Fix cargo add overwriting symlinked Cargo.toml files (#15281)

### What does this PR try to resolve?

This PR fixes a bug where `cargo add` breaks symlinks to Cargo.toml
files. Currently, when Cargo.toml is a symlink and `cargo add` is used
to add a dependency, the symlink is replaced with a regular file,
breaking the link to the original target file.

This issue was reported in #15241 where a user who relies on symlinked
Cargo.toml files found that `cargo add` breaks their workflow.

Fixes #15241

### How should we test and review this PR?

I've modified `LocalManifest::write()` to check if the path is a
symlink, and if so, follow it to get the actual target path. This
ensures we write to the actual file rather than replacing the symlink.

I've also added a test in `tests/testsuite/cargo_add/symlink.rs` that:
1. Creates a symlinked Cargo.toml file
2. Runs `cargo add` to add a dependency
3. Verifies the symlink is preserved and the dependency is added to the
target file

I've manually tested this fix and confirmed it works correctly.

crates/cargo-util/src/paths.rs

@@ -190,9 +190,20 @@ 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 = fs::read_link(path)
+            .with_context(|| format!("failed to read symlink at `{}`", path.display()))?;
+        &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.
@@ -981,6 +992,33 @@ mod tests {
         }
     }
 
+    #[test]
+    fn write_atomic_symlink() {
+        let tmpdir = tempfile::tempdir().unwrap();
+        let target_path = tmpdir.path().join("target.txt");
+        let symlink_path = tmpdir.path().join("symlink.txt");
+
+        // Create initial file
+        write(&target_path, "initial").unwrap();
+
+        // Create symlink
+        #[cfg(unix)]
+        std::os::unix::fs::symlink(&target_path, &symlink_path).unwrap();
+        #[cfg(windows)]
+        std::os::windows::fs::symlink_file(&target_path, &symlink_path).unwrap();
+
+        // Write through symlink
+        write_atomic(&symlink_path, "updated").unwrap();
+
+        // Verify both paths show the updated content
+        assert_eq!(std::fs::read_to_string(&target_path).unwrap(), "updated");
+        assert_eq!(std::fs::read_to_string(&symlink_path).unwrap(), "updated");
+
+        // Verify symlink still exists and points to the same target
+        assert!(symlink_path.is_symlink());
+        assert_eq!(std::fs::read_link(&symlink_path).unwrap(), target_path);
+    }
+
     #[test]
     #[cfg(windows)]
     fn test_remove_symlink_dir() {

tests/testsuite/cargo_add/mod.rs

@@ -150,6 +150,7 @@ mod script_bare;
 mod script_frontmatter;
 mod script_shebang;
 mod sorted_table_with_dotted_item;
+mod symlink;
 mod target;
 mod target_cfg;
 mod unknown_inherited_feature;

tests/testsuite/cargo_add/symlink.rs

@@ -0,0 +1,54 @@
+use cargo_test_support::prelude::*;
+use cargo_test_support::project;
+use cargo_test_support::registry;
+use std::fs;
+
+#[cargo_test]
+fn symlink_case() {
+    if !cargo_test_support::symlink_supported() {
+        return;
+    }
+
+    registry::init();
+    registry::Package::new("test-dep", "1.0.0").publish();
+
+    let project = project().file("src/lib.rs", "").build();
+
+    let target_dir = project.root().join("target_dir");
+    fs::create_dir_all(&target_dir).unwrap();
+
+    fs::copy(
+        project.root().join("Cargo.toml"),
+        target_dir.join("Cargo.toml"),
+    )
+    .unwrap();
+
+    fs::remove_file(project.root().join("Cargo.toml")).unwrap();
+
+    #[cfg(unix)]
+    {
+        use std::os::unix::fs::symlink;
+        symlink(
+            target_dir.join("Cargo.toml"),
+            project.root().join("Cargo.toml"),
+        )
+        .unwrap();
+    }
+
+    #[cfg(windows)]
+    {
+        use std::os::windows::fs::symlink_file;
+        symlink_file(
+            target_dir.join("Cargo.toml"),
+            project.root().join("Cargo.toml"),
+        )
+        .unwrap();
+    }
+
+    project.cargo("add test-dep").run();
+
+    assert!(project.root().join("Cargo.toml").is_symlink());
+
+    let target_content = fs::read_to_string(target_dir.join("Cargo.toml")).unwrap();
+    assert!(target_content.contains("test-dep"));
+}