Will we be able to use code snippets from Wikipedia after this proposal is implemented?
Of course, we don't use code snippets from Wikipedia that often. I was just wondering.
Things licensed under creative commons share alike (such as wikipedian content) will not be licensable under MIT without permission from the author (all of the authors actually). See "Will we be able to reference code from before the license change in new posts?"
Notwithstanding the provisions of sections 106 and 106A, the fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include—
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work.
The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors.
I would argue that most of the content on the Stack Exchange network could be considered criticism, comment, teaching, or research. However, this burden is on the poster of the content that they are using the content appropriately.
Again, note that code can only be copyrighted and licensed if it meets the threshold of originality. In the US, the threshold of originality simply requires a minimal degree of creativity.
Since the posts are currently all licensed CC BY-SA, I think the Creative Commons guidance is most appropriate:
May I apply a CC license to my work if it incorporates material used under fair use or another exception or limitation to copyright?
Yes, but it is important to prominently mark any third party material you incorporate into your work so reusers do not think the CC license applies to that material. The CC license only applies to the rights you have in the work. For example, if your CC-licensed slide deck includes a Flickr image you are using pursuant to fair use, make sure to identify that image as not being subject to the CC license. For more information about incorporating work owned by others, see our page about marking third party content. Read more considerations for licensors here.
If you are using something under fair use, you can't apply a any license to it since you don't own it. You would need to mark the work appropriately to indicate to readers that it is not available under whatever license the content would otherwise be available under.