banredirect: +b mask$#chan bounces a banned user to another channel (loop-guarded)

src/modules/banredirect.rs

@@ -0,0 +1,31 @@
+//! banredirect — a ban of the form `+b <mask>$<#channel>` bounces a matching,
+//! banned user into `#channel` instead of refusing them outright. The redirect
+//! fires at most once, guarded by `Server.in_redirect` (shared with the `+L`
+//! full-channel redirect), so it can never loop. Reference: InspIRCd's
+//! `m_banredirect`. Original native Rust.
+
+use crate::channels::glob_match;
+use crate::server::Server;
+use crate::Uid;
+
+/// The mask part of a ban, without any `$#chan` redirect suffix — used both for
+/// matching (a redirect ban must still *block*) and for reading the target.
+pub fn mask_part(mask: &str) -> &str {
+    mask.split('$').next().unwrap_or(mask)
+}
+
+/// If `uid` is caught by a redirect ban (`mask$#chan`) in `key` with no matching
+/// exception, return the target channel. The caller has already established the
+/// user is banned; this extracts the `$#chan` destination.
+pub fn redirect_target(s: &Server, uid: Uid, key: &str) -> Option<String> {
+    let ch = s.channels.get(key)?;
+    let who = s.users.get(&uid)?.prefix();
+    // a matching +e exception cancels the ban, hence the redirect
+    if ch.excepts.iter().any(|e| glob_match(mask_part(&e.mask), &who)) {
+        return None;
+    }
+    ch.bans.iter().find_map(|b| {
+        let (mask, redir) = b.mask.split_once('$')?;
+        (redir.starts_with('#') && glob_match(mask, &who)).then(|| redir.to_string())
+    })
+}

src/modules/mod.rs

@@ -6,6 +6,7 @@
 pub mod account_registration;
 pub mod antimixedutf8;
 pub mod antirandom;
+pub mod banredirect;
 pub mod blockamsg;
 pub mod channames;
 pub mod channelban;