Start on a new BridgeRelay type.

This is the one we'll actually use to connect to bridges. It
has a `Bridge` line, and an optional `BridgeDesc`.

Maybe this will turn into a `BridgeRelay<'a>` by analogy to `Relay`
some time; I'm not sure.

crates/tor-guardmgr/src/bridge.rs

@@ -12,6 +12,8 @@
 
 mod config;
 mod descs;
+mod relay;
 
 pub use config::Bridge;
-pub use descs::{BridgeDescEvent, BridgeDescList, BridgeDescProvider};
+pub use descs::{BridgeDesc, BridgeDescEvent, BridgeDescList, BridgeDescProvider};
+pub use relay::BridgeRelay;

crates/tor-guardmgr/src/bridge/relay.rs

@@ -0,0 +1,38 @@
+//! Implementation code to make a bridge something that we can connect to and use to relay traffic.
+
+use std::sync::Arc;
+
+use tor_linkspec::{HasRelayIds, RelayIdRef, RelayIdType};
+
+use super::{Bridge, BridgeDesc};
+
+/// The information about a Bridge that is necessary to connect to it and relay traffic.
+#[derive(Clone, Debug)]
+
+pub struct BridgeRelay {
+    /// The local configurations for the bridge.
+    ///
+    /// This is _always_ necessary, since it without it we can't know whether
+    /// any pluggable transports are needed.
+    bridge_line: Arc<Bridge>,
+
+    /// A descriptor for the bridge.
+    ///
+    /// If present, it MUST have every RelayId that the `bridge_line` does.
+    desc: Option<BridgeDesc>,
+}
+
+impl BridgeRelay {
+    /// Return true if this BridgeRelay has a known descriptor and can be used for relays.
+    pub fn has_descriptor(&self) -> bool {
+        self.desc.is_some()
+    }
+}
+
+impl HasRelayIds for BridgeRelay {
+    fn identity(&self, key_type: RelayIdType) -> Option<RelayIdRef<'_>> {
+        self.bridge_line
+            .identity(key_type)
+            .or_else(|| self.desc.as_ref().and_then(|d| d.identity(key_type)))
+    }
+}