tor-socksproto: Style and info fixes in errors.

(The use of Cow<> in the error message isn't strictly necessary
here, but it's a pattern I want to encourage.)

crates/tor-socksproto/semver.md

@@ -0,0 +1 @@
+BREAKING: changes to Error type.

crates/tor-socksproto/src/err.rs

@@ -1,4 +1,6 @@
 //! Declare an error type for tor_socksproto
+use std::borrow::Cow;
+
 use thiserror::Error;
 
 use tor_error::{ErrorKind, HasKind};
@@ -14,7 +16,7 @@ pub enum Error {
     Syntax,
 
     /// Failed to decode a SOCKS message.
-    #[error("Error decoding message")]
+    #[error("Error decoding SOCKS message")]
     Decode(#[from] tor_bytes::Error),
 
     /// The SOCKS client declared a SOCKS version number that isn't
@@ -27,8 +29,8 @@ pub enum Error {
 
     /// The SOCKS client tried to use a SOCKS feature that we don't
     /// support at all.
-    #[error("SOCKS feature not implemented")]
-    NotImplemented,
+    #[error("SOCKS feature ({0}) not implemented")]
+    NotImplemented(Cow<'static, str>),
 
     /// Tried to progress the SOCKS handshake when it was already
     /// finished.  This is a programming error.
@@ -54,7 +56,7 @@ impl HasKind for Error {
                 EK::Internal
             }
             E::Syntax | E::Decode(_) | E::BadProtocol(_) => EK::LocalProtocolViolation,
-            E::NotImplemented => EK::NotImplemented,
+            E::NotImplemented(_) => EK::NotImplemented,
             E::AlreadyFinished(e) => e.kind(),
             E::Bug(e) => e.kind(),
         }

crates/tor-socksproto/src/handshake.rs

@@ -173,7 +173,7 @@ impl SocksHandshake {
             (State::Socks5Wait, [5, NO_AUTHENTICATION])
         } else {
             // In theory we should reply with "NO ACCEPTABLE METHODS".
-            return Err(Error::NotImplemented);
+            return Err(Error::NotImplemented("authentication methods".into()));
         };
 
         self.state = next;
@@ -190,7 +190,9 @@ impl SocksHandshake {
 
         let ver = r.take_u8()?;
         if ver != 1 {
-            return Err(Error::NotImplemented);
+            return Err(Error::NotImplemented(
+                format!("username/password version {}", ver).into(),
+            ));
         }
 
         let ulen = r.take_u8()?;
@@ -437,7 +439,7 @@ mod test {
     fn socks5_init_nothing_works() {
         let mut h = SocksHandshake::new();
         let a = h.handshake(&hex!("05 02 9988")[..]);
-        assert!(matches!(a, Ok(Err(Error::NotImplemented))));
+        assert!(matches!(a, Ok(Err(Error::NotImplemented(_)))));
     }
 
     #[test]

crates/tor-socksproto/src/msg.rs

@@ -186,7 +186,9 @@ impl SocksRequest {
         auth: SocksAuth,
     ) -> Result<Self> {
         if !cmd.recognized() {
-            return Err(Error::NotImplemented);
+            return Err(Error::NotImplemented(
+                format!("SOCKS command {}", cmd).into(),
+            ));
         }
         if port == 0 && cmd.requires_port() {
             return Err(Error::Syntax);
@@ -283,7 +285,7 @@ mod test {
             1024,
             SocksAuth::NoAuth,
         );
-        assert!(matches!(e, Err(Error::NotImplemented)));
+        assert!(matches!(e, Err(Error::NotImplemented(_))));
 
         let e = SocksRequest::new(
             SocksVersion::V4,