chore: add autogenerated banner to cpp files

Author: asterite

acvm-repo/acir/codegen/acir.cpp

@@ -227,6 +227,7 @@ mod reflection {
         MsgPackCodeGenerator::add_preamble(&mut source);
         MsgPackCodeGenerator::add_helpers(&mut source, namespace);
         MsgPackCodeGenerator::replace_array_with_shared_ptr(&mut source);
+        MsgPackCodeGenerator::add_autogen_header(&mut source);
 
         if !should_overwrite()
             && let Some(old_hash) = old_hash
@@ -380,6 +381,23 @@ mod reflection {
     }
 
     impl<'a> MsgPackCodeGenerator<'a> {
+        /// Prepend a banner marking the file as auto-generated and pointing
+        /// readers at the generator. Without it, the committed `.cpp` files
+        /// look hand-written and an unsuspecting editor would lose their
+        /// changes the next time the codegen tests run.
+        pub(crate) fn add_autogen_header(source: &mut String) {
+            let header = "\
+// AUTO-GENERATED — DO NOT EDIT.
+//
+// Generated by the `cpp_codegen` tests in `acvm-repo/acir/src/lib.rs`.
+// To regenerate, run:
+//
+//     NOIR_CODEGEN_OVERWRITE=1 cargo test -p acir cpp_codegen
+//
+";
+            source.insert_str(0, header);
+        }
+
         /// Add the import of the Barretenberg C++ header for msgpack.
         pub(crate) fn add_preamble(source: &mut String) {
             let inc = r#"#include "serde.hpp""#;