fix: Preserve symlinks when running cargo add

When cargo add is used on a project where Cargo.toml is a symlink,
it now follows the symlink and writes to the target file instead of
replacing the symlink with a regular file.

This is achieved by updating write_atomic() to detect symlinks and
resolve them before writing.

Fixes #15241

Author: RaghavenderSingh

crates/cargo-util/src/paths.rs

@@ -190,9 +190,19 @@ pub fn write<P: AsRef<Path>, C: AsRef<[u8]>>(path: P, contents: C) -> Result<()>
 /// Writes a file to disk atomically.
 ///
 /// This uses `tempfile::persist` to accomplish atomic writes.
+/// If the path is a symlink, it will follow the symlink and write to the actual target.
 pub fn write_atomic<P: AsRef<Path>, C: AsRef<[u8]>>(path: P, contents: C) -> Result<()> {
     let path = path.as_ref();
 
+    // Check if the path is a symlink and follow it if it is
+    let resolved_path;
+    let path = if path.is_symlink() {
+        resolved_path = std::fs::read_link(path)?;
+        &resolved_path
+    } else {
+        path
+    };
+
     // On unix platforms, get the permissions of the original file. Copy only the user/group/other
     // read/write/execute permission bits. The tempfile lib defaults to an initial mode of 0o600,
     // and we'll set the proper permissions after creating the file.
@@ -201,10 +211,9 @@ pub fn write_atomic<P: AsRef<Path>, C: AsRef<[u8]>>(path: P, contents: C) -> Res
         use std::os::unix::fs::PermissionsExt;
 
         // these constants are u16 on macOS
-        let mask = u32::from(libc::S_IRWXU | libc::S_IRWXG | libc::S_IRWXO);
-        let mode = meta.permissions().mode() & mask;
-
-        std::fs::Permissions::from_mode(mode)
+        const PERMS_MASK: u32 = 0o777;
+        let mode = meta.permissions().mode() as u32;
+        std::fs::Permissions::from_mode(mode & PERMS_MASK)
     });
 
     let mut tmp = TempFileBuilder::new()
@@ -213,7 +222,7 @@ pub fn write_atomic<P: AsRef<Path>, C: AsRef<[u8]>>(path: P, contents: C) -> Res
     tmp.write_all(contents.as_ref())?;
 
     // On unix platforms, set the permissions on the newly created file. We can use fchmod (called
-    // by the std lib; subject to change) which ignores the umask so that the new file has the same
+    // by the std lib method below) which ignores the umask so that the new file has the same
     // permissions as the old file.
     #[cfg(unix)]
     if let Some(perms) = perms {