fece9c7356
incr.comp.: Make cross-crate tracking for incr. comp. opt-in. The current implementation of cross-crate dependency tracking can cause quite long compile times and high memory usage for some crates (see #39208 for example). This PR therefore makes that part of dependency tracking optional. Incremental compilation still works, it will only have very coarse dep-tracking for upstream crates. r? @nikomatsakis
This is the code to load/save the dependency graph. Loading is assumed to run early in compilation, and saving at the very end. When loading, the basic idea is that we will load up the dependency graph from the previous compilation and compare the hashes of our HIR nodes to the hashes of the HIR nodes that existed at the time. For each node whose hash has changed, or which no longer exists in the new HIR, we can remove that node from the old graph along with any nodes that depend on it. Then we add what's left to the new graph (if any such nodes or edges already exist, then there would be no effect, but since we do this first thing, they do not).