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.

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

@@ -68,7 +68,6 @@ mod ffi {
 
 use std::ffi::{CStr, CString, OsStr, OsString};
 use std::os::unix::ffi::OsStrExt;
-use std::ptr::addr_of;
 
 #[derive(Debug)]
 struct DirectoryIterator {
@@ -108,7 +107,7 @@ impl Iterator for DirectoryIterator {
         }
         // SAFETY: dirent is not NULL and dirent.d_name is NUL
         // terminated.
-        let d_name = unsafe { CStr::from_ptr(addr_of!((*dirent).d_name).cast()) };
+        let d_name = unsafe { CStr::from_ptr((*dirent).d_name.as_ptr()) };
         let os_str = OsStr::from_bytes(d_name.to_bytes());
         Some(os_str.to_owned())
     }
@@ -142,7 +141,6 @@ mod tests {
     use std::error::Error;
 
     #[test]
-    #[cfg(not(miri))]
     fn test_nonexisting_directory() {
         let iter = DirectoryIterator::new("no-such-directory");
         assert!(iter.is_err());
@@ -161,7 +159,6 @@ mod tests {
     }
 
     #[test]
-    #[cfg(not(miri))]
     fn test_nonempty_directory() -> Result<(), Box<dyn Error>> {
         let tmp = tempfile::TempDir::new()?;
         std::fs::write(tmp.path().join("foo.txt"), "The Foo Diaries\n")?;