It's not as bad as it sounds. The LLVM they ship in Xcode is basically stock, or at least it was in past releases where they posted the source - after all, Apple is the upstream for LLVM. And unlike the LLVM API, the LLVM bitcode format is stable and preserves backwards compatibility whenever possible, in the sense that newer versions of LLVM can read old bitcode. So there's a good chance that passing bitcode from rustc to Apple's tools will Just Work, and if it doesn't (like if rustc is using a newer LLVM than Xcode) then it should be fixable.
I agree that ostensibly it is possible. But what I would love is to see either: Apple show how to do it in a supported manner; or someone at least prove that it's possible. So far I haven't seen either.
To your comment about bitcode being stable, I don't think that bitcode stability and forward compatibility was something guarateed until 4.0.
That's using Apple's compiler to compile bitcode from rustc. The .bc file has significant chunks of Rust's libstd embedded, so the test isn't as trivial as it seems. To embed bitcode as they want for the App Store, you can use -fembed-bitcode, except it doesn't work because liballoc_jemalloc wasn't compiled with that option (but that's trivial to fix).
I also tried compiling for iOS, which seems to work, but I didn't bother to test the resulting binary.
(As for stability, according to the announcement[1], the previous policy was that bitcode would be readable "up to and including the next major release", which was already reasonable from the perspective of keeping a third-party compiler's output compatible.)
You could get rid of jemalloc, but on the other hand you could also just compile it with -fembed-bitcode. (It's part of the Rust standard library, so that's a bit more work. Rust code doesn't have this issue because .rlibs contain serialized ASTs, and passing -C lto makes rustc only use that rather than use the precompiled bits. In any case, it's just a matter of rebuilding.)
Incidentally, I just tried updating my nightly Rust, and it stopped working - clang started failing to read the bitcode, bailing out with a vague "error: Invalid record". This is not too surprising, because Rust just landed a big LLVM upgrade two weeks ago. Newer LLVM can read older bitcode files but not the other way around, and even though LLVM 4.0 was released months ago, Apple seems to only sync with trunk yearly, along with major Xcode releases. (You can tell based on the --version output.)
However, Rust can be built against an external LLVM (rather than the fork it uses by default), and AFAIK it tries to preserve compatibility with older versions. So it should still be possible to use the latest rustc, you just have to compile it yourself against a slightly older LLVM.
