Make ChannelMethod non-exhaustive

Enums with variants conditional on cargo features must be
non-exhaustive, because cargo features are supposed to be additive,
meaning that enabling a feature (which might happen due to some random
distant thing) ought not to break things using that enum.

There were surprisingly few places to fix this.

crates/tor-chanmgr/src/transport/proxied.rs

@@ -278,6 +278,12 @@ impl<R: TcpProvider + Send + Sync> TransportHelper for ExternalProxyPlugin<R> {
                 )))
             }
             ChannelMethod::Pluggable(target) => target,
+            other => {
+                return Err(crate::Error::UnusableTarget(bad_api_usage!(
+                    "Used unknown, unsupported, transport {:?} for a TCP connection.",
+                    other,
+                )))
+            }
         };
 
         let protocol = settings_to_protocol(encode_settings(pt_target.settings()))?;

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

@@ -76,7 +76,9 @@ struct Inner {
     /// Address and transport via which the bridge can be reached, and
     /// the parameters for those transports.
     ///
-    /// Restriction: This `addrs` may NOT contain more than one address.
+    /// Restriction: This `addrs` may NOT contain more than one address,
+    /// and it must be a variant supported by the code in this crate:
+    /// ie, currently, `Direct` or `Pluggable`.
     addrs: ChannelMethod,
 
     /// The RSA identity of the bridge.
@@ -366,6 +368,11 @@ impl FromStr for BridgeConfigBuilder {
                 let (transport, addr, settings) = target.into_parts();
                 (transport.into_inner(), vec![addr], settings.into_inner())
             }
+            other => {
+                return Err(BridgeParseError::UnsupportedChannelMethod {
+                    method: Box::new(other),
+                });
+            }
         };
 
         let ids = chain!(
@@ -518,6 +525,7 @@ impl FromStr for Inner {
                         source,
                     }
                 })?,
+                other => panic!("made ourselves an unsupported ChannelMethod {:?}", other),
             }
         }
 
@@ -556,6 +564,12 @@ impl Display for BridgeConfig {
                 write!(f, "{} {}", target.transport(), target.addr())?;
                 Some(target.settings())
             }
+
+            _ => {
+                // This shouldn't happen, but panicking seems worse than outputting this
+                write!(f, "[unsupported channel method, cannot display properly]")?;
+                return Ok(());
+            }
         };
 
         //  * One or more identity key fingerprints,

crates/tor-guardmgr/src/bridge/config/err.rs

@@ -5,6 +5,8 @@
 
 use thiserror::Error;
 
+use tor_linkspec::ChannelMethod;
+
 /// Error when parsing a bridge line from a string
 #[derive(Error, Clone, Debug)]
 #[non_exhaustive]
@@ -94,6 +96,16 @@ pub enum BridgeParseError {
         word: String,
     },
 
+    /// Channel method specified of unsupported type
+    ///
+    /// This can only occur with unusual (unsupported) combinations of cargo features,
+    /// or building an older `tor-guardmgr` against a newer `tor-linkspec`.
+    #[error("Channel method specified but not of supported type ({method:?})")]
+    UnsupportedChannelMethod {
+        /// The not-understood method
+        method: Box<ChannelMethod>,
+    },
+
     /// Parameters may only be specified with a pluggable transport
     #[cfg(feature = "bridge-client")]
     #[error("Parameters supplied but not valid without a pluggable transport")]

crates/tor-linkspec/semver.md

@@ -9,3 +9,4 @@ BREAKING: Replaced functions to strip addresses from ChanMethod.
 BREAKING: Remove impl Display for OwnedCircTarget.
 ADDED: Provide deconstructors for PtTargetSettings and PtTarget
 MODIFIED: Renaming PtTargetAddr to BridgeAddr (in progress, will become BREAKING)
+BREAKING: ChannelMethod is now non-exhaustive

crates/tor-linkspec/src/transport.rs

@@ -457,7 +457,7 @@ impl PtTarget {
 /// pluggable transport, this includes information about the transport, and any
 /// address and settings information that transport requires.
 #[derive(Clone, Debug, Eq, PartialEq, Hash)]
-#[allow(clippy::exhaustive_enums)]
+#[non_exhaustive]
 pub enum ChannelMethod {
     /// Connect to the relay directly at one of several addresses.
     Direct(Vec<std::net::SocketAddr>),