Support citext postgres type

[lancecarlson]

Likely this should be fine:

type Citext = Text

Also need to check if the right coersions exist and that codegen will place nice with citext[]

Notes from gitter^ :)

[killercup]

The citext module provides a case-insensitive character string type, citext. Essentially, it internally calls lower when comparing values. Otherwise, it behaves almost exactly like text.

https://www.postgresql.org/docs/9.6/static/citext.html

[sgrif]

This will have the same caveats as varchar when it comes to arrays.

Other than that this should be fine. The binary representation is identical to text.

[Lukazoid]

Should this not be reopened now support for citext has been removed?

[sgrif]

No, this isn't something anybody on the team is actively working on -- We'll be publishing a new policy soon about what we're doing with the issue tracker, but we're moving towards issues open here either being on the team's roadmap or things that are good for new contributors. If you'd like to discuss implementing this feature, you can start a thread at https://discourse.diesel.rs

[Boscop]

Does this mean that citext is not supported anymore? Why?
I really want to use citext for e.g. username and email address columns.

[sgrif]

@Boscop Because the support we had was broken, and requires a more complete solution. We only accepted it in the first place because it was initially thought that a type alias would be sufficient. In general we don't support extensions in Diesel proper, since they can easily live in third party crates. Supporting this type is no different than supporting any other custom type as I've mentioned in #1624 (comment). If you're interested in supporting a crate for this and have questions after taking a swing at it feel free to ask in Gitter