Various fixups to unsafe code (#37651)

A collection of fixups of possibly-unsound code and removing some small
useless writes.

Release Notes:

- N/A

crates/fs/src/fs.rs

@@ -20,6 +20,9 @@ use std::os::fd::{AsFd, AsRawFd};
 #[cfg(unix)]
 use std::os::unix::fs::{FileTypeExt, MetadataExt};
 
+#[cfg(any(target_os = "macos", target_os = "freebsd"))]
+use std::mem::MaybeUninit;
+
 use async_tar::Archive;
 use futures::{AsyncRead, Stream, StreamExt, future::BoxFuture};
 use git::repository::{GitRepository, RealGitRepository};
@@ -261,14 +264,15 @@ impl FileHandle for std::fs::File {
         };
 
         let fd = self.as_fd();
-        let mut path_buf: [libc::c_char; libc::PATH_MAX as usize] = [0; libc::PATH_MAX as usize];
+        let mut path_buf = MaybeUninit::<[u8; libc::PATH_MAX as usize]>::uninit();
 
         let result = unsafe { libc::fcntl(fd.as_raw_fd(), libc::F_GETPATH, path_buf.as_mut_ptr()) };
         if result == -1 {
             anyhow::bail!("fcntl returned -1".to_string());
         }
 
-        let c_str = unsafe { CStr::from_ptr(path_buf.as_ptr()) };
+        // SAFETY: `fcntl` will initialize the path buffer.
+        let c_str = unsafe { CStr::from_ptr(path_buf.as_ptr().cast()) };
         let path = PathBuf::from(OsStr::from_bytes(c_str.to_bytes()));
         Ok(path)
     }
@@ -296,15 +300,16 @@ impl FileHandle for std::fs::File {
         };
 
         let fd = self.as_fd();
-        let mut kif: libc::kinfo_file = unsafe { std::mem::zeroed() };
+        let mut kif = MaybeUninit::<libc::kinfo_file>::uninit();
         kif.kf_structsize = libc::KINFO_FILE_SIZE;
 
-        let result = unsafe { libc::fcntl(fd.as_raw_fd(), libc::F_KINFO, &mut kif) };
+        let result = unsafe { libc::fcntl(fd.as_raw_fd(), libc::F_KINFO, kif.as_mut_ptr()) };
         if result == -1 {
             anyhow::bail!("fcntl returned -1".to_string());
         }
 
-        let c_str = unsafe { CStr::from_ptr(kif.kf_path.as_ptr()) };
+        // SAFETY: `fcntl` will initialize the kif.
+        let c_str = unsafe { CStr::from_ptr(kif.assume_init().kf_path.as_ptr()) };
         let path = PathBuf::from(OsStr::from_bytes(c_str.to_bytes()));
         Ok(path)
     }