Function download_tarball 

pub fn download_tarball(
    client: &Client,
    version: &str,
    dest_dir: &Path,
    cli_label: &str,
    skip_sha256: bool,
    mp: Option<&FetchProgress>,
) -> Result<AcquiredSource>

Download a kernel tarball (stable or RC) and extract it.

cli_label prefixes diagnostic status output (e.g. "ktstr" or "cargo ktstr").

skip_sha256 propagates to download_stable_tarball only — stable tarballs publish a sha256sums.asc manifest the flag bypasses. RC tarballs (download_rc_tarball) have no published manifest so verification is impossible regardless of the flag; the RC path always runs unverified and emits its own warning, so skip_sha256 is a no-op on the RC arm. --kernel <path> and --kernel git+… sources do not reach this function at all.

mp is the progress group the determinate download bar is added to; None disables the bar (the single-shot kernel build paths and unit tests pass None).