Fix cargo add overwriting symlinked Cargo.toml files

crates/cargo-util/src/paths.rs

@@ -190,14 +190,22 @@ 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 actual_path = if path.is_symlink() {
+        std::fs::read_link(path)?
+    } else {
+        path.to_path_buf()
+    };
+
     // 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.
     #[cfg(unix)]
-    let perms = path.metadata().ok().map(|meta| {
+    let perms = actual_path.metadata().ok().map(|meta| {
         use std::os::unix::fs::PermissionsExt;
 
         // these constants are u16 on macOS
@@ -208,8 +216,8 @@ pub fn write_atomic<P: AsRef<Path>, C: AsRef<[u8]>>(path: P, contents: C) -> Res
     });
 
     let mut tmp = TempFileBuilder::new()
-        .prefix(path.file_name().unwrap())
-        .tempfile_in(path.parent().unwrap())?;
+        .prefix(actual_path.file_name().unwrap())
+        .tempfile_in(actual_path.parent().unwrap())?;
     tmp.write_all(contents.as_ref())?;
 
     // On unix platforms, set the permissions on the newly created file. We can use fchmod (called
@@ -220,7 +228,7 @@ pub fn write_atomic<P: AsRef<Path>, C: AsRef<[u8]>>(path: P, contents: C) -> Res
         tmp.as_file().set_permissions(perms)?;
     }
 
-    tmp.persist(path)?;
+    tmp.persist(&actual_path)?;
     Ok(())
 }
 

tests/testsuite/cargo_add/mod.rs

@@ -145,6 +145,7 @@ 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 +1,51 @@
+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());
+}