Some updates. I'm hoping to push emscripten support across the finish line in the next 2 or so months. That is, for the project to distribute emscripten-targetting compilers and stds (whether they contain awful bugs is another matter).
The short term plan is for kripken to port emscripten's LLVM branch forward to the same merge-base as ours, then for us to rebase onto their branch. We don't have any immediate plans to upgrade LLVM, but in the future we'll have try not to do it too often, because each time will force emscripten to upgrade as well. Likewise we'll have to upgrade when they do, but they don't anticipate needing to either. And we'll have to tolerate temporary breakage of our emscripten support if it gets difficult to do it simultaneously. It could end in disaster. Who knows.
Once we're on their LLVM, distributing emscripten bins is mostly a matter of turning some knobs on the CI.
In the longer term, kripken is very keen on creating a MIR->wasm backend for Rust, bypassing LLVM. If it works out then it could open up a lot of really interesting possibilities.
@tomaka is the best way to compile for emscripten still to use my 'emscripten' branch or do you have something more up to date? I'll aim to rebase it onto Rust master soon.