Function build_diff 

pub fn build_diff<G: GitRepository, V: VcsProvider + ?Sized>(
    plan: &ReleasePlan,
    git: &G,
    vcs: &V,
    config: &Config,
    env: &[(&str, &str)],
) -> Result<ReleaseDiff, ReleaseError>

Build the diff by querying actual state for every resource the plan would touch.

env is forwarded to publishers (they run check() which may shell out — for built-in publishers like cargo/npm, check is a pure HTTP call, but custom runs the user’s check command).