feat: serve gopher with dot-stuffing and a synthetic root menu

core/src/config.rs

@@ -125,6 +125,15 @@ impl Protocol {
     pub fn negotiates(self) -> bool {
         matches!(self, Protocol::Http)
     }
+
+    /// Whether a listener exists for this protocol.
+    ///
+    /// Gemini needs a TLS stack and per-host certificate selection, which is its
+    /// own piece of work; naming it in a configuration is refused here rather
+    /// than panicking on the first connection.
+    pub fn is_implemented(self) -> bool {
+        !matches!(self, Protocol::Gemini)
+    }
 }
 
 /// What validation established, and what `--check` prints.
@@ -254,6 +263,12 @@ impl ServerConfig {
     fn validate_listeners(&self, available: &[&str]) -> Result<BTreeSet<String>, Error> {
         let mut formats = BTreeSet::new();
         for (name, spec) in &self.listener {
+            if !spec.protocol.is_implemented() {
+                return Err(Error::config(format!(
+                    "listener '{name}': {} is not implemented yet",
+                    spec.protocol.as_str()
+                )));
+            }
             if spec.formats.is_empty() {
                 return Err(Error::config(format!("listener '{name}': formats must not be empty")));
             }