Function setup_dep_graph 

pub fn setup_dep_graph(
    sess: &Session,
    crate_name: Symbol,
    stable_crate_id: StableCrateId,
) -> DepGraph

Loads the previous session’s dependency graph from disk if possible, and sets up streaming output for the current session’s dep graph data into an incremental session directory.

In non-incremental mode, a dummy dep graph is returned immediately.