[bencode/de] Improved syntax error messages

src/bencode/de.rs

@@ -85,11 +85,13 @@ impl <'de, 'a> de::Deserializer<'de> for &'a mut Deserializer<'de> {
     where
         V: Visitor<'de>
     {
-        match self.peek_byte()? as char {
+        let b = self.peek_byte()? as char;
+        match b {
             'i' => self.deserialize_i64(visitor),
             'l' => self.deserialize_seq(visitor),
             'd' => self.deserialize_map(visitor),
-            _ => Err(Error::Syntax),
+            '0'..='9' => self.deserialize_str(visitor),
+            _ => Err(Error::Syntax(format!("any: invalid start char: {}", b))),
         }
     }
 
@@ -278,14 +280,15 @@ impl <'de, 'a> de::Deserializer<'de> for &'a mut Deserializer<'de> {
     where
         V: Visitor<'de>
     {
-        match self.peek_byte()? as char{
+        let b = self.peek_byte()? as char;
+        match b {
             'l' => self.deserialize_seq(visitor),
             '0'..='9' => {
                 let str = self.parse_byte_string()?;
                 let ba = ByteAccess::new(self, str);
                 visitor.visit_seq(ba)
             },
-            _ => Err(Error::Syntax),
+            _ => Err(Error::Syntax(format!("tuple: invalid start char: {}", b))),
         }
     }
 
@@ -397,12 +400,13 @@ impl<'a, 'de> SeqAccess<'de> for RegularAccess<'a, 'de> {
     where
         T: DeserializeSeed<'de>
     {
-        match self.de.peek_byte()? as char {
+        let b = self.de.peek_byte()? as char;
+        match b {
             'd' | 'i' | 'l' | '0'..='9' => {
                 seed.deserialize(&mut *self.de).map(Some)
             },
             'e' => Ok(None),
-            _ => Err(Error::Syntax)
+            _ => Err(Error::Syntax(format!("seq: next_element: invalid start/end char: {}", b)))
         }
     }
 }
@@ -427,11 +431,12 @@ impl<'a, 'de> MapAccess<'de> for RegularAccess<'a, 'de> {
     where
         V: DeserializeSeed<'de>
     {
-        match self.de.peek_byte()? as char {
+        let b = self.de.peek_byte()? as char;
+        match b {
             'd' | 'i' | 'l' | '0'..='9' => {
                 seed.deserialize(&mut *self.de)
             },
-            _ => Err(Error::Syntax)
+            _ => Err(Error::Syntax(format!("map: next_value: invalid start char: {}", b)))
         }
     }
 }

src/bencode/error.rs

@@ -9,7 +9,7 @@ pub enum Error {
 
     WontImplement,
     Eof,
-    Syntax,
+    Syntax(String),
     InvalidUtf8,
     ExpectedBytes,
     ExpectedBytesSep,
@@ -48,8 +48,8 @@ impl Display for Error {
             Error::Message(msg) => formatter.write_str(msg),
             Error::WontImplement => formatter.write_str("there is no reasonable way to (de)serialize this type"),
             Error::Eof => formatter.write_str("unexpected end of input"),
-            Error::Syntax => formatter.write_str("syntax error"),
+            Error::Syntax(msg) => formatter.write_fmt(format_args!("syntax error: {}", msg)),
-            Error::InvalidUtf8 => formatter.write_str("could not decoded as UTF-8"),
+            Error::InvalidUtf8 => formatter.write_str("could not decode as UTF-8"),
             Error::ExpectedBytes => formatter.write_str("expected byte string start char: any number"),
             Error::ExpectedBytesSep => formatter.write_str("expected byte separator char: ':'"),
             Error::ExpectedInteger => formatter.write_str("expected integer start char 'i'"),