consensus_encoding: remove Transitioning state

Simplify the Decoder2State by dropping the unnecessary Transitioning
variant. We can just use the Errored variant and not change the state
of the Decoder2 if the first decoder fails while transitioning to the
second.

Author: nyonson

consensus_encoding/src/decode/decoders.rs

@@ -295,26 +295,6 @@ enum Decoder2State<A: Decoder, B: Decoder> {
     Second(A::Output, B),
     /// Decoder has failed and cannot be used again.
     Errored,
-    /// Temporary state during transitions from First to Second, should never be observed.
-    Transitioning,
-}
-
-impl<A: Decoder, B: Decoder> Decoder2State<A, B> {
-    /// Transitions from the first state to second by extracting both decoders.
-    ///
-    /// We use `mem::replace` to atomically swap the entire state, giving us
-    /// ownership of the decoders so we can consume the first decoder while
-    /// holding a mutable reference to the state.
-    ///
-    /// If this method is called when not in the `First` state, we panic
-    /// with `#[track_caller]` to show where the bug occurred.
-    #[track_caller]
-    fn transition(&mut self) -> (A, B) {
-        match mem::replace(self, Decoder2State::Transitioning) {
-            Decoder2State::First(first, second) => (first, second),
-            _ => panic!("transition called on invalid state"),
-        }
-    }
 }
 
 impl<A, B, Err> Decoder2<A, B, Err>
@@ -347,12 +327,15 @@ where
                     }
 
                     // First decoder is complete, transition to second.
-                    let (first, second) = self.state.transition();
-                    let first_result = first.end().map_err(|error| {
-                        self.state = Decoder2State::Errored;
-                        Err::from(error)
-                    })?;
-                    self.state = Decoder2State::Second(first_result, second);
+                    // If the first decoder fails, the composite decoder
+                    // remains in an Errored state.
+                    match mem::replace(&mut self.state, Decoder2State::Errored) {
+                        Decoder2State::First(first, second) => {
+                            let first_result = first.end()?;
+                            self.state = Decoder2State::Second(first_result, second);
+                        }
+                        _ => unreachable!("we know we're in First state"),
+                    }
                 }
                 Decoder2State::Second(_, second_decoder) => {
                     return second_decoder.push_bytes(bytes).map_err(|error| {
@@ -363,9 +346,6 @@ where
                 Decoder2State::Errored => {
                     panic!("use of failed decoder");
                 }
-                Decoder2State::Transitioning => {
-                    panic!("use of decoder in transitioning state");
-                }
             }
         }
     }
@@ -388,9 +368,6 @@ where
             Decoder2State::Errored => {
                 panic!("use of failed decoder");
             }
-            Decoder2State::Transitioning => {
-                panic!("use of decoder in transitioning state");
-            }
         }
     }
 
@@ -401,7 +378,6 @@ where
                 first_decoder.read_limit() + second_decoder.read_limit(),
             Decoder2State::Second(_, second_decoder) => second_decoder.read_limit(),
             Decoder2State::Errored => 0,
-            Decoder2State::Transitioning => 0,
         }
     }
 }