Fail to build rustfmt since rustc 1.40.0-nightly (c27f7568b 2019-10-13)

[topecongiro]

git clone https://github.com/rust-lang/rustfmt
cd rustfmt
cargo b

Running the above commands will fail with the following error when using rustc 1.40.0-nightly (e413dc36a 2019-10-14) in Linux, macOS and Windows:

   Compiling crossbeam-channel v0.3.9
error[E0432]: unresolved import `crossbeam_utils::atomic`
 --> /Users/seiichi.uchida/.cargo/registry/src/github.com-1ecc6299db9ec823/crossbeam-channel-0.3.9/src/flavors/tick.rs:8:22
  |
8 | use crossbeam_utils::atomic::AtomicCell;
  |                      ^^^^^^ could not find `atomic` in `crossbeam_utils`

I have tested with multiple recent nightlies. It turns out that the build started to fail since rustc 1.40.0-nightly (c27f7568b 2019-10-13). Building with rustc 1.40.0-nightly (1721c9685 2019-10-12) or earlier succeeded without any error.

[sinkuu]

crossbeam_utils::atomic module is gated under cfg(all(target_has_atomic = "cas", target_has_atomic = "ptr")).

https://github.com/crossbeam-rs/crossbeam/blob/ae148dec8eff2ee40dc4d6c11ae63d79bfd79b6e/crossbeam-utils/src/lib.rs#L47-L51

This module got disabled because the value of cfg!(target_has_atomic) was changed between two nightlies:

/t/bar (master|…) $ cargo +nightly-2019-10-13 run
   Compiling bar v0.1.0 (/tmp/bar)
    Finished dev [unoptimized + debuginfo] target(s) in 0.38s
     Running `target/debug/bar`
[src/main.rs:3] cfg!(target_has_atomic = "cas") = true
[src/main.rs:4] cfg!(target_has_atomic = "ptr") = true

/t/bar (master|…) $ cargo +nightly-2019-10-14 run
   Compiling bar v0.1.0 (/tmp/bar)
    Finished dev [unoptimized + debuginfo] target(s) in 0.36s
     Running `target/debug/bar`
[src/main.rs:3] cfg!(target_has_atomic = "cas") = false
[src/main.rs:4] cfg!(target_has_atomic = "ptr") = true

[sinkuu]

target_has_atomic = "cas" was removed by #65214 (cfg_target_has_atomic is a nightly feature), so this is an expected breakage. crossbeam-utils crate has to be fixed.

[topecongiro]

@sinkuu Thank you for all the information! I will close this as it is an expected breakage.