Preserve lockfile top comment

src/cargo/ops/lockfile.rs

@@ -43,6 +43,16 @@ pub fn write_pkg_lockfile(ws: &Workspace, resolve: &Resolve) -> CargoResult<()>
 
     let mut out = String::new();
 
+    // Preserve the top comments in the lockfile
+    // This is in preparation for marking it as generated
+    // https://github.com/rust-lang/cargo/issues/6180
+    if let Ok(orig) = &orig {
+        for line in orig.lines().take_while(|line| line.starts_with("#")) {
+            out.push_str(line);
+            out.push_str("\n");
+        }
+    }
+
     let deps = toml["package"].as_array().unwrap();
     for dep in deps.iter() {
         let dep = dep.as_table().unwrap();

tests/testsuite/update.rs

@@ -374,3 +374,24 @@ fn update_precise() {
 ",
         ).run();
 }
+
+#[test]
+fn preserve_top_comment() {
+    let p = project().file("src/lib.rs", "").build();
+
+    p.cargo("update").run();
+
+    let mut lockfile = p.read_file("Cargo.lock");
+    lockfile.insert_str(0, "# @generated\n");
+    lockfile.insert_str(0, "# some other comment\n");
+    println!("saving Cargo.lock contents:\n{}", lockfile);
+
+    p.change_file("Cargo.lock", &lockfile);
+
+    p.cargo("update").run();
+
+    let lockfile2 = p.read_file("Cargo.lock");
+    println!("loaded Cargo.lock contents:\n{}", lockfile2);
+
+    assert!(lockfile == lockfile2);
+}