Remove the IO error from DecodeError

The `DecodeError` (badly named) consensus decodes an object from an
iterator that implements `Read`. The `Read` impl never returns a real IO
error, we use the `io::Error` to temporarily wrap the error returned by
the inner iterator and unwrap it in `IterReader::decode`. As such there
is no reason for the `DecodeError` to hold an `encode::Error`, it can
hold an `encode::ParseError`.

The value of this change is easily seen in the removal of calls to
`unreachable`.

bitcoin/src/consensus/encode.rs

@@ -1164,7 +1164,7 @@ mod tests {
         hex.push_str("abcdef");
         assert!(matches!(
             deserialize_hex::<Transaction>(&hex).unwrap_err(),
-            FromHexError::Decode(DecodeError::TooManyBytes)
+            FromHexError::Decode(DecodeError::Unconsumed)
         ));
     }
 }

bitcoin/src/consensus/error.rs

@@ -51,14 +51,17 @@ impl From<ParseError> for DeserializeError {
 }
 
 /// Error when consensus decoding from an `[IterReader]`.
+///
+/// This is the same as a `DeserializeError` with an additional variant to return any error yealded
+/// by the inner bytes iterator.
 #[derive(Debug)]
 pub enum DecodeError<E> {
-    /// Attempted to decode an object from an iterator that yielded too many bytes.
-    TooManyBytes,
     /// Invalid consensus encoding.
-    Consensus(Error),
+    Parse(ParseError),
+    /// Data unconsumed error.
+    Unconsumed,
     /// Other decoding error.
-    Other(E),
+    Other(E),                   // Yielded by the inner iterator.
 }
 
 internals::impl_from_infallible!(DecodeError<E>);
@@ -68,9 +71,8 @@ impl<E: fmt::Debug> fmt::Display for DecodeError<E> {
         use DecodeError::*;
 
         match *self {
-            TooManyBytes =>
-                write!(f, "attempted to decode object from an iterator that yielded too many bytes"),
-            Consensus(ref e) => write_err!(f, "invalid consensus encoding"; e),
+            Parse(ref e) => write_err!(f, "error parsing encoded object"; e),
+            Unconsumed => write!(f, "data not consumed entirely when deserializing"),
             Other(ref other) => write!(f, "other decoding error: {:?}", other),
         }
     }
@@ -82,8 +84,8 @@ impl<E: fmt::Debug + std::error::Error + 'static> std::error::Error for DecodeEr
         use DecodeError::*;
 
         match *self {
-            TooManyBytes => None,
-            Consensus(ref e) => Some(e),
+            Parse(ref e) => Some(e),
+            Unconsumed => None,
             Other(ref e) => Some(e),
         }
     }

bitcoin/src/consensus/mod.rs

@@ -38,12 +38,12 @@ impl<E: fmt::Debug, I: Iterator<Item = Result<u8, E>>> IterReader<E, I> {
     fn decode<T: Decodable>(mut self) -> Result<T, DecodeError<E>> {
         let result = T::consensus_decode(&mut self);
         match (result, self.error) {
-            (Ok(_), None) if self.iterator.next().is_some() => Err(DecodeError::TooManyBytes),
+            (Ok(_), None) if self.iterator.next().is_some() => Err(DecodeError::Unconsumed),
             (Ok(value), None) => Ok(value),
             (Ok(_), Some(error)) => panic!("{} silently ate the error: {:?}", core::any::type_name::<T>(), error),
 
             (Err(consensus::encode::Error::Io(io_error)), Some(de_error)) if io_error.kind() == io::ErrorKind::Other && io_error.get_ref().is_none() => Err(DecodeError::Other(de_error)),
-            (Err(consensus_error), None) => Err(DecodeError::Consensus(consensus_error)),
+            (Err(consensus::encode::Error::Parse(parse_error)), None) => Err(DecodeError::Parse(parse_error)),
             (Err(consensus::encode::Error::Io(io_error)), de_error) => panic!("unexpected IO error {:?} returned from {}::consensus_decode(), deserialization error: {:?}", io_error, core::any::type_name::<T>(), de_error),
             (Err(consensus_error), Some(de_error)) => panic!("{} should've returned `Other` IO error because of deserialization error {:?} but it returned consensus error {:?} instead", core::any::type_name::<T>(), de_error, consensus_error),
         }

bitcoin/src/consensus/serde.rs

@@ -17,7 +17,7 @@ use serde::de::{SeqAccess, Unexpected, Visitor};
 use serde::ser::SerializeSeq;
 use serde::{Deserializer, Serializer};
 
-use super::{Decodable, Encodable, Error, ParseError};
+use super::{Decodable, Encodable, ParseError};
 use crate::consensus::{DecodeError, IterReader};
 
 /// Hex-encoding strategy
@@ -387,10 +387,8 @@ where
     fn unify(self) -> E {
         match self {
             DecodeError::Other(error) => error,
-            DecodeError::TooManyBytes => E::custom(format_args!("got more bytes than expected")),
-            DecodeError::Consensus(Error::Parse(e)) => consensus_error_into_serde(e),
-            DecodeError::Consensus(Error::Io(_)) =>
-                unreachable!("iterator never returns I/O error"),
+            DecodeError::Unconsumed => E::custom(format_args!("got more bytes than expected")),
+            DecodeError::Parse(e) => consensus_error_into_serde(e),
         }
     }
 }
@@ -402,10 +400,8 @@ where
     fn into_de_error<DE: serde::de::Error>(self) -> DE {
         match self {
             DecodeError::Other(error) => error.into_de_error(),
-            DecodeError::TooManyBytes => DE::custom(format_args!("got more bytes than expected")),
-            DecodeError::Consensus(Error::Parse(e)) => consensus_error_into_serde(e),
-            DecodeError::Consensus(Error::Io(_)) =>
-                unreachable!("iterator never returns I/O error"),
+            DecodeError::Unconsumed => DE::custom(format_args!("got more bytes than expected")),
+            DecodeError::Parse(e) => consensus_error_into_serde(e),
         }
     }
 }