feat(style): enable and run cargo fmt

this PR updates the way mods are imported in src/device/mod to
avoid a bug in cargo fmt that struggles with cfg_attr(path)
(see: rust-lang/rustfmt#2407).

once that bug was avoided, that allowed us to run cargo fmt.
this PR includes those formatting updates and adds a cargo
fmt check to CI.

Author: ashleygwilliams

ci/azure-build.yml

@@ -5,6 +5,8 @@ parameters:
 steps:
   - script: |
       curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain stable
+      export PATH=$PATH:$HOME/.cargo/bin
+      rustup component add rustfmt
     displayName: 'Install cargo'
   - script: |
       export PATH=$PATH:$HOME/.cargo/bin
@@ -26,4 +28,8 @@ steps:
     - script: |
         export PATH=$PATH:$HOME/.cargo/bin
         cargo test --bin boringtun -- --test-threads=1
-      displayName: 'Test executable'
+      displayName: 'Test executable'
+  - script: |
+      export PATH=$PATH:$HOME/.cargo/bin
+      cargo fmt --all -- --check
+    displayName: 'Cargo fmt'

src/device/mod.rs

@@ -7,13 +7,25 @@ pub mod dev_lock;
 pub mod drop_privileges;
 mod integration_tests;
 pub mod peer;
-#[cfg_attr(any(target_os = "macos", target_os = "ios"), path = "kqueue.rs")]
-#[cfg_attr(target_os = "linux", path = "epoll.rs")]
+
+#[cfg(any(target_os = "macos", target_os = "ios"))]
+#[path = "kqueue.rs"]
+pub mod poll;
+
+#[cfg(target_os = "linux")]
+#[path = "epoll.rs"]
 pub mod poll;
-#[cfg_attr(any(target_os = "macos", target_os = "ios"), path = "tun_darwin.rs")]
-#[cfg_attr(target_os = "linux", path = "tun_linux.rs")]
+
+#[cfg(any(target_os = "macos", target_os = "ios"))]
+#[path = "tun_darwin.rs"]
 pub mod tun;
-#[cfg_attr(unix, path = "udp_unix.rs")]
+
+#[cfg(target_os = "linux")]
+#[path = "tun_linux.rs"]
+pub mod tun;
+
+#[cfg(unix)]
+#[path = "udp_unix.rs"]
 pub mod udp;
 
 use crypto::x25519::*;