_str()[colon_idx + 1..]))
     }
 
     /// Create a `matrix.to` URI for this room ID.
@@ -202,10 +198,6 @@ impl RoomId {
             None,
         )
     }
-
-    fn colon_idx(&self) -> usize {
-        self.as_str().find(':').unwrap()
-    }
 }
 
 #[cfg(test)]
@@ -287,8 +279,11 @@ mod tests {
     }
 
     #[test]
-    fn missing_room_id_delimiter() {
-        assert_eq!(<&RoomId>::try_from("!29fhd83h92h0").unwrap_err(), IdParseError::MissingColon);
+    fn missing_server_name() {
+        assert_eq!(
+            <&RoomId>::try_from("!29fhd83h92h0").expect("Failed to create RoomId."),
+            "!29fhd83h92h0"
+        );
     }
 
     #[test]

crates/ruma-identifiers-validation/src/room_id.rs

@@ -1,5 +1,10 @@
-use crate::{validate_delimited_id, Error};
+use crate::{server_name, validate_id, Error};
 
 pub fn validate(s: &str) -> Result<(), Error> {
-    validate_delimited_id(s, &['!'])
+    validate_id(s, &['!'])?;
+    if let Some(colon_idx) = s.find(':') {
+        server_name::validate(&s[colon_idx + 1..])?;
+    }
+
+    Ok(())
 }