Add support for RFC7162 (QRESYNC) VANISHED response.

[mordak]

RFC7162 defines a VANISHED response to the EXPUNGE command which must be used whenever a client has enabled QRESYNC. VANISHED may also appear in response to SELECT/EXAMINE or UID FETCH.

https://tools.ietf.org/html/rfc7162#section-3.2.10

The VANISHED response sends a sequence set of UIDs indicating UIDs which have been deleted from the mailbox.

I had to add parsers for sequence-range (eg 15:30) and sequence-set (eg. 1,10:20,25,28:31), and I put them in core.rs since they are actually defined in RFC 3501 and show up in a couple of other places, so may be generically useful.

I am not sure if the choice to return a Vec<RangeInclusive> is to your liking. Alternatives could be just a tuple of (start, end), or expand the range out into a big list of Vec<u32>. I experimented with both options and settled on the Vec<RangeInclusive> because:

• It is literally how the spec defines them
• It is impossible to misinterpret, whereas a user would need to read the RFC to know (start, end) is inclusive
• It is efficient in that large ranges can be represented and iterated with low memory overhead.

The downside is it can be a bit awkward to use, since you end up iterating a list of iterators. Of course, the nicest way to represent this for the user is to expand the sequence set out into a Vec<u32>, but this could be wasteful / dangerous in some circumstances.

[djc]

This is great, thanks! Would it be helpful to get a release including this?

[mordak]

Would it be helpful to get a release including this?

A release including this would be helpful, yes! :-)

[djc]

0.12.0 is out now.

[mordak]

0.12.0 is out now.

Thank you! Just opened the domino PR for rust-imap: jonhoo/rust-imap#172

And then my IMAP <-> Maildir sync engine will actually build against public crates!