source_encoding

        # Encoding::InvalidByteSequenceError.source_encoding

(from ruby core)
### Implementation from InvalidByteSequenceError
---
    ecerr.source_encoding              -> encoding

---

Returns the source encoding as an encoding object.

Note that the result may not be equal to the source encoding of the
encoding converter if the conversion has multiple steps.

    ec = Encoding::Converter.new("ISO-8859-1", "EUC-JP") # ISO-8859-1 -> UTF-8 -> EUC-JP
    begin
      ec.convert("\xa0") # NO-BREAK SPACE, which is available in UTF-8 but not in EUC-JP.
    rescue Encoding::UndefinedConversionError
      p $!.source_encoding              #=> #<Encoding:UTF-8>
      p $!.destination_encoding         #=> #<Encoding:EUC-JP>
      p $!.source_encoding_name         #=> "UTF-8"
      p $!.destination_encoding_name    #=> "EUC-JP"
    end