Function traverse_and_share 

fn traverse_and_share(
    interner: &UnitInterner,
    memo: &mut HashMap<Unit, Unit>,
    new_graph: &mut UnitGraph,
    unit_graph: &UnitGraph,
    unit: &Unit,
    unit_is_root: bool,
    unit_is_for_host: bool,
    to_host: Option<CompileKind>,
    compile_time_deps_only: bool,
) -> Unit

Recursive function for rebuilding the graph.

This walks unit_graph, starting at the given unit. It inserts the new units into new_graph, and returns a new updated version of the given unit (dep_hash is filled in, and kind switched if necessary).