Ambiguous usage of std::io::ErrorKind::InvalidInput

[mzabaluev]

The error code InvalidInput on std::io::Error is currently used for at least two different purposes:

• Indicate invalid programmatic usage, as in seeking to a negative position.
• Report invalid data, such as when read_to_string reads content that is not valid UTF-8.

I think these two conditions are sufficiently different to deserve separate error codes.

[alexcrichton]

cc rust-lang/rust#16666 (somewhat related)

[bluss]

This clarification is needed, but does it count as a breaking change?

To minimize impact to users it would be better to change the definition of InvalidInput to cover invalid data, and move programmer errors to a different error code?

[mzabaluev]

@bluss Yes, the change could go the other way around, and the name InvalidInput describes data format errors better.