test: Add failing test for cargo add breaking symlinks

This test demonstrates that cargo add replaces symlinked Cargo.toml
files with regular files, breaking the symlink. The test currently
fails as expected.

Author: RaghavenderSingh

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 +1,52 @@
+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();
+
+    // This will FAIL because cargo add replaces the symlink with a regular file
+    assert!(project.root().join("Cargo.toml").is_symlink());
+}