Function merge_kconfig_fragments 

pub fn merge_kconfig_fragments<'a>(
    baked: &'a str,
    extra: Option<&str>,
) -> Cow<'a, str>

Merge the user-supplied --extra-kconfig fragment on top of EMBEDDED_KCONFIG for the configure pass. Returns a std::borrow::Cow so the no-extras branch borrows baked without allocating; only the Some branch heaps the merged String.

The user fragment is appended AFTER the baked-in fragment so kbuild’s last-wins rule (scripts/kconfig/confdata.c::conf_read_simple — “If conflicting CONFIG options are given from an input file, the last one wins.”) makes user values override baked-in ones on conflict.

A single \n separator is interleaved between the two fragments. EMBEDDED_KCONFIG ends in a newline today, so the interleaved \n produces a blank line between the segments — kbuild’s .config parser ignores blank lines (every if (!line[0]) short-circuit in conf_read_simple), so the blank line is harmless. The separator is mandatory for the adversarial case where the operator hand-crafts an EMBEDDED_KCONFIG without a trailing newline AND a user fragment that starts with CONFIG_X — without the interleaved \n, the two would concatenate into a single malformed line. Always emit the separator so the merge is safe regardless of either side’s terminator.

The production configure path in crate::cli::kernel_build_pipeline calls this helper to build the bytes handed to configure_kernel. Tests that assert merge-ordering invariants call it directly so the production byte sequence is what kbuild’s last-wins rule operates on. (Note: cache_key_suffix_with_extra hashes extra ALONE for its xkc{...} segment — it doesn’t pass through this helper — so the cache-key suffix and the merged-fragment content evolve independently. The cache-key segment exists to discriminate extras-vs-no-extras at the cache layer; the merge ordering exists to give kbuild the right final value.)