Don't check for availability of usernames that aren't ASCII (#4469)

crates/matrix-synapse/src/lib.rs

@@ -209,6 +209,12 @@ impl HomeserverConnection for SynapseConnection {
         err(Debug),
     )]
     async fn is_localpart_available(&self, localpart: &str) -> Result<bool, anyhow::Error> {
+        // Synapse will give us a M_UNKNOWN error if the localpart is not ASCII,
+        // so we bail out early
+        if !localpart.is_ascii() {
+            return Ok(false);
+        }
+
         let localpart = urlencoding::encode(localpart);
 
         let response = self