Run builds on both Mac OS and Linux (#830)

* Run builds on both Mac OS and Linux

This would have helped us catch #570.

* Fix MacOS CI (#848)

* Revert unnecessary changes

The changes might be good, but I want to keep this PR small and
focused. If we end up with the extra `cfg` statements, we should
include a comment to let students know what they do: we’re targeting
people new to Rust, so we need to be careful with explanations.

---------

Co-authored-by: Dominik Maier <domenukk@gmail.com>

.github/workflows/build.yml

@@ -25,7 +25,12 @@ jobs:
         uses: dprint/check@v2.2
 
   cargo:
-    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        os:
+          - ubuntu-latest
+          - macos-latest
+    runs-on: ${{ matrix.os }}
     steps:
       - name: Checkout
         uses: actions/checkout@v3

src/exercises/day-3/safe-ffi-wrapper.rs

@@ -37,7 +37,7 @@ mod ffi {
     }
 
     // Layout as per man entry for dirent
-    #[cfg(target_os = "macos")]
+    #[cfg(all(target_os = "macos", target_arch = "aarch64"))]
     #[repr(C)]
     pub struct dirent {
         pub d_ino: u64,
@@ -48,6 +48,17 @@ mod ffi {
         pub d_name: [c_char; 1024],
     }
 
+    // Layout according to <https://github.com/rust-lang/libc/issues/414>
+    #[cfg(all(target_os = "macos", target_arch = "x86_64"))]
+    #[repr(C)]
+    pub struct dirent {
+        pub d_fileno: u32,
+        pub d_reclen: u16,
+        pub d_type: u8,
+        pub d_namlen: u8,
+        pub d_name: [c_char; 256],
+    }
+
     extern "C" {
         pub fn opendir(s: *const c_char) -> *mut DIR;
         pub fn readdir(s: *mut DIR) -> *const dirent;