The URL: https://rust-lang-nursery.github.io/rustc-guide/
Produces:

Maybe someone changed something in the settings of the repo? I would check, but I don't have access.
Hmm... that might have been me. I fat-fingered the delete-branch button earlier and restored the branch, but maybe github didn't catch on? (It's a good thing the master branch is protected!)
I just pushed a dummy commit to the gh-pages branch in hopes of spurring github to rerender...
@nikomatsakis Could you confirm that the GH pages is still enabled?
You may have to reconfigure it in the settings if the branch was deleted.
Oh, :disappointed:
I don't have permissions to fiddle with settings...
@nikomatsakis confirmed that the right branch is enabled, but it appears to be stuck building the site. I'm going to contact support.
Thanks for investigating @sunjay
I contacted Github support and the site is back up! Here's their reply:
Hi Sunjay,
We require that an admin of the repo create the initial Pages build. It looks like all the pushes on this repo to the gh-pages branch have happened from users that are not an admin or owner.
I went ahead and triggered a build for you manually, but going forward have the owner of the repo create a commit in new repos and the site will build automatically.
Now that the site has been built once, future pushes will automatically trigger a rebuild regardless of which user pushes the changes.
Hope this helps! Let us know if you need anything else.
So hopefully that clears up what happened.
Thanks @sunjay !
Most helpful comment
I contacted Github support and the site is back up! Here's their reply:
So hopefully that clears up what happened.