Give better error messages when decoding data with trailing newlines

src/decode.rs

@@ -212,7 +212,17 @@ fn decode_helper(
  // and the fast decode logic cannot handle padding
  0 => INPUT_CHUNK_LEN,
  // 1 and 5 trailing bytes are illegal: can't decode 6 bits of input into a byte
- 1 | 5 => return Err(DecodeError::InvalidLength),
+ 1 | 5 => {
+ // trailing whitespace is so common that it's worth it to check the last byte to
+ // possibly return a better error message
+ if let Some(b) = input.last() {
+ if decode_table[*b as usize] == tables::INVALID_VALUE {
+ return Err(DecodeError::InvalidByte(input.len() - 1, *b));
+ }
+ }
+
+ return Err(DecodeError::InvalidLength);
+ }
  // This will decode to one output byte, which isn't enough to overwrite the 2 extra bytes
  // written by the fast decode loop. So, we have to ignore both these 2 bytes and the
  // previous chunk.
@@ -865,4 +875,14 @@ mod tests {
  decode_config(b"+//+", crate::STANDARD_NO_PAD)
  );
  }
+
+ #[test]
+ fn decode_invalid_trailing_bytes() {
+ // The case of trailing newlines is common enough to warrant a test for a good error
+ // message.
+ assert_eq!(
+ decode(b"Zm9vCg==\n"),
+ Err(DecodeError::InvalidByte(8, b'\n'))
+ );
+ }
 }