Support atomic_fence instrinstic

[jonhoo]

I'd love to try miri on evmap, but sadly I get the following error:

error[E0080]: Miri evaluation error: unimplemented intrinsic: atomic_fence
    --> /home/jon/.rustup/toolchains/nightly-x86_64-unknown-linux-gnu/lib/rustlib/src/rust/src/libcore/sync/atomic.rs:2412:23
     |
2412 |             SeqCst => intrinsics::atomic_fence(),
     |                       ^^^^^^^^^^^^^^^^^^^^^^^^^^ Miri evaluation error: unimplemented intrinsic: atomic_fence
     |
note: inside call to `std::sync::atomic::fence` at src/write.rs:306:9
    --> src/write.rs:306:9
     |
306  |         atomic::fence(atomic::Ordering::SeqCst);
     |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Is this a known issue? Some cursory searching did not yield anything that seemed relevant.

[RalfJung]

This is just a missing shim. The shim should not do anything so this should be easy to fix.

[jonhoo]

I'm guess we also need to do something special to get the miri installed by rustup to pick this up?

[RalfJung]

Yeah, the miri submodule in rustc (at src/tools/miri) needs to be updated. Do you want to make a PR?

[jonhoo]

Sure, I'll do that, thanks!

[RalfJung]

Remember to run ./x.py (with no arguments) after updating the submodule (and comitting!) to make sure your Cargo.lock is up-to-date.

[jonhoo]

rust-lang/rust#65278