incr.comp.: Load dep-graph in the background

[michaelwoerister]

The dependency graph is loaded at the earliest point possibly during the compilation session (after the crate disambiguator has been set):

rust/src/librustc_driver/driver.rs

Line 654 in bd7021f

let dep_graph = if sess.opts.build_dep_graph() {

However, it is only needed for the first time a few passes later:

rust/src/librustc_driver/driver.rs

Line 890 in bd7021f

let hir_crate = lower_crate(sess, cstore, &dep_graph, &krate, &mut resolver);

This was intentional so that we have to possibility to load and decode it the graph on a background thread. One way to implement this is to make rustc_incremental::load_dep_graph() start a new thread and return the thread::JoinHandle and then only join that thread right before the dep-graph is actually needed.

[Yoric]

I'll try and do that.

[michaelwoerister]

@Yoric Cool, thank you!