Function build_plugin 

pub fn build_plugin(
    builder: &dyn ComponentBuilder,
    source_dir: &Path,
    name: &str,
    user_root: &Path,
    pinned: bool,
) -> BuildOutcome

Build name from source_dir into user_root, unless the cached artifact is already current.

pinned skips the staleness check entirely: build only if the artifact is absent. That is the escape hatch for a user who wants a known-good build to stay put regardless of what the source tree does.

Blocking. Run it on spawn_blocking — never the boot or actor thread (§5).